Deciphering the Degradation Mechanisms and Realize High‐Voltage Stability of A ₃ V ₂ (PO ₄ ) ₃ (A = Li ⁺ , Na ⁺ ) in Aqueous Dual‐Ion Batteries

Abstract Polyanionic A ₃ V ₂ (PO ₄ ) ₃ (A = Li ⁺ , Na ⁺ ) with open channels have been extensively utilized as cathode materials for aqueous zinc‐metal batteries (AZMBs), whereas suffering from severe capacity fading and rapid operation voltage decay during cycling. when used as In this work, it is disclosed that the rapid degradation is induced by an irreversible phase change from electrochemical active Li ₃ V ₂ (PO ₄ ) ₃ to nonactive monoclinic LiZnPO ₄ , as well as active Na ₃ V ₂ (PO ₄ ) ₃ to nonactive rhombic Zn ₃ (PO ₄ ) ₂ (H ₂ O) ₄ . Subsequently, a rational dual‐cation (Al‐Fe) doping strategy is proposed to suppress these detrimental transformations. Such dual‐cation doping entails stronger P–O and V–O bonds, thus stabilizing the initial polyanionic structures. Consequently, the optimized member of Li ₃ V _1.775 Al _0.075 Fe _0.225 (PO ₄ ) ₃ (LVAFP) exhibits desirable cycling stability (1000 cycles, 68.5% capacity retention) and notable rate capability (92.1% of the initial capacity at 10 C). Moreover, the dual‐cation doping methodology is successfully extended to improve the stability of Na ₃ V ₂ (PO ₄ ) ₃ cathode in aqueous dual‐ion batteries, signifying the versatility and feasibility of this strategy. The comprehensive identification of local structural evolution in these polyanions will broaden the scope of designing high‐performance alkali‐vanadyl‐phosphates for multivalent aqueous batteries.