Date of Award

Summer 7-1-2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical and Chemical Engineering

Advisor(s)

Hosein, Ian D

Second Advisor

Qiao, Quinn

Keywords

Battery, Electrochemistry, Lithium, Molecular Dynamics, Polymer Electrolytes, Sodium

Subject Categories

Chemical Engineering | Electrical and Computer Engineering | Engineering | Materials Science and Engineering | Power and Energy

Abstract

The establishment of sustainable energy sources highly depends on efficient storage devices to guarantee a consistent power supply. The growing demand for lithium-ion batteries (LIBs) for this purpose, combined with concerns about lithium availability, has motivated the search for viable storage alternatives, such as sodium and calcium. While several studies have investigated different lithium-free liquid electrolytes, the transport of these alternative ions in their polymer counterparts remains understudied. The advantages of solid polymer electrolytes include the possibility of higher energy density in solid-state devices and the elimination of safety concerns associated with liquid electrolytes, such as flammability, leakage, and dendrite growth. This dissertation describes my efforts to develop solid-state polymer electrolytes for lithium, sodium, and calcium conduction. It also presents these materials' electrochemical and polymer characterization and informs potential routes for future electrolyte design.In this dissertation, my first effort was to produce and characterize a novel solid polymer electrolyte (SPE) for calcium-ion conduction. Poly(ethylene glycol) diacrylate was photocrosslinked under blue light in the presence of calcium salt to form a stable network with high thermal stability and promising conductivity. Ionic conductivity of 3.4×10-4 S cm-1 at 110 °C and 3.0x10-6 S cm-1 at room temperature was achieved, similar to early-stage lithium SPEs. The electrolytes evaluated under thermogravimetric analysis (TGA) remained stable at temperatures of approximately 140 °C. XRD confirmed no salt precipitation within the polymer matrix, and Raman analysis indicated the complexation of the calcium ions to the PEGDA network. These results highlighted the potential of polymer electrolytes for solid-state calcium-ion batteries. I will also present my work on the synthesis, property characterization, and ion conductivity studies of a solid polymer electrolyte produced from polytetrahydrofuran (PTHF) photo-crosslinked with 3,4-epoxycyclohexylmethyl 3ʹ,4ʹ-epoxycyclohexane carboxylate (Epoxy). The preparation occurred via an active monomer mechanism that facilitated the reaction of the native hydroxyl and epoxide end-groups. Crosslinked samples were loaded with different lithium tetrafluoroborate (LiBF4) quantities and evaluated by electrochemical spectroscopy impedance (EIS) to determine their ionic conductivity. An increase in lithium salt loading led to increased ionic transport, reaching competitive conductivities of up to 10-3 S cm-1 at 60 °C. Diffraction analysis also revealed the amorphous nature of the electrolytes, which was confirmed by thermal analysis. Mechanical evaluation of the samples showed that the materials possessed suitable stiffness for battery applications. The results demonstrate a new synthetic route to tunable crosslinked networks for a broad range of chemical building blocks to achieve high lithium-ion conduction and attain desirable thermal and mechanical properties. This dissertation also includes molecular dynamics simulations employed to characterize the main factors that affect sodium-ion transport in two polymer hosts: polyethylene oxide (PEO) and poly(tetrahydrofuran) (PTHF). I analyzed the influence of oxygen density in each chain and its effect on diffusivity, conductivity, and cation-anion interactions. It is inferred that the weaker coordination in PTHF resulted in differences in the Na+ transport mechanism, with interchain hopping being more prominent in PTHF than in PEO. However, the faster diffusion observed in PTHF was hindered by the significantly larger formation of ion clusters in the PTHF electrolyte, which could lead to smaller transference numbers in battery settings. These findings elucidated the fundamental influences and correlations of varied polymer ether content to ion coordination and transport, which informed the following polymer design to improve electrolyte properties. In a subsequent effort to develop solid polymer electrolytes, semi-IPN membranes were fabricated for sodium-ion conduction using a boron-centered moiety with PTHF chains. The resulting SPEs combined the anion trapping properties of the boron with the weakly coordinating PTHF to achieve enhanced salt dissociation and transference number. The structure of the polymer was confirmed by 1H NMR, 13C NMR, and 11B NMR, and the FTIR spectra confirmed the crosslinking reaction with HMDI. The mechanical and thermal stabilities of the samples were evaluated by DSC and TGA, and FTIR spectroscopic analysis showed the efficiency of the boron-centers in complexing with ClO4-. Dielectric studies also indicated the predominance of free Na+ in samples. Boron-containing electrolytes with O/Na: 5 delivered the highest room-temperature ionic conductivity of 7.54 x 10-5 S cm-1. LSV revealed suitable electrochemical stability against Na metal. The measured transference number was 0.88, confirming that electrolytes benefit from the looser coordination of Na+ ions with PTHF chains combined with the anion trapping performance of boron moieties, indicating a potential direction in polymer electrolyte design. In summary, the contributions in this dissertation offer new methodologies to produce novel solid polymer electrolytes and provide critical insights into the coordination and cation transport in those systems. These findings can contribute to better design strategies in the field of solid-state batteries.

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