Diffusion Process in LixW3O9F(1≤x≤3)

Abstract

AbstractThe structure of LiW3O9F is built up of a peculiar stacking of hexagonal tungsten bronze-type layers. Li can be reversibly intercalated in this material, electrochemically or chemically up to the Li3W3O9F composition.Diffusion coefficients are determined by the Honders method using Li/LiClO4-PC/Lix W3O9F cells which are discharged galvanostatically for given periods of time. This method does not require the knowledge of the Darken factor nor of the material/electrolyte contact area, which are both rather delicate to determine precisely in pratice.The evolution of D with the intercalation amount exhibits a drastic increase (by three orders of magnitude) from x = 1 to x = 1.15 and a constant and rather high value afterwards. Different sites for the preexisting lithim (x ≤ 1) and the intercalated ones are therefore expected.7Li NMR spectra have been recorded for various intercalation amounts. For x = 1, two first order quadrupolar signals are observed, corresponding to the sites of the immobile lithium ion. When lithium is intercalated (x = 1.1), another signal is observed, as well as a modification of that of the first lithium, due to intra site mobility. A more important change is observed between x = 1.1 and x = 1.2, which suggests an exchange process. For larger values of x, a single signal is observed, corresponding to mobile Li+ ions.The sites in which lithium may be intercalated and the diffusion pathways are discussed on the basis of these results.