Green Car Congress reports that Stanford researchers take another approach to improve the durability of silicon anodes for Li-ion batteries. The research team leader, Dr. Yi Cui, has reported on development of a novel design of carbon-silicon core-shell nanowires. The researchers at Stanford believe that these carbon-silicon core-shell nanowires will mean the electrodes will be capable of high power and long life.

These nanowires have a high charge storage capacity of 2,000 mAh g-1 and good cycling life. They also have a high Coulombic efficiency of 90% for the first cycle and 98-99.6% for the following cycles. A paper on their work was published 5 August in the ACS journal Nano Letters.

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 silicon nanowires as anodes. These showed excellent performance. since crystalline silicon nanowires “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.”

As previously reported by GCC, Cui subsequently developed anodes based on crystalline-amorphous core-shell silicon nanowires directly grown on metal current collectors. 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)

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