Non-layered KFeO2 /KFe5O8 cathodes promoted by dual-phase crystal synergistic regulation strategies based on eutectoid transformation for potassium ion batteries

Potassium ion batteries (PIBs) have become a hotspot of research in the field of large-scale energy storage owing to their abundant resource endowments. Notably, the inherent advantage of PIBs development lies in their costeffectiveness. Among them, low cost is the inevitable advantage of PIBs development. Herein, low-cost pure ironbased non -layered KFeO 2 /KFe 5 O 8 biphase coexistence cathode materials for PIBs are synthesized through onestep calcination solid phase method. XRD results confirm the evident biphase coexistence of KFeO 2 /KFe 5 O 8 when the content of K source is 0.3. As the K source content increases, the biphase coexistence is transformed into pure KFeO 2 . The activation energy of KFeO 2 /KFe 5 O 8 biphase coexistence sample is lower than that of pure KFeO 2 by Kissinger method, providing evidence that the existence of KFe 5 O 8 can promote the reaction. TEM results show that the existence of KFe 5 O 8 does not affect the lattice fringes of KFeO 2 , and the presence of two types of lattice fringes once again verifies the fact that KFeO 2 /KFe 5 O 8 biphase coexistence. It is proved that the valence of Fe in KFeO 2 /KFe 5 O 8 and pure KFeO 2 is +3 by XANES. Comparative analysis with a high-potassium content pure KFeO 2 cathode material reveals that the synergistic effect of KFeO 2 and KFe 5 O 8 can bring excellent electrochemical performance and improve the thermodynamic stability of the system. Moreover, GITT results show that the K + diffusion coefficient of the KFeO 2 /KFe 5 O 8 biphase coexistence material is significantly higher. The results of this study are sufficient to prove the feasibility of cheap iron as cathode materials for PIBs, and provide a new design idea for the development of low-cost and high-performance cathode materials.