Investigating the Ion Transport Properties of Polymerized Ionic Liquids and Gel Polymer Electrolytes
Open Access
- Author:
- Kuray, Preeya
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 17, 2020
- Committee Members:
- Michael Anthony Hickner, Dissertation Advisor/Co-Advisor
Michael Anthony Hickner, Committee Chair/Co-Chair
Enrique Daniel Gomez, Outside Member
Lauren Dell Zarzar, Committee Member
Ralph H Colby, Committee Member
John C Mauro, Program Head/Chair - Keywords:
- polymer electrolytes
dielectric relaxation spectroscopy
solid state electrolytes
sodium ion battery
gel polymer electrolyte
ion transport - Abstract:
- Single-ion conducting solid polymer electrolytes (SPEs) are defined as ion-conducting polymers in which one ionic species is fixed to the polymer chain while the other is free for transport. These materials present the benefit of having a transference number close to unity, allowing for isolated study of ion transport in polymeric materials. In this thesis, the conductive properties of two different single-ion conducting polymer systems were studied: polymerized ionic liquids (PILs) and Na+ conducting gel polymer electrolytes (GPEs). Polymerized ionic liquids (PILs) are defined as single-ion conductors based on ionic liquids, in which either the cation or anion is bound to the polymer chain while the other is free for transport. In these materials, the ionic species can either be incorporated into the polymeric backbone (backbone PILs) or positioned as pendant groups to the chain (pendant PILs). This thesis focused on exploring how morphology and placement of the ionic group impacts conductivity for imidazolium-based pendant and backbone PILs with TFSI-, CPFSI-, and NfO- counter-anions. Wide-angle X-ray scattering (WAXS) was used to quantify 3 different polymeric correlation lengths: the backbone-to-backbone, pendant-to-pendant, and anion-to-anion correlation length. These distances corresponded to the length between polymer backbone chains, pendant groups, and counter-anions, respectively. Although pendant PILs exhibited all three correlation distances, backbone PILs only showed the anion-to-anion correlation peak due to the absence of a pendant group. It was also demonstrated that the anion-to-anion correlation peaks do not change between equivalent pendant and backbone PILs with the same counter-anion at the same temperature, implying that the anion-to-anion correlation length is the same between PILs with the same counter-anion. While the WAXS spectra of the backbone-CPFSI and backbone-TFSI PILs showed just one anion-to-anion scattering peak, the backbone-NfO sample exhibited an additional peak at 0.4 Å−1 that became more evident as temperature decreased before finally becoming constant in intensity below Tg. Temperature dependent small-angle X-ray scattering (SAXS) was used to further investigate the morphological changes of the backbone-NfO sample. In contrast to WAXS, SAXS experimental results showed a thermally reversible correlation peak at 0.053 Å−1, which became more pronounced as temperature increased. These results imply a morphological change of the backbone-NfO sample, which may be due to the NfO anions forming segregated fluorine domains due to aggregation of fluoroalkyl segments on the anion. These results indicate that the fluoroalkyl sections in the PIL sample play an important role in temperature-dependent morphology. Dielectric relaxation spectroscopy (DRS) was used to characterize the peak relaxation frequencies for the backbone and pendant PIL samples by fitting the derivative of the imaginary part of the dielectric constant to the Havraliak-Negami fitting function. From this fit, peak relaxation frequencies were obtained and plotted as a function of inverse temperature. The backbone PILs showed consistent VFT behavior, while the pendant PILs transitioned from VFT to Arrhenius behavior below Tg. The transition from VFT to Arrhenius in pendant PILs indicates decoupling of ion transport from segmental dynamics, which has been seen in other PIL systems. The Havriliak-Negami fit also yielded dielectric relaxation strength (∆ε) as a function of temperature for the PIL samples. At temperatures well above Tg, most of the samples showed a decrease in ∆ε with increasing temperature which followed the Onsager prediction of ∆ε decreasing with temperature due to thermal dipole randomization. For the backbone-NfO sample a sharp decrease in ∆ε was observed between 70 °C to 90 °C, which is similar to the temperature range where the morphological change occurred in the WAXS and SAXS spectrum. The reduction of ∆ε from the dielectric data may indicate the formation of ionic aggregates as temperature decreases. Ionic conductivity for the pendant and backbone PILs as a function of frequency and temperature were also measured using DRS. Experimental results showed that the backbone PILs exhibited a higher ionic conductivity on an absolute temperature scale, which is attributed to the lower glass transition temperatures compared to the equivalent pendant PILs. The lower Tg of the backbone PILs and higher conductivity compared to equivalent pendant PILs is attributed to the higher flexibility and more of a percolated pathway for charge transport than the pendant PILs, which is consistent with prior experimental and simulation studies. However, upon normalizing the temperature to the respective Tg of each material, pendant PILs exhibited a higher Tg-normalized conductivity than their equivalent backbone PIL counterparts. Experimental results also show that pendant PILs exhibit conductivity below Tg, implying they are conductive in the solid state. The second part of this thesis investigated the ion transport properties of Na+ conducting gel polymer electrolytes (GPEs). Single ion conducting GPEs are characterized as having a certain amount of ionic liquid or solvent incorporated into a single ion-conducting polymer matrix. Non-flammable and low vapor pressure solvents were chosen to plasticize the polymer matrix and enhance ion conductivity. For this study, carbonate-based solvents such as propylene carbonate, dimethyl adipate, diethyl 4-oxopimelate, and glycerol were used to determine the impact of the solvent on the conductivity profile and polymer dynamics of single-ion Na+-conducting photopolymerized GPE membranes. Propylene carbonate (PC) was chosen due to its high dielectric constant, low flashpoint, low vapor pressure, and ability to form solvation complexes with alkali metal ions, while the adipic-based linear carbonates (dimethyl adipate and diethyl 4-oxopimelate) were chosen due to their low viscosities and vapor pressures. Finally, glycerol was chosen due to its high dielectric constant and flashpoint, and ability to form coordinating complexes with Na+. The GPEs in this study were created through photopolymerization, which is an effective method of producing mechanically robust, free-standing GPEs. Using this method, curable monomers, a liquid electrolyte, and photo-initiators were cast onto a glass plate and then cured under UV radiation to form a polymer network in which the liquid electrolyte solidified within the gaps of the polymer matrix. Dielectric relaxation spectroscopy was used to characterize the Na+ conductivity, static dielectric constant, ion-conducting content, and mobility of the membranes with and without the solvents. Experimental results showed that all plasticizers investigated improved the conductivity of the Na+ GPE base membrane. The linear carbonate-solvated membrane yielded an increase in conductivity by an order of magnitude, while the glycerol-solvated membrane exhibited a 2 order of magnitude improvement in conductivity compared to the solvent-free membrane. The glycerol-solvated membrane likely showed the highest improvement in conductivity due to low solvent evaporation effects and high ion-solvent coordination. Experimental results also showed that the membranes solvated with propylene carbonate showed a modest increase in conductivity compared to the base membrane. It is possible that the low increase in conductivity of the propylene carbonate membranes stemmed from solvent evaporation effects during membrane processing. The electrode polarization (EP) model was then used to separate the measured ionic conductivity into contributions from number density of conducting ions and conducting ion mobility as a function of temperature to obtain the static dielectric constant (ε_s). At 85 °C, ε_s of the unsolvated membrane was 5.28, and upon incorporation of the coordinating solvents, ε_s improved to 8.7 for propylene carbonate and 39.0 for glycerol. ε_s values obtained for both the glycerol and propylene carbonate membranes stayed consistent across the measured temperature range. EP analysis showed that the addition of dimethyl adipate and diethyl 4-oxopimelate increased ε_s from 5.28 to 65.6 and 45.2, respectively, at 85 °C. ε_s changed by 57.5 over a span of 50 °C for the dimethyl adipate solvated membranes and by 29 over a span of 55 °C for the diethyl 4-oxopimelate solvated membranes. At 130 °C, experimental results showed that the conducting ion concentration for the glycerol, propylene carbonate, and unsolvated membranes was low, with ~0.0001% of ions (3.89 ∙ 1014 ~ 1.20 ∙ 1015 cm-3) being mobile. The conducting ion content for membranes solvated with dimethyl adipate and diethyl 4-oxopimelate was higher, with 0.001% and 0.005% of ions being conductive at 130 °C (4.85 ∙ 1015 ~ 1.51 ∙ 1015 cm-3. Below 100 °C, experimental results indicated that incorporation of solvent improved ionic mobility of each of the solvated membranes, which may be due to increased flexibility of the polymer matrix. Of all the studied membranes, glycerol and propylene carbonate yielded the highest improvement in mobility. Glycerol improved the mobility of the unsolvated membrane by 100 times at 100 °C while propylene carbonate improved mobility by 3 times at 100 °C. Incorporation of these solvents likely lowered the Tg of the system and increased chain flexibility, which may be confirmed using dynamic mechanical analysis. Dimethyl adipate yielded a smaller increase in mobility (2.3 times at 100 °C), while diethyl 4-oxopimelate only improved mobility at temperatures under 100 °C. In summary, the ion transport properties of two different types of ion-conducting polymer systems were explored. The results from the first part of this thesis demonstrated the importance of ionic group placement on ion transport properties in PIL systems. Additionally, experimental results showed that ion transport for backbone PILs was coupled to the segmental dynamics below Tg, whereas decoupling of ionic conductivity from segmental relaxation was observed for pendant PILs. The second part of this thesis investigated the conductive properties of photopolymerized single-ion Na+ conducting GPE membranes using propylene carbonate, dimethyl adipate, diethyl 4-oxopimelate and glycerol as plasticizers. Experimental results indicated that all plasticizers investigated improved the conductivity of the Na+ GPE base membrane, and that using glycerol as a plasticizer yielded a nearly 2-order of magnitude improvement in conductivity. This is likely due to low solvent evaporation effects of glycerol and high ion-solvent coordination interactions between Na+ and glycerol. Understanding these results may glean insight on possible plasticizers to develop for next generation solid-state polymer electrolytes.