Nitrile-Assistant Eutectic Electrolytes for Cryogenic Operation of Lithium-Ion Batteries at Fast Charges and Discharges

The charge/discharge characteristics at low temperature (LT = -20 ^oC) are enhanced by using ethylene carbonate (EC)-based electrolytes with the help of assistant solvents of nitriles. Conventional liquid electrolytes (e.g. a mixture of EC and dimethyl carbonate (DMC), abbreviated by L_ED) cannot support satisfactory capacity at the low temperature as well as high rates even if electric vehicles require low temperature operation. Introducing propionitrile or butyronitrile (Pn or Bn) into L_ED (resulting in L_EDPn or L_EDBn) as a co-solvent increases significantly the high-rate capacities at -20 ^oC. For example, L_EDPn delivers 62 % of available capacity at 1C and 46 % at 3C with 2.7 V cut-off while the control L_ED provides just 6 % and 4 % at the same rates. The successful operation at -20 ^oC with the nitrile-assistant electrolytes results from high ionic conductivity, low viscosity and freezing point depression caused by eutectic behavior of carbonates (EC/DMC) and Pn. Based on phase diagram of Pn with EC/DMC, we expect meaningful battery operation up to -110 ^oC, probably lower, at the eutectic composition.