Ions Transport Properties in Polymer Gel Electrolytes With Introduction of Ionic Liquid for Lithium Electrochemical Systems

The study 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 LiBF salt, 1-ethyl-3-methylimidazolium tetrafluoroborate, and various organic solvents: dioxolane (DOL), diglyme (G2), tetraglyme (G4), and ethylene carbonate (EC). The aim was to identify a composition with the highest mobility for the Li cation. Flexible films of the polymer gel electrolytes were analyzed using differential scanning calorimetry, thermogravimetric analysis, and Fourier transform infrared spectroscopy. The features of ionic and molecular transport were investigated using pulsed field gradient NMR in conjunction with electrochemical impedance spectroscopy. The total conductivity of these systems ranged from 1.8 to 4.1 mS cm at room temperature. Although the composition with EC exhibited high ionic conductivity, the mobility of the Li cation at room temperature increased in the following order: Li(EC) < Li(DOL) < Li(G4) < Li(G2). Calculating the hydrodynamic radius of the lithium cation revealed that for Li(EC) and Li(DOL), the radius decreased with rising temperature; for Li(G2), it remained nearly constant; while for Li(G4), it 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 assessing the compatibility of the polymer gel electrolytes with metallic lithium, it was found that electrolyte compositions containing tetraglyme, diglyme, and ethylene carbonate show promise for further research and potential application as electrolytes in lithium power sources.