Charge–Discharge Performance of Copper Metal Positive Electrodes in Fluorohydrogenate Ionic Liquids for Fluoride-Shuttle Batteries

Abstract

In search of room-temperature electrolytes for fluoride-shuttle batteries, fluorohydrogenate ionic liquids (FHILs) have emerged, showing high ionic conductivities and better operational practicality. To enhance the performance of these electrolytes, the charge–discharge behavior of copper metal as positive electrodes in FHILs was investigated in this study. In the [C2C1im][(FH)2.3F] (C2C1im = 1-ethyl-3-methylimidazolium) FHIL electrolyte, although the 1st discharge capacity of 599 mAh (g-Cu)−1 included the reductive reaction of surface oxide films, the 2nd discharge capacity of 444 mAh (g-Cu)−1 that corresponds to 53% of the theoretical capacity was achieved. However, the capacity declines to 167 mAh (g-Cu)−1 at the 20th cycle, indicating low capacity retention. In contrast, the adoption of [C2C1pyrr][(FH)2.3F] (C2C1pyrr = N-ethyl-N-methylpyrrolidinium) electrolyte confers improved cycleability across the cycles with a higher discharge capacity of 210 mAh (g-Cu)−1 at the 20th cycle. Scanning electron microscopy and energy-dispersive X-ray spectroscopy performed on the electrode surfaces confirm reduced electrode degradation characterized by suppressed aggregation of copper particles in [C2C1pyrr][(FH)2.3F] due to its low CuF2 solubility compared with [C2C1im][(FH)2.3F]. Herein, we demonstrate the use of FHILs with low CuF2 solubilities as a strategy for improving the charge–discharge performance of copper metal positive electrodes in fluoride-shuttle batteries.