Electrochemical properties and structural evolution of O3-type layered sodium mixed transition metal oxides with trivalent nickel

The electrochemical properties of NaNi_0.5Co_0.5O₂ and NaNi_0.5Fe_0.5O₂ and their structural transitions as a function of Na extraction associated with redox reactions are investigated in this work. Synthesized in the O3-type layered structure, both materials show reasonable electrochemical activities at room temperature, delivering approximately 0.5 Na per formula unit at C/10 discharge. More Na can be reversibly cycled in NaNi_0.5Co_0.5O₂ at elevated temperature and/or in an extended voltage window, while NaNi_0.5Fe_0.5O₂ shows significant capacity fading at a high voltage cutoff which is likely due to Fe⁴⁺ migration. In situ X-ray diffraction shows that the structural changes in the two materials upon desodiation are very different. NaNi_0.5Co_0.5O₂ goes through many different two-phase reactions including three different O3-type and three different P3-type structures during cycling, producing a voltage profile with multiple plateau-like features. In contrast, NaNi_0.5Fe_0.5O₂ has a smooth voltage profile and shows the typical O3-P3 phase transition without lattice distortion seen in other materials. This different structural evolution upon desodiation and re-sodiation can be explained by the electronic structure of the mixed transition metals and how it perturbs the ordering between Na ions differently.