Enhanced capacity and rate capability of carbon nanotube based anodes with titanium contacts for lithium ion batteries.

Carbon nanotubes are being considered for adoption in lithium ion batteries as both a current collector support for high-capacity active materials (replacing traditional metal foils) and as free-standing electrodes where they simultaneously store lithium ions. The necessity to establish good electrical contact to these novel electrode designs is critical for success. In this work, application of nickel and titanium as both separable and thin film electrical contacts to free-standing single-wall carbon nanotube (SWCNT) electrodes is shown to dramatically enhance both the reversible lithium ion capacity and rate capability in comparison with stainless steel. Scanning electron microscopy showed that evaporation of Ni and Ti can effectively coat the SWCNT bundles in a bulk electrode which is capable of providing an improved electrical contact. A thin film of titanium emerged as the preferred electrical contact promoting the highest capacity ever measured for a SWCNT free-standing electrode of 1250 mAh/g. In addition, the titanium contacting approach demonstrated a 5-fold improvement in lithium ion capacity at extraction rates greater than 1C for a high-energy density Ge-SWCNT electrode. The overall performance improvement with Ti contacts is attributed to a lower contact resistance, nanoscale "wetting" of SWCNT bundles to improve contact uniformity, and effective electron coupling between Ti and SWCNTs due to work function-energy level alignment. The experimental results provide the basis for a Ragone analysis (power vs energy parameters), whereby Ge-SWCNT-Ti anodes paired with a LiFePO(4) cathode can lead to a 60% improvement over conventional graphite anodes in both power and energy density for a complete battery.