Silicon nanowire anode for lithium-ion batteries: fabrication, characterization and solid electrolyte interphase

Depletion of fossil fuels and concerns over CO2 emission have driven the development of electric vehicles (EVs) with high-energy efficiencies and low emissions. Lithium-ion rechargeable batteries, compared to lead acid, nickel cadmium, nickel metal hydroxide, and other popular rechargeable batteries, are considered as the most promising candidates for EVs for their high operating voltage and high energy density. Silicon nanowires are considered as lithium-ion battery anodes for their ultra high capacity at 4200 mAh·g-1 (10× higher than conventional graphite anode), as well as stress accommodation for reversible lithiation and delithiation. Silicon nanowires were fabricated via metal assisted electroless etching, and conductive nickel monosilicides ohmic contacts were created via simple one-step thermal annealing procedure between nanowires and nickel electrodes for integration. Composite anodes were prepared from electrolessly fabricated silicon nanowires for lithium-ion batteries, and an addition of only 15 % silicon nanowires results in a two-fold increase in reversible capacities for 15 cycles. Silicon anodes with hydride, methylated and siloxane surface terminations were prepared and tested in lithium-ion cells; another silicon anode was cycled with 5 % trimethoxymethylsilane. Analyses showed methylated and siloxane terminations lead to passivated surfaces, and hydride-terminated nanowires were relatively more reactive with electrolytes. The addition of silane additive results in more OPFx compounds and Si-O-Si bonds at the silicon surface with significantly higher capacities (3287 mAh·g-1). AFM nano-indentation analyses also showed a significant increase in contact stiffness with silane additive, and the increase in contact stiffness may improve the anode’s ability to withstand large volume changes. Although the chemical composition of the SEI is altered with silane additives, performance improvements were mainly associated mechanical effects