Unveiling yavapaiite-type K x Fe(SO4)2 as a new Fe-based cathode with outstanding electrochemical performance for potassium-ion batteries

Highlights: • Yavapaiite-type KFe(SO₄)₂ is prepared for potassium-ion batteries. • First-principles calculation and various experimental techniques are introduced. • KFe(SO₄)₂ shows ~93 mAh g⁻¹ at C/20 with high average voltage of ~3.3 V. • KFe(SO₄)₂ exhibits ~80% retention after 300 cycles at 2 C. • High-performance of KFe(SO₄)₂ comes from small volume change and facile K⁺ diffusion. -- Abstract: We report KFe(SO₄)₂ as a novel cathode material for potassium-ion batteries. The synthesis of phase-pure KFe(SO₄)₂ is confirmed through Rietveld refinement of X-ray diffraction data, and the ion diffusion path and possible positions of K⁺ ions in the crystal structure are verified using bond-valence sum analysis. In addition, the theoretical average voltage, ion diffusion paths, and associated activation barrier energies obtained by first-principles calculations predict feasibility of the KFe(SO₄)₂ as a robust cathode for potassium-ion batteries. . Experimentally, the average working voltage of KFe(SO₄)₂ is approximately 3.3 V (vs. K⁺/K) assisted by Fe^3+/2+ redox pair, and the resulting specific capacity (94 mAh g⁻¹) obtained at C/20 approaches to the theoretical capacity, 94 mAh g⁻¹. In addition, the capacity retention of KFe(SO₄)₂ at 2C was approximately 80% of the initial capacity after 300 cycles, with a Coulombic efficiency of over 99%. X-ray absorption near-edge structure and operando synchrotron X-ray diffraction analyses reveals the reversible change of the crystal structure with a minimal volume difference (1.83%) by occurrence of Fe^3+/2+ redox reaction during dis/charge. The demonstrated excellent structural stability of KFe(SO₄)₂ endorses feasibility of the KFe(SO₄)₂ as a sustainable cathode for potassium-ion batteries.