KFeO2 with corner-shared FeO4 frameworks as a new type of cathode material in potassium-ion batteries

KFeO2 is presented as a new type of cathode material for potassium-ion batteries. In contrast to LiFeO2 and NaFeO2, KFeO2 has tetrahedrally coordinated Fe3+ ions linked by three-dimensional corner-sharing. When chemically oxidized, KFeO2 releases ca. 0.3 K+ (K0.7FeO2) and shows no apparent changes in X-ray diffraction patterns. Rietveld refinement of KFeO2 and K0.7FeO2 reveals negligible variation in the unit cell dimension and in the basic structure, but noticeable differences in FeO4 volumes and inter-tetrahedra angles due to the Jahn-Teller active Fe4+ (eg2t2g2) in K0.7FeO2. The refinement also shows no preferential extraction of K+ from two possible sites (K1 and K2), which coincides with the results from density-functional-theory calculation. In accordance with the compositional and structural analysis, KFeO2 delivers a reversible capacity of 60 mAh g−1 (ca. 0.28 K+) during the initial charge/discharge (C/D), showing a plateau-like voltage response at 3.43 V vs. K/K+. In-operando diffraction studies also show no distinctive signs of pattern evolution, which is likely due to the structural similarities of KFeO2 and K0.7FeO2. With repeated cycling, however, the reversible capacity of KFeO2 continuously decreases and falls to 30 mAh g−1 after 50 C/D cycles. The repeated changes in the FeO4 tetrahedra geometry appear to cause a decrease in the crystallinity of KFeO2, which eventually aggravates the facile K+ transport. The results of this study show that the practicality of KFeO2 can be realized via improvements of cyclability by mitigating the Jahn-Teller effect (for example, isovalent doping with Al3+or Ga3+).