Synthesis and characterization of a crystalline Na4Fe3(PO4)2(P2O7) cathode material for sodium-ion batteries

Na4Fe3(PO4)(2)(P2O7) (NFPP) as a promising cathode material for sodium-ion batteries possesses excellent structural stability, minimal volume change, low cost, and non-toxicity. However, its practical application is hindered by the formation of impurity phases and its intrinsically low electronic conductivity. Herein, crystalline high purity carbon-coated NFPP (NFPP/CC) is synthesized by performing a green and scalable combustion method to enhance its overall electrochemical performance. The effects of pre-treatment and the calcination atmosphere on the structure and purity of NFPP are systematically investigated for a variety of synthesis parameters. The electrochemical performance of NFPP cathodes is evaluated in both half-cells with the sodium metal anode and full-cells with the hard-carbon anode via galvanostatic charge-discharge cycling measurements. The "combustion" synthesized NFPP/CC cathode delivers a reversible discharge capacity of similar to 102 mA h g(-1) at 0.1C in an operating voltage window of 1.8-3.8 V (vs. Na/Na+) retaining 99.7% of its initial capacity over 100 cycles. Furthermore, it demonstrates enhanced rate capability in comparison to the NFPP/CC cathode synthesized via the conventional calcination route. This study sheds light on using the combustion method as a facile and effective strategy to simultaneously mitigate the formation of impurity phases, reduce the carbon content, enhance the quality of carbon coating, improve the homogeneity of nanoparticles and pores within the structure, and enhance the electronic conductivity and physical stability of NFPP cathodes, paving the way for their practical application in high-performance sodium-ion batteries.