Rational design of anti-freezing electrolytes for extremely low-temperature aqueous batteries

Designing anti-freezing electrolytes through choosing suitable H2O–solute systems is crucial for low-temperature aqueous batteries (LTABs). However, the lack of an effective guideline for choosing H2O–solute systems based on decisive temperature-limiting factors hinders the development of LTABs. Here we identified two decisive factors: thermodynamic eutectic temperature (Te) and kinetic glass-transition temperature (Tg), with Tg being applicable for LTABs only when H2O–solute systems have strong super-cooling ability. We proposed a general strategy wherein low-Te and strong-super-cooling ability electrolytes can be realized by creating multiple-solute systems via introducing assisted salts with high ionic-potential cations (for example, Al3+, Ca2+) or cosolvents with high donor numbers (for example, ethylene glycol). As a demonstration in Na-based systems, we designed electrolytes with ultralow Te (−53.5 to −72.6 °C) and Tg (−86.1 to −117.1 °C), showcasing battery performances including 80 Wh kg−1 and 5,000 cycles at 25 °C, and 12.5 Wh kg−1 at −85 °C. The work provides effective guidelines for the design of anti-freezing electrolytes for extremely low-temperature applications.