Stoichiometric Layered Potassium Transition Metal Oxide for Rechargeable Potassium Batteries

K-ion batteries are promising alternative energy storage systems for large-scale applications because of the globally abundant K reserves. K-ion batteries benefit from the lower standard redox potential of K/K<sup>+</sup> than that of Na/Na<sup>+</sup> and even Li/Li<sup>+</sup>, which can translate into a higher working voltage. Stable KC<sub>8</sub> can also be formed via K intercalation into a graphite anode, which contrasts with the thermodynamically unfavorable Na intercalation into graphite, making graphite a readily available anode for K-ion battery technology. However, to construct practical rocking-chair K-ion batteries, an appropriate cathode material that can accommodate reversible K release and storage is still needed. We show that stoichiometric KCrO<sub>2</sub> with a layered O<sub>3</sub>-type structure can function as a cathode for K-ion batteries and demonstrate a practical rocking-chair K-ion battery. In situ X-ray diffraction and electrochemical titration demonstrate that KxCrO<sub>2</sub> is stable for a wide K content, allowing for topotactic K extraction and reinsertion. We further explain why stoichiometric KCrO<sub>2</sub> is unique in forming the layered structure unlike other stoichiometric K-transition metal oxide compounds, which form nonlayered structures; this fundamental understanding provides insight for the future design of other layered cathodes for K-ion batteries.