Reversible Ammonium Ion Intercalation/de-intercalation with Crystal Water Promotion Effect in Layered VOPO₄ ⋅2 H₂ O

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.