Aligned Carbon Nanotube-Silicon Sheets: A Novel Nano-architecture for Flexible Lithium Ion Battery Electrodes

In the pursuit of high performance lithium ion batteries (LIBs), significant effort has been expended to explore high performance cathode and anode materials. Silicon has the greatest lithium storage capacity per unit mass, and is therefore one of the most promising potential candidates to replace graphite as the anode material in future generations of batteries. The main challenge in utilizing silicon comes from the structural failure induced by its large volume change (>300%) during electrochemical cycling, leading to capacity loss. New designs, in which silicon and carbon can act in a mutually beneficial way, so that silicon can fully contribute to the capacity while maintaining cyclic stability, are needed. With this in mind, this communication describes novel, binder-free, thin sheet anodes for LIBs using aligned carbon nanotube (CNT) based silicon films which were processed in a way that is conducive to future commercial production. The horizontal super-aligned CNT sheets provided high surface area and a porous structure to facilitate both the uniform chemical vapor deposition of silicon during fabrication and the electrochemical kinetics between the silicon and the electrolyte during use. The CNT-based silicon composite sheets had both high specific energy capacity and stable cycle performance. This work also revealed an interesting new mechanism of deformation for silicon coated CNT structures after electrochemical cycling. A spring-like deformation behavior of the aligned CNTs helped to explain the electrochemical stability of the crystalline silicon coatings. These findings will guide future work to optimize this unique nano-architecture for further increases in energy density and stability. This aligned CNT scaffold design may be extended to other anode and cathode materials utilized in thin and flexible LIBs.