ABSTRACT
The morphological and the electrical properties of carbon coated LiFePO4 (LFPC) active material functionalized by 4-ethynylbenzene tetrafluoroboratediazonium salt were investigated. For this purpose, FTIR, Raman, XPS, High Resolution Transmission Electron Microscopy (HRTEM) and Broadband Dielectric Spectroscopy (BDS) were considered. Electronic conductivities of LFPC samples at room temperature were found to decrease in a large frequency range upon simple immersion in polar solvents and to decrease further upon functionalization. Due to their high dipole moment, strongly physisorbed molecules detected by XPS likely add barriers to electron hopping. Significant alteration of the carbon coating conductivity was only observed, however, upon functionalization. This effect is most presumably associated with an increase in the sp(3) content determined by Raman spectroscopy, which is a strong indication of the formation of a covalent bond between the organic layer and the carbon coating. In this case, the electron flux appears to be redirected and relayed by short-range (intra chain) and long-range (inter chain) electron transport through molecular oligomers anchored at the LFPC surface. The latter are controlled by tunnelling and slightly activated hopping, which enable higher conductivity at low temperature (T < 250 K). Alteration of the electron transport within the carbon coating also allows detection of a relaxation phenomenon that corresponds to small polaron hopping in bulk LiFePO4. XPS and HRTEM images allow a clear correlation of these findings with the island type oligomeric structure of grafted molecules.
ABSTRACT
This work is the first detailed study concerning the multiscale electronic transport and its temperature dependence in the LiNi1/3Co1/3Mn1/3O2 (NMC) family, high-capacity electrode materials for lithium ion batteries. Powders with two different mean cluster sizes (3 µm and 10 µm) but the same particle sizes (0.4 to 1.3 µm) were measured. The detailed formula of the studied compound is Li1.04Ni(2+)0.235Ni(3+)0.09Mn(4+)0.315Co(3+)0.32O2. Different electrical relaxations are evidenced, resulting from the polarizations at the different scales of the powder architecture. When the frequency increases, three dielectric relaxations are detected in the following order 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 NMC clusters (micronic scale) induced by the existence of resistive junctions between them; and (c) polarization of NMC particles (at sub-micronic scale) induced by resistive junctions between them. High interatomic level conductivity of about 20 S m(-1) was evidenced and attributed to the contribution of the extended states and to a Brownian motion of the charge carriers with mean free path similar to the lattice constant. The ratio between sample and local conductivity is more than 10(5). The large conductivity drop of 3 to 4 orders of magnitude is observed from the particle to the cluster scale. A very large number of charge carriers are blocked by the interparticle junctions within the clusters. The conductivity drop from the cluster to the sample scale is comparatively very small, owing to the dense architecture of the NMC sample in which the spherical clusters are very piled up on each other.
ABSTRACT
The broadband dielectric spectroscopy (BDS) technique (40 to 10(10) Hz) is used here to measure the electronic transport across all observed size scales of a Li(1.1)V(3)O(8)-polymer-gel composite material for lithium batteries. Different electrical relaxations are evidenced, resulting from the polarizations at the different scales of the architecture: (i) atomic lattice (small-polaron hopping), (ii) particles, (iii) clusters of particles, and finally (iv) sample-current collector interface. A very good agreement with dc-conductivity measurements on a single macro-crystal [M. Onoda and I. Amemiya, J. Phys.: Condens. Matter, 2003, 15, 3079.] shows that the BDS technique does allow probing the bulk (intrinsic) electrical properties of a material in the form of a network of particles separated by boundaries in a composite. Moreover, this study highlights a lowering of the surface electronic conductivity of Li(1.1)V(3)O(8) particles upon adsorption of polar ethylene carbonate (EC) and propylene carbonate (PC) that trap surface polarons. This result is meaningful as EC and PC are typical constituents of a liquid electrolyte of lithium batteries. It is thus suggested that interactions between active material particles and the liquid electrolyte play a role in the electronic transport within composite electrodes used in a lithium battery.
