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

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-K_xFe₂(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-K_xFe₂(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-K_xFe₂(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 K_xFe₂(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-K_xFe₂(SO₄)₃ and the experimental results, including the average operation voltage, power-capability, and phase reaction.