Quantum size effect activating MoO_3-x voltage platforms enables high-capacity and long-cycle magnesium-ion batteries

Transition metal oxides (TMOs) demonstrate great potential as cathode candidates for magnesium ion batteries (MIBs) owing to their high specific capacity, variable valence, structural diversity, and low cost. However, in practice, they generally exhibit problems low specific capacity, poor cycle stability, and slope-like voltage curves, which severely limit energy efficiency and practical application prospects. In this study, a coupling regulation strategy of oxygen vacancies (OVs) and quantum dots (QDs) was proposed, and molybdenum trioxide quantum dots/carbon nanotube composites (MoO_3-x-QDs/CNT, denoted as MQT) rich in OVs were successfully constructed. The introduction of OVs effectively optimizes the electronic structure, lowers the energy barrier for Mg²⁺ ion migration, and stimulates the emergence of a voltage plateau. On the other hand, the QDs structure further shortens the diffusion path and amplifies the voltage plateau characteristics. Additionally, the in-situ composite CNT skeleton forms an efficient conductive network, enhancing structural stability during charge and discharge cycling. Benefiting from the coupling effect, the MQT electrode exhibits an obvious and stable charge-discharge voltage plateau at about ±0.4 V, which is the first time to achieve in the MoO₃-based cathode electrode, a distinct and stable charge-discharge plateau, but also exhibits excellent electrochemical properties: The reversible capacity of 312.8 mAh g⁻¹ is achieved at 200 mA g⁻¹, a stable discharge plateau appears at approximately −0.4 V. A coulombic efficiency of 99.8 % is maintained after 500 cycles at 500 mA g⁻¹. This study provides an effective approach for other TMOs materials to be used as high-performance cathodes in MIBs.