Potassium metal electrochemical cells are widely utilized to examine potassium insertion materials for nonaqueous potassium-ion batteries. However, large polarization during K plating–stripping and unstable rest potential are found at the potassium electrodes, which leads to an underestimation of the electrochemical performance of insertion materials. In this study, the electrochemical behavior of K-metal electrodes is systematically investigated. Electrolyte salts, solvents, and additives influence the polarization of K metals. Although a highly concentrated electrolyte of 3.9 M KN­(SO2F)2/1,2-dimethoxyethane realizes the smallest polarization of 25 mV among all the electrolytes investigated in this study, the polarization of K metals is still larger than those of Li and Na metals. The issue of inaccurate rest potential is solved by pretreating the K electrodes with a plating–stripping process, which is essential in evaluating the intrinsic electrode performance of potassium insertion materials.

Hard Optimal carbonization of glucose, sucrose, maltose, cellulose, glycogen, and amylopectin is studied for Na-ion application.

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.

Graphite is commonly known as a layered material to accommodate alkali metal ions between the graphene layers and is used as a negative electrode material of most Li-ion batteries and...