Fundamental Understanding of the Behavior Under Aging of Aqueous Magnesium-Based Concentrated Electrolytes

Abstract

The concept of water-in-salt electrolytes (WISEs)1, implying highly concentrated aqueous solutions, has completely rejuvenated the aqueous systems. These electrolytes can be depicted as few water molecules surrounded by cations and anions forming ion pairs. This change of solvation structure compared to standard electrolytes increases the electrochemical stability window. For example, using 20 molal (molality: m, moles per kg) lithium (bistrifluoromethanesulfonyl(imide) (LiTFSI) aqueous solutions, a stability window of 3 V is reached.1 While WISEs open the way for sustainable systems, these solutions only use expensive and toxic lithium salts. To move towards more sustainable elements, one can envision magnesium and associated salts: safer, less expensive and more abundant than lithium. Yet, the state-of-the-art on water-in-salt aqueous Mg batteries is scarce,2,3 with potential windows reaching only 2-2.3 V. There is no clear understanding of the mechanisms at work but the low solubility of Mg salts could account for the narrow potential window. Rationalizing reactivity of water-in-salt solutions on a fundamental level is critical to design efficient strategies to increase the voltage window of concentrated aqueous Mg batteries. We investigated the Mg(TFSI)2/H2O system with radiolysis, a technique which provides a chemical approach4 to exacerbate in a short time the degradation mechanisms. The aging process with radiolysis is much shorter than the electrochemical one and mimics the products of salt and solvent degradation observed during long battery operation. We performed radiolysis to study and compare Mg(TFSI)2 and LiTFSI-based electrolytes with different molalities. Degradation products formed after irradiation were identified using Nuclear Magnetic Resonance spectroscopy (NMR) for the liquid phase, and with gas chromatography coupled to mass spectrometry (GC-MS) and micro-gas chromatography (μ-GC) for the gas phase. For example, the evolution of the amount of H2 and CO2 gases in the case of LiTFSI or Mg(TFSI)2/H2O electrolytes show large discrepancies, hinting at a preferential anion degradation in the Mg system. Overall, the nature of the degradation species strongly depends on the molality, with degradation routes driven by the water or the anion at low or high molalities. Finally, radiolysis allows to identify minor species, providing a view of the long-term (un)stability of these electrolytes.