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 Mg0/2+ 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 Mg0/2+ electrochemical cycling. The best Mg0/2+ 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.
Keywords: Mg electrochemistry; Mg electrodeposition; bis(trifluoromethylsulfonyl)imide; borohydride; mixed electrolyte.