Thermodynamics and kinetics of electrochemical intercalation of lithium into Li 0.50WO 3.25 with a hexagonal tungsten bronze structure

Abstract

The kinetics and thermodynamics of electrochemical intercalation of lithium into Li 0.50WO 3.25 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 [Li 0.50WO 3.25]-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, Δ G I 0, was −252.6 kJ/mol in the range from x to 1 in Li x [Li 0.50WO 3.25] at 25°C. The chemical diffusion coefficients, D̃, of lithium intercalation into oxide, were measured by ac impedance method as functions of depth of lithium intercalation and temperature in 1 M LiClO 4/propylene carbonate solution. The calculated D̃ 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 Li 0.50WO 3.25 were compared with hexagonal WO 3 and hexagonal-type A n WO 3+ n/2 ( n=0.14–0.28, A=Na +, K +, NH 4 +).