title: Alluaudite Na2Co2Fe(PO4)3 as an electroactive material for sodium ion batteries.
abstract: 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.
authors.affiliation: Qatar Environment and Energy Research Institute, Qatar Foundation, P.O. Box 5825, Doha, Qatar. ressehli@qf.org.qa ibelharouak@qf.org.qa Hyahia@qf.org.qa.
authors.forename: R
authors.initials: R
authors.lastname: Essehli
authors.forename: I
authors.initials: I
authors.lastname: Belharouak
authors.forename: H
authors.initials: H
authors.lastname: Ben Yahia
authors.forename: K
authors.initials: K
authors.lastname: Maher
authors.forename: A
authors.initials: A
authors.lastname: Abouimrane
authors.forename: B
authors.initials: B
authors.lastname: Orayech
authors.forename: S
authors.initials: S
authors.lastname: Calder
authors.forename: X L
authors.initials: XL
authors.lastname: Zhou
authors.forename: Z
authors.initials: Z
authors.lastname: Zhou
authors.forename: Y-K
authors.initials: YK
authors.lastname: Sun