Electrochemical investigations of ionic liquids with vinylene carbonate for applications in rechargeable lithium ion batteries

Ionic liquids based on methylpropylpyrrolidinium (MPPY) and methylpropylpiperidinium (MPPI) cations and bis(trifluoromethanesulfonyl)imide (TFSI) anion have been synthesized and characterized by thermal analysis, cyclic voltammetry, impedance spectroscopy as well as galvanostatic charge/discharge tests. 10 wt% of vinylene carbonate (VC) was added to the electrolytes of 0.5 M LiTFSI/MPPY.TFSI and 0.5 M LiTFSI/MPPI.TFSI, which were evaluated in Li || natural graphite (NG) half cells at 25 °C and 50 °C under different current densities. At 25 °C, due to their intrinsic high viscosities, the charge/discharge capacities under the current density of 80 μA cm⁻² were much lower than those under the current density of 40 μA cm⁻². At 50 °C, with reduced viscosities, the charge/discharge capacities under both current densities were almost indistinguishable, which were also close to the typical values obtained using conventional carbonate electrolytes. In addition, the discharge capacities of the half cells were very stable with cycling, due to the effective formation of solid electrolyte interphase (SEI) on the graphite electrode. On the contrary, the charge/discharge capacities of the Li || LiCoO₂ cells using both ionic liquid electrolytes under the current density of 40 μA cm⁻² decreased continually with cycling, which were primarily due to the low oxidative stability of VC on the surface of LiCoO₂.