Factors affecting rate capability of graphite electrodes for lithium-ion batteries

Kinetic problems associated with rate capability of graphite electrode were examined by impedance measurement, current-pulse relaxation technique, and voltage-step chronoamperometry. The impedance and current-pulse relaxation data showed a diffusion behavior. The diffusion behavior was examined as to which process is reflected on the data, i.e., lithium-ion transport in a graphite solid matrix or in a liquid electrolyte, using a lithium electrode substituting for the lithiated graphite electrode and using the same electrolyte, separator, and cell. It was shown that the diffusion behavior observed for the lithiated graphite electrode was the same as that of lithium electrode reflecting lithium salt diffusion in the electrolyte. To examine specifically whether or not lithium transport in the solid matrix is responsible for the rate capability, the voltage-step chronoamperometry of lithiated graphite electrodes having different porosity in the electrode mix was carried out. The current per graphite sample weight was highly dependent on the porosity, confirming that the rate capability problem is not due to lithium transport in the solid matrix. From these results, factors affecting the rate capability of graphite negative electrodes for lithium-ion batteries are discussed. (C) 2003 The Electrochemical Society.