Measurements of chemical diffusion coefficient of lithium ion in graphitized mesocarbon microbeads using a microelectrode

The solid-state diffusion of lithium ion in carbon was directly investigated in single spherical particles of graphitized mesocarbon microbeads (MCMB, Osaka Gas Co.). In order to achieve electrochemical measurements focusing on a single particle, a molybdenum-filament microelectrode was maneuvered to make electrical contact with a single MCMB placed in an organic solvent mixture containing lithium ions as an electrolyte. This novel microelectrode technique gives electrochemical information about a MCMB particle, itself, without any dilution with additives such as organic binders. The chemical diffusion coefficient (D) of lithium ion was determined from potential-step chronoamperograms assuming spherical diffusion of lithium ions in the MCMB particle. The values of D were found to change the order between 10^-9 and 10^-11 cm²/s depending on the electrode potential. Plots of D showed pronounced minima at the potentials where the transformation of staging level of lithium intercalation compounds occurred.