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Ion Diffusion and Electrochemically Driven Transport in Homogenous and Nanostructured Polymer Electrolytes

  • Author(s): Timachova, Ksenia
  • Advisor(s): Balsara, Nitash P
  • et al.

There is a growing need for safe, reliable, economical, and energy dense electrical energy storage devices. Polymer electrolytes are of great interest due to their potential use in high specific energy solid-state batteries. Polymer electrolytes are mixtures containing dissociated ions in a polymer matrix. The transport of ions in polymer electrolytes is of significant practical interest in order to enable their use and commercialization in next generation energy storage devices. Most experimental work on the field has focused on studying bulk electrochemical transport properties such as ionic conductivity. In this work, ion transport is primarily studied using a spectroscopic technique called pulsed-field gradient NMR (PFG-NMR). Using this technique, we investigate the molecular mechanisms that dictate ion transport through polymer materials.

Polyethylene oxide (PEO) provides a useful model system to study the effects of molecular weight and salt concentration on ion transport. We present measurements of the electrochemical transport properties of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in PEO over a wide range of PEO molecular weights and salt concentrations. Individual self-diffusion coefficients of the Li and TFSI ions, D+ and D-, were measured using PFG-NMR both in the dilute limit and at high salt concentrations. Conductivities calculated from the measured D+ and D- values based on the Nernst-Einstein equation were in agreement with experimental measurements reported in the literature. We determined the molecular weight dependence of the cation transference number in these solutions. We introduce a new parameter, s, the number of lithium ions per polymer chain, that allows us to account for both the effect of salt concentration and molecular weight on cation and anion diffusion. Expressing cation and anion diffusion coefficients as functions of s results in a collapse of D+ and D- onto a single master curve.

We extend our analysis of ion transport to perfluoropolyether-based polymer electrolytes. Perfluoropolyethers (PFPEs) are polymer electrolytes with fluorinated carbon backbones that have high flash points and have been shown to exhibit moderate conductivities and high cation transference numbers when mixed with lithium salts. Ion transport in four PFPE electrolytes with different endgroups was characterized by differential scanning calorimetry (DSC), ac impedance, and PFG-NMR as a function of salt concentration and temperature. In spite of the chemical similarity of the electrolytes, salt diffusion coefficients measured by PFG-NMR and the glass transition temperature measured by DSC appear to be uncorrelated to ionic conductivity measured by ac impedance. We calculate a non-dimensional parameter, β, that depends on the salt diffusion coefficients and ionic conductivity. We also use the Vogel-Tammann-Fulcher relationship to fit the temperature dependence of conductivity. We present a linear relationship between the prefactor in the VTF fit and β; both parameters vary by four orders of magnitude in our experimental window. Our analysis suggests that transport in electrolytes with low dielectric constants (low β) is dictated by ion hopping between clusters.

A set of polyether polymers with varying density of ether groups were synthesized to study the effects of polymer mobility and solvation site density on ion transport. The mobility of the polymer backbones dictated by the glass transition temperature were measured using DSC, the diffusion and conductivity of the ions were measured using PFG-NMR and ac impedance, and the steady-state current responses of the electrolytes were calculated from the measurements. The results indicate that the steady-state current response of a polyether-based polymer electrolytes is proportional to the density of lithium solvation sites and inversely proportional to the mobility of the polymer backbone. This set of polyether polymers was used to predict a polymer structure that could have a better steady-state current response than PEO.

Unlike homopolymer electrolytes, where the structure and ion transport functions are linked together by the chemistry of the backbone, nanostructured block copolymer electrolytes can provide independently unable ion transport and structural properties. Consequently, we extend our characterization of ion transport to block copolymers composed of a flexible ion transporting block and a rigid mechanically robust block. These materials can microphase segregate into nanostructured domains. Practical uses of the materials, however, require an understanding of the microscopic and macroscopic ion transport properties across the microphase-separated systems. We present complete continuum characterization of ion transport in a lamellar polystyrene-b-poly(ethylene oxide) copolymer/LiTFSI electrolyte as a function of salt concentration. Electrochemical measurements are used to determine the Stefan-Maxwell salt diffusion coefficients D_(+,0 ), D_(-,0), and D_(+,-). Individual self-diffusion coefficients of the Li- and TFSI-containing species were measured by PFG-NMR. The NMR data indicate that salt diffusion is locally anisotropic, and this enables determination of a diffusion coefficient parallel to the lamellae, D_(||), and a diffusion coefficient through defects in the lamellae, D_⊥. We quantify anisotropic diffusion by defining an NMR morphology factor and demonstrate that it is correlated to defect density seen by transmission electron microscopy. We find agreement between the electrochemically determined Stefan-Maxwell diffusion coefficients and the diffusion coefficient D_⊥ determined by PFG-NMR. Our work indicates that the performance of nanostructured block copolymer electrolytes in batteries is strongly influenced by ion transport through defects.

Nanostructured block copolymer electrolytes composed of organic and inorganic moieties extend the application potential of polymer materials by providing more chemically discrete properties between the two blocks. Using these materials requires us to understand how ion transport changes across different morphologies. The self-diffusion of salt ions across a disorder to lamellar phase transition in a nanostructured poly(ethyelene oxide)-b-poly(silesquioxane) copolymer was studied using PFG-NMR and changes in the morphology were studied using small-angle x-ray scattering. The diffusion of the salt is isotropic when the polymer electrolyte is disordered and anisotropic when the polymer is microphase separated. The difference between the diffusion coefficient parallel to the lamellae, D_(||), and the diffusion coefficient perpendicular to the lamellae, D_⊥, measured using PFG-NMR increases above the phase transition, and the two diffusivities diverge as segregation strength increases with increasing temperature. The degree of anisotropy of diffusion increases with increasing segregation strength, paralleling the changes in the morphology measured by small-angle x-ray scattering.

Advanced measurement techniques that combine electrochemistry together with imaging and spectroscopy are emerging as powerful methods for characterizing transport in electrolyte and battery systems. Two imaging techniques, x-ray imaging and NMR imaging or magnetic resonance imaging (MRI) have proven to be effective in looking at salt distribution in electrolytes under the application of electric fields. Additionally, electrophoretic NMR (eNMR) can be used to directly measure ion mobilities. We provide some methods for implementing these techniques and discuss future work using these approaches. We show that the standard approach for analyzing eNMR data is valid only for dilute electrolytes and provide a theoretical framework for interpreting eNMR results for all binary electrolyte systems with univalent salts. We derive relationships between the velocities of the ion species and the solvent in terms of the electrochemical Stefan-Maxwell diffusion coefficients and provide modified expressions for correctly calculating the transference number and conductivity from eNMR data in concentrated electrolytes. Our approach suggests that it is necessary to measure not just the displacement of ion species during the application of current in an eNMR experiment but also the displacement of the uncharged solvent in order to correctly calculate ion mobilities and the transference number.

Our analysis of ion transport properties across a range of different polymer electrolyte chemistries and morphologies delves into molecular ion transport mechanisms and highlights the importance of studying ion transport on the micro- meso- and macro-scales. We make connections between the transport quantities measured using NMR and the more traditional transport parameters measured using electrochemistry. The measurement presented here provide an overview of how the chemical, physical, and morphological properties of polymer electrolytes dictate ion transport. Finally, we lay the groundwork for further investigation of polymer electrolyte materials using advanced techniques that combine electrochemical measurements with imaging and spectroscopy.

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