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 S2- /S2 2- redox. O3-KCrS2 is first charged to P3-K0.39 CrS2 and subsequently discharged to O'3-K0.8 CrS2 , delivering an initial discharge capacity of 71 mAh g-1 . The following charge/discharge (C/D) shows excellent reversibility between O'3-K0.8 CrS2 and P3-K0.39 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-K0.39 CrS2 and O'3-K0.8 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.