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

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(BF₄)₂ electrolyte that spontaneously generates protons is developed for ultralow-temperature copper metal batteries. Systematic studies demonstrate that the hydrolysis of BF₄⁻ generates more protons, rendering the Cu(BF₄)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.