Solvent-regulated growth and electronic structure evolution of (NH₄)₂V₄O₉ induced by ethylene glycol for durable aqueous zinc-ion batteries

Aqueous zinc-ion batteries (AZIBs) have attracted considerable attention as competitive systems for sustainable energy storage. Ammonium vanadate, recognized as a promising cathode material for AZIBs, demonstrates outstanding electrochemical performance; Nevertheless, it is hindered by inherent drawbacks, including structural instability, fast capacity decay, and slow reaction kinetics. In this work, a solvent-regulation strategy is developed, in which ethylene glycol (EG) is introduced to modulate the thermodynamic and kinetic evolution of the hydrothermal reaction, thus enabling the controllable preparation of a series of (NH₄)₂V₄O₉ (NVO) cathode materials. By varying the H₂O/EG volume ratio, NVO with diverse morphologies is obtained. Owing to the mild reducing ability and hydroxyl functional groups of EG, the vanadium valence-state distribution and oxygen coordination environment of NVO are effectively regulated. These characteristics, together with the enlarged specific surface area arising from the tailored morphology, jointly account for the improved electrochemical performance of the NVO cathodes. Notably, when the H₂O/EG volume ratio is adjusted to 8:2, the resulting NVO-82 cathode achieves a high specific capacity of 478.7 mAh g^-1 at 0.1 A g^-1. It retains 182.6 mAh g^-1 after 5000 cycles at 5 A g^-1, highlighting its remarkable electrochemical performance.