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

Abstract

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 PO43– group with the SiO44– 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 SiO44– 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 SiO44– 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.