Iron-Based Polyanionic Cathodes for Sustainable Sodium-Ion Batteries

Many efforts have been devoted to the development of high-performance cathode materials for sodium-ion batteries to make them fully competitive to lithium-ion batteries. Polyanionic compounds are considered as promising cathode materials with regard to their high working voltage, robust framework thermal stability and small volume change upon cycling [1]. Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) combines the benefits of both phosphate and pyrophosphate, offering potential advantages such as low-cost, environmental friendliness, high average working voltage (~ 3.1 V vs. Na⁺/Na), favorable theoretical capacity (129 mAh g^-1), low volume change (< 4%) as well as structural and thermal stability [2]. However, its practical use is limited because of the formation of impurity phases and low intrinsic electronic conductivity. Various strategies have been developed to enhance conductivity such as nanosizing, carbon coating, and metal-ion doping [3]. In this study, Na₄Fe_3-xM_x(PO₄)₂(P₂O₇) (M: Mn, Ni, or Zn) composites are synthesized by a simple sol-gel method followed by a low-temperature calcination to improve their capacity and energy density.
The crystal structure, composition and purity of NFPP composites are characterized by X-ray diffraction (XRD) method. The morphology, particle size, and distribution of elements are investigated by field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) equipped with an energy dispersive X-ray (EDX) detector. The electrochemical performance of doped NFPP cathodes are analyzed with cyclic voltammetry (CV), and galvanostatic cycling tests to determine the reversibility of Na⁺ deintercalation in the structure, kinetic properties, the actual capacity of the active cathode material. In order to gain a fundamental understanding of the effect of doping on the oxidation state, local structures, charge compensation mechanism of transition metals, and sodium storage mechanism, ex-situ X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) on cycled cathodes are conducted. By substituting minimal amount of Mn²⁺ and Ni²⁺ into Fe sites, the generation of Fe and Na vacancies suppresses the formation of electrochemically inactive maricite NaFePO₄, thereby decreasing the ion migration energy barriers during Na⁺ insertion/extraction. Substituting with Zn²⁺ has also been tested. However, Zn²⁺ doping strategy results in capacity decay due to the higher content of both NaFePO₄ and Na₂FeP₂O₇ impurity phases within the structure.
Ex-situ XAS reveals doping to cause characteristic lattice distortions, strongest with Zn and weakest with Ni. Low doping limits are crucial for NFPP stability. All NFPP composites show highly reversible changes in Fe-O local order and bond lengths associated with characteristic Fe²⁺ / Fe³⁺ redox during battery cycling. Fe L₃-edge RIXS data suggest a slight increase in charge carrier localization in the charged state of the Zn substituted NFPP, which may negatively impact battery performance. The O K-edge XAS results indicate a stronger charge compensation contribution for the Mn and Ni doped compound compared to the Zn-NFPP. Additionally, the Fe-O hybridization is strongest for the Mn-NFPP in both the charged and discharged state. We believe that the incorporation of suitable metal ions within the crystal structure will provide insights for the development of low-cost polyanionic cathodes due to the generation of vacancies facilitating the transport of sodium ions.

[1] Z. Liu, Y. Cao, H. Zhang, J. Xu, N. Wang, D. Zhao, X. Li, Y. Liu, J. Zhang, ACS Sustainable Chemistry & Engineering 2024, 12, 1132.
[2] A. Gezović, M. Milović, D. Bajuk-Bogdanović, V. Grudić, R. Dominko, S. Mentus, M. J. Vujković, Electrochimica Acta 2024, 476, 143718.
[3] Y. Zhang, L. Tao, C. Xie, D. Wang, Y. Zou, R. Chen, Y. Wang, C. Jia, S. Wang, Advanced Materials 2020, 32, 1905923.