One-step core-shell structuring of silicon graphene composite anode materials by aqueous reduced graphene oxide: Toward practical use of high-performance lithium-ion battery

Carbon coatings for silicon (Si)-based anode materials are essential for designing high-performance Li-ion batteries (LIBs). The coatings prevent direct contact with the electrolyte and enhance anode performance. However, conventional carbon coatings are limited by their volume expansion and structural degradation, which lead to capacity fading and reduced durability. This study introduces a scalable and practical one-step carbon coating strategy for directly coating silicon suboxide (SiOx)-based materials using aqueous quasi-defect-free reduced graphene oxide (QrGO) without post-treatment, unlike conventional graphene oxide (GO)-based coating methods. This simple process enables uniform encapsulation with QrGO for a highly adhesive and conductive coating. The QrGO-based composite anode material has several advantages, including reduced cracking due to volume expansion and enhanced charge carrier transport, as well as an increased Si content of 20 wt.% compared to the 5 wt.% in typical commercial Si-based active materials. In particular, the capacity retention of the QrGO-coated Si electrodes dramatically increases at high C-rate. The full cell exhibited long-term stability and capacity that were twice that of commercial SiOx-based cells. Therefore, the QrGO-based one-step coating process represents a scalable, transformative and commercially viable strategy for developing high-performance LIBs. Article category: Full Paper Subcategory: Lithium Ion Batteries One-step core-shell structuring of silicon graphene composite anode materials by aqueous reduced graphene oxide: Toward practical use of high-performance lithium-ion battery By Byeong Guk Kim, Jihyeon Ryu, Ki-Hun Nam, Sooyeon Jeong, Hye Jung Lee, Jungmo Kim, Dong Gyun Hong, Oh Sung Kwon, Sunhye Yang *, and Seung Yol Jeong * Mr. B. G. Kim, Ms. S. Jeong, Ms. H. J. Lee, Dr. J. Kim, Mr. D. G. Hong, Mr. O. S. Kwon, Dr. S. Y. Jeong Nano Hybrid Technology Research Center, Korea Electrotechnology Research Institute (KERI), Changwon 642-120, Republic of Korea E-mail: [email protected] (S. Y. Jeong) Ms. J. Ryu, Dr. K.-H. Nam, Dr. S. Yang Battery Research Division, Korea Electrotechnology Research Institute (KERI), Changwon 642-120, Republic of Korea E-mail: [email protected] (S. Yang) Mr. B. G. Kim, Ms. J. Ryu, Dr. K.-H. Nam, Dr. J. Kim, Mr. D. G. Hong, Dr. S. Yang, Dr. S. Y. Jeong Electric Energy Materials Engineering, KERI School, University of Science and Technology (UST), Daejeon, 34113, Republic of Korea Mr. O. S. Kwon Department of Materials Science and Engineering, Pusan National University, Busan, 46241, Republic of Korea

silicon, anode material, reduced graphene oxide, core-shell structure, lithium-ion battery Carbon coatings for silicon (Si)-based anode materials are essential for designing high-performance Li-ion batteries (LIBs). The coatings prevent direct contact with the electrolyte and enhance anode performance. However, conventional carbon coatings are limited by their volume expansion and structural degradation, which lead to capacity fading and reduced durability. This study introduces a scalable and practical one-step carbon coating strategy for directly coating silicon suboxide (SiO x )-based materials using aqueous quasi-defect-free reduced graphene oxide (QrGO) without post-treatment, unlike conventional graphene oxide (GO)-based coating methods. This simple process enables uniform encapsulation with QrGO for a highly adhesive and conductive coating. The QrGO-based composite anode material has several advantages, including reduced cracking due to volume expansion and enhanced charge carrier transport, as well as an increased Si content of 20 wt.% compared to the 5 wt.% in typical commercial Si-based active materials. In particular, the capacity retention of the QrGO-coated Si electrodes dramatically increases at high C-rate. The full cell exhibited long-term stability and capacity that were twice that of commercial SiO x -based cells. Therefore, the QrGO-based one-step coating process represents a scalable, transformative and commercially viable strategy for developing high-performance LIBs. 1. Introduction Silicon (Si) has emerged as a useful candidate for high-energy-density Li-ion batteries (LIBs), with potential applications in large-scale energy storage systems (ESSs), electric vehicles (EVs), and urban air mobility (UAM) . [1-3] The exceptional potential of Si arises from its theoretical capacity (~4200 mAh g –1, based on Li ₂₂ Si₅), which is nearly an order of magnitude higher than that of conventional graphite (Gr) anodes . [4-8] In addition, its abundance, economic feasibility, and compatibility with the stringent requirements of next-generation ESSs make Si a transformative material in the development of advanced LIBs. However, the application of Si is severely limited by its low electrical and ionic conductivity and substantial volumetric expansion (~400%) that occurs during lithiation/delithiation . [9-12] These factors lead to structural degradation, electrode delamination, and solid electrolyte interphase (SEI) instability, ultimately causing rapid capacity fading and reduced energy density. Consequently, the stand-alone use of Si as an anode material remains impractical. Various modifications have been attempted to address the inherent limitations of Si in LIBs. For example, silicon suboxide (SiO x ) enhances structural stability through its mixed oxide structure, which effectively mitigates volumetric expansion during lithiation/delithiation by buffering the associated stress . [13-15] Si-alloys (e.g., Si-Al, Si-Sn, and Si-Fe) incorporate metals to improve mechanical stability and electrical conductivity, thereby alleviating pulverization and capacity degradation . [16-18] Furthermore, silicon carbide (SiC) provides excellent thermal stability and mechanical strength, which resists cracking and improves long-term cycling performance . [19-21] Various carbon coating techniques have been developed to enhance the electrochemical performance of Si-based materials for LIBs. However, carbon coatings on Si-based materials are limited by their poor uniformity, thickness control, and adhesion, which become particularly pronounced under the substantial ~150% volumetric expansion of Si-based materials during lithiation/delithiation. [22-25] These issues often exacerbate coating delamination, SEI instability, and capacity degradation. Furthermore, the low electrical conductivity of carbon poses a major limitation for thick electrodes and reduces their overall efficiency. While carbon coatings improve the conductivity of Si-based materials compared to bare Si, the conductivity of carbon coatings is relatively low compared to more crystalline carbon forms. [26-27] Consequently, the content of Si-based materials in commercially available LIBs is restricted to ~5 wt.%. Achieving a higher Si content (>20 wt.%) while improving anode performance requires innovative strategies to maintain structural integrity and enhance electrochemical performance. Carbon coating techniques for active materials can be broadly categorized into bottom-up and top-down methodologies. The bottom-up approach, which includes thermal treatment using suitable precursors and chemical vapor deposition, is widely used to synthesize carbon layers that protect Si-based anodes by preventing direct contact with the electrolyte. [28-32] However, such coatings fail to fully mitigate the substantial volumetric changes during cycling, underscoring the need for alternative carbon materials with higher Si-based material contents. In contrast, the top-down approach utilizes nanocarbon materials such as graphene and carbon nanotubes as coatings to enhance electrical conductivity and SEI stability during cycling. [33-35] Graphene, with its superior electrical conductivity and mechanical robustness, provides efficient pathways for electron and ion transport and maintains structural integrity even under significant volumetric changes in Si. [36-37] Furthermore, graphene reduces Si-based materials and electrolyte interactions, thereby mitigating SEI thickening and capacity loss. These advancements highlight the critical role of innovative carbon materials in addressing the limitations of Si-based anodes and designing electrodes with high Si-based material contents for developing high-performance LIBs. The integration of graphene oxide (GO) with Si-based materials is a promising strategy to enhance anode performance. [38-39] However, GO composites are limited by their inherently low electrical conductivity. While graphene has exceptional conductivity, its oxidation to produce GO disrupts the conjugated π-system and introduces oxygen-containing functional groups, significantly reducing electrical conductivity. Consequently, an additional reduction step is required to produce reduced GO (rGO) with improved electrical conductivity. However, this process is limited by non-uniform reduction, residual reducing agents, and structural damage caused by gas evolution during the removal of oxygen-containing functional groups, which adversely affect electrochemical performance. [40-41] Therefore, achieving a uniform distribution of Si-based materials within the GO matrix and stable interfacial bonding during reduction are immediate challenges that require further investigation. Herein, we propose a one-step core-shell design for SiO x -based materials using aqueous quasi-defect-free reduced graphene oxide (QrGO), which is scalable for industrial fabrication and straightforward for use in LIBs. We selected SiO x as a representative commercial active Si-based material. Direct coating of the active materials was achieved through the aqueous dispersion of rGO, which generates a highly crystalline structure on graphene layers. For the SiO x core, the coating was applied as a water-dispersible QrGO. Unlike conventional GO-based methods, this approach employs structurally intact QrGO, which leverages enhanced stability and cation-π interactions in aqueous systems to form a robust and conductive graphene matrix. The QrGO-based aqueous solution was directly attached to SiO x using the straightforward spray-drying method. The core-shell architecture effectively mitigated the volume expansion of SiO x while maintaining superior electrical conductivity, addressing the critical limitations of Si-based anodes. Notably, this method enables a high Si content of 20 wt.%, significantly exceeding the 5 wt.% typically used in commercial anodes, while achieving excellent electrochemical performance. Full cells paired with Li[Ni 0.6 Co 0.2 Mn 0.2 ]O 2 (NCM622) cathodes achieved a capacity twice that of commercial SiO x /C after 500 cycles, underscoring the scalability and commercial viability of this approach. These findings highlight the transformative potential of the QrGO-based one-step coating process for developing durable high-performance Si/graphene anodes, which are essential for high-performance LIBs. 2. Results and Discussion Among Si-based materials, carbon-coated SiO x is most commonly used in LIBs. Commercial SiO x /C particles exhibited a smooth surface ( Figure 1 a), indicative of a uniform thin carbon shell structure. Transmission electron microscopy (TEM) further confirmed the presence of this layer encapsulating the SiO x core, with a carbon layer of approximately 50 nm surrounding the SiO x core (Figure 1b,c and Figure 1e,f). This surface carbon layer plays a pivotal role in enhancing the electrochemical performance and maintaining structural stability during cycling by acting as a passivation layer that prevents direct contact between the active materials and the electrolyte. However, despite its beneficial properties, the synthesized carbon shell cannot effectively mitigate the volume changes associated with Li-ion insertion/extraction due to the rigid and tight carbon layers. The carbon-coated SiO x exhibited extensive surface cracking after 50 cycles ( Figure S6 a, Supporting Information). To address these drawbacks, we developed a SiO x /C/QrGO composite featuring a distinctive wrinkled surface morphology attributed to the QrGO layer (Figure 1d). TEM confirmed the formation of a dual encapsulation structure, with an approximately 20 nm QrGO layer surrounding the SiO x /C core (Figure 1e and f). The wrinkled morphology of the QrGO layer is essential for accommodating the volume expansion of SiO x /C by promoting the flexibility of the coating and mitigating the dramatic mechanical stress during expansion and shrinkage. The QrGO-coated SiO x /C maintained a stable surface without cracking after 50 cycles (Figure S6b, Supporting Information). The QrGO layer served as a robust protective barrier, effectively preventing direct contact between SiO x /C and the electrolyte. The corrugated protective graphene layers not only enhance the structural stability of the SiO x /C composite but also minimize undesirable side reactions, thereby mitigating capacity degradation and ensuring superior long-term cycling performance. The core-shell structure of the SiO x /C/Thermally treated rGO (TrGO) composite was achieved in two primary steps ( Figure 2 a): The conventional method initially coats the SiO x /C particles with GO and recovers electrical conductivity through a thermal treatment that removes oxygen functional groups via a reduction process. However, the oxygen-containing functional groups on graphene decompose during high-temperature treatment, releasing gas-phase species such as CO and CO₂. [42-43] In our study, the generated gas was dramatically desorbed and caused a pop-up between the stacked graphene layers, leading to the expansion and explosion of the graphene sheets (Figure 2b and c). This phenomenon results in structural collapse and detachment of the graphene layers from the SiO x /C surface, ultimately separating the graphene shell from the SiO x /C (Figure 2d) . Our proposed method employs a one-step coating process using a highly conductive and concentrated aqueous dispersion of QrGO, enabling the efficient fabrication of SiO x /C/QrGO composites. This approach is easy, straightforward, and highly efficient for applying uniform coatings to SiO x /C particles. Notably, our one-step process does not require a post-thermal treatment because the QrGO is chemically reduced. The rGO retained fewer oxygen functional groups and exhibited high electrical conductivity. Although rGO is hydrophobic, its aqueous dispersions exhibit various viscosities. This is due to the interaction between the π -state and monovalent cation. [44] The QrGO-coated SiO x /C materials maintained their structural integrity even after thermal treatment, providing uniform and durable coatings (Figure 2e–g). Compared to the GO-coating method, the QrGO shells were not affected by the gas desorption associated with the post-thermal treatment. We confirmed the adhesion stability of QrGO on SiO x /C following a high-temperature treatment (600 °C) under the same conditions as the GO-coating process, without any structural deformation of the graphene layers (Figure 2f and g). In particular, these findings highlight the efficiency of the one-step coating process for fabricating QrGO-based composites with superior structural attributes compared to conventional materials. The core-shell structure was fabricated using a spray-drying technique with an aqueous QrGO-based solution specifically engineered for direct coating onto the active material. This solution exhibited excellent dispersibility and allowed precise concentration control with high viscosity ( Figure 3a ). The thickness and size of the QrGO layer and particles were confirmed using atomic force microscopy (AFM) (inset image in Figure 3a). The thickness of the QrGO layer was <1 nm, confirming the few-layered configuration of rGO. Thermogravimetric analysis (TGA) showed that rGO underwent rapid decomposition at ~500 °C, whereas QrGO exhibited a significantly higher thermal stability, with major decomposition occurring only above 600 °C (Figure 3b). In addition, the QrGO fabricated using the modified Brodie method exhibited metal-like behavior, unlike typical rGO fabricated using Hummers method. This demonstrates the superior crystallinity and structural robustness of QrGO, which contribute to its enhanced structural properties due to small amount of defect formation. [45] Zeta potential analysis revealed that QrGO retained strong negative charges even after reduction, unlike conventional HrGO, which lost its negative charge after reduction (Figure 3c). GO is characterized by a high density of oxygen-containing functional groups, such as hydroxyl, carboxyl, and epoxides, which contribute to its overall negative charge. Following reduction, the loss of oxygen-containing functional groups results in a decrease in the overall negative charge of rGO. [46] The zeta potential of HrGO was lower than the negative value of HGO (Figure 3c). Despite the reduction of QGO, QrGO exhibited an unusually high negative potential. This is due to the cation-π interaction, which increases the electronic density of the π-state. [44,47] Although cations and π-states are neutralized by electrostatic interactions, the overall charge distribution can have a strong negative value owing to the rearrangement of electrons on the graphene surface. Monovalent cations induce abundant electrons in the π-states of graphene, contributing to a negative surface charge distribution, which not only maintains dispersion in strongly polar solvents such as water but also promotes strong and uniform bonding with SiO x /C. Previous studies used GO because of its high dispersibility in aqueous solvents. In this work, an aqueous QrGO solution was used because of its strong negative charge in polar solvents. Therefore, the core-shell composite structure exhibited strong adhesion between the SiO x /C and QrGO interfaces (Figure 3d). The interaction mechanism was achieved via three crucial processes. (i) The graphene layer physically encapsulates the SiO x /C particles, forming a protective shell. During the spray drying process, as the dispersion medium evaporates at a sudden high temperature, extremely thin graphene layers adhere strongly to the particle surface. (ii) Non-covalent π–π interactions between the graphene layer and carbon-coated SiO x /C surface ensure strong bonding, enhancing both electron conductivity and structural integrity. Although the cation-π interactions in the QrGO layer provide a strong negative surface charge, non-interacting π-states can be generated on the graphene layers. The exposed π-states can interact with the π-states of carbon-coated layers on SiO x through interfacial interactions. (iii) Electrostatic interactions between the negatively charged QrGO and relatively positively charged SiO x /C surface promote uniformity and enhance interfacial interactions to form robust and efficient QrGO coatings on SiO x /C particles (Figure 3e and f). TGA showed that the carbon content of commercial SiO x /C was was significantly increased to 7 wt.% (Figure 3g). The smooth weight loss profile of the QrGO-coated composite indicated a uniformly distributed layer across the surface. In order to evaluate the impact of uniform layer distribution, the structural differences between composites with uniformly coated QrGO on graphite surfaces and those with non-uniform coatings were analyzed in Figure S1 (Supporting Information), followed by a comparison of their thermal behavior. TGA showed that uniformly coated QrGO@Gr composites lost weight continuously at lower decomposition temperatures due to the strong adhesion of QrGO to the Gr surface ( Figure S2, Supporting Information). In contrast, non-uniform QrGO@Gr composites showed distinct decomposition steps, reflecting the different decomposition rates of the weakly bonded QrGO and Gr layers. These results demonstrate the critical role of the uniform QrGO coating in achieving consistent thermal behavior and structural stability. Moreover, scanning electron microscopy (SEM) revealed the stable adhesion of QrGO in the coated SiO x /C composites after 10 min of strong sonication in an aqueous solution ( Figure S3 ). The QrGO layers on SiO x /C were maintained without structural deformation or slippage despite strong mechanical stress. The uniform and robust QrGO coating significantly enhanced the structural stability and quality of SiO x /C composites. Our approach provides a practical and scalable strategy for developing advanced electrode materials with enhanced performance and reliability. Figure 4 illustrates the influence of the coating on a copper current collector, comparing the SiO x /C and SiO x /C/QrGO composites. The commercial SiO x /C electrode exhibited a significant amount of particle aggregation and a rough surface morphology (Figure 4a). In contrast, the SiO x /C/QrGO composite exhibited a flat surface without particle agglomeration and corrugation (Figure 4b). Surface roughness was characterized using high-resolution optical microscopy and mapped in 3D using confocal microscopy. The commercial SiO x /C electrode showed an uneven particle distribution and rough surface, as indicated by the wide range of colors in Figure 4c. In contrast, the SiO x /C/QrGO composite exhibited a uniform particle distribution, as indicated by the lack of a white region (Figure 4c), and a flat surface, confirming the uniform surface roughness (Figure 4d). Through a quantitative analysis, we confirmed the difference in surface roughness between the SiO x /C (R a : 3.3) and SiO x /C/QrGO (R a : 1.1) electrodes. The commercial SiO x /C electrode exhibited a contact angle of 109.3°, confirming its hydrophobic surface state (Figure 4f). However, the SiO x /C/QrGO composite exhibited a significantly lower contact angle of 73.9°, indicating a hydrophilic surface (Figure 4h). The enhanced hydrophilic surface state of QrGO facilitated a uniform particle coating on the copper current collector and ensured consistent slurry viscosity. These improvements promote the uniform encapsulation of the SiO x /C particles in a protective shell that maintains strong negative charges and surface stability (Figure 3c). Collectively, these findings highlight the potential of the one-step core-shell design for fabricating SiO x /C/QrGO composites with superior electrochemical performance for advanced electrode applications. The impact of the uniformity and structural stability of the SiO x /C/QrGO electrode was evaluated through Galvanostatic discharge and charge (GDC) tests. The test was first conducted at 0.1C for the initial formation step, followed by cycling at 0.5C, as shown in Figure 5 . At a current density of 0.1C, the initial Li-insertion/extraction capacity was 2003/1584 mAh g –1 for the SiO x /C electrode and 2023/1596 mAh g –1 for the SiO x /C/QrGO electrode ( Figure S4, Supporting Information), indicating comparable initial electrochemical properties. Subsequently, under a current density of 0.5C as a severe lithiation/delithiation condition, SiO x /C/QrGO exhibited a higher capacity of 1388 mAh g –1 in the first cycle (Figure 5a) than the specific capacity of SiO x /C (1315 mAh g –1 ) in Figure 5b. According to GDC profiles in Figure 5a and b, the capacity of the pristine SiO x /C dramatically decreased compared to the QrGO-coated SiO x /C, as indicated by the arrows. This suggests that QrGO effectively covered the SiO x /C core and facilitated electron transport through the composite surface. The SiO x /C electrode showed a rapid capacity decline, retaining only 18.4% of its initial capacity after 50 cycles. In contrast, the SiO x /C/QrGO electrode retained 60% capacity, demonstrating its superior long-term stability (Figure 5c). This significant improvement was attributed to the QrGO coating, which enhanced the structural stability of the electrode and effectively mitigated the damage caused by volume expansion during repeated discharge/charge processes. The structural changes of the electrodes after cycling were observed through cross-sectional SEM images (Figure 5d–g). Severe cracks formed on the SiO x /C electrode surface, and its thickness increased from 42 to 82 μm (by expansion/contraction of SiO x /C during repeated Li-cycles. Consequently, voids formed throughout the electrode, disrupting electron transport pathways and increasing electrode resistance. Conversely, the SiO x /C/QrGO electrode maintained structural integrity without cracking and exhibited a modest thickness change from 42 to 50 μm (by ~120%, Figure 5e and g). Moreover, the electrode components maintained intimate contact. Figure S5 (Supporting Information) compares the changes in the Li-metal electrode of the SiO x /C || Li-metal and SiO x /C/QrGO || Li-metal cells after cycling. The SiO x /C electrode caused electrolyte decomposition owing to electrode cracking and volume expansion, leading to asymmetric dendritic structures on the Li-metal surface, which increases internal resistance and significantly compromises long-term stability. In contrast, the SiO x /C/QrGO electrode maintained a smoother and more uniform Li-plating/stripping surface, enabling stable performance. This improvement can be attributed to the QrGO coating, which effectively mitigated structural damage to the anode and stabilized the SEI, preventing surface cracking and particle fracture ( Figure S6, Supporting Information). To further validate the practical applicability of SiO x /C/QrGO as an LIB anode, the Gr-blended electrodes were prepared with Si-based active materials (SiO x /C and SiO x /C/QrGO, Si content: 20 wt.% in the electrode) and their electrochemical performance was evaluated. The initial Li-insertion/extraction capacities were 673/560 and 681/561 mAh g –1, respectively ( Figure 6 a and b). The electrodes exhibited similar initial Coulombic efficiency (ICE) of 83.1 and 82.4%, respectively. However, the SiO x /C/QrGO electrode demonstrated a gradual improvement in CE per cycle, achieving 98.2% after the initial three cycles and maintaining a stable CE of Supporting Information). This improvement can be attributed to the stabilization of the charge/discharge pathways as the SiO x /C-graphene interface and QrGO layer became electrochemically active with progressing cycles. Electrochemical impedance spectroscopy (EIS) further highlighted the benefits of the QrGO coatings (Figure 6c). The fitted equivalent circuit, as indicated in the inset of Figure 6c, reveals two semicircles at the high-to-mid frequencies and a straight line at the low frequencies. The intersection on the x -axis at R1 indicates the bulk resistance (R s ) originating from the electrolyte, separator, and electrical connections of the system. The first semicircle indicates R2, the solid electrolyte interface resistance (R SEI ) corresponding to the resistance associated with the SEI layer formed on the Si surface. The other semicircle denotes R3, which represents the resistance of the charge-transfer process (R ct ). The straight line at low frequencies represents the Warburg resistance (W o ), which reflects the diffusion impedance caused by the diffusion of lithium ions throughout the electrode. After formation, the R ct of the SiO x /C/QrGO electrode was significantly reduced to 6.59 Ω compared to 25.39 Ω for the SiO x /C electrode. Similarly, the R SEI of the SiO x /C/QrGO electrode (14.8 Ω) was lower than that of the SiO x /C electrode (20.5 Ω). These results indicate that the QrGO coating promoted the formation of a stable and uniform SEI and highly conductive network facilitating efficient Li-ion and electron transport. In terms of high-power charge/discharge capacity, the SiO x /C/QrGO electrode has high-capacity retention under various C-rate conditions compared to the SiO x /C electrode (Figure 6d-f). According to the GDC profiles in Figure 6d and e, the capacity retention of SiO x /C/QrGO@Gr was twice that of the pristine SiO x /C@Gr at high C-rate, specifically 495 mAh g –1 @4C (SiO x /C/QrGO@Gr) and 252 mAh g –1 @4C (SiO x /C@Gr). Moreover, the SiO x /C@Gr and SiO x /C/QrGO@Gr revealed similar capacity retention values at 0.2C. The capacity retention of SiO x /C/QrGO (90%) increased significantly compared to that of SiO x /C (40%) at 4C as a harsh lithiation/delithiation condition (Figure 6f). This superior performance underscores the importance of the uniform structure and excellent electrical conductivity provided by the QrGO coating, which enables stable Li-ion and electron transport even at high C-rate. During discharge, the voltage drop in SiO x /C@Gr increased, particularly after 60 cycles. We presumed that the SiO x /C@Gr electrode was deformed because of the continuous volume change of SiO x /C, resulting in a thicker SEI and the loss of electron pathways ( Figure S8, Supporting Information). In contrast, SiO x /C/QrGO@Gr showed similar GDC profiles and low voltage drop over 100 cycles, implying good electron transport and structural stability. The capacity retentions of SiO x /C@Gr and SiO x /C/QrGO@Gr were 11% and 87%, respectively (Figure 6g). These results demonstrate that QrGO passivation plays a crucial role in LIBs with high electrochemical performance. Regarding the practical use of electrodes, full cells were fabricated using the SiO x /C/QrGO@Gr anodes with NCM622 cathodes (assembled in a pouch-type configuration; inset in Figure 6h) and their electrochemical performance was evaluated at 25 °C at 3.0–4.2 V. The SiO x /C@Gr || NCM622 full cell exhibited a rapid capacity decline, retaining only 51% of its initial capacity after 300 cycles (Figure 6h). In contrast, the SiO x /C/QrGO@Gr || NCM622 full cell maintained a capacity of 76% even after 500 cycles, indicating the effectiveness of the QrGO coating in facilitating electron transport and restraining electrode deformation. Furthermore, we disassembled the cells after 500 cycles to examine the electrode condition, and the results are presented in Figure 6i and j. In the case of the SiO x /C@Gr full cell, the electrode surface exhibited delamination and pulverization after cycling. In contrast, the SiO x /C/QrGO@Gr full cell maintained its electrode surface morphology similar to its initial state even after 500 cycles. These results demonstrated that the one-step coating process using water-dispersible QrGO effectively addresses the volume expansion issues of Si-based anodes and highlights the potential of this strategy for developing next-generation LIBs with high energy densities, extended cycle lives, and excellent high-rate performances. Figure 1. (a) FE-SEM image of commercial SiO x /C (scale bar: 5 μm). (b) TEM morphology image of SiO x /C (scale bar: 20 nm) with corresponding (c) EDS mapping image of SiO x /C (scale bar: 20 nm). (d) SEM image of SiO x /C/QrGO (scale bar: 5 μm). (e) TEM morphology image of SiO x /C/QrGO (scale bar: 20 nm) with corresponding (f) EDS mapping image of SiO x /C/QrGO (scale bar: 20 nm). Figure 2. (a) Schematic diagram illustrating the fabrication process of SiO x /C/TrGO and SiO x /C/QrGO. FE-SEM images of (b) stacked graphene oxide (GO) film (inset: stacked GO, scale bar: 20 μm), (c) stacked reduced graphene oxide film after thermal treatment and reduction (TrGO) at 600 o C and the white arrows indicate the delaminated TrGO after thermal treatment of GO (inset: stacked TrGO and its surface delaminated TrGO, scale bar: 20 μm), (d) Fully separated TrGO and SiO x /C after thermal treatment (scale bar: 5 μm), (e) stacked quasi-defect free reduced graphene oxide (QrGO) film (inset: stacked QrGO, scale bar: 20 μm), (f) stacked QrGO film after thermal treatment at 600 o C (inset: stacked QrGO and its stable surface adhesion without delamination, scale bar: 20 μm), and (g) stable core-shell structure of SiO x /C and QrGO (scale bar: 5 μm). Figure 3. (a) Photograph of conductive aqueous QrGO paste (inset: AFM image of QrGO, scale bar: 1 μm). (b) TGA curves of HrGO based on the modified Hummers method (orange line) and QrGO based on the modified Brodie method (blue line). (c) Zeta potential of QGO, QrGO, HGO, and HrGO. (d) Schematic diagram of various encapsulation mechanisms between QrGO and the SiOx/C surfaces (left: Spray drying process, right: (i) Mechanical interaction, (ii) π–π interaction, and (iii) Coulombic interaction). FE-SEM images of (e) low-magnification of SiOx/C/QrGO (scale bar: 10 μm) and (f) high-magnification of SiOx/C/QrGO (scale bar: 2 μm). (g) TGA curves of SiOx/C (orange line) and SiOx/C/QrGO (blue line) (red arrow: effect of QrGO decomposition). Figure 4. Photograph of slurry coating on a Cu current collector for (a) SiO x /C (white circles: agglomerated active materials) and (b) SiO x /C/QrGO electrodes. Optical and 3D confocal microscopy images of (c) SiO x /C (white circles: different white contour regions indicate agglomerated particles, different colors: contour map on the electrode, arithmetic mean roughness (R a ): 3.3) and (d) SiO x /C/QrGO electrodes (similar color, R a : 1.1). 3D surface mapping images of (e) SiO x /C and (g) SiO x /C/QrGO electrodes. Contact angle measurements and water droplet images of (f) SiO x /C-based pellet (red line: 109.3°, upper side of image: photograph of sample) and (h) SiO x /C/QrGO-based pellet (red line: 73.9° upper side of image: photograph of sample), highlighting differences in surface morphology and wettability. Figure 5. Half-cell GDC profiles of (a) SiO x /C and (b) SiO x /C/QrGO at 0.5C for every 10 cycles. (c) Cycling performance of SiO x /C and SiO x /C/QrGO over 50 cycles at a 0.5C. FE-SEM images of the electrode surface for (d) SiO x /C (left: initial stage of electrode, inset: photograph of electrode), (right: electrode after 50 cycles at 0.5C, inset: photograph of electrode with severe delamination, white arrow: surface crack, scale bar: 500 μm) and (e) SiO x /C/QrGO (left: initial stage of electrode, inset: photograph of electrode), (right: electrode after 50 cycles at 0.5C, inset: photo-image of electrode without delamination, scale bar: 500 μm). Cross-sectional FE-SEM images showing electrode swelling in (f) SiO x /C (left: initial stage of electrode, right: electrode after 50 cycles at 0.5C, thickness dramatically increased from 42 μm to 82 μm, scale bar: 50 μm) and (g) SiO x /C/QrGO electrode (left: initial stage of electrode, right: electrode after 50 cycles at 0.5C, slight thickness change from 42 μm to 50 μm, scale bar: 50 μm). Figure 6. Half-cell GDC profiles of (a) SiO x /C@Gr and (b) SiO x /C/QrGO@Gr ( Si content: 20 wt.% in the electrode) for the first three cycles at 0.1C. (c) Nyquist plots of SiO x /C@Gr and SiO x /C/QrGO@Gr electrodes. GDC profiles of (d) SiO x /C@Gr and (e) SiO x /C/QrGO@Gr at different current rates (0.2C–4C). (f) Rate capability of SiO x /C@Gr and SiO x /C/QrGO@Gr at different current rates (0.2C–4C). Cycling performance of (g) SiO x /C@Gr and SiO x /C/QrGO@Gr half-cells at 0.5C and (h) NCM622-paired full cells at 1C. (i, j) Photographs of the SiO x /C@Gr and SiO x /C/QrGO@Gr anodes showing delamination after cycling in pouch cells. 3. Conclusions This study proposes a scalable and practical one-step coating strategy using QrGO, which is water-dispersible, highly crystalline, and exhibits excellent electrical conductivity. Conventional carbon-coating techniques are insufficient to effectively accommodate the volume changes of the Si-based core, leading to structural damage and interfacial instability. Additionally, the reduced conductivity of non-crystalline carbon materials remains a critical challenge in electrode design. In particular, the application of GO-based coatings is hindered by non-uniform reduction, residual reducing agents, and the structural damage associated with thermal treatments. In contrast, QrGO eliminates the requirement for post-processing reduction and effectively mitigates the cracking associated with anode volume expansion while enabling efficient ion and electron transport and maintaining high electrical conductivity. This innovative strategy achieves a high Si content of 20 wt.%, surpassing the typical 5 wt.% of commercial anodes. The SiO x /C/QrGO anode exhibited significantly improved capacity retention, even at high C-rates. The SiO x /C/QrGO || NCM622 full cell demonstrated twice the capacity of commercial SiO x /C after 500 cycles. The one-step QrGO coating process offers a commercially viable solution for high Si loading, improved energy density, extended cycle life, and superior high-rate performance. This study establishes a practical framework for the development of high-performance LIBs. 4. Experimental Preparation of QrGO : QrGO was synthesized using a modified Brodie method. High-purity graphite powder (10 g, Alfa Aesar, 99.999%, 200 mesh) was dispersed in concentrated nitric acid (f-HNO₃, 350 mL). Sodium chlorate (NaClO₃, 75 g) was gradually added to the suspension under continuous stirring at room temperature, and the reaction was allowed to proceed for 48 hours. The reaction was quenched by adding deionized water, and residual metal ions were removed through a purification process using hydrogen peroxide (H₂O₂) and hydrochloric acid (HCl). The resulting graphite oxide (GpO) was diluted to a concentration of 1 g/L in an alkaline NaOH solution adjusted to pH 10. Exfoliation and dispersion were performed using a high-speed homogenizer (UNIDRIVE X1000, CAT Scientific) operating at 10,000 rpm for 1 hour. The dispersion was then stabilized under ambient conditions for 6 hr to facilitate cation–π interactions. To obtain QrGO, the QGO dispersion was reduced by the gradual addition of aqueous hydroiodic acid (HI, 55%, 1 mM), followed by stirring at 70°C for 12 hrs. The pH of the mixture was neutralized to pH 7 using NaOH, and residual impurities were removed through repeated centrifugation at 10,000 rpm. [44,45] Preparation of SiO x /C/QrGO and SiO x /C/TrGO : Aqueous dispersions for spray drying were prepared by mixing commercial SiO x /C (Tainjin, China) with QrGO paste at a solid content ratio of 10:1. The dispersion was prepared at 1 wt.% solid content, as the solid content of the dispersion significantly influences the particle size during the spray-drying process. The dispersion was spray-dried in a chamber preheated to 220 °C, yielding micron-sized SiO x /C/QrGO composite particles. In order to compare the thermally reduced GO, the SiO x /C/TrGO was prepared by GO coating with similar process for SiO x /C/QrGO, followed by a thermal reduction process in an argon atmosphere at 600 °C for 1 hr.

characterization : In order to evaluate the surface delamination of graphene layers, GO and rGO bucky paper were prepared by filtration method. Followed by thermal treatment at 600oC for 1 hr, the pop-up effect occurs in case of GO thin film. Moreover, the morphologies of commercial SiO x /C and the synthesized SiO x /C/QrGO particles were characterized by using field-emission scanning electron microscopy (FE-SEM; S4800, Hitachi, Japan). The thickness of carbon layers with respect to the commercial SiO x /C and synthesized SiO x /C/QrGO were further analyzed by using high-resolution transmission electron microscopy (HR-TEM; Titan G2, FEI Company, USA). The elemental component of active materials was analyzed using an energy-dispersive X-ray spectroscopy (EDS) in TEM. The surface charge of graphene was analyzed through Zeta potential measurements (OTSUKA ELECTRONICS, ELSZ-2000ZS). The carbon content of SiO x /C and SiO x /C/QrGO was determined by thermogravimetric analysis (TGA; TA Instruments, TA Q50) at a heating rate of 10°C/min under ambient condition. The particle distribution and surface roughness of the electrodes were evaluated by using optical microscopy (OM; Olympus, BX53M-BF) and confocal microscopy (KEYENCE, VK9700K). Furthermore, to evaluate the hydrophobic or hydrophilic surface states of the composites, the powders were filtered onto filter paper, and their wettability was assessed through contact angle measurements (S-EO, Phoenix-300, Korea). Electrochemical characterization : The electrodes were prepared by using a typical slurry-casting method. For SiO x /C and SiO x /C/QrGO electrodes, the active material (SiO x /C or SiO x /C/QrGO), conductive additive (Super P, MTI), and binder (polyacrylic acid, Mw 250,000, Sigma Aldrich) were mixed in a weight ratio of 8:1:1. For the preparation of graphite-blended electrodes, the active material (SiO x /C@Gr or SiO x /C/QrGO@Gr), graphite, conductive additive, and binder (S-CMC/SBR) were mixed with 2:7:0.3:0.7 wt.%. The electrolyte which prepared 1.0 M LiPF₆ dissolved in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (3:7 v/v) containing 5 wt.% fluoroethylene carbonate (FEC) and 2 wt.% vinylene carbonate (VC). All electrodes were dried at 100 °C under vacuum condition prior to use. CR2032 coin cells were assembled in a dry room, with Li-metal foil as the counter electrode and a porous polypropylene membrane (Celgard 2400) as separator. The formation cycles were conducted at 25 °C over a voltage range of 0.005–1.5 V at 0.1 C. Rate performance tests were carried out at various C-rates (0.1–4 C), and cycle tests at 0.5 C were conducted using a battery testing system (WonATech, Korea). Electrochemical impedance spectroscopy (EIS) of SiO x /C and SiO x /C/QrGO anodes was performed by using a ZIVE MP2 (WonATech, Korea) with a voltage amplitude of 0.01 V over a frequency range of 1 MHz to 5 mHz. For cathode electrodes, the active material (NCM622), conductive additive (Super P, MTI), and binder (PVDF, S5130, Solvay) were mixed in a weight ratio of 9.6:0.2:0.2. The full cells were constructed in a pouch-type configuration with an N/P ratio of approximately 1.15. Electrochemical testing of the full cells was conducted within a voltage window of 3.0–4.2 V. Formation of the cells was carried out at 25 °C at 0.2 C, and cycle tests were conducted at 1 C. Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

This work was supported by the KERI Primary research program of MSIT/NST (No. 25A01014) and by the Technology Innovation Program (20019091) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea) and by the National Research Council of Science & Technology (NST) grant by the Korea government (MSIT) (No. GTL24011-000). Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff))

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It mitigates cracking, enhances charge transport, and supports a high silicon content of 20 wt.%, surpassing commercial 5 wt.%. QrGO-coated electrodes achieve superior capacity retention, with full cells delivering twice the capacity and long-term stability, offering a scalable solution for high-performance LIBs.

silicon, anode material, reduced graphene oxide, core-shell structure, lithium-ion battery By Byeong Guk Kim, Jihyeon Ryu, Ki-Hun Nam, Sooyeon Jeong, Hye Jung Lee, Jungmo Kim, Dong Gyun Hong, Oh Sung Kwon, Sunhye Yang *, and Seung Yol Jeong * One-step core-shell structuring of silicon graphene composite anode materials by aqueous reduced graphene oxide: Toward practical use of high-performance lithium- ion battery ToC figure Information & Authors Information Version history Peer review timeline Published ENERGY & ENVIRONMENTAL MATERIALS Version of Record15 Jul 2025Published Copyright This work is licensed under a Non Exclusive No Reuse License. Collection

Authors Metrics & Citations Metrics Article Usage 837views 305downloads Citations Download citation Byeong Guk Kim, Jihyeon Ryu, Ki-Hun Nam, et al. One-step core-shell structuring of silicon graphene composite anode materials by aqueous reduced graphene oxide: Toward practical use of high-performance lithium-ion battery. Authorea. 12 March 2025. DOI: https://doi.org/10.22541/au.174180253.35531575/v1 DOI: https://doi.org/10.22541/au.174180253.35531575/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu. Cited by - Synergistic Enhancement of Lithium-Ion Battery Anodes: A Comprehensive Study of SiO2, GO and GO–SiO2 Hybrids, International Journal of Experimental Research and Review, 49, (10-18), (2025).https://doi.org/10.52756/ijerr.2025.v49.002 - Nano Carbon-Based Hybrid Strategies for Mitigating Silicon Anode Expansion in Lithium-Ion Batteries: A Comprehensive Review, Materials, 18, 24, (5532), (2025).https://doi.org/10.3390/ma18245532 Loading...