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

Gavelin, Patrik LU (2002)
Abstract
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... (More)
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. (Less)
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author
opponent
  • Hvilsted, Soren, Danish Polymer Centre, Technical University of Denmark
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Polymer technology, biopolymers, Polymerteknik, Kemiteknik och kemisk teknologi, Chemical technology and engineering, transport dynamics, ion conductivity, solid polymer electrolyte, gel electrolyte, macromonomer copolymerization, amphiphilic graft copolymer
pages
71 pages
publisher
Birgitta Rydh, Dep. Polymer Science & Engineering, Chemical Center, Lund University
defense location
Chemical Center, Lund
defense date
2002-03-08 10:15
ISBN
91-628-5137-3
language
English
LU publication?
yes
id
a7e255d0-6b4e-40cb-9f6b-171de24f1b51 (old id 464406)
date added to LUP
2007-10-14 15:12:59
date last changed
2016-09-19 08:45:07
@phdthesis{a7e255d0-6b4e-40cb-9f6b-171de24f1b51,
  abstract     = {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.<br/><br>
<br/><br>
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.},
  author       = {Gavelin, Patrik},
  isbn         = {91-628-5137-3},
  keyword      = {Polymer technology,biopolymers,Polymerteknik,Kemiteknik och kemisk teknologi,Chemical technology and engineering,transport dynamics,ion conductivity,solid polymer electrolyte,gel electrolyte,macromonomer copolymerization,amphiphilic graft copolymer},
  language     = {eng},
  pages        = {71},
  publisher    = {Birgitta Rydh, Dep. Polymer Science & Engineering, Chemical Center, Lund University},
  school       = {Lund University},
  title        = {Amphiphilic Graft Copolymer Electrolytes for Lithium-Ion Polymer Batteries. - Preparation and Characterisation},
  year         = {2002},
}