Ionic liquid electrolyte additive regulates the multi-species-insertion titanium sulfide cathode for magnesium batteries

Abstract

Benefiting from the appealing Mg metal anodes, magnesium batteries (MBs) present attractive potential as sustainable batteries of tomorrow. However, the Mg metal anode-compatible electrolytes generally contain large-size and strongly bonded Mg-clusters ( i.e., Mg x Cl y 2x-y ), resulting in the inefficient cathode chemistries associated with the sluggish Mg-species insertion. Here, using the iconic TiS 2 cathode, we demonstrate the pronounced effect of ionic liquid on regulating Mg x Cl y 2x-y clusters in the MB electrolyte and promoting the high-kinetics multi-Mg-species insertion into TiS 2 . Specifically, the addition of 1-butyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide (PP14TFSI) ionic liquid into the conventional Mg bis(hexamethyldisilazide)/4MgCl 2 electrolyte induces a nontrivial two-plateau charge/discharge profile of the TiS 2 electrode, in which Mg 2+ insertion is mainly disclosed at the high-potential plateau and MgCl + insertion dominates the low-potential plateau. Molecular dynamic simulations indicate that the PP14TFSI additive can dissociate large Mg x Cl y 2x-y clusters to produce MgCl + , which can be effectively stabilized by PP14 + and TFSI – . Meanwhile, PP14TFSI catalyzes the Mg-Cl dissociation, thus creating the desirable Mg 2+ species. These electrolyte-regulation effects consequently enable the TiS 2 cathode with a decent specific capacity (81 mAh g –1 at 10 mA g –1 ), high rate capability (63 mAh g –1 at 200 mA g –1 ), and long-term durability (86% capacity retention after 500 cycles).