Regulating local microenvironment of frozen aqueous electrolyte with auxiliary salt for low-temperature zinc-ion hybrid capacitors

<h2>Summary</h2> The freezing of aqueous electrolytes seriously hinders aqueous energy-storage devices' low-temperature application in portable and wearable devices. Herein, going beyond the traditional strategy of preparing antifreeze electrolytes, we propose regulating the local microenvironment of frozen ZnSO₄ electrolyte with auxiliary salt to achieve normal operation even after freezing. The as-prepared frozen ZnSO₄ electrolyte achieves a high ionic conductivity of 1.07 mS cm⁻¹ even at ultralow temperatures of −50°C. Aided by low-temperature characterization techniques and theoretical simulation, the ion transport mechanism in frozen electrolyte is revealed. Furthermore, a zinc-ion hybrid capacitor using the frozen aqueous electrolyte obtains excellent electrochemical performance at −50°C. In an application demonstration, our device provides energy for a portable medical device at −50°C, demonstrating the possibility of real-time health monitoring in cold regions. This work provides mechanistic insights and opens a distinctive avenue for the design of low-temperature energy-storage devices.