THE DIFFUSION OF LITHIUM IN GRAPHITIC MATRIX A3-3 AT LOW CONCENTRATIONS AND HIGH-TEMPERATURES

The diffusion of Li into the bulk is the first step in the formation of intercalation compounds. Previously we found that Li migrates in Li doped, highly oriented pyrolytic graphite within the interplanar space, and in comparison to Poco graphite AXM-5Q1, Li is preferentially absorbed in a phenolic resin based glassy carbon. We now report on a study of Li diffusion in graphitic Matrix A3-3 which consists of graphite grains (like Poco graphite) and a phenolic resin based polymeric carbon. Specimens of Matrix A3-3 and POCO AXM-5Q1 were Li doped in a closed container at 1273 K and subsequently the Li content of the specimen was determined. The Li concentration in the binder carbon was estimated to be 10 times higher than in the graphite grains. The Li release kinetics were measured by using a mass spectrometric method and were found to conform to Fick's second law at high temperatures. At low temperatures the release rate was initially lower than expected for simple diffusion kinetics. We assume this is mainly due to a nonzero Li surface concentration. After considering the literature on the structure of polymeric carbons we conclude that micropores, and probably even more vacancies and other small defects in the graphitic layers within the bulk, are preferred Li binding sites. The temperature dependence of the diffusion coefficient of Li in graphitic Matrix A3-3 is given by D = 5.6 × 105 exp(-2.62/kT) cm2s-1 in the temperature range 1100-1400 K at concentrations in the range 1.5 μgLi/gC to 12 μgLi/gC. The activation energy E for Li diffusion in Poco AXM-5Q1, graphitic Matrix A3-3, and glassy carbon increases in the order given. © 1990.