Thesis presented December 11, 2023

Abstract:
State-of-the-art lithium–ion technology is reaching its limits regarding applications as energy storage devices for electric mobility. In fact, both high energy density and safety standards requested by the market are hardly attainable with the actual materials and components. In theory, the current limitations could be overcome by the use of metallic lithium as the negative electrode, which would increase the energy density of the cell but would also require a mean to prevent lithium dendritic growth. In this context, polymer electrolytes are promising materials as their solid state could hinder the dendritic growth. Nevertheless, in practice, they still lack sufficient ionic conductivity. It has been reported that, in some conditions, the fabrication of composite material by adding fillers inside a polymer electrolyte can enhance the ionic conductivity. Some studies attributed this effect to beneficial interactions occurring at the interface between fillers and the polymer-lithium salt system. Other studies, on a larger scale, highlighted modifications of the polymer mobility in presence of filler. Together, these results suggest that fillers create faster conduction pathways surrounding them, which on a macroscopic scale could enhance the electrolyte conductivity.
This thesis work aims at understanding the contribution of these effects on the transport properties, in order to clarify the role of fillers added inside a polymer electrolyte. The system under investigation is an electrolyte based on poly(trimethylene carbonate) (PTMC) and LiTFSI, inside which we mixed different proportion of alumina particles. We selected three kind of particles with different morphologies and crystalline phases. In order to study the impact of fillers at multiple scales, we combined characterizations in lab and at large-scale facilities. The ionic transport properties were studied by electrochemical techniques. The composite microstructure was probed by phase contrast X-ray imaging and small angle scattering - X-rays and neutrons. Several relevant microstructural parameters were identified, quantified, and then correlated with the properties of ion transport of the electrolyte. We showed that the density of hydroxyls on the surface of particles for a given volume of electrolyte could be increased two-fold depending on the filler type, and that this parameter is linked to the state of agglomeration of fillers and the homogeneity of their dispersion. This structural study is supplemented by a study on the dynamics of PTMC at the molecular scale by quasi-elastic neutron scattering (QENS). Our results show that the presence of lithium salt hinders the intrinsic mobility of PTMC, while in presence of alumina, the PTMC polymer backbone recovers a local mobility. At typical timescales of hundreds of picoseconds, relaxation times are divided by a factor two in presence of fillers.
All the characterization conducted shed light on the impact of inert filler on the transport properties of polymer electrolyte. Inside a composite electrolyte, the presence of filler induce multiple effects that combine and the complex outcome depend on multiple factors. We showed that the extent of interactions at the interface between particles and polymer as well as variations in the local mobility of polymer correlate with changes in the ionic transport properties of the electrolyte. The understanding of these mechanisms establish an important step toward the optimization of composite formulation for the production of better performing composite electrolytes.

Keywords:
Battery, composite electrolyte, polymer electrolyte, large scale facilities, nanoparticles, neutron scattering