Tailoring NH4+ storage by regulating oxygen defect in ammonium vanadate

Defect engineering is an effective strategy for modifying the energy storage materials to improve their electrochemical performance. However, the impact of oxygen defect and its content on the electrochemical performances in the burgeoning aqueous NH 4 + storage field remains explored. Therefore, for the first time in this work, an oxygen-defective ammonium vanadate [(NH 4 ) 2 V 10 O 25 ·8H 2 O, denoted as Od-NHVO] with a novel 3D porous flower-like architecture was achieved via the reduction of thiourea in a mild reaction condition, which is a facile method that can realize the intention to regulate the oxygen defect content, with the capability of mass-production. The as-prepared Od M -NHVO with moderate oxygen defect content can deliver a stable specific capacitance output (505 F g −1 , 252 mAh g −1 at 0.5 A g −1 with ∼80% capacitance retention after 10,000 cycles), which benefits from extra active sites, unimpeded NH 4 + -migration path and relatively high structure integrity. In contrast, low oxygen defect content will lead to the torpid electrochemical reaction kinetics while too high content of it will reduce the charge-storage capability and induce structural disintegration. The superior NH 4 + -storage behavior is achieved with the reversible intercalation/de-intercalation process of NH 4 + accompanied by forming/breaking of hydrogen bond. As expected, the assembled flexible Od M -NHVO//PTCDI quasi-solid-state hybrid supercapacitor (FQSS HSC) also exhibits high areal capacitance, energy density and reliable flexibility. This work provides a new avenue for developing materials with oxygen-deficient structure for application in various aqueous non-metal cation storage systems. • Defect engineering is employed to improve the NH 4 + -storage ability of the ammonium vanadate. • The oxygen defect content in oxygen-defective ammonium vanadate (Od-NHVO) can be controlled. • Od M -NHVO cathode with moderate oxygen defect content shows a high specific capacitance of 505 F g −1 . • Reversible storage process with forming/breaking of hydrogen bond is demonstrated. • Flexible Od M -NHVO//PTCDI quasi-solid-state HSC possesses prominent electrochemical performance and flexibility.