Effect of particle size on lithium intercalation rates in natural graphite

The intercalation rate of Li+-ions in flake natural graphite with particle size that ranged from 2 to 40 μm was investigated. The amount of Li+-ions that intercalate at different rates was determined from measurement of the reversible capacity during deintercalation in 1 M LiClO4/1:1 (volume ratio) ethylene carbonate–dimethyl carbonate. The key issues in this study are the role of particle size and fraction of edge sites on the rate of intercalation and deintercalation of Li+-ions. At low specific current (15.5 mA/g carbon), the composition of lithiated graphite approaches the theoretical value, x=1 in LixC6, except for the natural graphite with the largest particle size. However, x decreases with an increase in specific current for all particle sizes. This trend suggests that slow solid-state diffusion of Li+-ions limits the intercalation capacity in graphite. The flake natural graphite with a particle size of 12 μm may provide the optimum combination of reversible capacity and irreversible capacity loss in the electrolyte and discharge rates used in this study.