NaSICON NaFe2PO4(SO4)2 Revisited: Insights into the Crystal Structure and Electrochemical Performance

Abstract

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).