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

Abstract

Abstract 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 VOPO4 play 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.