Investigations of single-ion conducting polymer electrolytes by nuclear magnetic resonance spectroscopy.

Restricted (Penn State Only)
Author:
Lafemina, Nikki Hunter
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 05, 2015
Committee Members:
  • Karl Todd Mueller, Dissertation Advisor
  • David D Boehr, Committee Member
  • Benjamin James Lear, Committee Member
  • Professor Michael A Hickner, Committee Member
Keywords:
  • single-ion conducting polymer electrolytes
  • ionomers
  • diffusion
  • NMR
  • lithium diffusion
  • batteries
  • conductivity
Abstract:
Conventional polymer electrolytes for battery systems consist of a polymer and salt dissolved in an organic solvent. Conventional polymer electrolytes have higher energy densities and are safer than traditional lead-acid battery systems, but still have issues with solvent leaking and low transference numbers due to the ability of both the cation and the anion of the salt to move. In single-ion conducting polymers, or ionomers, one of the ions is covalently attached to the polymer backbone, eliminating any diffusive motion from this ion and providing for a transference number approaching unity for the non-bound ion. With the choice of an appropriate polymer, the diffusing ion is dissolved within the polymer matrix, eliminating the need for solvent and mitigating the issue of solvent leakage. However, one of the main limitations with ionomer systems is their typically low conductivity at room temperature (a desired operating range for many applications). The work in this dissertation elucidates the mechanisms by which ions move throughout ionomer systems and contribute to conductivity, an important step toward a predictive understanding of these systems. Ultimately, this understanding is critical for creating more efficient ionomer electrolyte systems for battery applications. Nuclear magnetic resonance (NMR) spectroscopy is an ideal tool to study the motion and dynamics of ionomer systems because this technique provides structural information and is able to elucidate the movement of species (charged and uncharged) in the system, where dielectric relaxation spectroscopy (DRS) can only measure the movement of charged species. Moreover, NMR and DRS studies are conducted with the ionomer systems in their natural state, in the absence of solvent, providing insight into how the ionomer systems would behave if they were to be used inside a battery system. Comparisons to DRS measurements are important to discern the contributions of all parts of the ionomer system to the overall motion and dynamics. The motion of the diffusing ion through the polymer matrix, as well as the polymer matrix itself, in two ionomer systems was studied to understand how the motion of the diffusing ion relates to the structure and dynamics of the polymer matrix and ultimately determines conduction. The first system consists of a poly(ethylene oxide) (PEO)-based sulfonate ionomer with lithium as the diffusing anion. Within this ionomer system two sample series were studied: one with varying fraction of the ionized unit and constant spacer molecular weight, and the other with a constant fraction of the ionized unit and varying molecular weight of the PEO spacer. 7Li and 1H T1 inversion recovery experiments and motional narrowing of the spectral linewidths for the two PEO-based ionomer series were studied as well as 7Li pulsed field gradient stimulated echo (PGSE) NMR to determine the extent and mechanism of motion within these ionomers on length scales of nanometers and micrometers, respectively. Motional narrowing of the 7Li linewidths indicates that as the ion content is decreased the lithium ions become more mobile. The local motions of the lithium ions are correlated to the polymer segmental motion, although the motion of the lithium ions is approximately an order of magnitude slower than the polymer segmental motion. Comparison between 7Li PGSE self-diffusion coefficients, ionic conductivity and lithium self-diffusion coefficients calculated from the ionic conductivity by the Nernst-Einstein equation indicated that the self-diffusion coefficient decreased with increasing ion content due to the presence of ionic aggregates with the exception of the PEO400-100%Li ionomer, which exhibited the greatest self-diffusion coefficient even with the highest ion content. The anomalous behavior of the PEO400-100%Li ionomer was determined to be due to the decreased ion-polymer interaction that results in a mechanism that cannot be described by a simple succession of ion hops or by the complete motion of the polymer backbone. It was concluded that greater changes in the self-diffusion coefficients can be obtained by changing the length of the PEO spacer rather than by changing the fraction of the ionized unit in order to achieve a certain ion content. Battery systems depend on the diffusion of ions between electrodes to maintain charge balance and as such an electrolyte system that exhibits increased ion diffusion coefficients may lead to a more efficient battery. The second system studied here consists of a polysiloxane backbone with pendent PEO and phosphonium side chains. The length of the PEO side chain was kept constant at three oligomeric units to aid in solvation of the counter anions. The identities and amounts of the counter anion were varied to be either a fluoride or a TFSI anion with concentrations between 5-26 mol %. Polymer and anion motion were investigated through 31P and 19F T1 relaxation rate measurements and motional linewidth analysis, as well as 19F PGSE NMR over a temperature range of 293-407 K. Comparison of results obtained from NMR spectroscopy and DRS showed that the self-diffusion coefficient is dominated by the motion of the anion with very little interaction with the polymer backbone. Weakly coordinating ions in the polysiloxane-based ionomers resulted in increased conductivity and self-diffusion coefficients at room temperature (10-5 S/cm and 10-11 m2/s, respectively) compared to the PEO-based ionomers (10-6 S/cm and 10-12 m2/s, respectively).