Investigating the Accelerated Aging of Water-in-Salt Electrolytes: A Focus on Magnesium Perchlorates via Radiolysis

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.