Effect of divalent transition metal ions on physical, morphological, electrical and electrochemical properties of alluaudite phases Na2M2+2Fe3+(PO4)3 (M = Mn, Co and Ni): Cathode materials for sodium-ions batteries

The limited resources of lithium stimulated the research work to develop new polyanionic cathode materials for sodium-ion batteries. The Na2M2+2Fe3+(PO4)3 (M = Mn, Ni and Co) phases were prepared by autocombustion method assisted by glycine. Structural, morphology, thermal, electrical and electrochemical properties have been investigated. Its structures were determined using Xray powder diffraction and Rietveld method refinements. The two compounds Na2M2+2Fe3+(PO4)3 (M = Mn and Co) are alluaudite-type. Both compounds crystallize in monoclinic system with the space group C2/c and similar parameters: a = 12.0337(3) Å, b = 12.6268(3) Å, c = 6.5070(1) Å, β = 114.563(2)° for Na2M2+2Fe3+(PO4)3 (NMFP) and a = 11.7597(3) Å, b = 12.4579(3) Å, c = 6.4607(1) Å, β = 113.968(1)° for Na2Co2Fe(PO4)3 (NCFP). The NaNiFe2(PO4)3 (NNFP) compounds crystallize in orthorhombic system with the space group Imma and unit cell parameters: a = 10.3993(1) Å, b= 13.1966(1) Å, c = 6.4955(1) Å. The composition and morphology of the compounds were checked by energy dispersive spectroscopy coupled with scanning electron microscope. The thermal analysis confirmed the allotropic transition of the three materials from monoclinic to orthorhombic symmetry with the changing of divalent transitions metal ion. The electrical conductivity results of indicated that NNFP has the lowest value of activation energy of value= 0.63 eV owing to the large size of open channels existed in the orthorhombic symmetry. The electrochemical cycling results showed that NMFP cathode delivered the maximum storage capacity of about 94.2 mAh/g which correspond to coloumbic efficiency of about 75.5% after the initial cycling.