(Text in French) Radiolysis and concentrated electrolytes for aqueous batteries: an accelerated aging tool to elucidate degradation mechanisms. Aqueous batteries have long been constrained by the electrochemical stability of water, limiting their theoretical operating voltage window to 1.23 V due to hydrogen and oxygen evolution reactions. The emergence of highly concentrated aqueous electrolytes, commonly referred to as water-in-salt electrolytes (WISEs), has significantly broadened this window. This breakthrough has paved the way for safer and more sustainable aqueous batteries, though most developments still rely on costly and toxic lithium salts. Magnesium emerges as a promising alternative due to its abundance, low toxicity, and low cost. However, studies on magnesium-based concentrated aqueous electrolytes have so far achieved a limited voltage window of around 2 V, without a clear understanding of the mechanisms limiting this performance. The objective of this Ph.D. work was to gain fundamental insight into the reactivity of magnesium-based concentrated aqueous electrolytes, with the aim of identifying strategies to expand their electrochemical stability window. To this end, radiolysis, the set of chemical reactions induced by ionizing radiation, was employed as a tool for accelerated aging. This innovative approach makes it possible to simulate in just a few hours the cumulative effects of several weeks of electrochemical cycling. Using this methodology, we were able to identify the major degradation pathways of the electrolytes, detect transient radical species, and distinguish the respective roles of water and anions in the overall reactivity of the system, depending on the salt concentration. The study was conducted first on lithium-based electrolytes, then extended to magnesium-based systems, laying the groundwork for the rational design of next-generation aqueous battery electrolytes.