Electrolyte Design for Fast‐Charging Lithium‐Based Batteries

ABSTRACT

Fast charging is essential for the widespread adoption of lithium (Li)‐ion batteries, but it is fundamentally limited by sluggish interfacial kinetics, Li plating, and electrolyte instability at high current densities. Over the past decade, electrolyte engineering has emerged as a key strategy to address these challenges. This review summarizes the development of fast‐charging electrolytes over the past ten years and outlines a design framework. Electrolyte formulations are first deconstructed into their main components—solvents, salts, and functional additives—and representative strategies for tuning solvation structure and interphase chemistry are discussed to suppress Li plating and improve interfacial kinetics. The discussion then extends to advanced electrolyte systems, particularly localized high‐concentration electrolytes (LHCEs), and their compatibility with different anode chemistries. Advanced characterization techniques are also summarized and categorized based on destructiveness, spatial and temporal resolution, quantitative analysis, and the chemical species or processes probed across multiple length scales. Recent progress in AI‐enabled electrolyte discovery and battery management system (BMS) strategies for optimized fast‐charging protocols is further highlighted. Finally, perspectives are presented on translating electrolyte innovations from academic research to practical applications, with emphasis on cell format, realistic operating conditions, and manufacturability.

A decade of progress in fast‐charging electrolytes for lithium batteries is reviewed. Electrolyte design strategies spanning solvents, salts, additives, and advanced systems, such as localized high‐concentration electrolytes (LHCEs), are summarized. Advanced diagnostic tools for lithium plating and interphase chemistry are discussed, with perspectives on bridging fundamental electrolyte discoveries with practical battery applications.

1. Introduction

Over the past two decades, battery‐powered systems have expanded from consumer electronics to other applications: These include electric vehicles (EVs), advanced air mobility (AAM) platforms, such as electric vertical take‐off and landing aircrafts (eVTOL) and unmanned delivery drones, and emerging high‐utilization applications, such as heavy‐duty trucks and humanoid robots, accompanied by a rapid escalation in both charging‐rate requirements and operational life cycles (Figure 1a) [1]. In consumer electronics, moderate power demand and limited cycling intensity have historically been acceptable; however, modern EVs increasingly target ultra‐fast charging times below 15–20 min, as demonstrated by recent technologies developed by companies, such as CATL, BYD, and Tesla. Meanwhile, eVTOL platforms pursued by companies like Joby and Archer demand not only extreme power output, particularly during takeoff and landing hovers (usually ∼4C), but also fast energy resupply to realize economical success [2, 3, 4]. Likewise, heavy‐duty trucks require not only high power but also sustained, repetitive fast‐charging cycles to enable continuous logistics operations [1, 2, 5, 6, 7, 8]. In this evolving landscape, fast charging is no longer merely a desirable feature, but a fundamental performance constraint that is expected to define the practical limits of next‐generation electrified systems beyond 2030 [3, 4].

FIGURE 1.

(a) Historical and emerging application scenarios of lithium‐based batteries, illustrating the continuous escalation of charging‐rate requirements and duty cycles. (b) Radar plot summarizing key battery performance metrics. (c) Comparative assessment of cathode, anode, and electrolyte design levers in terms of effectiveness, cost, feasibility, and compatibility. (d) Mechanistic illustration of Li⁺ transport, interfacial processes, and the emergence of Li⁺ concentration polarization at high C rates. Reproduced with permission [9]. Copyright 2024, Wiley‐VCH. (e) Classification of electrolyte design strategies into conventional and advanced paradigms, with distinct emphasis on each component, providing a conceptual map for rational electrolyte optimization toward fast‐charging performance.

This shift in application demands exposes a critical imbalance in conventional battery development priorities [10]. Historically, battery research has focused predominantly on calendar life, cycle life, and interfacial stability, whereas high‐rate capability has often been treated as a secondary objective (Figure 1b). As a result, many commercial battery systems exhibit excellent durability under moderate operating conditions, but suffer from severe degradation, performance losses, and heightened safety risks under aggressive fast‐charging protocols, underscoring that improvements in charging speed cannot come at the expense of thermal stability and safety for large‐scale deployment [11, 12, 13]. Consequently, next‐generation batteries must operate within a multidimensional performance space in which rate capability, lifetime, interphase robustness, thermal resilience, and safety are simultaneously optimized (Figure 1b). This inherent tension highlights the limitations of conventional single‐parameter optimization strategies and motivates a more holistic design guideline.

To address these challenges, it is instructive to shift the discussion from the battery‐system level to the material level. From this perspective, cathodes, anodes, and electrolytes constitute three distinct design levers for enabling fast charging, each associated with characteristic trade‐offs in effectiveness, cost, feasibility, and compatibility (Figure 1c) [3, 14, 15]. Importantly, the rate‐limiting process during fast charging is not fixed but evolves with charging rate and cell design. At moderate charging rates (typically ≤1–2 C), polarization can arise from a combination of solid‐state Li‐ion diffusion within active materials and charge‐transfer kinetics at the electrode/electrolyte interface. As charging rate increases, however, transport limitations become increasingly concentrated at the anode side, particularly in graphite‐based systems, where large Li⁺ concentration gradients and interfacial polarization accelerate the onset of Li plating. Under extreme fast‐charging conditions (>3–4C rate), electrolyte transport and interphase stability emerge as critical factors because they directly govern ion delivery to the anode surface and determine the severity of concentration polarization.

Cathode engineering can contribute to improved rate capability through strategies, such as particle‐size reduction, microstructural engineering, and interphase modification [16, 17]. Nevertheless, advances in cathode materials and electrode architectures have significantly improved Li‐ion transport kinetics, and high‐energy cathodes, such as layered oxides, are generally not considered the primary bottleneck under practical fast‐charging conditions. However, high‐energy cathodes like layered oxides are not primarily limited by their fast‐charging capability; moreover, cathode optimization often relies on complex synthesis and processing routes, which substantially increase material and manufacturing costs [13, 18].

In comparison, the anode electrode, including both graphite (Gr) and Li‐metal anodes, represents the primary kinetic bottleneck under fast‐charging operation [19, 20]. For Gr anodes, concentration polarization and sluggish intercalation kinetics can induce Li plating, while Li‐metal anodes suffer from dendrite growth and interfacial instability under fast charging [21, 22, 23]. Although anode engineering strategies, including particle‐size tuning, pitch coating, morphology optimization, and artificial interphase design, can partially mitigate these limitations, their effectiveness is often constrained by manufacturability, cost, and compatibility considerations, particularly at commercially relevant charging rates.

Compared with electrode materials, electrolyte engineering offers a uniquely powerful and system‐wide approach as it simultaneously governs bulk ion transport and interfacial reactions at both electrodes [24, 25, 26, 27, 28]. Because electrolyte properties directly influence ionic conductivity, Li⁺ transference, solvation/desolvation behavior, and interphase formation, electrolyte design becomes increasingly important as charging rates approach the practical limits of graphite‐based batteries. However, this leverage is accompanied by intrinsic constraints. Depending on the selection of salts, solvents, additives, and diluents, electrolyte formulations can range from cost‐effective to highly expensive, while also requiring stringent electrochemical and chemical compatibility with both the cathode and anode across wide operating voltage windows [1, 10, 14]. In addition, electrolyte design must reconcile multiple competing physicochemical properties, including ionic conductivity, solvation strength, interfacial stability, thermal robustness, and safety, within a single formulation, which fundamentally limits its feasibility [29]. Therefore, while electrolyte optimization emerges as one of the most effective strategies for overcoming fast‐charging limitations, it also represents one of the most complex and constrained design spaces in lithium‐based batteries.

At the mechanistic level, fast charging involves a sequence of coupled processes: (i) Li⁺ extraction from the cathode lattice, (ii) solvation by anions and solvents in the electrolyte, (iii) transport through the bulk electrolyte driven by the electric field and concentration gradients, (iv) desolvation at the solid‐electrolyte interphase (SEI), and (v) subsequent intercalation into the anode host or deposition on the anode surface (Figure 1d) [13, 28, 30, 31]. Under moderate charging conditions, these processes remain kinetically less constrained; however, at high current densities, limitations in Li⁺ transport and interfacial reaction kinetics induce pronounced concentration polarization, which can drive the graphite anode potential below 0 V vs. Li/Li⁺ and trigger Li plating, thereby accelerating active Li loss and performance degradation [1, 19, 28, 30, 31, 32]. As battery systems evolve toward higher energy densities and increasingly incorporate Li‐metal anodes, these challenges become even more severe [33, 34, 35]. The high local current densities aggravate non‐uniform Li deposition and dendrite formation under fast‐charging conditions. Concurrently, intensified interfacial reactions accelerate SEI and cathode–electrolyte interphase (CEI) evolution, accompanied by gas generation and heat accumulation, increasing the risk of degradation and thermal instability [16, 36, 37, 38, 39, 40]. Collectively, these phenomena highlight the central role of electrolyte chemistry in governing Li⁺ solvation, transport, and interfacial dynamics under fast‐charging conditions.

Given this pivotal role, electrolyte design can be further decomposed into its fundamental chemical components. As illustrated in Figure 1e, electrolyte strategies can be broadly categorized into two groups: conventional electrolytes and advanced electrolytes, the latter predominantly represented by localized high‐concentration electrolytes (LHCEs) [10, 24, 25, 28, 41]. Conventional electrolytes typically emphasize solvent engineering and functional additives, focusing on tuning solvent polarity, viscosity, and electrochemical stability, as well as additive‐driven regulation of SEI/CEI formation [14, 15, 33, 42, 43]. In contrast, advanced electrolytes prioritize salt chemistry and concentration effects, combined with the use of inert or weakly coordinating diluents to decouple solvation structure from bulk transport properties. This shift in design philosophy fundamentally reshapes Li⁺ solvation environments and transport pathways, enabling enhanced high‐rate performance and interfacial robustness, albeit often at the cost of increased formulation complexity and economic burden [44, 45]. Together, these two electrolyte families define the current landscape of fast‐charging electrolyte research over the past two decades.

In this review, we systematically examine solvents, salts, diluents, and functional additives in relation to their physicochemical properties, interphase formation behavior, electrochemical performance, safety characteristics, and application feasibility (Figure 2). In parallel, we summarize state‐of‐the‐art characterization techniques for identifying and quantifying Li plating and interphase evolution across multiple spatial and temporal scales, highlighting the complementarity between qualitative and quantitative approaches, destructive and non‐destructive methodologies, and material‐level versus cell‐level diagnostics. Beyond materials chemistry, we highlight the increasing integration of machine learning (ML) and artificial intelligence (AI) in accelerating electrolyte discovery and optimization, as well as the role of battery management systems (BMS) in translating material‐level advances into system‐level fast‐charging strategies. By comparing diverse electrolyte systems across battery chemistries and application scenarios, we aim to extract mechanistic insights and practical design principles that can guide the rational development of next‐generation fast‐charging energy‐storage technologies.

FIGURE 2.

Conceptual framework for fast‐charging electrolyte design, linking electrolyte composition with ion‐transport kinetics, interfacial chemistry, and system‐level performance metrics.

2. Non‐Aqueous Liquid Electrolyte for Fast Charging LIBs

2.1. Solvents

To begin with, solvents have long been recognized and extensively investigated as the most critical component of electrolyte systems as they constitute the dominant fraction of the electrolyte and therefore fundamentally govern key physicochemical properties, including Li⁺ transport kinetics and interfacial reaction pathways [24, 28]. As summarized in Figure 3a, carbonate‐, ester‐, and ether‐based solvents remain the most extensively investigated in fast‐charging lithium batteries [46]. Among the major solvent families, carbonate‐ and ester‐based solvents exhibit good compatibility with both graphite anodes and high‐voltage oxide cathodes, owing to their satisfactory film‐forming ability and oxidative stability. In contrast, ether‐based solvents are predominantly employed in advanced electrolyte formulations, such as LHCEs, where salts and diluents play a dominant role, and are more widely adopted in Li‐metal systems due to their favorable Li⁺ coordination environments and high reductive stability [42, 47]. Alternative solvent families, including nitriles and formates, have received comparatively limited attention despite their promising physicochemical properties, primarily due to their insufficient chemical and electrochemical stability and poor compatibility with conventional electrode materials [48]. Finally, fluorinated solvents have attracted increasing interest as fluorination can facilitate robust interphase formation and reduce Li⁺ desolvation energy [49].

FIGURE 3.

(a) Overview of major solvent families and their functional roles in regulating interphase formation and ion‐transport properties. (b) Comparison of representative interphase components formed from solvent decomposition on graphite and Li‐metal anodes, and their adsorption and desolvation energy [50]. (c) Schematic illustration of strongly and weakly coordinating solvents, showing the relationship between solvent coordination strength and Li⁺ desolvation barriers. (d) Influence of solvent‐dependent viscosity and solvation structure on Li⁺ diffusion and ionic conductivity, distinguishing fast and slow ion‐transport regimes. (e) Comparison between Li⁺–solvent and anion‐solvent interactions and its impact on Li⁺ transference number. (f) Correlations between solvent dielectric constant, donor number, and coordination strength of various solvents. Reproduced with permission [51]. Copyright 2025, Royal Society of Chemistry. (g) Electrolyte ion transport mechanism of highly concentrated electrolytes, weakly solvating electrolytes, and small‐ligand solvation electrolytes. Reproduced with permission [52]. Copyright 2025, Wiley‐VCH.

Beyond compositional diversity, solvent design can be rationalized through two overarching functional dimensions: interphase regulation and ionic transport. Specifically, interphase regulation governs the chemistry, architecture, and spatial homogeneity of SEI/CEI layers through solvent decomposition pathways. In parallel, ionic transport is dictated by Li⁺ solvation/desolvation kinetics, ionic conductivity, and transference number, governed predominantly by solvent intrinsic physicochemical properties, as discussed below.

From an interphase perspective, (non‐)fluorinated solvents primarily contribute to the formation of organic interphase components, such as lithium ethylene monocarbonate (LEMC) and polyethylene carbonate, which typically exhibit reasonable ionic conductivity and mechanical flexibility. However, these organic species generally possess lower chemical stability and mechanical robustness compared with inorganic interphase constituents, including LiF, Li₂CO₃, Li₂O, and Li₃P [50]. Among them, Li₃P, characterized by strong Li⁺ adsorption, low desolvation energy, and relatively high ionic conductivity, has been proposed as an effective complement to organic interphase components (Figure 3b) [50]. It is generally recognized that inorganic‐rich interphases can offer adequate chemical and mechanical stability; however, an excessive inorganic fraction can be counterproductive for fast‐charging‐oriented interphase design [53]. Under fast‐charging conditions, rapid electrode volume changes and localized strain accumulation favor interphases with higher elasticity rather than excessive rigidity. Consequently, solvent chemistry plays a critical role in governing interphase composition and functional properties.

From the transport perspective, solvent primarily governs three factors dictating the Li⁺ transport feasibility—‐desolvation energy, ionic conductivity, and Li⁺ transference number. First, the solvent affinity with Li⁺ directly determines its binding energy with Li⁺. Strongly coordinating solvents that usually possess a high donor number (DN) exhibit higher desolvation energies and activation barriers, resulting in slower interfacial reaction kinetics and increased polarization (Figure 3c,f) [51]. Second, the electrolyte ionic conductivity (σ) is governed by the diffusion coefficients (D), which is in turn controlled by solvent viscosity (η) and solvation cluster size (r), as described by Nernst–Einstein and Einstein‐stokes equations, respectively (Figure 3d). Lower viscosity (usually with a low dielectric constant) and smaller solvation shells enabled by small ligand incorporation generally promotes higher diffusion coefficients and reduced polarization (Figure 3d,g) [52, 54]. Third, the Li⁺ transference number quantifies the fraction of total current carried by Li⁺ and is dictated by the relative strength of Li⁺–solvent versus anion‐solvent interactions. Stronger anion‐solvent coordination can effectively reduce the anion mobility and thereby enhance Li⁺ transport selectivity, mitigate concentration polarization, and ultimately improve its transport kinetics (Figure 3e) [55].

Overall, these insights underscore that solvent design for fast‐charging electrolytes is governed not by a single parameter, but by a delicate balance between interphase chemistry and transport kinetics. Successful fast‐charging electrolytes typically combine high ionic conductivity (>8–10 mS cm⁻¹), moderate viscosity (<5–10 mPa·s), sufficient Li⁺ transference, and robust interphase‐forming capability, thereby simultaneously promoting rapid ion transport and suppressing Li plating under high‐rate operation. These fundamental relationships provide the mechanistic basis for the solvent‐engineering strategies discussed in the following sections.

2.1.1. Carbonate/Ester System for Graphite Anode

2.1.1.1. DMC/FEC‐Based Electrolyte Skeleton

Linear carbonates (e.g., dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC)), particularly DMC‐based systems incorporating fluoroethylene carbonate (FEC), have emerged as a representative and widely adopted solvent skeleton for fast‐charging electrolyte design [9, 56, 57]. The DMC/FEC solvent skeleton is attractive because it addresses two important requirements of fast charging: (i) efficient bulk Li⁺ transport and (ii) sufficiently robust interphase formation. Linear carbonates usually exhibit low viscosity and high ionic conductivity, along with good electrochemical compatibility with graphite anodes and high‐voltage oxide cathodes, but they suffer from limited film‐forming ability [58, 59, 60, 61, 62]. In contrast, FEC provides strong interphase‐forming capability, while maintaining lower viscosity and a reduced donor number compared to ethylene carbonate (EC), which improves ionic conductivity and lowers Li⁺ desolvation energy at the electrode interface [63]. As a result, the DMC/FEC solvent skeleton offers a rational starting point for balancing transport kinetics and interfacial stability under fast‐charging conditions [60, 61, 62].

First, Hall et al. systematically examined the physicochemical properties of a broad range of co‐solvent classes, including linear carbonates (DMC, EMC, DEC) and FEC, which enable fast‐charging capability through fundamentally different pathways [60]. Specifically, linear carbonates primarily act as bulk Li⁺ transport promoters, where their intrinsically low viscosities (0.6–0.8 cP at 25°C) enhance ionic conductivity and Li⁺ diffusivity, mitigating concentration polarization near the graphite surface and alleviating anode overpotential buildup during fast charge. Although linear carbonates exhibit low dielectric constants (ε ≈ 2–3), their effectiveness as co‐solvents arises from decoupling salt solvation from transport, enabling high conductivity without strong Li⁺–solvent coordination excessively. Compared to linear carbonates, FEC mainly functions as an interfacial‐kinetics modifier, the decomposition of which forms robust interphases at both electrodes. Lastly, both linear carbonates and FEC exhibit sufficient oxidative stability to remain compatible with high‐voltage oxide cathodes.

Similarly, Yao et al. showed that replacing EC with FEC in a DMC‐based electrolyte fundamentally improves fast‐charging behavior by accelerating anode‐side Li⁺ desolvation, as directly evidenced by impedance analysis and full‐cell performance [58]. In conventional EC/DMC electrolytes, the average anode charge‐transfer resistance (R _ct) during charging remains on the order of 7–9 Ω, which is sufficiently large that high charging currents readily drive the Gr potential below 0 V vs. Li/Li⁺, triggering Li plating (Figure 4a) [58]. As a result, commercial Gr electrodes can only reach 12% SOC at 2C rate without severe polarization. By contrast, FEC/DMC electrolytes dramatically reduce the anode R _ct to 2 Ω (Figure 4a) [58]. Chemically, the electron‐withdrawing F substituent in FEC weakens Li⁺–solvent coordination, lowers the desolvation energy barrier, and accelerates interfacial charge transfer at the Gr surface, thereby allowing the anode to sustain substantially higher effective charging rates prior to Li plating. Evidently, 21700 NCA | Gr/SiO _x cylindrical cells (245 Wh kg^{− 1}) sustain 25 min charging to 80% SOC with a five times improvement in fast‐charge lifetime compared with conventional carbonate electrolytes (Figure 4b) [58].

FIGURE 4.

(a) Interfacial resistances for graphite electrodes over a 0%–90% state‐of‐charge window during Li⁺ intercalation. Reproduced with permission [58]. Copyright 2022, Wiley‐VCH. (b) Fast‐charging capacity retention of high‐energy NCA | Gr/SiO _x 21700 cylindrical cells with a commercial electrolyte and an XFC‐formulated electrolyte; the inset shows a photograph of 21700 cell. Reproduced with permission [58]. Copyright 2022, Wiley‐VCH. (c) Binding energy of Li⁺‐EMC (top) and Li⁺‐MA (bottom); long‐term cycling performance at −5°C under 0.5C rate. Reproduced with permission [57]. Copyright 2021, Wiley‐VCH. (d) ⁷Li and ¹⁹F NMR spectra of FCE0, FCE20, and FCE65 electrolytes, highlighting differences in solvation environments. Reproduced with permission [9]. Copyright 2024, Wiley‐VCH. (e) Conceptual schematic depicting Li⁺ coordination with solvent molecules and anions. (f) Cycling performance of 1‐Ah pouch cells of LP57 and FCE at different C rates. Reproduced with permission [9]. Copyright 2024, Wiley‐VCH. (g) High‐resolution cross‐sectional SIMS imaging of the spatial distribution of total and selected secondary‐ion species from cathodes cycled in FCE electrolytes. Reproduced with permission [64]. Copyright 2025, American Chemical Society. (h) TOF‐SIMS 3D reconstructions of Li⁻ fragments from anodes recovered from LP57‐ and FCE‐based cells. Reproduced with permission [64]. Copyright 2025, American Chemical Society. (i) SEM images of graphite electrodes extracted from in‐house NMC811 pouch cells containing LP57 and FCE electrolytes after cycling. Reproduced with permission [64]. Copyright 2025, American Chemical Society. (j) Cycling stability of Gr | NCM811 pouch cells tested at 45°C under a 4C fast‐charging protocol. Reproduced with permission [65]. Copyright 2025, Wiley‐VCH. (k) Energy density retention of ETMA‐based pouch cells during repeated 4C charging. Reproduced with permission [65]. Copyright 2025, Wiley‐VCH. (l) Optical images of graphite anodes after cycling in ETMA‐containing electrolytes. Reproduced with permission [65]. Copyright 2025, Wiley‐VCH. (m) Corresponding normalized TOF‐SIMS depth profiles of representative LiF⁻ and C₂H₃O⁻ fragments. Reproduced with permission [65]. Copyright 2025, Wiley‐VCH. (n) Accelerating rate calorimetry (ARC) measurements of pouch cells with different electrolytes. Reproduced with permission [65]. Copyright 2025, Wiley‐VCH.

Son et al. further elucidated the interphase chemistry of FEC/DMC‐based electrolytes and explored the effect of FEC content [59]. From an interfacial perspective, FEC undergoes preferential reductive decomposition to form an ion‐conductive, Li₂CO₃‐rich SEI, which suppresses Li plating and promotes uniform Li⁺ intercalation into graphite. However, the authors also emphasized that high FEC fractions (≥70 vol%), despite increasing ionic conductivity, significantly raise electrolyte viscosity and slow interfacial kinetics, resulting in rapid capacity decay even at moderate charge rates. Moreover, replacing DMC with EMC leads to inferior rate capability and accelerated degradation, as the bulkier ethyl group restricts Li⁺ mobility and solvation efficiency under high‐current conditions. As a result, their Gr | NMC622 cells employing an optimized FEC/DMC (30/70 vol%) electrolyte exhibit stable fast‐charging performance, retaining ∼79% of the capacity after 1,000 cycles at 2C–1C charge–discharge rates.

Collectively, these studies define FEC/DMC as a baseline fast‐charging carbonate framework. Building on this foundation, ester co‐solvents are commonly incorporated to further optimize transport kinetics and interfacial behavior, as discussed in the following sections.

2.1.1.2. Non‐Fluorinated Esters

Non‐fluorinated aliphatic ester co‐solvents have been extensively investigated as an effective strategy to enhance the fast‐charging capability of Li‐ion batteries by mitigating electrolyte transport limitations, which dominate polarization in thick, high‐energy electrodes. Across multiple systematic studies, ester incorporation consistently lowers electrolyte viscosity and increases ionic conductivity, thereby reducing electrolyte ohmic drop and concentration overpotential during high‐rate charging, delaying Li⁺ depletion at the graphite surface and suppressing the onset of Li plating [60, 66, 67]. Quantitatively, replacing a fraction of linear carbonates with low‐viscosity esters, such as methyl acetate (MA), ethyl acetate (EA), or formates reduces viscosity from 0.6–0.7 cP to 0.3–0.5 cP and increases ionic conductivity by 20%–50% at 1.0–1.5 M LiPF₆, reaching 10–14 mS cm^{− 1} at room temperature. Comparative fast‐charging studies further reveal that although formates (e.g., ethyl formate (EF), methyl formate (MF)) provide the highest conductivity owing to their extremely low viscosity, they suffer from accelerated Li inventory loss, non‐uniform Li plating, and rapid capacity decay during long‐term fast charging, highlighting that conductivity alone is insufficient to guarantee durability.

Among ester families, acetates, particularly MA, exhibit the most favorable balance between transport enhancement and electrochemical stability [57, 68, 69]. For instance, Zou et al. have demonstrated that ester co‐solvents like MA can substantially enhance fast‐charging performance by improving electrolyte transport properties even in the absence of traditional film‐forming additives (Figure 4c) [57]. Despite that, practical deployment of esters often involves the use of interphase formation agents to resolve the interfacial stability issue of non‐fluorinated esters, particularly at the graphite anode. To address this trade‐off, subsequent studies have introduced fluoroethylene carbonate (FEC) into ester‐containing electrolytes to simultaneously preserve the transport benefits of esters and stabilize the electrode–electrolyte interphase. Cho et al. demonstrated that a methyl propionate (MP)‐based ester electrolyte incorporating FEC (1 M LiPF₆ in MP:FEC = 90:10 vol%) enables stable operation of NMC | Gr pouch cells at subzero temperatures, achieving reversible cycling at 0.5C rate at −20°C and retaining 60% of room‐temperature capacity at −40°C [70].

Complementary insights were provided by systematic studies from the Dahn group, where FEC was paired with low‐viscosity ester co‐solvents, such as MA or MP in high‐energy NMC | Gr pouch cells [66, 68, 69]. In these works, electrolytes composed of 1.2 M LiPF₆ in EC: EMC(: DMC) blends containing 20–40 wt% MA or MP, together with 2 wt% FEC, exhibited significantly reduced electrolyte viscosity, increased ionic conductivity, and lower charge‐transfer resistance compared with ester‐free counterparts. As a result, cells containing FEC‐ester electrolytes tolerated higher charging rates (up to 2–3C at 20°C) with minimal Li plating, whereas analogous formulations without esters or without FEC suffered from rapid capacity loss under identical fast‐charging protocols. Importantly, ultra‐high precision coulometry and microcalorimetry revealed that although both esters and FEC improve fast‐charging kinetics, excessive ester content (≥40 wt%) exacerbates parasitic reactions and gas evolution, leading to accelerated long‐term degradation. Consequently, an optimal compromise was identified at moderate ester fractions (20 wt% MA) combined with low‐level FEC additives, balancing high‐rate charging capability with acceptable cycle life.

Based on the carbonate‐MA skeleton, MA content and the choice of electrolyte additives were further studied systematically in practical pouch full cells. Li et al. investigated MA as a co‐solvent in single‐crystal NMC532 | Gr pouch cells employing electrolytes formulated as 1.2 M LiPF₆ in EC: EMC (3:7, w/w) with 0–40 wt% MA and various additive packages, including 2 wt% FEC with or without 1 wt% ethylene sulfate (DTD) [69]. Ma et al. examined MA‐containing electrolytes in NMC532 | Gr pouch cells by blending EC: EMC: DMC (25: 5: 70, vol%) with 20–40 wt% MA and either 2 wt% VC or 2 wt% FEC additives [68]. Specifically, the use of DTD can significantly enhance the cell cyclability at both 4.2 and 4.3 V, at a cost of slightly higher gas generation during formation cycles. Compared with FEC, VC additive can reduce parasitic side reactions, reduce gas generation, and improve Coulombic efficiency, thereby improving the cycle life. Nevertheless, long‐term cycling and calorimetric analysis confirmed that both high ester content and FEC introduction incur lifetime penalties, again identifying 20 wt% MA as an optimal compromise between fast‐charging capability and cyclability, particularly at high temperatures beyond 40°C.

Ester‐fluorinated ether combinations have been explored to further extend low‐temperature operability and suppress electrolyte freezing. Yao et al. developed EC‐free electrolytes consisting of MA, fluorinated ethers, and LiFSI salt, forming an anion‐dominated solvation structure that enables stable SEI formation in the absence of EC [71]. In these systems, MA serves as a low‐viscosity, high‐voltage‐compatible solvent (η ≈ 0.37 cP, anodic stability ≈ 5.2 V), while fluorinated ethers expand the liquid‐phase range and reduce freezing points. Therefore, pouch‐type NCM523 | Gr cells exhibit >600 cycles at 4C rate and maintain 76% capacity at −50°C.

2.1.1.3. Fluorinated Esters

Fluorinated ester co‐solvents provide an effective strategy for fast‐charging electrolytes by simultaneously regulating Li⁺ solvation and improving transport kinetics within carbonate‐based systems [49, 72, 73]. The electron‐withdrawing fluorinated groups weaken Li⁺–solvent interactions and reduce the effective Li⁺ solvation radius, leading to enhanced Li⁺ mobility and suppressed concentration polarization under high current density. Crucially, molecular structure plays a determining role: short‐chain fluorinated esters lower electrolyte viscosity, while preserving sufficient salt dissociation, whereas longer alkyl chains increase solvated‐ion size and polarization, ultimately limiting rate capability [74]. When judiciously incorporated, fluorinated esters, therefore, enable a balanced solvation‐transport landscape that mitigates anode polarization and supports stable fast charging.

Our group recently designed an EC‐free, fast‐charging electrolyte (FCE) based on a DMC/FEC carbonate skeleton with the addition of methyl trifluoroacetate (MTFA) [9, 64]. MTFA is the fluorinated analogue of methyl acetate, in which three hydrogen atoms are replaced by fluorine. Interestingly, we found that MTFA does not act as a primary solvating solvent but instead functions as a weakly coordinating diluent in this electrolyte system. As a result, Li⁺ coordination remains dominated by carbonate solvents, while anion participation is effectively suppressed, leading to a reduced Li⁺ desolvation barrier under high‐rate charging conditions. This solvation‐regulation effect is directly supported by ⁷Li and ¹⁹F NMR spectroscopy (Figure 4d). With increasing MTFA content (FCE65), the ⁷Li signal exhibits a clear upfield shift, indicating enhanced electronic shielding around Li⁺. Technically, if this shielding arose from increased Li⁺–PF₆ ⁻ coordination, a corresponding downfield shift in the ¹⁹F signal would be expected. Instead, the ¹⁹F resonance also shifts upfield. These results indicate that MTFA preferentially interacts with PF₆ ⁻ anions, while simultaneously promoting stronger Li⁺ coordination with DMC (Figure 4e). As shown in Figure 4f, 1 Ah Gr | NMC811 pouch cells employing the FCE electrolyte exhibit outstanding fast‐charging durability under 3C and 4C charging rate (0–100% SOC), retaining 82.1% capacity at 3C and 80.0% at 4C rates after 3,000 cycles. In contrast, cells with the commercial LP57 electrolyte suffer rapid degradation, reaching 80% capacity retention after only 1,000 cycles at 3C charging rate. On the cathode side, high‐resolution TOF‐SIMS cross‐sectional analysis shows that electrodes cycled in FCE form a much thinner CEI with significantly reduced organic species compared with LP57 (Figure 4g). On the anode side, 3D Li⁻ rendering reveals substantially less inactive “dead” Li in FCE, while SEM images of cycled graphite electrodes confirm a dendrite‐free surface, in stark contrast to the pronounced needle‐like Li dendrites observed with LP57 (Figure 4i).

Yang et al. identified ethyl trimethylacetate (ETMA) as an effective solvent for high‐temperature fast charging by combining weak Li⁺ coordination, high thermal stability, and favorable interphase chemistry [65]. ETMA was screened with a data‐knowledge‐dual‐driven framework and implemented as a carboxylate‐based electrolyte (e.g., 1.5 M LiFSI in ETMA/FEC/PS, with ETMA as the primary solvent) specifically designed for high‐rate operation at elevated temperatures. Chemically, ETMA exhibits moderate solvating power and low viscosity, resulting in weaker Li⁺–solvent interactions and a reduced Li⁺ desolvation barrier at the graphite interface. Consistent with this, molecular dynamics simulations and temperature‐dependent EIS reveal lower activation energies for both Li⁺ desolvation (E _{a , ct} ≈ 58 kJ mol^{− 1}) and Li⁺ transport through the SEI (E_{a, SEI} ≈ 52 kJ mol^{− 1}) compared with conventional EC/DMC electrolytes. Meanwhile, the ETMA‐derived solvation structure favors anion‐ and FEC‐derived decomposition, forming a thin, inorganic‐rich (LiF/Li₂O‐dominated) SEI that remains chemically stable at high temperatures, suppresses parasitic reactions, and mitigates Li plating. As a result, Gr | NCM811 pouch cells (181 Wh kg^{− 1}) achieve 15‐min charging to >80% capacity at 4C rate and 45°C, retaining 94.3% capacity after 500 cycles and 77.3% after 4, 100 cycles, substantially exceeding the USDOE fast‐charging durability target, with no observable Li plating and stable interfacial impedance (Figure 4j–l). TOF‐SIMS depth profiling further confirms that the SEI formed in ETMA‐based electrolytes contains higher inorganic (LiF⁻) and lower organic fractions (C₂H₃O⁻) (Figure 4m). In Figure 4n, ARC measurements demonstrate markedly enhanced thermal safety that ETMA raises the self‐heating onset temperature (T _{self‐heating}) from 124 to 226°C and reduces the maximum thermal runaway temperature (T _Max) from 409 to 303°C, underscoring the critical role of electrolyte‐enabled interfacial stability in mitigating thermal hazards during fast charging.

In addition, Mo et al. demonstrated that ethyl difluoroacetate (EDFA) enables fast charging by striking an optimal balance between weakened Li⁺–solvent binding and sufficient salt dissociation, achieved through moderate fluorination of the ester molecule [75]. Chemically, the asymmetric ─CHF₂ group introduces a local dipole that lowers Li⁺–solvent binding energy relative to non‐fluorinated esters, while avoiding the poor salt solubility and ion aggregation observed in fully fluorinated analogs. This solvation structure accelerates Li⁺ desolvation at the graphite interface and promotes the formation of a thin, inorganic‐rich (Li₂CO₃/LiF‐dominated) SEI with low interfacial resistance, enabling rapid Li⁺ transport under high current densities. As a result, the optimized EDFA–FEC electrolyte delivers outstanding rate performance, allowing graphite anodes to reach 196 mAh g^{− 1} at 6C rate, while also supporting durable full‐cell operation. In particular, 1.2 Ah NCM811 | Gr cells exhibit excellent low‐temperature capability, retaining 58.3% capacity at −30°C and delivering a discharge capacity of 790 mAh at −40°C. Similarly, Wang et al. demonstrated that 2,2‐difluoroethyl acetate (DFEA) is also highly effective for fast charging: structurally, EDFA and DFEA differ in the position of fluorination, with the ─CF₂– group located on the acyl versus alkoxy moiety, respectively [76]. Li | Gr cells with the DFEA‐based electrolyte (1.2 M LiPF₆ in DFEA with 10wt% FEC) sustain 10,000 cycles at 1 A g^{− 1} with 88.0% capacity retention, alongside excellent rate capability.

2.1.1.4. Advanced Ester‐Based Electrolytes

In this section, ester‐based electrolytes are designed beyond the conventional FEC/DMC carbonate framework, with esters serving as the dominant solvent matrix. Co‐solvents are commonly introduced to assist in forming conductive interphases and to extend the electrochemical stability window.

Xu et al. demonstrated that fluorinated esters, such as methyl difluoroacetate (MDFA) and methyl 2,2‐difluoro‐2‐(fluorosulfonyl)acetate (MDFSA) are highly effective for fast‐charging Li‐ion batteries by simultaneously lowering Li⁺ desolvation barriers and promoting kinetically matched, LiF‐rich interphases [26]. From a chemical perspective, the strong electron‐withdrawing ─CF₂‐ and ─SO₂F groups reduce solvent donor number and Li⁺–solvent binding energy, favoring contact ion pairs and aggregates that accelerate interfacial charge transfer, while suppressing Li plating. In practice, optimized electrolytes, such as 1 M LiTFSI in MDFA/MDFSA–TTE (and analogous EDFA‐based formulations), enable NMC811 | Gr full cells to deliver strong rate capability (up to 4C charging), maintain 80.1% capacity after 400 cycles at room temperature, and exhibit stable long‐term cycling across a wide temperature range, including sub‐zero operation, with high Coulombic efficiency.

Zou et al. designed a non‐flammable fluorinated ester electrolyte where LiPF₆ is dissolved in a MDFA/ ethoxy (pentafluoro) cyclotriphosphazene (PFPN)/FEC mixed‐solvent system, and the fast‐charging advantage is fundamentally tied to solvation/desolvation engineering plus interfacial stabilization [77]. Specifically, MDFA offers a moderate LiPF₆ solubility (up to 2.7 M) and, owing to its electron‐withdrawing fluorine substituents, weakens Li⁺–solvent coordination to lower the Li⁺ desolvation barrier and sustain high‐rate charge transfer. PFPN functions as a non‐coordinating flame‐retardant that interacts weakly with MDFA (and FEC), indirectly reshaping the Li⁺ solvation environment and regulating contact ion‐pair behavior without impeding ion transport. As a result, Gr | NCM811 with MDFA/PFPN/FEC electrolyte achieves 85.2% capacity retention after 200 cycles and 80.3% after 500 cycles with 99.90% average CE and delivers 78.4% of initial discharge capacity at 5C rate (157.4 mAh g^{− 1}) with stable return to 1C cycling after rate testing; its fast‐charge performance is consistent with its low contact angle (23.0°), high ionic conductivity (10.3 mS cm^{− 1}), and reduced interphase impedance. Importantly, under a practical scenario, a 240 mAh Gr | NCM811 pouch cell with the same electrolyte retains 81.8% after 500 cycles at 1C rate, whereas the EC/EMC baseline (25.4% retention) collapses later in life.

2.1.2. Ether Systems for Both Graphite and Li‐Metal Anodes

Ether‐based electrolytes have attracted growing interest for fast‐charging lithium batteries owing to their favorable ion‐transport characteristics and adaptable molecular chemistry. Compared with conventional carbonate solvents, ethers generally exhibit lower viscosity and weaker Li⁺–solvent interactions, which facilitate rapid ion transport and mitigate kinetic limitations under high‐rate charging [78, 79, 80]. Of note, ether solvents are most commonly employed within high‐concentration and localized high‐concentration electrolyte (HCE/LHCE) frameworks, where salts and diluents play a dominant role in defining the overall solvation structure and interfacial chemistry. Outside the HCE/LHCE framework, the fast‐charging behavior of ether‐based electrolytes can be systematically tuned through molecular‐level solvent design, giving rise to three broad classes—fluorinated ethers, non‐fluorinated ethers, and nitrogen (N)‐containing ethers. These categories represent distinct strategies for balancing Li⁺ transport, electrochemical stability, and interfacial compatibility, while the roles of ethers within HCE/LHCE systems are addressed separately in later sections.

2.1.2.1. Fluorinated Ethers

Fluorinated ethers have emerged as promising solvents for fast‐charging ether electrolytes by concurrently tuning electronic structure, solvation behavior, and interphase chemistry [80, 81, 82, 83]. Compared with non‐fluorinated ethers, fluorinated ethers exhibit a lower highest occupied molecular orbital (HOMO) energy, improving oxidative stability, and decompose preferentially into inorganic‐rich species that form mechanically robust and ionically conductive interphases [82, 84]. Fluorination also weakens Li⁺–solvent coordination, thereby lowering the Li⁺ desolvation barrier at the interfaces. However, excessive fluorination can impede salt dissociation and reduce ionic conductivity, making the trade‐off between interfacial stabilization and bulk transport a central design criterion for fluorinated‐ether‐based fast‐charging electrolytes.

Addressing the transport penalties associated with extensive fluorination, Zhang et al. proposed a partially fluorinated ether strategy to precisely modulate Li⁺ coordination [80]. As revealed by coordination schematics and electrostatic potential analyses, mono‐fluorinated ethers balance Li⁺ solvation strength and transport kinetics through controlled fluorination. The –CH₂F group enables stable Li–F/Li–O chelation, promoting salt dissociation and high ionic conductivity while lowering Li⁺ desolvation barriers and accelerating interfacial charge transfer (Figure 5a). Consistent with this design principle, the monofluoride bis(2‐fluoroethyl) ethers (BFE) exhibit high bulk ionic conductivity (8 mS cm^{− 1} at 30°C) together with oxidative stability up to 4.7 V, and impedance analysis confirms reduced desolvation and charge‐transfer resistances. These kinetic advantages translate directly into fast‐charging performance under practical, high‐stress conditions: high‐voltage Li | NCM811 cells with high areal capacity (3.5 mAh cm^{− 2}), limited Li excess (N/P = 2.8), and lean electrolyte retain > 90% capacity after 200 cycles at a high current density of 7 mA cm^{− 2}, whereas cells with conventional non‐fluorinated ether electrolytes fail within 50–75 cycles (Figure 5b).

FIGURE 5.

(a) Fluorination‐dependent Li⁺ coordination chemistry, illustrating the molecular design of mono‐fluorinated BFE that enables simultaneous Li–O and dual Li–F coordination, together with its facile one‐step synthesis at 30°C. Reproduced with permission [80]. Copyright 2023, Springer Nature. (b) Cycling behavior of Li | NCM811 coin cells with high areal capacity and low N/P ratio under elevated current density. Reproduced with permission [80]. Copyright 2023, Springer Nature. (c) Molecular design strategy for fluorinated ether electrolytes, highlighting how controlled fluorination and structural tuning balance fast‐charging kinetics and environmental considerations. Reproduced with permission [82]. Copyright 2024, American Chemical Society. (d) Fast‐charging behavior of Gr | NMC811 cells evaluated over a range of charge rates. Reproduced with permission [82]. Copyright 2024, American Chemical Society. (e) Electrochemical cycling performance of Gr | NMC811 cells at a 1C rate. Reproduced with permission [82]. Copyright 2024, American Chemical Society. (f) Post‐cycling SEM images of graphite electrodes operated at −20°C. Reproduced with permission [82]. Copyright 2024, American Chemical Society. (g) Comparison of transport properties, electrochemical stability, and graphite intercalation behavior of representative ether solvents. Reproduced with permission [85]. Copyright 2025, Wiley‐VCH. (h) High rate cycling performance of Gr | NMC full cells. Reproduced with permission [85]. Copyright 2025, Wiley‐VCH. (i) Comparison of key properties of EC/DEC and selected ether‐based solvents. Reproduced with permission [79]. Copyright 2024, American Chemical Society. (j, k) Electrochemical performance of EC/DEC‐ and THP‐based electrolytes. Reproduced with permission [79]. Copyright 2024, American Chemical Society. (l) TEM characterization of graphite particles cycled in different electrolytes. Reproduced with permission [79]. Copyright 2024, American Chemical Society.

Complementary insights were provided by Ma et al., who introduced a partially fluorinated ether (─CH₂F) design to replace highly fluorinated diluents by precisely tuning solvent–Li⁺ interactions [82]. By replacing highly fluorinated terminal groups and shortening the ether backbone, partial fluorination maintains weak Li⁺ coordination, while substantially reducing viscosity and improving salt dissociation (Figure 5c). As a result, the optimized E1CH2F electrolyte delivers an ionic conductivity of 5.74 mS cm^{− 1} at room temperature (20°C), markedly higher than that of the more heavily fluorinated analogue E3F1 (0.93 mS cm^{− 1}) and retains superior conductivity at −30°C relative to conventional carbonate electrolytes. As a result, Gr | NMC811 cells employing E1CH2F electrolytes sustain stable fast charging at rates up to 6C with significantly higher capacity retention than carbonate‐based electrolytes (Figure 5d). Moreover, Gr | NMC811 full cells with 2 M lithium bis(fluorosulfonyl)amide (LiFSA) in E1CH2F with 5 wt% FEC achieve 76% capacity retention after 300 cycles at a 1C rate, outperforming EC/DMC‐based controls, which retain only 56% capacity after 200 cycles (Figure 5e). Morphological analysis further confirms suppressed Li plating on graphite in E1CH2F electrolytes at sub‐zero temperatures (Figure 5f).

Moreover, Kumar et al. demonstrated that incorporating a small dose of fluorinated ether, bis(2,2,2‐trifluoroethyl) ether (BTFE), into conventional carbonate electrolytes is an effective strategy to enable fast charging, while stabilizing electrode–electrolyte interfaces [81]. In their formulation, BTFE (2 wt%) was introduced into a baseline 1 M LiPF₆/EC:DMC (1:1, v/v) electrolyte, either alone or in combination with 1 wt% lithium difluoro(oxalato)borate (LiDFOB). BTFE preferentially participates in interfacial reactions, promoting the formation of LiF‐rich and chemically robust interphases, while suppressing HF generation and current‐collector corrosion. These effects reduce interfacial resistance, enhance Li⁺ transference, and mitigate polarization under high‐rate operation. As a result, Li | LiFePO₄ cells employing the BTFE‐containing electrolyte sustain stable cycling at an aggressive 6C rate for up to 500 cycles, delivering an initial discharge capacity of 123 mA h g^{− 1} with a minimal capacity decay rate (0.01% per cycle).

2.1.2.2. Non‐Fluorinated Ethers

In contrast to fluorinated ethers, non‐fluorinated ether systems provide an alternative design principle: by introducing steric hindrance (e.g., methylation/branching or ring engineering), ethers exhibit a weaker solvating power with Li⁺ ion, lowering solvation/desolvation energy barriers, increasing the size of solvated structures, and minimizing the solvent co‐intercalation into graphite; meanwhile, chemical modifications like methylation can also reduce the number of reactive α‐H on α‐C that are prone to oxidation at high voltages, thereby improving solvent oxidative stability without relying on C–F chemistry [79, 85, 86].

Based on this theory, Liu et al. designed a trimethoxymethane (TMM)‐based single‐salt, single‐solvent steric‐coordinated electrolytes (SCEs), which was derived from a weakly solvating four‐membered ring dimethoxymethane (DMM) by methoxylation to maintain sufficient salt dissociation and avoid solvent co‐intercalation [85]. As illustrated schematically in Figure 5g, linear five‐membered‐ring ethers, such as DME exhibit strong Li⁺ coordination that favors high conductivity, but suffers from co‐intercalation into graphite, whereas reducing the backbone to a four‐membered acetal (DMM) weakens Li⁺ coordination to the extent that salt dissociation and bulk transport become insufficient. Introducing steric hindrance via methoxy substitution in TMM restores an intermediate coordination environment in which steric shielding around the ether oxygen lowers the effective Li⁺ coordination number and desolvation barrier, while preserving adequate electron density for salt dissociation, thereby achieving a balance between transport kinetics, oxidative stability, and graphite compatibility. As a result, Gr | NCM811 full cells paired with the TMM electrolyte maintain the highest reversible specific capacity under 5C rate with the slowest capacity decay over 1,000 cycles, whereas the EC‐ and DMM‐based electrolytes exhibit markedly faster fading (Figure 5h).

Tao et al. designed a weakly solvating ether electrolyte, 1 M LiPF₆ in 2‐methyltetrahydrofuran (2MeTHF), to decouple Li⁺ desolvation kinetics from interfacial transport limitations under fast‐charging and low‐temperature conditions [86]. Li⁺ forms a contact‐ion‐pair‐dominant solvation structure with weakened Li⁺–solvent interactions, which lowers the desolvation barrier while steering SEI formation toward a thin fluorinated interphase with finely dispersed LiF domains rather than dense, continuous LiF layers. Such a dispersed LiF microstructure minimizes the Li⁺ diffusion barrier across the SEI, mitigating metallic Li plating. As a result, Gr | Li cells can retain ≥ 90% capacity at −20°C (0.2 C) and deliver ≥ 250 mA h g⁻¹ at 2C rate with a stable cyclability. Gr | NMC622 pouch cells operated at −20°C achieve 125 mA h g^{− 1} at 0.1C rate and retain 90% of the capacity after 500 cycles and 87% after 1,000 cycles.

Finally, Liao et al. identified tetrahydropyran (THP) as a weakly solvating six‐membered cyclic ether that can enable fast charging of graphite by simultaneously suppressing solvent co‐intercalation and polymerization, while preserving rapid Li⁺ desolvation, as summarized in Figure 5i [79]. As a result, 3 Ah Gr | LiFePO₄ pouch cells equipped with 1 M LiFSI in THP can deliver 2.96 Ah at 4C charge, retain 83.1% capacity after 1,000 cycles at 1 C, and sustain 500 cycles under 2C–1C charge–discharge rate, whereas EC/DEC‐based cells rapidly fail under identical protocols (Figure 5j,k). In addition, THP suppresses parasitic heat generation during fast charging, exhibiting a surface temperature rise of 35.4°C at 2C rate, compared with 39.4°C for EC/DEC, indicative of reduced side reactions. Consistent with this mechanism, TEM images reveal a dense, thinner SEI (30 nm) on graphite with THP, in contrast to a ∼45 nm SEI in EC/DEC, while DME induces severe graphite exfoliation due to solvent co‐intercalation (Figure 5l).

2.1.2.3. Nitrogen (N)‐Containing Ethers

Ether‐ and amide‐based electrolytes incorporating N‐containing functional groups have recently emerged as an effective approach for enabling fast‐charging Li‐metal batteries (LMBs), owing to their tunable Li⁺ coordination strength, favorable oxidative stability, and interfacial chemistry. Compared with conventional ether electrolytes that often suffer from limited oxidative stability or excessive solvent‐derived interphase reactions, N‐containing molecules allow more precise regulation of solvation structures through Lewis basicity, dipole strength, and electron‐withdrawing substituents [87, 88, 89].

A representative strategy is the strong/weak solvent co‐solvation design reported by Wang et al., where a strongly coordinating ether (1,2‐dimethoxyethane, DME) is paired with a weakly solvating N‐containing amide (N,N‐dimethylsulfamoyl, FSN) [87]. The reduced coordinating strength of FSN promotes anion participation in the primary solvation sheath, forming anion‐rich clusters, while excess DME molecules remain outside the first shell to preserve high ionic conductivity. This decoupled solvation structure enables both rapid Li⁺ transport and the formation of inorganic‐rich SEI/CEI layers. As a result, high‐loading Li | NMC622 pouch cells (3.75 mA h cm^{− 2}) exhibit 80% capacity retention after 100 cycles at 1C rate. These results demonstrate that co‐solvation with weakly coordinating N‐containing solvents can simultaneously support fast kinetics and long‐term cycling stability.

Interestingly, Fang et al. reported an amide‐based eutectic electrolyte composed of a single salt and a single solvent, in which Li⁺ solvation chemistry dynamically evolves with temperature [88]. At room temperature, the polar amide N‐methyl‐2,2,2‐trifluoroacetamide (NMTFA) participates in Li⁺ coordination, enabling sufficiently fast desolvation kinetics to support high‐rate operation, with the eutectic electrolyte constructed by dissolving LiDFOB in NMTFA at a molar ratio of 1: 5. As the temperature increases, enhanced thermal motion progressively weakens Li⁺–amide interactions, driving a transition toward anion‐dominated solvation structures that favor the formation of thermally stable, inorganic‐rich SEI layers. As a result, Li | LiFePO₄ cells retain 99.38% of their capacity after 200 cycles at 1C rate at 25°C, while under extreme conditions (100°C, 20C rate), they still maintain 83% of their initial capacity after 1,000 cycles. The robust capacity retention under both high‐rate and high‐temperature conditions highlights the advantage of temperature‐responsive N‐containing amide solvents in stabilizing fast‐charging lithium‐metal batteries.

While the above studies focus on liquid electrolytes, Yang et al. further translated N‐containing solvation regulation into a fluorinated eutectic‐based polymer electrolyte, enabling structural and mechanical stabilization of the optimized solvation chemistry [89]. Incorporation of trifluoroacetamide (TFA) weakens Li⁺–amide coordination through strong electron‐withdrawing effects, promoting anion‐enriched solvation environments, while polymerization and synergistic additives stabilize both SEI and CEI layers. Consequently, Li | LiFePO₄ cells deliver 84.7% capacity retention after 1,000 cycles at 10 C rate, and high‐voltage Li | LiCoO₂ cells retain 92.3% of capacity after 1,000 cycles at 3C rate under 4.5 V, demonstrating that moderated Li⁺–solvent interactions can be preserved in polymer matrices without sacrificing rate capability or cycle life.

Overall, these studies demonstrate that N‐containing solvents emerge as a versatile molecular lever that synchronizes solvation kinetics with interfacial stability, establishing a coherent design framework for fast‐charging lithium‐metal batteries.

2.1.3. Fluorinated Benzene

Fluorinated benzenes, such as fluorobenzene (FB) and difluorobenzene (DFB), have recently emerged as an unconventional yet powerful class of electrolyte components for fast‐charging lithium batteries [90, 91, 92]. Unlike conventional solvents or weakly solvating diluents that directly alter the primary Li⁺ solvation sheath, fluorinated benzenes primarily function by modulating intermolecular interactions within the electrolyte, rather than coordinating Li⁺ themselves. Owing to their low donor number, high chemical stability, and strong dipole character, these molecules weaken Li⁺–solvent binding indirectly through solvent–solvent interactions, thereby facilitating interfacial desolvation, suppressing parasitic reactions, and expanding operational temperature windows without relying on high salt concentrations.

Notably, the two contemporaneous studies by Qin et al. converge on a shared design principle that deliberately weakening Li⁺–solvent interactions to accelerate charge‐transfer kinetics, while addressing this objective from complementary perspectives [90, 92]. In one work, non‐coordinating molecules, such as fluorobenzene (FB), chlorobenzene (ClB), bromobenzene (BrB), and cyclohexane (CH), are introduced as non‐solvating solvents that remain outside the Li⁺ primary solvation shell, but interact with polar solvents (e.g., DME and PC) through dipole–dipole coupling [90]. This indirect modulation partially offsets Li⁺–solvent Coulombic attraction without altering the solvation topology, effectively loosening the solvation sheath, suppressing solvent co‐intercalation into graphite, and facilitating faster Li⁺ desolvation at the electrode interface. As a result, LiFePO₄ | Li half cells employing the LiFSI–3DME–7FB electrolyte—prepared by dissolving 1 LiFSI in 3 mol DME, followed by the addition of 7 mol FB to form a homogeneous solution—exhibit excellent rate capability. The cells operate stably over prolonged cycling and retain 96.5% of their initial capacity after over 500 cycles at 1C rate, validating the effectiveness of dipole–dipole–mediated solvation weakening in enabling fast‐charging performance.

In parallel, the second study examines the same solvation‐weakening concept from a formulation‐optimization and performance‐scaling standpoint. Here, FB is again employed as a non‐solvating cosolvent in a PC‐based electrolyte, where it weakens Li⁺–PC interactions via intermolecular van der Waals and dipole interactions, reducing the Li⁺ desolvation energy barrier to ∼0.41 eV and substantially lowering the interfacial resistance [92]. The optimized electrolyte, LPF157 (LiFSI: PC: FB = 1:5:7 by molar ratio, ∼0.9 M LiFSI), demonstrates that this interaction motif is robust across different lithium salts (LiPF₆, LiClO₄, LiTFSI), while maintaining good ionic conductivity and wettability. Electrochemically, Gr | Li cells retain 80% of the capacity after 500 cycles at 1C rate and deliver 250 mA h g^{− 1} at 6C rate, whereas Li | NCM811 cells can deliver 140 mA h g^{− 1} after 400 cycles at 1C rate and maintaining 145 mA h g^{− 1} at 6C rate, confirming that solvation‐weakening interactions can simultaneously support fast charging and long‐term durability.

In addition, Huang et al. also demonstrated that fast charging can also be enabled by deliberate engineering solvent–solvent interactions to decouple Li⁺ desolvation from bulk transport limitations [91]. In this work, conventional ethylene carbonate (EC) is replaced by weakly solvating FEC, and the low‐melting‐point diluent 1,2‐difluorobenzene (2FB) is introduced to construct an intermolecular interaction‐mediated electrolyte. The resulting formulation, based on a commercial carbonate backbone (LiPF₆/FEC/EMC/2FB = 1/3/1/6 by molar ratio; denoted EF‐2FB), leverages solvent–solvent interactions between FEC/EMC and 2FB to weaken Li⁺–solvent coordination without incorporating 2FB into the primary solvation sheath. This interaction both lowers the Li⁺ desolvation energy barrier and depresses the electrolyte freezing point, thereby facilitating rapid Li⁺ transfer under fast‐charging and low‐temperature conditions. Specifically, Li | NMC811 cells with the EF‐2FB electrolyte deliver 154 mA h g^{− 1} at 5C rate at −30°C. At room temperature, the EF‐2FB electrolyte further enables 86.7% capacity retention after 200 cycles at 1C rate. Additionally, the cells with EF‐2FB electrolyte exhibit 79% capacity retention under 5 C rate, outperforming the cell without the addition of 2FB.

2.1.4. Nitrile‐Based Solvents

Nitrile‐based solvents are attractive electrolyte components because they combine high dielectric constants, low viscosities, and very low melting points, which are advantageous for low‐temperature operation and high‐rate charge/discharge [54, 93, 94, 95, 96, 97, 98]. However, most nitriles are intrinsically poor SEI formers on graphite and are prone to co‐intercalation or parasitic reduction. As a result, practical nitrile‐containing electrolytes are typically formulated in combination with carbonate solvents and film‐forming additives, which compensate for their interfacial deficiencies and enable balanced transport properties and electrochemical stability.

For instance, Cho et al. reported that introducing short‐chain nitriles (acetonitrile (An), propionitrile (Pn), and butyronitrile (Bn)) as assistant co‐solvents into conventional EC/DMC electrolytes is an effective strategy to enhance fast‐charging performance in Li‐ion batteries [96]. From a transport perspective, introducing nitriles into EC/DMC electrolytes markedly increases ionic conductivity and lowers activation energy for ion migration. For example, at −20°C, ionic conductivity increases from 3.1 mS cm^{− 1} in baseline EC/DMC electrolytes to ∼6.4 mS cm^{− 1} with propionitrile and 8.4 mS cm^{− 1} with acetonitrile, directly reducing ohmic polarization at high C rates. Lower viscosity further accelerates ion mobility, delaying Li⁺ depletion near electrode surfaces during fast charging and significantly suppressing concentration polarization. Specifically, voltage profile analysis shows that capacity loss at high rates cannot be explained solely by IR drop; nitrile electrolytes maintain flatter potential plateaus, indicating more uniform Li⁺ flux through the electrolyte and improved mass transport. Furthermore, NCA | Gr full cells employing 1 M LiPF₆ in EC/DMC/X (1:1:1 by vol%, X = An, Pn, or Bn) with 2 wt% FEC exhibit substantially improved rate performances. The nitrile‐assisted electrolytes delay premature voltage cutoff under high C‐rates, enabling more complete utilization of active materials. As a result, Pn‐ and Bn‐based electrolytes maintain high‐capacity retentions of 85% and 87%, respectively, at 2C‐2C cycling after 400 cycles, while the baseline carbonate electrolyte retains 76% capacity.

Huang et al. demonstrated that acetonitrile enhances fast charging not simply by lowering viscosity, but by enabling a solvent‐assisted Li⁺ hopping transport mechanism [97]. In the optimized electrolyte, AN is largely excluded from the primary Li⁺ solvation sheath, which remains FEC‐dominated, while free AN molecules establish an AN‐rich transport environment. This decoupled solvation architecture simultaneously preserves effective SEI formation and enables ultrafast Li⁺ transport. Molecular dynamics simulations reveal that AN markedly increases the solvation exchange frequency, shortening the Li⁺ diffusion time to 77 ps, nearly an order of magnitude faster than in pure FEC electrolytes. Consequently, Li⁺ transport becomes dominated by a hopping mechanism rather than vehicular motion, substantially suppressing concentration polarization under extreme fast‐charging conditions. The resulting electrolyte achieves an ionic conductivity of 12 mS cm^{− 1}, while maintaining an inorganic‐rich, low‐resistance SEI derived from FEC. Importantly, their 1 Ah Gr | NMC811 pouch cells employing the optimized 1 M LiPF₆ in FEC/AN (7/3, vol%) electrolyte deliver 3× higher capacity at 8C rate compared to conventional EC/DMC electrolytes, retain 85% capacity after 100 cycles under 6C‐1C charge‐discharge rate, and sustain 70% capacity after 500 cycles at 20C rate and 60°C, demonstrating the scalability and practical relevance of the nitrile‐enabled hopping transport mechanism. Consistent with the electrochemical results, transmission electron microscopy (TEM) analysis reveals a pronounced difference in SEI morphology. Graphite cycled in conventional carbonate electrolytes forms a thick and heterogeneous SEI (19 nm), whereas the optimized FEC/AN electrolyte produces a uniform and ultrathin SEI (3 nm). This compact, inorganic‐rich interphase minimizes interfacial resistance and facilitates rapid Li⁺ transport across the interface, directly supporting the observed ultrafast charging performance.

Lu et al. further proposed a ligand‐channel‐facilitated Li⁺ transport mechanism enabled by small‐sized nitrile solvents with low solvation energy, exemplified by fluoroacetonitrile (FAN) [54]. In contrast to conventional carbonate electrolytes dominated by vehicular transport, FAN primarily occupies the secondary solvation sheath, intermittently interacting with Li⁺ in the primary sheath and promoting rapid solvation–desolvation. This dynamic solvation environment establishes continuous Li⁺ transport pathways, enabling fast Li⁺ migration and suppressing concentration polarization under high current densities. The weakly solvating nature of FAN further allows anions to participate in the primary solvation structure, favoring the formation of an anion‐derived, LiF‐rich inorganic SEI that decouples fast bulk transport from interfacial stability. The FAN‐based electrolyte (LiFSI/FAN) exhibits ultrahigh ionic conductivity, reaching 40 mS cm^{− 1} at 25°C, markedly exceeding those of LiTFSI/FAN and conventional LiPF₆/EC–EMC electrolytes. This conductivity advantage directly translates into superior cell high‐rate behaviors. The charge–discharge profiles demonstrate that Gr | NMC811 cells with FAN‐based electrolyte maintain well‐defined voltage plateaus up to 6C rate, whereas EC/EMC‐based cells suffer from severe polarization and premature voltage cutoff as the rate increases. The transport advantages of FAN‐based electrolytes are further validated by long‐term cycling of Gr | NMC811 coin full cells at 6C charge and discharge rates. Cells with FAN‐based electrolyte deliver an initial discharge capacity of 180 mA h g^{− 1} and retain 80% capacity after 3,000 cycles. In sharp contrast, EC/EMC‐based cells show rapid degradation, retaining only 14% capacity after 3,000 cycles under identical conditions.

Zhou et al. showed that isobutyronitrile (IBN) enhances fast charging by compensating the intrinsic transport limitations of weakly solvating ester solvents [94]. In this work, the MDFA provides weak Li⁺–solvent interactions that facilitate rapid desolvation, while the IBN, with its higher dielectric constant and low viscosity, ensures sufficient salt dissociation and bulk ionic conductivity. IBN resides in a favorable solvent design space that combines low melting point and moderate‐to‐high permittivity, enabling a broad liquid‐phase range and stable ion transport over a wide temperature window. Correspondingly, the optimized M4 electrolyte maintains high ionic conductivity across −60 to 25°C, delivering 12.6 mS cm^{− 1} at 25°C and substantially outperforming conventional carbonate electrolytes at subzero temperatures, thereby directly alleviating transport limitations under fast‐charging conditions. Specifically, their 1.2 Ah NCM811 | Gr pouch cells employing the M4 electrolyte exhibit 80.1% capacity retention after 150 cycles at 1C rate and 25°C, while maintaining 75.3% capacity retention at 3C rate and stable operation from −50 to 70°C. The strong agreement between transport properties and pouch‐cell electrochemical performance highlights the critical role of nitrile‐enabled conductivity enhancement in enabling practical fast‐charging lithium‐ion batteries.

2.2. Salts

Beyond solvent engineering, salt design is another critical factor for enabling fast‐charging electrolytes. While solvents mainly control Li⁺ solvation strength, desolvation kinetics, and solvent‐derived interphase chemistry, salts determine the concentration and speciation of charge carriers in the electrolyte. First, the chemical identity of the lithium salt—especially the anion size, charge delocalization, and coordination strength—affects Li⁺–anion interactions, salt solubility, and the balance between free ions, contact ion pairs, and larger aggregates, thereby influencing ion transport and polarization at high charging rates [99, 100]. More importantly, salt decomposition products directly participate in interphase formation, making salt chemistry a key factor in determining SEI/CEI composition, interfacial resistance, and electrochemical stability during fast charging [99, 100]. At the same time, salt concentration provides another way to tune electrolyte behavior: increasing salt concentration changes the Li⁺ solvation structure, reduces the amount of free solvent, and shifts interfacial reactions from solvent‐dominated to anion‐dominated pathways [101, 102]. In particular, high salt concentrations (typically ≥ 3 M) are commonly used in HCEs/LHCEs. As a result, the role of salts in fast‐charging electrolytes should be discussed by considering both salt chemistry and salt concentration, and their combined effects on ion transport, interfacial chemistry, and Li plating.

2.2.1. Salt Physicochemical Properties

We begin the discussion of salt effects by comparing the properties of four commonly used lithium salts— lithium hexafluorophosphate (LiPF₆), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium difluoro(oxalato)borate (LiDFOB)—across key parameters relevant to fast charging, including ionic conductivity (σ), lithium transference number (t_Li+), SEI/CEI formation, and safety (Figure 6a) [103, 104, 105, 106, 107, 108]. Specifically, LiPF₆ serves as the conventional baseline, offering moderate conductivity and interphase formation, but limited safety and Li⁺ transport. In comparison, LiFSI and LiTFSI show larger overall performance envelopes, reflecting higher ionic conductivity, improved Li⁺ transference, and more favorable interphase chemistry, all of which are beneficial for fast‐charging applications. However, despite these advantages, LiFSI and LiTFSI remain substantially more expensive than LiPF₆ and are often limited by safety and corrosion concerns. LiDFOB exhibits a more balanced profile, with improved SEI/CEI formation and safety, but lower conductivity, highlighting its role primarily as a functional or interphase‐forming salt on both electrodes.

FIGURE 6.

(a) Radar plot comparison of representative lithium salts used in Li‐ion battery electrolytes. (b) Ionic conductivity of LiFSI and LiPF₆ in EC: EMC (30:70 wt%) as a function of salt concentration and temperature. Reproduced with permission [104]. Copyright 2019, Elsevier. (c) Rate‐dependent charge‐discharge voltage profiles of NMC811 | graphite cells with LiPF₆ and LiFSI electrolytes. Reproduced with permission [104]. Copyright 2019, Elsevier. (d) Long‐term cycling performance under 12 min fast‐charging conditions for cells with LiFSI and LiPF₆ electrolytes and the image of Li plating on graphite electrodes. Reproduced with permission [104]. Copyright 2019, Elsevier. (e) DSC data of charged NMC811 cathode (240 mAh g⁻¹) mixed with 1 M LiFSI or LiPF₆ in EC: EMC (3:7 by wt%), respectively. (f) Discharge capacity of pouch cells (normalized to C/10 capacity) with different electrolyte concentrations under various charging rates. Reproduced with permission [101]. Copyright 2022, Elsevier. (g) Cycling stability at 6C rate and 30°C. Reproduced with permission [109]. Copyright 2023, American Chemical Society. (h) Cycling performance of 1 Ah pouch cells at 1C and 2C rates at −20°C. Reproduced with permission [109]. Copyright 2023, American Chemical Society. (i) Low‐temperature discharge performance of full cells with EC: EMC electrolytes containing different salt compositions. Reproduced with permission [103]. Copyright 2025, Wiley‐VCH. (j) Charge‐rate performance of cells with a fixed 0.1C discharge rate. Reproduced with permission [110]. Copyright 2025, American Chemical Society. (k) Long‐term cycling stability at 1C rate. Reproduced with permission [110]. Copyright 2025, American Chemical Society.

Du et al. compared LiPF₆ and LiFSI in a practical carbonate electrolyte and showed that anion selection directly governs fast‐charging performance [104]. With 1.5 M salt in EC: EMC (30:70 wt%) in NMC811 | Gr pouch cells, the LiFSI‐bearing electrolyte exhibited higher ionic conductivity (10.36 vs 9.35 mS cm^{− 1} at 30°C) and a higher Li⁺ transference number (0.495 vs. 0.382) than the LiPF₆‐based one (Figure 6b). Under aggressive charging protocols (up to 5C rate), LiPF₆ reached the upper cutoff voltage earlier, indicating a higher concentration polarization, whereas LiFSI sustained a longer constant‐current period and delivered higher charge capacities (Figure 6c) [104]. As a result, the LiFSI‐based cell achieved a discharge capacity of 153.2 mAh g^{− 1}, representing a 13% improvement over the LiPF₆ counterpart (135.4 mAh g^{− 1}). Moreover, the LiFSI electrolyte enabled markedly improved cycling stability at 5C rate, retaining 134.3 mAh g^{− 1} (84%) after 500 cycles, compared with 110.6 mAh g^{− 1} (77%) for LiPF₆, accompanied by a substantially reduced Li plating area on the graphite electrode (Figure 6d) [104]. Meanwhile, Badwekar et al. compared LiFSI and LiPF₆ electrolytes in EC/EMC (30:70 wt%) and showed that LiFSI consistently leads to lower internal heat generation and reduced temperature rise during high‐rate charging (up to 5C rate) due to its lower internal impedance [105]. Yet, the thermal stability of LiFSI‐containing electrolyte has been shown inferior to the LiPF₆‐based electrolytes during thermal runaway (Figure 6e).

Furthermore, Shah et al. clearly isolated the role of anion size in ion transport by comparing LiFSI, LiTFSI, and lithium bis(pentafluoroethanesulfonyl)imide (LiBETI) in short chain perfluoroethers [111]. Their results establish a clear anion‐size rule: smaller anions, such as FSI, deliver the highest ionic conductivity and the lowest interfacial resistance against Li metal due to their higher mobility and more facile interfacial charge transfer, whereas larger anions progressively suppress anion motion, leading to higher steady‐state Li⁺ transference numbers (t⁺ _ss), following BETI⁻ > TFSI⁻ > FSI⁻. This study further highlights that electrolyte optimization cannot rely on conductivity alone. By evaluating the product σ × t⁺ _ss as a metric for effective Li⁺ current‐carrying capability, they identified a practical optimum at 20 wt% salt for common systems.

Extending these insights to more realistic formulations, a recent work from our group demonstrated that the salt performance depends strongly on the solvent composition and operating temperature, such that trends observed in single‐salt systems can shift substantially in mixed‐salt electrolytes [103]. In EC/EMC electrolytes cycled between −40 and 60°C, LiFSI consistently behaved as a high‐transport salt, exhibiting the highest ionic conductivity among the single‐salt electrolytes across all EC: EMC ratios and closely matching LiPF₆ in first‐cycle efficiency (82%–86%), while delivering higher reversible capacities of 180 mA h g^{− 1} at C/10 rate in NMC811 | Gr full cells. In contrast, LiDFOB showed lower conductivity and first‐cycle efficiency (down to 72%–78%), but formed strongly passivating interphases, leading to suppressed impedance growth and superior capacity retention after high‐temperature (60°C) cycling, particularly in EC‐free electrolytes. Electrochemical impedance analysis further revealed that LiPF₆‐ and LiFSI‐based electrolytes primarily exhibit faster charge‐transfer resistance growth in EC‐rich formulations than EMC‐rich electrolytes. Importantly, combining LiFSI with LiDFOB preserved the high‐rate transport advantages of LiFSI, while leveraging the interphase‐stabilizing role of LiDFOB.

Wang et al. demonstrated that incorporating lithium trifluoroacetate (LiTFA), a carboxylate‐containing lithium salt with strong Li⁺ coordination capability, improves fast‐charging performance by modulating the Li⁺ solvation sheath [112]. In Li | NCM622 full cells with an electrolyte of 1 M LiTFA in DME/FEC, the TFA⁻ anion partially replaces solvent molecules within the primary solvation shell, lowering the Li⁺ desolvation energy barrier and facilitating fast ion transfer across the SEI. Owing to its lower LUMO energy than carbonate solvents, TFA⁻ undergoes preferential reduction during SEI formation to generate LiF‐ and Li₂O‐rich interphases, which reduce nucleation overpotential and enable uniform Li⁺ deposition. As a result, LiTFA‐containing cells exhibit enhanced cycling stability and Coulombic efficiency.

Finally, Berhaut et al. provided a cautionary example showing that a high Li⁺ transference number does not automatically translate into high ionic conductivity, based on a lithium 4,5‐dicyano‐2‐trifluoromethyl‐imidazolide (LiTDI) in the EC/DMC (50/50 wt%) system [108]. At 1 M, LiTDI exhibits the lowest viscosity among the salts compared, yet also the lowest conductivity (6.84 mS cm^{− 1} at 25°C), highlighting that reduced viscosity alone is insufficient to guarantee fast ion transport. Detailed analyses combining conductivity–viscosity correlations (Walden analysis) and PGSE‐NMR measurements revealed that LiTDI is only weakly dissociated in EC/DMC, with an ion‐pair dissociation fraction of ∼31% at 25°C, compared with ≥ 64% for LiPF₆, LiTFSI, and lithiumtris(pentafluoroethane)‐trifluorophosphate (LiFAP) under identical conditions. Despite this poor dissociation, LiTDI delivers a relatively high Li⁺ transference number (0.58), primarily because the large, charge‐delocalized TDI⁻ anion diffuses more slowly than PF₆ ⁻. This apparent decoupling between t₊ and conductivity underscores a key design trade‐off: increasing t₊ via bulky or weakly mobile anions can come at the cost of reduced salt dissociation and suppressed conductivity. Importantly, they also emphasize that these transport characteristics are strongly solvent‐dependent, suggesting that alternative solvent mixtures may be required to unlock the potential of such salts, rather than evaluating anion effects in a single carbonate formulation alone.

2.2.2. Salt Concentration and Blending Effects

Salt concentration and salt blending represent two complementary strategies for tuning fast‐charging electrolytes. Increasing salt concentration generally strengthens Li⁺ solvation and promotes the formation of more robust electrode‐electrolyte interphases, which can suppress Li plating and improve cycling stability. However, excessive salt concentrations increase electrolyte viscosity, enhance ion pairing and aggregation, and ultimately reduce ionic conductivity and transport kinetics. Consequently, optimal salt concentration must balance bulk ionic transport with interfacial stabilization. In contrast, salt blending provides a practical means of combining the complementary functionalities of different lithium salts within a single electrolyte formulation. Rather than maximizing a single property, blended‐salt systems are typically designed to achieve a more favorable balance among ionic transport, interphase stability, cycling durability, and operational robustness. Therefore, both concentration optimization and salt blending aim to reconcile the inherent trade‐offs between transport and stability, which ultimately govern fast‐charging performance.

Wu et al. systematically investigated the role of salt concentration in fast‐charging performance with NMC622 | Gr pouch cells with a mixed‐salt electrolyte composed of LiFSI/LiPF₆ (9:1 molar ratio) in EC/EMC/DMC (40/30/30 wt%) [113]. By varying the total salt concentration from 0.75 to 2.0 M, they revealed a clear concentration‐dependent trade‐off between ionic transport and interfacial stability. While the electrolyte viscosity increased monotonically with salt concentration (from 1.8 mPa·s at 0.75 M to 5.6 mPa·s at 2.0 M), ionic conductivity peaked at 1.5 M and declined at higher concentrations due to enhanced ion pairing. Correspondingly, cells with electrolytes in the 0.75–1.50 M range delivered similar attainable capacities under fast charging (up to 4 C rate), whereas further increasing the concentration to 1.75–2.0 M led to reduced fast‐charging capability (Figure 6f). Importantly, cell capacity retention after 200 cycles improved monotonically with increasing salt concentration, with the 1.50 M electrolyte achieving the best overall balance between fast‐charging capacity and durability. Post‐mortem analysis revealed pronounced Li plating on graphite electrodes at low concentrations (≤1.25 M) and substantially reduced plating at ≥1.5 M.

Dual‐salt electrolytes exemplify a synergistic design strategy, where balanced solvation and interphase chemistry collectively enable enhanced electrochemical stability, wide‐temperature operability, and improved durability beyond conventional single‐salt systems [112, 114]. Specifically, Wang et al. demonstrated that a dual‐salt electrolyte combining LiPF₆ and LiFSI enables fast charging of energy‐dense Li‐ion batteries by simultaneously enhancing ionic transport and thermal stability [115]. Particularly, the authors replaced a conventional 1 M LiPF₆ electrolyte in EC/EMC (3:7 wt%) with a dual‐salt formulation containing 0.6 M LiPF₆ and 0.6 M LiFSI. While the dual salt did not accelerate SEI growth relative to LiPF₆ alone, it significantly improved electrolyte transport properties, benefiting from LiFSI's higher Li⁺ transference number (0.56 vs 0.38 for LiPF₆) and superior thermal stability. As a result, cells employing the dual‐salt electrolyte exhibited reduced polarization, delayed Li‐plating onset, and markedly enhanced high‐rate capability, achieving 90% relative capacity at 6 C discharge rate compared with 70% for the baseline LiPF₆ electrolyte in NMC811 | graphite cells with high areal capacities of 3.4–4.2 mAh cm⁻².

Complementing this transport‐focused perspective, Yao et al. emphasized that fast‐charging performance can be limited by interfacial charge‐transfer kinetics rather than bulk conductivity alone, and that dual‐salt electrolytes offer a practical route to balance cathodic and anodic kinetics [58]. By combining LiTFSI and LiPF₆, they synchronized Li⁺ release at the cathode and intercalation at the graphite anode, enabling Li‐plating‐free extreme fast charging in energy‐dense pouch and cylindrical cells. Extending the benefits of dual‐salt design to durability under extreme conditions, Zhao et al. demonstrated exceptional long‐term stability for LiPF₆‐LiFSI electrolytes under sustained high‐rate cycling. At 30°C, graphite | LCO cells operating at a 6C charge–discharge rate retained 80% capacity after 4,500 cycles and 70% after 6,000 cycles, highlighting the robustness of the dual‐salt system under prolonged high‐current operation (Figure 6g) [109]. More importantly, this durability advantage persists under kinetically limited conditions. Even at a 2C charge rate at −20°C, the LiPF₆‐LiFSI electrolyte maintained 90% capacity retention over 850 cycles in 1 Ah pouch cells (Figure 6h) [109].

Building on these findings, a study from our group further showed that dual‐salt formulations play a decisive role in preserving low‐temperature discharge capability after thermal aging [103]. At −40°C, dual‐salt electrolytes consistently outperformed their single‐salt counterparts by maintaining lower discharge polarization and higher reversible capacity, even after cycling at 25 and 60°C. Among the combinations studied, electrolytes containing LiDFOB paired with a secondary salt (LiFSI or LiPF₆) exhibited the most robust behavior, delivering initial −40°C capacities on the order of 130 mAh g^{− 1} and retaining 120–127 mAh g^{− 1} after cycling, depending on solvent composition (Figure 6i). Even in the absence of LiDFOB, dual‐salt systems, such as LiPF₆‐LiFSI still show measurable improvements in post‐cycling −40°C performance in low‐EC electrolytes, underscoring the general effectiveness of salt‐combination strategies (Figure 6i).

Consistent with this broader trend toward multi‐anion electrolyte design, Zhang et al. demonstrated that a highly diverse anion species provide further advantages under dynamic operating conditions [110]. As illustrated in Figure 6j, cells employing diverse‐anion containing electrolyte maintain a higher accessible capacity and reduced polarization across increasing charge rates, preserving 95.4% capacity retention over nearly 650 cycles at 1C rate; while cells with the conventional electrolyte exhibit progressive capacity loss and polarization buildup, retaining only 76.3% of its initial capacity after 410 cycles at 1C rate (Figure 6k).

2.3. Additives

Beyond bulk electrolyte components, electrolyte additives play a disproportionately important role in enabling fast charging, primarily by regulating interphase chemistry at minimal material cost and formulation complexity [116]. Unlike solvents and salts, which govern bulk transport properties and solvation thermodynamics, additives normally operate at low concentrations (typically ≤ 5 wt%) and selectively decompose at electrode surfaces to form protective, ion‐conductive interphases. These interphases suppress parasitic reactions, mitigate Li plating (and dendrite formation), and stabilize high‐voltage cathodes under aggressive charging conditions [117, 118, 119]. From a chemical design perspective, fast‐charging additives reported to date can be broadly classified into six structure‐based categories, including carbonate‐, boron‐, nitrile‐, phosphorus‐, sulfur‐based additives, and other emerging types (Figure 7). Notably, sulfur‐based additives dominate existing literature, with phosphorus‐based compounds as the second most studied class, highlighting significant opportunities for exploration of alternative additive chemistries in other categories.

FIGURE 7.

Classification of electrolyte additives for fast‐charging Li‐ion batteries based on their chemical functionality, including carbonate‐, boron‐, nitrogen‐, phosphorus‐, sulfur‐based, and other categories. Representative salt‐type additives, such as LiBF₄, KPF₆, and LiPF₆ are highlighted in orange.

Importantly, additive efficacy is strongly electrode specific. Most additives primarily stabilize graphite or Li‐metal anodes by promoting the formation of inorganic‐rich SEI layers, whereas others enhance CEI robustness through improved oxidative stability at high potentials. A smaller subset exhibits dual‐electrode functionality, enabled by wider electrochemical stability windows that allow simultaneous protection of both anode and cathode (Table 1). Although most additives predominantly regulate interphase chemistry, some also participate in modulating the Li⁺ solvation structure, thereby influencing desolvation kinetics and interfacial reaction pathways [120]. Collectively, these findings underscore electrolyte additives as a critical and cost‐effective lever for enhancing fast‐charging performance. Accordingly, the following sections highlight representative and high‐performing additive designs across different structural categories.

TABLE 1.

Classification of various types of additives for fast‐charging electrolytes.

Classification	Additive name	Abbreviation	Refs.
Carbonate‐based	Vinylene Carbonate	VC	[120, 121, 122, 123, 124, 125, 126, 127]
	Fluoroethylene Carbonate	FEC	[120, 123]
Boron‐based	Triphenylboroxin	TPBX	[120]
	Lithium bis(oxalato)borate	LiBOB	[27, 128, 129]
	Lithium difluoro(oxalato)borate	LiDFOB	[129, 130]
	Lithium tetrafluoroborate	LiBF4	[131]
	Potassium trifluoro(trifluoromethyl)borate	PTB	[34]
Nitrogen‐based	1,3,6‐Hexanetri‐carbonitrile	HTCN	[132]
	Isopropyl nitrate	ISPN	[133]
	1,2‐Bis(2‐cyanoethoxy)ethane	DENE	[132]
	Dopamine	DA	[128]
	Lithium Nitrate	LiNO3	[131, 134]
	Cesium nitrate	CsNO3	[135]
Phosphorus‐based	Tris(trimethylsilyl) phosphite	TMSPi	[124, 129, 136]
	Tris(trimethylsilyl) phosphate	TMSPA	[129]
	Lithium difluorobis(oxalato)phosphate	LiDFBOP	[137]
	Lithium difluorophosphate	PFO	[121, 125, 129, 138, 139, 140]
	Potassium hexafluorophosphate	KPF6	[141]
	Ethoxy(pentafluoro)cyclotriphosphazene	PFPN	[142]
	Lithium hexafluorophosphate	LiPF6	[27]
Sulfur‐based	1,3‐propane sultone	PS	[122]
	Ethylene sulfite	ES	[124]
	Prop‐1‐ene‐1,3 sultone	PES	[122, 136]
	Ethylene sulfate	DTD	[122, 136, 138]
	Diphenyl sulfone	DPS	[143]
	Bis(4‐florophenyl) sulfone	BFS	[143]
	(trimethylsilyl)isothiocyanate	TMSNCS	[144]
	Fluorosulfonyl isocyanate	FI	[145]
	Vinylsulfonyl fluoride	VSF	[146]
	Isosorbide 2,5‐dimethanesulfonate	ISDMS	[147]
Other types	Hexafluoroglutaric anhydride	HFA	[127]
	Octa‐decylamine	ODA	[148]
	Triglyme	G3	[134]
	2‐fluoroanisole (FMOP)	FMOP	[149]

2.3.1. Carbonate‐Based Additives

Carbonate‐based additives, represented almost exclusively by vinylene carbonate (VC) and fluoroethylene carbonate (FEC), are among the earliest and most widely adopted film‐forming additives for fast‐charging Li‐ion batteries. Both molecules feature activated carbonate motifs that undergo preferential reductive decomposition at graphite or Li‐metal anodes, enabling rapid formation of passivating SEI layers during initial charging. VC primarily polymerizes upon reduction, forming organic‐rich interphases that suppress continuous solvent decomposition, but often suffer from limited mechanical robustness under high current densities [121, 123, 126]. In contrast, FEC decomposes to generate LiF‐rich, inorganic‐dominated SEI components, which exhibit higher mechanical strength, lower interfacial impedance growth, and improved resistance to Li plating. However, excessive FEC content can increase interfacial resistance and gas evolution, highlighting a strong concentration‐dependent trade‐off between interphase stability and charge‐transfer kinetics. Of note, VC or FEC alone is generally insufficient to sustain aggressive fast‐charging conditions; consequently, these additives are typically employed in combination with other functional additives or replaced by more effective additives that can independently deliver measurable fast‐charging benefits.

2.3.2. Boron (B)‐Based Additives

Boron‐based additives represent a distinct class of electrolyte modifiers characterized by their intrinsic Lewis acidity and strong affinity toward anions and electron‐rich species. Across different molecular architectures, these additives share a common ability to preferentially participate in interfacial reactions, scavenge detrimental species, such as HF, and promote the formation of inorganic‐rich, mechanically robust interphases. Importantly, beyond interphase stabilization, certain boron‐containing species also actively regulate Li⁺ solvation environments, lowering desolvation barriers and facilitating rapid interfacial charge transfer under fast‐charging conditions.

Among oxalate‐derived borates, the most widely studied class, lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxalato)borate (DFOB) serve as representative examples. As reported by our group, LiBOB and DFOB undergo reductive decomposition at approximately 1.95 and 2.08 V, respectively [129]. Graphite | NMC811 full cells incorporating LiBOB and DFOB deliver reversible capacities of 150 and 144 mA h g^{− 1}, respectively, at a 1C charge–discharge rate after 500 cycles. Differential capacity (dQ/dV) analysis of discharge voltage decay associated with the H3–H2 phase transition reveals that both additives effectively mitigate cathode degradation; however, DFOB exhibits two distinct discharge voltage plateaus at 3.4–3.6 V corresponding to the cathode phase transition and a preserved graphite delithiation plateau, whereas the latter is absent in LiBOB‐containing cells. This indicates that DFOB provides more balanced interfacial protection at both electrodes, whereas LiBOB owns an inferior anode passivation ability. Consistent with this observation, a pronounced contrast is also observed in their rate performance (Figure 8a). In Li | graphite cells, DFOB maintains capacities comparable to the baseline LP57 electrolyte at low rates (C/10 and C/3 rates), while delivering substantially higher capacity at 1C rate, indicative of enhanced graphite fast‐charging kinetics. In contrast, LiBOB shows significantly reduced capacities relative to LP57 even at the C/3 rate, reflecting a detrimental impact on graphite anode performance. On the cathode side, although both additives improve the rate capability of Li | NMC811 cells compared to LP57, DFOB consistently outperforms LiBOB, reinforcing its superior cathode‐stabilization efficiency.

FIGURE 8.

(a) Rate capability of Li | graphite (left) and Li | NMC811 (right) cells with different electrolytes. Reproduced with permission [129]. Copyright 2025, Elsevier. (b) Cycling performance at 10C rate at room temperature. Reproduced with permission [120]. Copyright 2024, Wiley‐VCH. (c) TEM characterization of the CEI formed on cycled cathodes in different electrolytes. Reproduced with permission [120]. Copyright 2024, Wiley‐VCH. (d) High‐rate cycling comparison between AD3 (LP57 + 0.5 wt% nitrile additive) and baseline LP57 electrolytes. Reproduced with permission. Copyright 2026, Wiley‐VCH (to be submitted) [150]. (e) Long‐term 5 C rate cycling of NCM622 cells with Base‐PTA and ISPN‐PTA electrolytes. Reproduced with permission [133]. Copyright 2024, Wiley‐VCH. (f) Cycling stability of Li | NMC811 cells with 3 wt% CsNO₃ additive. Reproduced with permission [135]. Copyright 2023, Springer Nature. (g) Optical images of cycled Li metal with baseline electrolyte (top) and CsNO₃‐containing electrolyte (bottom). Reproduced with permission [135]. Copyright 2023, Springer Nature. (h) First‐cycle dQ/dV⁻¹ profiles showing phase transitions and additive decomposition. Reproduced with permission [129]. Copyright 2025, Elsevier. (i) 5 C fast charge/discharge performance of Li | LCO cells in three electrolytes. Reproduced with permission [140]. Copyright 2024, Royal Society of Chemistry. (j) Low‐temperature (‐20°C) cycling of graphite | LiNi_0.5Co_0.2Mn_0.3O₂ cells with and without 1% LiDFBOP additive. Reproduced with permission [137]. Copyright 2018, Wiley‐VCH. (k) 5C‐rate cycling after three formation cycles at 0.5C rate (3.0–4.3 V). Reproduced with permission [143]. Copyright 2020, Elsevier. (l) Fast‐charging performance of NCM811 | graphite full cells at 3C charge and 1C discharge rates. Reproduced with permission [147]. Copyright 2025, Wiley‐VCH.

Beyond interphase stabilization, Xiao et al. demonstrated that triphenylboroxin (TPBX), an emerging boron‐containing additive, enables extremely fast charging in high‐Ni Li‐ion batteries by simultaneously regulating Li⁺ desolvation and CEI chemistry (generating B–O‐containing species through ring‐opening polymerization) [120]. The TPBX‐containing formulation induces a PF₆ ⁻‐dominated solvation structure with increased contact ion pairs and aggregates, lowering the Li⁺ desolvation energy to 29.72 kJ mol^{− 1} (compared to 37.20 kJ mol^{− 1} in the blank electrolyte), thereby accelerating interfacial charge‐transfer kinetics. As a result, in Figure 8b, NCM811 full cells deliver 160 mA h g^{− 1} capacity and sustain 800 cycles at 10C rate, markedly outperforming the conventional EC‐based baseline. High‐resolution TEM further confirms that a uniform amorphous CEI of approximately 6 nm forms on the primary particle surface in the TPBX electrolyte, whereas a much thicker interphase (19 nm) develops in the blank system (Figure 8c).

Similarly, Xia et al. reported that potassium trifluoro(trifluoromethyl)borate (PTB) serves as an effective boron‐containing additive for enabling fast charging and high‐voltage stability in Li‐metal batteries through rational solvation‐structure engineering [34]. In the optimized electrolyte (1.1 M LiFSI in DME/TTE with Li⁺:DME = 1:4, supplemented with 0.5 wt% PTB), PTB strengthens Li⁺–DME and Li⁺–FSI⁻ interactions, increasing coordinated solvent species, while promoting preferential reduction of FSI⁻ anions. This solvation modulation facilitates the formation of a robust, inorganic‐rich SEI dominated by LiF and B–F species, while the CF₃BF₃ ⁻ moiety participates in constructing a thin, LiF‐rich CEI on the NMC90 cathode. As a result, Li | NMC90 cells achieve 84.1% capacity retention after 170 cycles at a high charging current density of 4.0 mA cm^{− 2} with a 4.5 V cut‐off, together with markedly enhanced rate capability compared to the baseline electrolyte. The improved fast‐charging behavior originates from the synergistic formation of F‐rich SEI/CEI layers and reduced interfacial resistance, enabling accelerated Li⁺ transport and stable electrode–electrolyte interfaces under aggressive current densities. Interestingly, Wang et al. reported that in a carbonate baseline electrolyte (1 M LiPF₆ in FEC/EMC, 3:7 by volume), the incorporation of 2 wt% LiBF₄ enables stable dissolution of LiNO₃ without additional co‐solvents, owing to the mild Lewis acidity of LiBF₄, which facilitates coordination between the electron‐deficient boron center and NO₃ ⁻ anions [131]. As a consequence, Li | NCM811 full cells deliver 185.6 mA h g^{− 1} at a high 5C charge rate and retain 80.3% capacity after 250 cycles.

2.3.3. Nitrogen (N)‐Based Additives

Nitrogen‐containing additives constitute a versatile class of functional molecules for fast‐charging electrolyte design, operating through chemistry‐dependent coordination and redox pathways. Depending on the functional group, these additives can coordinate with Li⁺ to modulate solvation environments, undergo preferential reduction to construct N‐rich interphases, or participate in surface‐reactive film formation.

Li et al. reported a nonsacrificial nitrile additive strategy to stabilize high‐voltage LiNi_0.83Co_0.07Mn_0.1O₂ (NCM83) cathodes with 1,3,6‐hexanetricarbonitrile (HTCN), in comparison with 1,2‐bis(2‐cyanoethoxy)ethane (DENE) [132]. In a baseline electrolyte of 1 M LiPF₆ in EC/DEC (1: 1 v/v), 3 wt% HTCN (BE‐HTCN) or DENE (BE‐DENE) was introduced. From a chemical perspective, HTCN exhibits a strong coordination with surface transition‐metal (TM) ions and superior oxidative stability, as supported by DFT‐calculated adsorption energies and lower HOMO levels relative to DENE. Unlike DENE, which undergoes preferential oxidation at high voltages, HTCN functions as a nonsacrificial additive that anchors on the cathode surface through its multiple ─C≡N groups, constructing a stable and uniform CEI. Electrochemically, NCM83 | graphite full cells delivered 81.42% capacity retention after 300 cycles at 1C rate in BE‐HTCN, significantly outperforming BE (64.01%) and BE‐DENE (60.05%). Recent work from our group demonstrates that introducing a trace amount (0.5 wt%) of a nitrile‐based additive, 3‐(trifluoromethyl)benzonitrile (AD3), into the commercial LP57 electrolyte significantly enhances fast‐charging stability. In 1 Ah pouch Gr | NMC811 cells cycled under 2C–1C charge–discharge conditions, the AD3‐containing electrolyte delivers 88% capacity retention after 1,000 cycles, compared with 80% for the baseline LP57 electrolyte (Figure 8d). Beyond its enhanced fast‐charging capability, the AD3 electrolyte also exhibits excellent low‐temperature performance (0°C) and high oxidative stability up to 4.4 V [150].

Su et al. introduced a redox‐active nitrate ester, isopropyl nitrate (ISPN), to enable fast‐charging Li‐metal batteries via coupled SEI reconstruction and solvation regulation [133]. Compared to the common LiNO₃ additive, ISPN exhibits a non‐resonant nitrate configuration that lowers its reduction barrier relative to NO₃ ⁻, enabling preferential decomposition on the Li‐metal surface. This process leads to the formation of a uniform LiF–Li₃N composite SEI, in which Li₃N provides high ionic conductivity, while LiF contributes mechanical robustness and electronic insulation. Beyond interphase formation, DFT and MD analyses reveal ISPN promotes the partial incorporation of PF₆ ⁻ into the primary solvation sheath, generating an anion‐enriched solvation structure and lowering the Li⁺ desolvation barrier (from −493.40 to −423.45 kJ mol^{− 1}). Consequently, the electrolyte exhibits an ionic conductivity of 5.3 × 10^{− 4} S cm^{− 1} with a high Li⁺ transference number (0.68). As a result, Li | LFP cells with ISPN sustain stable cycling for 850 cycles at 10 C rate and Li | NCM622 cells retain 97.59% capacity after 165 cycles at 5C rate (Figure 8e).

Rahman et al. demonstrated that CsNO₃ also serves as a dual‐functional additive [135]. In a low‐concentration ether electrolyte (1.5 M LiFSI in DME) containing 3 wt% CsNO₃, the large Cs⁺ cation (1.67 Å) exhibits weak solvation interaction with DME, which promotes anion participation in the primary solvation sheath and facilitates anion‐derived interphase formation on the Li metal. Unlike conventional LiF‐rich SEI strategies, the resulting SEI is dominated by crystalline CsFSI species and is notably LiF‐free, while the NO₃ ⁻ anion preferentially accumulates on the NMC811 cathode to form nitrate‐derived protective species. Electrochemically, Li | NMC811 full cells with high cathode loading (21 mg cm^{− 2}) sustain stable cycling at 1C rate (4 mA cm^{− 2}) and even 2C rate (8 mA cm^{− 2}), maintaining above 80% capacity retention after 220 cycles (Figure 8f). Hence, visual inspection of cycled Li metal shows pronounced surface degradation in the baseline electrolyte, while the CsNO₃‐containing system preserves a smooth and deposit‐free surface, evidencing more uniform Li deposition enabled by Cs⁺‐induced electrostatic shielding (Figure 8g). Similarly, nitrate‐based additives, such as LiNO₃, have also been shown to regulate solvation structures and construct Li₃N‐, LiN _x O _y ‐, and Li₂O‐rich interphases, providing fast‐ion‐conductive protective layers on both electrodes [131, 134].

Distinct from nitrile‐ and nitrate‐based additives, Cheng et al. reported dopamine (DA) as a surface‐reactive nitrogen‐containing additive that enables fast charging through interfacial polymerization‐driven CEI engineering [128]. In a conventional carbonate electrolyte, dopamine undergoes oxidative polymerization during the charging process to form a conformal polydopamine layer on the NMC811 cathode prior to severe electrolyte decomposition. Unlike inorganic‐dominated interphases, this polymer‐derived CEI introduces an ion‐conductive organic network that facilitates rapid Li⁺ transport, while immobilizing trace H₂O/HF via hydrogen bonding. As a result, the cells retain over 90% capacity after 200 cycles at 1C rate and delivers 118 mA h g^{− 1} at an ultrahigh 20C rate.

2.3.4. Phosphorous (P)‐Based Additives

Phosphorus‐containing electrolyte additives have emerged as an important class of interphase‐regulating components for fast‐charging lithium batteries. Molecular phosphate‐ and phosphite‐based additives tend to undergo preferential electrochemical decomposition prior to bulk electrolyte components and promotes the formation of Li _x PO _y F _z ‐rich interphases that suppress parasitic reactions. In addition to interphase formation, certain fluorophosphate‐derived species can further influence Li⁺ solvation structures and interfacial charge‐transfer kinetics.

Common phosphorus‐containing additives, including lithium difluorophosphate (PFO), tris(trimethylsilyl) phosphate (TMSPA), and tris(trimethylsilyl) phosphite (TMSPi), exhibit early electrochemical reactivity within practical voltage windows of around 3 V (Figure 8h), which is approximately 1 V higher than that of boron‐based additives [129]. This behavior suggests that P‐containing additives possess enhanced reductive stability compared to their boron counterparts. Among them, TMSPi is particularly effective in enabling fast charging by simultaneously stabilizing both the CEI and SEI, resulting in thinner and more ionically conductive interphases that facilitate Li⁺ transport under high‐rate operation [129]. Consistently, multiple studies have shown that TMSPi participates in the formation of a thin, inorganic‐rich SEI containing Li _x PO _y F _z species, which reduces interfacial resistance and suppresses metallic Li deposition under aggressive charging conditions [122, 136]. In contrast, PFO and TMSPA mainly enhance cathode rate capability but tend to form relatively resistive SEI on graphite, which limits their effectiveness in suppressing Li plating during fast charging [129]. As demonstrated by Zhang et al., a fluorinated electrolyte comprising 1 M LiPF₆ in FEMC/DFEC/FEC/TTE supplemented with 0.02 M PFO (denoted as FPE) undergoes preferential oxidative decomposition on the cathode surface to generate Li₃PO₄ ⁻ and LiF‐rich thin CEI film (3 nm) with low impedance and high Li⁺ conductivity [140]. As a result, Li | LCO cells operated up to 4.6 V exhibit stable fast‐charging at 5C rate with 73.2% capacity retention after 1,000 cycles (Figure 8i), alongside high‐rate capacities of 160 and 140 mAh g^{− 1} at 5C and 10C rates, respectively. Of note, emerging studies suggest that synergistic interactions between PFO and complementary polymer‐forming additives (e.g., VC and DTD) can further efficiently enhance the fast‐charging performance by forming a hybrid SEI composed of polymer and fluorophosphate species [121, 138].

Furthermore, dual‐functional P/B‐containing additives, such as lithium difluorobis(oxalato) phosphate (LiDFBOP) have been developed by Liao et al., which possesses both higher HOMO and lower LUMO than carbonate solvents, enabling preferential oxidation at the cathode (3.6 V) and reduction at the anode (2.5–1.9 V) [137]. Graphite | LiNi_0.5Co_0.2Mn_0.3O₂ full cells with the modified electrolyte (1 M LiPF₆ in EC/EMC with 0.5–1 wt% LiDFBOP) exhibits improved high‐rate capability, delivering 556 mAh at 5C rate compared to 412 mAh for the baseline electrolyte, while also demonstrating enhanced low‐temperature cycling stability with capacity retention improved from 73% to 93% after 50 cycles at −20°C (Figure 8j).

Beyond film‐forming P–O–F additives that lower impedance via PO _x F _y ‐rich interphases, fast charging can also be enabled by regulating interfacial Li⁺ flux without relying on extensive additive decomposition. Accordingly, Moharana et al. demonstrated that incorporating a small amount of KPF₆ as a secondary salt additive improves fast‐charging capability by modifying Li⁺ intercalation behavior and suppressing Li dendrite growth on graphite anodes [141]. Specifically, the larger K⁺ cations preferentially accumulate at defect sites (e.g., particle boundaries) on graphite due to their faster diffusion rate, acting as electrostatic blockers that hinder Li⁺ crowding and restrict dendritic Li growth during high‐rate charging. Similarly, Zheng et al. showed that incorporating a trace amount of LiPF₆ (0.05 M) into a LiTFSI–LiBOB dual‐salt electrolyte (0.6/0.4 M in EC/EMC 4:6 wt%) enables Li | NMC full cells to achieve 97.1% capacity retention after 500 cycles at 1.75 mA cm^{− 2} [27]. The additive‐derived PF₅/POF₃ species catalyze EC polymerization to form polycarbonate‐rich SEI layers that are flexible and Li⁺‐conductive, thereby lowering interfacial polarization and supporting stable Li deposition under fast‐charging conditions.

Long et al. reported that introducing ethoxy(pentafluoro)cyclotriphosphazene (PFPN) into an ether‐based electrolyte (1 M LiFSI in DOL) improves fast‐charging performance [142]. Specifically, molecular orbital analysis shows that PFPN exhibits lower LUMO and weaker Li⁺ binding energy (−1.21 eV) than DOL, enabling preferential generation of LiF‐ and P–F‐containing SEI species and facilitating rapid Li⁺ transport. The inorganic‐rich, thin (∼10–12 nm) interphase suppresses organic solvent decomposition, while lowering interfacial impedance and accelerating Li⁺ desolvation kinetics. As a result, at 5C rate, the LFP | Li cell with the PFPN‐containing electrolyte delivers an initial capacity of 139.5 mAh g^{− 1} and remains stable over 180 cycles without noticeable degradation, whereas the baseline electrolyte exhibits significant capacity decay to 103.9 mAh g^{− 1}.

2.3.5. Sulfur (S)‐Based Additives

Sulfur‐containing ring‐opening additives, such as prop‐1‐ene‐1,3‐sultone (PES), ethylene sulfite (ES), ethylene sulfate (DTD), and 1,3‐propane sultone (PS), have been widely adopted to regulate SEI chemistry at the graphite interface for fast‐charging applications, whose decomposition generates sulfur‐derived species (e.g., ROSO₂Li, Li₂SO₃, Li₂SO₄), suppresses EC‐driven Li₂CO₃ formation, and construct dense. yet ionically conductive and mechanically robust, Li₂SO _x ‐rich SEI layers.

Specifically, Liu et al. demonstrated that PES‐ and ES‐containing electrolytes in 220 mAh NMC111 | graphite pouch cells (1 M LiPF₆ in EC/EMC, 3:7 wt%) suppress EC reduction and improve SEI stability, enabling extended lifetime under both 4.2 and 4.4 V operation through modified interphase chemistry [124]. Similarly, the incorporation of DTD in multi‐additive systems has been shown to generate ROSO₂Li‐ and Li₂SO₄‐rich interphases that stabilize Li₂CO₃ against Lewis acid attack and suppress sustained electrolyte decomposition during high‐rate charging [124, 136, 138]. Likewise, Hekmatfar et al. showed that PS‐derived SEI components scavenge HF and moisture impurities, suppress excessive LiF formation, and improve Li⁺ transport across the interphase, thereby reducing the propensity for Li plating during high‐rate charging in NMC532 | graphite full cells [122]. Shi et al. also demonstrated that incorporating fluorosulfonyl isocyanate (FI) improves fast‐charging performance by altering the SEI formation pathway at the graphite interface. The resulting interphase significantly reduces the SEI resistance (21 Ω vs. 58 Ω), thereby facilitating Li⁺ transport under high‐rate lithiation [145].

Ou et al. further demonstrated that incorporating vinyl sulfonyl fluoride (VSF) enables fast charging by simultaneously regulating both SEI and CEI formation through molecular coupling of sulfonyl fluoride (─SO₂F) and vinyl functional groups [146]. Specifically, VSF participates in the solvation structure and undergoes controlled reductive decomposition at the anode and oxidative decomposition at the cathode to form LiF‐ and LiS _x O _y ‐rich interphases. The resulting ultrathin and uniform interphases significantly reduce interfacial resistance and charge‐transfer activation energy (from 56.85 to 44.27 kJ mol^{− 1}), thereby accelerating Li⁺ interfacial kinetics. As a result, Li | NCM811 cells filled with 1 M LiPF₆ + 0.2 M LiDFOB in FEC/EMC (1:3 v/v) with 1.3 wt% VSF exhibit markedly improved high‐rate performance, retaining 125.3 mAh g^{− 1} after 1,000 cycles at 5C rate. Likewise, Zhang et al. reported that incorporating sulfone‐based additives that own a lower LUMO and higher HOMO compared to carbonate solvents, such as diphenyl sulfone (DPS) and bis(4‐fluorophenyl) sulfone (BFS), into a carbonate electrolyte (1 M LiPF₆ in EC/EMC/DEC = 1:1:1) improves fast‐charging performance in Li | NMC811 cells, by concurrently stabilizing both the SEI and CEI with LiF, Li₂SO₃, and ROSO₂Li [143]. As a result, Li | NMC811 cells with 0.5 wt% DPS exhibit significantly improved high‐rate capability, retaining 89 mA h g^{− 1} after 500 cycles at 5C rate, compared to 70 mA h g^{− 1} with BFS and 25 mAh g^{− 1} without additives (Figure 8k).

Beyond bifunctional additives that regulate both interphase chemistry, Han et al. demonstrated that incorporating (trimethylsilyl)isothiocyanate (TMSNCS) further enhances fast‐charging capability by preserving interphase integrity through chemical scavenging of electrolyte‐derived reactive species [144]. In NCM62 | graphite full cells with a baseline electrolyte of 1.15 M LiPF₆ in EC/DMC (5:95 vol%) containing 10 wt% FEC and 0.1 wt% TMSNCS, TMSNCS deactivates HF and stabilizes PF₅ generated from LiPF₆ decomposition. This suppresses LiF overaccumulation and mitigates SEI/CEI degradation during repeated high‐rate cycling. Consequently, TMSNCS‐containing cells exhibit reduced interfacial resistance and improved charge‐transfer kinetics, achieving 91.8% capacity retention after 300 cycles at a 2C charge rate, compared to 78.9% for the baseline electrolyte.

Building upon the sulfur‐driven SEI regulation enabled by FI, VSF, and TMSNCS, which primarily rely on preferential reductive decomposition to construct Li⁺‐conductive sulfur‐containing interphases, Kim et al. further introduced that isosorbide 2,5‐dimethanesulfonate (ISDMS) can improve fast‐charging capability through an alternative solvation‐mediated pathway [147]. Unlike conventional sulfur additives, ISDMS contains oxygen‐rich polar motifs that competitively coordinate with Li⁺ ions and promote the participation of PF₆ ⁻ anions into the primary solvation sheath, facilitating the formation of a less resistive PF₆ ⁻‐incorporated SEI enriched with P–F and SO₃‐derived species. As a result, NCM811 | graphite full cells employing 1 wt% ISDMS in 1.15 M LiPF₆ EC/DEC (1:3 v/v) exhibit improved fast‐charging adaptability, achieving 76.7% capacity retention after 500 cycles at a 3C charge rate compared to 59.7% for VC‐based electrolytes (Figure 8l).

2.3.6. Other Types

Some advanced additives do not fit within the aforementioned classifications and are therefore discussed separately in this section. Similar to the previously described additives, these species can enhance fast‐charging performance by weakening Li⁺–solvent binding, tuning solvation coordination, and promoting anion participation in the interphase formation process.

Wang et al. reported that 2‐fluoroanisole (FMOP), a weakly solvating aromatic ether featuring an electron‐donating ─OCH₃ group and steric hindrance from the methyl substituent, improves fast‐charging capability by regulating the primary Li⁺ solvation structure (Figure 9a) [149]. In Li | NCM811 full cells with a baseline electrolyte of 1 M LiPF₆ in EC/DEC (1:1 vol%) containing 4 wt% FMOP, the weak coordination between FMOP and Li⁺ disrupts Li⁺–carbonate interactions and promotes PF₆ ⁻ participation in the inner solvation sheath, forming more contact ion pairs (CIPs) and aggregates (AGGs). This solvation transition facilitates preferential anion‐derived SEI formation enriched with inorganic components. As a result, Li | NCM811 cells exhibit improved high‐rate performance, retaining 80% capacity after 250 cycles at a 5C charge–discharge rate (Figure 9b). Similarly, Xu et al. demonstrated that incorporating triglyme (G3), an ether‐based coordinating ligand with multiple oxygen donor sites that can promote LiNO₃ dissolution, causes a synergistic decomposition to generate cross‐linked oligomeric ethers and Li₃N‐rich interphase and facilitate uniform Li deposition under high‐rate lithiation [134]. The 1.2 Ah LiFePO₄ | graphite pouch cells with a baseline electrolyte of 1 M LiPF₆ in EC/DMC (1:1 vol%) containing 5 vol.% FEC and 5 vol.% G3–LiNO₃ synergistic additive demonstrates stable cycling for over 150 cycles at a 3C charge rate (Figure 9c).

FIGURE 9.

(a) Illustration of the change in solvation structure induced by the introduction of the FMOP solvent. Reproduced with permission [149]. Copyright 2025, Elsevier. (b) Cycling stability of Li | NCM811 full cells at a 5C rate. Reproduced with permission [149]. Copyright 2025, Elsevier. (c) Cycling performance of pouch cells with the blank electrolyte and the GLN‐containing electrolyte at 3C rate. Reproduced with permission [134]. Copyright 2023, Wiley‐VCH. (d) Fast‐charging cycling performance and corresponding Coulombic efficiency at 3C rate between 2.7 and 4.3 V at 45°C. Reproduced with permission [127]. Copyright 2024, Wiley‐VCH. (e) Long‐term cycling stability of Ti‐SiO_x@C anodes with different electrolytes. Reproduced with permission [148]. Copyright 2022, Elsevier.

An et al. demonstrated that incorporating 0.1 wt% hexafluoroglutaric anhydride (HFA) into 800 mAh graphite | NCM811 pouch cells with an electrolyte of 1 M LiPF₆ in EMC:2,2,2‐trifluoroethyl acetate (30:70 vol%) and 2 wt% VC promotes the formation of fluorinated inorganic‐rich SEI/CEI layers on both electrodes [127]. As a result, the pouch cells exhibit stable cycling under a 3C charge (20 min) rate, retaining 82% capacity after 700 cycles with suppressed impedance growth compared to the baseline electrolyte (Figure 9d) [127]. Similarly, Yang et al. showed that incorporating octadecylamine (ODA) enhances fast‐charging performance by stabilizing the anode SEI through the formation of Li₃N‐rich inorganic species [148]. As a result, NCM811 | Ti–SiO _x @C full cells retain 72.8% capacity after 1,000 cycles at a 5C charge–discharge rate, compared to 38.9% for the VC‐only electrolyte (Figure 9e) [148].

3. Advanced Electrolytes

Fast charging of lithium batteries is fundamentally governed by the local solvation environment of Li⁺ ions in the electrolyte, which dictates the interfacial charge‐transfer kinetics at electrode surfaces. As illustrated in Figure 10a, conventional carbonate electrolytes are dominated by solvent‐separated ion pairs (SSIPs), where Li⁺ is primarily coordinated by solvent molecules. This solvent‐rich solvation sheath promotes solvent‐driven interfacial reactions, resulting in resistive organic‐dominated SEI/CEI formation. Increasing the salt concentration transforms the solvation structure from SSIPs to contact ion pairs (CIPs) and anion‐rich aggregates (AGGs), which favor anion‐participated reactions and inorganic interphase formation. Localized high‐concentration electrolytes (LHCEs) exploit this effect by introducing non‐solvating diluents that preserve AGG‐dominated coordination while reducing viscosity and improving transport properties, thereby enabling robust LiF‐rich SEI/CEI and widened electrochemical stability windows (Figure 10a). Importantly, recent studies indicate that similar anion‐participated solvation environments and interphases may also be achieved in non‐localized systems through superconcentration, weakly solvating solvents, or targeted interphase engineering. These findings suggest that the regulation of Li⁺ solvation structure and interfacial chemistry represent the central design principle for enabling fast charging in next‐generation lithium batteries [151].

FIGURE 10.

(a) Schematic comparison of Li⁺ solvation structures in conventional electrolytes, high‐concentration electrolytes (HCE), and localized high‐concentration electrolytes (LHCE), highlighting increased anion participation and anion‐derived interphases in LHCE. (b) Rate capability at different charge rates with a constant C/5 discharge rate. Reproduced with permission [45]. Copyright 2020, Wiley‐VCH. (c) SEM image of a cycled anode in LHCE (left) and 6C pouch‐cell cycling comparing REC and LHCE electrolytes (right). Reproduced with permission [152]. Copyright 2023, Wiley‐VCH. (d) Optical images of graphite electrodes after cycling in different electrolytes. Reproduced with permission [44]. Copyright 2024, Wiley‐VCH. (e) High‐rate cycling performance in various electrolytes. Reproduced with permission [44]. Copyright 2024, Wiley‐VCH. (f) Long‐term cycling at 4C charge and C/3 discharge. Reproduced with permission [153]. Copyright 2023, Wiley‐VCH. (g) Cycling stability and Coulombic efficiency at 4C rate in different electrolytes. Reproduced with permission [154]. Copyright 2020, Wiley‐VCH. (h) Schematics illustration of the relationship between ΔESP of the diluent and the cluster size of an ion pair. (i) Cycling performance of Li | |NCM811 full cells when charged at 4C rate in CC/CV mode and discharged at nC in CC mode in each cycle. Reproduced with permission [155]. Copyright 2026, Springer Nature.

3.1. LHCEs

To start with, Zhang et al. first adopted the carbonate‐based LHCE formulations for fast‐charging Li‐ion full cells [45]. The developed LiFSI‐ and carbonate‐based LHCEs with a fluorinated ether (TTE) for Gr | NMC811 full cells can sustain stable high‐voltage cycling up to 4.4 V, equivalent to 4.5 V in Li‐metal cells. Among the formulations investigated, the electrolyte comprising 1.4 M LiFSI in DMC–EC–TTE (AE003) exhibits a superior fast‐charging capability, maintaining stable capacity at charge rates up to 3C rate, whereas conventional LiPF₆/carbonate electrolytes suffer rapid capacity decay at ≥ 3C rate due to pronounced interfacial impedance growth (Figure 10b). In addition to cathode stabilization, LHCEs are well‐known for their anode stabilization capability by forming anion‐derived SEIs on both graphite and Li‐metal anodes. Therefore, the full‐cell cycle stability will not be compromised even when severe Li plating has occurred. Yue et al. validated this theory experimentally and demonstrated that a similar LHCE composed of 1.6 M LiFSI in DMC with 1 vol% EC and TTE enables uniform Li deposition and dissolution with a plating reversibility as high as 99.95%, even when Li plating contributes up to 40% of the total lithiation capacity during fast charging in 1.2‐Ah NMC532 | graphite pouch cells, due to the formation of an anion‐derived LiF‐rich SEI that maintains structural integrity before and after Li plating (Figure 10c) [152]. As a result, the pouch cell can achieve 88.8% state‐of‐charge within 10 min (6C charging rate) and retain 84.4% of their capacity after 150 cycles under 6C charge/1C discharge rate conditions, whereas cells with the conventional carbonate electrolyte exhibit rapid failure due to dendrite‐induced polarization (Figure 10c).

Our group further validated the fast‐charging capability of LHCEs in practical Graphite | LiNiO₂ full cells by systematically tuning the solvation structure, solvent composition, and salt chemistry [44]. Among the LHCEs investigated, substituting LiPF₆ with LiFSI in a 1.5 M FEC:DMC:TTEE formulation (LDF) markedly enhances high‐rate performance and mitigates Li plating on graphite during aggressive charging (Figure 10d). Under 5C‐2C charge‐discharge rate, the LDF electrolyte retains 84% of its initial discharge capacity (134 mAh g^{− 1}) after 200 cycles, outperforming conventional LP57 electrolyte (73%) (Figure 10e).

Kautz et al. also pointed out that the compact solvation nature of conventional LHCEs can limit fast‐charging performance due to the low salt dissociation degree and insufficient ionic carrier mobility [153]. By tuning the molar ratio between solvating solvent (DMC) and non‐coordinating diluent (TTE) in LiFSI‐based carbonate LHCEs, a controlled solvation electrolyte (LiFSI–4.8DMC–0.2EC–TTE) was developed to simultaneously maintain high salt dissociation and minimize free solvent molecules. This optimized coordination environment improves bulk ionic conductivity, while still promoting anion‐derived interphase formation on both graphite and NMC811 electrodes. As a result, Gr | NMC811 full cells cycled at 2.5–4.4 V deliver 165 mAh g^{− 1} at 4C rate and 160 mAh g^{− 1} at 5C rate and maintain stable cycling over 300 cycles under 4C fast‐charging conditions (Figure 10f).

Park et al. systematically compared the effects of chain length and degree of fluorination of diluents on the fast charging performance of a model LHCE, LiFSI:DMC:diluent (1:3:2.5 by mol) with 5 wt% FEC [156]. It was revealed that longer diluents with increased ─CF₂‐ moieties possess a higher viscosity, promote excessive anion decomposition, and hinder FEC‐guided SEI formation; while shorter diluents with less ─CF₂‐ moieties preserve the original Li⁺ coordination environment and enable the formation of anion/additive co‐derived SEI layers. As a result, cells employing optimized LHCEs with smaller diluents like BTFE achieve stable 10‐min charging over 500 cycles with 88% capacity retention.

Additionally, ether‐based LHCEs have also demonstrated viable for Li‐ion full cells. Jiang et al. demonstrated an ether‐based LHCE composed of 1.5 M LiFSI in dimethoxyethane with bis(2,2,2‐trifluoroethyl) ether (BTFE) diluent promotes the formation of CIPs and AGGs, enabling preferential anion reduction and the formation of an inorganic‐rich SEI dominated by LiF, Li₂O, and Li₃N species [154]. This robust interphase effectively inhibits ether solvent co‐intercalation into graphite and reduces Li⁺ desolvation barriers during lithiation. As a result, graphite | Li cells deliver 220 mA h g^{− 1} at 4C rate and maintain 85.5% capacity retention after 200 cycles under 4 C rate (Figure 10g).

Guo et al. demonstrated that the Li⁺ solvation sheath size in an ether–TTE LHCE (1 M LiFSI) can be regulated by tailoring the alkyl chain length of linear ether solvents (diethyl ether (DEE), dipropyl ether (DPE), and dibutyl ether (DBE)), thereby modulating Li⁺–solvent interaction strength and desolvation kinetics [157]. Despite exhibiting weaker solvation strength, the larger DPE‐ and DBE‐based electrolytes formed thicker and LiF‐deficient SEI layers that impeded Li⁺ transport. In contrast, the DEE‐based LHCE forms a smaller solvation complex (1.02 nm), which facilitates stable ternary graphite intercalation compounds (GICs) and promotes preferential FSI⁻ reduction to generate an inner LiF‐rich and ultrathin SEI (2.5 nm). This interphase significantly lowers the Li⁺ diffusion barrier across the SEI (30.2 kJ mol^{− 1}), enabling graphite anodes to retain 200 mA h g^{− 1} at 4C rate and maintain 190 mA h g^{− 1} after 500 cycles under 4C rate.

Wang et al. further expanded the LHCE design concept to ionic liquid (IL)‐based systems by tailoring the Li⁺ solvation structure to enable fast‐charging graphite compatibility [158]. A nonflammable LHCE was formulated by dissolving LiFSI into N‐methyl‐N‐propylpiperidinium bis(fluorosulfonyl)imide followed by dilution with fluorinated ethers, which promotes the formation of anion‐rich aggregates (AGGs) in the primary solvation sheath. Preferential reduction of FSI⁻ in these AGG complexes generates a thin (1.5 nm) inorganic‐rich SEI that suppresses the co‐intercalation of IL cations into graphite and reduces interphase impedance compared to conventional carbonate electrolytes. As a result, graphite | Li cells exhibit enhanced fast‐charging capability with superior rate performance, retaining capacities above 190 mA h g^{− 1} after 300 cycles at 3C rate.

Compared to graphite full cells, LHCEs have been demonstrated particularly outstanding in fast‐charging Li‐metal batteries. This is because the formation of a mechanically robust LiF‐rich SEI preserves interfacial integrity during Li plating/stripping, promoting uniform Li deposition with high ionic flux, effectively reducing diffusion polarization, and suppressing dendrite growth. For instance, Chen et al. demonstrated that Li | NMC full cells equipped with a representative LHCE consisting of 1.2 M LiFSI in dimethyl carbonate/bis(2,2,2‐trifluoroethyl) ether (DMC/BTFE, 1:2 by mol) can deliver 150 mA h g^{− 1} with 95% capacity retention over 300 cycles at 1C rate [159].

Cao et al. proposed a novel LHCE composition by replacing conventional diluent TTE with a fluorinated orthoformate (TFEO) in the LiFSI‐ DME‐diluent system [83]. Cells employing 1 M LiFSI in DME–TFEO electrolyte exhibit markedly improved rate capability in Li | NMC811 full cells compared to conventional carbonate electrolytes. While the reference electrolyte suffers rapid capacity decay beyond 2C charge and fails to sustain stable cycling upon returning to low rates due to Li anode degradation, the LHCE maintains stable discharge capacities of 175 mAh g^{− 1} at 2C rate, 160 mAh g^{− 1} at C rate, and 137 mAh g^{− 1} at 4C charging rate. These improvements are attributed to suppressed Li depletion and volumetric expansion of the Li metal anode, which mitigates pulverization‐induced polarization and facilitates stable high‐rate cycling in Li metal batteries. Xia et al. further demonstrated that dual‐anion solvation engineering can simultaneously improve ion transport kinetics and interfacial stability in LHCE‐type electrolytes [160]. By introducing a small amount of LiPF₆ (0.1 M) into a LiFSI‐ DME‐TTE electrolyte (1.0 M LiFSI + 0.1 M LiPF₆), PF₆ ⁻ anions partially compete with FSI⁻ in the primary solvation sheath, strengthening Li⁺–DME interactions and reducing the fraction of free ether solvent. As a result, Li | NMC90 full cells employing a thin Li metal anode (40 µm), high cathode loading (4.0 mAh cm^{− 2}), and lean electrolyte can maintain 93.7% capacity retention after 250 cycles at a high charging current density of 4.0 mA cm^{− 2} and deliver 151 mAh g^{− 1} at 5C charging rate.

Kim et al. recently demonstrated that beyond increasing anion participation in the solvation sheath, the distribution of ion‐pair species can also strongly influence fast‐charging performance [155]. By promoting the formation of contact ion pairs (CIPs) and small aggregates (AGG1), while suppressing larger aggregates (AGG2), a fluorinated‐silane‐based LHCE facilitated Li⁺ transport through the swollen SEI and mitigated interfacial Li⁺ depletion under high‐rate operation. As a result, the electrolyte delivered markedly improved fast‐charging durability in Li||NCM811 full cells under 4C charge conditions, highlighting ion‐cluster engineering as an emerging design principle beyond conventional anion‐rich solvation structures.

3.2. Beyond LHCEs

While LHCEs enable fast charging by inducing anion‐participated solvation structures, recent efforts suggest that similar localized environments may also be achieved in non‐LHCE systems through another interphase‐targeted designs.

Liu et al. reported an anchored weakly‐solvated electrolyte (AWSE) based on polyoxymethylene ether solvents that form a four‐membered Li⁺ coordination structure with reduced ion–dipole interaction, thereby lowering Li⁺ desolvation energy, while maintaining sufficient ionic dissociation at moderate salt concentration [161]. The resulting anion‐participating solvation sheath enables the formation of inorganic‐rich SEI/CEI on both graphite and NCM811 electrodes, allowing Ah‐scale Gr | NCM pouch cells to retain 70.85% capacity after 1,000 cycles at room temperature (Figure 11a). Importantly, the weakly solvated solvation structure further promotes CIPs/AGGs formation at sub‐zero temperatures, facilitating interfacial Li⁺ transport and enabling 75.86% capacity retention after 400 cycles at −20°C. Wang et al. further demonstrated that a weakly solvating electrolyte (WSE) based on cyclopentyl methyl ether (CPME) can inherently suppress solvent co‐intercalation into graphite by weakening Li⁺–solvent coordination via steric hindrance from a monodentate ether structure (Figure 11b) [162]. This reduced binding energy promotes anion participation within the primary solvation sheath and lowers the Li⁺ desolvation barrier, enabling both rapid interfacial charge transfer and the formation of a thin LiF‐rich SEI (Figure 11c). As a result, practical LFP | Gr pouch cells exhibit stable cycling over 1,000 cycles with 80% capacity retention at 1C rate (Figure 11d) and maintain 84%, 62%, and 31% discharge capacity at −20, −40, and −60°C, respectively.

FIGURE 11.

(a) Long‐term cycling performance at 1C rate following initial rate tests, compared with a commercial EC‐based electrolyte. Reproduced with permission [161]. Copyright 2024, Wiley‐VCH. (b) Molecular structures and electrostatic potential maps of different ether solvents. Reproduced with permission [162]. Copyright 2023, American Chemical Society. (c) TEM images of the SEI formed in EC/CPME‐based electrolyte. Reproduced with permission [162]. Copyright 2023, American Chemical Society. (d) Cycling performance of LFP | Gr pouch cells in FEC/CPME electrolyte at 1 Crate and room temperature. Reproduced with permission [162]. Copyright 2023, American Chemical Society. (e) Schematics of Li⁺ diffusion pathway through fluorophosphate SEI layer. Reproduced with permission [163]. Copyright 2024, Wiley‐VCH. (f) High‐rate cycling performance of coin cells at 5C rate. Reproduced with permission [163]. Copyright 2024, Wiley‐VCH. (g) Nail penetration test of a 5 Ah Li | NCM811 pouch cell. Reproduced with permission [163]. Copyright 2024, Wiley‐VCH. (h) Cycling performance of Li | NCM811 full cells at 10C rate. Reproduced with permission [164]. Copyright 2023, Wiley‐VCH.

Yamada et al. demonstrated that super‐concentrated electrolytes fundamentally alter Li⁺ solvation environments by forming a polymeric network of anions and Li⁺ cations, in which both solvent molecules and anions are coordinated within the primary solvation sheath [165, 166]. As such, reversible Li⁺ intercalation can be achieved even in otherwise reduction‐vulnerable solvents (e.g., acetonitrile or ether), while significantly lowering interfacial polarization and enabling ultrafast Li⁺ intercalation beyond conventional EC‐based electrolytes. Suo et al. reported a “water‐in‐salt” electrolyte (WiSE) by dissolving LiTFSI at ultrahigh concentrations (>20 M), fundamentally altering the Li⁺ solvation environment toward anion‐containing primary solvation sheaths [167].

Tan et al. constructed a fluorophosphated SEI featuring LiF/Li₃P inorganic grain boundaries via a sol electrolyte containing LiF–P nanoparticles, enabling fast Li⁺ diffusion through interfacial grain‐boundary pathways (Figure 11e) [163]. Compared with conventional fluorinated SEI, the LiF/Li₃P grain boundaries exhibit significantly reduced Li diffusion energy barriers (0.55–0.65 eV), homogenizing Li⁺ flux and suppress localized ion oversaturation during fast charging. As a result, Li | NCM811 cells maintain stable cycling over 1,000 cycles at 5C rate, while pouch cells deliver 400 Wh kg^{− 1} with 86.5% capacity retention over 150 cycles (Figure 11f). These results indicate that the electrolyte exhibits excellent safety, as cells containing it successfully passed the nail penetration test without combustion or exothermic reactions (Figure 11g). Ding et al. demonstrated that introducing ionic liquids into a diluted ether‐based electrolyte (1 M LiFSI + 0.3 M LiNO₃ in PP13TFSI:DME) enables the formation of an anion‐enriched solvation sheath without relying on high salt concentrations [164]. The participation of FSI⁻ and NO₃ ⁻ anions in the primary solvation shell weakens Li⁺–solvent interaction, facilitating faster desolvation kinetics and promoting the formation of inorganic‐rich SEI/CEI on both Li metal and NCM811 surfaces. As a result, Li | NCM811 full cells achieve stable cycling over 1,000 cycles at 10C rate with 81.5% capacity retention (Figure 11h).

4. Advanced Characterization Techniques

Fast charging places batteries under demanding conditions, including high current densities, strong polarization, and rapid heat generation, which can accelerate Li plating and interphase evolution. These processes often occur dynamically and unevenly across different length scales, making them difficult to capture by conventional post‐mortem characterization alone. As a result, specialized techniques that can probe battery behavior during/after fast charging are increasingly needed.

As summarized in Table 2, existing diagnostic tools can be broadly categorized into electrochemical diagnostics, chemical titration approaches, optical and mechanical sensing methods, and advanced spectroscopy/spectrometry‐based techniques [168, 169, 170, 171, 172]. These methods differ significantly in their analytical capabilities, including whether they provide qualitative or quantitative information, the measurement scale (cell vs. electrode level), their temporal resolution (ex situ versus in situ/operando), and the extent to which the measurements are destructive. From a functional perspective, these techniques can also be classified according to the specific information they provide, including (i) Li‐plating onset detection, (ii) plated‐Li quantification, (iii) spatial localization and morphology analysis, and (iv) interphase and chemical‐state characterization. Because no single technique can simultaneously provide all of these capabilities, complementary methods are often required to establish a comprehensive understanding of Li‐plating behavior under fast‐charging conditions.

TABLE 2.

Summary of characterization techniques for fast‐charging cell applications.

Name of Technique		Quantitative (N) or Qualitative (L)	Scale	In situ	Non‐Destructiveness	Function	Refs.
Voltage Relaxation (VR)	Voltage Relaxation	L	cell	✓	✓	detect the SOC onset of Li plating on Gr	[173]
	Differential Voltage Analysis	L	cell	✓	✓	identify and quantify Li plating on Gr	[174]
	dQ/dV	L	cell	✓	✓	detect the Li plating event on Gr	[175]
	Mass Spectrometry Titration and Voltage Relaxation	N	electrode	✗	✗	detect the onset of and quantify Li plating	[176]
	Battery Swelling Force Measurement; Voltage Relaxation; and CE	L	cell	✓	✓	detect Li plating in large‐format prismatic hard‐case cells	[177]
SOC Sweep Cycling Protocol combined with Titration		N	cell	✓	✓	quantify irreversible Li plating	[178]
Impedance Relaxation (IR)		L	cell	✓	✓	detect the likelihood of metallic Li plating on Gr	[179]
Dynamic Electrochemical Impedance Spectroscopy (DEIS)		N	cell	✗	✓	detect Li plating onset	[180]
Transient Cell Current Analysis		L	cell	✓	✓	determine Li plating‐induced degradation	[181]
Titration	H2 Microsensor Titration	N	electrode	✗	✗	quantify Li plating on Gr	[182]
	Mass Spectrometry Titration and Voltage Relaxation	N	electrode	✗	✗	detect the onset of and quantify Li plating	[176]
	Mass Spectrometry Titration	N	electrode	✗	✗	detect the onset of and quantify Li plating	[183]
	DEMS and Mass Spectrometry Titration	N	electrode	✗	✗	detect the onset of and quantify Li plating	[53]
	Titration Mass Spectroscopy	N	electrode	✗	✗	quantify Li plating on Gr	[184]
	SOC Sweep Cycling Protocol combined with Titration	N	cell	✓	✓	quantify irreversible Li plating	[178]
Optical Cell Imaging	In situ Optical Microscopy Cell	L	cell	✓	—	detect the onset of and map the Li plating	[185]
	Operando High‐Resolution Optical Microscopy	L	electrode	✓	—	record dynamic lithiation process of Gr and detect Li plating	[186]
	Transparent Capillary Full Cells	L	cell	✓	—	visualization of de/lithiation of Gr and Li plating	[187]
Pressure Sensing	Battery Swelling Force Measurement; Voltage Relaxation; and CE	L	cell	✓	✓	detect Li plating in large‐format prismatic hard‐case cells	[177]
	Differential Pressure Sensing	L	cell	✓	✓	detect onset of Li plating	[188]
Differential Scanning Calorimetry (DSC)		N	electrode	✗	✗	quantify the metallic Li by measuring the heat of Li melting	[189]
In Situ Thermal Spectrum		L	cell	✓	✓	detect Li plating on Gr	[190]
In Operando Acoustic Detection		L	cell	✓	✓	detect Li plating and determine the degree of Li plating in different regions	[191]
Operando Electron Paramagnetic Resonance (EPR)	Operando Electron Paramagnetic Resonance (EPR) Spectroscopy	N	cell	✓	—	detect the onset of and quantify Li plating on Gr	[192]
	operando EPR and imaging	N	cell	✓	—	quantify and visualize spatial distribution of Li	[193]
In Situ Neutron Diffraction (ND)	In Situ Neutron diffraction	L	cell	✓	✓	detect Gr local structural change during lithiation	[194]
	Neutron and X‐ray Tomograph	L	electrode	✓	✓	3D visualization of Gr electrode degradation	[195]
High‐energy synchrotron X‐ray Diffraction (HE‐XRD)	High‐energy Synchrotron XRD	L	cell	✗	✓	detection and visualization of Li plating homogeneity on Gr anode	[196]
	In Situ High Energy Synchrotron XRD	N	cell	✓	✓	visualize and quantify Li plating	[197]
Operando Nuclear Magnetic Resonance (NMR) and X‐ray Diffraction (XRD)		N	cell	✓	✓	detect the onset of and quantify Li plating	[198]
Surface‐Enhanced Raman Scattering (SERS)		L	electrode	✗	✗	detect and map Li plating	[199]
X‐ray Computed Tomographic (CT) Imaging		L	cell	✗	✓	visualize the interior features within the fast‐charging cells	[200]
Fluorescence Probe Analysis		N	electrode	✗	✗	visualize and quantify Li plating	[201]
Time‐of‐Flight Secondary Ion Mass Spectrometry (TOF‐SIMS)		L	electrode	✗	✗	visualize Li plating on Gr and quantify interphase thickness and homogeneity	[64]

Electrochemical‐based methods, such as voltage relaxation, differential voltage analysis, impedance relaxation, and transient current analysis, are widely used because they can be performed directly at the cell level in a non‐destructive manner during operation. These approaches are particularly valuable for identifying the onset of Li plating, estimating plating probability, or tracking degradation evolution. In contrast, chemical titration techniques—including mass spectrometry titration (MST), DEMS‐assisted titration, and hydrogen microsensor titration—provide more direct and quantitative measurements of plated Li but typically require cell disassembly and therefore operate at the electrode level.

Other complementary methods provide spatially resolved chemical/chemo mechanical information. Optical imaging techniques can visualize Li plating or dynamic lithiation processes, while pressure sensing and thermal diagnostics capture mechanical swelling and heat evolution, respectively. More advanced operando probes, such as neutron diffraction, synchrotron x‐ray diffraction, EPR, and x‐ray computed tomography, enable the observation of structural evolution and Li distribution across different length scales. Finally, surface‐sensitive techniques such as TOF‐SIMS and SERS provide detailed insights into interphase chemistry and morphology. Together, these complementary methods form a multi‐scale diagnostic toolkit for understanding fast‐charging behavior.

Specifically, electrochemical diagnostics, such as dynamic electrochemical impedance spectroscopy (DEIS) (Figure 12a), extract plating information directly from the evolving interfacial response during charge [180]. As illustrated by the equivalent circuit, Li deposition introduces an additional charge‐transfer pathway associated with metallic Li (R _ct,Li) in parallel with the normal Li intercalation process in graphite (R _ct,LixC6). As a result, the fitted charge‐transfer resistance exhibits a characteristic deviation once plating begins, which is reflected in the SOC‐dependent resistance evolution shown in the lower panel and enables identification of the plating onset. This can also be realized mechanically by monitoring the pressure evolution. In a constrained multilayer pouch cell (Figure 12b), normal graphite lithiation leads to only small pressure changes, whereas the formation of metallic Li and dendritic structures produces a much larger expansion [188]. Monitoring the pressure evolution therefore allows the transition from Li intercalation to Li plating to be detected in real time.

FIGURE 12.

(a) Equivalent circuit showing the effect of Li plating on charge‐transfer resistance (R _ct) and its SOC dependence. Reproduced with permission [180]. Copyright 2024, American Chemical Society. (b) Configuration for operando pressure measurements with a constrained pouch cell stack with a load‐cell sensor. Reproduced with permission [188]. Copyright 2022, Springer Nature. (c) Quantification of inactive Li with titration‐gas chromatography. Reproduced with permission [56]. Copyright 2024, Wiley‐VCH. (d) Schematic of the TOF‐SIMS M6 instrument and high‐resolution cross‐sectional mapping of CEI and graphite anodes with selected secondary‐ion fragments. Reproduced with permission [64]. Copyright 2025, American Chemical Society. (e) Fluorescence visualization of Li plating on graphite employing 4’‐hydroxychalcone under 365 nm UV light, enabling spatial mapping of plated Li. Reproduced with permission [201]. Copyright 2025, Wiley‐VCH. (f) Operando EPR cell design and representative electrochemical response during C/10 and 1C rate cycling. Reproduced with permission [192]. Copyright 2018, Elsevier. (g) Operando XRD and NMR setups with representative NMR spectra revealing the emergence of the Li metal signal during cycling. Reproduced with permission [198]. Copyright 2025, Wiley‐VCH. (h) Schematic of operando acoustic detection of Li plating in commercial pouch cells. Reproduced with permission [191]. Copyright 2020, Elsevier.

To quantify the amount of inactive Li at the electrode level, chemical titration approaches become particularly useful. As illustrated in Figure 12c, the cycled graphite electrode (at the delithiation state) is sealed and reacted with water, where metallic Li reacts to generate hydrogen [56]. The evolved H₂ is subsequently measured by gas chromatography, allowing the amount of irreversible metallic Li and lithiated graphite species to be quantified. At the material level, spatially resolved surface analysis, such as TOF‐SIMS combined with FIB imaging, can reveal the distribution and morphology of plated Li and visualize the thickness and composition of EEIs (Figure 12d) [64]. Direct visualization strategies have also been developed to simplify Li plating detection. As shown in Figure 12e, a fluorescence probe can be sprayed onto the electrode surface, where the probe selectively reacts with metallic Li to produce a bright yellow fluorescence signal, while remaining quenched on graphite [201]. This approach allows plated Li regions to be rapidly visualized across the electrode surface under UV illumination, providing an intuitive way to map plating heterogeneity after fast charging.

Electron paramagnetic resonance (EPR) has also been applied to probe Li plating. As illustrated in Figure 12f, operando EPR detects paramagnetic species by monitoring their microwave absorption in a magnetic field, allowing metallic Li to be distinguished from graphite intercalation compounds based on their characteristic spectral signatures [192]. During charging, the emergence and growth of the Li metal signal indicate the onset and accumulation of plated Li.

A further step forward is achieved by techniques that monitor Li plating during cell operation. Operando NMR (Figure 12g) directly detects metallic Li through its distinct chemical shift compared with graphite intercalation compounds (GICs) [198]. The NMR spectra illustrate the emergence of the Li metal peak as SOC increases, enabling time‐resolved tracking of plating onset and accumulation. Finally, acoustic ultrasound diagnostics (Figure 12h) offer a nondestructive method for detecting plating in practical cells [191]. As illustrated by the time‐of‐flight signals, changes in ultrasonic wave propagation occur when Li plating alters the mechanical properties of the electrode stack. By comparing the acoustic response under slow and fast charging conditions, the onset of Li plating during operation can be identified.

Each technique provides only a partial view of Li plating. Electrochemical, mechanical, and acoustic diagnostics are primarily suited for detecting plating onset during operation, whereas titration‐based methods enable quantitative determination of inactive Li after cycling. In contrast, imaging and spectroscopic techniques reveal the spatial distribution, morphology, and chemical environment of plated Li and interphase species. In practice, combining these approaches is often necessary to reliably identify plating and understand its origin under fast‐charging conditions.

5. Conclusion and Future Outlook

Fast charging remains one of the most critical barriers to the widespread adoption of Li‐ion batteries. Over the past decade, substantial progress has been achieved through electrolyte engineering, including solvent design, salt optimization, functional additives, and advanced electrolyte systems, such as high‐concentration and localized high‐concentration electrolytes. These strategies primarily aim to regulate Li⁺ solvation structures and interphase chemistry to suppress Li plating, enhance interfacial kinetics, and improve stability under high current densities. Collectively, the studies reviewed herein suggest that successful fast‐charging electrolytes are governed not by any single component or formulation strategy, but by a delicate balance among ionic transport, desolvation kinetics, solvation structure, and interphase stability. In particular, emerging concepts, such as weakly solvating solvents, ion‐cluster engineering, and multifunctional electrolyte formulations highlight the growing importance of controlling Li⁺ solvation and interfacial processes across multiple length scales.

Despite these advances, translating laboratory discoveries into practical battery systems remains challenging. As illustrated in Figure 13, the governing mechanisms of fast charging evolve across multiple scales—from material‐level ion transport and interfacial chemistry to cell‐level polarization and heat generation, and further to module‐ and pack‐level heterogeneity. Electrolyte design must, therefore, be evaluated not only at the material level but also under realistic cell configurations and operating conditions. Importantly, the apparent fast‐charging performance of an electrolyte can vary substantially with cell format and design. Many electrolyte systems are initially demonstrated in Li || Cu cells, half cells, or low‐loading coin cells, where short transport distances, excess lithium inventory, and favorable thermal conditions can mask practical limitations. In contrast, high‐energy full cells, Ah‐level pouch cells, and commercial‐format batteries impose much more stringent requirements owing to higher electrode loadings, lean electrolyte conditions, limited heat dissipation, and nonuniform current distribution. Cell‐level parameters, including areal capacity, N/P ratio, electrolyte‐to‐capacity ratio, stack pressure, electrode architecture, and thermal management, can strongly influence polarization, Li‐plating propensity, and charging efficiency. Therefore, validating electrolyte performance under practically relevant cell configurations is essential for bridging the gap between laboratory‐scale demonstrations and real‐world fast‐charging applications. Moreover, emerging battery chemistries, including Li‐metal, solid‐state, and sodium (Na)‐ion systems, introduce new electrolyte requirements, such as dendrite suppression, interfacial compatibility, and mechanical compliance [202, 203, 204, 205].

FIGURE 13.

Overview of factors influencing fast‐charging performance across scales and systems. Key challenges evolve from material‐level interfacial chemistry to cell and module‐pack level thermal and electrical heterogeneity. Different battery chemistries (Li‐ion, Li‐metal, solid‐state, and Na‐ion) impose distinct electrolyte requirements. Emerging technologies—including automated characterization, advanced BMS control, and data‐driven electrolyte–BMS co‐optimization—provide new opportunities to improve fast‐charging performance. Reproduced with permission [206, 207]. Copyright 2023, Wiley‐VCH. Copyright 2025, Wiley‐VCH.

Future progress will likely rely on the integration of advanced characterization techniques, automated experimentation, and data‐driven approaches. High‐resolution and non‐destructive diagnostics will enable multi‐scale understanding of fast‐charging processes, while machine learning and artificial intelligence (AI)‐enabled electrolyte discovery may accelerate materials screening [208, 209, 210, 211, 212, 213]. In parallel, battery management systems (BMS) capable of adaptive charging protocols and real‐time state estimation will play a crucial role in unlocking the full potential of fast‐charging electrolytes [214, 215, 216, 217, 218, 219, 220, 221].

Bridging academic innovation with industrial implementation requires coordinated efforts in electrolyte formulation, cell engineering, and system‐level optimization. Rather than pursuing individual electrolyte components or isolated performance metrics, future electrolyte development will increasingly rely on design principles that simultaneously optimize ionic conductivity, viscosity, Li⁺ transport, solvation chemistry, and interphase formation. Ultimately, the co‐design of electrolytes, electrodes, and charging strategies across scales will be essential to enable safe, durable, and truly fast‐charging batteries for practical applications.

Conflicts of Interest

The authors declare no conflicts of interest.

Biographies

Chen Liu received her Ph.D. in Materials Science and Engineering from the University of Texas at Austin in 2026 under the supervision of Professor Arumugam Manthiram. She received her B.S. degree in Mechanical Engineering from The Ohio State University in 2022. Her research focuses on high‐Ni layered oxide cathodes and fast‐charging electrolytes for lithium‐ion batteries.

Zehao Cui received his B.S. degree in Chemistry (2019) from the Department of Chemistry and Chemical Engineering at Xiamen University, China. He is a research associate at the University of Texas at Austin. His work focuses on the development of high‐energy‐density oxide cathodes and advanced electrolytes for lithium batteries, under the supervision of Professor Arumugam Manthiram.

Arumugam Manthiram holds the George T. and Gladys H. Abell Endowed Chair of Engineering at the University of Texas at Austin. His research focuses on the development of electrodes and electrolytes for a wide range of rechargeable battery systems, including lithium‐ion, sodium‐ion, lithium‐sulfur, sodium‐sulfur, and all‐solid‐state batteries. His broader interests extend to new materials design, scalable synthesis, advanced characterization, and elucidation of structure–property–performance relationships.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.