Exploring the Relationship between Polymer Topology and Ionic Conductivity

This dissertation will be mainly exploring the relationship between polymer topology and ion transport properties of single-ion conductors (SICs) in lithium-ion battery application. Specifically, we strive to understand the impact of precise 5-carbon spacing on ion transports behavior of precision single-ion conductor. In chapter 2, the investigation was conducted on blending lithium sulfonate salt of precise 5-carbon spacing polymer electrolyte (p5PhS-Li) with poly(ethylene oxide) (PEO), a popular solvating polymer. The highest ionic conductivity of this type of SIC was achieved on the order of 10⁻⁷ S/cm at 90 °C. Results from differential scanning calorimetry (DSC) also indicated that polymer blends are at least partial miscible. The conclusion was made due to strong ionic interactions between sulfonate anions and lithium cations that lead to small magnitude of interaction parameter as well as melting point depression in PEO with complicating interpretation of transference number. We were strived to improve the ionic conductivity of single-ion conductors by altering the chemical structures of anions from sulfonate to trifluoromethylsulfonylimide salt (TFSI) that has been shown to increase electrochemical, thermal stabilities and ionic conductivity in chapter 3. Upon characterizing with ¹H NMR, ¹⁹F NMR and ¹³C NMR, the efficiency of post polymerization reaction was obtained as high as 90 %. The conversion of sulfonated into TFSI-containing SIC (p5PhTFSI-Li) was shown to improve thermal stability as well as plasticize by an appearance of glass transition temperature (Tg) with higher TFSI content corresponds to lower (Tg). The ionic conductivity of true SIC p5PhTFSI-Li was lower than previously studied p5PhS-Li which contradicted to our hypothesis. The improvement in ionic conductivity was only observed when p5PhTFSI-Li was doped with PEO. Study by DSC also revealed that no crystallinity in PEO was detected, and these blends exhibited a single Tg which is attributed to the miscible behavior of the components. X-ray scattering also complemented with DSC study as ionic aggregates are diluted by the introduction of PEO. Realizing the immediate effect of PEO addition on the ionic conductivity of SICs, chapter 4 of the thesis further expands wider range of blend composition between PEO and p5PhTFSI-Li. Study by DSC reveals one single Tg for every blend composition which is consistent with results obtained from chapter 3. The addition of p5PhTFSI-Li retarded crystallization kinetics of PEO until it fully disrupted the crystalline phase of PEO, which proves that these two components provide greater compatibility than PEO/p5PhS-Li. Highest ionic conductivity of 6.37 x 10⁻⁴ S cm⁻¹ was also obtained at 42 wt% of p5PhTFSI-Li, which is on par with that observed in literature TFSI-based SICs. Transference number was also observed to approach unity for experimented compositions. The future of p5PhTFSI-Li is wide open as the material will be investigated in block polymer as well as electrochemical stability. Last but not least, a side project was researched on catechol-containing precision polymer in underwater adhesion applications in chapter 5. Even though the research was not timely done, the synthesis of catechol-containing precision polymer was investigated on monomer synthesis, thermodynamics of polymerization, efficiency of polymerization and copolymerization with a similar comonomer structure. This project will leave opportunities for incoming graduate student to take over and analyze the adhesion performance the catechol-containing precision polymer.