Magnesium storage enhancement of molybdenum dioxide in hybrid magnesium lithium batteries

Hybrid Mg/Li-ions batteries (MLIBs), combining the fast kinetics of Li and the advantages of magnesium ion batteries (MIBs), are regarded as one of the most prospective electrochemical energy storage systems. Nevertheless, the electrochemical properties of batteries are generally limited by the diffusion kinetics of highlypolarized divalent Mg ions in the cathode materials. Herein, rutile-type molybdenum dioxide (R-MoO2) was synthesized by the hydrothermal treatment and subsequent thermal treatment and used as the cathode materials in MIBs and MLIBs. The initial discharge capacities of R-MoO2 cathodes in MIBs and MLIBs are 50 and 253 mAh center dot g - 1 at the current density of 50 mA center dot g - 1, respectively. The specific capacities are 17 and 111 mAh center dot g - 1 after 100 cycles. The R-MoO2 cathode possesses good Coulombic efficiency and rate capability in MLIBs. Compared with MIBs, the electrochemical performances of MLIBs were greatly enhanced. The energy storage mechanisms were investigated by the experiments results and density functional theory (DFT) calculations. Moreover, the discharge-charge mechanisms were also explored by ex-situ characterizations. The diffusion/ migration of Mg2+ and Mg storage abilities are improved by introducing the Li+, promoting electrochemical performances. The energy storage mechanisms are Mg2+ intercalation in MIBs and Mg2+/Li+ co-intercalation in MLIBs. This work further investigates the energy storage mechanism of MLIBs and provides the theoretical groundwork for the follow-up development of high-performance cathode materials.