Highly concentrated electrolyte solutions for 4 V class potassium-ion batteries.

Stable cycling of a 4 V-class potassium-ion battery is demonstrated with a highly concentrated potassium bis(fluorosulfonyl)amide 1,2-dimethoxyethane solution as an electrolyte. Not only graphite and K2Mn[Fe(CN)6] half cells but also graphite//K2Mn[Fe(CN)6] full cells filled with the electrolyte exhibit higher coulombic efficiency and better cyclability than those of KPF6/carbonate ester solutions.