Microstructural Characteristics and Electrochemical Performance of Core-Shell-Structured Si Powders Fabricated by Ball Milling with Different Process Control Agents

Scalable wet high-energy ball milling was used to refine commercial micro-silicon (Si) with water and ethanol as process control agents (PCA). After grinding, the original particle size of 15 microns was reduced to approximately 1 micron for both PCAs. The sample with water as PCA produced more amorphous silicon oxide on the surface of Si particles compared to ethanol, resulting in relatively inferior electrochemical performance. This is attributed to the fact that the water-milling-induced amorphous layer comprises a significant portion of high-valence Si than the ethanol-milling-induced surface layer. Various silicon powders ground with ethanol at different ratios (e.g., 1:3, 1:4, 1:9, and 1:12) were evaluated to establish process–performance relationships. The ball-milled Si powders with a silicon-to-ethanol ratio of 1:9 showed the best electrochemical performance, displaying an initial coulombic efficiency of about 90% and a reversible specific capacity of 3000 mA h/g for the first cycle. After 55 cycles, the reversible specific capacity was maintained at 2200 mA h/g at 0.105 A/g with a corresponding retention rate of 86%, showing good cycling stability. This amorphous shell layer structure introduced through the in situ surface during the grinding process played a role in boosting the electrochemical performance of the micro-silicon anode. In addition, the amorphous shell formed by SiO x on the milled micro-Si surface and its effect on the electrochemical performance are discussed in this paper.