Olivine NaMn0.66Fe0.34PO4 as a Cathode Material for Advanced Sodium Ion Batteries

Sodium-ion batteries continue to rise in the energy storage landscape, their increasing adoption being driven by factors such as cost-effectiveness and sustainability. As a consequence, there is a growing emphasis on the development of new electrode materials. Among these, olivine phosphates emerge as a promising family of cathode materials. However, viable synthesis routes are still lacking. In this study, cathode materials of olivine NaMn_1-xFe_xPO₄ (x=0.34 and 1) were prepared by directly sodiating Mn_1-xFe_xPO₄ through a solid-state process at 300 °C. X-ray diffraction, Mössbauer spectroscopy and electrochemical measurements were employed to study their structural and electrochemical features. NaMn_0.66Fe_0.34PO₄ exhibits two pseudo-plateaus profile with an average potential of ~3.2 V vs. Na⁺/Na⁰ with a reversible capacity reaching 75 mAh/g at C/20 via a monophasic (de)intercalation mechanism. In parallel, the intermediate composition Na_0.5Mn_0.66Fe_0.34PO₄ could be prepared via the solid-state reaction of NaMn_0.66Fe_0.34PO₄ and Mn_0.66Fe_0.34PO₄. Such a solvent-free sodiation process not only provides a simplified preparation of NMFP, but also offers easy scalability compared to the more laborious electrochemical sodiation route, making it an interesting prospect for future industrialization. Finally, this research confirms that the olivine NMFP is indeed an attractive candidate as a cathode material for SIBs.