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 cost-effectiveness. Among them, low cost is the inevitable advantage of PIBs development. Herein, low-cost pure iron-based non-layered KFeO2/KFe5O8 biphase coexistence cathode materials for PIBs are synthesized through one-step calcination solid phase method. XRD results confirm the evident biphase coexistence of KFeO2/KFe5O8 when the content of K source is 0.3. As the K source content increases, the biphase coexistence is transformed into pure KFeO2. The activation energy of KFeO2/KFe5O8 biphase coexistence sample is lower than that of pure KFeO2 by Kissinger method, providing evidence that the existence of KFe5O8 can promote the reaction. TEM results show that the existence of KFe5O8 does not affect the lattice fringes of KFeO2, and the presence of two types of lattice fringes once again verifies the fact that KFeO2/KFe5O8 biphase coexistence. It is proved that the valence of Fe in KFeO2/KFe5O8 and pure KFeO2 is +3 by XANES. Comparative analysis with a high‑potassium content pure KFeO2 cathode material reveals that the synergistic effect of KFeO2 and KFe5O8 can bring excellent electrochemical performance and improve the thermodynamic stability of the system. Moreover, GITT results show that the K+ diffusion coefficient of the KFeO2/KFe5O8 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.