Solid-state electrochemistry of the Li single wall carbon nanotube system

Reversible insertion of lithium into purified single wall carbon nanotubes was achieved electrochemically. Nanotubes exhibited reversible capacities o n the order of 460 mAh/g, corresponding to a stoichiometry of Li1.23C6. The material also presented very high irreversible capacities (1200 mAh/g) whi ch we ascribe to the large specific surface area (350 m(2)/g). Galvanostati c charge-discharge and cyclic voltammetry indicated that there is no well-d efined redox potential for lithium insertion or removal in the nanotube lat tice, ruling our the hypothesis of a staging mechanism via well-defined int erstitial sites. In situ X-ray diffraction revealed an irreversible loss of crystallinity, suggesting that duping disrupts the intertube binding, anal ogous to exfoliation in layer hosts. In situ resistance measurements showed a 20-fold decrease upon doping, consistent with charge transfer between li thium and carbon. Electrochemical impedance spectra were interpreted in ter ms of a Randles-type equivalent circuit. The data showed a continuous decre ase in charge transfer resistance upon doping, consistent with the decrease in electronic resistivity of the electrode. It also showed that the high l ithium capacities are not due to double layer capacitance effects, but to a n actual ion insertion/extraction process in the bulk material. (C) 2000 Th e Electrochemical Society. S0013-4651(99)09-084-9. All rights reserved.