Reversible ammonium ion intercalation/de-intercalation with crystal water promotion effect in layered VOPO${}_4$⋅2 H${}_2$O

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. (© 2023 Wiley‐VCH GmbH)