Facile aqueous synthesis of high performance Na2FeM(SO4)3 (M = Fe, Mn, Ni) alluaudites for low cost Na-ion batteries

Abstract

A new method of synthesis of stoichiometric Na₂FeM(SO₄)₃ (M = Fe, Mn, Ni) materials is developed. The proposed unique methodology is based on a facile processing of dissolution of Na₂SO₄ and transition metal (Fe, Mn, and Ni) sulfates in an aqueous solution, solvent evaporation, and annealing. This novel method is characterized by a very low degree of complexity but still ensures the highest purity, resulting in the high purity of Mn and Ni substituted alluaudite compounds. Synthesized materials possess nanometric size grains with uniform composition distribution. Na₂FeM(SO₄)₃ exhibits great transport properties with low charge transport activation energy related to sodium ion diffusion. Electrochemical characterization of the processed Na₂Fe₂(SO₄)₃ compound in both half-cell and full cell formats demonstrates the highest power density among all polyanion Na cathodes.