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

Abstract

• LiHMDS with water-resistant capability is found to assist the solvation of MgCl 2 in THF solvent by ion association. • A small amount of LiHMDS in MgCl 2 /THF contributes to reduce the de-solvation energy of Mg 2+ by forming [Mg x Li y HMDS z Cl 2x+ y -z · n THF] aggregates. • An increasing amount of LiHMDS in MgCl 2 /THF makes the functional [Mg x Li y HMDS z Cl 2x+ y -z · n THF] start to dissociate. • LiCl as co-additive can avoid the dissociation of [Mg x Li y HMDS z Cl 2x+ y -z · n THF]. • A Mg//Mo 6 S 8 full cell can be cycled for over 10,000 cycles with a superior capacity retention of 83 mA h g –1 even under an ultrahigh rate of 31.1 C (1 C = 128.8 mA g –1 ). 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 MgCl 2 -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 H 2 O or ∼3% impurities introduced by using impure reagents (MgCl 2 , 97%) during its synthesis. Experimental characterization and theoretical calculations reveal that the single-salt additive of organic LiHMDS in MgCl 2 /THF is a “double edged sword”:the upside is that, with a small amount added, it contributes to reduce the de-solvation energy of Mg 2+ by forming water-resistant [Mg x Li y HMDS z Cl 2x+ y -z · n THF] aggregates with MgCl 2 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 [Mg x Li y HMDS z Cl 2x+ y -z · n THF] aggregates and avoid their dissociation. With this hybrid electrolyte, a Mg//Mo 6 S 8 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.