Electrochemical Intercalation of Cesium into Graphite in Ionic Liquid Electrolyte (Supporting Information)

Graphite is widely used as the negative electrode for alkali-metal ion secondary batteries because of its ability to accommodate various ions between its graphene layers resulting in the formation of graphite intercalation compounds (GICs). In this study, we investigated the intercalation of Cs⁺ ions into graphite using an ionic liquid (IL)-based electrolyte, Cs[FTA]–[C₄C₁pyrr][FTA] (FTA = (fluorosulfonyl)(trifluoromethylsulfonyl)amide, C₄C₁pyrr = N-butyl-N-methylpyrrolidinium). In this electrolyte, the graphite negative electrode imparted an initial reversible capacity of 180 mAh g⁻¹ at 298 K. The formation of Cs-GICs was analyzed using an X-ray diffraction technique to reveal the formation of stage-1 CsC₈. The formation potentials of various alkali metal GICs were also compared with respect to the potential of ferrocenium/ferrocene redox couple, revealing that these GICs particularly for stage-1 compounds form in a similar potential range with small differences of 0.2–0.3 V.