Investigation of Conductivity and Ionic Transport of VO₂(M) and VO₂(R) via Electrochemical Study

Vanadium dioxides exist as a variety of polymorphs, each with differing structural and electrochemical capabilities. The monoclinic to rutile transition is an interesting system for study as the transition temperature is easily accessible at moderate temperature and corresponds to an increase in electrical conductivity by 2 orders of magnitude. The transition from monoclinic to rutile is characterized structurally herein using synchrotron-based X-ray diffraction and related to lithium-ion electrochemistry using electrochemical impedance spectroscopy and intermittent pulsatile galvanostatic discharge tests. The experimental results indicate a decrease in ohmic resistance for lithium-based cells tested under higher temperatures. Complementary density functional theory calculations described the experimentally measured intercalation voltages and identified a possible Li-induced Li_xVO₂(M) to Li_xVO₂(R) phase transition during the discharging process rationalizing the favorable impact on the function of a lithium-based electrochemical cell. To our knowledge, this is the first observation of the metal-insulator transition of VO₂ in a full electrochemical cell.