Synergistic effect of iron substitution by manganese and carbon addition in Na2FePO4F to enhance its electrochemical performance as a cathode for sodium-ion batteries.

Fluorophosphate materials are attracting increasing interest as cathodes for sodium-ion batteries (NIBs) thanks to their unique combination of structural stability, high electrochemical potential, and enhanced safety. Their robust crystalline structure, resulting from the combination of phosphates and fluorides, enables efficient Na⁺ ion insertion while maintaining good cyclability during electrochemical cycling. Among them, Na2FePO4F offers an ideal balance between performance, cost, and durability, providing an alternative to vanadium- or cobalt-based materials 1,2. However, the low electronic conductivity of fluorophosphates and their limited potential require improvements to enhance their performance. Partial substitution of iron with manganese increases the working voltage while maintaining good structural stability, thereby enhancing electrochemical performance. The integration of carbon, particularly in the form of carbon nanotubes (CNTs), further improves their electronic conductivity, paving the way for competitive and sustainable energy storage solutions.
In this work, Na2Fe0.5Mn0.5PO4F material and its composite with carbon nanotubes (CNTs) were synthesized via a one-step spray-drying process and evaluated as cathode materials for sodium-ion batteries (NIBs). Their structural and electrochemical properties were examined using X-ray diffraction (XRD), scanning electron microscopy (SEM), and galvanostatic cycling. Mössbauer spectroscopy was used to analyze iron oxidation states, phase purity, and the local electronic environment, revealing a mixed Fe²⁺/Fe³⁺ state with a fraction of iron oxide impurities. The Na2Fe0.5Mn0.5PO4F/CNT composite exhibited improved electronic conductivity and enhanced electrochemical performance, delivering a reversible capacity of 118 mAh/g at C/20 with 97% retention after 50 cycles. These results highlight the synergistic effect of partial iron substitution with manganese and CNT integration on charge transport and cycling stability, confirming its potential as a high-performance cathode active material for next-generation sodium-ion batteries.