In a paper published online in the journal Nature Nanotechnology in 2007 , Yi Cui and his colleagues reported the use of single crystalline silicon nanowires as the anode material could offer good electrical connection and ability to overcome the mechanical breaking caused by up to 300% volume expansion. The nanowires achieved the theoretical charge capacity for silicon anodes and maintain a discharge capacity of 75% of the maximum, with little fading under 10 charge/discharge cycles.

The new study builds on that by demonstrating a Si crystalline-amorphous (c-a) core-shell NW design resulting in significant improvement over power rate and cycling life.

Both crystalline Si (c-Si) and amorphous Si (a-Si) can store Li⁺ ions with similar specific capacity. Previous studies, mostly involving thin films, have suggested that a-Si has a superior cycling performance compared to c-Si. One explanation for this is that the volume expansion of a-Si upon Li insertion is homogeneous and causes less pulverization as in the crystalline material. Another interesting behavior of a-Si is that it reacts with lithium at slightly higher potential (~220 mV) than c-Si does (~120 mV), which leads to our idea of using c-a core-shell Si NWs as anode material. When limiting the charging potential, it should be possible to utilize only the amorphous shell material for Li⁺ storage while preserving the crystalline core as mechanical support and efficient electron transport pathways.

—Cui et al. (2009)

Cui and his colleagues developed a simple method to grow c-a core-shell Si NWs directly on stainless steel (SS). The c-a core-shell NWs were assembled into half-cells with Li metal as counter electrode. No binders or conducting carbon were used. The half-cells were studied using electrochemical potential spectroscopy (EPS) and galvanostatic charge/discharge.

A comparison of c-a NW half-cells at the low-voltage cutoffs of 10 mV, 70 mV, and 150 mV showed that with charge cutoff at 150mV, the reaction involves just the external amorphous shell of the NWs and preserved the c-Si core for mechanical supports and efficient conductive pathways.

...c-a core-shell NWs with a 150 mV cutoff and the same current density [0.85 A g^-1 (0.2C according to the theoretical capacity of Si)] show dramatically improvement of cycle life. The c-a NW electrode was charged to 150 mV in 1.2 h, showing a capacity of ¡~1060 mAh g^-1 with 85% discharge capacity retention over 100 cycles. Despite the discharge capacity of 150 mV cutoff is lower than that (2500 mAh g^-1) of 10 mV cutoff, the value is 3 times of carbon and is already high enough to be technologically significant. What is more important is to have good capacity retention over long cycle life. In addition, the Coulombic efficiency (86.2% for the first cycle, 99-99.7% for cycles 5-70, 98.4-99% for cycle 70-100) is significantly higher for the 150 mV cutoff, which provides a strong evidence of little fatigue of NWs during cycling.

—Cui et al. (2009)

The team also found that the c-a core-shell NWs showed show excellent electrochemical performance at high rate of charging and discharging (6.8 A g^-1, ~20 times of carbon at 1 h rate).

The research was supported by the Global Climate and Energy Project at Stanford; US Office of Naval Research; and King Abdullah University of Science and Technology (KAUST), along with graduate fellowships from the National Science Foundation graduate fellowship and Stanford.

In March 2008, KAUST named Dr. Yi Cui as one of its 12 Global Research Partnership (GRP) Investigators. (Earlier post.)

Resources

• Candace K. Chan, Hailin Peng, Gao Liu, Kevin McIlwrath, Xiao Feng Zhang, Robert A. Huggins & Yi Cui; “High-performance lithium battery anodes using silicon nanowires” Nature Nanotechnology, Published online: 16 December 2007 | doi:10.1038/nnano.2007.411

• Li-Feng Cui, Riccardo Ruffo, Candace K. Chan, Hailin Peng, and Yi Cui (2009) Crystalline-Amorphous Core-Shell Silicon Nanowires for High Capacity and High Current Battery Electrodes. Nano Lett., 9 (1), 491-495 doi: 10.1021/nl8036323