Segmental dynamics and ion association in poly(ethylene oxide) based single ion conductors

Open Access
Author:
Sinha, Kokonad
Graduate Program:
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
November 15, 2011
Committee Members:
  • Janna Kay Maranas, Dissertation Advisor
  • Janna Kay Maranas, Committee Chair
  • Michael John Janik, Committee Member
  • Scott Thomas Milner, Committee Member
  • James Spiro Vrentas, Committee Member
  • James Patrick Runt, Committee Member
Keywords:
  • polymer conduction
  • solid polymer electrolytes
  • single ion conductors
  • ionomers
  • quasi elastic neutron scattering
  • polymer dynamics
Abstract:
Increased use of hand held electronic devices have fueled the need for smaller and more flexible lithium ion batteries. The mechanical flexibility of currently used batteries is reduced because of the use of liquid electrolytes, which require a rigid casing. Therefore, scientists are turning to replacing the liquid electrolyte with a solid polymer electrolyte (SPE), thereby eliminating the need for the casing. SPEs contain a polymer host such as polyethylene oxide (PEO) and a lithium salt, which provides the cation. Ionic conductivity in SPEs is coupled to the polymer mobility. The cation is solvated by ether oxygen atoms (EO) and the lithium moves by hopping from one EO-rich site to another. This movement is aided by the segmental mobility of the polymer. However, high anion mobility contributes to conductivity and results in reverse electrode polarization which degrades battery life. We therefore use single ion conductors (ionomers) where only the cation conducts. In this case a sulfonate anion is covalently bonded to a PEO backbone through an isophthalate comonomer unit. We use the nomenclature PEOx-Y %M for these ionomers (e.g. PEO400-50%Li). These samples allow us to vary the degree of sulfonation (Y, the percentage of sulfonated isophthalate groups), the spacer length (x, molecular weight of PEO between isophthalate groups) and the cation identity (M, which could be Li+, Na+ and Cs+). We use quasi-elastic neutron scattering (QENS) to study the PEO backbone dynamics of these ionomers as a function of ion content and ion identity. The ion content, defined as the molar ratio of cations to EO atoms, can be changed in two ways - by varying the degree of sulfonation (Y in [0,100]), and by changing the MW of the spacer (x = 400, 600, or 1100). When we compare the dynamics of nonionic polymers (i.e. no acid groups - PEO600-0%) to that of pure PEO, we observe that the isophthalate group reduces the mobility of its neighboring atoms. Therefore, the overall dynamics is composed of two fractions - a fast fraction in the mid-region of the spacer away from the effect of the isophthalate group (bridge atoms), and a slow fraction which neighbors the isophthalate group (anchor atoms). When we introduce ions by increasing the degree of sulfonation (PEO600-Y%Na), no new relaxations are observed, but the two fractions are affected differently. The bridge atoms appear to saturate at a low ion content of 0.01. In contrast, the anchor atoms have considerably reduced mobility when the ion content is above 0.01 due to crosslinking between ionic groups at the isophthalate units. Correspondingly, the anchor atom relaxation determines the glass transition temperature of the ionomer. We also compare the results to the PEO/LiClO4 system, by comparing both polymer dynamics and conductivity as a function of ion content. The optimal ion content for ionomers is half that of the salt system, which we explain based on the differing behavior of polymer dynamics in the two systems. We further investigate the effects of changing ion content on the component dynamics by changing the spacer length (at 100% sulfonation). The trend in bridge and anchor atom dynamics depends mainly on the absolute ion content, and not on the way it is varied. On comparing the dynamics and conductivities of two samples which have similar ion content, we conclude that it is better to achieve a particular ion content by varying the spacer length (at 100% sulfonation) rather than having partial sulfonation. Using QENS to calculate the ionic composition of bridge and anchor atoms, we incorporate the findings of small angle X-ray scattering (SAXS) and present a visual schematic of the dynamic patterning of bridge and anchor atoms. SAXS measurements also show that formation of ionic aggregates is characteristic of these ionomers and in some cases microphase separation occurs. We also study the effect of different cations (Li+, Na+ and Cs+) on the bridge and anchor atom dynamics of PEOx-100 %M ionomers. The ion content of these systems is above 0.01, therefore, the bridge atom relaxation times are similar, and ion identity primarily affects the anchor atom relaxations. By correlating QENS data with SAXS measurements, we study the effect of aggregate size and extent of microphase separation on anchor atom dynamics. Li based samples have high extent of microphase separation, whereas Na and Cs based samples display a distribution of aggregate sizes. These morphological differences affect the nature of interaction between the cation and the anchor atoms. Based on the binding energies and atomic radii of these cations, we identify the factors which govern the anchor atom dynamics at different temperatures. We conclude that when the reduced temperature [T - Tg] is less than 60 C, the anchor atom dynamics are controlled by the cation coordination number (number of EO atoms that solvate a cation in a PEO environment). Comparatively, when [T - Tg] is greater than 60 C, the dynamics are controlled by the cation-EO binding energies. Using QENS data, we further investigate the effect of spacer length on the degree of microphase separation in these samples. We conclude by identifying conditions that are favorable for conductivity in single ion conductors. Molecular dynamics (MD) simulations on PEO600-100%Na show the existence of string like aggregates which assist in charge transfer. We recognize that to improve the conductivity of single ion conductors and solid polymer electrolytes in general, we need to focus on both favorable cation coordination states (like string like aggregates) as well as high polymer host mobility. While the presence of favorable cation coordination states help in charge transport, amorphous polymer domains are necessary for ion transport from one coordination state to another. We finally propose a sample design that allows this partial decoupling of ion transport and conductivity. This design can help us create string like aggregates as a function of several variables (such as length, concentration, chain length etc.). By studying the effect of such factors on dynamics and conductivity, we can build better single ion conductors.