AbstractPotassium‐ion batteries (KIBs) have attracted wide interest for energy storage because of the abundance of the electrode materials involved; however, their electrochemical performances are far behind what can be achieved from lithium‐ion batteries (LIBs) or sodium‐ion batteries (SIBs). Herein, key promising electrode and electrolyte materials for potassium‐ion batteries are identified, the coupled electrochemical reactions in the cell are investigated, and the compatibility between different materials is demonstrated to play the most important role. K2Mn[Fe(CN)6] cathode can deliver a high capacity of ≈125 mAh g−1 and exceptional cycling stability over 61 000 cycles (≈9 months) if the side reactions from the anode can be prevented. Graphite is a good anode material but is subjected to degradation in traditional carbonate electrolytes. New concentrated electrolytes are developed and evaluated. A stable KIB system is demonstrated by coupling a stable K2Mn[Fe(CN)6] cathode, a prepotassiated graphite anode with a concentrated electrolyte to achieve a high energy density of ≈260 Wh kg−1 (based on the active mass of cathode and anode) and good cycling of over 1000 cycles.