Polymeric single Li+-ion conductors: preparation and characterisation

Abstract: Lithium-ion batteries are a common type of light-weight, rechargeable batteries. There are safety concerns because these batteries typically use an organic solvent electrolyte which presents fire hazards. By using dry polymer electrolytes, the fire hazards can be eliminated. The single-ion conductor is a type of polymer electrolyte in which the ion-containing group has been covalently bonded with the polymer structure, thereby immobilising the anion. This eliminates problems present in common liquid electrolytes, such as dendritic growth in the battery that can lead to short circuiting. The purpose of this Master thesis was to synthesise and characterise single-ion conductors. The polymers were random copolymers with a poly(ethylene oxide) backbone. The ion-containing repeating units contained a side chain with one quaternary ammonium cation and two sulfonate anions. The non-ionic repeating units carried a triethylene glycol monomethyl ether side chain. Copolymerisation by anionic and cationic ring-opening mechanism failed. Instead poly(epichlorohydrin) was synthesised using an anionic ring-opening mechanism followed by several post-polymer reactions to yield the target polymers. These polymers did not form a melt at sufficiently low temperatures (below 140 °C) in order to make samples for conductivity measurements. Instead blends between homopolymer only containing ionic repeating units and poly(ethylene glycol-ran-propylene glycol) were made and the conductivity of these blends were measured. The highest measured conductivity was 3.3∙10-6 S/cm at 90 °C for a blend with an EO/Li ratio of 8. This conductivity, while similar to other single-ion conductors, is too low for the blends to be used for battery applications. Suggestion for future work include the preparation of polymers with sulfonate groups that are more rigidly bonded to the polymer to inhibit the tight coordination of lithium ions by the polymer, thereby improving the conductivity. Copolymers with large amount of non-ionic repeating units should be synthesised to further decrease their glass transition temperature and make measurements by impedance spectroscopy possible.