Negative electrode materials are critical components of lithium-ion batteries (LIBs) and directly influence their performance and energy density [1–4]. The continuous demand for high-performance LIBs in modern electronics and electric vehicles has driven extensive research on novel negative electrode materials [5]. Lithium metal was initially explored as a negative electrode material due to its exceptionally high theoretical capacity. However, its practical application has been quickly abandoned owing to safety concerns, particularly dendritic lithium growth during repeated charge-discharge cycles, which has led to internal short circuits [6]. Carbonaceous negative electrodes address these safety issues by enabling lithium-ion intercalation and offering structural stability during repeated lithium-ion insertion and extraction [7–9]. This advancement has laid the foundation for high-capacity and long-life lithium secondary batteries, leading to their successful commercialization. Despite their structural stability and commercial success, carbon-based materials suffer from relatively low theoretical capacities (~372 mAh g^–1 for graphite), which limits further improvements in energy density. Therefore, there is a strong need to develop alternative negative electrode materials with higher capacities to meet the increasing demand for electric vehicles and large-scale energy storage systems [10–12].

To overcome this limitation, various high-capacity negative electrode materials have been investigated, including transition metal oxides, silicon-based materials, and metal silicides. Silicon-based materials, in particular, have attracted considerable interest owing to their high theoretical capacities (~3579 mAh g^–1) [13,14]. However, they suffer from drawbacks such as low initial coulombic efficiency, severe volume expansion, and poor cycle stability. Various composite materials and structural modifications have been explored to address these issues. In this context, transition metal silicates have emerged as promising negative electrode materials. Among them, Mn₂SiO₄ is newly proposed in this study as a promising negative electrode material. This olivine-structured mineral, also known as tephroite, stores lithium through a conversion reaction mechanism, potentially offering relatively high capacity. In line with commonly adopted strategies for improving the performance of electrode materials, carbon coating has been applied to Mn₂SiO₄ in this study to enhance its electrical conductivity and structural stability. Based on this background, our research aimed to explore the synthesis and electrochemical properties of Mn₂SiO₄ as a potential negative electrode material for LIBs. Specifically, we focus on the solid-state synthesis of Mn₂SiO₄ and its electrochemical evaluation, particularly on the effects of carbon composition on performance enhancement [15–20]. In this study, we aim to maximize the advantages of Mn₂SiO₄ and explore its potential to overcome the limitations of existing negative electrode materials, thereby contributing to the ongoing development of high-performance LIBs.

Fig. 1a shows a schematic of the Mn₂SiO₄ synthesis process. The precursors MnCO₃ and SiO₂ were mixed in a stoichiometric ratio and ball milled, followed by heat treatment under different conditions [21,22]. This solid-state reaction is scalable and efficient for preparing Mn₂SiO₄. The XRD patterns of the synthesized samples are shown in Fig. 1b. The characteristic peaks of Mn₂SiO₄, marked with stars (★), are predominantly observed at 2θ values of approximately 24.5°, 31.2°, 34.4°, 35°, and 50.3° [23,24]. These peaks correspond to the orthorhombic structure of Mn₂SiO₄ (space group Pbnm) and are in good agreement with JCPDS card no. 35-0748. The XRD analysis revealed that the optimal synthesis condition for obtaining high-purity Mn₂SiO₄ was heat treatment at 1000°C under an argon atmosphere. The successful synthesis of pure Mn₂SiO₄ is significant, as impurities such as MnO (JCPDS Card no: 75-0626) and Li₂SiO₃ (JCPDS Card no: 29-0829), which normally appear at 700°C under argon, and Mn₂O₃ (JCPDS Card no: 089-4836) and Si-containing intermediate phases such as MnSiO₃ or Mn₃Si₂O₇, which form at 1000°C in air, can substantially deteriorate the electrochemical performance of the material [25–31]. The absence of these impurity peaks in the XRD pattern of the sample synthesized at 1000°C under an argon atmosphere indicates that this method effectively minimizes the formation of undesired crystalline phases. The selection of an argon atmosphere and 1000°C heat treatment were the optimal synthesis conditions for obtaining pure Mn₂SiO₄. This sample exhibited well-defined characteristic peaks with high intensities, indicating the successful formation of Mn₂SiO₄. Fig. 1c shows the FE-SEM images of the samples prepared under different conditions. The sample treated at 700°C under argon exhibited insufficient growth, resulting in smaller and less aggregated particles, while the sample synthesized at 1000°C in air shows a relatively uniform particle distribution with an average size of ~1 μm. Although the high-temperature treatment facilitated particle growth, the XRD patterns revealed that the air atmosphere may also promote the formation of oxide impurities (e.g., Mn₂O₃, MnSiO₃, and Mn₃Si₂O₇). The sample treated at 700°C in argon showed incomplete aggregation owing to insufficient thermal energy, whereas the sample synthesized at 1000°C in air exhibited features indicative of intermediate or amorphous phases, likely induced by oxidative conditions. By contrast, the sample synthesized at 1000°C under argon comprised larger particles (3–4 μm) with well-organized aggregation [26,32–35]. This morphology indicates sufficient particle growth and crystallization at higher temperatures. The argon atmosphere prevents oxidation and promotes the formation of a pure Mn₂SiO₄ phase, allowing for more uniform particle growth and agglomeration. The combined analysis of the XRD and FE-SEM results confirms that heat treatment at 1000°C under argon is optimal for synthesizing high-purity, well-crystallized Mn₂SiO₄ [36]. This synthesis method yields a material with the desired crystal structure and morphology, both of which are critical for its potential application as a negative electrode material in lithium-ion batteries. Furthermore, the particle size and morphology observed in the FE-SEM images play a crucial role in determining the electrochemical performance of the material. The large, well-organized aggregated particles obtained at 1000°C under argon may provide better structural stability during lithiation-delithiation processes, potentially leading to improved cycling performance in battery applications.

Fig. 2a illustrates the XRD patterns of pure Mn₂SiO₄ and the Mn₂SiO₄/C composite prepared with 10 wt% sucrose addition. Sucrose is known to be an effective carbon precursor, forming a uniform carbon composite during thermal decomposition. XRD analysis confirmed that the optimal synthesis conditions for the Mn₂SiO₄/C composite were consistent with those for phase-pure Mn₂SiO₄, namely, heat treatment at 1000°C under an argon atmosphere [37]. The XRD patterns of the carbon composite retain the characteristic peaks of Mn₂SiO₄, indicating that the carbon composite process does not significantly alter the crystalline structure of Mn₂SiO₄ [25]. However, a slight increase in background signals and a modest decrease in peak intensity are observed in the composite sample. These changes can be attributed to the presence of amorphous carbon and the carbon composite on particle surfaces [32,38]. It is noteworthy that the carbon composite process does not introduce additional impurity phases such as MnO or Mn₂O₃, which are often observed in similar syntheses. This indicates that the argon atmosphere effectively prevents oxidation during the heat treatment process, even in the presence of the carbon precursor. Fig. 2b presents FE-SEM images comparing the morphologies of the pure Mn₂SiO₄ and Mn₂SiO₄/C composite samples. The pure Mn₂SiO₄ particles exhibit a relatively uniform morphology with particle sizes in the range of 3–4 μm. By contrast, the Mn₂SiO₄/C sample exhibited carbon components closely integrated with the Mn₂SiO₄ particles, which appeared to enhance the interparticle connectivity and improve the overall structural cohesion of the composite. This observed morphological difference is crucial for understanding potential electrochemical performance improvements in lithium-ion battery applications. Preservation of the Mn₂SiO₄ crystal structure, coupled with the formation of a carbon composite layer, presents a promising approach for enhancing the electrical conductivity of a material while maintaining its structural integrity. This strategy is particularly relevant for improving the performance of Mn₂SiO₄ as a negative electrode material in lithium-ion batteries. The carbon composite is expected to facilitate electron transfer not only between the Mn₂SiO₄ particles and the current collector but also among adjacent Mn₂SiO₄ particles, thereby enhancing the overall electrical conductivity. This improvement contributes to a better rate capability and cycling stability. In addition, the carbon layer may serve as a protective barrier that mitigates direct contact between the electrolyte and Mn₂SiO₄, further improving the electrochemical stability during cycling.

Fig. 3 presents a comprehensive overview of the electrochemical behaviors of the Mn₂SiO₄ and Mn₂SiO₄/C composite electrodes, as evidenced by their voltage profiles (Fig. 3a,b) and differential capacity (dQ/dV) plots (Fig. 3c–f). The voltage profiles directly illustrate the charge-discharge behavior, while the dQ/dV plots provide enhanced resolution for identifying the phase transitions and reaction mechanisms. These plots provide valuable insights into the lithium-ion storage mechanisms and the impact of the carbon composition on the electrochemical performance of Mn₂SiO₄ [39–42]. The voltage profile of carbon-free Mn₂SiO₄ (Fig. 3a) exhibited a discharge capacity of ~220 mAh/g. This value is lower than the theoretical capacity of Mn₂SiO₄, which may be attributed to its inherently limited electrical conductivity and potential structural instability issues arising during the lithiation-delithiation processes [43,44]. The sloping nature of the voltage profile indicates solid-solution behavior with gradual phase transitions during lithium insertion [33]. In contrast, the Mn₂SiO₄/C composite (Fig. 3b) shows an improved discharge capacity of ~260 mAh/g under identical conditions, which corresponds to an 18% increase compared with that of the carbon-free sample [45]. This enhanced capacity was primarily attributed to the carbon composite, which improved the electrical conductivity of the electrode [46]. The shape of the voltage profile is similar to that of Mn₂SiO₄, indicating that the carbon composite does not significantly alter the lithium storage mechanism. Differential capacity plots (dQ/dV) were obtained to further analyze the electrochemical reactions (Fig. 3c–f). The dQ/dV plot of carbon-free Mn₂SiO₄ (Fig. 3c) exhibits multiple reduction peaks during the first cycle. The broad reduction features observed at ~0.7 V were ascribed to irreversible reactions related to the formation of a solid electrolyte interphase layer (SEI). In contrast, the sharp peaks below 0.5 V are attributed to a conversion reaction between lithium and Mn²⁺, resulting in the formation of metallic Mn⁰ and Li₂O. The oxidation peak observed at approximately 1.3–1.4 V corresponds to the reoxidation of Mn⁰ to Mn²⁺ [36]. Notably, in the second cycle of the carbon-free electrode, the reduction peak shifts from 0.7 V to ~1.0 V (Fig. 3d). This shift was observed for both carbon-free and composite electrodes (Fig. 3d,f). The significant overpotential observed in the first cycle originated from the initial conversion reaction of the Mn₂SiO₄ phase. In the second cycle, this overpotential is notably reduced owing to the activation of the material during the first cycle, resulting in a shift in the reduction peak to a higher potential. In the second cycle, the reduction peak near 1.0 V appears at a slightly higher potential for the Mn₂SiO₄/C composite than for the carbon-free sample (Fig. 3f). This shift can be attributed to the improved electrical conductivity of the carbon matrix, which slightly reduces the overpotential during the conversion reaction.

Fig. 4a illustrates the capacity variation of each sample as a function of current density, demonstrating the rate capability of pure Mn₂SiO₄ and its carbon composite. The specific capacities of both pure Mn₂SiO₄ and the carbon composite exhibited a decreasing trend as the current density increased from 25 to 2500 mA/g. However, the capacity curve of the carbon composite (Mn₂SiO₄/C) consistently maintained a higher position than that of pure Mn₂SiO₄, indicating a superior rate capability. This improved rate performance is attributed to the enhanced electronic conductivity of the carbon composites. Furthermore, the carbon composite appears to mitigate the structural degradation of the active material during high-current charge-discharge cycles, enabling the retention of a relatively high capacity, even at elevated current densities [33,46,47]. The experimental results reveal an initial capacity of ~220 mAh/g for Mn₂SiO₄ in the 3rd cycle at 25 mA/g, which corresponds to ~41% of its theoretical capacity (531 mAh/g). This suboptimal capacity utilization is likely due to the inherently low electrical conductivity of Mn₂SiO₄ and sluggish lithium-ion diffusion kinetics, which impede complete electrochemical reactions [43,48]. To address these limitations, a carbon composite was fabricated using sucrose as a carbon precursor. The resulting Mn₂SiO₄/C composite demonstrated a capacity of ~250 mAh/g during the 3rd cycle at 25 mA/g. This corresponds to a capacity increase of ~14%, which can be ascribed to the role of the carbon composite in improving the electronic conductivity. Moreover, the Mn₂SiO₄/C composite exhibited a better rate capability than pure Mn₂SiO₄. This improvement is attributed to the provision of electron conduction pathways by the carbon network, leading to enhanced electrochemical kinetics. Fig. 4b shows the long-term cycling performances of the Mn₂SiO₄ and Mn₂SiO₄/C composite electrodes over 100 cycles. The Mn₂SiO₄/C electrode exhibited a significantly higher capacity and better capacity retention than its carbon-free counterpart. Notably, both electrodes showed a slight increase in capacity over cycling, which may be attributed to gradual electrochemical activation and improved accessibility of the active material. Although Mn₂SiO₄ demonstrates a lower gravimetric capacity than graphite (360 mAh/g), it excels in terms of volumetric capacity. With a theoretical density of 4.25 g/cc, significantly higher than that of graphite (2.2 g/cc), Mn₂SiO₄ offers promising volumetric energy density. Calculations indicate that the potential volumetric capacity of graphite is 792 mAh/cc (360 mAh/g × 2.2 g/cc), whereas Mn₂SiO₄ achieves 935 mAh/cc (220 mAh/g × 4.25 g/cc). This indicates that Mn₂SiO₄ has considerable potential for volume-constrained applications.

Mn₂SiO₄ and Mn₂SiO₄/C composites were successfully synthesized and evaluated as negative electrode materials for lithium-ion batteries. High-purity Mn₂SiO₄ was obtained through a solid-state reaction involving heat treatment of MnCO₃ and SiO₂ precursors at 1000°C under an argon atmosphere. The resulting Mn₂SiO₄ exhibits limited electrochemical performance, primarily owing to its low electrical conductivity. To address this limitation, a carbon composite was synthesized using sucrose as the carbon precursor and employing the same optimized synthesis conditions that improved the rate capability of Mn₂SiO₄. The Mn₂SiO₄/carbon composite demonstrated better overall performance than pure Mn₂SiO₄. The improved characteristics were attributed to increased electronic conductivity and structural stability. In conclusion, Mn₂SiO₄ and its carbon composites have demonstrated potential as high-capacity negative electrode materials for lithium-ion batteries. The performance enhancement achieved through carbon compositing is particularly encouraging, and future studies on the unique structural attributes of Mn₂SiO₄ are expected to be useful. Furthermore, optimization strategies, such as nanostructure control, doping, and coating, are expected to yield additional improvements, and in-depth observations of structural changes during charge-discharge processes will provide further insights into the unique cycling behaviors of Mn₂SiO₄-based electrodes.