F and Si dual-doping induced oxygen vacancies in a Na4Fe3(PO4)2P2O7 cathode enables boosting electrochemical performance for sodium storage

Abstract

The iron-based mixed polyanionic compound Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) with high theoretical capacity and excellent structural stability has attracted comprehensive attention as a promising cathode material for sodium-ion batteries (SIBs). Nevertheless, the constrained conductivity and restricted diffusion kinetics during sodium storage pose an unprecedented challenge to rate capability and cycling stability. Herein, an oxygen vacancy strategy by dual-anionic doping (F⁻ and SiO₄⁴⁻) is first proposed and the corresponding function mechanism in structural and electronic optimization is unraveled. Experimental analyses and theoretical calculations reveal that the combination of F⁻ with higher electro-negativity and the SiO₄⁴⁻ group with a larger ionic radius can efficiently expand the ion diffusion channels and induce more oxygen vacancies, thus boosting electronic conductivity, as well as accelerating sodium ion transfer kinetics. Benefitting from the synergy of F/Si dual doping and abundant oxygen vacancies, NFPP-0.1F/0.05Si outperforms both the undoped and single-anionic-doped NFPP samples, and delivers a remarkably high capacity of 119.6 mA h g⁻¹ at 0.1C, a conspicuous rate performance of 67.7 mA h g⁻¹ at 10C, and good cycling stability with a capacity retention of 80.86% over 4000 cycles at 50C. More encouragingly, the full cells (NFPP-0.1F/0.05Si‖hard carbon) also exhibit outstanding long-term cycling performance at 50 mA g⁻¹, and demonstrate a reversible capacity of 77.8 mA h g⁻¹ and still retain 78.3% of their initial capacity after 200 cycles, manifesting great practical potential.