Simultaneous optimization of solvation structure and water-resistant capability of MgCl2-based electrolyte using an additive combination of organic and inorganic lithium salts

The inefficient operation of Mg batteries associated with the high sensitivity of electrolyte to impurities (water, air, etc.) seriously impedes their practical use. Here, we report a water-resistant MgCl2-based electrolyte consisting of low-cost organic lithium hexamethyldisilazide (LiHMDS) and inorganic lithium chloride (LiCl) dual-salt additives. The electrolyte displays excellent electrochemical performance for reversible Mg stripping and plating, with overpotential of 0.15 V and 0.30 V at 5 mA cm–2 and 10 mA cm–2, respectively, and Coulombic efficiency (CE) up to 100%. It keeps its reactivity even with the presence of 1000 ppm H2O or ∼3% impurities introduced by using impure reagents (MgCl2, 97%) during its synthesis. Experimental characterization and theoretical calculations reveal that the single-salt additive of organic LiHMDS in MgCl2/THF is a “double edged sword”:the upside is that, with a small amount added, it contributes to reduce the de-solvation energy of Mg2+by forming water-resistant [MgxLiyHMDSzCl2x+y-z· nTHF] aggregates with MgCl2 salts; the downside is that, while its amount increases, it starts to dissociate those functional aggregates. On the other hand, adding an inorganic salt LiCl as co-additive can reconstruct [MgxLiyHMDSzCl2x+y-z· nTHF] aggregates and avoid their dissociation. With this hybrid electrolyte, a Mg//Mo6S8 full cell can achieve a discharge specific capacity of 83 mA h g–1 even after 10,000 cycles at a high rate of 31.1 C (1 C = 128.8 mA g–1). This solvation structure reconstruction approach has far-reaching significance for the electrolyte design for rechargeable magnesium batteries.