Reversible Ammonium Ion Intercalation/de‐intercalation with Crystal Water Promotion Effect in Layered VOPO4⋅2 H2O**

The non-metal NH4+ 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 NH4+ ion storage in layered VOPO4 ⋅2 H2 O is impossible due to the removal of NH4+ from NH4 VOPO4 leads to a phase change inevitably. Herein, we update this cognition and demonstrated highly reversible intercalation/de-intercalation behavior of NH4+ in layered VOPO4 ⋅2 H2 O host. Satisfactory specific capacity of 154.6 mAh g-1 at 0.1 A g-1 and very stable discharge potential plateau at 0.4 V based on reference electrode was achieved in VOPO4 ⋅2 H2 O. A rocking-chair ammonium-ion full cell with the VOPO4 ⋅2 H2 O//2.0 M NH4 OTf//PTCDI configuration exhibited a specific capacity of 55 mAh g-1 , 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 NH4+ ions in layered hydrated phosphates through crystal water enhancement effect.