Stable Discharge Plateau Induced by Reversible Ammonium Ion Intercalation in Layered VOPO4·2H2O

The non-metal ammonium (NH₄⁺) ion carrier has attracted tremendous interests for aqueous energy storage owing to the light molar mass and small hydrated ionic size. Nevertheless, the NH₄⁺storage is severely challenged by that the as-reported cathode materials generally fail to satisfy the requirements on high capacity and stable working potential simultaneously up to date. Herein, we demonstrated that VOPO₄∙2H₂O with a layered framework and two-dimensional channels can reversibly host NH₄⁺ion with a high specific capacity of 148.6 mAh g^{− 1}at 0.1 A g^{− 1}. More importantly, very stable discharge potential plateau at 0.4 V was achieved, which comes from the reversible intercalation/de-intercalation of NH₄⁺in the interlayer spacing followed by an alternated stacking configuration. We revealed the crystal water molecules in the interlayer of VOPO₄play a critical role in the NH₄⁺ion storage performance. Theoretical DFT calculations suggest a unique crystal water substitution process by ammonium ion during the intercalation process. Our results realize the first inorganic compound with very stable working voltage for NH4⁺storage, and also contributes the fundamental understanding of the intercalation/de-intercalation of NH4⁺ions in layered hydrated phosphates for clean energy storage.