A full metal-free flexible ammonium-ion battery with biodegradable hydrogel electrolyte

Metal-free ammonium-ion batteries (AIBs) are emerging as viable substitutes to conventional lithium-ion batteries, due to the abundance and environmental friendliness of their base materials as well as potential for low cost. AIBs utilize ammonium cations (NH4+) as charge carriers, which can be sustainably sourced and open the door to nonmetal batteries. Herein, we report, for the first time, a full metal-free flexible battery using a polyaniline (PANI) anode and polypyrrole (PPy) cathode coupled with a quasi-solid-state (QSS) hydrogel electrolyte that is prepared with edible xanthan gum. The composition of the hydrogel is facilely tuned for maximized properties and it is found that the electrolyte synthesized with 3m concentrated (NH4)2SO4 solution endows the battery with the best performance. For comparison purposes, an aqueous AIB is also assembled, showing an initial capacity of 68.903 mA h g-1 with 89.67% retention of capacity after 100 cycles at a specific current of 0.1 A g-1. Furthermore, the QSS battery with the optimized hydrogel electrolyte exhibits an initial capacity of 44.321 mA h g-1 with 74.56% retention of capacity after 100 cycles at 0.1 A g-1. The electrochemical performances of the battery are also assessed upon mechanical deformations, such as bending and twisting. Following 20 electrochemical cycles in a flat state, the battery is then bent at 90 degrees for the next 20 charge/discharge cycles, demonstrating a capacity retention of 89%. Subsequently, while bent at 180 degrees for an additional 20 cycles, a capacity retention of 90% is achieved. Similarly, after 20 electrochemical cycles while being flat, the battery is twisted for 20 cycles, achieving a good capacity retention of 92.48%. These findings showcase the inherent qualities of the as-prepared QSS metal-free battery with good mechanical strength and flexibility along with promising electrochemical performances. This study lays the groundwork for realizing fully biodegradable metal-free flexible batteries and eventually metal-free electronics that are safer, more sustainable and more cost-effective. In this work, a full flexible metal-free battery is developed using a QSS hydrogel electrolyte sandwiched between a PANI anode and PPy cathode enabling the battery to operate and maintain its performance while bent or twisted.