α-Na2Ni2Fe(PO4)3: a dual positive/negative electrode material for sodium ion batteries

A new orthophosphate α-Na2Ni2Fe(PO4)3 was synthesized using a solid state reaction route, and its crystal structure was determined from powder X-ray diffraction data. The physical properties of α-Na2Ni2Fe(PO4)3 were studied by magnetic and electrochemical measurements and by Mossbauer and Raman spectroscopy. α-Na2Ni2Fe(PO4)3 crystallizes according to a stuffed α-CrPO4-type structure with the space group Imma and the cell parameters a = 10.42821(12), b = 13.19862(15), c = 6.47634(8) A, and Z = 4. The structure consists of a 3D-framework of octahedra and tetrahedra sharing corners and/or edges with channels along [100] and [010], in which the sodium atoms are located. The 57Fe Mossbauer spectrum indicates that the Fe3+ cation is distributed over two crystallographic sites implying the presence of a Ni2+/Fe3+ statistical disorder. Magnetic susceptibility follows the Curie–Weiss behavior above 100 K with θ = −114.3 K indicating the occurrence of predominant antiferromagnetic interactions. Electrochemical tests indicate that during the first discharge to 1 V vs. Na+/Na in a sodium cell, one Na+ ion could be inserted into the α-Na2Ni2Fe(PO4)3 structure. This has led to the formation of a new phase Na3Ni2Fe(PO4)3 which was found to be promising as a positive electrode material for sodium batteries. When α-Na2Ni2Fe(PO4)3 is further discharged to 0.03 V, it delivers a capacity of 960 mA h g−1. This corresponds to the intercalation of more than seven sodium atoms per formula unit which is an indication of a conversion-type behaviour with the formation of metallic Fe and Ni. When cycled in the voltage range 0.03–3 V vs. Na+/Na, at 20 °C, under the current rates of 50, 100, 200, and 400 mA g−1, reversible capacities of 238, 196, 153, and 115 mA h g−1, were obtained, respectively.