Mixed anion/cation redox in K0.78Fe1.60S2 for a high-performance cathode in potassium ion batteries

Cathode materials in potassium ion batteries (KIBs) generally exhibit low charge storage capabilities when compared with cathode materials implemented in lithium or sodium ion batteries. In this work, K0.78Fe1.60S2 is described as a high capacity KIB cathode that exhibits mixed anion/cation redox behaviors during charge/discharge (C/D). When charged to 3.2 V vs. K/K+, K+ extraction occurs along with simultaneous oxidations of S2- to S22- and Fe(II) to Fe(III). During subsequent discharge to 1.5 V, this process is reversed, in addition to a further reduction of Fe(II) to Fe(I). After a few C/D cycles, K0.78Fe1.60S2 reversibly delivers 0.69 K+ with a capacity of 100.5 mAhg-1 (i.e., K0.20Fe1.60S2 ⇆ K0.89Fe1.60S2). The evolution of S2- and Fe(II) valence states along with a lack of discernable changes in crystallographic dimensions clearly confirms the concomitant redox of anions and cations with C/D. Density functional theory calculation also validates the possibility of mixed redox reactions in K0.78Fe1.60S2. Unique structural features of K0.78Fe1.60S2 (layers consisting of edge-shared FeS4 tetrahedra with partial Fe vacancies) result in high K+ diffusion coefficients that are unprecedented (ca. 10-9 cm2s-1), which contributes to an excellent rate capability (56.3 mAhg-1 at 1000 mAg-1 vs. 100.5 mAhg-1 at 20 mAg-1). Nudged elastic band calculations also reveal that the diffusion preferentially occurs along [100] directions with a low activation energy barrier of 0.41 eV.