Reversible Ammonium Ion Intercalation/de-intercalation with Crystal Water Promotion Effect in Layered VOPO4•2H2O

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 VOPO(4 center dot)2H(2)O is impossible due to the removal of NH4 (+) from NH(4)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 center dot 2H(2)O host. Satisfactory specific capacity of 154.6 mAhg 1 at 0.1 Ag 1 and very stable discharge potential plateau at 0.4 V based on reference electrode was achieved in VOPO4 center dot 2H(2)O. A rocking-chair ammonium-ion full cell with the VOPO(4 center dot)2H(2)O//2.0 M NH4OTf//PTCDI configuration exhibited a specific capacity of 55 mAhg (-1), an average operating voltage of about 1.0 V and excellent long-term cycling stability over 500 cycles with a coulombic efficiency of approximate to 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/deintercalation of NH4+ ions in layered hydrated phosphates through crystal water enhancement effect.

Automated summary

The non-metal NH4+ carrier is attracting a lot of interest for aqueous energy storage due to its light molar mass and fast diffusion in aqueous electrolytes. Previous studies suggested that NH4+ ion storage in layered VOPO4 center dot 2H2O was impossible due to a phase change caused by the removal of NH4+ from NH4VOPO4. However, this belief has been updated and highly reversible intercalation/de-intercalation behavior of NH4+ in layered VOPO4 center dot 2H2O has been demonstrated. The results provide new insight into the intercalation/deintercalation of NH4+ ions in layered hydrated phosphates through crystal water enhancement effect.