New network-gel-electrolytes consisting of polyethylene glycol diacrylate, LiBF4, and 1-buthyl-3-methylimidazolium tetrafluoroborate, added with alkylene carbonates: the ion transfer mechanism and properties

Abstract

Polymer gel-electrolytes based on polyethylene glycol diacrylate, LiBF4 salt, ethylene carbonate, propylene carbonate, and 1-buthyl-3-methylimidazolium tetrafluoroborate ionic liquid, are synthesized by using radical polymerization. For the optimal composition (polyethylene glycol diacrylate, 19 wt %; LiBF4, 10 wt %; 1-buthyl-3-methylimidazolium tetrafluoroborate, 44 wt %; ethylene carbonate, 27 wt %) the conductivity is maximal: 2.5 × 10−3 S/cm at 20°C; 1.1 × 10−2 S/cm at 100°C. The system remains thermostable up to 107°C. It was shown, by using NMR with the pulsed magnetic field gradient, that the polymer matrix is solely involved in the Li+ ion solvation in the electrolyte composed of polyethylene glycol diacrylate, LiBF4, and 1-buthyl-3-methylimidazolium tetrafluoroborate; and the Li+ self-diffusion coefficients do not depend on the ionic liquid content. When the electrolyte is added with alkylene carbonates, both the polymer matrix and the solvent are involved in the Li+ ion solvation. The highest conductivity of 10−3 S/cm (at 20°C) was reached when 27 wt % of ethylene carbonate have been added; here Li+ ions are entirely solvated by ethylene carbonate molecules, the diffusion coefficient being equal to 10−11 m2/s.