Enhanced dynamics of Al3+/H+ ions in aqueous aluminum ion batteries: Construction of metastable structures in vanadium pentoxide upon oxygen vacancies

In recent years, aqueous aluminum ion batteries have been widely studied owing to their abundant energy storage and high theoretical capacity. An in-depth study of vanadium oxide materials is necessary to address the precipitation of insoluble products covered cathode surface and the slow reaction kinetics. Therefore, a method using a simple one-step hydrothermal preparation and oxalic acid to regulate oxygen vacancies has been reported. A high starting capacity (400 mAh g⁻¹) can be achieved by O_vV₂O₅, and it is capable of undergoing 200 cycles at 0.4 A g⁻¹, with a termination discharge capacity of 103 mAh g⁻¹. Mechanism analysis demonstrated that metastable structures (Al_xV₂O₅ and H_xV₂O₅) were constructed through the insertion of Al³⁺/H⁺ during discharging, which existed in the lattice intercalation with V₂O₅. The incorporation of oxygen vacancies lowers the reaction energy barrier while improving the ion transport efficiency. In addition, the metastable structure allows the electrostatic interaction between Al³⁺ and the main backbone to establish protection and optimize the transport channel. In parallel, this work exploits ex-situ characterization and DFT to obtain a profound insight into the instrumental effect of oxygen vacancies in the construction of metastable structures during in-situ electrochemical activation, with a view to better understanding the mechanism of the synergistic participation of Al³⁺ and H⁺ in the reaction. This work not only reports a method for cathode materials to modulate oxygen vacancies, but also lays the foundation for a deeper understanding of the metastable structure of vanadium oxides.