Rational Design of Fluorinated Electrolytes for Low Temperature Lithium-Ion Batteries

Nonaqueous carbonate electrolytes are commonly used in commercial lithium-ion battery (LIB). However, the sluggish Li⁺ diffusivity and high interfacial charge transfer resistance at low temperature (LT) limit their wide adoption among geographical areas with high latitudes and altitudes. Herein, a rational design of new electrolytes is demonstrated, which can significantly improve the low temperature performance below −20 °C. This electrolyte is achieved by tailoring the chemical structure, i.e., altering the fluorination position and the degree of fluorination, of ethyl acetate solvent. It is found that fluorination adjacent to the carbonyl group or high degree of fluorination leads to a stronger electron-withdrawing effect, resulting in low atomic charge on the carbonyl oxygen solvating sites, and thus low binding energies with Li⁺ ions at LT. The optimal electrolyte 2,2,2-trifluoroethyl acetate (EA-f) shows significantly improved cycle life and C-rate of a NMC622/graphite cell when cycled at −20 °C and −40 °C, respectively. In addition to superior LT performance, the electrolyte is nonflammable and tolerant for high voltage charging all owing to its fluorine content. This work provides guidance in designing next-generation electrolytes to address the critical challenge at subzero temperatures.