Thermodynamics and kinetics of electrochemical intercalation of lithium into Li0.50WO3.25 with a hexagonal tungsten bronze structure

The kinetics and thermodynamics of electrochemical intercalation of lithium into Li0.50WO3.25 with a hexagonal tungsten bronze structure prepared by a solution technique were studied with an ac impedance method, open-circuit potential and X-ray diffraction measurements. The open-circuit-potential versus x-in-Li-x[Li0.50WO3.50]-curve consists of two straight lines with different slopes, which were related to the structural variation in the oxide with lithium intercalation. The standard Gibbs energy for lithium intercalation, Delta G(1)(0), was -252.6 W/mol in the range from x to 1 in Li-x[Li0.50WO3.25] at 25 degrees C. The chemical diffusion coefficients, (D) over tilde, of lithium intercalation into oxide, were measured by ac impedance method as functions of depth of lithium intercalation and temperature in 1 M LiClO4/propylene carbonate solution. The calculated (D) over tilde values were of the order of 10(-9) cm(2)/s and the activation energy for lithium diffusion was 35-44 kJ/mol, being typical of diffusion in layered intercalation materials. Furthermore, the lithium diffusion coefficient and the activation energy for lithium diffusion in Li0.50WO3.25 were compared with hexagonal WO3 and hexagonal-type A(n)WO(3+n/2) (n = 0.14-0.28, A = Na+, K+, NH4+).