Graphene coating on silicon anodes enabled by thermal surface modification for high-energy lithium-ion batteries

Silicon is a high-energy density anode material for lithium-ion batteries, but it possesses shortcomings such as poor electronic conductivity, interfacial instability and mechanical fracturing that hinder its battery cycling. Carbon coating has been an important strategy for stabilizing silicon anodes, but the effects of the silicon surface properties on carbon coating morphology and the consequent silicon cycling stability have not been clearly elucidated. Herein, we find that thermal oxidation of the silicon anodes followed by chemical vapor deposition of carbonaceous precursors leads to a well-ordered graphene coating, whereas disordered graphite coating is formed on the native silicon surface. Graphene-coated silicon exhibits superior cycling performance, retaining a discharge capacity of ~1300 mAh g−1 after 300 cycles, whereas the disordered graphite-coated silicon suffers continuous degradation, retaining only ~ 600 mAh g−1 after 300 cycles. Cryogenic electron microscopy reveals the mechanism behind the difference in cycling stabilities; graphene coated silicon is able to withstand the large mechanical strains induced during extended cycling, whereas disordered graphite coating is ruptured, exposing silicon surfaces to the electrolyte, leading to extensive buildup of SEI and poor cycling performance. Characterization of the silicon surface reveals that thermal treatment yields an oxygen-rich surface layer, which is hypothesized to play a decisive role in dictating the carbon coating. This work highlights the effect of silicon surface properties on carbon coating microstructure, and presents thermal treatment as a facile avenue to attain graphene coating on silicon anodes. Lithium-ion batteries have become an indispensable element of modern society, powering our mobile devices, electric vehicles, and the grid. For longer-ranged vehicles and longer-running devices, higher-energy density batteries are in demand. Silicon is a promising next-generation anode material, as it offers a capacity of 3570 mAh/g, 10 times that of the current state-of-the-art graphite anode material. However, the adoption of Si anodes is hindered by mechanical fracturing due to volume expansion, poor electrical conductivity, and interfacial instability in contact with the electrolyte. Carbon coating on Si particles plays an integral role in improving these features. The quality of carbon has a critical impact on the performance of the coating: Graphene, compared to disordered carbon, has been suggested to provide superior electrical conductivity, interphase stability, and mechanical integrity. This study deploys cryogenic electron microscopy to reveal the mechanism behind the improved electrochemical performance of graphene-coated silicon.