The Ion Transport Properties in Ionic-Liquid-Added Polymer Gel Electrolytes for Lithium Electrochemical Systems

The study is focused on the competitive ionic and molecular transport characteristics of four polymer gel electrolyte compositions synthesized through the radical polymerization of polyethylene glycol diacrylate, incorporating LiBF₄, 1-ethyl-3-methylimidazolium tetrafluoroborate, and various organic solvents: dioxolane, diglyme, tetraglyme, and ethylene carbonate. The aim was to identify a composition with the Li⁺ cation highest mobility. Flexible films of the polymer gel electrolytes were analyzed using differential scanning calorimetry, thermogravimetric analysis, and Fourier-transform infrared spectroscopy. The ionic and molecular transport features were investigated using pulsed field gradient NMR in conjunction with electrochemical impedance spectroscopy. The total conductivity of the systems ranged from 1.8 to 4.1 mS cm^–1 at room temperature. Despite the high ionic conductivity of the EC-containing composition, the Li⁺ cation mobility at room temperature increased in the following order: Li⁺(ethylene carbonate)₄ < Li⁺(dioxolane)₄ < Li⁺(tetraglyme) < Li⁺(diglyme)₂. Calculation of the lithium cation hydrodynamic radius revealed that for Li⁺(ethylene carbonate)₄ and Li⁺(dioxolane)₄, the radius decreased with rising temperature; for Li⁺(diglyme)₂, remained nearly constant; for Li⁺(tetraglyme), exhibited an abnormal increase. This unusual behavior is likely due to the re-solvation of the lithium cation from the polymer matrix into tetraglyme. In the assessing of the polymer gel electrolytes compatibility with metallic lithium, the electrolyte compositions containing tetraglyme, diglyme, and ethylene carbonate are found to show promise for further research and potential application as electrolytes in lithium power sources.