Low-cost and High-power K4[Mn2Fe](PO4)2(P2O7) as a novel cathode with outstanding cyclability for K-ion batteries

De/intercalation of K+ ions results in more severe structural change and volume expansion/shrinkage than that of other monovalent ions such as Li+ and Na+ because of the larger K+-ion size. Thus, it is important to develop novel cathode materials with stable three-dimensional crystal structure and large K+ diffusion pathways to prevent severe structural change during repeated K+ de/intercalation. Here, we introduce K4[Mn2Fe](PO4)2(P2O7) composed of three-dimensionally interconnected [Mn, Fe]O6 octahedra and PO4 tetrahedra as a promising cathode material for K-ion batteries. We demonstrate the outstanding electrochemical properties and reaction mechanism of K4[Mn2Fe](PO4)2(P2O7) in a K-ion battery system through combined studies using various experimental techniques and first-principles calculation. K4[Mn2Fe](PO4)2(P2O7) delivers ∼110 mA h g−1 with a high average operation voltage of 3.5 V (vs. K+/K) at C/20 (1C = 117 mA g−1). Even at 5C, its specific capacity is ∼85 mA h g−1, corresponding to ∼77% of the capacity measured at C/20. In addition, K4[Mn2Fe](PO4)2(P2O7) exhibits an outstanding capacity retention of ∼83% after 300 cycles at C/3 with a high coulombic efficiency of more than 99%. These findings confirm the importance of a stable three-dimensional crystal structure for high-performance K-ion batteries.