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

Abstract

The polymer gel electrolytes studied in this paper are composed of two kinds of organic solvents (γ-butyrolactone (GBL) or propylene carbonate (PC)), two kinds of lithium salts (LiBF 4 or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, LiN(SO 2CF 3) 2)) and chemically cross-linked poly(ethyleneoxide) (PEO). The ionic conductivity and the thermal properties were measured by the AC impedance method and differential scanning calorimetry (DSC), respectively. The individual self diffusion coefficients ( D) of the electrolyte components measured by the pulsed-gradient spin-echo (PGSE)-NMR method were compared with the ionic conductivities to clarify the ionic conduction mechanisms, the ion compositions and the degree of dissociation. The results suggest that although the degrees of ion dissociation in the gel electrolytes are larger than in the liquid electrolytes, the ionic conductivities of the gels are lower than that of the liquids due to the slower diffusion of the lithium ion. From diffusion behaviours observed at various salt concentrations in the gels (especially in the PC-based gels), it was found that the lithium ions are trapped in the polymer matrix.