Comparative Study on Charge–Discharge Behavior of Graphite Positive Electrode in FSA- and FTA-Based Ionic Liquid Electrolytes with Different Alkali Metal Cations

Abstract

Dual-carbon batteries (DCBs), in which both the positive and negative electrodes are composed of carbon-based materials, are promising next-generation batteries owing to their limited usage of scarce metals and high operating voltages. In typical DCBs, metal cations and anions in the electrolytes are consumed simultaneously at the negative and positive electrodes, respectively, which can rapidly deplete the charge carrier ions in the electrolytes. In this study, to solve this challenge, we focused on ionic liquids (ILs) as DCB electrolytes because they are solely composed of ions and are therefore intrinsically highly concentrated electrolytes. Charge–discharge behavior of the graphite positive electrodes was investigated in several IL electrolytes containing alkali metal cations (Li⁺, Na⁺, and K⁺) and amide anions (FSA− and FTA−; FSA = bis(fluorosulfonyl)amide, FTA = (fluorosulfonyl)(trifluoromethylsulfonyl)amide). It was found that FTA-based ILs conferred superior cycling stability and higher capacities to graphite electrodes compared to FSA-based ILs, which was explained by the suppression of the corrosion of the aluminum current collector at high voltages. The highest reversible capacity of approximately 100 mAh g⁻¹ was obtained for the K-ion system using FTA-based ILs at 20 mA g⁻¹, which involved the formation of FTA–graphite intercalation compounds, as confirmed by ex situ X-ray diffraction.