BMIMBF4 Ionic Liquid Mixtures Electrolyte for Li-ion Batteries

Abstract

The ionic liquid (IL) 1-butyl-3-methyl-imidazolium tetrafluoroborate (BMIMBF4) was prepared on basis of 1-methylimidazole, and was combined with γ-butyrolactone (γ-BL). The BMIMBF4/γ-BL mixture was used as Li/LiFePO4 cell electrolyte. The viscosities, conductivities and thermal stability of the mixtures were investigated. The results indicates that BMIMBF4/γ-BL (40/60) -based electrolyte has high conductivity (10.76 mS/cm) at 25 °C, low viscosity and high thermal stability. In addition, linear sweep voltammetry test of BMIMBF4/γ-BL (40/60)-1 M LiBF4 demonstrated that the electrochemical window was 5.3 V. Moreover, cyclic voltammetry and charge–discharge of BMIMBF4/γ-BL (40/60)-1 M LiBF4 electrolyte have been investigated in Li/LiFePO4 cells. The addition of vinylene carbonate (VC) significantly improved the capacity and cyclic performance of the cell. The initial discharge capacity of Li/LiFePO4 in optimized mixture electrolyte with VC, was 144.2 mAh/g at a current density of 0.1 mAh/g, 40 mAh/g higher than that of the without. The cell with the additive had a 140.3 mAh/g discharge capacity without any considerable fading during 20 cycle numbers.