Synthesis and physical properties of Na${}_x$TO${}_2$ (T=Mn, Co) nanostructures for cathode materials in Na--ion batteries

Abstract

We prepared Na${}_x$TO${}_2$ (T = Mn, Co) nanostructures to use as cathode material in Na-ion batteries. The Rietveld refinement of x-ray diffraction data confirms the hexagonal symmetry. Scanning and transmission electron microscopy measurements show hexagonal and rod-shape morphology for T = Co and Mn, respectively. The magnetic measurements indicate the presence of variable oxidation states of Mn/Co ions, but no long range ordering till 5~K. We find that the Na${}_{0.6}$MnO${}_2$ is highly insulating whereas Na${}_{0.7}$CoO${}_2$ is semiconducting in nature. The conduction in Na${}_{0.7}$CoO${}_2$ takes place through both variable range hopping (VRH) as well as activation mechanisms in different temperature ranges. However, for Na${}_{0.6}$MnO${}_2$ the VRH prevails at elevated temperatures. Furthermore, the coin cells have been fabricated and tested the electrochemical behavior using cyclic voltammetry, which confirms the reversibility of Na--ions during intercalation/de-intercalation.

to appear in Materials Research Bulletin