Synthesis, Structure, and Electrochemical Behaviour of O3-type NaNi_1/3Mn_1/3Al_1/3O₂

O3-type layered oxides are being actively investigated as potential positive electrodes for Na-ion batteries due to their high specific capacity. In this work, the role of synthesis conditions on the phase formation in NaNi_1/3Mn_1/3Al_1/3O₂ (NMA111) sample was investigated, and an O3-type phase (R $\overline{3}$ m), along with a minor β-phase (Pn2₁ a), was obtained in the pellet (covered with a sacrificial powder) heated at 850 °C. NMA111 exhibited a mixed conducting behaviour at room temperature, with ionic and electronic conductivities estimated to be ~5.82 × 10^-6 S cm^-1 and ~2.15 × 10^-6 S cm^-1, respectively. The oxidation states and local structure of Mn and Ni were confirmed by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy studies. The electrochemical behaviour of NMA111 was also investigated using the GCD, CV, GITT, and in-situ EIS techniques. The cathode showed an initial discharge capacity of ~87 mAh g^-1 (which corresponds to 0.33 Na-ions reversible intercalation) at 0.1C in the 2.0 to 4.0 V range, and the voltage profile suggested a solid-solution type insert mechanism. A discharged capacity of 70 mAh g^-1 at 2C (~80 % of the initial capacity at 0.1C), and a capacity retention of 70% after 100 cycles at 0.3C were obtained in the NMA111 cathode. The ex-situ XPS measurements confirmed that the Ni^2+/3+ redox couple is responsible for the charge compensation in NMA111 during the charge-discharge process.