Determination of the Formation Potential of Solid-Electrolyte Interphase in Some Amide-Type Ionic Liquids Containing Metal Salts

The formation potentials of the solid-electrolyte interphase (SEI) on a platinum and glassy carbon electrode in two amide-type ionic liquids, 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)amide (BMPFSA) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA), containing some metal salts were investigated by cyclic voltammetry in the presence of ferrocene (Fc). It is considered that the SEI with the thickness of less than a few nm can be observed by monitoring the shift in the peak potential of Fc+/Fc. The SEIs began to form at ∼2.4 V vs. Li|Li(I) in the presence of lithium salts because of a lowering of the lowest unoccupied molecular orbital level of the anion by the interaction with lithium ion. In the case of BMPTFSA containing Ni(TFSA)2, the SEI began to form at -0.7 V vs. Ag|Ag(I), suggesting that the deposition of Ni may be hindered by the SEI formation.