Nitrile-assistant eutectic electrolytes for cryogenic operation of lithium ion batteries at fast charges and discharges

Abstract

The charge/discharge characteristics of lithium ion batteries at low temperature (LT = -20 degrees C) 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 as L-ED) cannot support a satisfactory capacity at low temperature as well as at high rates even if electric vehicles require low-temperature operation. Introducing propionitrile or butyronitrile (Pn or Bn) into LED (resulting in L-EDPn or L-EDBn) as a co-solvent increases significantly the high-rate capacities at -20 degrees C. For example, L-EDPn delivers 62% of the available capacity at 1 C and 46% at 3 C with a 2.7 V cut-off while the control L-ED provides just 6% and 4% at the same rates. Successful operation at -20 degrees C with nitrileassistant electrolytes results from high ionic conductivity, low viscosity and freezing point depression caused by the eutectic behavior of the carbonates (EC/DMC) and Pn. Based on the phase diagram of Pn with EC/DMC, we expect a meaningful battery operation up to -110 degrees C, probably lower, at the eutectic composition.