Synergistic structure engineering and electrochemical activation modulating vanadium oxide cathode toward superior zinc-ion storage

Aqueous zinc-ion batteries (ZIBs) have emerged as competitive systems for grid-scale energy storage due to their high safety and low cost. However, the lack of suitable high-performance cathode composites limits the practical progress of ZIBs. Herein, a novel cathode material, ammonium cation-inserted and oxygen vacancy co-modulated VO2 (named NVE), was firstly synthesized through the ethylene glycol (EG) sacrificial solvent-assisted structure transformation with the ammonium vanadate (named NV) as the precursor. The sacrificial solvent promotes the structural transformation of ammonium vanadate to NH4+-intercalated vanadium oxide and enhances the number of oxygen vacancies in the resulting material. Moreover, the electrochemical activation process was performed for in-situ construction of NH4+-inserted hydrated vanadium oxide (V2O5·nH2O) material. The electrochemical activation process with high anodic voltage enables multiple electron reactions, increasing the utilization of vanadium elements and achieving high capacity. Additionally, the formed hydrogen bonds between the V-O host and inserted NH4+ ions can enhance the structural integrity, while the oxygen vacancies decrease the interaction between the V-O host and inserted Zn2+ ions to boost ion diffusion. Consequently, the resulting activated cathode demonstrates higher capacity (441 mAh/g at 0.1 A/g), superior cycling durability (85.6 % retention over 4000 cycles), and exceptional rate capability (205 mAh/g at 20 A/g). Besides, the fabricated devices based on the activated NVE cathode show decent capacity and excellent flexible stability. Furthermore, the reversible electrochemical Zn2+ storage mechanism upon battery cycling was evaluated by several kinetic measurements and in/ex-situ characterizations. This work provides novel perspectives for the fabrication of advanced cathode materials for superior aqueous ZIBs.