The development of high-energy cathode materials is crucial for the commercialization of emerging K-ion batteries (KIBs). While layered potassium-transition-metal-oxides (K_x(MO_2, M=transition metals) have been investigated as potential cathodes, they have two intrinsic challenges. First, most of the K-layered oxides have K-poor composition (x<1.0 in K_xMO_2) and it leads practical difficulty of realizing KIBs because all the K ions should come from the cathode in a rocking-chair KIB. Second, K-layered oxides have too high a voltage slope, resulting in low specific capacity and average voltage. Both the problems are attributable to much stronger K~+-K~+ interaction than Na~+-Na~+ and Li~+-Li~+ in the layered oxide structure. In contrast, polyanion cathodes can be better alternatives because 3-dimensional arrangement of K ions in their frameworks can significantly reduce the strength of effective interaction between K ions. As a result, polyanion cathodes can have K-rich (or K-stoichiometric) composition and high working voltage.
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