Synthesis And Characterization Of Polysiloxane-based Single–ion Conductors

Open Access
Author:
Liang, Siwei
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 08, 2013
Committee Members:
  • Ralph H Colby, Dissertation Advisor
  • Ralph H Colby, Committee Chair
  • Michael Anthony Hickner, Committee Member
  • James Patrick Runt, Committee Member
  • Harry R Allcock, Special Member
Keywords:
  • polyelectrolyte
  • single-ion conductor
  • ionomer
  • lithium-ion battery
  • phosphonium ionomer
  • rheology of ionomer
  • dielectric relaxation
Abstract:
Polysiloxane-based single-ion conductors containing different side groups and bulky ionic side chains have been discussed in this thesis. Firstly, three borate monomers: lithium triphenylstyryl borate (B1), a variant with three ethylene oxides between the vinyl and the borate (B2) and a third with perfluorinated phenyl rings (B3) were synthesized and used to prepare polysiloxane ionomers based on cyclic carbonates via hydrosilylation. B1 ion content variations show maximum 25 ºC conductivity at 8mol%, reflecting a tradeoff between carrier density and Tg increase. Ethylene oxide spacers (B2) lower Tg, and increase dielectric constant, both raising conductivity. Perfluorinating the four phenyl rings (B3) lowers the ion association energy, as anticipated by ab initio estimations. This increases conductivity, a direct result of 3X higher measured carrier density. The ~ 9 kJ/mol activation energy of simultaneously conducting ions is less than half that of ionomers with either sulfonate or bis(trifluoromethanesulfonyl) imide anions, suggesting that ionomers with weak-binding borate anions may provide a pathway to useful single-ion Li+ conductors, if their Tg can be lowered. Then two groups of novel non-volatile plasticizers containing pendant cyclic carbonates and short ethylene oxide chains have been successfully synthesized, as confirmed by 1H and 29Si NMR spectra. After mixing with polysiloxane-based ionomer, the resulting polymer electrolyte blends show improved conductivity. At room temperature the d. c. conductivity has been improved to between 10-4 to 10-5 S/cm. Electrode polarization in dielectric relaxation spectroscopy reveals that part of the increased conductivity comes from lowering Tg, which raises the mobility of the conducting ions. The number density of simultaneously conducting ions is also boosted by the plasticizers, particularly for those containing more of the strongly solvating oligo-(ethylene oxide). Polysiloxane phosphonium single-ion conductors with ion contents ranging from 5 to 22 mol% were synthesized via hydrosilylation reaction. The parent Br- anion was exchanged to F- or bis(trifuoromethanesulfonyl)imide (TFSI-). Results of X-ray scattering experiments suggest the absence of ion aggregation in our phosphonium ionomers, which keeps glass transition temperatures (Tg) low. DSC Tg of the phosphonium ionomers are all below -70 oC, suggesting a weak dependence of Tg on both ion content and ion type; while conductivities weakly increase with ion content but exhibit a strong dependence on anion type. The highest conductivity at 30 oC, (2 x 10-5 S/cm) was obtained for the TFSI anion and is attributed to its relatively delocalized negative charge and its large size, both weakening the interaction between TFSI and phosphonium cation. The linear viscoelastic (LVE) properties of polysiloxane-based phosphonium-containing ionomers with ion contents f = 0 to 0.22 have been studied. The master curves of those ionomers have been constructed with reduced frequency spanning 14 decades. The ionic association has been witnessed as a delayed polymer relaxation with increasing ion content, although there is no ion aggregate peak in X-ray scattering and LVE suggests only limited ionic associations with no ion clusters in our phosphonium ionomers. All observations are consistent with weak interchain ionic interactions determined by bulky weak-binding phosphonium salts. Ionomer LVE can be well fit by the KWW model on short time scales and the sticky Rouse model on long time scales, which proves that the ion association lifetime in our ionomers is shorter than that of polymer chain relaxation, despite the fact that the chains are short.