Structural and Kinetic Characterization of Lithium Intercalation into Carbon Anodes for Secondary Lithium Batteries

Abstract

Electrochemical intercalation of lithium into carbons has been studied using mesophase‐pitch‐based carbon fibers with different heat‐treatment temperatures, coke, and graphites as anodes for secondary lithium batteries. The variations in the average layer spacing and the voltage profile for the carbons with intercalating depend on the degree of graphitization. The intercalation into a more disordered carbon fiber heated at 900°C has been characterized as intercalation into the layer structure for 0 < x < 0.5 in Li x C 6 , but additional lithium insertion into an unorganized carbon loses the layer structure. The polarization resistance ( R P ) estimated from the impedance spectrum decreased by increasing degree of graphitization. The variation in R P with intercalation revealed the intercalation processes in various disordered carbons to be single‐phase reactions with different stoichiometries of lithium intercalation. The chemical diffusion coefficient D Li of lithium ions in carbons decreased by increasing the composition x in Li x C 6 up to x = 0.5 . The chemical diffusion coefficient was considerably affected by the texture and the degree of graphitization of the carbons. The graphitized carbon fiber heated at 3000°C for 0.1 < x < 0.5 in Li x C 6 exhibited one order magnitude larger values of D Li than those of graphites. The rapid diffusion in the graphitized carbon fiber has been attributed to the radial texture in the cross section. It has been found that the activation energy for the diffusion process decreased by increasing the degree of graphitization.