Low-Cost High-Energy Potassium Cathode

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 size 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 K_xMnFe(CN)₆ (0 ≤ x ≤ 2) as a potassium cathode: high-spin Mn^III/Mn^II and low-spin Fe^III/Fe^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.