NaSICON NaFe2PO4(SO4)2 revisited: insights into the crystal structure and electrochemical performance

With the aim to meet the needs for positive electrode materials for Na-ion batteries, based on abundant elements, synthesis routes using two different Fe3+ precursors were explored for the preparation of a pure mixed phosphate-sulfate NaSICON-type compound, NaFe23+PO4(SO4)2 (NFPS). Interestingly, a structural model described in the R-3 space group is found from careful analysis of X-ray, neutron, and electron diffraction. Atoms P and S (and thus, PO4 and SO4 anionic groups) are statistically distributed within the polyanionic framework, with a splitting of the conventional Na2 3b site into two positions 3a and 18f. Na-half cells with NFPS as a positive electrode material delivered 126 mAh/g, nearly the full theoretical capacity, at a cycling rate of D/30–C/30 (i.e., the exchange of 2 Na+ in 30 h).