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

Abstract

AbstractThe 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 H2O is impossible due to the removal of NH4+ from NH4VOPO4 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 H2O 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 H2O. A rocking‐chair ammonium‐ion full cell with the VOPO4⋅2 H2O//2.0 M NH4OTf//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.