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

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_xCl_y^2x-y), resulting in the inefficient cathode chemistries associated with the sluggish Mg-species insertion. Here, using the iconic TiS₂ cathode, we demonstrate the pronounced effect of ionic liquid on regulating Mg_xCl_y^2x-y clusters in the MB electrolyte and promoting the high-kinetics multi-Mg-species insertion into TiS₂. Specifically, the addition of 1-butyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide (PP14TFSI) ionic liquid into the conventional Mg bis(hexamethyldisilazide)/4MgCl₂ electrolyte induces a nontrivial two-plateau charge/discharge profile of the TiS₂ electrode, in which Mg²⁺ 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_xCl_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 Mg²⁺ species. These electrolyte-regulation effects consequently enable the TiS₂ 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).