Coupling abundant active sites and an ultra-short ion diffusion path: R–VO ₂ /carbon nanotube composite microspheres boosted the performance of aqueous ammonium-ion batteries

Ammonium (NH₄ ⁺) ions as charge carriers have exhibited tremendous potential in aqueous batteries because of their abundant resources, ultrafast reaction kinetics, and negligible dendrite risks. However, the choices of cathode materials have resulted in relatively low capacities in aqueous ammonium ion batteries (AAIBs). Herein, double tunnel NH₄ ⁺ ion insertion behavior with hydrogen bond building-breaking in rutile-phase VO₂ (R-VO₂) microspheres was revealed for the first time, and the capacity contribution was confirmed to be dominated by surface-control according to kinetic analysis and density functional theory (DFT) calculations. Composite microspheres with R-VO₂ and carbon nanotubes (R-VO₂/CNTs) were acquired to comprehensively improve the capacity, rate performance, and cycling stability of R-VO₂ microspheres. In addition, R-VO₂/CNT composite microspheres exhibited excellent capacity (950 mAh g^-1) within -1.3-0.8 V at 0.05 A g^-1, which was maintained at 170 mAh g^-1 at 5 A g^-1, and achieved excellent capacity retention of 113% at the 5000th cycle and 0-0.4 V. To explore their practical application, a full cell was constructed by coupling a R-VO₂/CNT composite microsphere cathode with a urea-perylene diimide polymer (UP) anode. Excellent capacity (130 mAh g^-1) with imperceptible capacity decay following 2500 cycles at 1 A g^-1 was achieved within the 0-0.9 V practical voltage range. In summary, R-VO₂/CNT composite microspheres have been demonstrated for potential application in sustainable energy storage in AAIBs.