A Layered Iron(III) Phosphate Phase, Na₃Fe₃(PO₄)₄: Synthesis, Structure, and Electrochemical Properties as Positive Electrode in Sodium Batteries

Abstract

For the first time, a powder of Na 3 Fe 3 (PO 4 ) 4 was obtained by solid state reaction. It crystallizes in monoclinic space group C 2/ c in good agreement with previous studies of a single crystal. The Rietveld refinement of the XRD pattern showed line broadening of some diffraction lines associated with size and strain effects. Its layered structure can be described by complex layers of corner-sharing FeO 6 octahedra connected by PO 4 tetrahedra through corner and edge sharing. The Na + ions are located in the interslab space. The local environments of Fe, Na, and P were characterized by 57 Fe Mössbauer spectroscopy and 23 Na and 31 P MAS NMR. A Second ORder Graphic Extrapolation (SORGE) diagram as introduced by Massiot et al. allowed us to fully interpret the 23 Na MAS NMR spectrum that exhibits three signals for two crystallographic Na sites. The electrochemical properties of Na 3 Fe 3 (PO 4 ) 4 were tested in sodium cells. Ex situ and in situ X-ray diffraction data and Mössbauer spectroscopy measurements indicate that the intercalation−deintercalation process of Na + ions in Na 3 Fe 3 (PO 4 ) 4 is reversible and that the structural framework is maintained during cycling. Some degree of disorder is, however, observed for a large intercalated Na + amount.