### Abstract

The electroactive orthophosphate Na<inf>2</inf>Co<inf>2</inf>Fe(PO<inf>4</inf>)<inf>3</inf> 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 [MO<inf>6</inf>] octahedra (M = Fe, Co) resulting in chains parallel to [-101]. These chains are linked together via the [PO<inf>4</inf>] tetrahedra to form two distinct tunnels in which sodium cations are located. The electrochemical properties of Na<inf>2</inf>Co<inf>2</inf>Fe(PO<inf>4</inf>)<inf>3</inf> were evaluated by galvanostatic charge-discharge cycling. During the first discharge to 0.03 V, Na<inf>2</inf>Co<inf>2</inf>Fe(PO<inf>4</inf>)<inf>3</inf> delivers a specific capacity of 604 mA h g<sup>-1</sup>. 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<sup>+</sup>-ion leading to the formation of a new phase Na<inf>3</inf>Co<inf>2</inf>Fe(PO<inf>4</inf>)<inf>3</inf>. This phase could then be cycled reversibly with an average voltage of 3.6 V vs. Na<sup>+</sup>/Na and a capacity of 110 mA h g<sup>-1</sup>. This result is in good agreement with the theoretical capacity expected from the extraction/insertion of two sodium atoms in Na<inf>3</inf>Co<inf>2</inf>Fe(PO<inf>4</inf>)<inf>3</inf>.

Original language | English |
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Pages (from-to) | 7881-7886 |

Number of pages | 6 |

Journal | Dalton Transactions |

Volume | 44 |

Issue number | 17 |

DOIs | |

Publication status | Published - 7 May 2015 |

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### ASJC Scopus subject areas

- Inorganic Chemistry

### Cite this

*Dalton Transactions*,

*44*(17), 7881-7886. https://doi.org/10.1039/c5dt00971e