Synthesis,structure and electrochemical performance of Eldfellite, NaFe(SO₄)₂, doped with SeO₄, HPO₄ and PO₃F

NaFe(SO₄)₂ materials partially substituted with SeO₄^2-, HPO₄^2- and PO₃F^2- were prepared and investigated as possible cathode materials for sodium-ion batteries. Neutron diffraction and Raman spectroscopy studies confirmed the successful incorporation of SeO₄^2- and HPO₄^2-, while the Raman studies suggested that significant hydrolysis of PO₃F^2- had occurred during the synthesis. The effective diffusion coefficients were determined from conductivity and electrochemical kinetics studies, showing low Na ion diffusion. Galvanostatic and cyclic voltammetry investigations demonstrated more significant degradation and a narrowing of the electrochemical stability window for the doped materials, which was most significant for the SeO₄ and HPO₄ doped samples. Therefore, while doped NaFe(SO₄)₂ were successfully prepared, the best electrochemical performance was shown for the undoped system.