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 Mössbauer 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) Å, 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 Mössbauer 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.