Nucleation and phase-boundary movement upon stage transformation in lithium-graphite intercalation compounds

Nucleation and phase-boundary movement upon stage transformation in lithium-graphite intercalation compounds were investigated by potential-step chronoamperometry and alternating current impedance spectroscopy. Highly oriented pyrolytic graphite was used as a host graphite material. The current response obtained by potential step was strongly dependent on the kind and direction of stage transformation. A current hump due to the nucleation and nuclear growth of a new stage was observed during the stage transformations from dilute stage 1 to stage 4 and from stage 2 to stage 1 upon lithium intercalation, and from stage 4:to dilute stage 1 upon deintercalation. The rate-determining steps for nucleation and for phase-boundary movement were discussed in the stage transformation between dilute stage 1 and stage 4. It was shown that the nucleation rate was controlled by the reaction at the graphite/electrolyte interface in both directions. The rate of the phase-boundary movement was affected greatly by the diffusivity of lithium ions in a growing phase; that is, it was controlled mainly by the diffusion in stage 4 in the direction from dilute stage 1 to stage 4, whereas by the interfacial reaction in the reverse direction. (C) 1999 Elsevier Science Ltd. All rights reserved.