Non-Flammable Liquid and Quasi-Solid Electrolytes toward Highly-Safe Alkali Metal-Based Batteries

Rechargeable alkali metal (i.e., lithium, sodium, potassium)-based batteries are considered as vital energy storage technologies in modern society. However, the traditional liquid electrolytes applied in alkali metal-based batteries mainly consist of thermally unstable salts and highly flammable organic solvents, which trigger numerous accidents related to fire, explosion, and leakage of toxic chemicals. Therefore, exploring non-flammable electrolytes is of paramount importance for achieving safe batteries. Although replacing traditional liquid electrolytes with all-solid-state electrolytes is the ultimate way to solve the above safety issues, developing non-flammable liquid electrolytes can more directly fulfill the current needs considering the low ionic conductivities and inferior interfacial properties of existing all-solid-state electrolytes. Moreover, the electrolyte leakage concern can be further resolved by gelling non-flammable liquid electrolytes to obtain quasi-solid electrolytes. Herein, a comprehensive review of the latest progress in emerging non-flammable liquid electrolytes, including non-flammable organic liquid electrolytes, aqueous electrolytes, and deep eutectic solvent-based electrolytes is provided, and systematically introduce their flame-retardant mechanisms and electrochemical behaviors in alkali metal-based batteries. Then, the gelation techniques for preparing quasi-solid electrolytes are also summarized. Finally, the remaining challenges and future perspectives are presented. It is anticipated that this review will promote a safety improvement of alkali metal-based batteries.