Enabling the Electrochemical Activity in Sodium Iron Metaphosphate [NaFe(PO₃)₃] Sodium Battery Insertion Material: Structural and Electrochemical Insights

Sodium-ion batteries are widely pursued as an economic alternative to lithium-ion battery technology, where Fe- and Mn-based compounds are particularly attractive owing to their elemental abundance. Pursuing phosphate-based polyanionic chemistry, recently solid-state prepared NaFe(PO₃)₃ metaphosphate was unveiled as a novel potential sodium insertion material, although it was found to be electrochemically inactive. In the current work, employing energy-savvy solution combustion synthesis, NaFe²⁺(PO₃)₃ was produced from low-cost Fe³⁺ precursors. Owing to the formation of nanoscale carbon-coated product, electrochemical activity was enabled in NaFe(PO₃)₃ for the first time. In congruence with the first principles density functional theory (DFT) calculations, an Fe³⁺/Fe²⁺ redox activity centered at 2.8 V (vs Na/Na⁺) was observed. Further, the solid-solution metaphosphate family Na(Fe_1-xMn_x)(PO₃)₃ (x = 0-1) was prepared for the first time. Their structure and distribution of transition metals (TM = Fe/Mn) was analyzed with synchrotron diffraction, X-ray photoelectron spectroscopy, and Mössbauer spectroscopy. Synergizing experimental and computational tools, NaFe(PO₃)₃ metaphosphate is presented as an electrochemically active sodium insertion host material.