Comparison of the Structure and Phase Changes of Carbon-Coated SiO and Li-Doped Carbon-Coated SiO During Repeated Charge–Discharge Cycling

Carbon-coated SiO (SiO–C), which is a high-capacity anode material, experiences a significant capacity drop in the initial charge–discharge cycles. In contrast, Li-doped SiO–C (Li–SiO–C), which has been recently developed, exhibits a significantly smaller capacity drop. To explain this difference, we performed a detailed investigation of the structures and phase changes associated with the charge–discharge cycling of these materials by comparing their Si structures and electronic states obtained from solid-state magic-angle spinning nuclear magnetic resonance and Si K-edge X-ray absorption fine structure measurements. The results show that, in the case of SiO–C, the Li4SiO4 generated during charge is partially decomposed during discharge in the initial charge–discharge cycles. These generation and decomposition behaviors are most intense during the first 20 cycles. We believe that this phenomenon is the cause of the increased irreversible capacity observed in the initial cycles of SiO–C. In addition, we confirmed that Li2SiO3, a component of Li–SiO–C, is relatively stable electrochemically, although some of it gradually converts into Li4SiO4 during charge–discharge cycling. The presence of Li2SiO3 at the outset implies that less Li4SiO4 is generated during charging compared to SiO–C, which we believe explains the lack of a significant capacity drop in the initial cycles of Li–SiO–C.