A full flexible NH4+ ion battery based on the concentrated hydrogel electrolyte for enhanced performance.

Nonmetal ammonium  NH 4 +  ions have recently been explored as effective charge carriers in battery systems due to their abundancy, light weight, small hydration shells in water. The research concerning the use of NH 4 +  ion redox chemistry in flexible batteries, is still in its infancy. For the first time, we report a flexible full  NH 4 +  ion battery (AIB) composed of a concentrated hydrogel electrolyte sandwiched between NH4V3O8·2.9H2O nanobelts cathode and polyaniline (PANI) anode. The hydrogel electrolyte is simply synthesized by using ammonium sulfate, xanthan gum and water. As a reference, AIB based on the liquid aqueous electrolyte is prepared first, which exhibits a capacity of 121 mAh g-1  and capacity retention of 95% after 400 cycles at a specific current of 0.1 A g -1. The full flexible AIBs are then fabricated using hydrogel electrolytes with varied salt concentrations, to maximize the electrochemical performance. It is found that the battery based on the hydrogel electrolyte prepared from 3 M ammonium sulfate solution shows the best electrochemical performance i.e., a capacity of 60 mAh g-1 while maintaining a capacity retention of 88% after 250 cycles at a specific current of 0.1 A g  -1. Moreover, the flexible AIB retains excellent electrochemical performance when bent at different angles, demonstrating remarkable mechanical strength and flexibility. Therefore, this study sheds new light on the utilization of concentrated hydrogel electrolyte in the AIB chemistry, for future developments of new electrochemical energy storage technology.