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

Abstract

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, K_0.78Fe_1.60S₂ 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, K_0.78Fe_1.60S₂ reversibly delivers 0.69 K⁺ with a capacity of 100.5 mA h g⁻¹ (i.e., K_0.20Fe_1.60S₂ ⇆ K_0.89Fe_1.60S₂). 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 calculations also validate the possibility of mixed redox reactions in K_0.78Fe_1.60S₂. Unique structural features of K_0.78Fe_1.60S₂ (layers consisting of edge-shared FeS₄ tetrahedra with partial Fe vacancies) result in high K⁺ diffusion coefficients that are unprecedented (ca. 10⁻⁹ cm² s⁻¹), which contributes to an excellent rate capability (56.3 mA h g⁻¹ at 1000 mA g⁻¹vs. 100.5 mA h g⁻¹ at 20 mA g⁻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.