Reversible Ammonium Ion Intercalation/de-intercalation with Crystal Water Promotion Effect in Layered VOPO ⋅2 H O.

The non-metal NH carrier has attracted tremendous interests for aqueous energy storage owing to its light molar mass and fast diffusion in aqueous electrolytes. Previous study inferred that NH ion storage in layered VOPO ⋅2 H O is impossible due to the removal of NH from NH VOPO leads to a phase change inevitably. Herein, we update this cognition and demonstrated highly reversible intercalation/de-intercalation behavior of NH in layered VOPO ⋅2 H O host. Satisfactory specific capacity of 154.6 mAh g at 0.1 A g and very stable discharge potential plateau at 0.4 V based on reference electrode was achieved in VOPO ⋅2 H O. A rocking-chair ammonium-ion full cell with the VOPO ⋅2 H O//2.0 M NH OTf//PTCDI configuration exhibited a specific capacity of 55 mAh g , an average operating voltage of about 1.0 V and excellent long-term cycling stability over 500 cycles with a coulombic efficiency of ≈99 %. Theoretical DFT calculations suggest a unique crystal water substitution process by ammonium ion during the intercalation process. Our results provide new insight into the intercalation/de-intercalation of NH ions in layered hydrated phosphates through crystal water enhancement effect.