Ammonium vanadium oxide framework with stable NH4+ aqueous storage for flexible quasi-solid-state supercapacitor

Aqueous energy storage systems show great promise because of their merits of high safety, low cost and environmentally friendliness. Huge efforts, in recent years, have been focused on these systems employing metal cations as charge carriers. In aqueous rechargeable batteries, a paradigm shift has recently moved to nonmetal ions as charge carriers (e.g.: H+, NH4+). Herein, we report an ammonium vanadium oxide framework (NVO) displaying excellent NH4+ storages. The capacitance and cycle performance of NVO are greatly improved using NH4Cl/PVA gel electrolyte. In three-electrode configuration, at 0.5 A g−1, the NVO electrode delivers a specific capacitance of 339 F·g−1 (610 C·g−1, 169 mAh·g−1) in NH4Cl/PVA electrolyte, which is much higher than the value of 268 F·g−1 (483 C·g−1, 134 mAh·g−1) in NH4Cl electrolyte. It shows 71% capacitance retention after 14,000 cycles in NH4Cl/PVA electrolyte. The intercalation pseudocapacitance and double layer capacitance are contributed to the charge storage mechanism of NVO. The flexible quasi-solid-state NVO//AC hybrid supercapacitor is assembled and displays outstanding electrochemical performances with high capacitance, long lifespan and practical applications with good mechanical stability and flexibility in various bending states and excellent series and parallel performance. This work not only demonstrates that NVO has great potential for NH4+ storages, but also sheds a light on NH4Cl/PVA electrolyte for flexible aqueous NH4+-supercapacitors.