Ph'D defense ROSELYNE JEANNE-BROU

Abstract :
Solid Polymer Electrolytes (SPEs) are promising to replace the conventional flammable liquid electrolyte in batteries to move toward an all-solid-state system comprising a lithium (Li) metal negative electrode. Indeed, they can combine high mechanical properties limiting Li dendrite growth and ionic conductivity high enough for the application. Many materials have been investigated mostly based on Poly(ethylene oxide) (PEO), the reference material, complexed with a Li salt (such as LiTFSI) such as composites (PEO mixed with nanoparticles), neutral and functionalized block copolymers, and crosslinked electrolytes. However, their ionic conductivities are generally below that of the PEO homopolymer above its melting temperature (at about 55 – 60 °C). In addition, it has been mainly reported in the literature an anisotropic effect in ionic conductivity for PEO homopolymer electrolyte, i.e. according to the in-plane (//) and through-plane (Ʇ) and under a series of external fields (mechanical stretching, electromagnetic field, etc.). Therefore, in an attempt to optimize SPE for the application, it is necessary to investigate the isotropic and anisotropic ionic transport properties corresponding to the ionic conductivity, the transference number, and the diffusion coefficient depending on the SPE nature (from homopolymer to functionalized block copolymer electrolytes).
This thesis work focuses first on the study of ionic transport properties (ionic conductivity, but also transference number, and diffusion) according to the two main directions of space (// vs. Ʇ). Series of physico-chemical and electrochemical characterizations were performed to study those ionic transport parameters. The transference number and the diffusion evolve with the ionic conductivity of the SPEs according to the orientations // vs. Ʇ;. In addition, simulations under COMSOL have permit to model in 2-dimensions (2D) the concentration gradients depending on the cell geometry (// vs. & Ʇ). For the diffusion, a 1D analytical model was developed within the framework of John Newman's methodology to establish the model of the experimental relaxations of the potential as a function of time (//). The impact of the chain conformation via polymer chain elongation of the SPEs on the ionic conductivity was also investigated thanks to a lab-made specific instrumentation enabling the coupling of impedance measurements and mechanical elongation in a controlled inert atmosphere. This instrument was designed and realized by a collaboration between LEPMI and the IUT of Chambéry / Le-Bourget-du-Lac.
The second part of the thesis concerns the physico-chemical, materials and electrochemical characterizations of single-ion conducting SPEs based on hybrid crosslinked SPEs synthesized by ICR (Aix-Marseille University). In particular, a methodology based on the subtraction of impedance spectra was developed to determine the main ionic transport contributions and to correlate them with the SPEs’ nanostructuration analyzed by small-angle X-ray scattering (SAXS) carried out by LLB (Gif Sur Yvette). At last, Li metal-based batteries were assembled and cycled as a proof-of-concept to establish the performances with an in-situ LiFePO4 based positive electrode/cathode.