Lithium ion interactions in polymer gel electrolytes : Effect on structure, dynamics and morphology

Sammanfattning: Polymer electrolytes are an essential part of the modern all-solid lithium battery. Several properties, such as mechanical and thermal stability, good ionic conductivity and compatibility with other materials used in the battery are necessary for a successful material. This thesis is focused on the ionic interactions in two electrolyte systems, based on a cross-linked polyethylene oxide (nona(ethylene oxide)dimethacrylate) or a fluorinated copolymer (polyvinylidene fluoride-hexafluoropropylene). Changes of thermal properties and morphology of and coordination in the electrolyte on addition of a lithium salt (LiTFSI) and different types and concentrations of solvents are studied. These properties are shown to control the ionic conductivity of the resulting material, influencing the conductivity mechanism. The presumably inert polyvinylidene fluoride is shown to undergo large structural changes, where polar conformations of the polymer backbone appear on salt addition, inducing a different crystalline phase. The lithium cation in the polyethylene oxide based electrolyte can coordinate either to the polymer chain or to the solvent. In for example systems containing propylene carbonate, the lithium ion binds more strongly to the polymer than to the solvent. This is in contrast to the systems with dimethyl sulphoxide where the ion prefers the solvent. This variation in coordination also strongly affects the mobility of the cation, reflected both in diffusion constants and in conductivity data.A combination of techniques was necessary to use for a deeper understanding of these complex materials. Coordination has been studied by FTIR and high resolution NMR, dynamics by NMR diffusion measurements and impedance spectroscopy, and morphology by diffraction techniques and DSC.