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

Abstract

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.