Potassium has as rich an abundance as sodium in the earth, but the development of a K-ion battery is lagging behind because of the higher mass and larger ionic site of K+ than that of Li+ and Na+, which makes it difficult to identify a high-voltage and high capacity intercalation cathode host. Here we propose a cyanoperovskite KxMnFe(CN)(6) (0 <= x <= 2) as a potassium cathode: high-spin Mn-III/Mn-II and low-spin Fe-III/F-II couples have similar energies and exhibit two close plateaus centered at 3.6 V; two active K+ per formula unit enable a theoretical specific capacity of 156 mAh g(-1); Mn and. Fe are the two most-desired transition metals for electrodes because they are cheap and; environmental friendly. As a powder prepared by an inexpensive precipitation method, the cathode delivers a specific capacity of 142 mAh g(-1). The observed voltage, capacity, and its low cost make it competitive In large-scale electricity storage applications.