Tailoring Triphylite-NaFePO4 Cathodes: A Pathway to Sustainable Sodium-Ion Batteries

In this study, the properties of triphylite-NaFePO₄ (NFP) sodium-ion battery cathode powders produced via a chemical conversion route from various starting LiFePO₄ (LFP) powders are presented and compared. Lithium was first extracted from the LFP using sodium persulfate, where over 95% of Li was removed from the LFPs. The delithiated samples were then sodiated using different sodiating agents, namely, sodium thiosulfate and sodium iodide. The resulting NFP materials were characterized using X-ray diffraction (XRD) and inductively coupled plasma atomic emission spectroscopy (ICP-AES), before final electrochemical testing in a sodium half-cell configuration. The findings confirm that sodium iodide is an effective reagent for the complete chemical sodiation of FePO₄ without compromising the integrity of the carbon coating of the starting LFP. Furthermore, this work provides key insights to the processing parameters and the influence of the starting LFP morphological and electrochemical properties. Raman spectroscopy showed that the LFPs with a lower I_D/I_G ratio and platelet-like morphology resulted in better performing NFPs. Electrochemical results show that the best-performing LFP with a capacity of 155 mAh/g (93% of the theoretical capacity) produced the highest capacity NFP, delivering a capacity of 127 mAh/g (∼90% of the theoretical capacity).