Edge Structure and Formation of a Solid Electrolyte Interphase Film in Amorphous Carbon-Coated Spheroidized Graphite

Lithium-ion batteries use spheroidized graphite coated with amorphous carbon as the negative electrode material. In this study, we measured the physical properties of spheroidized graphite with varying amounts of an amorphous carbon coating to elucidate its effect on the battery performance. To this end, electrochemical evaluation and surface analysis of the graphite electrode were performed. The specific surface area was significantly reduced by the amorphous carbon coating because the pores, including the surface inside the spheroidized graphite particles, were occluded by the coating layer. By contrast, the capacitance at the graphite electrode/electrolyte interface did not correspond to the specific surface area, indicating that the amorphous carbon coating served as an edge plane. Consequently, efficiency at the initial charging–discharging cycle was improved, inducing a reduction in the charge-transfer resistance of lithium insertion/desorption. The solid electrolyte interphase formed on graphite was homogenized by the amorphous carbon coating, and the thickness was significantly reduced. The amorphous carbon coating suppressed the reductive decomposition of the electrolyte and increased the number of active sites for lithium insertion/desorption by reducing the number of bare overactive edges present on the surface of the spheroidized graphite particles. The results confirm that the design of an amorphous carbon coating that suppresses the overactivity of the edge during the reductive decomposition of electrolyte components while increasing the active points for lithium insertion and desorption is crucial for enhanced battery performance.