Low-Melting-Point Ionic Liquid Electrolyte for an Intermediate-Temperature Sodium-Copper Chloride Battery

Sodium-metal chloride batteries are usually operated at a relatively high temperature (270-350 °C) to achieve adequate electrochemical performance. Such a high operating temperature may cause several issues and limit their widespread applications. Lowering the operating temperature may alleviate these issues, which can be achieved by reducing the ohmic resistance of a Na-β″-Al2O3 solid electrolyte (BASE) and incorporating a low-melting-point catholyte. Herein, a planar sodium-copper chloride battery is evaluated at intermediate temperatures (from 100 to 175 °C) with a thin BASE disk (500 μm) and a room-Temperature ionic liquid (RTIL). The RTILs at various concentrations (0.1-0.75 mol L-1) are prepared by dissolving different amounts of sodium bis(trifluoromethanesulfon)imide (NaTFSI) into 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][TFSI]) and used as the catholyte. The effect of NaTFSI addition in [EMIm][TFSI] on the physical properties of the catholyte is examined. With 0.5 mol L-1 NaTFSI in [EMIm][TFSI] ionic liquid to assemble the Na/CuCl2 battery, the fabricated cell delivers a capacity up to 146.5 mAh g-1 when cycled at 175 °C with the current density of 2 mA cm-2 and retains 94.5% capacity after 20 cycles. Moreover, the battery can run steadily at 130 °C and show a reversible capacity of 79.2 mAh g-1 when the operation temperature is set as low as 100 °C. To the best of our knowledge, these are the lowest operation temperatures reported thus far for sodium-metal-chloride-based cells.