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 ¼ À20 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 L ED (resulting in L EDPn or L EDBn ) as a co-solvent increases significantly the high-rate capacities at À20 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 À20 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 À110 C, probably lower, at the eutectic composition. Broader contextLithium ion batteries for electric vehicles should provide required energy within limited spaces with guaranteed safety, delivering enough power to satisfy the requirements of electric motors, be charged within a short time period so that drivers can endure and be operated in a wide range of temperature including the cryogenic range up to À20 C and more severely À30 C. The inferior charge and discharge characteristics of LIBs at low temperatures are one of the most serious problems that should be overcome by research and development. The disability of LIBs at low-temperature operations is caused by the limited ionic transport properties of the electrolyte, sluggish Li + desolvation with slow charge transfer kinetics and phase transition of the electrolyte, leading to severe ohmic and concentration polarization. In this work, we present nitrile-assistant carbonate-based eutectic electrolytes showing signicantly enhanced charge and discharge characteristics at À20 C and high rates up to 3 C. The successful operation at À20 C with the nitrile-assistant electrolytes results from high ionic conductivity, low viscosity and freezing point depression caused by the eutectic behavior of the carbonates and nitriles. Fig. 8 Comparison of discharge capacities at À20 C obtained with different cut-off voltages. Pouch full cells were discharged up to 3.0 V or 2.7 V. The discharge capacity values (Q dCh ) were normalized by the value obtained at room temperature with 0.2 C.This journal is