KCrS₂ Cathode with Considerable Cyclability and High Rate Performance: The First K⁺ Stoichiometric Layered Compound for Potassium-Ion Batteries

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 S^2- /S₂ ^2- redox. O3-KCrS₂ is first charged to P3-K_0.39 CrS₂ and subsequently discharged to O'3-K_0.8 CrS₂ , delivering an initial discharge capacity of 71 mAh g^-1 . The following charge/discharge (C/D) shows excellent reversibility between O'3-K_0.8 CrS₂ and P3-K_0.39 CrS₂ , 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-K_0.39 CrS₂ and O'3-K_0.8 CrS₂ , 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.