Amphiphilic Graft Copolymer Electrolytes for Lithium-Ion Polymer Batteries. - Preparation and Characterisation

Sammanfattning: In the present work amphiphilic graft copolymers were prepared by free radical copolymerization of macromonomers in solution. The graft copolymers consisted of methacrylate backbones bearing ethylene oxide (EO)n side chains of varying length as ionophilic groups, and fluorocarbon (CF2)6 side chains or hydrocarbon groups as ionophobic groups. Homogenous polymer gel electrolytes were prepared by adding an electrolyte solution consisting of a solvent and a lithium salt, where the anion was fluorinated, to the copolymers. The gel electrolytes based on the amphiphilic copolymers bearing (CF2)6 side chains were found to have significantly higher ion conductivity than corresponding gels based on amphiphilic copolymers containing hydrocarbon groups. The lithium ions in the gels were found to coordinate cooperatively to the (EO)9 units in competition with the solvent. For copolymers bearing (EO)1, (EO)2 and (EO)4 side chains the coordination to the polymer was weaker. However, the ion conductivity was influenced only slightly by the varying strength of coordination of lithium ions by the different (EO)n side chains, in competition with the solvent. It was concluded that the ionophobic-ionophilic balance of the amphiphilic graft copolymers had to be controlled for achieving high ion conductivities in the gel electrolytes. From diffusion measurements it was concluded that the (CF2)6 side chains of the amphiphilic graft copolymer associated to form ionophobic microdomains in the gels. The (EO)n side chains seemed to stabilize the microdomains, and a high ionophobic content in combination with an appropriate EO content and (EO)n side chain length resulted in high ion conductivities. Moreover, the data suggested that the aggregating behaviour of the semi-fluorinated amphiphilic graft copolymers had a more intramolecular than intermolecular character at high concentrations of electrolyte solution. The self-diffusion coefficients of the lithium ions were significantly higher for the semi-fluorinated amphiphilic graft copolymer gels than for the gels based on a non-amphiphilic homopolymer. Furthermore, the lithium transference numbers were also higher, by almost a factor three. This effect was partly due to a higher EO content in the non-amphiphilic homopolymer, resulting in cooperative coordination of lithium ions by ether oxygens in competition with the oxygens in the solvent molecules. From 19F NMR data it was observed that the fluorinated anions in the semi-fluorinated amphiphilic copolymer gels were most probably present both in a solvent-rich environment, and an environment dominated by the ionophobic microdomains formed by the associating (CF2)6 side chains. If the TFSI anions associated to the microdomains formed in the semi-fluorinated amphiphilic copolymer gels, or were “captured” by the microdomains, the lithium salt dissociation might increase, leading to improved lithium mobility in the gel electrolytes.