Recent progress and challenges in potassium-ion battery anodes: towards high-performance electrodes

ABSTRACT

With the growing demand for lithium-ion batteries (LIBs) and the rising cost of lithium (Li) resources, potassium-ion batteries (KIBs) have emerged as promising alternatives due to their abundant potassium (K) reserves, cost-effectiveness, and electrochemical similarities to LIBs. However, the commercialization of KIBs hinges on the development of high-performance anode materials with improved capacity, stability, and rate capability. This review provides a comprehensive overview of KIB anode materials, with a particular focus on the key reaction mechanisms, including intercalation, alloying, and conversion. Various material strategies, including carbonaceous structures, metal-based alloys, and conversion-type compounds, are discussed in terms of their advantages, limitations, and recent advancements. Additionally, challenges such as volume expansion, sluggish kinetics, and long-term stability are examined, along with strategies to overcome these issues. Future research directions are also highlighted, including electrolyte optimization, interface engineering, and K metal anodes, which hold great potential for increasing energy density but require solutions for dendrite formation and Coulombic efficiency. By summarizing the latest developments and challenges, this review aims to provide insights into the future design and optimization of KIB anodes for next-generation energy storage systems.

GRAPHICAL ABSTRACT

IMPACT STATEMENT

This review highlights recent advancements and challenges in potassium-ion battery anodes, offering insights into material design strategies crucial for developing cost-effective, high-performance alternatives to lithium-ion batteries.

1. Introduction

Since the commercialization of lithium-ion batteries (LIBs), they have been widely utilized as power sources for a broad range of applications, from small-scale electronic devices such as mobile phones and laptops to large-scale systems including electric vehicles (EVs) and energy storage systems (ESSs) [1–6]. As the demand for LIBs continues to grow, driven by the increasing need for high energy density storage solutions in large-scale applications, the price of lithium (Li) has surged significantly due to its limited geological distribution and constrained reserves [7–10]. Reflecting its growing scarcity and economic importance, Li is now often regarded as a high-value material in the global market. Consequently, substantial research efforts have been dedicated to developing next-generation battery technologies that can serve as viable alternatives to LIBs. Among these, sodium-ion batteries (NIBs) and potassium-ion batteries (KIBs) have garnered considerable attention due to their cost-effectiveness, natural abundance, and chemical and physical similarities to lithium-based systems (Figure 1a) [8,11–14].

Figure 1.

(a) Comparative chart highlighting the distinct characteristics of Li, Na, and K. (b) The relative abundance of Li, Na, and K in the Earth’s crust along with their standard reduction potentials.

With advancements in next-generation battery technologies, NIBs have reached a level of technological maturity close to commercialization, and major battery manufacturers worldwide are actively exploring their implementation in EVs. Given this progress, KIBs are increasingly being recognized as a strong rival to NIBs and as a promising alternative for large-scale energy storage applications. As interest in KIBs continues to grow, the number of research publications on this technology has been steadily increasing, highlighting the rapid advancements in the field [15–19]. Potassium (K) offers several advantages that make KIBs particularly attractive. For instance, while Li is present in the Earth’s crust at approximately 0.002%, sodium (Na) is significantly more abundant at ~2.3%, and K is also plentiful at ~1.5%, ensuring its cost competitiveness (Figure 1b) [20–23]. Moreover, the redox potential of K/K⁺ (−2.93 V vs. SHE) is comparable to that of Li/Li⁺ (−3.04 V vs. SHE) and is more favorable than that of Na/Na⁺ (−2.71 V vs. SHE), indicating that KIBs can achieve higher energy densities than NIBs [24–27]. Although K⁺ (1.38 Å) has a larger ionic radius than Li⁺ (0.76 Å) and Na⁺ (0.96 Å), its Stokes radius in common electrolytes such as ethylene carbonate (EC) and propylene carbonate (PC) is smaller (3.6 Å) than that of Li⁺ (4.8 Å) and Na⁺ (4.6 Å), leading to lower desolvation energy and superior ionic conductivity [28–32]. This highlights the need to consider the compatibility between electrode materials and electrolytes, as it plays a decisive role in governing the electrochemical behavior of KIBs. A well-matched electrode – electrolyte pair ensures stable interfacial reactions, suppresses unwanted side reactions, and promotes efficient K⁺ transport. In particular, the formation and stability of the solid electrolyte interphase (SEI) are strongly influenced by electrolyte composition, which in turn affects the cycling stability and Coulombic efficiency of the anode [33,34]. Recent studies have demonstrated that the electrolyte plays a pivotal role not only in facilitating K⁺ transport but also in dictating the nature and stability of the SEI layer, which critically governs the long-term cycling performance of KIB anodes [33,35,36]. The choice of solvent (e.g., carbonate- vs. ether-based systems), potassium salts (e.g., potassium hexafluorophosphate (KPF₆), potassium bis(fluorosulfonyl)imide (KFSI)), and functional additives (e.g., fluoroethylene carbonate (FEC), 1,2-dimethoxyethane (DME)) directly influence the SEI’s chemical composition, mechanical integrity, and ionic permeability. An optimally engineered SEI must be electronically insulating, ionically conductive, and mechanically robust to mitigate side reactions and suppress dendrite growth. Furthermore, advanced characterization techniques, such as in-situ transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), have revealed that SEI formation pathways vary substantially with electrolyte formulation, ultimately impacting Coulombic efficiency and capacity retention [37,38]. Therefore, a comprehensive understanding of electrolyte – electrode interfacial chemistry is essential for the rational design of high-performance KIB anodes. Additionally, in LIBs, the reaction between Li⁺ and aluminium (Al) at 0.3–0.5 V (vs. Li/Li⁺) necessitates the use of a heavier and more expensive copper (Cu) current collector for the anode [39–41]. In contrast, K⁺ does not react with Al within the operating voltage range of KIB anodes, allowing for the use of a lightweight and cost-effective Al current collector for both the anode and cathode. These advantages collectively establish KIBs as highly suitable candidates for large-scale energy storage applications.

Since the commercialization of LIBs by Sony in 1991, graphite, with a theoretical capacity of ~372 mA h g⁻¹, has remained the predominant anode material [42–48]. However, ongoing research continues to explore novel anode materials with higher capacities and wider operating voltage windows to enhance energy storage capabilities. For instance, silicon (Si) has gained significant interest due to its ability to alloy with Li⁺, delivering an exceptionally high theoretical capacity of ~3,579 mA h g⁻¹ [49–53]. This has led to the commercialization of Si/graphite composites containing ~5% Si, and further efforts are being made to develop pure Si anodes and even Li metal anodes. In addition, high-power applications necessitate fast-charging anode materials such as lithium titanate (Li₄Ti₅O₁₂, LTO) and niobium oxide (Nb₂O₅), which have been actively developed and integrated into LIB systems [54–59]. The evolution of anode materials within LIBs highlights the continuous pursuit of improved electrochemical performance through material innovation. Notably, the nanoscale engineering of electrode materials has been widely reported to enhance capacity, rate performance, and cycling stability [60–66]. These fundamental insights into electrode materials are equally crucial for next-generation battery systems, including KIBs.

As research on KIBs progresses, significant efforts have been devoted to optimizing both anode and cathode materials to maximize electrochemical performance [67–71]. While K metal-based battery technologies such as K – oxygen (K – O₂) and K – sulfur (K – S) batteries have also been explored, this review focuses specifically on KIBs, which are considered the most commercially viable alternative to NIBs. In particular, this paper provides a comprehensive overview of the evolution of KIB anode materials and the various strategies employed to enhance their electrochemical properties. Similar to LIBs, KIB anodes operate via three primary reaction mechanisms: intercalation, alloying, and conversion reactions. Various candidate materials have been extensively investigated for each reaction mechanism, including conventional carbon-based materials such as graphite, as well as metal and metal oxide anodes. In this review, we systematically discuss the characteristics of each reaction mechanism, the advantages and limitations of different anode materials, and recent research trends. Furthermore, we provide insights into future research directions and the development of advanced anode materials that will further propel KIB technology toward commercialization.

2. Reaction mechanisms for K⁺ in KIB anodes

One of the key advantages of potassium-ion batteries (KIBs) is their operational similarity to lithium-ion batteries (LIBs), allowing for a relatively seamless transition toward commercialization without requiring substantial modifications to existing cell designs, provided that suitable electrode materials, electrolytes, and separators are developed. Similar to LIBs, KIBs store and release energy through the reversible movement of K⁺ between the cathode and anode. During charging, K⁺ are extracted from the cathode and migrate through the electrolyte toward the anode, where they combine with electrons traveling through the external circuit and are stored within the electrode structure (Figure 2a). The discharge process is the reverse, wherein the stored K⁺ migrate back to the cathode while electrons flow through the external circuit, generating power (Figure 2b). The choice of anode material plays a vital role in optimizing KIB performance, as different reaction mechanisms govern K⁺ storage and influence electrochemical behavior across various capacity ranges (Figure 3) [72–77]. Intercalation and de-intercalation, alloying and de-alloying, and conversion reactions are the primary mechanisms involved. The following sections provide a detailed discussion of these mechanisms.

Figure 2.

Schematic illustration of the (a) charging and (b) discharging processes in a KIB.

Figure 3.

Working potential and specific capacity of previously reported KIB anode materials categorized by their reaction mechanisms.

2.1. Intercalation reaction mechanism

The intercalation/de-intercalation mechanism in KIBs involves the reversible insertion and extraction of K⁺ within a layered structure or a three-dimensional (3D) framework. During charging, K⁺ intercalate into the crystal structure of the anode material, while de-intercalation occurs during discharge as K⁺ migrate back to the cathode (Figure 4). For effective intercalation/de-intercalation reactions, the electrode material must possess sufficient interlayer spacing and high structural stability [78–80].

Figure 4.

Schematic representation of KIB anode materials based on the intercalation reaction mechanism.

Due to the larger ionic radius of K⁺ (1.38 Å) compared to Li⁺ (0.76 Å), conventional KIB anode materials often suffer from severe volume expansion and structural degradation upon K⁺ intercalation. While graphite is the most widely used intercalation-type anode in LIBs, it exhibits limited capacity and poor electrochemical stability in KIBs due to the sluggish kinetics of K⁺ intercalation and the formation of KC₈ (the fully intercalated K⁺ -graphite compound) [81]. To address these challenges, alternative carbonaceous materials with expanded interlayer spacing, such as expanded graphite and porous carbon, have been investigated [82,83]. These materials provide enhanced structural flexibility and improved charge transport, facilitating more efficient K⁺ intercalation/de-intercalation.

Beyond structural considerations, several critical parameters influence the effectiveness of K⁺ intercalation. These include the diffusion coefficient of K⁺ within the electrode host, the activation energy for ion migration, and the charge transfer resistance at the electrode – electrolyte interface. High K⁺ mobility is favored by wider interlayer gaps, defect engineering, and optimized electrolyte formulations. Theoretical studies using density functional theory (DFT) have shown that tuning the electronic structure of carbonaceous or layered materials can lower the migration barrier for K⁺ [84,85]. Experimentally, materials with low activation energy and high ionic conductivity, such as heteroatom-doped carbons, have demonstrated improved rate capability and cycling stability [86,87]. Therefore, evaluating these physical and electrochemical parameters is vital for designing high-performance intercalation-type anodes for KIBs.

Layered transition metal chalcogenides (TMCs) and layered oxides have also been explored as promising intercalation-type KIB anodes due to their ability to accommodate K⁺ within their structural frameworks while maintaining high reversibility [88–92]. The key parameters influencing K⁺ insertion include the diffusion coefficient of K⁺ and charge transfer kinetics, prompting extensive research on electrode structural optimization. Despite their high reversibility and stability, intercalation-type anodes may experience capacity degradation over long-term cycling due to the large ionic size of K⁺. To mitigate this issue, nanostructuring strategies have been actively pursued. A more in-depth review of these strategies is provided in Part 3.

2.2. Alloying reaction mechanism

The alloying/de-alloying reaction mechanism in KIBs involves the reversible formation and decomposition of potassium-metal (K-M) alloys through electrochemical reactions between K⁺ and metal-based anode materials (Figure 5). Representative alloying-type anodes include Si, Sn, and Sb, which can form various K-M alloys upon reaction with K⁺ [93–95]. During the alloying reaction, K⁺ migrate to the anode and react with metal atoms to form K-rich alloy phases, while the de-alloying process during discharge results in the release of K⁺ and the restoration of the original metal phase. One of the primary advantages of alloying-type anodes is their ability to host multiple K⁺ per host atom, thereby enabling the transfer of multiple electrons per host atom. This leads to significantly higher theoretical capacities compared to intercalation-based materials. However, a major drawback is the severe volume expansion (>200%) that occurs during the charge/discharge process [96–98].

Figure 5.

Schematic representation of KIB anode materials based on the alloying reaction mechanism.

For instance, Sn-based anodes undergo multiple phase transitions (Sn ↔ K₄Sn₉, KSn, K₅Sn₄), while Sb-based anodes exhibit similar behavior (Sb ↔ K₃Sb, K₅Sb₄), both experiencing substantial volume expansion [99–102]. This structural instability can lead to electrode pulverization, loss of electrical contact, and solid electrolyte interphase (SEI) instability, ultimately deteriorating long-term cycling performance. To overcome these challenges, several strategies have been explored. One approach is nanostructuring, where reducing the particle size mitigates mechanical stress induced by volume expansion and shortens K⁺ diffusion paths, enhancing reaction kinetics [103]. Another widely studied strategy is the use of carbon-based composites, which serve as a buffering matrix to accommodate volume fluctuations and improve electrical conductivity [104,105]. While alloying-type anodes offer high capacity and low operating voltage, addressing the volume expansion issue remains a critical research focus. Further advancements are required to improve rate capability and long-term cycling stability, and these aspects will be reviewed in detail later in this paper.

2.3. Conversion reaction mechanism

The conversion reaction mechanism in KIBs involves the electrochemical transformation of metal-based compounds (e.g., metal oxides (MOₓ), metal sulfides (MS₂), and metal nitrides (MN)) into metallic nanoparticles and K compounds upon reaction with K⁺ (Figure 6) [106,107]. The general reaction follows the form:

$${\mathrm{M}}_{\mathrm{a}}\mathrm{X}+\mathrm{b}{\mathrm{K}}^{+}+\mathrm{b}{\mathrm{e}}^{-}\leftrightarrow {}_{\mathrm{a}}\mathrm{M}+{\mathrm{K}}_{\mathrm{b}}\mathrm{X}\left(\mathrm{where}\mathrm{M}=\mathrm{metal};\mathrm{X}=\mathrm{O},\mathrm{S},\mathrm{N},\mathrm{etc}.\right)$$

Figure 6.

Schematic representation of KIB anode materials based on the conversion reaction mechanism.

where the metal cation is reduced, leading to the formation of nanosized metallic clusters within the electrode structure. Conversion-type anodes provide exceptionally high theoretical capacities due to their ability to involve multiple electron transfer reactions per K⁺. Various materials have been explored for this mechanism, including transition metal oxides (e.g., Fe₃O₄, MnO₂, Co₃O₄), metal sulfides (e.g., MoS₂, FeS₂, VS₂), metal nitrides (e.g., TiN, VN), and metal halides (e.g., FeF₃) [108–111].

While conversion-type anodes enable higher electron storage capacity compared to intercalation-based materials, they suffer from several key challenges, including large structural rearrangements during cycling, leading to mechanical instability, high reaction overpotential, which can reduce energy efficiency, and high initial irreversible capacity loss, which lowers Coulombic efficiency. During cycling, the repeated formation and oxidation of metallic nanoparticles can compromise electrode integrity and degrade long-term cycling performance. Some layered sulfides, such as MoS₂, exhibit a dual-storage mechanism combining both intercalation and conversion reactions, which can enhance structural stability and reversibility [112]. To mitigate these issues, various nanostructuring approaches have been adopted to reduce diffusion limitations and accommodate volume changes, thereby improving cycling stability. Additionally, hybrid electrode designs incorporating carbon composites have been explored to enhance electronic conductivity and prevent aggregation of metal nanoparticles [113]. The development of such hybrid architectures remains a crucial area of research for advancing conversion-type anodes.

Each reaction mechanism – intercalation, alloying, and conversion – offers distinct advantages and challenges in the context of KIB anodes. While intercalation-based materials provide high structural stability and reversibility, they often suffer from limited capacity due to the large ionic radius of K⁺. Alloying-based materials deliver higher capacity, but severe volume expansion remains a significant drawback. Conversion-based anodes exhibit even higher capacities due to multi-electron transfer reactions, but suffer from structural degradation and high overpotentials. From a thermodynamic standpoint, the intercalation reaction is generally characterized by a low driving force and high reversibility, owing to minimal changes in the host structure [21]. In contrast, alloying and conversion reactions are thermodynamically more favorable, with large negative Gibbs free energy changes, which allow for high capacity but also lead to irreversible structural transformations [20]. Kinetically, intercalation reactions benefit from relatively low activation energy barriers and fast ion diffusion, particularly when the host materials have large interlayer spacings or open frameworks. Alloying reactions involve nucleation and growth of new phases, which introduces diffusion-limited kinetics, while conversion reactions typically suffer from sluggish reaction rates due to phase separation, particle aggregation, and high charge transfer resistance. Furthermore, intermediate phases such as KC₃₆, KC₂₄, and KC₈ in graphite-based intercalation, or K₄Sn₉, KSn, and K₅Sn₄ in Sn-based alloying, have been observed experimentally via in-situ XRD and TEM analyses [114]. These phases evolve dynamically during cycling and influence the reversibility and capacity retention of the anodes. Understanding these transient species is therefore critical for optimizing the cycling performance of KIB anodes.

To develop commercially viable KIB anodes, ongoing research must focus on improving long-term stability, rate performance, and energy efficiency through advanced material design, nanostructuring, and hybrid composite strategies. These aspects will be further examined in the subsequent sections of this review. In order to effectively implement such strategies, a deeper understanding of the fundamental mechanisms governing electrochemical performance is essential.

Beyond descriptive classification of material types, it is imperative to establish mechanistic connections between material properties and electrochemical behavior. For instance, the diffusion kinetics of K⁺, which are influenced by factors such as ionic radius, host structure, and interlayer spacing, play a crucial role in determining rate performance and can be significantly improved through nanostructuring or defect engineering. Electronic conductivity, particularly critical for alloying and conversion-type materials, dictates the efficiency of charge transfer processes and is often improved via conductive carbon matrices or heteroatom doping. Structural stability, another key factor, underpins long-term cyclability and is influenced by the extent of volume change and the mechanical robustness of the active material. The optimization of these physicochemical parameters collectively governs a material’s ability to deliver high capacity, excellent rate performance, and long-term cycling stability. As such, future research should aim to unify these mechanistic insights to guide the rational design of next-generation KIB anode materials.

3. Strategies for enhancing the electrochemical performance of anode materials and recent research trends

3.1. Anode materials based on the intercalation mechanism

Carbonaceous materials have been extensively studied and widely utilized as anode materials in lithium-ion batteries (LIBs), making them a natural starting point for potassium-ion battery (KIB) research due to their well-established synthesis methods and relatively low research barriers. Among various carbonaceous materials, graphite is one of the most representative intercalation-type anode materials, characterized by its intrinsic hexagonal layered structure with AB stacking [29,114,115]. When fully intercalated with K⁺, graphite exhibits a theoretical capacity of ~279 mA h g⁻¹ with a volume expansion of approximately 60%. The first study utilizing graphite as a KIB anode was reported by Ji et al. in 2015, where they demonstrated that K⁺ intercalation in graphite leads to the formation of KC₈, similar to the LiC₆ phase in LIBs [81]. Ex-situ X-ray diffraction (XRD) analysis confirmed the sequential formation of KC₃₆, KC₂₄, and KC₈ during K⁺ insertion, with the reverse phase transformations occurring during K⁺ extraction, restoring the original graphite structure (Figure 7). However, graphite exhibited poor rate capability and rapid capacity fading, prompting researchers to explore soft carbon as an alternative. Soft carbon demonstrated enhanced cycle life and superior rate performance compared to graphite, marking an important milestone in KIB research and laying the foundation for the development of various carbon-based anodes. However, a key limitation of intercalation-type anodes is their relatively low theoretical capacity, which arises from the limited number of available sites for K⁺ insertion. Furthermore, the large ionic radius of K⁺ can induce sluggish diffusion kinetics and lattice stress during cycling, potentially leading to structural degradation. These drawbacks necessitate the careful design of host materials with expanded interlayer spacing, enhanced conductivity, and mechanical robustness to maintain long-term cycling stability and rate capability.

Figure 7.

(a) First cycle galvanostatic charge-discharge profiles at 0.1 C. (b) XRD patterns of graphite anode materials at different states of charge (SOCs) corresponding to the points marked in (a). Reproduced with permission from ref [81]. Copyright (2015) American chemical society.

Given the limitations of graphite in KIBs, extensive research has been conducted on alternative carbonaceous materials such as expanded graphite, heteroatom-doped carbon, and porous carbon. Among these, expanded graphite has been widely studied due to its low-cost synthesis from natural graphite and tunable interlayer spacing. Kang et al. optimized the mass ratio of natural graphite and potassium permanganate oxidant via a wet chemical oxidation route to synthesize mildly-expanded graphite (MEG-x), where x represents the mass ratio of potassium permanganate to natural graphite (Figure 8) [116]. The synthesized MEG exhibited a remarkable rate capability, delivering ~88 mA h g⁻¹ at 1500 mA g⁻¹ (operating voltage: 0.01–2.0 V vs. K/K⁺). Furthermore, long-term cycling tests at 100 mA g⁻¹ demonstrated a capacity retention of ~ 72.3% after 200 cycles, highlighting that precise control over the interlayer spacing of graphite can effectively mitigate its limitations, making it a viable anode material for KIBs.

Figure 8.

High-resolution TEM (HR-TEM) images and interlayer distance measurements of (a) NG and (b–d) MEGs with varying degrees of oxidation. Reproduced with permission from ref [116]. Copyright (2021) Elsevier.

In recent years, significant attention has been given to heteroatom-doped carbonaceous materials, particularly those doped with nitrogen (N), sulfur (S), and boron (B). One of the most accessible and scalable synthesis methods involves doping soft carbon nanofibers with nitrogen (NCNF-x, where x = 650, 950, and 1100 indicates the carbonization temperature). As revealed by the transmission electron microscopy (TEM) analysis in Figure 9a–d, N is uniformly doped throughout the carbon nanofibers, and various N configurations, including pyrrolic N (N-5), pyridinic N (N-6), and quaternary N (N-Q), are present, as shown in Figure 9e [117]. The synthesized NCNF anodes exhibited exceptional electrochemical performance for KIBs. Specifically, NCNF-650 delivered a reversible specific capacity of 248 mA h g⁻¹ at 25 mA g⁻¹ (operating voltage: 0.01–3.0 V vs. K/K⁺) and maintained an impressive capacity of 101 mA h g⁻¹ even at a high current of 20 A g⁻¹ (Figure 9f). Additionally, after 4000 cycles at 2 A g⁻¹, a stable specific capacity of 146 mA h g⁻¹ was retained. The authors attributed this outstanding performance to the pyrrolic and pyridinic N species, which enhanced the electronic structure and facilitated K⁺ storage, as confirmed by quantitative analysis and theoretical simulations.

Figure 9.

(a-d) TEM images with elemental mapping of C, N, and O for NCNF-650 (scale bar: 100 nm). (e) Schematic illustration of the N-doping species. (f) Rate performance of NCNFs at current densities ranging from 0.05 to 20 A g⁻¹. Reproduced with permission from ref [117]. Copyright (2018) Springer nature.

Another promising approach for enhancing KIB performance is the utilization of porous carbon materials. Hwang et al. recently demonstrated that precise control over the nanochannel orientation in mesoporous carbon significantly improves K⁺ storage capacity [118]. They synthesized two distinct types of model carbon electrodes using a multiscale phase separation method combining block copolymer microphase separation and homopolymer macrophase separation. The synthesized materials were classified as open-end mesoporous carbon spheres (oe-MCS), where the channel ends were open, and closed-end mesoporous carbon spheres (ce-MCS), where the channel ends were closed (Figure 10a). The oe-MCS exhibited enhanced K⁺ adsorption capacity, reduced K⁺ diffusion length, and improved ion transport, resulting in superior electrochemical performance. Compared to ce-MCS, which achieved ~145 mA h g⁻¹ and ~80 mA h g⁻¹ at 0.05 and 1.0 A g⁻¹, respectively, oe-MCS demonstrated significantly higher capacities of ~210 mA h g⁻¹ at 0.05 A g⁻¹ and ~120 mA h g⁻¹ at 1.0 A g⁻¹ (operating voltage: 0.01–3.0 V vs. K/K⁺) (Figure 10b). Notably, cyclic voltammetry (CV) analysis, combined with the i = av^b equation (where i and v represent measured current and scan rate, respectively, and a and b are adjustable variables), revealed that the superior rate performance of oe-MCS was primarily due to a higher proportion of surface-controlled reactions compared to ce-MCS. This study underscored the critical role of pore channel orientation in determining the electrochemical properties of porous carbon anodes for KIBs.

Figure 10.

(a) Schematic illustration of the fabrication process for MCS materials. (b) Rate performance of MCS materials at current densities ranging from 0.05 to 1.0 A g⁻¹. Reproduced with permission from ref [118]. Copyright (2025) Wiley.

3.2. Anode materials based on the alloying reaction mechanism

Similar to LIBs, efforts to enhance the capacity of KIB anodes have been ongoing. Given that graphite exhibits a lower theoretical capacity in KIBs (~279 mA h g⁻¹) compared to LIBs (~372 mA h g⁻¹), the development of high-capacity anode materials is crucial to fully utilize KIBs’ voltage range and energy density. Alloying reaction-based anode materials are particularly attractive as they enable multi-phase transformations, allowing multiple K atoms to be stored per metal atom, thus significantly increasing capacity [99,119,120].

The most commonly studied alloying-type anodes include elements from Group 14 (Si, Ge, Sn, and Pb) and Group 15 (P, Sb, and Bi) [98,121,122]. As illustrated in Figure 11a, these materials exhibit substantially higher theoretical capacities than graphite. However, one of the major challenges associated with alloying-type anodes is their severe volume expansion during charge/discharge cycling, leading to poor cycle life [95,97,103,123]. Although alloying-type anodes offer high theoretical capacities, they typically undergo substantial volume changes, often exceeding 20%, during K⁺ insertion and extraction. This pronounced volumetric fluctuation induces mechanical stress, resulting in electrode pulverization, disruption of electrical connectivity, and instability of the SEI. These degradation mechanisms significantly accelerate capacity fading over repeated cycles. To address these critical limitations, a variety of advanced strategies in electrode engineering, including structuring at the nanoscale, incorporation of elastic and conductive support materials, and the design of mechanically robust composite architectures, have been proposed to effectively buffer the volume changes and maintain electrode integrity.

Figure 11.

(a) Gravimetric capacity of KIB anode materials based on the alloying reaction mechanism. (b–c) TEM images and SAED patterns of red P@CN electrodes at 0 V (vs. K/K⁺) during potassiation. (d) Calculated formation energy during the charge/discharge process of red P@CN electrodes. Reproduced with permission from ref [124]. Copyright (2018) Wiley.

Xu et al. synthesized red phosphorus (P) nanoparticles (10 ~ 20 nm) anchored on a 3D carbon nanosheet framework (Red P@CN) to enhance electronic conductivity and structural stability [124]. The highly conductive 3D porous carbon network facilitated efficient charge transport, enabling Red P@CN to achieve a reversible capacity of ~655 mA h g⁻¹ at 100 mA g⁻¹ (operating voltage: 0.01–2.0 V vs. K/K⁺) and maintain ~324 mA h g⁻¹ even at 2 A g⁻¹. Notably, the first experimental and theoretical study on the K⁺ storage mechanism of red P was conducted in this work. TEM (Figure 11b) and density functional theory (DFT, Figure 11d) calculations confirmed the formation of crystalline KP as a potassiation product, which was thermodynamically stable. The proposed reaction, P + K⁺ + e⁻ → KP, corresponded to a theoretical capacity of ~843 mA h g⁻¹, providing fundamental insights into the design of high-performance red P anodes.

Further advancements in alloying-type anodes have focused on structural engineering to improve cycle life. Ryan et al. addressed the volume expansion issues of Sb-based anodes by directly growing highly dense copper silicide (Cu₁₅Si₄) nanowires (NWs) on a Cu mesh substrate to form a 3D current collector (Figure 12a) [125]. This architecture provided strong anchoring effects for Sb and created buffer spaces between NWs to accommodate volume changes. The resulting Sb@Cu₁₅Si₄ NW composite exhibited a high reversible capacity of ~648 mA h g⁻¹ at 50 mA g⁻¹ (operating voltage: 0.01–1.5 V vs. K/K⁺) and retained ~250 mA h g⁻¹ after 1,250 cycles at 200 mA g⁻¹, corresponding to an impressively low capacity decay rate of ~ 0.028% per cycle (Figure 12b). This study demonstrated that careful electrode design can significantly enhance the long-term stability of alloying-type anodes. In addition, Li et al. utilized a Sb-based electrode material as a promising anode for KIBs. They designed highly dense Ti₃C₂T_x MXene and graphene dual-encapsulated nano-Sb monolith architectures (HD-Sb@Ti₃C₂T_x-G), which form an elastic network with high electrical conductivity and a compact dually encapsulated structure, achieving a high volumetric capacity of approximately 1,780 mA h cm⁻³ and a specific capacity of ~565.0 mA h g⁻¹ in KIBs. Notably, the electrode exhibited excellent long-term stability with a capacity retention of ~ 82% over 500 cycles, along with a high areal capacity of ~8.6 mA h cm⁻² [126]. To address the intrinsic challenges of Sb and to harness its high theoretical capacity, considerable efforts have been devoted to confining Sb within well-designed structural networks [127].

Figure 12.

(a) Schematic illustration of the synthesis of the Cu₁₅Si₄ NW array current collector and thermal evaporation deposition of Sb, along with magnified views showing the pristine Cu mesh fiber, Cu₁₅Si₄ NW grown fiber, and Sb@Cu₁₅Si₄ NW fiber. (b) Long-term cycling stability of Sb@Cu₁₅Si₄ NW arrays compared to Sb@Cu mesh samples. Reproduced with permission from ref [125]. Copyright (2022) Wiley.

Similarly, Zhu et al. synthesized crystalline silicene (c-silicene) from Zintl phase CaSi₂ to develop a stable silicon-based KIB anode [128]. In-situ synchrotron XRD and TEM analyses revealed a reversible K-Si phase transition, with KSi forming as the dominant discharge product (Figures 13a–f). The c-silicene anode exhibited a reversible capacity of ~180 mA h g⁻¹ at 10 mA g⁻¹ (operating voltage: 0.01–3.0 V vs. K/K⁺) and an outstanding cycle life, maintaining a Coulombic efficiency of 99.4% over 3,000 cycles at 500 mA g⁻¹ (Figure 13g). These findings challenge the conventional perception of Si as an ‘inert’ material for KIBs and highlight its potential as a high-performance alloying-type anode.

Figure 13.

In-situ SXRD and TEM characterization. (a) Galvanostatic charge-discharge curves between 0.01 and 3.0 V at 100 mA g⁻¹ during in-situ measurements. (b) in-situ SXRD patterns of c-silicene during the discharge/charge process. (c) in-situ SXRD patterns of the discharge products. (d) TEM images and elemental mapping of K and Si for c-silicene. (e) High-resolution TEM (HR-TEM) images of c-silicene and KSi. (f) Selected area electron diffraction (SAED) pattern of KSi. (g) Long-term cycling performance at a high current density of 500 mA g⁻¹. Reproduced with permission from ref [128]. Copyright (2022) Elsevier.

3.3. Anode materials based on the conversion reaction mechanism

Similar to LIB research, the development of conversion-type anode materials for KIBs has progressed at a slower pace compared to intercalation- and alloying-based anodes. This is primarily due to the inherent challenges associated with conversion reactions, such as significant structural rearrangements during cycling, high reaction overpotentials, and poor energy efficiency [129–131]. These factors lead to substantial capacity fading and hinder the practical application of conversion-type anodes. In particular, the repeated formation and dissolution of electrochemically active phases during charge/discharge cycles induce extensive structural reorganization, which results in high polarization, poor Coulombic efficiency, and instability of the SEI. Moreover, the mechanical stress caused by these transformations often leads to electrode pulverization and electrolyte decomposition, ultimately contributing to short cycle life. Large voltage hysteresis and sluggish reaction kinetics further limit the rate capability and energy efficiency of these systems. To overcome these limitations, extensive research efforts have been directed toward developing novel strategies that enhance their electrochemical performance and long-term cycling stability [132,133]. Among the most promising approaches are the incorporation of nanoconfined active phases within conductive carbon matrices, core – shell structural design, and interface engineering, all of which aim to alleviate structural stress and stabilize electrode – electrolyte interactions.

Guo et al. synthesized amorphous FeVO₄ (a-FVO) as a promising anode material for KIBs via a low-temperature synthetic process, which follows a reaction mechanism distinct from conventional conversion reactions [134]. Compared to its crystalline counterparts, FVO-600, FVO-700, and FVO-800, which were obtained through heat treatment at 600, 700, and 800 °C, respectively, a-FVO exhibited a smaller particle size and a larger specific surface area. As a result, a-FVO demonstrated superior specific capacity (~350 mA h g⁻¹ at 100 mA g⁻¹ in the potential range of 0.05–3.0 V vs. K/K⁺) and enhanced cycling stability compared to its crystalline counterparts (Figure 14a). To further improve electrochemical performance, the authors employed a simple ball-milling approach using Ketjen black carbon as an additive to fabricate an a-FVO/carbon composite (a-FVO/C). This composite electrode exhibited excellent rate capability (~180 mA h g⁻¹ at 2 A g⁻¹) and remarkable cycling stability, maintaining ~ 99.8% Coulombic efficiency over 2,000 cycles (Figure 14b–e). As another example of metal oxide electrode materials based on conversion reactions, Rahman et al. synthesized a hybrid composite of Co₃O₄ and Fe₃O₄ nanoparticles dispersed within a Super P carbon matrix (Co₃O₄-Fe₃O₄/C) using a ball-milling-assisted molten salt method, which is a cost-effective and scalable approach [135]. The resulting Co₃O₄-Fe₃O₄/C composite exhibited a reversible capacity of ~220 mA h g⁻¹ at 50 mA g⁻¹ (operating voltage: 0.01–3.0 V vs. K/K⁺). Ex-situ XRD analysis confirmed that the conversion reaction was not fully completed during the first cycle but progressively improved, reaching full conversion after approximately 10 charge/discharge cycles. The authors proposed that the reaction mechanisms followed Co₃O₄ + 8K⁺ +8e⁻ ↔ 3Co + 4K₂O and Fe₂O₃ + 6K⁺ +6e⁻ ↔ 2Fe + 3K₂O. The excellent electrochemical performance was attributed to the synergistic combination of highly conductive carbon chains and electrochemically active metal oxides. However, despite the promising capacity, long-term cycling stability remained a challenge, indicating that further material optimization is necessary.

Figure 14.

(a) Galvanostatic charge-discharge curves during the second cycle. (b) Rate performance of the a-FVO/C composite electrode compared to the a-FVO electrode. Discharge-charge profiles of (c) the a-FVO/C composite electrode and (d) the a-FVO electrode at various current densities (100, 200, 300, 500, 1000, and 2000 mA g⁻¹). (e) Cycling performance of the a-FVO/C composite electrode at 300 mA g⁻¹. Reproduced with permission from ref [134]. Copyright (2019) Elsevier.

Ren et al. explored the potential of Sb₂S₃-based anodes for KIBs by synthesizing amorphous and crystalline Sb₂S₃ nanoparticles (a-Sb₂S₃@NSC and c-Sb₂S₃@NSC) embedded within N/S co-doped carbon composites (Figure 15a–c) [136]. The synthesis was carried out via a combination of carbonization and sulfidation processes followed by an organic coating method. When applied as KIB anodes, c-Sb₂S₃@NSC exhibited a discharge capacity of ~312 mA h g⁻¹ after 50 cycles at 200 mA g⁻¹ and a relatively low rate capability of ~68 mA h g⁻¹ at 2 A g⁻¹ (Figure 15d). In contrast, a-Sb₂S₃@NSC demonstrated superior electrochemical performance, delivering a high discharge capacity of ~416 mA h g⁻¹ and an improved rate capability of ~225 mA h g⁻¹ (operating voltage: 0.01–2.5 V vs. K/K⁺). The authors attributed the enhanced electrochemical properties of a-Sb₂S₃@NSC to the presence of abundant defects and structural disorder within the amorphous phase, which lowered entropy energy and activation barriers associated with K⁺ incorporation. This structural advantage facilitated improved K⁺ transport and storage. Furthermore, the synergistic interactions between amorphous Sb₂S₃ nanoparticles and the N/S co-doped carbon matrix effectively enhanced electron/ion transport kinetics and provided buffering effects to accommodate volume changes during cycling.

Figure 15.

(a) Schematic illustration of the synthesis of a-Sb₂S₃@NSC and c-Sb₂S₃@NSC. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images with corresponding EDS elemental mapping for (b) c-Sb₂S₃@NSC and (c) a-Sb₂S₃@NSC. (d) Rate capabilities at various current densities from 0.1 to 2 A g⁻¹. Reproduced with permission from ref [136]. Copyright (2024) Elsevier.

Another promising approach for enhancing the performance of conversion-type anodes is the engineering of two-dimensional (2D) materials with optimized electrode architectures. Huang et al. synthesized octahedrally coordinated MoS₂ (1T phase) on graphene oxide and assembled it into a hydrogel [112]. By employing a capillary tension densification strategy, they successfully fabricated a compact 1T-MoS₂/graphene (CTMG) anode with superior electrochemical properties (Figure 16a). When utilized as a KIB anode, CTMG exhibited a dual-storage mechanism involving both intercalation and conversion reactions, leading to significantly improved rate capability. The pseudocapacitive storage behavior of 1T-MoS₂, coupled with its exceptional K⁺ transport properties, enabled fast reaction kinetics, resulting in high reversible capacities of 511 mA h g⁻¹ at 0.1 A g⁻¹ and 327 mA h g⁻¹ at 1 A g⁻¹ (operating voltage: 0.01–3.0 V vs. K/K⁺). Moreover, its compact architecture facilitated a high volumetric capacity of 512 mA h cm⁻³ at 0.1 A g⁻¹, demonstrating the potential of densified electrode structures for practical KIB applications. Beyond achieving high capacity, long-term cycling stability remains a critical challenge for conversion-type anodes. Remarkably, CTMG maintained a capacity of ~246 mA h g⁻¹ after 800 cycles at 1 A g⁻¹, highlighting the feasibility of extending the cycle life of conversion-based anodes. Ex-situ XRD and TEM analyses confirmed the co-existence of intercalation and conversion mechanisms, providing valuable insights into the reaction pathways of MoS₂-based anodes (Figures 16b–f). In addition, a study demonstrated that ultrathin metallic cobalt selenide (CoSe₂) nanosheets were successfully assembled onto three-dimensional nitrogen-doped carbon foam (CSNS/NCF) via Co – C and Co – N – C bonding, significantly enhancing the K⁺ storage capacity. Specifically, the CSNS/NCF framework not only offered a large surface area for sufficient electrolyte contact and rapid diffusion of electrons and K⁺, but also provided a robust network with strong chemical bonding for stable K⁺ storage. Benefiting from these advantages, the optimized CSNS/NCF exhibited an excellent specific capacity of 335 mA h g⁻¹ after 200 cycles at a current density of 50 mA g⁻¹ [137]. These studies collectively demonstrated that conversion-type materials, when properly engineered, can achieve competitive stability and high energy storage capacity, making them highly promising for next-generation KIBs. Lastly, Table 1 summarizes the representative anode materials reported to date for KIBs, comparing their first-cycle de-potassiation specific capacities, initial Coulombic efficiencies (ICE), rate capabilities, and cycling stabilities.

Figure 16.

(a) Schematic representation of the CTMG synthesis process. (b) Initial discharge voltage – capacity curve and corresponding potassiation reactions of the CTMG electrode. (c) ex-situ XRD patterns of the cycled CTMG electrode. (d) Schematic illustration of K⁺ storage in CTMG. TEM images of the charged CTMG electrode after (e) the initial cycle and (f) 100 cycles. Reproduced with permission from ref [112]. Copyright (2020) Wiley.

Table 1.

Summary of the electrochemical performance of representative anode materials reported for KIB.

Anodes	De-potassiation capacity/ Currenta	ICEb	Rate capability/ Currentc	Stabilityd	Electrolyte	Ref.
Graphite	~263/28	57	~80/280	~50/50/140	0.8 M KPF6 in EC/DEC	[81]
Soft carbon	~273/7	n/a	~140/1400	~81/50/560	0.8 M KPF6 in EC/DEC	[81]
MEG-1	~267/25	55	~50/1500	~80/100/100	0.5 M KPF6 in EC/DEC	[116]
MEG-2	~269/25	53	~48/1500	~72/200/100	0.5 M KPF6 in EC/DEC	[116]
MEG-3	~276/25	52	~88/1500	~79/200/100	0.5 M KPF6 in EC/DEC	[116]
NCNF-650	~368/25	50	~101/20000	~67/100/25	0.8 M KPF6 in EC/PC	[117]
NCNF-950	~297/25	38	~75/20000	~74/100/25	0.8 M KPF6 in EC/PC	[117]
NCNF-1100	~281/25	36	~25/20000	~77/100/25	0.8 M KPF6 in EC/PC	[117]
oe-MCS	~210/50	27	~109/1000	~87/500/1000	1.0 M KFSI in EC/DEC	[118]
ce-MCS	~145/50	27	~77/1000	~75/500/1000	1.0 M KFSI in EC/DEC	[118]
Red P@CN	~655/100	59	~324/2000	~65/40/100	0.8 M KPF6 in EC/DEC	[124]
Sb@Cu₅Si NWs	~557/50	n/a	~105/4000	~65/1250/200	4.0 M KFSI in DME	[125]
HD-Sb@Ti3C2T -G	~558/100	89	~106/5000	~82/500/500	4.0 M KTFSI in EC/DEC	[126]
3D SbNPs@C	~461/200	70	~288/1000	~78/50/1000	0.8 M KPF6 in EC/DEC	[127]
c-silicene	~180/10	33	~60/500	~100/3000/500	3.0 M KFSI in DME	[128]
a-FVO	~360/100	48	~50/2000	~61/200/100	1.5 M KFSI in EC/DEC	[134]
Co₃O₄-Fe₃O₄/C	n/a	54	n/a	n/a	0.75 M KPF6 in EC/DEC	[135]
a-Sb₂S₃@NSC	~505/100	69	~225/2000	~86/50/200	4 M KFSI in EC/DEC	[136]
c-Sb₂S₃@NSC	~502/100	60	~68/2000	~64/50/200	4 M KFSI in EC/DEC	[136]
CTMG	~511/100	n/a	~234/2000	~75/800/1000	1 M KFSI in DME	[112]
CSNS/NCF	~352/50	70	~226/2000	~95/50/200	0.8 M KPF6 in EC/DEC	[137]

^aDe-potassiation specific capacity and current are expressed in units of mA h g⁻¹ and mA g⁻¹, respectively.

^bICE denotes the initial Coulombic efficiency and is expressed as a percentage (%).

^cRate capability and current are reported in units of mA h g⁻¹ and mA g⁻¹, respectively.

^dStability refers to capacity retention (%), number of cycles (cycle count), and current density (mA g⁻¹).

n/a stands for ‘not available’ and indicates that the information was not provided.

4. Conclusion and future perspective

This review provides a comprehensive summary of KIB anode materials, highlighting the key research strategies that have been employed to enhance their electrochemical performance. Specifically, the fundamental characteristics of intercalation, alloying, and conversion reactions in anodes were discussed, along with a detailed examination of various anode materials based on these mechanisms. Furthermore, recent advancements in electrode design aimed at improving the electrochemical properties of KIB anodes were analyzed. Based on the current research trends, several future perspectives can be considered for the continued development of KIB technology.

First, as KIB research progresses, it has become increasingly evident that factors beyond the electrochemical performance of anode materials significantly impact overall battery performance. While optimizing anode materials remains crucial, it is equally important to enhance the electrochemical properties of cathode materials to ensure a well-balanced system. To maximize the performance of electrode materials, further research should focus on the microscopic aspects of binders, electrolytes, and salts used in electrolytes, as well as the interfacial reactions between electrode materials and K⁺ and the diffusion behavior of K⁺ within the electrode bulk. In addition to these microscopic considerations, macroscopic factors such as cell balancing between the cathode and anode, optimization of operating voltage and applied current to mitigate capacity degradation, and the design of cells, modules, and battery packs must be systematically addressed. Achieving a fully optimized KIB system will require a multi-scale approach that integrates material-level innovations with cell- and system-level engineering.

Second, intercalation-based anode materials in KIBs have shown rapid progress and are expected to achieve electrochemical performance comparable to that of lithium-ion battery (LIB) anodes in the near future. However, alloying- and conversion-type anodes still face significant challenges compared to their LIB counterparts, particularly in terms of capacity retention, rate capability, cycling stability, and reaction reversibility. To overcome these limitations, future research should explore the potential of K metal anodes, which offer an exceptionally high theoretical capacity and could significantly enhance the energy density of KIBs. However, K metal anodes present their own challenges, including safety concerns, dendrite formation, and low Coulombic efficiency. Addressing these issues will be critical for their practical implementation.

Ultimately, achieving safe and long-cycle-life KIBs will require an integrated research approach encompassing highly stable electrolytes, precise SEI control, advanced 3D electrode architectures, and solid-state electrolyte-based all-solid-state KIBs. As part of this approach, future electrolyte optimization should particularly focus on designing ether-based electrolytes that can effectively suppress dendrite formation and extend the electrochemical stability window. Additionally, the incorporation of SEI-modifying additives such as FEC or KFSI can enhance interfacial stability and improve Coulombic efficiency over extended cycling. Interface engineering strategies should include the development of artificial SEI layers and atomic layer deposition (ALD)-based surface coatings that mitigate side reactions and promote uniform K deposition, especially for K metal anodes. In particular, developing protective coatings that facilitate a stable interface between K metal anodes and electrolytes will be a key research direction. In addition, critical challenges such as the relatively large volume change of K⁺ storage materials, the instability of electrode/electrolyte interfaces, and the limited electrochemical stability window of conventional electrolytes must be addressed to ensure the long-term durability of KIB systems. Moreover, to achieve industrial scalability, further efforts are needed to develop cost-effective, environmentally benign, and scalable synthesis methods for electrode and electrolyte materials. Environmental constraints, such as resource sustainability and safe disposal of potassium-containing waste, should also be carefully considered in future research and commercialization strategies. By tackling these challenges, KIBs can move closer to commercialization, establishing themselves as viable next-generation energy storage systems with high energy density and cost-effectiveness.

Biographies

Jeseon Lee is currently pursuing a bachelor’s degree in Chemical & Biochemical Engineering at Dongguk University, Republic of Korea, which he is expected to complete in August 2025. Since March 2025, he has been conducting research through an integrated bachelor’s and master’s program. His research interests focus on the precise nanoscale control of battery electrode materials, including the structural tuning of crystal phases, to develop high capacity, high power, and long cycle life electrode materials.

Juyeon Lee is currently pursuing a bachelor’s degree in Chemical & Biochemical Engineering at Dongguk University, Republic of Korea, with an expected graduation in August 2025. Since March 2025, she has been engaged in research through an integrated bachelor’s and master’s program. Her research interests lie in the development of highly stable electrode materials by controlling the formation of uniform coating layers on battery electrode materials.

Eunho Lim received his Ph.D. in 2017 from POSTECH, Republic of Korea. He then worked as a postdoctoral researcher at the INM–Leibniz Institute for New Materials, Germany. From 2018 to 2024, he served as a senior researcher at the Korea Research Institute of Chemical Technology (KRICT). Since September 2024, he has been an assistant professor in the Department of Chemical & Biochemical Engineering at Dongguk University. His research focuses on the rational design of electrode materials and electrocatalysts for use in energy storage and conversion devices.

Funding Statement

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) [RS-2025-00513443].

Disclosure statement

No potential conflict of interest was reported by the author(s).

Author contribution

Jeseon Lee and Juyeon Lee equally contributed this work. Jeseon Lee, Juyeon Lee, and Eunho Lim conceived the topic and scope of the review. Jeseon Lee and Juyeon Lee conducted the literature search and analysis, and contributed to data visualization and figure preparation. Jeseon Lee and Juyeon Lee drafted the manuscript under the supervision of Eunho Lim. All authors contributed to the revision of the manuscript and approved the final version.