A High‐Rate Aqueous Proton Battery Delivering Power Below −78 °C via an Unfrozen Phosphoric Acid

The sluggish ion diffusion and electrolyte freezing with volumetric changes limit the low‐temperature performance of rechargeable batteries. Herein, a high‐rate aqueous proton battery (APB) operated at and below −78 °C via a 62 wt% (9.5 m) H 3 PO 4 electrolyte is reported. The APB is a rocking‐chair battery that operates with protons commuting between a Prussian blue cathode and an MoO 3 anode. At −78 °C, the APB full cells exhibit stable cycle life for 450 cycles, high round‐trip efficiency of 85%, and appreciable power performance. The APB delivers 30% of its room‐temperature capacity even at −88 °C. The proton storage mechanism is investigated by ex situ synchrotron XRD, XAS, and XPS. The APB pouch cells demonstrate no capacity fading at −78 °C, and thus offers a safe and reliable candidate for high‐latitude applications.