Phase Relations in a NaFeO2-SnO2 (0–50 mol.% SnO2) System and the Crystal Structure and Conductivity of Na0.8Fe0.8Sn0.2O2

Abstract

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 Na0.8Fe0.8Sn0.2O2 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 Na0.8Fe0.8Sn0.2O2 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 Na0.8Fe0.8Sn0.2O2 as the active material of cathodes in Na and Na-ion power sources that presents practical interest.