Freeze‐Vulnerable Plastic Crystal to Cryogenic Liquid Electrolyte for Lithium Batteries by Deep Freezing‐Point Depression

Low-T_f solvents (T_f = freezing point) are considered and employed for low-temperature lithium-ion battery (LIB) electrolytes to keep electrolytes in the liquid phase at low temperatures. Unfortunately, T_f is synchronized with T_b (boiling point) so low T_f brings T_b down and therefore discourages the thermal stability of electrolytes using low-T_f solvents. In this work, 1) the hot wing of LIB-working temperature by employing a high-T_b (inevitably high-T_f) solvent and 2) the cold wing by using a significant T_f depression is secured. Sulfolane is employed as the high-T_f (therefore, high-T_b) and high-K_f (K_f = cryoscopic constant) solvent since its mesomorphic state between solid and liquid. That abnormally and significantly decreases the enthalpy of fusion, and resultantly grants extremely high K_f at 66.4 K m^-1. By employing sulfolane with 2 m lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), the liquid-phase temperature window down to <-80 °C for the cold wing and simultaneously guaranteed its flash point at >+150 °C for the hot wing is successfully extended. LIB cells with lithium iron phosphate and lithium metal worked in a good stand with 2 m LiTFSI/sulfolane at room temperature, -30 °C as an ambient cold, -74 °C as a deep cold, and +80 °C as a deep hot.