DIFFUSION OF LITHIUM IN CARBON

To estimate the solid state diffusion coefficient of lithium in lithiu m intercalated carbon electrodes, a new model based on the GITT (galva nostatic intermittent titration technique) method was developed after a careful consideration of the models and methods used in the literatu re. The concept of CPE (constant phase element) which is often used in the interpretation of ac impedance spectroscopy was introduced to acc ount for the roughness of the electrode, leading to a model closer to the powder pellet electrode often used in lithium batteries. Diffusion coefficient of lithium in three kinds of carbon, i.e. pyrolytic poly( furfural alcohol) (PFA), coke and MCMB (mesophase carbon, 1000 degrees C) were calculated by fitting the voltage response due to a constant low current perturbation with the new proposed model. The results were compared with those obtained according to an old model, which involve d an ideal planar electrode. Introduction of the CPE element led to an increase of two orders of magnitude in the D values. The D similar to x(x in LixC6) and diffusion activation energy E(a) similar to x relat ionships were also obtained and were interpreted in terms of the struc tures of the carbons.