A new iron-based sulfate cathode material for high-performance potassium-ion battery

Iron-based sulfates have emerged as promising cathode materials for potassium-ion batteries due to its 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 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 <em>in-situ</em> X-ray diffraction (XRD) and <em>ex-situ</em> 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.