High-Performance, Activation-Free Magnesium-Ion Batteries Enabled by Ionic Liquid Electrolyte Additive

Magnesium-ion batteries (MIBs), as a highly promising next-generation energy storage technology, benefit from the high theoretical volumetric capacity (3833 mAh L^-1) of magnesium metal and its intrinsic safety. However, its commercialization is still hindered by sluggish de-solvation kinetics during cycling, which prolongs the activation periods to reach maximum capacity and impairs rate performance. To overcome this bottleneck, we add 4-ethyl-4-methylmorpholinium cation (EMM⁺) as an additive into the conventional all-phenyl-complex (APC) electrolyte. Density functional theory computations confirm that EMM⁺ shows a strong affinity for chloride ions (-0.513 eV), which weakens the Mg-Cl coordination and, thus promotes Mg²⁺ de-solvation. In CuS-based MIBs, the modified APC-EMM electrolyte eliminates the activation cycles that are required with pure APC, and achieves a high specific capacity of 405.1 mAh g^-1 at 100 mA g^-1, while maintaining excellent rate performance (220.1 mAh g^-1 at 1 A g^-1). Notably, this electrolyte also shows significant improvements in capacity, activation kinetics, and cycling stability when applied to other cathode materials, including CuSe, Cu₇Te₄, Mo₆S₈, and perylene-3,4,9,10-tetracarboxylic dianhydride. This study establishes a de-solvation-accelerated electrolyte design concept as a universal paradigm for the development of high-performance MIBs.