The layered transition metal oxides (TMOs) have been investigated as cathode materials for Li-and Na-ion batteries because of their high specific capacity and rate capability.^[1-3] In this respect, researchers have recently studied the layered TMOs as cathode materials for K-ion batteries, and they have so far exhibited only moderate specific capacity and rate capability.^[4-9] However, all the layered K-TMOs reported to date are K-deficient phases (x ≤ 0.7 in K_xTMO₂),^[4-9] which limits their use in practical rocking-chair batteries because in a typical alkali-intercalation battery system all the alkali is brought in through the cathode. The use of K-deficient phases in cathodes requires a pre-potassiation process of the electrodes in order to insert enough K in the cells. Therefore, it is vital to understand the factors that destabilize (or stabilize) the layered structure of K_xTMO₂ (x = 1) and then design a stoichiometric K_xTMO₂ (x = 1) cathode material for K-ion batteries.

In this work, we find that the strong electrostatic repulsion between K ions due to the short K⁺-K⁺ distance destabilizes the layered structure in a stoichiometric composition of KTMO₂.^[10] The stoichiometric KCrO₂ is thermodynamically stable in the layered structure despite a short K⁺-K⁺ distance unlike other KTMO₂ compounds that form non-layered structures. The unique stability of layered KCrO₂ is attributable to the unusual ligand field preference of Cr³⁺ in octahedral sites that can compensate for the energy penalty from the short K⁺-K⁺ distance. Therefore, we develop the stoichiometric layered KCrO₂ cathode material for KIBs and investigate its K-storage properties. In K-half cells, the KCrO₂ cathode delivers a reversible specific capacity of ~90 mAh/g with an average voltage of ~2.73 V (vs. K/K⁺). The practical feasibility of a KCrO₂ cathode is confirmed in a full-cell system using a graphite anode. In-situ diffraction and electrochemical characterization further demonstrate multiple phase transitions via reversible topotatic reactions occurring as the K content changes.

References

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