Controlling MoO₂ and MoO₃ phases in MoO_x/CNTs nanocomposites and their application to anode materials for lithium-ion batteries and capacitors

Molybdenum oxides (MoO₂ and MoO₃) are attractive anode materials for Li- and Na- ion batteries. Although there have been extensive studies on them individually, systematic and comparative studies are still lacking. In this work, we demonstrate a facile and straightforward synthesis method to control the phase and oxidation state in the MoO_x/CNTs nanocomposites via hydrothermal reaction followed by heat-treatment. By changing the gas atmosphere during the annealing process, well-dispersed MoO₂/CNTs and MoO₃/CNTs nanocomposites are formed without altering their overall morphology. This strategy enables us to investigate the true structure-property correlation of MoO_x/CNTs nanocomposites by comparing the structure and electrochemical properties of MoO₂/CNTs and MoO₃/CNTs. When tested as anode materials for lithium-ion batteries, both HT-MoO_2&3/CNTs electrodes show much-improved cycling stability and rate performance compared to the rod-shaped bulk MoO₃ electrode. In situ Mo K-edge x-ray absorption spectroscopy (XAS) has been further employed to compare and elucidate Li⁺ storage mechanisms of both electrodes. When employed to the negative electrode of a high-power lithium-ion capacitor (LIC), the LIC full-cell composed of HT-MoO₃/CNTs negative and activated carbon positive electrodes demonstrates impressive energy and power densities (~ 90 Wh kg⁻¹ with 2000 W kg⁻¹) and excellent cycling stability (96.8 % capacity retention after 300 cycles), revealing the versatility of the MoO_x/CNTs electrodes in energy applications.