Lattice thermal conductivity in bulk and nanosheet NaxCoO2

Abstract

In this paper we present the results of calculations of the lattice thermal conductivity of layered complex metal oxide NaxCoO2 within the Green–Kubo theory. Using NaxCoO2 we identify the two competing mechanisms responsible for the favorable scaling properties of the Green–Kubo method for calculating the lattice thermal conductivity. The artificial correlations of the heat flux fluctuations due to the finite size of the supercells are partially cancelled by the missing long wavelength acoustic phonon modes. We compute the lattice thermoelectric properties of bulk NaxCoO2 with varying stoichiometry, structural defects, and temperature. We also calculate the thermal conductivity of NaxCoO2 in the nanosheet geometry. While the dependence of thermal conductivity on Na fractions x in the middle range (0.5<x<0.8) is relatively weak, introducing Co vacancies results in significant lattice thermal conductivity reduction. The material exhibits strong anisotropy of lattice thermal conductivity due to a layered crystal structure and relatively weak bonding between layers. This structure leads to the possibility of manufacturing relatively large nanosheets of NaxCoO2. However, the weak inter-layer binding also results in the insensitivity of thermal conductivity to the nanosheet thickness.