Reversible Ammonium Ion Intercalation/de‐intercalation with Crystal Water Promotion Effect in Layered VOPO 4 ⋅2 H 2 O**

Abstract The non‐metal NH 4 + 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 4 + ion storage in layered VOPO 4 ⋅2 H 2 O is impossible due to the removal of NH 4 + from NH 4 VOPO 4 leads to a phase change inevitably. Herein, we update this cognition and demonstrated highly reversible intercalation/de‐intercalation behavior of NH 4 + in layered VOPO 4 ⋅2 H 2 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 VOPO 4 ⋅2 H 2 O. A rocking‐chair ammonium‐ion full cell with the VOPO 4 ⋅2 H 2 O//2.0 M NH 4 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 NH 4 + ions in layered hydrated phosphates through crystal water enhancement effect.