Anionic Group Doping of Na₄Fe₃(PO₄)₂P₂O₇ Stabilizes Its Structure and Improves Electrochemical Performance for Sodium Ion Storage

Due to its low cost and high stability, the iron-based mixed polyanionic compound Na₄Fe₃(PO₄)₂P₂O₇ is widely studied for use as a sodium-ion battery cathode material. However, its development is limited by a low electrical conductivity and restricted diffusion kinetics. In this work, we chose to replace the PO₄^3– group with the SiO₄^4– group to enhance the electronic conductivity and diffusion kinetics, while this structural substitution maintains the integrity of the material. Furthermore, this substitution at a nonactive site improves the electrochemical performance without reducing the theoretical capacity. Additionally, it stabilizes the crystal structure during the repeated charging and discharging process. As a result, this SiO₄^4– doped Na₄Fe₃(PO₄)₂P₂O₇ exhibits improved electrochemical properties. For instance, it achieves a capacity of 119.4 mA h g^–1 at a rate of 0.1 C and 60.7 mA h g^–1 at 50 C after 5000 cycles with 84.2% of its capacity retained. Moreover, theoretical calculations revealed the doping form and occupancy of SiO₄^4– in the host material structure. It is believed that this work provides a new perspective on doping modification with anionic groups to improve the electrochemical performance of cathode materials for sodium ion storage.