An intermediate temperature sodium copper chloride battery using ionic liquid electrolyte and its degradation mechanism

Sodium metal chloride batteries possessing many merits, such as high energy density and long cycle life, are usually operated above 300 °C. Such high operating temperature may accelerate corrosion and aging, increase operating complexity, require an extra thermal management system, and limit their widespread applications. Lowering the working temperature may alleviate these issues and broaden their usage. Herein, a sodium copper chloride battery running at 175 °C is designed with the room temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide dissolved with sodium trifluoromethanesulfonate, to replace sodium chloride saturated sodium tetrachloroaluminate as the catholyte. The cathode delivers the high specific capacity of 141.4 mAh g⁻¹ and the high energy density of 374.7 Wh kg⁻¹. In addition, the capacity retention reaches 92.1% after 50 cycles with an average coulombic efficiency as high as 99.6%. The examination of the cathode and solid electrolyte collected after 50 cycles shows that the degradation mechanism of the battery is attributed to (1) the accumulation of a large amount of non-conductive copper chloride in the three dimensional network structure of the copper foam and (2) the loss of β″-alumina in the solid electrolyte during the charge/discharge process.