EQCM measurements of solvent transport during Li + intercalation in V 2O 5 xerogel films

Abstract

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 V 2O 5 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 V 2O 5 film. Based on previous descriptions of solvent swelling of V 2O 5 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.