Mesoporous MoO2 thin films for high rate Li+ storage: Effect of crystallinity and porous structure

Abstract

MoO2 has attracted much recent attention as a high capacity energy storage material. While much of the current work on MoO2 has been focused on the high capacity four-electron reduction, this reaction is limited to slow charging process due to the large volume change and phase transitions involved. In this study we focus on one-electron insertion reactions and demonstrate that ordered mesoporous thin films of MoO2 can show signatures of pseudocapacitive charge storage. Mesoporous MoO2 (mp-MoO2) thin films were treated at different temperatures between 350 and 700 °C to explore the role of crystallinity and nanoscale structure on charging dynamics. The porosity and pore size decreased while the crystallinity and grain size increased as the calcination temperature increased. Materials processed at 600 °C showed the best electrochemical performance due to an optimized combination of high crystallinity and small grain size. These materials could be charged and discharged in 24 s while still achieving a Li+ storage capacity of 158 mAh/g. While such thin film systems do not constitute a practical energy storage device, the work provides structural design parameters for the production of future bulk nanoporous materials.