Ideally, the SEI film should be a good Li⁺ion conductor and an electronic insulator, with long-term cycling stability required for practical applications^[33-34]. However, this is not the case in reality. Therefore, constructing a coating with good ionic conductivity and high mechanical strength on the graphite surface through surface coating is an effective method to enhance the performance of graphite anodes.

As shown in Figure 4a, Xin et al.^[35] constructed a MoO _x-MoN _x (MoON) coating on the graphite surface. The resulting MoON@Gr anode delivers a reversible capacity of 340.3 mAh·g^-1 after 4000 cycles at a 6 C rate, while retaining the intact structure of graphite and exhibiting a stable solid electrolyte interphase at the interface. Studies have shown that the MoON coating possesses high ionic conductivity (5 mS/cm), a high electron work function (4.66 eV), a low Li⁺migration energy barrier (0.91 eV), a high Young’s modulus (9.65 GPa), and a low desolvation energy (1.18 eV), which are the key factors responsible for the significant enhancement in the performance of the graphite anode.

Sun et al.^[36]used MD and DFT methods to investigate the impact of different SEI components on the solvation structure of Li⁺. They found that, due to the strong interaction between Li⁺ and Li₃P, a low-solvent-coordination solvation structure of Li⁺ can form near the inner Helmholtz plane. In addition, the high ionic conductivity of Li₃P facilitates the desolvation process of Li⁺ and enables rapid Li⁺ transport through the SEI. Based on these findings, Sun et al.^[36] developed a P-S-Graphite anode (Fig. 4b) that in situ generates an SEI containing Li₃P during cycling. The results show that an Ah-scale battery with a P-S-graphite anode and an NCM622 cathode exhibits excellent performance under high-rate charging conditions, sustaining 2,500 cycles at a 8 C rate with a capacity retention of 81.7%.

Dasgupta et al.^[37]used atomic layer deposition to coat solid electrolyte Li₃BO₃-Li₂CO₃(LBCO) onto graphite, forming an artificial solid electrolyte interphase (SEI) (Fig. 4c). Compared with naturally formed SEIs, the LBCO coating exhibits lower impedance and can delay the deposition of metallic lithium. Ultimately, the LBCO-coating strategy enabled 4 C charging without sacrificing energy density. Suh et al.^[38]coated MoS₂onto the surface of natural graphite (Fig. 4d). During initial charging, MoS₂reacts with Li⁺, forming an SEI layer rich in Li₂S and metallic Mo clusters on the graphite particle surface. This process generates a robust SEI layer with high ionic conductivity, effectively enhancing the performance of the graphite anode.

Graphite's unique layered structure provides space for lithium-ion storage, but its anisotropic nature means that lithium ions can only be inserted from the edge planes, which limits the lithium-ion transport rate^[39].By expanding the interlayer spacing of graphite^[40]and designing porous structures^[41], the structure of graphite can be optimized to increase the lithium-ion diffusion rate and enhance cycling performance^[14].

Cheng et al.^[42]used KOH etching to treat natural graphite, thereby fabricating a porous-structured graphite anode. As shown in Figure 5a, etching a porous surface into the graphite significantly increases the number of sites for lithium-ion insertion/extraction and reduces the diffusion distance of lithium ions. The electrochemical performance comparison of the resulting porous graphite is shown in Figure 5b: at a 10 C rate, the capacity of the porous graphite shows virtually no decay, and the multi-channel structured graphite anode exhibits better charge-discharge capability, cycling performance, and higher Coulombic efficiency than the pristine graphite material.

To address the capacity decay caused by the expansion of graphite materials, Zou et al.^[43]used an intercalation reaction between a mixture of H₂SO₄and H₂O₂and natural flake graphite to prepare mildly expanded graphite (MEG). The working mechanism of MEG is illustrated in Figure 6: the intercalation reaction used increases the interlayer spacing of MEG slightly from 0.3358 nm to 0.3370 nm, and the preparation process introduces numerous defects, such as pores, channels, and stacking faults. The increased interlayer spacing and porous structure buffer the volume changes of graphite during cycling, thereby significantly enhancing the cycling stability of the graphite anode. The porous structure also provides more sites for lithium storage, greatly increasing the reversible capacity of MEG.

During the first week of charge and discharge, the electrolyte decomposes on the surface of the graphite anode to form the SEI, whose structure and composition are largely determined by the properties of the electrolyte. Under low-temperature conditions, the viscosity of the electrolyte increases, ionic conductivity decreases, and the desolvation process of Li⁺is slow, which is considered a key factor limiting the low-temperature performance of graphite anodes^[44]. Therefore, optimizing the electrolyte through electrolyte engineering is an effective approach to enhancing the high- and low-temperature performance, rate capability, and cycle stability of graphite anodes. Optimization of the electrolyte can be carried out by focusing on parameters such as ionic diffusivity, electrical conductivity, viscosity, desolvation energy, and other critical factors.

Currently, mainstream ester-based electrolytes have high viscosity and melting points, making them unable to meet the low-temperature performance requirements of lithium-ion batteries^[45].The low viscosity and low melting point of ether solvents make them the best choice for fast charging and low-temperature lithium-ion batteries (LIBs). However, linear ether solvent molecules irreversibly co-intercalate into graphite layers during cycling, severely disrupting the crystal structure of graphite. Cyclic ethers can effectively address this issue. Wang et al.^[46]developed cyclopentyl methyl ether (CPME) as a non-co-intercalating ether solvent. CPME contains a cyclopentyl group with significant steric hindrance, which imparts weak solvation ability and a wide liquid-phase temperature range to the electrolyte.

Electrolyte additives also play a very important role in the electrolyte, and the addition of different additives can bring about different performance improvements to the graphite anode^[47-48]. For example, fluoroethylene carbonate (FEC) has a reduction potential (1.37 V vs. Li/Li⁺) higher than that of the EC solvent, causing it to decompose prior to the initial lithiation of the graphite anode. Ultimately, the decomposition of the FEC additive forms inorganic compounds such as LiF and Li₂O, which exhibit high ionic conductivity. These compounds can increase the density, robustness, and chemical/electrochemical stability of the SEI film, improve surface kinetics, suppress the continuous decomposition of the electrolyte, thereby reducing side reactions and extending the cycle life of the graphite anode^[49].

To enhance the sodium storage performance of graphite anodes, increasing the interlayer spacing of graphite is one of the most commonly used structural modification methods. Expanded graphite (EG) is a key material prepared by oxidizing and partially reducing graphite. Strong oxidants are used to oxidize the graphite, resulting in a material whose primary structure remains layered, while the interlayer spacing of the graphite is significantly increased and its structure undergoes complex changes. Oxidation introduces a large number of oxygen-containing functional groups into the graphite interlayers, which not only increases the interlayer spacing but also enhances the conductivity of the graphite, thereby creating favorable conditions for sodium ion insertion into the graphite anode^[66]..

Wang et al.^[67]used an improved Hummers method to prepare graphene oxide, which was partially reduced through heat treatment to effectively regulate the interlayer spacing of graphite, yielding an expanded graphite with an interlayer spacing of approximately 0.43 nm. At a current density of 20 mA/g, the expanded graphite anode synthesized via the improved Hummers method and heat treatment exhibits a high reversible capacity of about 300 mAh/g in sodium-ion batteries, demonstrating the superior electrochemical performance of expanded graphite in sodium-ion batteries. The structural evolution of graphite, graphene oxide, and expanded graphite is shown in Figure 7^[68].

Surface modification methods such as coating protective layers and ball-milling activation can enhance the sodium storage performance of graphite anodes by expanding the interlayer spacing of graphite, introducing more active sites, or reducing the surface area, thereby improving the Coulombic efficiency of graphite and reducing the risk of sodium dendrite formation^[67,69]..

In 2023, Liang et al.^[70]in situ synthesized a ZIF-8 coating layer on the surface of graphite, which provided additional sodium-ion storage sites and significantly enhanced the electrochemical performance of the anode material. At a high current density of 20 A/g, the material exhibited a first-cycle Coulombic efficiency of 86% and a reversible capacity of 90 mAh/g, demonstrating its excellent rate performance. Moreover, the composite material showed good stability and capacity retention: after 15,000 cycles and 20,000 cycles, the capacity retention rates were 96% and 94%, respectively, indicating a long operational lifespan. In addition, researchers have also employed mechanical activation to enhance the electrochemical performance of graphite anodes. Lee et al.^[71]obtained mechanically activated graphite through ball milling. The resulting mechanically activated graphite exhibited a high reversible capacity of 290.5 mAh/g at a current density of 10 mA/g, and a reversible capacity of 157.7 mAh/g at a current density of 1 A/g, representing a significant improvement over pristine graphite.

To improve the performance of graphite anodes in sodium-ion batteries, intercalation modification is also a commonly used approach. Intercalation modification refers to a method of inserting specific guest molecules or ions into the interlayer spaces of layered materials, thereby enhancing the electrochemical performance of graphite materials. In 2021, Li et al.^[72]prepared a highly stable composite material, AlCl₃-MGIC, by intercalating AlCl₃ into microcrystalline graphite. After using DEGDME as the solvent, AlCl₃-MGIC exhibited excellent electrochemical performance. At a current density of 200 mA/g, after 100 cycles, the AlCl₃-MGIC anode still retained a specific capacity of 202 mAh/g; after 900 cycles, the capacity retention of the AlCl₃-MGIC anode reached as high as 98%, with a specific capacity of 198 mAh/g. In 2023, Lan et al.^[73]introduced metal chlorides enhanced by sulfonyl chloride into graphite, yielding the intercalated compound material BiCl₃-GICs. Raman spectroscopy analysis showed that the electronic interaction between graphite and BiCl₃ weakened during the discharge process in the first cycle, creating favorable conditions for Na⁺ insertion. At a current density of 1 A/g, BiCl₃-GICs exhibited a high reversible capacity of 213 mAh/g; as the current density increased to 5 A/g, BiCl₃-GICs still maintained a reversible capacity of 170 mAh/g, fully demonstrating the superior high-rate performance of BiCl₃-GICs. Liu et al.^[74]replaced traditional ester-based electrolytes with short-chain ether solvents (DME/DEGDME) to induce the formation of ternary graphite intercalation compounds (t-GICs). Through a co-intercalation reaction of Na⁺ with the solvent, graphite, which was originally hydrophobic toward sodium, was transformed into a sodium-affinity matrix. After modification, the interlayer spacing of graphite increased from 0.335 nm to 1.158 nm, providing more active sites for sodium storage; at the same time, the operating voltage of the modified graphite decreased to 0.18 V, a significant improvement compared to conventional graphite anodes (0.6–0.8 V). At a C-rate of 6, the optimal t-GIC electrode exhibited a high reversible capacity of 588.4 mAh/g, 4.9 times that of a conventional graphite anode (120 mAh/g), and this electrode maintained a Coulombic efficiency of over 99.7% over 550 cycles, with a high capacity retention rate. Chen et al.^[75]modified the graphite anode in sodium-ion batteries by intercalating Bi nanoparticles into graphite. They first obtained an intermediate product, K-Bi-GIC, through co-melting of a K-Bi alloy with graphite, and then, through potassium removal using ethanol, retained Bi nanoparticles on the nanoscale (~10 nm) within the graphite layers, forming a “sandwich structure” in which graphite serves as a conductive protective layer and Bi nanoparticles act as active centers. At a current density of 80 mA/g, the Bi@Graphite electrode exhibited a high reversible capacity of 164 mAh/g, representing a 45% improvement over pristine graphite (110 mAh/g).

Graphite anodes have large lateral dimensions and high carbon-layer stacking in the longitudinal direction, resulting in long ion-diffusion distances and low ion-mobility between graphite anode layers. This leads to poor cycling and rate performance of graphite^[82]. Reducing the particle size of graphite through mechanical grinding and other methods can, to a certain extent, shorten the diffusion distance of potassium ions in the graphite anode, thereby improving the electrochemical performance of the graphite anode^[83].

In 2019, Carboni et al.^[84]used a simple ball-milling process to reduce the particle size of graphite from 30–40 μm to 20–25 μm, with the concomitant exfoliation of graphene layers during this process. At a current density of 25 mA/g, the capacity of the pristine graphite was 115 mAh/g, whereas the ball-milled graphite exhibited a capacity of 211 mAh/g, representing an 83.5% increase in capacity. Furthermore, the pristine graphite lost 50% of its capacity after 10 cycles, while the ball-milled graphite anode retained more than 150 mAh/g even after 100 cycles, demonstrating a significant improvement in cycling stability. Rahman et al.^[85]subjected commercial graphite to ball milling in the presence of acetone. In electrochemical tests at a current density of 100 mA/g, the modified graphite achieved an initial reversible capacity of 227 mAh/g, with a capacity retention of as high as 98% after 500 cycles. This result also demonstrates that the specialized ball-milling treatment can enhance graphite performance to a certain extent. After ball milling, the charge–discharge curve of the graphite anode still exhibits a distinct low-voltage plateau, reflecting the formation of potassium-ion intercalation compounds and indicating that ball milling has only a minor impact on the structure of graphite. Ball-milling is low-cost, fast, and can serve as a method for the large-scale preparation of modified graphite.

In addition to ball milling, some scholars have employed other methods to modify and regulate the microcrystalline structure of graphite. Xiao et al.^[86]constructed 2.5–5 nm nanoscale pores in synthetic graphite, providing rapid K⁺transport channels. By introducing defects, they reduced the graphite microcrystallite size from 39.4 nm to 24.6 nm while preserving the long-range ordered structure of graphite. At a 0.1 C rate, the modified graphite exhibited a reversible capacity as high as 306 mAh/g, representing an approximately 17.7% improvement over the pristine synthetic graphite (260 mAh/g). At a 2 C rate, the reversible capacity of the modified graphite was 29 mAh/g, an 81.25% improvement over the pristine synthetic graphite (16 mAh/g), demonstrating a significant enhancement in electrochemical performance.

In the research on graphite anodes, heteroatom doping is widely recognized as an effective method for enhancing the electrochemical performance of carbon materials. During the process of doping heteroatoms into graphite anodes, defect formation often occurs, and the defects generated in graphite can provide more active sites for potassium ion insertion. Moreover, if larger-sized atoms such as S or P are used for doping, the interlayer spacing of graphite can be expanded, facilitating the insertion and extraction of potassium ions and enhancing the cycling and rate performance of graphite anodes^[87-89]..

Among the numerous studies on heteroatom doping, nitrogen doping of graphite anodes has made significant progress. First, nitrogen has a higher electronegativity than carbon; when nitrogen atoms are incorporated into the graphite anode, the interaction between the graphite anode and potassium ions is enhanced. Second, after nitrogen doping, three main structural configurations emerge in the graphite anode: pyrrolic nitrogen, pyridinic nitrogen, and graphitic nitrogen. Pyrrolic and pyridinic nitrogens provide active sites for potassium ions, facilitating their insertion into the graphite anode, while graphitic nitrogen enhances conductivity by providing charge carriers. At the same time, nitrogen doping improves the wettability of graphite, ensuring that the solid–liquid interface is fully utilized^[90]. Wang et al.^[91]prepared nitrogen-doped graphite foams with varying nitrogen concentrations and configurations using an improved chemical vapor deposition method. The shift of the (002) peak to lower diffraction angles in the XRD patterns confirms the increased interlayer spacing of graphite after nitrogen doping. The researchers conducted electrochemical tests using the highest-nitrogen-content nitrogen-doped graphite foam NGF-8.47 obtained in the experiment as the anode in a potassium-ion battery. At a current density of 40 mA/g, NGF-8.47 delivered a reversible capacity of 231.4 mAh/g. When the current density was increased to 200 mA/g, the capacity of undoped graphite rapidly decayed to 6 mAh/g, whereas NGF-8.47 retained a capacity of 112 mAh/g, demonstrating the enhancement of graphite’s electrochemical performance through nitrogen doping. Ma et al.^[92]improved the potassium-ion storage performance of carbon nanosheets through edge nitrogen/sulfur co-doping, using thiourea as both the nitrogen and sulfur source. By controlling the amount of thiourea and the high-temperature carbonization conditions, they obtained a series of N/S co-doped carbon nanosheets. At a current density of 0.1 A/g, the reversible capacity of the N/S co-doped carbon nanosheets reached 368.2 mAh/g, exceeding the theoretical capacity of KC₈(270 mAh/g). At a current density of 1 A/g, after 2,800 cycles, the N/S co-doped carbon nanosheets still maintained a reversible capacity of 205.8 mAh/g. At a high current density of 10 A/g, the N/S co-doped carbon nanosheets exhibited a reversible capacity of 106.1 mAh/g, demonstrating excellent fast-charging performance.

Coating modification is also a commonly used method for modifying graphite materials. Coating modification refers to a modification approach in which materials such as carbon nanotubes and graphene are uniformly coated onto the surface of graphite materials. The coating layer on the surface of graphite materials can be regarded as the “shell” of the graphite anode, which can enhance the cycling stability of graphite by mitigating volume expansion during charge and discharge. To further enhance the effectiveness of the coating layer, heteroatom doping can be introduced into the coating layer, providing more active sites for potassium-ion insertion, facilitating the intercalation and deintercalation of potassium ions, and improving the rate performance of graphite materials.

Tian et al.^[90]used urea as a carbon source and synthesized nitrogen-doped carbon nanosheets (NC)-coated multilayer graphite (NC@MG) by designing a multifunctional layer on the graphite surface, thereby enhancing the structural and cycling stability of the graphite anode in potassium-ion batteries. At the same time, the generated carbon nanosheets provided more adsorption sites for potassium ions on the anode surface, increasing the capacity of the graphite anode. Electrochemical tests were conducted using this NC@MG as the anode material in potassium-ion batteries at a current density of 50 mA/g, and NC@MG exhibited a high capacity of 232.2 mAh/g, which is higher than that of the pristine graphite anode. Moreover, after 1000 cycles, the capacity retention of NC@MG reached 97.5%, fully demonstrating the enhancement of rate performance and cycling stability of the graphite anode through coating modification. Wang et al.^[93]followed the preparation process shown in Figure 8and used chemical reduction to coat graphene oxide (GO) onto the surface of expanded graphite (EG), forming an rGO-modified composite structure. Raman spectroscopy and XRD results confirmed that the introduction of rGO increased the interlayer spacing of graphite from 0.335 nm to 0.342 nm, facilitating the insertion and extraction of potassium ions. The composite material exhibited a relatively high capacity of 450 mAh/g at a current density of 50 mA/g, and even when the current density was increased to 800 mA/g, the composite still retained a capacity of 126 mAh/g, representing a significant improvement over uncoated expanded graphite. Xiong et al.^[94]employed an innovative approach that enhances potassium-ion storage performance by in situ chemical regulation of the graphitization degree of carbon fibers. First, MnO₂nanosheets were grown via KMnO₄oxidation etching, and then nitrogen-doped carbon-coated graphite was prepared via oxidative polymerization, organically combining a highly graphitized core with a defect-rich interfacial layer. At a current density of 0.1 A/g, the optimized CNF@NC-5 electrode still maintained a high reversible capacity of 294.9 mAh/g after 100 cycles; at a high current density of 2 A/g, the CNF@NC-5 electrode retained a capacity of 98.3 mAh/g after 2000 cycles, with significantly improved electrochemical performance.