Nitrile‐Assisted Hybrid‐Solvation Electrolyte Enables Wide‐Temperature, High‐Voltage, and Ultrafast‐Charging Lithium‐Metal Batteries

ABSTRACT

The stable operation of lithium metal batteries (LMBs) requires simultaneous stabilization of anode and cathode interfaces, a challenge that intensifies under extreme operating conditions due to divergent formation mechanisms. Here, we present a hybrid‐solvation electrolyte design employing isobutyronitrile (IBN) as the primary solvent to regulate both Li⁺ solvation and interfacial protection. Functioning as a bifunctional modulator, IBN drives dual‐source interfacial chemistry at the anode where anion‐enriched solvation and coordinated‐solvent decomposition co‐generate an inorganic‐ and nitrogen‐rich solid–electrolyte interphase (SEI) while lowering Li⁺ desolvation barriers. At the cathode surface, excess IBN molecules form an adsorption‐derived protective layer that effectively suppresses solvent oxidation and stabilizes the cathode–electrolyte interface (CEI) under high‐voltage and high‐temperature conditions. Enabled by this design, Li||LiNi_0.8Co_0.1Mn_0.1O₂ coin cells exhibit robust operation across wide temperatures (−40°C∼60°C) and high voltages (4.6 V), alongside ultrafast charging capabilities (20 C). Upscaling to practical pouch cells under lean‐electrolyte conditions (1.2 g Ah⁻¹) yields a high energy density of 403 Wh kg⁻¹ with a 12‐min fast‐charging/discharging capability. The hybrid solvation design framework integrates solvent‐ and anion‐driven chemistries in a unified electrolyte, enabling high‐energy LMB operation under demanding conditions.

A hybrid‐solvation electrolyte enables dual interfacial regulation in lithium‐metal batteries (LMBs). Weakly coordinated isobutyronitrile (IBN) reshapes Li⁺ solvation to promote anion‐derived solid–electrolyte interphase (SEI) formation, while interfacial enrichment and evolution of nitrile species stabilize the high‐voltage cathode, unlocking wide‐temperature operation, ultrafast charging, and high‐energy pouch‐cell performance.

1. Introduction

Lithium metal batteries (LMBs) represent a transformative energy storage technology, offering gravimetric energy densities exceeding 400 Wh kg⁻¹ by integrating lithium metal anodes (LMAs) with high‐voltage nickel‐rich cathodes, such as LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) [1, 2]. However, the aggressive chemistries of both electrodes pose significant challenges to interfacial stability, particularly under extreme conditions of temperature, voltage, and current. LMAs suffer from dendrite formation and electrolyte depletion due to unstable solid–electrolyte interphases (SEIs), issues exacerbated at low temperatures and high rates [3, 4, 5]. Nickel‐rich cathodes, meanwhile, require robust cathode–electrolyte interphases (CEIs) to mitigate transition‐metal dissolution, oxygen release, and surface reconstruction, which intensify at high voltages and elevated temperatures [6, 7, 8, 9].

The simultaneous optimization of SEI and CEI presents a fundamental asymmetry problem rooted in their distinct formation mechanisms (Figure 1a) [10, 11]. SEI formation occurs through reductive decomposition controlled by the Li⁺ solvation structure, where coordinated species undergo electron transfer at the anode interface. This solvation‐mediated process can be regulated by adjusting the Li⁺ coordination environment to promote beneficial decomposition pathways [12, 13]. In contrast, CEI formation involves surface‐mediated oxidative processes that are largely independent of bulk solvation chemistry. At high voltages, the electric field‐induced Li⁺ depletion near cathode surfaces restricts anion accessibility, leading to solvent‐dominated oxidation [14, 15]. This often results in unstable organic species and corrosive byproducts, such as HF from fluoroethylene carbonate (FEC) oxidation, which accelerate transition‐metal dissolution and capacity degradation [6, 16]. This mechanistic asymmetry creates mismatched interfacial properties and divergent degradation pathways that become severe under fast‐charging, extreme‐temperature, and high‐voltage operation [17].

FIGURE 1.

A nitrile‐assisted hybrid‐solvation strategy to overcome the asymmetric interfacial challenge in lithium metal batteries. (a) Schematic of the proposed hybrid‐solvation strategy using isobutyronitrile (IBN) that enables the simultaneous optimization of both electrode interfaces. (b) Molecular property screening of various electrolyte solvents based on calculated dipole moment (μ) and minimum electrostatic potential (ESP_min). (c) Design strategy of nitrile‐based solvents. Schematic illustrating the modulation of Li⁺ coordination ability through enhanced steric hindrance. By introducing branched substituents, IBN achieves an optimized balance between steric effects and coordination strength, yielding high reductive stability, low viscosity, and accelerated interfacial kinetics.

Electrolyte solvation engineering has emerged as a critical strategy to address these interfacial challenges. High‐concentration electrolytes (HCEs) and localized high‐concentration electrolytes (LHCEs) shift the solvation sheath toward anion coordination by increasing the global/local salt‐to‐solvent ratios, yielding abundant contact ion pairs (CIPs) and aggregates (AGGs) that promote the formation of inorganic‐rich interphases [18, 19, 20]. However, these systems often suffer from elevated viscosity or diminished ionic conductivity, which limits charging rates and power densities [21, 22]. Weakly solvating electrolytes (WSEs) offer an alternative pathway, employing solvents with low donor numbers to shift soft solvation equilibrium toward anion coordination at moderate concentrations [23, 24, 25]. While WSEs successfully generate anion‐derived SEIs, they remain constrained by the fundamental SEI–CEI asymmetry. Most WSEs suffer from oxidative decomposition induced by a free lone pair of noncoordinating solvent molecules, leading to organic‐rich CEI formation and high‐voltage instability (Figure S1) [26, 27]. This limitation necessitates new electrolyte design paradigms that can independently optimize both interfaces.

In this work, we present a hybrid‐solvation strategy using a nitrile‐based solvent to optimize both Li⁺ solvation and interfacial chemistry. Specifically, we employ isobutyronitrile (IBN) as the multifunctional primary solvent in a mixed FEC/ethyl methyl carbonate (EMC)/IBN system, paired with a dual‐salt combination of LiPF₆ and lithium difluoro(oxalato)borate (LiDFOB). Unlike conventional WSE solvents, IBN acts through dual pathways (Figure 1a): (i) as a weakly coordinating solvent that promotes anion‐enriched Li⁺ solvation and reduces desolvation barriers, and (ii) as an active participant in interfacial chemistry that generates beneficial nitrogen‐containing species while providing independent cathode surface protection. This bifunctional approach enables simultaneous optimization of both electrode interfaces via separate but synergistic mechanisms depending on the coordination state of the solvent. Coordinated IBN facilitates anion‐derived inorganic interphase (LiF, Li₂O, and borates) formation while contributing nitrogen species (Li₃N) that enhance ionic conductivity and mechanical stability. Uncoordinated IBN preferentially adsorbs on cathode surfaces, where it contributes to an adsorption‐initiated nitrile‐derived interfacial protection that stabilizes the CEI under high‐voltage and high‐temperature conditions. This integrated design framework demonstrates exceptional performance in Li||NCM811 cells across extreme operating conditions, establishing a new paradigm for functional nitrile‐assisted electrolyte engineering in high‐energy, all‐climate LMBs.

2. Results and Discussion

2.1. Design of Hybrid‐Solvation Electrolyte

To optimize both electrode interfaces, we developed a hybrid‐solvation strategy centered on the bifunctional properties of nitrile solvents. In contrast to conventional approaches that rely on either solvation‐controlled (LHCEs/WSEs) or surface‐mediated (additives) mechanisms, our design integrates both pathways through targeted molecular engineering [28]. Nitrile solvents are characterized by low donor numbers and minimal Li⁺ coordination strength, thereby promoting CIP and AGG formation even at moderate salt concentrations [29]. This solvation configuration supports the formation of anion‐derived interphases. Nitriles also show exceptional oxidative stability, essential for high‐voltage operation. This expands the electrochemical window while maintaining thermal stability. Despite historical concerns regarding reductive instability at LMAs, strategic incorporation of nitriles within controlled solvation structures can regulate their decomposition pathways at the anode interface, generating beneficial nitrogen‐containing products (e.g., Li₃N, Li_xNO_y) that enhance ionic conductivity and mechanical stability [30, 31, 32]. Critically, at the cathode interface, nitrile molecules exhibit preferential surface enrichment driven by dipole‐surface interactions, which helps suppress oxidative attack on other electrolyte components and promotes the formation of a durable nitrile‐derived CEI [33, 34]. This cooperative solvation–adsorption mechanism enables independent optimization of both interfaces: anion‐derived inorganic SEI through solvation control and oxidation‐resistant CEI by surface protection.

To find the optimal nitrile solvent, we systematically screened a homologous series: acetonitrile (AN), butyronitrile (BN), valeronitrile (VN), 2‐methylbutyronitrile (MBN), and IBN, aiming to balance steric hindrance, transport kinetics, and physical properties. Two complementary molecular descriptors, dipole moment (μ) and minimum electrostatic potential (ESP_min), were used to guide the screening. Specifically, ESP_min reflects the local electronic characteristics of the major Li⁺−binding site, whereas μ describes the overall molecular polarity and charge‐separation response. For monodentate solvent frameworks paired with conventional salts possessing relatively high lattice energies (e.g., LiPF₆), the global dipole moment serves as a primary indicator of bulk salt dissociation capability. It should be noted that multidentate linear ethers (e.g., DME) paired with low‐lattice‐energy imide salts (e.g., LiTFSI) represent a notable exception to this trend; these systems overcome low bulk polarity through a strong localized solvation enthalpy driven by the structural chelate effect. As shown in Figures 1b, S2 and Table S1, cyclic carbonates (e.g., ethylene carbonate; EC) exhibit high dipole moments and strongly negative ESP_min values (−48.41 kcal mol⁻¹), resulting in dominant Li⁺ coordination that produces organic‐rich interphases. Conversely, fluorinated ethers (e.g., 1,1,2,2‐tetrafluoroethyl‐2,2,3,3‐tetrafluoropropyl ether; TTE) display minimal polarity and weak electron‐donating capability, functioning primarily as inert diluents with insufficient salt solubility. Nitriles occupy an intermediate regime with moderate dipole moments and weakened Li⁺ coordination. Among assessed nitriles, IBN demonstrates the lowest dipole moment (4.3 D) and least negative ESP_min (−41.72 kcal mol⁻¹), attributed to steric hindrance from its branched isopropyl group, which may distort the molecular geometry and reduce the effective polarity of the nitrile group [35].

Despite weak coordination, linear nitriles like acetonitrile (AN) suffer from reductive instability against LMAs due to their low‐lying lowest unoccupied molecular orbital (LUMO) levels (Figure S3) [29, 36]. Within the nitrile series tested, LUMO energy levels generally increase with alkyl substitution, whereas the effect of branching depends on the specific substitution topology near the nitrile group. In particular, the compact α‐branching of IBN appears to be more effective in raising the LUMO level than the more extended branching pattern in MBN. As a result, IBN exhibits the highest LUMO level (−0.39 eV) among the solvents screened, which contributes to its superior reversibility in lithium plating/stripping (Figures S3 and S4). This enhanced reductive stability is attributed to the steric hindrance and electron‐donating effect of the branched isopropyl group, which raises the antibonding orbital energy and suppresses electron injection from the lithium metal [37, 38]. When combined with stable cosolvents EMC and FEC (LUMO > −1 eV), the hybrid electrolyte favors salt decomposition over parasitic solvent reduction (Figure S3). At the cathode, IBN's deep lowest occupied molecular orbital (HOMO) level gives oxidative inertness up to 5.0 V versus Li/Li⁺ (Figure S5a), while potentiostatic floating tests confirm minimal parasitic current across 4.3–5.0 V (Figure S5b). This enhanced stability aligns with surface‐specific “solvophobic” effects previously reported for nitrile systems, where preferential adsorption creates an oxidation‐blocking layer [39].

These attributes position IBN as an ideal candidate for solvation modulation, effectively balancing coordination strength and physical properties (Figure 1c). Compared to its linear isomer BN, IBN's α‐methyl branching provides steric hindrance that weakens Li⁺‐solvent interaction and raises the LUMO level, solving stability issues seen with linear structures. Compared to the longer‐chain homolog VN, IBN maintains a lower viscosity (0.456 cP) than VN (0.795 cP), enabling faster ion transport (Table S2). Compared to sterically bulkier analogues (e.g., MPN), the appropriate steric hindrance of IBN ensures sufficient solubility of the lithium salt without impeding dissociation. Thus, IBN avoids strong coordination complexes while ensuring an adequate dielectric environment to promote salt dissociation, making it the optimal choice for the hybrid‐solvation system. Nevertheless, the favorable electrochemical functions of IBN should not be directly equated with the intrinsically superior physical safety of the neat solvent, and further safety evaluation under abuse conditions remains necessary.

2.2. Solvation Structure Characteristics

To validate the hybrid‐solvation design, we systematically investigated the Li⁺ solvation environment in an FEC/EMC/IBN ternary electrolyte (denoted FEI), comparing it with two reference systems: a conventional EC/EMC electrolyte (EE) and a fluorinated WSE composed of FEC/EMC (FE). All electrolytes contained identical salt concentrations (total lithium salt concentration of 1 M, with a molar ratio of LiPF₆ to LiDFOB of 8:2) to isolate solvent‐specific effects. The FEI composition was determined through systematic formulation screening, and the corresponding optimization results are provided in Figures S6, S7 and Table S3. Quantum chemical calculations confirm that IBN exhibits weaker Li⁺ coordination ability (−46.12 kcal mol⁻¹) than carbonates, with an elongated Li─N bond distance of 1.88 Å versus 1.72−1.74 Å for carbonate coordination (Figure S8). Despite this weak individual coordination interaction, IBN effectively dilutes the stronger carbonate coordination interaction at approximately 60 vol% of solvent matrix, thereby creating spatial vacancies that facilitated more efficient participation of anions in the solvation shell. Furthermore, the dipole‐dipole interactions between the electron‐deficient nitrile group and the electron‐rich carbonyl oxygen in FEC may promote their cooperative arrangement around Li⁺, potentially enhancing FEC's participation in the solvation structure (Figure S9) [40, 41].

Molecular dynamics (MD) simulations quantify the impact of IBN on solvation structure reconfiguration (Figure S10). Analysis of radial distribution functions (RDFs) and integrated coordination numbers (CN, Figure 2a,b) shows that Li⁺ coordination in the reference FE electrolyte is dominated by carbonyl oxygens from FEC (CN ∼1.3) and EMC (CN ∼3.0). Anions are largely excluded from this first solvation shell (PF₆ ⁻: CN ∼0.7; DFOB⁻: CN ∼0.2). Introducing IBN in the FEI system fundamentally shifts this distribution: the CN of EMC decreases to ∼1.0, while those of FEC and IBN increase to ∼1.6 and ∼1.4, respectively. Critically, anion coordination numbers rise to ∼1.2 for PF₆ ⁻ and ∼0.4 for DFOB⁻, representing a near‐doubling of anion participation. The Li⁺‐N (IBN) RDF (Figure 2a) exhibits a broad, diffuse peak at 2.25 Å, indicating a dynamic coordination mode. This behavior disrupts the stable solvent shell without forming rigid complexes [42]. Such flexible binding permits anions to enter the solvation sheath while avoiding the high viscosity typically found in concentrated electrolytes.

FIGURE 2.

Isobutyronitrile modulates Li⁺ solvation to create an anion‐enriched coordination environment. Radial distribution functions (RDFs) and integrated Li⁺ coordination numbers (CNs) for Li⁺−solvent and Li⁺−anion pairs in the (a) fluorinated WSE (FE) and (b) the IBN‐based hybrid‐solvation electrolyte (FEI), derived from molecular dynamics (MD) simulations. (c) Deconvoluted Raman spectra of the PF₆ ⁻ symmetric stretching mode for the baseline carbonate (EE), FE, and FEI electrolytes. (d) ⁷Li nuclear magnetic resonance (NMR) spectra. (e) ¹⁹F NMR spectra of the PF₆ ⁻ anion. (f) Fourier‐transform infrared (FTIR) spectroscopy analysis for the C≡N stretching region of the electrolytes.

Raman spectroscopy quantifies the evolution of ion pairing across the electrolyte series (Figure 2c). Deconvoluting the PF₆ ⁻ symmetric stretching mode (700∼760 cm⁻¹) reveals that the EE electrolyte is dominated by solvent‐separated ion pairs (SSIPs, 59.24%), with moderate contributions from (CIPs/AGGs, 30.56%). Replacing EC with FEC (the FE system) reduces SSIPs to 50.22% and increases CIPs/AGGs to 43.56%, a shift driven by FEC's weaker solvating power and lower dielectric constant [43]. The FEI electrolyte shows the strongest trend toward ion association: SSIPs fall to 36.48%, while CIPs/AGGs rise to 49.47%. These data confirm that IBN promotes an anion‐enriched solvation structure, which is analogous to HCEs but at moderate salt concentrations. To validate this steric regulation strategy, we compared solvation structures across the homologous nitrile series (Figure S11). While linear acetonitrile (AN) favors solvent‐separated states due to strong solvation, extending the alkyl chain (BN, VN) or introducing branching (MBN) gradually enhances ion association. Notably, IBN achieves the lowest SSIP fraction and highest anion participation (particularly for DFOB⁻) among all screened solvents because the isobutyl group provides optimal steric shielding to minimize solvent coordination.

Multinuclear NMR spectroscopy provides complementary evidence for the hybrid solvation model. The ⁷Li NMR spectra (Figure 2d) reveal distinctive chemical shift patterns. The FE electrolyte shows a slight downfield shift relative to EE, despite FEC being a weaker coordinating solvent. This apparent contradiction arises from the electron‐withdrawing fluorine substituent, reducing local electron density [44]. In contrast, the FEI system exhibits a pronounced upfield shift, indicating enhanced shielding consistent with weaker average solvent coordination and closer anion proximity. This interpretation is supported by ¹⁹F and ³¹P NMR data for PF₆ ⁻ and DFOB⁻ anions (Figures 2e and S12). In FEI, these anions exhibit systematic downfield shifts, reflecting reduced magnetic shielding as they interact more intimately with Li⁺. FTIR spectroscopy (Figures 2f and S13) confirms weakened Li⁺−carbonate interactions, evidenced by higher C═O stretching frequencies. Additionally, the splitting of the C≡N band reveals an equilibrium between coordinated and free IBN populations.

These mutually corroborating computational and spectroscopic results establish a unified model of hybrid solvation in which IBN serves dual roles: acting as a weak cosolvent partially occupying the coordination site of Li⁺ on the one hand and acting as a diluent to reduce the effective concentration of the strongly coordinating carbonate on the other. This creates a dynamic solvation environment in which the solvent binding of Li⁺ is attenuated while maintaining sufficient salt decomposition through residual carbonate and anion interactions. The resulting balanced coordination preserves compatibility with commercial separators (Figure S14) while leaving uncoordinated IBN molecules available for beneficial interfacial processes. Importantly, this hybrid structure significantly enhances ionic transport, evidenced by the lithium transference number increasing from 0.43 (EE) and 0.48 (FE) to 0.65 in FEI (Figure S15). This enhancement stems from reduced solvent drag and improved Li⁺ mobility within a more flexible solvation environment, which lowers the activation energy for Li⁺ hopping between coordination sites [45].

2.3. Lithium Plating/Stripping Performance

The optimized solvation structure in the FEI electrolytes directly translates into exceptional lithium plating/stripping reversibility. Galvanostatic cycling curves (1 mA cm⁻²/1 mAh cm⁻², 25°C) of Li||Li symmetric cells demonstrate remarkable differences in interfacial stability across the three electrolyte systems (Figure 3a). The FEI‐based cell maintains exceptionally low and stable polarization (∼50 mV) throughout 1500 h of continuous cycling, without any signs of short‐circuiting. In stark contrast, both EE and FE systems suffer from progressive voltage hysteresis and premature short‐circuiting, indicative of unstable SEI formation and dendritic lithium growth. This performance hierarchy becomes more pronounced under elevated current densities (2 mA cm⁻²/2 mAh cm⁻²), where EE and FE cells fail catastrophically after merely ∼200 and ∼900 h, respectively, while FEI continues stable operation beyond 1500 h (Figure S16). Electrochemical impedance spectroscopy (EIS) reveals that FEI maintains symmetrical impedance profiles during plating/stripping cycles, indicating robust SEI stability, whereas EE and FE show asymmetric impedance growth reflecting ongoing SEI deterioration that continuously consumes active lithium (Figure S17). Additionally, in rate‐dependent cycling tests progressively ramped from 0.2 to 5 mA cm⁻², the FEI cell maintains the lowest overpotential throughout, underscoring its superior interfacial compatibility and efficient Li⁺ transport kinetics through the SEI (Figure 3b).

FIGURE 3.

Hybrid‐solvation electrolyte enables stable lithium plating/stripping. (a) Galvanostatic cycling performance of Li||Li symmetric cells at a current density of 1 mA cm⁻² with a capacity of 1 mAh cm⁻² at 25°C. (b) Rate capability of Li||Li symmetric cells at current densities ranging from 0.2 to 5 mA cm⁻². (c, d) Coulombic efficiency (CE) (c) and long‐term cycling stability (d) of Li metal plating/stripping on a Cu substrate in Li||Cu cells with different electrolytes (measured at 0.5 mA cm⁻²/1 mAh cm⁻²; the inset shows a magnified view of the initial Li nucleation overpotential. (e) Tafel plots derived from steady‐state polarization measurements on Li||Li cells. (f) Long‐term cycling performance of Li||Li symmetric cells at a low temperature of −30°C (0.5 mA cm⁻²/0.5 mAh cm⁻²). The distribution of relaxation times (DRT) contour plots during a discharge‐charge cycle for Li||Li symmetric cells containing (g) FEI and (h) FE electrolytes at −30°C.

To quantify lithium reversibility, Li||Cu cells were tested using Aurbach's method. [46] As shown in Figure 3c, the significantly reduced nucleation overpotential (∼61.5 mV) in FEI produces uniform deposition with exceptional Coulombic efficiency (CE) of 99.23% at 0.5 mA cm⁻²/1 mAh cm⁻², substantially exceeding EE (92.50%) and FE (95.96%) (Figure 3c). Extended cycling tests of Li||Cu half‐cells (Figures 3d and S18) demonstrate sustained performance. Under complete plating/stripping conditions at 0.5 mA cm⁻²/1 mAh cm⁻², FEI maintains an average CE of 98.88% over 150 cycles with minimal scatter, while EE and FE exhibit progressive deterioration to 84.70% and 94.91%, respectively (Figure 3d). The nearly constant voltage plateaus throughout testing confirm effective suppression of dead lithium accumulation and parasitic electrolyte decomposition (Figure S18).

The charge‐transfer kinetics of lithium plating/stripping were evaluated via Tafel analysis (Figure 3e). FEI exhibits the smallest plating overpotential and the steepest slope near equilibrium, corresponding to the highest exchange current density (I ₀, 0.57 mA cm⁻²) and faster charge transfer kinetics. Cyclic voltammetry confirms these trends (Figure S19), where FEI displays the highest peak currents and smallest redox peak separations, consistent with facile charge transfer and minimal polarization. Temperature‐dependent impedance analysis from −30°C to 60°C demonstrates consistently lower activation energies for both SEI diffusion (38.24 kJ mol⁻¹) and charge transfer (51.13 kJ mol⁻¹) compared to FE (43.56 and 59.03 kJ mol⁻¹) and EE (51.21 and 64.93 kJ mol⁻¹) (Figures S20–S22). These reduced energy barriers directly reflect the effectiveness of the hybrid‐solvation design in facilitating Li⁺ desolvation and transport through the SEI layer.

The practical advantages of FEI become particularly evident under extreme temperature conditions. At −30°C (0.5 mA cm⁻²/0.5 mAh cm⁻²), where lithium plating/stripping kinetics are severely compromised, FEI‐based Li||Li cells cycle continuously for over 1200 h without short‐circuiting or significant polarization increase (Figure 3f). In contrast, EE and FE systems fail within ∼700 and ∼1000 h, respectively, alongside significantly larger and more unstable overpotential evolution. This superior stability is rooted in the interfacial kinetics, as revealed by operando EIS and the corresponding distribution of relaxation times (DRT) analysis (Figures 3g,h and S23 and S24). The DRT plots show that the charge‐transfer resistance (R _ct) for the FEI system is minimal and remains exceptionally stable throughout the stripping and plating process. Conversely, the R _ct in the FE and EE systems is substantially larger and intensifies dramatically during plating, confirming the growth of a resistive, unstable interface. Collectively, these results prove that the FEI electrolyte forms a robust, highly Li⁺−conductive SEI that maintains fast charge‐transfer kinetics, enabling stable Li plating/stripping even at sub‐zero temperatures.

2.4. Anode–Electrolyte Interphase Analysis

To uncover the molecular‐level origins of enhanced lithium reversibility in FEI electrolytes, we performed comprehensive depth‐resolved surface analyses of SEI chemistry and spatial distribution. X‐ray photoelectron spectroscopy (XPS) depth profiles reveal distinct atomic ratios of key elements on cycled LMAs (Figures 4a,b and S25). Detailed XPS spectra for the EE, FE, and FEI electrolytes are presented in Figures S26 and S27. Compared to EE and FE, FEI‐derived SEI exhibits a markedly higher fraction of inorganic components, as an F/C atomic ratio exceeding 2.0 (vs. <1.6 for FE and <1.4 for EE) and persistent B and N signals throughout the sputtering depth (Figures 4a,b and S25). This inorganic enrichment stems from enhanced anion decomposition in the hybrid‐solvation structure, where increased PF₆ ⁻ and DFOB⁻ coordination promotes their preferential reduction over solvent molecules. [47] Deconvolution of high‐resolution spectra confirms this mechanism (Figures 4c and S28). F 1s analysis shows 87% LiF formation (at 300 s depth) in FEI versus 80% (FE) and 28% (EE), while parasitic species, such as C─F and P─F diminish, with depth, indicating minimized solvent decomposition. In addition, B 1s spectra reveal enhanced B─O and Li─B─O formation (∼62% of B signal) from more complete DFOB⁻ decomposition. [48]

FIGURE 4.

Comprehensive characterization of the solid–electrolyte interphase (SEI) on Li metal anodes. (a) Atomic ratios of various elements within the formed SEI as a function of Ar⁺ sputtering time for FEI electrolytes. (b) Ratios of inorganic‐to‐organic components (F/C) from XPS depth profiling for the EE, FE, and FEI electrolytes. (c) Quantitative analysis of the chemical species in the SEI at different depths for the FEI electrolyte. (d, e) TOF‐SIMS depth profiles (d) and corresponding 3D reconstructions (e) comparing the chemical fragment distribution in the SEI from FEI and FE. (f‐i) SEM images comparing the Li deposition morphology after the initial plating (f, g) and after 50 cycles (h, i) for the FEI and FE electrolytes.

The unique nitrogen incorporation distinguishes FEI‐derived SEI from conventional systems. N 1s spectra identify three distinct components: Li_xN (∼30%), Li_xNO_y (∼70%), originating from controlled IBN decomposition within the Li⁺ solvation shell (Figures 4c and S26) [32, 49]. These nitrogen species provide exceptional ionic conductivity pathways while maintaining mechanical flexibility, directly explaining the reduced interfacial resistance and enhanced cycling stability. The O 1s region further supports the dual‐source interphase model, where FEI displays significant Li₂O enrichment (∼44%) with reduced organic carbonyl species, contrasting with the organic‐dominated oxygen signals in EE and FE systems [50].

The spatial distribution of SEI components was further resolved by time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) depth profiling (Figures 4d and S29–S31) with 3D chemical mapping (Figure 4e). In FEI, key inorganic fragments (LiF₂ ⁻, BO⁻, LiO⁻) and nitrogen species (CN⁻) penetrate deeper and are more uniformly distributed than in FE, where these signals decay rapidly and concentrate within ∼50 nm of the surface (Figure 4d). In contrast, organic fragments like C⁻, C₂HO⁻, and CO₂Li⁻ are significantly reduced in FEI, while FE shows strong surface localization of organic ions, indicative of a stratified, organic‐rich outer layer (Figures 4d and S29). Three‐dimensional reconstructions (Figures 4e and S32 and S33) corroborate that FEI effectively suppresses organic accumulation throughout the SEI volume, instead establishing a homogeneous inorganic/nitrogen‐rich matrix that ensures uniform Li⁺ flux distribution and prevents localized hot spots for dendrite nucleation. In contrast, FE still suffers from surface‐layered SEI and insufficient structural depth.

The superior SEI chemistry directly manifests in lithium deposition morphology. After initial plating (1 mA cm⁻², 1 mAh cm⁻²) at 25°C, FEI produces dense, uniform deposits with fine‐grained texture and minimal surface roughness (Figure 4f). In comparison, FE yields granular morphologies with visible grain boundaries and micro‐crevices (Figure 4g), while EE results in mossy, filamentous structures characteristic of incipient dendrite formation (Figure S34a). After 50 cycles, these differences become more pronounced: FEI‐plated lithium remains remarkably coherent and smooth, maintaining its initial morphology with minimal dead lithium accumulation (Figure 4h). Conversely, FE and EE electrodes exhibit signs of pitting, voids, and uneven growth (Figures 4i and S34b). Under low‐temperature conditions (−30°C), FEI maintains uniform, compact deposits with smaller particle sizes due to increased nucleation overpotential yet retains SEI integrity after 30 cycles (Figure S35). In contrast, FE and EE cycled LMAs exhibit porous, heterogeneous dead lithium agglomerates with mossy structures and large gaps, indicating extensive parasitic product accumulation.

Atomic force microscopy (AFM) analysis quantifies the SEI's superior mechanical characteristics crucial for dendrite suppression. Surface roughness (R _a) measurements after 50 cycles (Figure S36) reveal R _a = 13.9 nm for FEI, substantially smoother than FE (25.7 nm) and EE (50.7 nm). Force modulation microscopy determines Young's modulus of 3.85 GPa for FEI‐derived SEI, significantly exceeding FE (2.49 GPa) and EE (1.35 GPa) (Figure S37). The enhanced mechanical strength, combined with the elastic properties of nitrogen‐containing phases, creates an SEI capable of accommodating volume changes while resisting dendrite penetration [51]. These findings demonstrate that the hybrid‐solvation strategy produces a unique dual‐source SEI combining anion‐derived inorganics (LiF, Li₂O, and borates) with IBN‐derived nitrogen species, ensuring stable, dendrite‐free operation under extreme conditions.

2.5. Wide‐Condition Electrochemical Performance

Building upon the optimized hybrid‐solvation structure and robust LMA interface, we evaluated the FEI electrolyte's effectiveness in regulating cathode chemistry across diverse practical conditions. In Li||NCM811 cells cycled under standard conditions (3.0−4.3 V, 1C), FEI demonstrates an initial discharge capacity of 187 mAh g⁻¹ and superior stability with 72.5% capacity retention after 500 cycles (Figures 5a and S38), surpassing FE (64.2%) while EE fails catastrophically within 250 cycles due to insufficient interfacial protection. Rate capability evaluation from 1 to 20 C reveals FEI's exceptional fast‐charging potential (Figures 5b and S39). The FEI electrolyte maintains higher specific capacities across all rates, particularly at extreme currents (> 5 C), where kinetic limitations typically dominate. At 20 C, FEI retains 64% of its 1 C capacity, significantly outperforming EE (15%) and FE (57%). Long‐term high‐rate cycling further demonstrates FEI's durability, where FEI maintains 75.25% capacity after 1500 cycles compared to 66.24% for FE at 5 C, while EE short‐circuits before 1000 cycles (Figure S40). Most remarkably, at 20 C continuous cycling, FEI exhibits an initial capacity of 119 mAh g⁻¹ with 82.35% retention over 500 cycles, whereas FE and EE suffer severe degradation with only 25.47% and 12.22% retention, respectively (Figures 5c and S41).

FIGURE 5.

Comprehensive electrochemical performance of Li||NCM811 full cells with EE, FE, and FEI electrolytes. (a) Long‐term cycling performance at 1 C and 25°C. (b) Rate capability from 1 to 20 C. (c) Long‐term cycling performance at 20 C and 25°C. (d) High‐voltage cycling performance up to 4.6 V. Representative galvanostatic charge‐discharge profiles and initial Coulombic efficiency (ICE) for cells with (e) FEI and (f) FE electrolytes, cycled between 3.0 and 4.6 V. (g) High‐temperature cycling performance at 60°C. (h) Cycling performance at −30°C and 0.1 C. (i) Long‐term cycling of a Li||NCM90 pouch cell prototype with the FEI electrolyte at a charge/discharge rate of 0.2/0.5 C and 25°C.

The enhanced cycling stability becomes more pronounced under elevated voltage conditions. When cycled to 4.6 V, FEI sustains 87.58% and 78.98% capacity after 200 and 400 cycles, respectively. In contrast, FE and EE exhibit rapid deterioration with capacity retention of 57.62% and 41.97% after 200 cycles, respectively (Figure 5d). Voltage profiles (Figures 5e,f and S42) reveal minimal polarization growth and well‐preserved charge‐discharge plateaus for FEI, achieving a high initial CE of 92.08%, indicative of efficient SEI/CEI formation without excessive parasitic reactions. CV test corroborates FEI's superior oxidative stability, showing minimal anodic current increase and negligible peak shifts over successive cycles, contrasting with the progressive electrolyte oxidation and CEI degradation observed in reference systems (Figure S43).

High‐temperature operation (60°C) typically accelerates electrolyte decomposition and transition‐metal dissolution, yet FEI demonstrates exceptional thermal stability. After 300 cycles at 60°C and 4.3 V, FEI retains 90.08% capacity, dramatically outperforming FE (30.48%) and EE (21.03%) (Figure 5g). Differential capacity (dQ/dV) analysis reveals that FEI preserves sharp, symmetric H1→M and H2→H3 phase transition peaks throughout cycling, indicating minimal interfacial impedance buildup and maintained structural integrity (Figure S44). Conversely, FE and EE display fading redox peaks and widening profiles, implying extensive electrolyte breakdown and HF etching (e.g., thermal decomposition of FEC), which result in CEI instability at elevated temperatures [52].

At the opposite extreme, low‐temperature performance was also assessed at −40°C, −30°C, and −20°C, respectively. At −30°C with a 0.1 C discharge rate, the FEI‐based cell delivers an impressive initial capacity of 150 mAh g⁻¹ (corresponding to 75% of its room‐temperature capacity) and exhibits excellent stability with 97.4% retention after 150 cycles. In comparison, FE and EE deliver only 125 and 103 mAh g⁻¹ initially, with retentions declining to 67.46% and 76.70%, respectively (Figure 5h). Voltage profiles (Figure S45) show that FEI preserves distinct redox plateaus with minimal hysteresis, confirming maintained Li⁺ mobility and interfacial functionality despite cryogenic conditions. At a 0.2 C discharge rate, FEI still sustains 97.70% retention over 200 cycles (Figure S46). Most remarkably, at −40°C, where EE and FE cells fail within ∼20 cycles, FEI maintains stable operation for over 80 cycles with 125 mAh g⁻¹ reversible capacity (Figure S47). At −20°C under 0.5 C discharge, FEI demonstrates 98.42% retention after 200 cycles, far exceeding FE (78.26%) and EE (53.70%) (Figure S48). This exceptional cryogenic performance is directly attributed to the favorable properties of IBN, including its low viscosity and reduced Li⁺ desolvation energy, coupled with a nitrogen‐enriched interface that ensures high ionic conductivity in freezing conditions. Finally, long‐term voltage‐hold cycling at 3.6−4.2 V (Figure 5i) demonstrates that FEI enables flat, stable capacity output with Coulombic efficiency exceeding 99.9% throughout.

Finally, a 2.0 Ah practical lithium metal pouch cell using LiNi_0.9Co_0.05Mn_0.05O₂ (NCM90) as cathode material was prepared to evaluate the utility of the hybrid solvation electrolyte (Figure S49 and Table S4). The assembled cell used a depleted FEI electrolyte (E/C ratio of 1.20 g Ah⁻¹) and achieved an impressive energy density of 403 Wh kg⁻¹ based on total cell mass. When cycled at a charge/discharge rate of 0.2/0.5 C and 25°C, the cell demonstrated excellent stability, retaining 90.0% of its initial capacity after 100 cycles (Figure 5i). The rate capability tests were conducted at incremental discharge rates from 0.2 to 10 C. As shown in Figure S50, even at rates up to 5 C, the cell retains a reversible capacity of 1.25 Ah, allowing a full charge/discharge cycle to be completed in approximately 12 min. Additionally, the cell exhibits excellent low‐temperature performance, retaining 77.15% of its room‐temperature discharge capacity at −40°C (Figure S51), which emphasizes the practical applicability of lithium‐metal batteries using FEI electrolyte in cold environments.

2.6. Cathode Interfacial Chemistries and Kinetics

To elucidate the origin of superior interfacial stability conferred by the FEI electrolyte in Li||NCM811 cells, we systematically examined CEI structure, chemistry, and dynamic impedance response on NCM811 cathodes. SEM and transmission electron microscopy (TEM) images (Figures 6a,b and S52–S54) reveal that FEI‐cycled cathode particles preserve their pristine, dense morphology with an ultrathin, conformal CEI layer (2.32–3.44 nm thick) after 4.6 V cycling. This minimal thickness suggests highly controlled electrolyte decomposition and effective surface passivation. In stark contrast, CEIs formed in FE and EE electrolytes are significantly thicker (4.25−10.63 nm and 7.87−11.34 nm, respectively) and exhibit apparent surface reconstruction, micro‐cracking, and heterogeneous coverage, indicative of uncontrolled parasitic reactions and structural deterioration (Figures 6a,b and S53 and S54). X‐ray diffraction (XRD) patterns provide quantitative evidence of bulk structural preservation (Figure S55). The FEI‐cycled cathode maintains a high I(003)/I(104) intensity ratio of 1.50, closely matching pristine NCM811 (1.54), confirming minimal cation mixing and preserved layered ordering. Remarkably, the FE sample exhibits severe degradation (0.99) below even EE (1.33), attributed to the HF attack generated by FEC decomposition under high voltage. In addition, ICP‐MS analysis of the post‐cycling electrolyte after 200 cycles shows that the concentrations of dissolved Ni, Co, and Mn in the FEI system are substantially lower than those in the FE and EE electrolytes (Figure S56), supporting that the FEI‐derived CEI suppresses transition‐metal dissolution and mitigates cathode degradation during prolonged high‐voltage cycling.

FIGURE 6.

Comprehensive analysis of the cathode–electrolyte interphase (CEI) formed in FEI and FE electrolytes on NCM811 cathodes. (a) SEM and (b) cross‐sectional TEM images comparing the morphology and thickness of the CEI. (c–g) XPS analysis, including (c) F/C ratio and high‐resolution depth profiles of (d, e) F 1s and (f, g) P 2p, revealing the chemical composition of the CEI. (h) The adsorption energy of EMC, EC, FEC, and IBN at the (001) plane of NCM811. (i) Calculated hydrogen transfer energy for different solvents. DRT analysis comparing the evolution of interfacial and charge‐transfer resistances for the (j) FEI and (k) FE electrolytes.

Depth‐resolved XPS analysis reveals unique CEI chemistry enabled by the hybrid‐solvation design (Figure 6c–g). FEI‐derived CEI exhibits a higher F/C ratio throughout the sputtering depths than that of FE, suggesting greater fluorine retention and lower organic decomposition (Figures 6c and S57). The F 1s spectra (Figure 6d) reveal that intense LiF signals (∼685.5 eV) are present throughout the sputtering depth in FEI, indicating the formation of a robust inorganic framework with enhanced thermal and electrochemical stability [53]. Conversely, FE shows weaker LiF signals with higher proportions of organic fluorinated species (C─F, B─F, P─F bonds), confirming extensive FEC decomposition at high voltages (Figure 6e). In addition, the P 2p spectra of FEI (Figure 6f) maintain stable –P_yF_z and minimal –PO_yF_z signals, indicating moderated PF₆ ⁻ decomposition with beneficial incorporation into the CEI matrix. Conversely, FE displays pronounced –P_xO_y and PO_yF_z components, characteristic of LiPF₆ hydrolysis and uncontrolled decomposition (Figure 6g) [54]. B 1s analysis (Figures S58 and S59) further demonstrates controlled LiDFOB decomposition in FEI. More importantly, the peak observed in the N 1s spectrum at approximately 400 eV can be attributed to the nitrogen species within the cyano group (─C≡N) that coordinates with metallic nickel, confirming the penetration of ─CN moieties into the deeper layers of the electrode (Figures S58 and S59) [54]. By establishing strong coordination interactions with Ni ions, the ─CN groups effectively suppress the high reactivity of Ni⁴⁺ under elevated voltages, thereby enhancing the structural and thermal stability of the cathode material.

The molecular‐level protection mechanism of IBN at the cathode interface operates through an adsorption‐initiated and progressively evolved pathway. Density functional theory (DFT) calculations reveal that IBN exhibits the strongest adsorption energy (−2.04 eV) on NCM811 surfaces, substantially exceeding those of FEC (−1.45 eV), EC (−0.72 eV), and EMC (−0.61 eV) (Figure 6h), indicating its strong tendency to enrich at the cathode interface. Moreover, the hydrogen transfer energy for IBN is −0.23 eV, which is significantly lower than that of the other solvents, suggesting a more favorable interfacial stabilization behavior under high‐voltage conditions (Figure 6i). To capture the interfacial evolution of nitrile species at the cathode, N 1s XPS spectra were collected for NCM811 electrodes after soaking in the FEI electrolyte, after charging to 4.3 and 4.6 V, and after 50 cycles at 4.6 V (Figure S60). A distinct TM–NC–R signal is already present on the soaked electrode, indicating that IBN‐derived species preferentially enrich at the NCM811 surface prior to electrochemical polarization. Upon charging and prolonged cycling, the N 1s spectra progressively evolve to include both retained Free –CN/TM–NC–R features and newly formed nitrile‐derived interfacial species. This spectroscopic evolution indicates that the cathode‐side role of IBN cannot be reduced to either a purely intact molecular adsorption layer or a purely decomposition‐derived CEI. Rather, the protective effect arises from an adsorption‐initiated nitrile interphase that undergoes subsequent evolution under high‐voltage operation, thereby suppressing parasitic oxidation, mitigating transition metal dissolution, and preserving the structural integrity of the NCM811 cathode [54, 55, 56, 57].

Operando EIS and galvanostatic intermittent titration technique (GITT) measurements quantify the interfacial benefits of FEI electrolyte. Operando EIS at various states of charge (Figure S61) confirms that the cell with the FEI electrolyte consistently maintains the lowest interfacial impedance across the entire 3.8–4.6 V operational window compared to the FE and EE electrolytes. DRT analysis further elucidates the interfacial kinetics by deconvoluting the impedance contributions from Li⁺ migration through the CEI (R _CEI) and the charge‐transfer reaction (R _ct) (Figures 6j,k and S62). Throughout the delithiation process, the R _CEI for all cells initially declines, corresponding to the formation of the CEI, but subsequently increases as the voltage rises, reflecting its progressive degradation. Notably, the FEI system exhibits substantially smaller R _CEI and R _ct values, demonstrating the formation of a highly ionically conductive and chemically stable CEI. This stability is attributed to the preferential adsorption of IBN on the NCM811 surface, which effectively shields the cathode from parasitic reactions with carbonate solvents and promotes the formation of nitrile‐derived protective interfacial species. In stark contrast, the R _ct in the FE and EE systems increases sharply upon approaching 4.6 V, signifying severely hindered Li⁺ desolvation kinetics at high potentials. This finding corroborates that sluggish ion transport in the FE and EE electrolytes promotes Li⁺ accumulation on the cathode surface. This, in turn, triggers non‐uniform Li⁺ flux, compromises CEI integrity, and initiates a vicious cycle of escalating polarization, continuous electrolyte decomposition, and irreversible structural degradation.

The favorable CEI dynamics are further reflected in Li⁺ diffusivity trends from GITT (Figure S63). FEI exhibited the least polarization and fastest voltage relaxation across the entire 3.4–4.6 V range, with average Li⁺ diffusion coefficients (D_Li ⁺) of 7.20 × 10⁻⁹ cm²/s, surpassing FE (4.67 × 10⁻⁹ cm²/s) and EE (3.01 × 10⁻⁹ cm²/s). This suggests that the CEI in FEI supports enhanced ion mobility and reduced kinetic barriers during charge‐discharge, particularly in phase transition regions above 4.0 V. Even at a demanding temperature of −30°C, where charge‐transfer resistance typically governs cell performance, the FEI electrolyte demonstrates superior low‐temperature stability (Figure S64). By maintaining a significantly smaller desolvation impedance throughout the entire discharge process (4.3to 3.1 V), it effectively mitigates energy loss and enhances the battery's cycling capability in cold conditions. These observations affirm that the FEI‐derived CEI is not only chemically robust but also structurally responsive, preserving low interfacial impedance even at a high state of charge and cryogenic conditions. Mechanistically, this is attributed to the dual interphase contributions of IBN. Specifically, free IBN molecules preferentially enrich at the cathode interface and interact with surface transition‐metal sites, thereby suppressing interfacial degradation and providing precursors for the subsequent formation of nitrile‐derived protective species during high‐voltage cycling. Simultaneously, the weakly coordinating IBN and anion‐regulated solvation structure promote the controlled decomposition of PF₆ ⁻ and DFOB⁻, generating inorganic‐rich CEI components that anchor the CEI both chemically and mechanically. This chemical convergence yields a CEI that accommodates high‐voltage cycling, fast charging, and thermal/mechanical stress, unlike conventional CEIs that suffer from compositional mismatch or fragmentation under aggressive operation.

3. Conclusion

This work introduces a hybrid‐solvation electrolyte design that addresses the fundamental challenge of simultaneous anode and cathode interface stabilization in LMBs. Through strategic incorporation of bifunctional IBN, we demonstrate that IBN's weak coordination shifts Li⁺ solvation toward anion‐enriched structures, promoting preferential salt decomposition into inorganic‐rich interphases (LiF, Li₂O, and B–O domains). Coordinated IBN undergoes controlled decomposition to generate highly conductive nitrogen species (Li₃N, LiN_xO_y) that enhance interfacial transport, while uncoordinated IBN preferentially enriches at cathode surfaces and initiates nitrile‐derived protective interfacial chemistry under high‐voltage conditions. This dual‐source interfacial chemistry enables exceptional performance: Li||NCM811 coin cells achieve 78.98% capacity retention after 400 cycles at 4.6 V, fast‐charging capability up to 20 C with 82.35% retention over 500 cycles, and reliable operation from −40°C to 60°C. The developed FEI electrolyte also enables stable cycling of Li||NCM90 pouch cells with a high energy density of 403 Wh kg⁻¹. The bifunctional solvent concept transcends traditional trade‐offs between ionic conductivity and interfacial stability, establishing a new paradigm for electrolyte engineering where molecular‐level design enables simultaneous optimization of both electrode interfaces.

Author Contributions

Yuhao Liang: conceptualization, methodology, investigation, writing – original draft, writing – review and editing, formal analysis. Ting He: data curation, formal analysis, investigation, visualization, writing – original draft. Zimo Huang: writing – review and editing, investigation, and formal analysis. Wei Chen: investigation, validation, and resources. Juncheng Wang: software, writing – review and editing. Hao Long: writing – review and editing, visualization, and data curation. Xueming Chen: investigation, visualization, writing – review and editing. Meng Li: software, formal analysis, and resources. Qifeng Zheng: formal analysis, writing – review and editing, methodology. Hao Chen: funding acquisition, conceptualization, methodology, writing – review and editing, formal analysis. Shanqing Zhang: supervision, funding acquisition, project administration, writing – review and editing, conceptualization.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: anie72827‐sup‐0001‐SuppMat.doc.

Data Availability Statement

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