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 mum and sphere-like graphite (three-dimensional), 12 to 40 mum 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 mum may provide the optimum combination of reversible capacity and irreversible capacity loss in the electrolyte and discharge rates used in this study. (C) 2003 Elsevier B.V. All rights reserved.