A full metal-free battery operating under cold condition enabled by an antisolvent

Metal-free batteries provide a more sustainable and environmental-friendly alternative to current popular batteries using nonrenewable metals. Alcohols as anti-solvents have been used in the electrolytes in Li-ion batteries (LIBs) and zinc-ion batteries (ZIBs), as they bring new properties with regards to the solvation sheath and water activity to the electrolytes. In this work, we explore the application of methanol (CH3OH) as an antisolvent in the electrolyte in a metal-free ammonium-ion battery (AIB) for operation at low temperature, with polyaniline (PANI) as anode and polypyrrole (PPy) as cathode. The addition of CH3OH alters the solvation sheath of the NH4+ and reduces the activity of H2O by forming hydrogen bonds in the solution, thereby decreasing the freezing point of the electrolyte. Through this strategy, a lower freezing point of the electrolyte is realized without using ultrahigh concentration of salts in aqueous solution. The resulted full metal-free battery using a 4 m ammonium acetate electrolyte is able to operate at -20°C, while long cycling at subzero temperatures has never been reported for ammonium-ion batteries before. Further, the CH3OH-H2O volume ratio can be optimized to 65-35, leading to an ammonium-ion battery that exhibits a capacity of 73.23, 48.32 and 32.88 mAh/g, as well as a capacity retention of 87.79%, 90.12% and 74.33% after 100 cycles at 0.1 A/g, at 25°C, 0°C and -20°C, respectively. In this work, an antisolvent is used in non-metal ammonium-ion batteries for the first time, engendering the battery anti-freezing properties. As such, this study opens the door to the realization of novel metal-free batteries with lower cost and better performance for applications in frigid environments.