Spontaneous Proton Chemistry Enables Ultralow‐temperature and Long‐life Aqueous Copper Metal Batteries

Abstract Aqueous copper metal batteries with acidic electrolytes are regarded as promising candidates for low‐temperature energy storage, benefiting from fast kinetics of protons and acid resistance of copper. Here, a Cu(BF 4 ) 2 electrolyte that spontaneously generates protons is developed for ultralow‐temperature copper metal batteries. Systematic studies demonstrate that the hydrolysis of BF 4 − generates more protons, rendering the Cu(BF 4 ) 2 among the most effective aqueous electrolyte capable of breaking hydrogen bonds in water molecules. This electrolyte endows a polyaniline/Cu battery to deliver a short charging time of 21 s and a charge/discharge capability of up to 10 A g −1 at −30 °C, along with a high discharge specific capacity of 70 mAh g −1 and a supercapacitor‐comparable power density of 3000 W kg −1 . Furthermore, it can exhibit a long and stable cycling lifespan over 10 000 cycles at −50 °C and works well at −70 °C. This work provides an opportunity for intrinsically acidic electrolytes.