Charge–Discharge Performance of Positive Electrodes for Fluoride-Shuttle Batteries Using Fluorohydrogenate Ionic Liquid Electrolytes

Abstract

1. Introduction Fluoride-shuttle batteries (FSBs) are attracting increasing attention because they possess superior energy densities compared to conventional lithium-ion batteries. In the past decade, fluoride-ion conducting solid electrolytes have been widely studied for FSBs [1,2]. However, most of them require operating temperature higher than 373 K. To decrease the operating temperature, ether-based organic solvents and ionic liquids were recently applied to FSBs [3–5]. We have focused on fluorohydrogenate ionic liquids (FHILs) for FSBs because FHILs exhibit ionic conductivities much higher than conventional liquid electrolytes. For example, [C2C1im][(FH)2.3F] (C2C1im = 1-ethyl-3-methylimidazolium) shows an ionic conductivity as high as 100 mS cm−1 at room temperature (298 K) [6,7]. We have already reported charge–discharge behaviors of CuF2 electrodes in this IL [8]. In the present study, charge–discharge behaviors of copper-based positive electrodes were further investigated in several FHILs at room temperature. 2. Experimental All experiments were conducted at room temperature (ca. 298 K). Three-electrode cells were used for electrochemical measurements. The working electrode was composed of the active material (CuF2 or metallic copper), acetylene black as a conductive agent, and polytetrafluoroethylene as a binder. Both the counter and reference electrodes were CuF2/Cu electrodes. Two FHILs, [C2C1im][(FH)2.3F] and [C2C1pyrr][(FH)2.3F] (C2C1pyrr = N-ethyl-N-methylpyrrolidinium), were used as electrolytes. Cyclic voltammetry was performed at 10 mV s−1 before charge–discharge tests. The working electrodes after the tests were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS). Prior to these analyses, the electrolytes were removed by washing with dehydrated ethanol, and the samples were transferred to all the analytical instruments without air exposure. The solubility of CuF2 was measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The solution for the ICP-AES measurement was prepared by immersion and stirring of excess CuF2 particles in each FHIL for 1 day. 3. Results and Discussion Fig. 1 shows charge–discharge curves of a metallic copper electrode in [C2C1im][(FH)2.3F] electrolyte at a rate of 0.05C (= 42.2 mA (g-Cu)−1). The initial charge and discharge capacities are 439 and 599 mAh (g-Cu)−1, respectively. The results of XRD and XPS analyses suggested the reversible reactions during charge–discharge cycling.Cu + 6[(FH)2F]− ⇌ CuF2 + 4[(FH)3F]− + 2e− (1)The discharge capacity larger than the charge capacity in the initial cycle may be attributed to the presence of oxide film on the surface of copper particles. The discharge capacity rapidly decreases after the 2nd cycle, and reaches 167 mAh (g-Cu)−1 at the 20th cycle, which corresponds to the capacity retention ratio of 28%. The SEM observation indicates that this steep decline in reversible capacities is likely ascribed to aggregation of active materials.Concerning [C2C1pyrr][(FH)2.3F] electrolyte, the same charge–discharge test was performed at 0.05C rate. Fig. 2 summarizes the cycling properties of discharge capacities in the two electrolytes. Initial discharge capacity obtained in [C2C1pyrr][(FH)2.3F] is 400 mAh (g-Cu)−1, which is lower than that in [C2C1im][(FH)2.3F]. However, the discharge capacity is 210 mAh (g-Cu)−1 at the 20th cycle, exhibiting the improved capacity retention of 53%. This improved cycleability is probably originated from the lower solubility of CuF2 formed in the charging reaction. According to the ICP-AES results, the solubilities of CuF2 were ca. 100 and 20 ppm for [C2C1im][(FH)2.3F] and [C2C1pyrr][(FH)2.3F], respectively. Due to the higher solubility in [C2C1im][(FH)2.3F], the dissolution and reprecipitation of CuF2 occur more severely on the surface of the working electrode, leading to more intense aggregation of the active material. The utilization of [C2C1pyrr][(FH)2.3F] mitigates such degradation of the electrode. Acknowledgments This study is based on results obtained from a project, "Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING2)", JPNP16001, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).