Influence of the carboxymethyl cellulose binder on the multiscale electronic transport in carbon–LiFePO4 nanocomposites

Abstract

The broadband dielectric spectroscopy (BDS) technique (40 to 10¹⁰ Hz) is used here to measure the electronic transport across all observed size scales of nanocomposite materials for lithium batteries composed of an active material (e.g. carbon-coated LiFePO₄) and a polymeric binder. Data acquisitions as functions of temperature were also carried out, in order to determine the activation energies of the conductivity and relaxation frequencies at the different scales of the materials' architectures. Different electrical relaxations are evidenced, resulting from the polarizations at the different scales of the architecture. When the frequency increases, four dielectric relaxations are detected in the following order and due to: (a) space-charge polarization (low-frequency range) owing to the interface between the sample and the conductive metallic layer deposited on it; (b) polarization of C–LiFePO₄ clusters (micronic and/or submicronic scales) induced by the existence of resistive junctions between them; (c) electron hopping between sp² domains (nanometric scale) within the carbon coating around the LiFePO₄ particles; and (d) electron transfer through inter-graphitic layers within sp² domains. The influence of the binder (sodium-carboxymethyl cellulose) on the coating conductivity has been evidenced. The establishment of (non-covalent) bonds between the adsorbed binder and the carbon coating results in the decrease of the hopping distance and at the same time in an increase of the potential barrier for hopping. These phenomena could be due to an electron trapping by the adsorbed polymeric species at the surface of the coating, resulting in a decrease of the coating conductivity.