Abstracts Category : Other

Single-Ion-Conducting Block Copolymer Electrolytes for Lithium Batteries: Morphology, Ion Transport, and Mechanical Properties

by Adriana Araceli Rojas

Lithium metal batteries have high theoretical specific energies, which make it a favorable candidate to meet our need for energy storage applications for electric vehicles and grid storage. However, there are significant safety concerns that limit our use of lithium metal electrodes. For this reason, polymer electrolytes have been a favorable choice of electrolyte, as they are they are more thermally and electrochemically stable against lithium metal. Block copolymer electrolytes are a promising candidate for these battery systems because of their ability to microphase separate into unique nanostructures. Given a high molecular weight block copolymer, the ion transport and moduli can be significantly improved relative to its hompolymer counterpart. As a result, block copolymers have been effective at slowing the growth of lithium dendrites. However, the main problem with block copolymer electrolytes where a salt was physically integrated, is the problem of concentration gradients that form over the length of the electrolyte. Concentration gradients are a result of low transference numbers, that is, the lithium ion of interest will carry a low fraction of current relative to the anion. To eliminate concentration gradients, single-ion-conducting block copolymer electrolytes were synthesized and characterized: poly(ethylene oxide)-b-poly(styrenesulfonyllithium(trifluoromethylsulfonyl)imide) (PEO-b-PSLiTFSI). In this class of copolymers, the anion (TFSI-1) was covalently bonded to the polystyrene backbone, allowing only the lithium ion to move. The work enclosed elucidates the relationship between the morphology, ion transport, and mechanical properties of this single-ion-conducting block copolymer electrolyte. In the first phase of this dissertation, the synthesis of the monomer, the PEO macroinitiator, and the subsequent nitroxide mediated polymerization procedure are detailed. Improvements to the polymerization are described, and the characterization steps for ion-exchange and polymer structure are discussed. The subsequent work discusses the relationship between ion transport and morphology using small angle X-ray scattering (SAXS) and impedance spectroscopy. It was demonstrated that the placement of the charged group in the non-ion-conducing block (PS) rendered fundamentally different nanostructure morphology. Unlike uncharged block copolymers, it was found that PEO-b-PSLiTFSI completely disordered (homogenized). There was no presence of concentration fluctuations. When the copolymer underwent an order-to-disorder transition, the ionic conductivity was found to increase three orders of magnitude. It was demonstrated that there are favorable interactions between the lithium ions and the ethyl ethers in PEO. Next, the effect of ion concentration on morphology and ion transport were explored. It was found that copolymers of low ion concentration (r = [Li+][EO]-1) were microphase separated at room temperature. However, at high r, the copolymers were found to be disordered (homogenous) at low