High-Performance Magnesium Electrochemical Cycling with Hybrid Mg-Li Electrolytes.

Kinetics and coulombic efficiency of the electrochemical magnesium plating and stripping processes are to a significant extent defined by the composition of the electrolyte solution, optimization of which presents a pathway for improved performance. Adopting this strategy, we undertook a systematic investigation of the Mg process in different combinations of the Mg-Li-borohydride-bis(trifluoromethylsulfonyl)imide (TFSI) electrolytes in 1,2-dimethoxyethane (DME) solvent. Results indicate that the presence of BH is essential for high coulombic efficiency, which coordination to Mg was confirmed by Raman and NMR spectroscopic analysis. However, the high rates observed also require the presence of Li and a supplementary anion such as TFSI. The Li + BH + TFSI combination of ionic species prevents passivation of the magnesium surface and thereby enables efficient Mg electrochemical cycling. The best Mg performance with the stabilized coulombic efficiency of 88 ± 1% and one of the highest deposition/stripping rates at ambient temperature reported to date are demonstrated at an optimal [Mg(BH)]:[LiTFSI] mole ratio of 1:2.