Anionic Group Doping of Na<sub>4</sub>Fe<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>P<sub>2</sub>O<sub>7</sub> Stabilizes Its Structure and Improves Electrochemical Performance for Sodium Ion Storage
Mingzu Liu, Min Li, Bolun Zhang, Houmou Li, Jiaxin Liang, Xinyu Hu, Haimei Liu, Zi‐Feng Ma
Abstract
Due to its low cost and high stability, the iron-based mixed polyanionic compound Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 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 PO 4 3– group with the SiO 4 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 SiO 4 4– doped Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 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 SiO 4 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.