A binary ionic liquid of K[FSA]-[C3C1pyrr][FSA] (2:8 in molar ratio) in which N-methyl-N-propylpyrrolidinium and bis(fluorosulfonyl)amide are abbreviated as [C3C1pyrr] and [FSA], respectively, is applied as an electrolyte of K-ion batteries for the first time. A graphite composite electrode with sodium carboxymethylcellulose as the binder undergoes reversible potassium intercalation, forming a KC8 compound in a K half-cell filled with the binary ionic liquid as electrolyte. Because the K[FSA]-[C3C1pyrr][FSA] does not corrode an Al foil current collector even with polarization of up to 6 V versus K+/K, a K2Mn[Fe(CN)6] positive electrode yields a highly reversible capacity of 134 mAh g-1 at ca. 4.0 V. We demonstrate a K-ion battery of graphite//K2Mn[Fe(CN)6] configuration with an ionic liquid electrolyte that exhibits stable charge and discharge properties over 200 cycles at room temperature. The redox performance is significantly improved by replacing the conventional KPF6 carbonate ester solution with the ionic liquid.