Effect of particle morphology on lithium intercalation rates in natural graphite

The intercalation rate of Li+-ions in flake natural graphite (two-dimensional) with particle size from 2 to 40 μm and sphere-like graphite (three-dimensional), 12 to 40 μm in particle size, was investigated. The amount of Li+ ions that intercalate at different rates was determined from measurement of the reversible capacity during de-intercalation in 1 M LiClO4/1:1 (volume ratio) ethylene carbonate—dimethyl carbonate. The key issues in this study are the role of the particle size and fraction of edge sites on the rate of intercalation and de-intercalation 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 3D 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.