Silicon is of interest as an anode material because of its highest known capacity (4,200 mAh g^-1). One of the challenges with silicon anodes, however, is that lithium ion insertion into and extraction from silicon are accompanied by a volume change of up to 300%, which induces a strong stress on the silicon particles and causes pulverization and rapid capacity fading.

Numerous approaches to addressing this issue have been investigated, including the preparation of nanosize active materials; active/inactive composite materials; and Si-based carbon composites. These approaches improved Si-based anode performance, but only to a limited extent, the authors noted.

Cui and his team earlier developed crystalline Si nanowires (SiNWs) as anodes, which showed excellent performance. (Earlier post.) SiNWs can relax the strain and overcome the problem of pulverization, maintain direct electrical connection with current collector, and have short diffusion distance for lithium insertion.

Cui subsequently developed anodes based on crystalline-amorphous core-shell SiNWs directly grown on metal current collectors. (Earlier post.) By limiting the charging potential above 150 mV versus lithium metal, they showed that amorphous shell can be selectively used for lithium ion storage with capacity of ~1,000 mAh g^-1 while the crystalline cores remained intact and functioned as efficient electron transport pathways and stable mechanical support.

In this report, we have synthesized C-Si core-shell NWs by chemical vapor deposition (CVD) of amorphous Si (a-Si) onto carbon nanofibers (CNFs)...Similar to crystalline Si core in the crystalline-amorphous core-shell SiNWs, carbon cores function as efficient electron transport pathways and stable mechanical support.

However, the difference is that the carbon core, due to its small capacity, has little structure or volume change with charge potential down to 10 mV versus lithium metal. Charging to this low potential allows a much higher usage (>2,000 mAh g^-1) of the specific charge capacity of a-Si.

—Cui et al. (2009)

In addition, the researchers noted, CNFs are commercially available in a large quantity, which makes the mass production of C-Si core-shell NWs easily achievable. They demonstrated the high mass loading of these core-shell NWs and achieved an area capacity of ~4 mAh/cm²—comparable to commercial values. A full cell using the anodes with LiCoO₂ as cathode discharges at 3.3 V and is commercially applicable, they concluded.

Resources

• Li-Feng Cui, Yuan Yang, Ching-Mei Hsu and Yi Cui (2009) Carbon-Silicon Core-Shell Nanowires as High Capacity Electrode for Lithium Ion Batteries. Nano Lett., Article ASAP doi: 10.1021/nl901670t