P′3-type Na0.52CrO2 is proposed as a viable cathode material for potassium-ion batteries (KIBs). The in-situ-generated title compound during the first charge of O3-NaCrO2 in K+-containing electrolytes can reversibly accommodate 0.35 K+-ions with no interference with Na+. In addition to the sequential interlayer slippage that occurs with Na+-insertion, K+-insertion into Na0.52CrO2 induces a sudden phase separation, which ultimately results in a biphasic structure when fully discharged (K+-free O3-NaCrO2 and K+-rich P3-K0.6Na0.17CrO2). A reversible transition between monophasic (Na0.52CrO2) and biphasic states during repeated K+-insertion/deinsertion is also maintained, which contributes to superior electrochemical properties of the title compound when used as a KIB cathode. Na0.52CrO2 delivers a specific capacity of 88 mA h g–1 with an average discharge potential of 2.95 V versus K/K+. This high level of energy density (260 W h kg–1 at 0.05C) is not substantially decreased at fast C-rates (195 W h kg–1 at 5C). When cycled at 2C, the first reversible capacity of 77 mA h g–1 gradually decreases to 52 mA h g–1 during initial 20 cycles, but no further capacity fading is observed for subsequent cycles (51 mA h g–1 after 200 cycles). Density-functional-theory computation reveals that the rearrangement of Na+ is an energetically favored process rather than a homogeneous distribution of Na+/K+.

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KCrS2 is presented as a stable and high‐rate layered material that can be used as a cathode in potassium‐ion batteries. As far as it is known, KCrS2 is the only layered material with stoichiometric amounts of K+, which enables coupling with a graphite anode for full‐cell construction. Cr(III)/Cr(IV) redox in KCrS2 is also unique, because LiCrS2 and NaCrS2 are known to experience S2−/S2 2− redox. O3‐KCrS2 is first charged to P3‐K0.39CrS2 and subsequently discharged to O′3‐K0.8CrS2, delivering an initial discharge capacity of 71 mAh g−1. The following charge/discharge (C/D) shows excellent reversibility between O′3‐K0.8CrS2 and P3‐K0.39CrS2, retaining ≈90% of the initial capacity during 1000 continuous cycles. The rate performance is also noteworthy. A C/D rate increase of 100‐fold (0.05 to 5 C) reduces the reversible capacity only by 39% (71 to 43 mAh g−1). The excellent cyclic stability and high rate performance are ascribed to the soft sulfide framework, which can effectively buffer the stress caused by K+ deinsertion/insertion. During the transformation between P3‐K0.39CrS2 and O′3‐K0.8CrS2, the material resides mostly in the P3 phase, which minimizes the abrupt dimension change and allows facile K+ diffusion through spacious prismatic sites. Structural analysis and density functional theory calculations firmly support this reasoning.