A kinetic study of electrochemical lithium insertion into oriented V2O5 thin films prepared by rf sputtering

The kinetics of the electrochemical lithium insertion reaction in crystalline V₂O₅ thin films in liquid electrolyte has been investigated using ac impedance spectroscopy. The experimental data are obtained for sputtered films characterized by a common morphology corresponding to an arrangement of V₂O₅ platelets perpendicular to the substrate (h 0 0 or 1 1 0 preferred orientation). The results are discussed as a function of the Li content for 0 < x < 1 in Li_xV₂O₅, the film thickness in the range of 0.6-3.6 μm and temperature 15-55 °C. The moderate evolution of the chemical diffusion coefficient D vs. the lithium content is related with the specific structural response of these pure thin film materials which exhibit a single phase behavior. A comparison of the kinetic parameters for different thickness values allows to indicate the same Li diffusion rate whatever the film thickness and the diffusion pathway does not correspond to the thickness but to the length of the edge (≈1 μm) of V₂O₅ platelets. For the first time, an experimental evaluation of the activation energy for Li diffusion in crystalline V₂O₅ is obtained. A value of 0.98 eV is found for a diffusion phenomenon along the b direction. This work demonstrates the excellent capacity-rate performance as well as the efficient and homogeneous behavior of these oriented films can be explained by their specific microstructure.