Lithium-ion batteries have succeeded in powering small portable electric devices due to their high energy density. However, it is still debatable whether mineral resources can meet the increasing demand associated with the growth of Li-ion into large scale energy storage systems. In this respect, K- and Na-ion batteries are considered alternative energy storage options because of the natural abundance of K and Na resources. K-ion batteries are particularly interesting because (i) potassium’s standard redox potential is lower than that of Na and is even below lithium’s potential in non-aqueous electrolytes (i. e. propylene carbonate (PC) and ethylene carbonate/diethyl carbonate electrolyte (EC/DEC)), which translates into potentially higher voltage than Na- and Li-ion batteries. (ii) The most important and interesting feature of K-ion battery system is the capability of graphite storing K ions unlike the Na-system, which attracts much attention and sparks research interest in the K-ion battery system.

The discovery of novel positive electrodes is a critical step toward realizing K-ion batteries. Many layered transition metal oxides (i. e. K_xTMO₂, TM = Co and Mn) are evaluated as cathodes for K-ion batteries, and they have so far exhibited moderate specific capacity and rate capability. However, all the layered K-TMOs reported to date have K-deficient compositions (x ≤ 0.7 in K_xTMO₂), which limits their practical use in K-ion batteries because in a typical alkali-intercalation battery system all the alkali should be brought from the cathode. In this presentation, factors that destabilize the layered structure of K_xTMO₂ (x = 1) with a stoichiometric composition will be discussed and then rationalize design principles of stoichiometric KTMO₂. 

Unfortunately, layered potassium-transition-metal-oxides (K_xMO₂, M=transition metals) have too high a voltage slope because of their strong K⁺-K⁺ interaction, which results in low specific capacity and average voltage. Therefore, it is required to develop new cathode materials that have high capacity and voltage, and thus high energy. This presentation will discuss design criteria of high energy K-cathodes.