EQCM measurements of solvent transport during Li⁺ intercalation in V₂O₅ xerogel films

The electrochemical quartz crystal microbalance technique has been used to monitor the mass changes that occur in acetonitrile electrolytes during electrochemically driven Li⁺ intercalation into V₂O₅ xerogel films prepared by aqueous sol-gel methods. The mass measurements made with the EQCM reveal that during reduction of the vanadium sites in the sol-gel oxide at low voltammetric scan rates (≤5 mV/s) the net compositional change corresponds to insertion of one Li⁺ per electron injected, with no accompanying solvent (i.e. no net mass change that can be attributed to solvent transfer into or out of the film). However, at higher scan rates (>50 mV/s), increased mass changes during reduction are attributed to simultaneous acetonitrile transport into the V₂O₅ film. Based on previous descriptions of solvent swelling of V₂O₅ thin films in acetonitrile solutions, these results suggest the following: (1) during reduction at low scan rates the intercalation of solvated Li⁺ takes place simultaneously with some solvent expulsion, such that the net mass change corresponds essentially exclusively to intercalation of one Li⁺ per electron, (2) at higher scan rates, the rapid electromigration of Li⁺ into the film drives simultaneous solvent insertion, such that the net compositional change corresponds to a net gain of considerable amounts of solvent during the Li⁺ intercalation process. This solvent swelling leads to structural changes that allow larger amounts of solvent to transfer into the films during Li⁺ intercalation for some time after the high scan rate perturbation. The implications of these findings for Li⁺ secondary battery cathode materials are discussed.