Physical and Electrochemical Properties of Ionic-Liquid- and Ester-Based Cosolvent Mixtures with Lithium Salts
Seungmin Oh, Michael Keating, Elizabeth J. Biddinger
Abstract
The ionic liquid (IL) 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide ([Pyr14][TFSI]) with organic solvent methyl propionate (MP) was investigated to gain a fundamental understanding of the thermal, physical and electrochemical properties. Estimation of the coulombic efficiencies on a copper electrode was additionally carried out using cyclic voltammetry as an initial screening to test the promise for lithium metal batteries. The addition of MP to [Pyr14][TFSI] suppressed the melting point, dramatically increased conductivity (from 0.56 mS/cm at 25 °C in 0.8 m LiTFSI in [Pyr14][TFSI] to 11 mS/cm at 25 °C in 0.8 m LiTFSI in a mixture of 1:7 mol ratio of [Pyr14][TFSI] to MP) and still maintained the electrochemical window (>5 V). While the addition of MP reduced the melting point and increased conductivity, the presence of cyclic carbonates in the cosolvent was necessary to enable deposition and stripping of lithium with high coulombic efficiencies. The fluorinated ethylene carbonate (FEC) appeared to enhance the SEI formation and suppress the Claisen reaction that MP enabled.