Fast Energy Storage in Two-Dimensional MoO2 Enabled by Uniform Oriented Tunnels

While pseudocapacitive electrodes have potential to store more energy than electrical double -layer capacitive electrodes, their rate capability is often limited by the sluggish kinetics of the Faradaic reactions or poor electronic and ionic conductivity. Unlike most transition -metal oxides, MoO2 is a very promising material for fast energy storage, attributed to its unusually high electronic and ionic conductivity; the one-dimensional tunnel is ideally suited for fast ionic transport. Here we report our findings in preparation and characterization of ultrathin MoO2 sheets with oriented tunnels as a pseudocapacitive electrode for fast charge storage/release. A composite electrode consisting of MoO2 and 5 wt % GO demonstrates a capacity of 1097 C g(-1) at 2 mV s(-1) and 390 C g(-1) at 1000 mV s(-1) while maintaining similar to 80% of the initial capacity after 10,000 cycles at 50 mV s(-1), due to minimal change in structural features of the MoO2 during charge/ discharge, except a small volume change (similar to 44%), as revealed from operando Raman spectroscopy, X-ray analyses, and density functional theory calculations. Further, the volume change during cycling is highly reversible, implying high structural stability and long cycling life.