INFLUENCE OF THE ELECTROLYTIC MEDIUM COMPOSITION ON THE STRUCTURAL EVOLUTION OF THIN ELECTROCHROMIC MOO3 FILMS PROBED BY X-RAY ABSORPTION-SPECTROSCOPY

We have performed an X-ray absorption spectroscopy study at the Mo K edge Of MoO3 thin films to determine the effect of the composition of the electrolyte on the structural modifications occurring in the layer during the electrocoloration process. Both Li+ and H+ cations containing solutions were studied. The MoO3 layers were obtained by thermal decomposition of electrodeposited MoS3 and the structure of the resulting material was confirmed by X-ray absorption spectroscopy. Upon Li+ (H+) incorporation into the MoO3 material, a 5% (2.3%) increase of the shortest Mo-Mo separation distance is observed. This structural deformation occurs in a direction perpendicular to the layered structure of the material. In potential sweep experiments, the structural modification induced by the incorporation of the cations within the MoO3 material is not totally reversible, and a buildup of chemical disordering is observed with the number of sweeps. This chemical disorder affects both the first O and the second Mo coordination shell and is more important when Li+ rather than H+ cations are incorporated into the MoO3 material. The perturbing effect of H+ and Li+ is best understood if the cations are located near the bridging O atoms within the MoO3 layer rather than between the MoO3 sheets of the layered compound. The cation-specific structural deformation and chemical disorder induced into the MoO3 material are responsible for the specific deterioration of the electrochemical and electrochromic properties of the layer in potential sweep experiments. This effect most probably arises from a variation of the diffusivity of the cation according to the extent of structural deformation and disorder in the material.