Adjusting oxygen vacancy of VO₂·xH₂O nanoarray architectures for efficient NH₄⁺ storage

Aqueous rechargeable batteries are the promising energy storge technology due to their safety, low cost, and environmental friendliness. Ammonium ion (NH₄⁺) is an ideal charge carrier for such batteries because of its small hydration radius and low molar mass. In this study, VO₂·xH₂O with rich oxygen defects (d-HVO) is designed and synthesized, and it exhibits unique nanoarray structure and good electrochemical performances for NH₄⁺ storge. Experimental and calculation results indicate that oxygen defects in d-HVO can enhance the conductivity and diffusion rate of NH₄⁺, leading to improved electrochemical performances. The most significant improvement is observed in d-HVO with 2 mmol thiourea (d-HVO-2) (220 mAh·g⁻¹ at 0.1 A·g⁻¹), which has a moderate defect content. A full cell is assembled using d-HVO-2 as the anode and polyaniline (PANI) as the cathode, which shows excellent cycling stability with a capacity retention rate of 80% after 1000 cycles and outstanding power density up to 4540 W·kg⁻¹. Moreover, the flexible d-HVO-2||PANI battery, based on quasi-solid electrolyte, shows excellent flexibility under different bending conditions. This study provides a new approach for designing and developing high-performance NH₄⁺ storage electrode materials.