Anion‐Dependent Structure and Properties of Concentrated Aqueous Magnesium Electrolytes

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Abstract
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.
Funding Information
  • Agence Nationale de la Recherche (ANR‐21‐CE50‐0005‐01)

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