MSE Ph.D. Thesis Defense: “Solvent Swollen Nanostructured Single-Ion Containing Polymers for Enhanced Lithium-Ion Conductivity”

Lithium-ion batteries are a dominant energy storage technology, and improvements to the electrolyte may further enhance energy density and safety. Single-ion conducting polymer electrolytes (SIPEs) show improved ion transport and stability, but fail to meet conductivities for use as an alternative to conventional liquid electrolytes. Strong coupling between lithium-ion transport and segmental dynamics in polymer electrolytes with dissolved salt limits the achievable ionic conductivities, while SIPEs sequester ionic groups into channels facilitating decoupling and improving ion transport. This dissertation explores the incorporation of solvent into single-ion conducting polymers and correlates solvent chemistries, solvent content, and nanoscale morphologies with lithium-ion conductivity.

Solvent-swollen periodic nanostructures are investigated using a multiblock copolymer comprised of a 12-carbon alkyl block strictly  alternating with a lithium-neutralized sulfosuccinate-based polar block (PESLi12). Using a combination of X-ray scattering, broadband dielectric spectroscopy, and all-atom molecular dynamics simulations, bulk, isotropic PES12Li displays layered nanostructures and is
selectively-swollen with a polar solvent incorporated primarily in the ion-containing polar sublayers. Ion transport is enhanced through partial coordination of lithium counterions with solvent molecules. Increasing solvent content increases interactions between lithium-ions and solvent molecules which further improves lithium-ion conductivities.

The influence of domain alignment on solvent swelling is explored using custom-fabricated environmental chambers for grazing incidence X-ray scattering and broadband dielectric spectroscopy which enable characterization of the morphologies, compositions, dielectric relaxations, and ionic conductivities of polymer thin films exposed to controlled water- or solvent-vapor environments. Thin films of PES12Li are prepared with layers oriented parallel to the substrate that persist under flowing solvent vapor. The in-plane ionic conductivities improve in the aligned nanostructures with selective-swelling by four solvents. When solvent-swelling is performed in the presence of grain boundaries, solvent uptake is reduced, while ionic conductivities are improved relative to the thin films.

In contrast to the periodic nanostructures of PESLi12, aperiodic solvent-swollen nanostructured aggregates in lithium-neutralized partially sulfonated polystyrene ionomers are captured using Gaussian random field (GRF) reconstructions. Wider, more interconnected channels correlate with improved lithium-ion conductivities. Importantly, solvent choice impacts ion transport with sulfolane yielding the highest room-temperature ionic conductivity when the total solvent content is low. Ultimately, this thesis establishes that solvent swelling of nanostructured SIPEs produces selective swelling of polar aggregates and significantly enhances lithium-ion conductivities. Meso- and nano-scale morphologies play a significant role in determining solvent uptake, and readily-accessible sulfonated polystyrenes can be swollen and rapidly characterized using GRF reconstructions. The structure-property relationships revealed by this work facilitate the development of solvent-swollen nanostructured SIPEs for use in next-generation lithium-ion batteries.