Layered P2-type Na_2/3Ni_1/3Mn_2/3O₂: conductivity and electrochemical characteristics

The cathode material of Na_2/3Ni_1/3Mn_2/3O₂ composition (NNMO) was obtained by the carbonate coprecipitation followed by solid-phase reaction and characterized by XRD analysis, ICP-MS, and electron and impedance spectroscopy. NNMO crystallizes in a P2-type layered hexagonal structure (sp. gr. P63/mmc), consists of spherical agglomerates of ~ 1–3 microns in size, and forms from the plate-like primary grains. The NNMO ionic conductivity value at room temperature was 1.8*10⁻⁴ and 1.3*10⁻⁴ S cm⁻¹ when measured perpendicularly and parallel to the compression axis, respectively. The estimated in dc mode values of electronic conductivity were by 2–3 orders of magnitude less than ionic conductivity. The obtained materials were tested as cathodes in sodium-ion battery cells versus sodium metal. The discharge capacity of NNMO was 160 mAh g⁻¹ and 86 mAh g⁻¹ in the potential range of 1.5–4.0 V and 2.3–4.0 V, respectively (20 mA g⁻¹). NNMO was shown to be stable under cycling in the potential range of 2.3–4.0 V.