Charge–Discharge Behavior of Graphite Negative Electrodes in FSA-Based Ionic Liquid Electrolytes: Comparative Study of Li-, Na-, K-Ion Systems

K-ion batteries utilizing ionic liquid (IL) electrolytes are promising candidates for next-generation batteries because of the abundance of potassium resources, low redox potential of potassium, and high safety of ILs. Our major interest is in the comprehensive understanding of electrochemical alkali metal intercalation/deintercalation into graphite negative electrodes, because graphite can easily form graphite intercalation compounds (GICs) with various ionic species, but not with sodium. In this study, we investigated the potassium storage mechanism of graphite negative electrodes in bis(fluorosulfonyl)amide (FSA)-based ILs, and compared the electrochemical GIC formation of Li-, Na-, and K-ion systems. Charge–discharge tests of graphite in K[FSA]–[C₃C₁pyrr][FSA] IL (C₃C₁pyrr = N-methyl-N-propylpyrrolidinium) at 313 K yielded an initial discharge capacity as high as 268 mAh (g-C)⁻¹, leading to the formation of several K-GICs including stage-3 KC₃₆, stage-2 KC₂₄, and stage-1 KC₈. The rate capability and long-term cycling tests indicated stable potassiation/depotassiation behavior for 225 cycles. A comparison of the electrochemical behavior of graphite among M[FSA]–[C₃C₁pyrr][FSA] (M = Li, Na, and K) ILs at 298 K indicated that the formation of binary M-GICs is localized in the potential range below −2.85 V vs. Fc⁺/Fc (Fc = ferrocene), which possibly hinders Na-GIC formation