Design of Layered Potassium Transition Metal Oxide Cathodes for K-Ion Batteries
Authors: Kim Haegyeom , Seo Dong-Hwa , Urban Alexander , Lee Jinhyuk , Kwon Deok-Hwang , Bo Shou-Hang , Shi Tan , Papp Joseph K , McCloskey Bryan D. , Ceder Gerbrand 
Published: February 2020

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
x
TMO2),[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
x
TMO2 (x = 1) and then design a stoichiometric K
x
TMO2 (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.

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