(503e) Nanostructured and Amorphous Silicon Composites: Reversible High Capacity Lithium-Ion Anodes

Due to their flexible design, high energy density and rate capability, lithium-ion batteries have become the state-of-the art energy storage systems for portable electronic devices such as laptops, cell phones, grid energy storage, and vehicle electrification. Currently, graphite with a theoretical capacity of 372 mAh/g is still used as the commercial anode material of choice for the above mentioned applications. Silicon, with a theoretical capacity of 4200 mAh/g has attracted tremendous interest to replace graphite as the high energy density energy source to be used in the next generation of lithium ion batteries. Silicon however undergoes colossal volume expansion (>300%) during lithium cycling and leads to pulverization of the active material resulting in loss of electrical contact with the current collector causing rapid decrease in capacity and consequent failure of the battery. Several approaches involving the use of nanostructures such as nanoparticles, nanotubes, nanowires and amorphous forms of silicon which undergo non-homogenous volume expansion with the availability of free volume has led to minimize the catastrophic failure resulting in better capacity retention and battery life.

Herein we report 3-D hetero-structures of vertically aligned multi-walled carbon nanotubes (VACNTs) and nano-crystalline silicon (nc-Si) as reversible, high capacity and rate capability anodes by chemical vapor deposition (CVD). A simple floating catalyst approach was used to grow dense VACNTs on quartz substrates and later silicon was decorated as droplets on the CNTs by thermal cracking of silane gas (SiH₄). These hierarchical based structures resulted in capacities in excess of 2500 mAh/g with a low first cycle irreversible loss (<15%) when cycled at a current density of 100 mA/g between 0.2 to 1.2 V vs. Li⁺/Li. Also, thin films of amorphous silicon were synthesized directly on copper substrates by electro-reduction of silicon halides from an organic solvent. The obtained amorphous films exhibited a first discharge capacity greater than 3400 mAh/g and an excellent cyclability with a reversible capacity of ~1300 mAh/g for 100 cycles. Impedance studies conducted on these films at the end of several cycles showed a consistent invariant charge transfer resistance with cycling, which can be correlated to the excellent long term cyclability obtained for 100 cycles. Results of these studies will be presented and discussed.