The layered Na 3 Fe 3 (PO 4 ) 4 phase prepared by solid-state reaction was studied as positive electrode in lithium batteries. Up to 1.9 Li + ions/f.u. could be intercalated, and only 1.7 Li + ions could be extracted between 4.5 and 2 V vs Li + /Li with an average voltage of around ~2.8 V. In situ and ex situ X-ray diffraction data and Mössbauer spectroscopy measurements indicate that the intercalation/deintercalation process occurs through a solid solution process and is reversible. To improve its electrochemical performances, Na 3 Fe 3 (PO 4 ) 4 was also prepared via a hydrothermal method that leads to a significant particle size reduction. However, no clear improvements of the cycling performances were observed using this material as positive electrode in sodium and lithium batteries.