ION CONDUCTION MECHANISMS IN POLYMER ELECTROLYTES FOR LITHIUM BATTERIES AND FUEL CELLS, AND CRYSTAL ENGINEERING OF CYCLOPHOSPHAZENES

Open Access
Author:
Lee, David Kim Yong
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 04, 2010
Committee Members:
  • Harry R Allcock, Dissertation Advisor
  • Harry R Allcock, Committee Chair
  • Alan James Benesi, Committee Member
  • John V Badding, Committee Member
  • Michael Anthony Hickner, Committee Member
Keywords:
  • polymer electrolytes
  • fuel cells
  • lithium batteries
  • phosphazenes
  • ion conduction mechanism
  • crystal engineering
Abstract:
The work described in this thesis is divided into two parts. The first part focuses on the synthesis and characterization of polyphosphazenes for polymer electrolytes in lithium batteries and fuel cells. The overall goal is to gain an understanding of the ion conduction mechanisms in these materials to aid future designs of ion conducting polymers. The second part of this thesis describes the design and synthesis of cyclophosphazenes with asymmetric spirocyclic side groups. The reaction mechanism for the selective formation of the cis-isomer is proposed and the inclusion behavior of crystals formed by these molecules was studied. The theme that ties the two parts of this thesis is the chemistry of phosphazenes. Chapter 1 outlines the fundamental concepts for polymer electrolytes used in lithium batteries and fuel cells. The properties of the polymers that are used as electrolytes and the current understanding of the ion conducting mechanism in these materials are described. Furthermore, the chemistry and applications of phosphazenes is also outlined. Chapter 2 is a study on the ion conduction mechanism in a polyphosphazene electrolyte. Lithium trifluoromethanesulfonate (LiTf), lithium bis(trifluoromethanesulfonyl)imidate (LiTFSI), magnesium trifluoromethanesulfonate (MgTf2) and magnesium bis(trifluoromethane sulfonyl)imidate (MgTFSI2) were dissolved in poly[bis(2-(2-methoxyethoxy)ethoxy)phosphazene] (MEEP) to compare the effect on solvent-free polymer ionic conductivity of monovalent versus divalent cations, and two anions with different degrees of dissociation. The polymer electrolytes with the bis(trifluoromethanesulfonyl)imidate anion had higher ionic conductivities even though the glass transition temperatures, which reflected polymer molecular motion, were higher than those of their counterparts with the trifluoromethanesulfonate anion. Furthermore, polymer electrolytes with magnesium salts achieved their maximum conductivity at lower salt concentrations than the counterparts with lithium salts. The temperature dependence of the ionic conductivity of the solid solutions was fitted to the Vogel-Tamman-Fulcher (VTF) equation. The pseudo-activation energy term, B, of the VTF equation showed a strong dependence on the anion present. The result suggests that the dominant mobile species is the anion, while the cation remains relatively bound to the polymer. The manuscript has been submitted to the journal Solid State Ionics. Chapter 3 describes a synthetic method to produce a proton conductive polymer membrane with a polynorbornane backbone and inorganic-organic cyclic phosphazene pendent groups that bear sulfonic acid units. This hybrid polymer combines the inherent hydrophobicity and flexibility of the organic polymer with the tuning advantages of the cyclic phosphazene to produce a membrane with high proton conductivity and low methanol crossover at room temperature. The ion exchange capacity (IEC), the water swelling behavior of the polymer, and the effect of gamma radiation crosslinking were studied, together with the proton conductivity and methanol permeability of these materials. A typical membrane had an IEC of 0.329 mmol g-1 and had water swelling of 50 wt%. The maximum proton conductivity of 1.13 x 10-4 S cm-1 at room temperature is less than values reported for some commercially available materials such as Nafion. However the average methanol permeability was around 10-9 cm s-1, which is one hundred times smaller than the value for Nafion. Thus, the new polymers are candidates for low-temperature direct methanol fuel cell membranes. The author was responsible for the synthesis of the materials used in this study and this work was done in collaboration with Shih-To Fei, Richard M. Wood, David A. Stone and Hwei-Liang Chang. The manuscript is published in the J. Membrane Science (year 2008, volume 320, pages 206-214). Chapter 4 deals with the characterization of water absorbed in proton conducting membranes. The proton conducing membranes were hydrated with 2H2O and 2H T1 NMR relaxation was used to probe the molecular dynamics of the water. The state of water in the proton conducting membrane was correlated to the chemical and morphological properties of the polymer. An understanding of the state of water in proton conducting membranes is as important as the morphological characterization of the proton conducting membrane because water is the medium for proton transport. Furthermore, this vital information will aid in the design of future proton conducting membranes, especially ones that can operate at low humidity and at temperatures above 100 ˚C. This work was done in collaboration with Professor Alan Benesi and Professor Michael Hickner. The intended target for submission is Journal of Physical Chemistry B. Chapter 5 describes layer-by-layer (LbL) films of poly[bis(methoxyethoxyethoxy)phosphazene] (MEEP) and poly (acrylic acid) (PAA) that are assembled by utilizing the hydrogen bonding between these two polymers. These films show controlled thickness growth, high ionic conductivity, and excellent hydrolytic stability. The ionic conductivity of these films is studied by changing the assembly pH of initial polymer solutions and thereby controlling the hydrogen bonding characteristics. Despite similar film composition, MEEP/PAA LbL films assembled at higher pH values have enhanced water uptake and transport properties, which play a key role in increasing ion transport within the films. At fully humidified conditions, the ionic conductivity of MEEP/PAA is 7 x 10-4 S cm-1, over one order of magnitude higher than previously studied hydrogen bonded LbL systems. Finally, free standing films are isolated from low-energy surface substrates, which allows for bulk characterization of these thin films. This work was done in collaboration with Avni A. Argun, J. Nathan Ashcraft, Marie K. Herring, and Professor Paula T. Hammond from the Massachusetts Institute of Technology. The manuscript has been accepted in Chemistry of Materials. (year 2010, volume 22, pages 226–232) Chapter 6 describes the synthesis and characterization of two novel cyclic phosphazenes with asymmetric spiro rings. The phosphazene molecules were synthesized via reactions of hexachlorocyclotriphosphazene with chiral amino alcohol residues. The reactions showed preferential formation of the cis isomer possibly due to the delocalization of the lone pair electrons of the spirocylic nitrogen, which reduces its ability to solvate protons. Crystals of these phosphazenes were analyzed by x-ray crystallography which confirmed the formation of cis isomers and showed their ability to include guest molecules within the crystal lattices. The selective inclusion of epoxides by one of the phosphazenes was an effective method for the separation of thermally sensitive guest molecules. This work was carried out in collaboration with Anne Jackson and Toshiki Fushimi, and has been published in Dalton Transactions. (DOI: 10.1039/b925734a) Appendix A describes the exploratory synthesis of a novel sulfonated polyphosphazene. The sulfonic acid group of this new polyphosphazene is tethered to the polyphosphazene backbone by a flexible side chain. The synthetic strategy of the polymer is compared to sulfonated polyphosphazenes synthesized previously. The logic behind this work is to examine the effect of distancing the sulfonic acid group from the polymer to the proton conductivity of the polymer.