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

TL;DR

This study synthesizes Na$_x$TO$_2$ (T=Mn, Co) nanostructures, characterizes their physical properties, and evaluates their potential as cathode materials in Na-ion batteries, demonstrating reversible Na-ion intercalation.

Contribution

It introduces a synthesis method for Na$_x$TO$_2$ nanostructures and provides comprehensive physical and electrochemical characterization for battery applications.

Findings

Na$_{0.6}$MnO$_2$ is highly insulating.

Na$_{0.7}$CoO$_2$ is semiconducting with dual conduction mechanisms.

Electrochemical tests confirm reversible Na-ion intercalation.

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.