Iron-based sulfates have emerged as promising cathode materials for potassium-ion batteries due to their low cost, high working potential, and environmentally friendly. However, the relatively large ionic radius and sluggish diffusion coefficient of K-ion pose significant challenges to the electrochemical performance and structural stability of cathode materials in potassium-ion batteries (PIBs). In this work, we successfully synthesis a new iron-based sulfate cathode material, potassium sodium iron sulfate (K_1.66Na_1.02Fe_1.66(SO₄)₃, KNFS), through an electrochemical ion exchange method. As a cathode material, it exhibits a reversible specific capacity of 83 mAh·g⁻¹ and an average working potential of 3.84 V (vs. K/K⁺) at 0.1 C in PIBs. Even at 2 C, it still demonstrates a reversible specific capacity of 52 mAh·g⁻¹ with a capacity retention ratio of 88.2% after 300 cycles. The in-situ X-ray diffraction (XRD) and ex-situ X-ray absorption spectroscopy reveal that the K-ion storage mechanism in KNFS is predominantly governed by the reversible Fe³⁺/Fe²⁺ redox couple, which provides a theoretical specific capacity of 94 mAh·g⁻¹ and involves minimal volume change (2.57%). The first-principles calculations combined with XRD results indicate that the KNFS cathode exhibits a typical alluaudite-type crystal structure with multiple fast K-ion migration channels along the three-dimensional orientation.

The off-stoichiometric iron-based phosphate (Na_3.12Fe_2.44(P₂O₇)₂, denoted as Na_3.12) as a low cost and high structure stability cathode material has been widely studied for sodium-ion batteries (SIBs). However, the lower theoretical specific capacity (117 mAh·g⁻¹) has seriously limited its practical application. In this work, we incorporate varying proportion of sodium-iron phosphate (NaFePO₄) into the Na_3.12 to form a series of new high specific capacity mixed-phosphates Na_3.12+xFe_2.44+x(P₂O₇)₂(PO₄)_x cathode materials for SIBs. After optimizing the introduction amount of NaFePO₄ into Na_3.12, the practical reversible of Na_3.12+xFe_2.44+x(P₂O₇)₂(PO₄)_x increased from 92 to 125.2 mAh·g⁻¹. The nano-size Na_5.12Fe_4.44(P₂O₇)₂(PO₄)₂ cathode material shows a reversible specific capacity of 125.2 mAh·g⁻¹ at 0.1 C in SIBs. Even at 60 C, it still exhibits a reversible specific capacity of 93.3 mAh·g⁻¹ and keeps a capacity retention ratio of 87% after 3000 cycles at 20 C. Thereby, we present a novel approach to design a series of off-stoichiometric mixed-phosphates cathode materials for SIBs.