Nanocomposite Polymer Electrolytes: Modulation of Mechanical Properties Using Surface-Functionalized Fumed Silica

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Title: Nanocomposite Polymer Electrolytes: Modulation of Mechanical Properties Using Surface-Functionalized Fumed Silica
Author: Yerian, Jeffrey Alan
Advisors: John H Van Zanten, Committee Member
Jan Genzer, Committee Member
Peter S. Fedkiw, Committee Co-Chair
Saad A. Khan, Committee Co-Chair
Abstract: Rechargeable lithium metal batteries are potential next-generation power sources for portable electronic devices and electric vehicles due to their high-energy density, low self-discharge rate, and environmentally benign construction materials. However, the high reactivity of lithium metal with the electrolyte impedes their commercialization. The addition of surface-functionalized fumed silica to composite polymer electrolytes (CPEs) forms a network structure that reduces reactivity with lithium metal and mitigates dendrite formation during cycling. Further improvements in performance can be achieved with additional enhancement of mechanical properties without reducing electrochemical properties. Novel surface-functionalized fumed silica, such as crosslinkable fumed silica and mixtures of fumed silica, are studied in a wide range of solvents (low-molecular weight polyethylene glycol dimethyl ether (PEGdm), high-molecular weight polyethylene oxide (PEO), and mineral oil) to understand how the fumed silica network affects rheological and electrochemical properties of these composites. Crosslinkable-based fumed silica can be subsequently reacted in CPEs to form covalent bonds between fumed silica particles rather than physical interactions. A chemically similar monomer dissolved in the electrolyte, e.g., methacrylate monomer, is needed to tether the crosslinkable silica particles. The CPEs consist of crosslinkable fumed silica + PEGdm (Mn = 250 and 500) + lithium bis(trifluromethylsulfonyl)imide [LiN(CF3SO2)2] (LiTFSI) (Li:O = 1:20) + methacrylate monomer of varying alkyl length (0 - 40 wt%). The conductivity of CPEs is independent of silica surface group before and after crosslinking and decreases by only a factor of two after crosslinking. While the interfacial stability of crosslinked CPEs with lithium is comparable to uncrosslinked CPEs, charge-discharge cycles of Li/CPE/Li cells indicate they are less stable towards lithium metal than uncrosslinked composites. Increasing monomer concentration reduces conductivity, but increases elastic modulus. Shorter aliphatic methacrylate monomers, e.g., methyl and ethyl, have lower conductivity and elastic modulus than longer aliphatic methacrylate monomers, e.g., butyl, hexyl, and dodecyl. The CPEs exhibit room-temperature conductivity near 10-3 S cm-1, elastic modulus greater than 105 Pa, and a yield stress approaching 104 Pa. These properties are comparable, if not better, than typical crosslinked polymer electrolytes and plasticized or gel electrolyte systems at end-use temperature. The addition of fumed silica to high-molecular weight PEO increases the elastic modulus; decreases the frequency dependence of the elastic modulus; and increases the percent recoverable strain. The extent of elasticity enhancement depends on the fumed silica surface chemistry and concentration. The largest increase in elasticity is observed for hydrophilic fumed silica primarily due to interactions between hydroxyl groups on the silica and ether oxygen on the PEO backbone. These interactions facilitate bridging of fumed silica particles through entanglements of adsorbed PEO, which increases the elastic modulus. Blends of different fumed silica types are studied in mineral oil and PEGdm (250) to determine how the presence of the second silica type affects the strength and mechanism of gel formation. In mineral oil, hydrophilic and hydrophobic silica blends exhibit elastic moduli between the elastic moduli of single-component hydrophilic and hydrophobic silicas. In contrast, mixtures of hydrophilic and hydrophobic silica in PEGdm (250) exhibit elastic moduli that are lower than elastic moduli for either single-component silica system. The difference in behavior for the blends stems from the interactions that facilitate gel formation. In mineral oil, both hydrophilic and hydrophobic fumed silica interact via hydrogen bonding so that the elastic modulus of mixed silica systems is comparable to the weighted-average of the single-component systems. In PEGdm (250), interactions of hydrophobic and hydrophilic silica interfere, which reduces the network strength compared to the single-component systems.
Date: 2003-08-18
Degree: PhD
Discipline: Chemical Engineering

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