Sophie Le Caër

Highly concentrated aqueous electrolytes have revitalized the field of aqueous batteries. Herein, we investigated the influence of anions on the physicochemical characteristics, solvation structure, and electrochemical behavior of magnesium aqueous electrolytes. Four salts, featuring acetate, nitrate, perchlorate, and imide anions, were selected to span a wide range of chaotropic and kosmotropic behaviors according to the Hofmeister classification. By combining ¹ H, ¹⁷ O NMR, and infrared spectroscopy with viscosity measurements, a strong correlation emerges between the anion’s nature and its ability to weaken (chaotropicity) or reinforce (kosmotropicity) the water hydrogen‐bond network. For the nitrate, at the boundary between chaotropic and kosmotropic behavior, and the weakly chaotropic perchlorate, results suggest a uniform solvation around [Mg(H ₂ O) _x ] ²⁺ clusters; NO ₃ ⁻ may retain some interaction with water around Mg ²⁺ , while ClO ₄ ⁻ appears more decoupled from Mg ²⁺ . In contrast, the strongly chaotropic TFSI ⁻ (bis(trifluoromethanesulfonyl)imide anion) induces a more pronounced perturbation of the hydrogen‐bond network, with TFSI ⁻ largely outside the Mg ²⁺ hydration shell. Finally, the anion‐dependent solvation contributes to electrochemical performance. A 4 m magnesium perchlorate electrolyte exhibits markedly enhanced capacity retention and Coulombic efficiency than a 4 m magnesium acetate electrolyte. However, the overall electrochemical behavior cannot be described by a simple monotonic relationship with the Hofmeister classification.

A fractional diffusion model describes delayed H ₂ release from irradiated portlandite, with a subdiffusive exponent consistent with atomistic simulations.

Magnesium-based batteries hold great promise for energy storage, owing to the abundance of magnesium and its theoretical high energy densities. However, the use of organic electrolytes in these batteries presents significant safety concerns, particularly regarding corrosion, flammability and toxic degradation products. Water-in-salt electrolytes (WiSE) offer a safer solution, expanding the electrochemical stability window of conventional aqueous solutions through high salt concentrations. Among the various salts, magnesium perchlorate stands out due to its good solubility, low cost, and chaotropic properties, which modify the water network significantly. To investigate the aging mechanisms of Mg(ClO₄)₂ WiSE electrolytes, radiolysis-induced degradation processes were studied using picosecond pulse radiolysis experiments, with a focus on the effects of salt molality on the formation of reactive species. These experiments, in conjunction with quantum chemical calculations, provide insights into the formation of transient species and their evolution during electrolyte degradation. Moreover, the radiolytic yields of stable species, including dihydrogen (H₂), dioxygen (O₂), chlorate (ClO₃^-), and chloride (Cl^-), were measured. O₂ is exclusively derived from perchlorate anions, while H₂ originates from water. As the concentration of Mg(ClO₄)₂ increases, the radiolytic yields of chlorinated species rise, in particular for ClO₃^-. The results obtained highlight that excited perchlorate anions in concentrated solutions generate a range of reactive species, namely ClO₃^-, ClO₂^-, as well as ClO₄^• and ClO₃^• radicals. Notably, ClO₃^- anions are formed from excited perchlorate anions. On the other hand, ClO₄^•, also directly generated through ionizing radiation/matter interaction, leads to the formation of O₂ and ClO₂^•. The ClO₂^• radical quickly loses O₂, resulting in the formation of the chlorine radical, and eventually of the chloride anion. The study also reveals that hydrogen peroxide formation is consistent across varying molalities, indicating a balance between contributions from both water and perchlorate anions. These findings provide critical insights into the radiolysis processes in Mg(ClO₄)₂ electrolytes, contributing to the understanding of electrolytes aging to further develop more efficient magnesium-ion batteries.

(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.

Minor actinide (MA) selective materials that are resistant to radiation are necessary to enable separation of MAs from lanthanides in nuclear wastes. Zirconium(IV) phosphonates (ZrPs) are a class of amorphous coordination polymers with promising applications as a solid-phase sorbent for MA-lanthanide separations. In this study, a zirconium phosphonate sorbent (ZrPTP) that intramolecularly incorporates the MA-selective 2,6-bis(1,2,3-triazol-4-yl)pyridine (PTP) ligand was synthesised and evaluated for radiation tolerance with high energy electron irradiation to doses of 2 MGy. The MA sorption selectivity of ZrPTP before and after irradiation was compared using Am and Eu. ZrPTP demonstrated maintained selectivity for Am over Eu even after a 2 MGy dose. Synchrotron radiation characterisation techniques and solid-state NMR were employed to accurately assess the average local structure before and after irradiation, where minor amounts of Zr–O bonds, aliphatic C–H bonds, and triazole groups were broken, showcasing excellent radiation stability for doses up to 2 MGy. Our results demonstrate the importance of chemistry in ZrP coordination polymers maintaining not only selective separation properties, but also maintaining radiation stability as well.