An alluaudite compounds Na2Fe2(SO4)3 vs. Na2.5Fe1.75(SO4)3 as earth abundant cathode materials for Na-ion batteries

Abstract In this paper, two compounds Na2Fe2(SO4)3 and Na2.5Fe1.75(SO4)3 cathode materials were synthesised and studied in terms of their structural, transport and electrochemical properties in Na/Na+/Na2+2yFe2−y(SO4)3 (y = 0; 0.25) cells. The materials were fabricated using a low temperature solid state reaction. XRD measurements accompanied with Rietveld analysis confirmed the presence of a phase with C2/c symmetry for both obtained compositions. By changing the stoichiometry and synthesis conditions of Na2+2yFe2−y(SO4)3 the compound with a minimal amount of impurities can be obtained. Na2.5Fe1.75(SO4)3 material exhibits good transport properties with low charge transport activation energy. Electrochemical studies show improved performance of Na2.5Fe1.75(SO4)3 compared to Na2Fe2(SO4)3.