Résumé :
Aqueous batteries were long believed to be constrained by the stability window of water (1.23 V), but this is no longer true thanks to water-in-salt electrolytes (WISEs). Using highly concentrated solutions leads to an extraordinary increase of the potential window of lithium aqueous batteries. It is mainly explained by the decrease of free water molecules and by the crucial role of the anion on the formation of the solid interphase electrolyte (SEI). While WISEs open the way for sustainable systems, these solutions only use expensive and toxic salts. Magnesium and associated salts are an interesting alternative : safer, less expensive and more abundant than lithium. Yet, the few Mg batteries based on WISEs reported only slightly improve the potential window to 2 V, without a clear understanding of the mechanisms at work. The aim of the thesis is to rationalize reactivity of concentrated aqueous solutions to design strategies for increasing the potential window of aqueous Mg batteries. One key parameter revealed in the case of imide or perchlorate-based electrolytes is the amount of ions in solution, whereas the nature of the cation (Li or Mg) has little influence. In addition, radiolysis experiments were used to propose degradation mechanisms for these electrolytes. These results were also correlated with the electrochemical behaviour. Finally, we demonstrated the benefits of using a concentrated aqueous electrolyte composed essentially of magnesium perchlorate. This electrolyte exhibits good electrochemical performance. This work shows the interest and possibility of implementing an effective strategy to limit water reactivity in aqueous magnesium batteries.
Keywords: aqueous electrolytes, electrochemistry, radiolysis, magnesium, batteries, spectroscopie