Microwave‐Assisted Rapid Hydrothermal Synthesis of Vanadium‐Based Cathode: Unravelling Charge Storage Mechanisms in Aqueous Zinc‐Ion Batteries

This contribution uses a rapid microwave-assisted hydrothermal synthesis method to produce a vanadium-based K_1.92Mn_0.54V₂O₅ ⋅ H₂O cathode material (quoted as KMnVOH). The electrochemical performance of KMnVOH is tested in an aqueous electrolyte, which exhibits a remarkable specific capacity of 260 mAh g^-1 at 5 C and retains 94 % of its capacity over 2000 cycles. In contrast to the aqueous electrolyte, the KMnVOH electrode tested in the organic electrolyte provides a modest discharge capacity of 60 mAh g^-1 at C/10, and the electrogravimetric analysis indicates that the charge storage mechanism is solely due to non-solvated Zn²⁺ intercalation. In aqueous electrolyte tests, Zn species insertion, interfacial pH increase, and subsequent formation of Zn_x(OH)_y(CF₃SO₃)_2x-y ⋅ nH₂O (ZHT) are supported by in-situ EQCM. Ex-situ XRD measurements also confirm the ZHT formation and its characteristic plate-like structure is observed by SEM. The ion diffusion coefficient values in aqueous and non-aqueous electrolytes are very similar according to the GITT analysis, while it is expected to be higher in aqueous electrolytes. These results may further emphasize the complex redox dynamics in the aqueous electrolyte, namely the difficulty of intercalation of bare Zn²⁺, strong Zn²⁺ solvation in the bulk electrolyte, solvent or proton intercalation, and ZHT formation.