Ion conduction mechanisms in polymer electrolytes for lithium batteries and fuel cells, and crystal engineering of cyclophosphazenes.

摘要

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.;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.;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.;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. 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.;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.;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. (Abstract shortened by UMI.)