High Energy Density Semiconductor-Carbon Nanotube Anodes for Lithium Ion Batteries

Alternative active materials for lithium ion batteries are under investigation to meet increasing energy storage demands.  Ultra high capacity Si and Ge are promising anode materials that demonstrate capacities much higher than state-of-the-art graphite.   In addition, promising developments have been made recently with electrode designs employing carbon nanotubes (CNTs) as a conductive additive or a free-standing electrode (absent of any binder or metal current collector) to serve as a physical support for silicon or germanium increasing energy and power densities.  Several approaches have shown viability such as semiconductor film deposition using chemical vapor deposition (CVD) and a nanoscale hybridapproach.  The performance of Si and single wall CNTs (SWCNT s) anodes, fabricated by low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD), was measured to have anode extraction capacities between 1000-1500 mAh/g, even at Si weight loadings around 30%.  However, dramatic differences in the rate capability and cycling for anodes from these two deposition techniques were demonstrated.  Alternatively, the mixing of Ge nanoparticles (NPs) with SWCNTs to form a hybrid free-standing anode has enabled a tunable 3-dimensional electrode structure. These hybrid anodes exhibit capacities over 800 mAh/g at rates up to 1C.  The balance between rate and capacity using Si and Ge has led to the development of a mixed hybrid anode that contains both materials in concert with a CNT electrode.  The addition of a Si coating enhances the Ge NP anode capacity along with reducing the surface area and the first cycle loss.  Full batteries incorporating hybrid anodes and conventional cathodes show potential improvements in gravimetric energy density over state-of-the-art by more than 50%.