Structural characterization and electrochemical intercalation of Li+ in layered Na0.65Ni0.5Mn0.5O2 obtained by freeze-drying method

Abstract

New data on the structure and reversible lithium intercalation properties of sodium-deficient nickel–manganese oxides are provided. Novel properties of oxides determine their potential for direct use as cathode materials in lithium-ion batteries. The studies are focused on Na x Ni0.5Mn0.5O2 with x = 2/3. Between 500 and 700 °C, new layered oxides Na0.65Ni0.5Mn0.5O2 with P3-type structure are obtained by a simple precursor method that consists in thermal decomposition of mixed sodium–nickel–manganese acetate salts obtained by freeze-drying. The structure, morphology, and oxidation state of nickel and manganese ions of Na0.65Ni0.5Mn0.5O2 are determined by powder X-ray diffraction, SEM and TEM analysis, and X-ray photoelectron spectroscopy (XPS). The lithium intercalation in Na0.65Ni0.5Mn0.5O2 is carried out in model two-electrode lithium cells of the type Li|LiPF6(EC:DMC)|Na0.65Ni0.5Mn0.5O2. A new structural feature of Na0.65Ni0.5Mn0.5O2 as compared with well-known O3–NaNi0.5Mn0.5O2 and P2–Na2/3Ni1/3Mn2/3O2 is the development of layer stacking ensuring prismatic site occupancy for Na+ ions with shared face on one side and shared edges on the other side with surrounding Ni/MnO6 octahedra. The reversible lithium intercalation in Na0.65Ni0.5Mn0.5O2 is demonstrated and discussed.