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^0/2+ process in different combinations of the Mg²⁺-Li⁺-borohydride-bis(trifluoromethylsulfonyl)imide (TFSI^-) electrolytes in 1,2-dimethoxyethane (DME) solvent. Results indicate that the presence of BH₄^- is essential for high coulombic efficiency, which coordination to Mg²⁺ 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⁺ + BH₄^- + TFSI^- combination of ionic species prevents passivation of the magnesium surface and thereby enables efficient Mg^0/2+ electrochemical cycling. The best Mg^0/2+ 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(BH₄)₂]:[LiTFSI] mole ratio of 1:2.