Influences of the degree of cross-linking on electrochemical and thermal properties in polyether-based electrolytes doped with LiN(SO₂CF ₃)₂

Solid polymer electrolytes of partially cross-linked poly(ethyleneoxide- propyleneoxide) random copolymers [poly(EO-PO)] synthesized from the precursor macromonomers terminated by different ratios of methyl/acryloyl functional groups were studied by doping LiN(SO₂CF₃)₂ with two salt concentrations. The glass transition temperature, T_g, ionic conductivity, direct current characteristics and lithium transference number were measured together with the ion self-diffusion coefficients at die long time intervals by the ⁷Li and ¹⁹F pulsed field gradient nuclear magnetic resonance (NMR) method. The ionic conductivity increased linearly with the decrease of the degree of cross-linking. The relative limiting current density, evaluated by linear current sweep amperometry, was enhanced, especially in the salt concentrated electrolytes. The lithium transference numbers of 0.05-0.13 for the polymer electrolytes studied in the present work suggest that the ion conduction is carried out mainly by anion transfer. The T_g was insensitive to the cross-linking degree but sensitive to the salt concentration, and closely related to the rates of the local segmental motion of the polymer chains obtained from NMR. In the framework of the reduced temperature, T -T_g, the ionic conductivity and the anion diffusion are shown to correlate well, and the mechanisms of the ionic conduction are discussed.