KINETICS OF ELECTROCHEMICAL INSERTION OF LITHIUM INTO WO

A kinetic study of the electrochemical insertion of lithium into the original form of WO3, formed by the heat-treatment of H2WO4 at 448 K, were carried out using an ac impedance method. The chemical diffusion coefficients, D, of lithium in the oxide were measured as functions of the lithium composition and temperature by using several kinds of electrolytes. The obtained values of D, based on a geometric surface area, depended strongly on the lithium composition and the kind of electrolytes, The D values, obtained using 1M LiClO4 and 1M LiAsF6 solutions of propylene carbonate, were of the order of 10(-11) cm/s at 298 K for 0.2 < x< 1.0 in LixWO3, corresponding to a lithium self-diffusion coefficient of the order of 10(-12) cm2/s, while using the IM LiBF4 and IM LiPF6 solutions considerably lower values of D, 10(-13)-10(-12) cm2/s, were obtained for 0.2 < x < 1.0. The activation energy for the diffusion of lithium in WO3 obtained using the former electrolyte (20-40 kJ/mol) is typical of diffusion in intercalation materials, whereas that obtained using 1M LiBF4 solution (approximately 60 kJ/mol) corresponds to the kinetics controlled by the surface layer.