Application of Ionic Liquid as K-Ion Electrolyte of Graphite//K2Mn[Fe(CN)6] Cell

A binary ionic liquid of K[FSA]-[C 3 C 1 pyrr][FSA] (2:8 in molar ratio) in which N -methyl- N -propylpyrrolidinium and bis(fluorosulfonyl)amide are abbreviated as [C 3 C 1 pyrr] 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 KC 8 compound in a K half-cell filled with the binary ionic liquid as electrolyte. Because the K[FSA]-[C 3 C 1 pyrr][FSA] does not corrode an Al foil current collector even with polarization of up to 6 V versus K + /K, a K 2 Mn[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//K 2 Mn[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 KPF 6 carbonate ester solution with the ionic liquid.