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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 (K1.66Na1.02Fe1.66(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.
Xu et al. (Tue,) studied this question.