Surface-modified silicon nanowire anodes for lithium-ion batteries

Silicon nanowires with hydride, methyl and siloxane surfaces terminations were evaluated as anodes in lithium-ion half cells using LiPF₆ in EC/DMC electrolytes. Voltammetry, FT-IR and XPS analyses show hydride-terminated nanowires react with the electrolyte and methyl termination tends to passivate silicon surfaces. Silicon anodes pretreated with trimethoxymethylsilane show decreased lithium capacities similar to methylated anodes; however, the addition of 5% trimethoxymethylsilane as an electrolyte additive resulted in the formation of significantly more OPFx compounds while improving capacity retention relative to hydride-terminated nanowires (2348 mAh g⁻¹ at 15 cycles at C/10 rates). FTIR analysis show trimethoxymethylsilane additives covalently bond silicon surfaces and other SEI components. AFM nano-indentation tests also suggest the alkoxy silane additives in the electrolyte function as a binder to improve silicon's ability to withstand the large reversible volume changes. The results indicate silicon surface terminations play a key role in chemical and mechanical behaviors that control reversibility.