Optimizing ammonium vanadate crystal structure by facile in situ phase transformation of VO₂/NH₄V₄O₁₀ with special micro-nano feature for advanced aqueous zinc ion batteries

Ammonium vanadium (NH4V4O10) is regarded as a potential cathode material for aqueous zinc ion batteries (AZIBs) and receives extensive research owing to its high theoretical capacity and large interlayer distance. However, partial NH4+ ions may be extracted from the interlayer accompanied with repeated Zn2+ insertion/extraction in the cycle process, leading to crystal structure collapse. Herein, a hydrothermal method was used to synthesize the VO2/NH4V4O10 composite first by adding excessive oxalic acid. Then, electrochemical oxidation induced the composite material to transform into NH4+-defected NH(4)V(4)O(10)in situ, improving the specific capacity, zinc ion diffusion efficiency, and crystal structure stability. The electrochemical test results demonstrate that the VO2/NH4V4O10-8.5 electrode displays a high specific capacity of 494.0 mA h g(-1) at 0.1 A g(-1). Additionally, a free-standing VO2/NH4V4O10-8.5/CNT/GN membrane electrode with 3D electron conducting network was prepared, delivering excellent rate performance with 291.2 mA h g(-1) at 10 A g(-1) and satisfactory cycle life with 71.8% retention at 5 A g(-1) after 2000 cycles. Subsequent ex situ characteristic methods were utilized to reveal the structural evolution and zinc ion storage mechanism. This work provides a new perspective to design high-performance cathode materials for AZIBs.