Boosting the capacity and stability of a MoO₃ cathode via valence regulation and polypyrrole coating for a rechargeable Zn ion battery

Molybdenum trioxide (MoO3) is emerging as a hugely competitive cathode material for aqueous zinc ion batteries (ZIBs) for its high theoretical capacity and electrochemical activity. Nevertheless, owing to its undesirable electronic transport capability and poor structural stability, the practical capacity and cycling performance of MoO3 are yet unsatisfactory, which greatly blocks its commercial use. In this work, we report an effective approach to first synthesise nanosized MoO3-x materials to provide more active specific surface areas, while improving the capacity and cycle life of MoO3 by introducing low valence Mo and coated polypyrrole (PPy). MoO3 nanoparticles with low-valence-state Mo and PPy coating (denoted as MoO3-x@PPy) are synthesized via a solvothermal method and subsequent electrodeposition process. The as-prepared MoO3-x@PPy cathode delivers a high reversible capacity of 212.4 mA h g(-1) at 1 A g(-1) with good cycling life (more than 75% capacity retention after 500 cycles). In contrast, the original commercial MoO3 sample only obtains a capacity of 99.3 mA h g(-1) at 1 A g(-1), and a cycling stability of 10% capacity retention over 500 cycles. Additionally, the fabricated Zn//MoO3-x@PPy battery obtains a maximum energy density of 233.6 W h kg(-1) and a power density of 11.2 kW kg(-1). Our results provide an efficient and practical approach to enhance commercial MoO3 materials as high-performance cathodes for AZIBs.