Jean-Claude Badot

Multiscale electronic transport mechanism and true conductivities in amorphous carbon–LiFePO4 nanocomposites

Composite and nanostructured materials have hierarchical architecture with different levels: (a) macroscopic (substructure of porous clusters); (b) mesostructural (particles constituting the clusters); and (c) microscopic and nanometric (coatings, bulk of the particles). The identification of the key parameters that affect the electronic transport across all observed size scales is required, but is not possible using conventional dc-conductivity measurements. In this paper, the powerful broadband dielectric spectroscopy (BDS) from low-frequencies (few Hz) to microwaves (few GHz) is applied to one of the most important composite materials for lithium batteries. LiFePO4 is wrapped in a carbon coating whose electrical properties, although critical for battery performance, have never been measured due to its nanometre-size and the powdery nature of the material. We provide a description of the electronic transport mechanism from the nanoscale (sp2 crystallites) up to the sample macroscopic scale for this material. Moreover, the true conductivities and their respective drop when going from one scale to another are given, for the very first time, in the case of a composite powdery material for lithium batteries.

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

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.