Surface-redox pseudocapacitive charge storage mechanism based on quantum size effect enabling stable and fast magnesium batteries

The all-phenyl complex (APC) electrolyte, which is considered to have excellent compatibility with the magnesium metal anode. Nevertheless, the high breaking energy of the Mg-Cl bond and the sluggish diffusion rate of Mg²⁺ in the cathode, results in poor kinetics and rapid failure of the electrode. Herein, the dual synergistic effect of quantum dots (QDs) and oxygen vacancies (OVs) is proposed to change the cathode/electrolyte interface behavior of MoO₃ for Mg²⁺ storage, and realize the co-rapid reversible storage of MgCl⁺-dominant/Mg²⁺. The rapid storage of MgCl⁺ is enabled by the surface-redox pseudocapacitive charge storage mechanism based on quantum size effect, avoiding the high breaking energy of Mg-Cl bond. On the other hand, the introduction of OVs increases the intrinsic conductivity of the material, reduces the migration energy barrier of Mg²⁺ and enhances the adsorption capacity of MgCl⁺. The in-situ/ex-situ characterizations demonstrate the structural stability of MoO_3-x QDs/reduced graphene oxide (MQR) composite and the rapid reversible storage of MgCl⁺-dominant with a small amount of Mg²⁺. Therefore, the MQR electrode has high reversible capacity (discharge capacity of 118.6 mAh g⁻¹ at 0.1 A g⁻¹), excellent rate capacity (discharge capacity of 53.4 mAh g⁻¹ at 3.0 A g⁻¹) and long cycle stability (109.5 mAh g⁻¹ after 3000 cycles at 0.5 A g⁻¹) in RMBs. This research provides a novel idea for the design of transition metal oxides as magnesium energy storage cathode material.