Influence of defects on the phase-boundary movement in a stage transformation of lithium-graphite intercalation compounds

The influence of defects on the phase-boundary movement in the stage transformation from random stage (dilute stage 1) to stage 4 of lithium-graphite intercalation compounds was investigated using highly oriented pyrolytic graphite (HOPG). Potential-step chronoamperometry and in situ optical microscopy coupled with micro Raman spectroscopy were used to clarify macroscopic and microscopic influence of defects. Potential-step chronoamperometry confirmed that the diffusion limited rate of the phase-boundary movement in the bulk of HOPG was independent of the crystallinity of the samples. This result indicates that defects have macroscopically no appreciable influence on the phase-boundary movement. In situ optical microscopy on the basal plane of HOPG revealed that the nucleation and growth of the stage-4 phase took place quite inhomogeneously in the initial stage. The rate of the phase-boundary movement in the surface region of HOPG was roughly evaluated to be in the range of 5 to 10 mu m min(-1). The phase-boundary movement was seriously retarded by cracks formed on the basal plane during the stage transformation. (C) 1999 Elsevier Science Ltd. All rights reserved.