A Non‐Volatile, Thermo‐Reversible, and Self‐Protective Gel Electrolyte Providing Highly Precise and Reversible Thermal Protection for Lithium Batteries

Abstract The safety issue represents a long‐standing obstacle that retards large‐scale applications of high‐energy lithium batteries. Among different causes, thermal runaway is the most prominent one. To date, various approaches have been proposed to inhibit thermal runaway; however, they suffer from some intrinsic drawbacks, either being irreversible (one‐time protection), using volatile and flammable electrolytes, or delayed thermal protection (140–150 °C). Herein, this work exploits a non‐volatile, non‐flammable, and thermo‐reversible polymer/ionic liquid gel electrolyte as a built‐in safety switch, which provides highly precise and reversible thermal protection for lithium batteries. At high temperature, the gel electrolyte experiences phase separation and deposits polymer on the electrode surfaces/separators, which blocks Li + insertion reactions and thus prevents thermal runaway. When the temperature decreases, the gel electrolyte restores its original properties and battery performance resumes. Notably, the optimal protection effect is achieved at 110 °C, which is the critical temperature right before thermal runaway. More importantly, such a thermal‐protection process can repeat multiple times without compromising the battery performance, indicating extraordinary thermal reversibility. To the authors' knowledge, such a precise and reversible protection effect has never been reported in any electrolyte systems, and this work opens an exciting avenue for safe operation of high‐energy Li batteries.