High-power rhombohedral-Fe2(SO4)3 with outstanding cycle-performance as Fe-based cathode for K-ion batteries

Abstract We report rhombohedral-Fe2(SO4)3 as a promising cathode material for potassium-ion batteries. Detailed structure information for rhombohedral-Fe2(SO4)3 was obtained using X-ray diffraction with Rietveld refinement. Based on the structural information, possible atomic sites for K ions and ion diffusion pathway in the structure were determined using bond-valence energy landscape analyses. At C/20 (1C = 112 mA g−1), rhombohedral-KxFe2(SO4)3 delivered a specific capacity of ∼100 mAh g−1 an average operation voltage of ∼3.3 V (vs. K+/K), corresponding to reversible de/intercalation of ∼1.78 mol K+ ions. Moreover, even at 5C, rhombohedral-KxFe2(SO4)3 exhibited capacity retention of ∼80 % of the capacity measured at C/20, indicating the outstanding power-capability. After 300 cycles at 2C, rhombohedral-KxFe2(SO4)3 maintained ∼83 % of its initial specific capacity with high Coulombic efficiency of more than 99%. Operando XRD analyses revealed that the XRD peaks of rhombohedral KxFe2(SO4)3 monotonously shifted during K+ de/intercalation, indicating a single phase reaction. First-principles calculation confirmed the good agreement between the theoretical properties of rhombohedral-KxFe2(SO4)3 and the experimental results, including the average operation voltage, power-capability, and phase reaction.