NaMnFe2(PO4)3 Alluaudite Phase: Synthesis, Structure, and Electrochemical Properties As Positive Electrode in Lithium and Sodium Batteries

The NaMnFe2(PO4)3 alluaudite phase was prepared either by classical solid-state chemistry or by the sol-gel method. Its structure was investigated by Rietveld refinement of XRD and neutron diffraction data in order to precisely determine the cationic distribution. This 3D skeleton structure exhibits two types of 1D tunnels where the Na+ ions are localized. The material obtained by the sol-gel method exhibits a composition very close to the ideal one, contrary to previous works reported in literature. This study leads us to propose the following formulation for our sample: (Na-0.81)(A(1))(Na-0.19)(A(2)')((Mn0.87Fe0.13II)-Fe-II)(M(1))((Fe1.87Mn0.13III)-Mn-III)(M(2))(PO4)(3). The iron/manganese exchange was revealed from neutron data and was supported by Mssbauer spectroscopy (presence of small amount of Fe2+ ions in the M(1) type site). The stability of the various Na+ ions sites within the tunnels were studied by DFT calculations. The intercalation/deintercalation properties of NaMnFe2(PO4)(3) as positive electrode were tested in lithium and sodium batteries. The cycling curves exhibit a significant polarization even for the material prepared by sol-gel method. For this material, 1.5 Li+ ions and electrons can be intercalated in the structure at low rate.