Li-ion batteries have a major role to play in the ecological transition, particularly in the development of electric vehicles, but also in providing optimised storage systems. However, the most commercialised system, lithium-ion batteries, are reaching their limits in terms of energy density and safety. One way of improving their safety is to replace the flammable organic electrolyte with a water-based electrolyte. Until now, water had not been considered as a solvent because of its narrow theoretical electrochemical stability window of 1.23 V, making the energy density of the battery too low for any application. Recently, this limitation has been overcome by using super-concentrated electrolyte, better known as water-in-salt (WISE). This approach involves introducing a very large quantity of lithium salt into water, exceeding the solvent in mass and volume. A high salt concentration considerably diminishes the molecular activity of water by reducing the quantity of ‘free’ water molecules, thus limiting the decomposition reactions of water, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). As a result, the electrochemical stability window can be extended to 3 V in the case of a 21 mol/kg solution of lithium bis(trifluoromethanesulphonyl)imidide (LiTFSI). While this electrolyte holds many promises in terms of transport properties with high ionic conductivity, batteries made with WISE encounter numerous problems, in particular numerous degradation reactions such as drying out of the electrolyte during cycling, corrosion of the current collectors and gas evolution, all of which lead to a reduction in electrochemical activity right up to the end of the cell's life. The aim of this thesis is to study the causes of such transport properties, as well as the reactions that lead to electrochemical performance decay of these batteries. Firstly, the impact of salt concentration is studied using a multi-scale approach that allows the dynamics of water molecules to be tracked by combining results from pulsed-field nuclear magnetic resonance (NMR) and quasi-elastic neutron scattering (QENS). These data, correlated with measurements of ionic conductivity and viscosity as a function of salt concentration, were used to establish a model for the mobility of the species present in the aqueous electrolyte. Once the dynamics of the aqueous electrolyte had been studied, we turned our attention to the degradation mechanisms. To do this, we studied two active materials, LiFePO4 as the positive electrode material and TiS2 for the negative electrode. By combining the results of multiple characterisation techniques such as X-ray photoelectron spectroscopy (XPS) in laboratory and in synchrotron, X-ray and neutron diffraction, etc., we have demonstrated that a passivation layer, rich in LiF for LiFePO4 and TiO2 for TiS2, is created at the electrode/electrolyte contact and is therefore independent of cycling conditions. Once the electrodes and electrolytes had been characterised, the electrochemical performance of the LiFePO4/TiS2 system could be studied in two-electrode and three-electrode configurations to allow elucidating the parameters controlling cycling failure. Several parameters were tested, including potential cut-off, electrode balancing and cycling rate. Whatever the battery configuration, performance losses are always visible and understanding these degradation requires a multi-technique approach. Gas formation is monitored by mass spectroscopy coupled to electrochemistry, iron oxidation is monitored by X-ray absorption spectroscopy at the iron K-edge and surface degradation is monitored by XPS. Ultimately, the degradation appears to be linked to the HER reaction, but also to OER, oxidation of the electrode's carbon nanoparticles and other unidentified parasitic reactions. The methodology presented here provides a better understanding of aqueous lithium-ion batteries based on water-in-salt electrolytes.

Supervision : Claire VILLEVIEILLE, Lauréline LECARME, Sandrine LYONNARD & Quentin BERROD​​​