Phase Relations in a NaFeO₂-SnO₂ (0-50 mol.% SnO₂) System and the Crystal Structure and Conductivity of Na_0.8Fe_0.8Sn_0.2O₂

With the view of developing new materials for sodium and sodium-ion power sources, NaFeO₂-SnO₂ (0-50 mol.% SnO₂) powders were synthesized using a solid state method, and their phase composition and crystal structure were studied. A phase of the Na_0.8Fe_0.8Sn_0.2O₂ composition with a layered rhombohedral structure of the α-NaFeO₂ type was found when the tin dioxide content was 20 mol.%. The phase produced was of an O3 structural type. O3-type phases have sufficiently good performance when used as cathode materials in sodium-ion batteries and, moreover, often have a rather high sodium-cation conductivity. A two-dimensional migration map was built using Voronoi-Dirichlet partition and TOPOS software package. The sodium-ion conductivity of Na_0.8Fe_0.8Sn_0.2O₂ at room temperature was rated low (10^-8 S × cm^-1 at 20 °C), which may be the result of channels too narrow for Na⁺ migration. The results obtained show that the application of the compound studied in this work as a solid electrolyte in sodium power sources is unlikely. It is the potential use of Na_0.8Fe_0.8Sn_0.2O₂ as the active material of cathodes in Na and Na-ion power sources that presents practical interest.