Phase diagram of NaFeyCo1-yO2 and evolution of its physico- and electrochemical properties with changing iron content

In this paper series of novel NaFeyCo1-yO2 (y = 0.1–1) cathode materials were synthesized and studied in terms of their structural properties, electrical conductivity and electrochemical performance in Na/Na+/NaxFeyCo1-yO2 cells. The materials were fabricated by a high-temperature solid-state reaction. XRD measurements with Rietveld analysis confirmed the presence of an O3-type phase with R-3m symmetry for all compositions with the existence of the monophase region within 0.3 ≤ y ≤ 0.7 Fe substitution range. The linear variation of the unit cell parameters of the R-3m structure was found for the broad Fe concentration. The combined studies of thermogravimetric analysis, XPS and Mössbauer spectroscopy pointed at the formation of the oxygen vacancies along with spontaneous magnetization at low temperatures. The electrical conductivity of the NaFeyCo1-yO2 exhibits thermally activated character with low activation energies and decreases with the increasing of iron content. Electrochemical investigation of Na/Na+/NaxFeyCo1-yO2 revealed changes in the character of discharge curve as a function of iron content and enabled us to select the NaFe0.3Co0.7O2 as a material exhibiting optimal chemical composition assuring high, stable discharge capacity (180 mAh g−1) along with high operational potential. The EIS and GITT studies were performed in order to investigate the kinetics of the electrode processes. The relation between sodium diffusion coefficients and chemical composition confirmed enhanced performance in case of the NaFe0.3Co0.7O2 material.