Ionic conductivity, DSC and self diffusion coefficients of lithium, anion,polymer, and solvent of polymer gel electrolytes: the structure of the gels and the diffusion mechanism of the ions

The polymer gel electrolytes studied in this paper are composed of two kind s of organic solvents (gamma-butyrolactone (GBL) or propylene carbonate (PC )), two kinds of lithium salts (LiBF4 or lithium bis(trifluoromethanesulfon yl)imide (LiTFSI, LiN(SO2CF3)(2))) and chemically cross-linked poly(ethylen eoxide) (PEO). The ionic conductivity and the thermal properties were measu red by the AC impedance method and differential scanning calorimetry (DSC), respectively. The individual self diffusion coefficients (D) of the electr olyte components measured by the pulsed-gradient spin-echo (PGSE)-NMR metho d were compared with the ionic conductivities to clarify the ionic conducti on mechanisms, the ion compositions and the degree of dissociation. The res ults suggest that although the degrees of ion dissociation in the gel elect rolytes are larger than in the liquid electrolytes, the ionic conductivitie s of the gels are lower than that of the liquids due to the slower diffusio n of the lithium ion. From diffusion behaviours observed at various salt co ncentrations in the gels (especially in the PC-based gels), it was found th at the lithium ions are trapped in the polymer matrix. (C) 2000 Elsevier Sc ience Ltd. All rights reserved.