"Synthesis and Development of a New Lyotropic Liquid Crystal Polyelectrolyte Material for Ion- Conductive Applications"

Kerr, Robert L.

Graduate Thesis Or Dissertation

"Synthesis and Development of a New Lyotropic Liquid Crystal Polyelectrolyte Material for Ion- Conductive Applications" Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/r207tp52w

Abstract

The objectives of this thesis research were to synthesize and develop a new type of nanostructured, ion-conductive material utilizing lyotropic (i.e., amphiphilic) liquid crystals (LLCs) as starting materials, in order to solve some inherent limitations and problems with current polymer electrolyte materials used in Li battery applications. Some of these limitations include poor low- temperature performance, high flammable liquid loadings, lower than desired energy density, lithium dendrite formation, and poor charge-discharge cycling. The first part of this research was focused on the design and synthesis of a new lithium-ion-containing polymerizable LLC that could self-assemble and be cross-linked into a bicontinuous cubic (Q) LLC phase in the presence of a conventional liquid electrolyte used in battery manufacture, instead of the typical solvent used in LLC phase formation, water. The presence of Li ions in the LLC monomer and the 3D-interconnected solvent nanopore structure of the targeted Q phase were both important for ensuring high Li-ion conductivity over a range of conditions in the resulting material. A non-aqueous LLC monomer system with these features was unprecedented prior to this work. Some secondary considerations in the design of this non-aqueous LLC monomer system were the use of inexpensive, industrially friendly, and readily available reactants, reagents, and reactions so that it would be attractive for potential future commercialization, if successful. To this end, a new three-tailed acrylate LLC monomer based on a gallic acid core with a tethered lithium sulfonate headgroup was successfully synthesized. After structural characterization and purity analysis, this monomer was found by polarized light microscopy (PLM) and powder X-ray diffraction (XRD) to form LLC phases with water and separately propylene carbonate (PC), a known battery liquid electrolyte. The phases formed in each solvent included a lamellar (L) phase and a well-defined type II (i.e., inverted) bicontinuous cubic (Q_II) phase, as well as several other unidentified mixed LLC phases. UV light-initiated radical photopolymerization of the acrylate groups of the monomer afforded cross-linked networks with retention of the original LLC phase structures. The system formed with PC as the liquid component provided the first example in the literature of LLC phase formation and polymerization around a known battery liquid electrolyte. The cross-linked Q_II-phase network formed with pure PC was then tested using AC impedance spectroscopy to determine ion conductivity. The room-temperature ion conductivity of this material was found to be to be just under 10^-6 S cm^-1 which is below the value typically required for battery applications (≥10^-4 S cm^-1). Subsequently, the PC was doped with a small amount of a free lithium salt (0.254 M LiClO₄), which is common practice in the Li battery community for increasing the Li-ion conductivity of an electrolyte material, The lithium sulfonate LLC monomer was found to exhibit similar phase formation behavior and polymerization characteristics when 0.245 M LiClO₄ in PC was used instead of pure PC as the LLC solvent. The room-temperature ionic conductivity of the cross-linked Q_II-phase LLC system formed around 0.245 M LiClO₄ in PC was found to be in the range of 10^-4 to 10^-3 S cm^-1. Variable-temperature (VT) AC impedance spectroscopy experiments were also carried out to determine ionic conductivity as a function of temperature because conventional Li- battery polymer electrolyte materials based on poly(ethylene oxide) drop significantly at low temperatures due to the glass transition of the polymer matrix.. These VT conductivity studies provided unexpected positive results in that the Q_II-phase, cross-linked LLC-(0.245 M LiClO₄-PC) nanocomposite material exhibited less than 1 order of magnitude change in ionic conductivity when cooled to -65 °C and heated up to 55 °C. Additional NMR DOSY studies on the material were performed to provide evidence of low-temperature mobility of the doped electrolyte within the nanostructured channels of the cross-linked Q_II-phase. These studies corroborated that the doped PC solution in the LLC nanochannels was still mobile down to -35 °C. A lithium metal battery was successfully made to demonstrate proof-of-concept of the efficacy of this ion-conductive membrane in battery operation using a proprietary cathode material provided by an outside battery company. A single-discharge lithium metal test battery employing the LLC-PC electrolyte material provided a voltage of just over 3 V upon testing, which demonstrates that it can function as a battery electrolyte and membrane separator. Research after this point was focused on evaluating the effect of several additional, known, battery liquid electrolytes and lithium salts on LLC phase behavior and ionic conductivity performance of the final polymerized composite material. Diethylcarbonate (DEC) and dimethylcarbonate (DMC) were chosen for the next evaluation because they are also common battery electrolyte solvents typically used in lithium-containing batteries. In fact, doped DEC and DMC or mixtures of the two are used more than PC in the lithium battery industry. Other Li salts such as LiBF₄, LiPF₆, LiOTf, and LiTf₂N are commonly employed as dopants in lithium-containing batteries. LiBF₄ and LiPF₆ were chosen as the salts for further analysis in combination with PC, DEC, and DMC as the liquid electrolytes with the LLC monomer, although LiPF₆ was used in a limited fashion due to its tendency to form HF under certain conditions. It was found that high loading level mixed phase morphology LLC composites performed as good as the pure Q_II-phase composite. This was not unexpected as many highly liquid-loaded composite materials. Generally, no discernable trends or unusually results were found. Generally high liquid loadings and high dopant salt concentrations result in better conductivity levels, which is not novel. What is still notable is that the pure Q_II-phase composite performs as well as or better than the 50 wt% loading level LLC composites, which are on the edge of macrophase separation, and at electrolyte levels nearly 70% less. Eight of the outstanding composite samples were selected and assembled into working Li-metal batteries. As expected the Q_II-phase samples performed very well, as did the high liquid loading level composites. With these formulation variation results in hand, the final part of this thesis work was centered on modifying the design of the original lithium sulfonate LLC monomer to give it potentially better thermal and chemical stability for potential long-term use in batteries. Thermal and chemical stability were of concern due to the acrylate decomposition temperature and the hydrolyzable ester linkage on the tails. Consequently, 1,3-diene- and isoprene-terminated tails were synthesized to replace the original long acrylate tails on the LLC monomer. Unfortunately, the isoprene-terminated tails were not successfully coupled onto the desired substrate. Due to time and funding this parallel path investigation was stopped. The 1,3-diene tails were successfully coupled onto the respective substrate and the monomer synthesized. The initial LLC phase behavior and photopolymerization characteristics of the new monomer with LiClO₄-doped PC solution were then investigated and found to be promising for future work and optimization.

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