Influence of Residual Water Traces on the Electrochemical Performance of Hydrophobic Ionic Liquids for Magnesium‐Containing Electrolytes

Abstract

AbstractA trace amount of water is typically unavoidable as an impurity in ionic liquids, which is a huge challenge for their application in Mg‐ion batteries. Here, we employed molecular sieves of different pore diameters (3, 4, and 5 Å), to effectively remove the trace amounts of water from 1‐methyl‐1‐propylpiperidinium bis(trifluoromethylsulfonyl)imide (MPPip‐TFSI) and 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP‐TFSI). Notably, after sieving (water content <1 mg ⋅ L−1), new anodic peaks arise that are attributed to the formation of different anion‐cation structures induced by minimizing the influence of hydrogen bonds. Furthermore, electrochemical impedance spectroscopy (EIS) reveals that the electrolyte resistance decreases by ∼10 % for MPPip‐TFSI and by ∼28 % for BMP‐TFSI after sieving. The electrochemical Mg deposition/dissolution is investigated in MPPip‐TFSI/tetraglyme (1 : 1)+100 mM Mg(TFSI)2+10 mM Mg(BH4)2 using Ag/AgCl and Mg reference electrodes. The presence of a trace amount of water leads to a considerable shift of 0.9 V vs. Mg2+/Mg in the overpotential of Mg deposition. In contrast, drying of MPPip‐TFSI enhances the reversibility of Mg deposition/dissolution and suppresses the passivation of the Mg electrode.