Growth mechanism and magnetic and electrochemical properties of Na_0.44MnO₂ nanorods as cathode material for Na-ion batteries

Nanorods of Na_0.44MnO₂ are a promising cathode material for Na-ion batteries due to their large surface area and single crystalline structure. We report the growth mechanism of Na_0.44MnO₂ nanorods via solid state synthesis and their physical properties. The structure and the morphology of the Na_0.44MnO₂ nanorods are investigated by X-ray diffraction (XRD), scanning and tunneling electron microscopy (SEM and TEM), and energy-dispersive X-ray (EDX) techniques. The growth mechanism of the rods is investigated and the effects of vapor pressure and partial melting of Na-rich regions are discussed. The magnetic measurements show an antiferromagnetic phase transition at 25 K and the μ_eff is determined as 3.41 and 3.24 μ_B from the χ-T curve and theoretical calculation, respectively. The electronic configuration and spin state of Mn³⁺ and Mn⁴⁺ are discussed in detail. The electrochemical properties of the cell fabricated using the nanorods are investigated and the peaks in the voltammogram are attributed to the diffusion of Na ions from different sites. Na intercalation process is explained by one and two Margules and van Laar models.