Alluaudite Na₂Co₂Fe(PO₄)₃ as an electroactive material for sodium ion batteries

The electroactive orthophosphate Na₂Co₂Fe(PO₄)₃ was synthesized using a solid state reaction. Its crystal structure was solved using the combination of powder X-ray- and neutron-diffraction data. This material crystallizes according to the alluaudite structure (S.G. C2/c). The structure consists of edge sharing [MO₆] octahedra (M = Fe, Co) resulting in chains parallel to [-101]. These chains are linked together via the [PO₄] tetrahedra to form two distinct tunnels in which sodium cations are located. The electrochemical properties of Na₂Co₂Fe(PO₄)₃ were evaluated by galvanostatic charge-discharge cycling. During the first discharge to 0.03 V, Na₂Co₂Fe(PO₄)₃ delivers a specific capacity of 604 mA h g^-1. This capacity is equivalent to the reaction of more than seven sodium ions per formula unit. Hence, this is a strong indication of a conversion-type reaction with the formation of metallic Fe and Co. The subsequent charge and discharge involved the reaction of fewer Na ions as expected for a conversion reaction. When discharged to 0.9 V, the material intercalated only one Na⁺-ion leading to the formation of a new phase Na₃Co₂Fe(PO₄)₃. This phase could then be cycled reversibly with an average voltage of 3.6 V vs. Na⁺/Na and a capacity of 110 mA h g^-1. This result is in good agreement with the theoretical capacity expected from the extraction/insertion of two sodium atoms in Na₃Co₂Fe(PO₄)₃.