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Introduction

The rapid development of portable electronics, electric vehicles, and grid-scale energy storage systems^1,2 has driven an unprecedented demand for fast-charging lithium-ion batteries (LIBs).^3,4 Benefiting from the relatively high theoretical capacity (372?mAh g^?1), excellent cycling stability, and low cost, graphite (Gr) has been the dominant anode material in commercial LIBs for decades.^5,6 Nevertheless, sluggish Li-ion diffusion kinetics at the Gr/electrolyte interface, including Li-ion desolvation on the surface of the solid electrolyte interphase (SEI) followed by transfer across the SEI film, have greatly limited the fast-charging capability of LIBs.^7,8 On one hand, the sluggish interfacial Li-ion diffusion kinetics inevitably results in fast capacity decay of LIBs at high charging rates (minimal capacity retention at >4?C).⁹ On the other hand, owing to the low equilibrium potential (～0.1?V vs Li/Li⁺) of Gr, the large voltage polarization during fast charging process can easily induce undesired metallic Li plating at the Gr surface,¹⁰ leading to inferior cycling stability and even safety concerns.^11,12 Therefore, elaborately optimizing the Gr/electrolyte interface components to enable efficient Li-ion transport is crucial for developing fast-charging LIBs.

To date, extensive efforts have been devoted to dealing with the issues and enhancing the fast-charging performance of Gr anodes.^13–15 On one hand, electrolyte engineering strategies,¹⁶ such as the utilization of weakly solvating electrolytes,^17,18 high-concentration or localized high-concentration electrolytes,^19,20 and the introduction of electrolyte additives,^21,22 have been implemented to modulate the Li-ion solvation configuration.²³ This modulation can decrease the activation energy of Li-ion desolvation and promote the formation of anion-derived SEI layers with rapid Li-ion diffusion capabilities, thereby leading to enhanced lithiation kinetics.^24,25 Unfortunately, electrolytes containing high-concentration Li salts typically have high viscosity, which leads to relatively low ionic conductivity and poor electrode–electrolyte contact; and the compatibility between weakly solvating electrolytes/electrolyte additives and high-voltage cathodes remains a significant concern.²⁶ On the other hand, artificial interphase coating of the Gr surface has recently emerged as a promising avenue for improving the interfacial Li-ion diffusion kinetics.^27,28 It has been demonstrated that various functional inorganic coatings on the Gr surfaces (e.g., MoS₂,²⁹ MoO_x-MoN_x,³⁰ MoO_x-MoP_x,³¹ soft carbon,³² Li₃PO₄,³³ La-doped TiNb₂O₇,³⁴ etc.) can be prelithiated and contribute to the formation of inorganic-rich SEI layers with a low Li-ion desolvation barrier and fast Li-ion diffusion capability. Despite the achievements, the chemically inert feature of the Gr surface brings great difficulty for homogeneous and tight deposition of these functional inorganic species, which inevitably creates hotspots in the Gr/electrolyte interface and leads to inhomogeneous interfacial Li-ion flux.^35,36 Increasing the loadings of functional inorganic species can achieve homogeneous surface coating to some extent, but this strategy inevitably results in excess reversible Li consumption, leading to decreased initial Coulombic efficiency (ICE).³⁷ Therefore, it is highly desirable, yet remains a great challenge, to develop an efficient strategy to construct an ultrathin yet homogeneous artificial interphase coating for fast-charging Gr materials.

Herein, minimal sulfur has been homogeneously grafted on the surface of Gr anodes to facilitate efficient Li-ion diffusion kinetics at the Gr/electrolyte interface for fast-charging LIBs (Figure?1). The key to this novel artificial interphase coating is introducing topological defects (e.g., pentagons, heptagons, and octagons) in the carbon lattice of Gr, which can regulate the electronic structure and produce numerous reactive C=C bonds, thereby enabling homogeneous and covalent grafting of sulfur species on the surface of Gr via simple free radical reaction. During initial cycling, the surface-grafted sulfur species can be preferentially reduced to produce a Li₂S-based interphase layer, which concurrently suppresses solvent decomposition and reduces the barrier of Li-ion transfer from electrolyte to Gr anode. As a result, the as-obtained Gr@S anode demonstrates outstanding rate performance, delivering a 2.7-fold increase in reversible capacity at 4?C and 94.1% capacity retention after 800 cycles at 1?C. When paired with a LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) cathode in a full-cell configuration, the Gr@S anode achieves 80% state of charge (SOC) within 8.7?min at a 4?C charging rate. These results underscore surface engineering as a scalable and cost-effective strategy to accelerate the development of fast-charging and long-life LIBs.

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Experimental Methods

Synthesis of Gr@S

A mixture of commercial Gr (1?g) and melamine powder (1?g) was ground and homogenized, followed by heating at 700?°C for 2?h and then at 1150?°C for 2?h under nitrogen flow with a heat rate of 5?°C min^?1. The resulting defective Gr (DGr, 0.5?g) was then mixed with elemental sulfur (2?g), and heated at 155?°C for 12?h and then at 200?°C for 6?h at a heating rate of 5?°C min^?1. After natural cooling to room temperature, the product was centrifuged in carbon disulfide to remove residual sulfur, then the collected solid was vacuum-dried to obtain Gr@S. Gr@S-L (low sulfur content) and Gr@S-H (high sulfur content) were synthesized following the same procedure as Gr@S, with DGr-to-sulfur mixing ratios of 1:1 and 1:16, respectively. Gr/sulfur composite (Gr/S) was synthesized via direct mixing of Gr with elemental sulfur under identical heating conditions to that for Gr@S.

Theoretical calculations

All calculations were carried out using the Vienna Ab initio Simulation Package (VASP 6.3.0).³⁸ The exchange-correlation functional used was Perdew–Burke–Ernzerhof, and ion-electron interactions were described using the projector augmented wave method.^39,40 The vdWs interaction was included using the empirical DFT-D3 method.⁴¹ Four layers of Li₂CO₃ (001), Li₂S (110), and LiF (100) were used to investigate the adsorption and desolvation of Li ions, respectively.⁴² Atoms in the upper layer of the surface were allowed to move freely, while the bottom two layers were fixed to simulate the surface structure.^43,44 The Monkhorst–Pack grid was used for k-points, with a 2×2×1 mesh and a cutoff energy of 450?eV. The convergence criteria were set to 0.02?eV ?^?1 for force and 10^?5?eV for energy.

The desolvation energy was calculated using the following equation:

where ${\mathit{E}}_{{\text{solvent}+\text{substrate}+\text{Li}}^{+}}$, ${\mathit{E}}_{{\text{substrate}+\mathrm{Li}}^{+}}$ and ${\mathit{E}}_{\text{solvent}}$ represent the density functional theory (DFT) energies of the solvent+substrate+Li⁺, substrate+Li⁺, and the solvent system, respectively.

Results and Discussion

The preparation process of Gr@S is schematically illustrated in Figure?2a. Firstly, topological defects are introduced onto the surface of Gr by simple temperature-programmed heat treatment of Gr and melamine, leading to DGr.^45,46 During this process, nitrogen heteroatoms are first doped into the carbon lattice at a relatively low temperature and then removed at a higher temperature, which leads to rearrangement of neighbor C atoms to produce topological defects ( Supporting Information Figure S1).^47,48 The defect structures can result in uneven distribution of the electronic conjugation structure in the carbon lattice and increase the number of reactive C=C bonds, thereby reducing the energy barrier for the free radical reaction.⁴⁵ Subsequently, the mixture of DGr and elemental sulfur is subjected to heat treatment at 200?°C. During this process, elemental sulfur molecules undergo a ring-opening reaction to produce a linear polysulfane with diradical chain ends, which can react with the active C=C bonds in topological defects to produce sulfur grafted Gr (Gr@S). According to nitrogen adsorption-desorption analysis, the Brunauer–Emmett–Teller surface areas (S_BET) of Gr, DGr, and Gr@S are all approximately 1?m² g^?1, suggestive of a nearly nonporous structure ( Supporting Information Figure S2). Scanning electron microscopy images demonstrate that the morphologies of DGr and Gr@S closely resemble that of pristine Gr, and no obvious sulfur particles are observed on the surface of Gr@S ( Supporting Information Figure S3). Elemental mappings confirm the uniform distribution of sulfur species across the Gr surface (Figure?2b and Supporting Information Figure S4). As revealed by X-ray diffraction (XRD) analysis (Figure?2c), the crystalline structure of Gr remains intact after sulfur grafting, with no detectable sulfur peaks, ruling out the presence of elemental sulfur. Raman spectroscopy analysis (Figure?2d) reveals that the defect density increases significantly in DGr, with the I_D/I_G ratio increasing from 0.23 for Gr to 0.57 for DGr, and the defects are effectively reduced after sulfur grafting.

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Elemental analysis (Figure?2e) indicates a sulfur content of approximately 2.51?wt % in Gr@S, a level sufficient to achieve surface modification without compromising the capacity of the Gr anode or inducing excessive side reactions during charge-discharge cycles. The slight weight loss observed for Gr@S upon heating to 800?°C under nitrogen atmosphere further supports this finding ( Supporting Information Figure S5). X-ray photoelectron spectroscopy (XPS) analysis (Figure?2f and Supporting Information Figure S6) reveals that sulfur in Gr@S predominantly exists as C–S and S–S bonds, indicating strong chemical bonding to the Gr structure. This stable bonding enhances the resistance of sulfur to detachment under electrochemical conditions.⁴⁵ Further analysis using sputtering ( Supporting Information Figure S7) demonstrates that the sulfur content decreases after a sputtering time of 1280?s, confirming that sulfur is localized on the Gr surface as short chains. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) (Figure?2g and Supporting Information Figure S8) provides additional insight into the sulfur chain configuration. Mass spectra reveal sulfur ionic species ranging from S^? to S₆^?, verifying the presence of short-chain sulfur. These results validate the effectiveness of the sulfur grafting strategy in introducing surface modification and forming chemically stable sulfur chains.

The electrochemical properties of Gr@S as anode materials for LIBs are evaluated by using coin-type half-cells in a voltage window of 0.005–2?V. The electrolyte consists of 1?M LiPF₆ in ethylene carbonate/ethyl methyl carbonate (v/v?=?3:7) with 5?wt % fluoroethylene carbonate. Cyclic voltammetry (CV) measurements show that Gr@S and Gr anodes present distinct redox peaks in the low-voltage range, indicating effective Li-ion insertion and extraction (Figure?3a). An irreversible reduction peak around 1.6?V is observed for Gr@S anode, which can be ascribed to the reduction of surface sulfur species to produce an Li₂S-containing SEI film. In comparison with a Gr anode, the Gr@S anode exhibits stronger peak densities with smaller voltage polarization between the oxidation and reduction peaks, suggesting that the Li₂S-containing SEI film can effectively facilitate the Li-ion insertion/extraction kinetics. Galvanostatic charge-discharge tests are further conducted to reveal the Li storage behavior of Gr@S anode. As shown in Figure?3b, several identical potential plateaus are observed for both Gr@S and Gr anodes, which are ascribed to the formation of different staged LiC_x. The Gr@S and Gr anodes deliver reversible capacities of 363 and 354?mAh g^?1, respectively, at 0.1?C. The similar ICE of 89% for both Gr@S and Gr anodes indicates that the surface sulfur species contribute little to the loss of active Li.

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Benefiting from reduced interfacial impedance ( Supporting Information Figure S9), the Gr@S anode also demonstrates excellent rate performance at different current densities from 0.2 to 4?C. As depicted in Figure?3c and Supporting Information Figure S10, the Gr@S anode delivers reversible capacities of 357, 344, 312, 260, and 193?mAh g^?1 at 0.2, 0.5, 1, 2, and 3?C, respectively. Moreover, it still maintains a high specific capacity of 144?mAh g^?1 at 4?C, suggestive of efficient Li-ion insertion/extraction kinetics. Additionally, a stable capacity of 355?mAh g^?1 can be recovered after switching the current density back to 0.2?C, highlighting the excellent robustness and stability of the Gr@S anode. In sharp contrast, the Gr and DGr anodes deliver considerably lower specific capacities of 53 and 75?mAh g^?1 at 4?C, respectively, accompanied with much larger voltage polarizations (Figure?3c and Supporting Information Figures S11 and S12). As revealed by the differential capacity versus voltage (dQ/dV) curves at various rates, the Gr@S anode obviously exhibits positive shifts in the reduction peaks and negative shifts in the oxidation peaks in comparison with the Gr anode (Figure?3d and Supporting Information Figure S13). The large difference in rate capabilities strongly indicates the superiority of surface-grafted sulfur in the Gr@S anode for favorable charge transfer and kinetically efficient Li storage behavior. To the best of our knowledge, the impressive rate capability of the Gr@S anode surpasses that of many previously reported Gr anodes for LIBs (Figure?3e and Supporting Information Table S1),^{29,32,49–65} such as 61?mAh g^?1 at 3?C for MoS₂-NG,²⁹ 20?mAh g^?1 at 3?C for photo-graphite,⁵⁴ 180?mAh g^?1 at 2?C for HF-PC,⁵⁹ 100?mAh g^?1 at 2?C for G@Cu-CuNWs,⁶³ and 69?mAh g^?1 at 2?C for aligned graphite.⁶⁵

It should be noted that the content of surface sulfur species plays a significant role on the high performance of the Gr@S anode ( Supporting Information Figure S14). As shown in Supporting Information Table S2, control samples with different sulfur contents are prepared by controlling the addition amount of elemental sulfur and the preparation process. The initial charge-discharge profiles reveal that the Gr@S anode with a lower sulfur content of 0.28?wt % (Gr@S-L) exhibits no distinct plateau with a low reversible capacity of 258?mAh g^?1 at 0.1?C ( Supporting Information Figure S15). Meanwhile, the Gr@S-H anode with a higher sulfur content of 5.67?wt % exhibits a noticeable reduction peak at approximately 1.7?V, which indicates the formation of byproducts and consequently leads to a significantly lower ICE (69%). Therefore, both the Gr@S-L and Gr@S-H anodes display considerably inferior rate performance compared to the Gr@S anode ( Supporting Information Figure S16). Moreover, comparative studies show that the sample prepared by direct sulfur grafting on pristine Gr (Gr/S) achieves only 1.16?wt % sulfur content and thus yields much lower reversible capacities at various current densities, highlighting the necessity of the topological defect engineering strategy ( Supporting Information Figures S17 and S18).

In order to simulate practical operating conditions and assess the cycling stability of the Gr@S anode, a constant current–constant voltage (CC–CV) discharge mode is employed. This mode mimics real-world discharge behavior by initiating a 1?C discharge to the cutoff voltage, followed by a constant voltage discharge to the cutoff current (0.01?C), ensuring full capacity utilization. As illustrated in Figure?3f, the Gr@S anode displays exceptional stability, maintaining 94.1% of its initial capacity after 800 cycles at 1?C; while the Gr anode suffers considerable capacity degradation, with noticeable fading occurring after only 200 cycles. The corresponding charge/discharge profiles reveal that the Gr@S anode retains its Li-ion intercalation plateaus over 800 cycles, while those of the Gr anode shorten after 200 cycles and disappear after 600 cycles ( Supporting Information Figure S19a,b). Furthermore, the Gr@S anode exhibits higher and more stable mid-discharge voltages than the Gr anode, indicating faster and more stable Li-ion intercalation in the Gr@S anode ( Supporting Information Figure S19c). Moreover, during the cycling test, the Gr@S anode maintains a more stable Coulombic efficiency of nearly 100% in contrast to the Gr anode, demonstrating a highly robust and reversible Li-ion insertion/extraction process for the Gr@S anode. Upon increasing the current density to 4?C, the Gr@S anode still maintains decent cycling behavior with a specific capacity of 311?mAh g^?1 after 400 cycles ( Supporting Information Figure S20).

To understand the advantages of surface-grafted sulfur in enabling efficient Li storage, high-resolution transmission electron microscopy (HRTEM) and XPS characterization are used to examine the structure and composition of the SEI of the Gr@S anode after 30 charge-discharge cycles at 1?C. As depicted in Figure?4a, an amorphous SEI layer measuring 7?nm in thickness is detected on the surface of Gr, which is consistent with previous reports on Gr. In contrast, an SEI layer with a thickness of merely 3?nm is observed on the surface of Gr@S (Figure?4b). The diminished thickness of the SEI on Gr@S suggests a shorter Li-ion diffusion distance within the SEI, which is advantageous for rapid Li-ion transport through it. According to XPS analysis, higher contents of F and S as well as lower contents of C and O are detected in the SEI of the Gr@S anode in comparison to the Gr anode (Figure?4c and Supporting Information Figures S21 and S22). The high-resolution C 1s spectra confirm that the SEI of the Gr@S anode exhibits lower quantities of organic components (such as C–O, C=O, and ROCO₂^?) than that of the Gr anode, suggestive of fewer organic decomposition products (Figure?4d). Additionally, the high-resolution F 1s spectra demonstrate an increased LiF content in the SEI of the Gr@S anode (Figure?4e), and the high-resolution S 2p spectrum (Figure?4f) reveals that a certain amount of Li₂S is located in the interior of the SEI of the Gr@S anode,^66,67 which can be ascribed to the reduction of surface sulfur species. The results clearly indicate that the surface-grafted sulfur can concurrently suppress the solvent decomposition and facilitate the formation of an inorganic-rich SEI. Through DFT calculations, the roles of LiF and Li₂S are further validated. According to experimental results and previous reports, a solvation model for Li ion is established ( Supporting Information Figure S23), and three crystal models including Li₂CO₃ (001),⁶⁸ Li₂S (110),⁶⁹ and LiF (100)⁷⁰ are also considered ( Supporting Information Figures S24–S26). Notably, the LiF and Li₂S in the SEI can significantly decrease the desolvation barriers of solvated Li ions from 978?kJ mol^?1 for Li₂CO₃ to 793 and 782?kJ mol^?1 for LiF and Li₂S, respectively, thereby accelerating interfacial Li-ion diffusion (Figure?4g,h).

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To further investigate the kinetic advantages enabled by the inorganic-rich SEI of the Gr@S anode, the activation energies of Li-ion desolvation (E_{a, de}) and diffusion through the SEI (E_{a, SEI}) are calculated based on temperature-dependent electrochemical impedance spectroscopy from 30 to 70?°C ( Supporting Information Figure S27). According to the classic Arrhenius law, E_{a, de} and E_{a, SEI} can be determined by the fitted charge transfer resistance (R_CT) and SEI resistance (R_SEI) of Nyquist plots, respectively ( Supporting Information Figure S28 and Table S3). As shown in Figure?5a,b, both the E_{a, de} (48.10?kJ mol^?1) and E_{a, SEI} (51.82?kJ mol^?1) of Gr@S anode are much lower than those of Gr anode (E_{a, de}?=?67.78?kJ mol^?1, E_{a, SEI}?=?63.18?kJ mol^?1), strongly indicating that the surface-grafted sulfur-derived inorganic-rich SEI can effectively reduce the barrier of Li-ion transfer from electrolyte to Gr anode. Moreover, kinetic analysis using the Randles–Sevcik equation reveals that the Gr@S anode has a higher Li-ion diffusion coefficient in the graphitic layers than the Gr anode, as evidenced by a higher slope (1.59) compared to Gr (1.12) in the linear relationship between the square root of the scan rate and anodic peak current ( Supporting Information Figures S29 and S30). Ion diffusion dynamics for both anodes are also examined using the galvanostatic intermittent titration technique (GITT). As shown in Figure?5c,d, both the Gr@S and Gr anode exhibit comparable GITT profiles and corresponding calculated diffusivity curves, reflecting their similar Li-ion insertion/extraction behaviors. Notably, the corresponding Li-ion diffusion coefficient (D_Li⁺) of the Gr@S anode during the whole lithiation and delithiation process is apparently larger than that of the Gr anode, demonstrating enhanced Li-ion diffusion kinetics in the Gr@S anode ( Supporting Information Figure S31).

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To evaluate the fast-charging capability of the Gr@S anode, full cells are assembled with Gr@S as the anodes and LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) as the cathodes, operating within a voltage range of 2.5–4.3?V. Charging is performed via a CC–CV protocol at rates from 0.2 to 8?C, with discharge fixed at 0.2?C. As shown in Figure?6a,b, the NCM811||Gr@S full cell achieves reversible capacities of 146, 134, and 125?mAh g^?1 at 2, 4, and 6?C, respectively, and retains 121?mAh g^?1 even at 8?C, indicating excellent rate performance. In sharp contrast, the NCM811||Gr full cell shows significantly lower capacities and larger voltage polarizations under identical conditions. Similar results are demonstrated when the Gr@S and Gr anodes are paired with commercial LiFePO₄ (LFP) cathodes to form LFP||Gr@S and LFP||Gr full cells, respectively ( Supporting Information Figure S32). The severe voltage polarizations of the LFP||Gr full cell lead to abundant dendritic metallic Li deposits in the Gr anode during rate capability testing, which will cause irreversible consumption of active Li ( Supporting Information Figure S33a,b). In sharp contrast, the Gr@S anode of the LFP||Gr@S full cell after rate capability testing retains a smooth and clean surface, suggestive of no metallic Li deposits ( Supporting Information Figure S33c,d). The superior fast-charging performance of the Gr@S anode is further confirmed by its charging kinetics. As shown in Figure?6c and Supporting Information Figure S34, the NCM811||Gr@S full cell reaches 97%, 95%, and 91% SOC within 30, 15, and 7.5?min at 2, 4, and 8?C, respectively, while only 75% SOC is achieved for the NCM811||Gr full cell at 8?C in 7.5?min. Notably, at a 4?C charging rate, the Gr@S anode attains 80% SOC in 8.7?min, nearly 40% faster than the Gr anode’s 14.0?min. Even at an extreme 8?C rate, the Gr@S anode reaches 80% SOC in just 4.4?min, compared to 10.1?min for the Gr anode (Figure?6d). This enhanced performance is attributed to the Li₂S-containing SEI, which promotes efficient Li-ion desolvation and transport. These findings highlight the essential role of the inorganic-rich SEI in enabling stable and high-rate Li-ion transport, positioning Gr@S as a promising anode material for fast-charging LIBs.

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Conclusion

In summary, we have developed a class of minimal sulfur-grafted Gr anodes through a topological defect-engineering strategy to address the critical challenge of sluggish Li-ion diffusion at Gr/electrolyte interfaces for fast-charging LIBs. The engineered topological defects in the Gr lattice provide numerous reactive C=C bonds, enabling the uniform and covalent grafting of sulfur species via a free radical reaction to form an ultrathin artificial interphase coating. During cycling, this interphase coating evolves into an inorganic-rich SEI containing Li₂S and LiF, which effectively lowers Li-ion desolvation and interfacial transfer barriers. As a result, the Gr@S anode achieves a 2.7-fold increase in reversible capacity at 4?C and retains 94.1% capacity after 800 cycles at 1?C. Remarkably, in full cells paired with LiNi_0.8Co_0.1Mn_0.1O₂ cathodes, Gr@S achieves 80% SOC in only 8.7?min at 4?C. This work highlights the pivotal role of defect-driven surface engineering in constructing stable, ion-conductive interphases, offering a scalable and cost-effective pathway toward high-performance fast-charging LIBs.

Supporting Information

Supporting Information is available and includes materials, characterization data, supplementary figures, and tables.

Conflict of Interest

There is no conflict of interest to report.

Funding Information

The authors acknowledge the support from the project of the National Key Research and Development Program of China (grant no. 2021YFF0500600), the National Natural Science Foundation of China (grant nos. 52172061 and 52472058), the Natural Science Foundation of Guangdong (grant no. 2024B1515020023), and the Science and Technology Program of Guangzhou (grant no. 2024A04J10003).