Bonding dependent lithium storage behavior of molybdenum oxides for next-generation Li-ion batteries

Abstract

Owing to their high reactivity toward lithium, molybdenum oxides have been widely studied as anode materials for lithium-ion batteries. The two most common molybdenum oxides, MoO2 and MoO3, are reported to undergo sequential insertion and conversion reactions during lithiation. Accordingly, the theoretical capacity of MoO3 is higher than that of MoO2 because of the higher oxidation state of Mo. However, some nanostructured MoO2 exhibits abnormally high reversible capacity close to that of MoO3, which cannot be explained by the conversion reaction mechanism. Herein, the charge storage behavior of the two molybdenum oxides is comparatively analyzed. Structural investigation using synchrotron X-rays demonstrates that the conversion reaction occurs in MoO3, whereas MoO2 accommodates a large amount of lithium in the form of metallic lithium. As a result, MoO2- and MoO3-based composite materials exhibit similar reversible capacities of similar to 1017 and 1110 mA h g(-1), although the changes in the oxidation state of Mo during lithiation are much smaller in the MoO2/graphene composite. First-principles calculations demonstrate that the origin of the different electrochemical properties is the different metal-oxygen bonding properties of MoO2 and MoO3. Understanding the bonding-dependent electrochemical properties in this work could provide a new perspective for designing high-performance electrode materials.