Next-generation lithium batteries with high energy densities are desired for powering the future fleet of electric vehicles. The implementation of these batteries hinges upon the development of novel electrolyte materials that exhibit stability against the lithium metal anode in addition to favorable transport properties. Conventional liquid electrolytes, such as the carbonate-based solvents that are standard in lithium-ion batteries, have high ionic conductivities but lack compatibility with lithium metal. In these systems, brittle, unstable interface layers form on the lithium surface leading to uneven plating and rapid cell failure. Polymer electrolytes offer a promising alternative to conventional liquid electrolytes, as they form stable interfaces with lithium metal and exhibit solid-like material properties. However, despite four decades of persistent research, the transport properties of the most promising solvent-free polymer electrolytes remain insufficient for use in commercial batteries. Our ability to design new polymers with improved electrolyte properties is compromised by a lack of understanding of ion transport in these materials. In this work, we employ a wide variety of electrochemical characterization techniques, supplemented with theory and simulations, to identify the factors that govern ion transport in polymer electrolytes.

The performance of battery electrolytes depends on three independent transport properties: ionic conductivity, diffusion coefficient, and transference number. We perform a complete characterization of all three transport properties in mixtures of 5 kg/mol polyethylene oxide (PEO) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) salt over a wide range of salt concentrations. Three different approaches were used to measure the transference number: the steady-state current measurement, pulsed-field gradient NMR, and a new approach proposed by Balsara and Newman. The latter approach is rigorous and based on concentrated solution theory, while the other two approaches only yield the true transference number in ideal solutions. The values obtained from the steady-state current method and pulsed-field gradient NMR are positive at all concentrations. In contrast, the transference number obtained by the approach of Balsara and Newman exhibits a complex dependence on the addition of salt, with negative values obtained at intermediate salt concentrations. Negative transference numbers suggest that ion transport is dominated by highly-mobile ionic clusters. These ion-ion interactions are neglected in the approaches that are derived using dilute solution theory.

There are a variety of techniques that can, in theory, be used to measure the transference number of concentrated electrolytes. We perform a comparison of two different electrochemical approaches: the method proposed by Balsara and Newman and a more well-established technique by Ma and coworkers. Both approaches are experimentally intensive and rely on concentrated solution theory. In high molecular weight PEO electrolytes, the data from the two techniques are in perfect agreement. In contrast, in low molecular weight PEO there is a disagreement between the two approaches, which is attributed to the presence of a complex interface layer on the surface of the lithium electrodes. The parameters measured in technique of Ma and coworkers are thought to be inherently sensitive to the nature of the electrode-electrolyte interface, which may not be representative of the bulk electrolyte. For this reason, the Balsara and Newman approach is taken as the more robust measure of transference number.

Complete characterization of ion transport in an electrolyte enables full cell modeling. In this work, the theory is presented for predicting the cycling characteristics of a lithium-polymer-lithium cell containing an electrolyte with known transport properties. Using the ionic conductivity, diffusion coefficient, and transference number of PEO/LiTFSI electrolytes as inputs to our model, we calculate salt concentration and potential profiles in the electrolyte under a constant dc polarization. At steady-state, these profiles are nonlinear due to the strong concentration dependence of the transport properties of the electrolyte. Predictions of the limiting current in PEO/LiTFSI electrolytes were obtained using the model. Experimentally-measured cycling data from a series of symmetric cells with different salt concentrations were used to test the validity of the model. The time-dependence and steady-state value of the potential measured during cycling experiments were in excellent agreement with model predictions, requiring no adjustable parameters or simplifying assumptions.

Our modeling work supports the notion that the transport properties of PEO/LiTFSI electrolytes are insufficient for immediate commercialization. Thus, there is a great deal of interest in developing next-generation polymer electrolytes with improved transport properties. Designing new polymers is hindered by the complex relationship between the transport of ions in the polymer and the structure of the monomer. For example, the ionic conductivity of a polymer electrolyte depends on a variety of interconnected factors: interactions between the polymer chains and the salt, extent of dissociation of the salt, and polymer dynamics in the vicinity of the ions. All of these are affected by the monomer structure. In this work, we attempt to unravel these factors through systematic analysis of electrolytes comprised of newly-designed polymers and LiTFSI salt. In all cases, PEO/LiTFSI electrolytes are used as a baseline for comparison.

A set of aliphatic polyesters with systematic variations to the monomer structure were characterized using ionic conductivity and glass transition temperature measurements over a wide range of salt concentrations. A novel analysis approach was introduced to factor out the effect of segmental motion on conductivity; the parameter calculated in this analysis is referred to as the reduced conductivity. The dependence of the reduced conductivity on salt concentration helps to clarify the relationship between monomer structure and ionic

conductivity, and highlights differences between PEO and the polyesters. This study also demonstrates that polymers, such as polyesters, which are comprised of multiple polar groups are not an ideal choice for fundamental studies due to the complexity of solvation and ion transport in these systems.

Linear polyethers (CxEOy) were synthesized as a systematic set wherein aliphatic linkers were added to a PEO backbone. The carbon linkers change the glass transition temperature and dilute the polar groups relative to PEO; both factors influence ionic conductivity. The analysis introduced in the previous study was used to factor out the effect of glass transition temperature on conductivity; the results show a clear dependence of the reduced conductivity on the mole fraction of oxygen of the polymer. MD simulations were used to study the solvation site around Li+, which were found to be similar in all polymers. A comparison of experimental measurements and simulation results highlights the importance of solvation-site connectivity, a parameter which is thought to affect the hopping rate of Li+. A polymer with a higher solvation-site connectivity than PEO is predicted to exhibit superior transport properties.

A newly-synthesized polymer, P(2EO-MO), was characterized using a variety of electrochemical and NMR experiments in addition to MD simulations. The maximum conductivity of P(2EO-MO) is comparable to that of PEO, but the glass transition temperature exhibits a more precipitous increase with the addition of salt. The transference number measured using the steady-state current method and NMR are about a factor of two higher in P(2EO-MO) compared to PEO. In lieu of complete electrolyte characterization, the product σt+,SS is identified as the most important transport characteristic for comparison of electrolytes. This product is higher in P(2EO-MO), thus it is predicted to be a more efficacious electrolyte than PEO for battery applications. MD simulations reveal that the promising transport properties of P(2EO-MO) are likely attributed to the high solvation-site density which facilitates the transport of Li+ in this material.

Rechargeable batteries are ubiquitous in our daily lives thanks to advancements in technology and low manufacturing costs for large scale production of lithium-ion batteries. State of the art, commercially available batteries are comprised of a graphite anode and liquid electrolyte. There is considerable interest in developing next-generation lithium batteries based on a lithium metal anode and solid-state electrolyte. While several commercial ventures have indicated that there is demand for such a product and significant capital to back them, lithium metal batteries have not been widely implemented due to high cost and poor cyclability compared to lithium-ion technology. We are motivated to study solid electrolytes which are compatible with lithium metal anodes. The majority of this thesis is focused on a nanostructured block copolymer electrolyte with phase-separated mechanically rigid and ion conducting domains: polystyrene-block-poly(ethylene oxide) (SEO) with lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI). Polystyrene (PS) provides mechanical rigidity which aides stable lithium deposition during battery charging and poly(ethylene oxide) (PEO) solvates the lithium salt and conducts ions between the electrodes. A variety of morphologies are accessible by tuning the volume fraction of one phase and the chain length. The viability of SEO/LiTFSI electrolytes in lithium metal batteries has been well established in the literature. However, there are many open questions about the mechanisms by which SEO facilitates the transportation of lithium ions between electrodes. Our goal is to shed light on the phenomena which govern the performance of block copolymer electrolytes via rigorous characterization of their ion transport properties and transient structure. The primary tools used in our analysis are electrochemical techniques involving a potentiostat to study the rate of ion transport under different conditions and small angle X-ray scattering (SAXS) to characterize block copolymer structure at equilibrium and while ions flow through the electrolyte. Unlike simple electronic conductors (e.g., a copper wire) which have only one charge carrier (electrons), a battery electrolyte has two charge carriers (cations and anions). When implementing an electrolyte into a lithium-ion or lithium metal battery, the electrodes are designed such that only lithium cations can cross the electrode/electrolyte interface. The unavoidable consequence is that passing ionic current through the electrolyte (i.e., charging or discharging the battery) results in accumulation of ions near one electrode and depletion of ions near the other. In other words, a salt concentration gradient develops along the axis perpendicular to the planar electrodes. Under galvanostatic conditions (constant current), thermodynamics requires that a larger driving force (over potential) be required to maintain the same current as the salt concentration gradient evolves. Under potentiostatic conditions (constant voltage), thermodynamics requires that a smaller current density be passed through the electrolyte as the salt concentration gradient evolves. Consequently, the nature of the salt concentration gradients which develop during battery operation are closely tied to material performance. In this dissertation, we characterize salt concentration gradients in symmetric cells composed of two lithium electrodes sandwiched around a polymer electrolyte. After introducing lithium-based batteries, polymer electrolytes, and SAXS in Chapter 1, we develop a framework for quantifying electrolyte performance in the presence of small salt concentration gradients. Measurement of the ionic conductivity, κ, by ac impedance spectroscopy characterizes ion transport in the absent of concentration gradients. In the presence of concentration gradients, electrolyte performance is quantified by the product of κ and the current fraction, ρ_+. The current fraction is the ratio of the steady state current, i_ss, to the initial current, i_0, in a potentiostatic experiment. We rank order a list of electrolytes reported on in the literature based on κρ_+ and find that there is a tradeoff between total ionic conductivity and selective cation transport. In Chapter 3, we explore the relationship between ρ_+ and the cation transference number, t_+, to show how the two parameters dictate ion transport at early and long times when an electrolyte of initially uniform salt concentration is subjected to ionic current. Negative transference numbers have been reported in the literature, however the result has not been definitively proven. Our analysis leads us to propose a cell based on anion exchange membranes which could be used to conclusively prove the result. In Chapter 4, we return our attention to SEO/LiTFSI block copolymer electrolytes and extend our discussion of electrochemical characterization beyond Chapter 2. By measuring κ, ρ_+, the salt diffusion coefficient, D, and the equilibrium potential of concentration cells, U, we fully characterize ion transport in a series of SEO/LiTFSI electrolytes with various morphologies and chain lengths to solve for the transference number with respect to the solvent velocity, t_+^0. We obtain negative values of t_+^0 over a wide range of salt concentrations and morphologies. Universal relationships are uncovered for ρ_+ and dU/(d ln⁡m ) which apply for any PEO/LiTFSI or SEO/LiTFSI electrolyte. The result allows us to predict salt concentration gradients for any SEO/LiTFSI electrolyte by simply measuring κ as a function of salt concentration. An important consideration when studying any electrochemical system is the interactions which take place at the electrode/electrolyte interface. For PEO-containing electrolytes in contact with a lithium electrode, we found that lithium metal is sparingly soluble in PEO. The primary evidence is obtained from nuclear magnetic resonance spectroscopy (NMR), which is discussed in Chapter 5. We show that this phenomenon is detectable by phase transitions in a low molecular weight SEO/LiTFSI electrolyte. We take this result into account when studying the phase behavior of SEO/LiTFSI electrolytes in lithium symmetric cells in the Chapters 6 and 7. In Chapter 6 and 7, we describe simultaneous polarization and SAXS experiments on SEO/LiTFSI electrolytes to study the transient structure in the presence of ionic current and salt concentration gradients. Our methodology allows us to study the nanostructure of the electrolyte as a function of time and distance from an electrode during polarization. In Chapter 6, we focus on a low molecular weight (3.1 kg mol-1) SEO/LiTFSI electrolyte which has accessible phase transitions over a wide salt concentration window. Under dc polarization, the initially lamellar morphology transforms into a disordered morphology near the negative electrode where salt concentration is lower, and pockets of the gyroid morphology form near the positive electrode where salt concentration is higher. The emergence of the gyroid phase is unexpected as it is only thermodynamically favored at significantly higher salt concentrations than should be accessible based on the initial salt concentration. We hypothesize the presence of the gyroid morphology is indicative of localized salt dense pockets which form at bottlenecks in the block copolymer structure. In Chapter 7, we study a higher molecular weight (39 kg mol-1) SEO/LiTFSI electrolyte which exhibits a lamellar morphology at all salt concentrations studied. The domain spacing, d, increases with salt concentration, and we study the change in d as a function of position in the cell. By measuring d at several current densities, we show that the spatial gradient in domain spacing aligns with model predictions at low to moderate current densities based on the measurements made in Chapter 4. The behavior at large current densities indicates that the rearrangement of the structure may limit the maximum sustainable current density which can be passed through the electrolyte. In our sample which consists of a distribution of lamellar grain orientations, we find those with PS/PEO interfaces aligned perpendicular to the current are distorted more than those with interfaces aligned parallel to the current. While grains with interfaces oriented perpendicular to the current do not provide pathways for current to pass between the electrodes, we show that the distortion of the structure is critical for facilitating the formation of a salt concentration gradient. In this dissertation, we discuss the physics that are responsible for the development of a salt concentration gradient when current flows through an electrolyte. Careful characterization of transport parameters provides microscopic insight into the mechanisms of ion transport and the magnitude of salt concentration gradients. Structural characterization of block copolymer electrolytes subjected to ionic current provides fresh insight into the mechanisms which limit the rate of ion transport between the electrodes. Our characterization of the dynamic relationship between salt concentration gradients, ion transport, and nanostructure in block copolymer electrolytes provides guiding principles for the design of novel electrolyte materials aimed at enabling the lithium metal anode.

Lithium-ion batteries are essential for the decarbonization of our power generation systems and electrification of the transportation sector. Their popularity stems from their impressive characteristics, including high energy density, low rates of self-discharge, and high cell voltage. However, the performance of these batteries is starting to reach a plateau. To enable the next generation of lithium-ion batteries, high energy electrodes such as lithium metal and silicon anodes are needed. Conventional battery electrolytes, consisting of a lithium salt dissolved in mixtures of organic solvents, are unstable against these electrode materials. A compelling alternative comes in the form of polymer electrolytes, which offer enhanced safety and greater chemical stability.

This dissertation focuses on understanding ion transport within polymer electrolytes, including a conventional polymer electrolyte and novel polymer electrolytes. Full electrochemical characterization, following Newman’s concentrated solution theory, allows for the measurement of transport parameters that provide a full description of ion transport in an electrolyte. Using concentrated solution theory, predictions of salt concentration profiles can be made. These are useful for understanding the conditions under which an electrolyte can stably operate. With higher amounts of current applied to an electrolyte, salt concentration gradients can grow until the salt is depleted at the cathode. The current density at which that occurs is known as the limiting current, which is an important parameter to judge the practical limitations of an electrolyte and its utility in battery systems.

Chapters 2, 3, and 4 present studies on ion transport in a conventional homopolymer electrolyte, LiTFSI salt dissolved in poly(ethylene oxide) (PEO). These works build on our understanding of ion transport in a model system that has been well characterized. In Chapter 2, the ion transport properties of PEO/LiTFSI electrolytes are measured across a wide range of salt concentration at three temperatures of interest. Included in this work is a full description of the experiments and calculations required for full electrochemical characterization. The dependence of the ion transport properties on temperature varies with each property, and thus predicting the effect of temperature on overall performance is impossible without the results of our full characterization study. Chapter 3 provides a comparison of salt concentration profiles predicted from concentrated solution theory with those measured using operando Raman spectroscopy. In Chapter 4, theoretical predictions are made of electrolyte performance at current densities above the limiting current density and are compared to experimental measurements.

Chapters 5 and 6 focus on potential next-generation polymer electrolytes. PEO has been the benchmark polymer electrolyte material since its discovery, and it is of utmost importance to discover new polymers for battery electrolytes. In Chapter 5, the dependence of the limiting current density on electrolyte thickness is measured for a conventional and a single-ion-conducting block copolymer. Chapter 6 presents the results of full electrochemical characterization of a novel homopolymer electrolyte, and these results are compared to those of PEO electrolytes.

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.

It is becoming increasingly clear that the next generation of rechargeable batteries for the emerging clean energy landscape will require electrolytes that are fundamentally different from those used in today’s lithium-ion batteries. In addition, it has been shown that the most promising approach to significantly increase the energy density of rechargeable batteries is through the implementation of the Li metal anode. Current electrolytes consist of mixtures of organic carbonates and a lithium salt. They pose safety concerns due to their flammability and are also incompatible with Li metal. Polymer electrolytes, which are mixtures of polymers and a lithium salt, are both less flammable than the organic solvents used currently and have been shown to be compatible with Li metal. Although linear polymers are able to exert some stress on the battery electrodes, which is essential to enabling rechargeable batteries with Li metal anodes, their viscoelastic nature prevents them from enduring stress in the long-time limit. One approach to improving the mechanical properties of polymer electrolytes is to use microphase separated block copolymers, which allows for decoupling of the ionically conducting and mechanically reinforcing properties.

The phase behavior of block copolymers, wherein two chemically distinct homopolymer chains are covalently bound, is dependent on two properties: segregation strength, χN, and copolymer composition, f_A. Segregation strength is the product of the Flory-Huggins interaction parameter, χ, and the overall chain length, N; it combines both enthalpic contributions from the monomer-monomer interactions, as well as entropic contributions which are directly related to chain length through the configurational entropy. The copolymer composition is measured in terms of the volume fraction of “block A”, f_A, and dictates the degree of curvature needed to minimize the areal contact between the unlike phases. For a given copolymer composition, when a copolymer has a segregation strength less than (χN)_odt, the copolymer melt will form a homogeneous disordered mixture. When the same copolymer has χN≥(χN)_odt, the copolymer will microphase separate into an ordered morphology such as body center cubic spheres, hexagonally packed cylinders, gyroid phases, or lamellae. The addition of salt to block copolymers allows them to conduct ions, but significantly alters their thermodynamics and resulting phase behavior. It is well-known for symmetric block copolymers, wherein the volume fractions of the two blocks are equal to 0.5, that the addition of salt will increase the segregation strength of the copolymer. For example, a copolymer that forms a disordered morphology in the salt-free state will undergo a disorder-to-order transition and form an ordered lamellar phase once a critical salt concentration is reached. In the simplest case, the increase in χ is linear with respect to salt concentration.

It is well-known that ion transport in polymer electrolytes is linked to the segmental motion of the polymer backbone. At short times, the polymer repeat units can be modeled as beads linked together by springs that are characterized by a friction coefficient, ζ. Motion in this regime is known as Rouse dynamics. At longer times, the motion of a segment of a polymer chain is influenced by the presence of neighboring chains that form entanglement constraints represented by tubes of diameter, d. As time progresses, the polymer chain must reptate through the entanglement constraints until it is fully free of its tube and can undergo self-diffusion. The motion of the polymer chain while it is entirely or partially constrained by its tube is known as reptation. Previous studies of dynamics in the Rouse regime have shown that the monomeric friction coefficient decreases as salt concentration increases in polymer electrolytes due to the coordination between the polymer backbone and the cations. In fact, for some polymer electrolytes, the ionic conductivity can be explained entirely by the segmental motion of the polymer. However, no studies have been conducted on polymer electrolytes at time-scales that correlate with reptation.

The block copolymer electrolyte used in this research is a well-studied model system: polystyrene-block¬-poly(ethylene oxide) mixed with lithium bis(trifluoromethanesulfonyl) imide (LiTFSI). SEO/LiTFSI was chosen as the model system because it is well-known that the Li salt preferentially segregates into the poly(ethylene oxide) (PEO) domains and the electrochemical properties of PEO/LiTFSI have been fully characterized. The objective of this dissertation is to develop a molecular-level understanding of the effect of salt on block copolymer self-assembly, thermodynamics, and dynamics. Specifically, I will use theories developed for salt-free polymer systems and apply them to data collected on block copolymer electrolytes to determine how the presence of salt changes polymer behavior. In cases where the original theories cannot explain the new data, I will derive new theories to describe the observed phenomenon.

In order to conduct these studies, a library of SEO copolymers with precise molecular weights and a wide range of copolymer compositions was synthesized. The first section describes the methods used to synthesize the SEO copolymers with precise molecular weights and compositions and low polydispersities.

The research in this dissertation covers a large area of polymer science; therefore, several different experimental techniques were used. Coincidentally, all experimental techniques were centered around scattering applications of both X-rays and neutrons. The second section provides an introduction to the fundamentals of scattering as well as a brief review on the specific techniques used in these studies.

The phase behavior of SEO/LiTFSI was determined using small angle X-ray scattering (SAXS). We studied the effect of salt concentration, molecular weight, copolymer composition, and temperature on the morphology of SEO/LiTFSI in sections four and five. In SEO copolymers with a majority polystyrene phase, the addition of salt induced the formation of ordered morphologies. However, unique phase behavior was seen in one copolymer, with a PEO volume fraction, f_EO, of 0.20. In this sample, a reentrant phase transition was found such that the disordered copolymer first formed an ordered morphology, followed by a disordered morphology, followed by a different ordered morphology with increasing salt concentration. The first ordered morphology seen at a low salt concentration was a novel type of coexistence wherein two distinct lattices of the same lattice type was observed. The nature of this type of coexistence was further probed with electron tomography to visualize the lattice structure and resonant soft X-ray scattering (RSoXS) to quantify the volume of salt in each lattice.

The goal of section five was to use the morphology data gathered from the SAXS experiments and assemble large experimental datasets of the phase behavior of SEO/LiTFSI. For simplicity, the phase diagrams were constructed at a single temperature of 100 °C. A simple framework based upon of mean-field theory developed for salt-free block copolymers was used to create a phase diagram plotting copolymer morphology as a function of χN and the volume fraction of the salt-containing phase, f_(EO,salt). It was found that the phase behavior of SEO/LiTFSI is qualitatively similar to that of salt-free block copolymer systems. In addition, the effect of copolymer composition and salt concentration was examined on the domain spacing of SEO mixed with two different Li salts. Expressions for the domain spacing as a function of copolymer chain length, composition and salt concentration were developed for both the weak (χN≤10) and strong segregation regimes (χN>10).

The following section focuses on the quantification of the thermodynamics of SEO/LiTFSI. Through application of Leibler’s Random Phase Approximation, the Flory-Huggins interaction parameter of the neat, χ_(0,SC), and salt-containing, χ_(eff,SC), SEO copolymers were measured. An expression for χ_(eff,SC) was developed as a function of N,f_EO and salt concentration. It was then used in conjunction with mean-field theory to calculate the critical chain length for ordering, N_crit, as a function of copolymer composition and salt concentration. Two regimes of phase behavior emerged: at f_EO>0.27, the addition of salt stabilizes the ordered phase and at f_EO<0.27, the addition of salt stabilizes the disordered phase. The copolymer composition wherein segregation is independent of salt concentration (f_EO=0.27) was termed the “isotaksis point” and simple theoretical model to predict this composition was developed.

The preceding sections focused on the effect of salt on the morphology of the block copolymer electrolytes, which corresponds to length-scales on the order of magnitude of the domain spacing of the microphases, ~10 nm. The effect of salt concentration on the polymer conformation of the corresponding homopolymer electrolyte, PEO/LiTFSI, was determined using small angle neutron scattering (SANS), which corresponds to length-scales less than 1 nm. Kratky analysis of the SANS profiles was used to determine the statistical segment length as a function of salt concentration. All parameters in the analysis were determined independently. It was found that the effect of salt concentration in non-monotonic: at low salt concentrations, the chain sizes decreases before reaching a minimum at intermediate salt concentrations and then increasing at higher salt concentrations. This relationship is hypothesized to be related to the preferred environment of Li ions within PEO where each Li ion prefers to be coordinated by six ether oxygens. As salt concentration increases, the ether oxygens become saturated with Li ions, which are hypothesized to lead to the formation of ion clusters, which could increase the chain dimensions. Additional SANS studies were used to probe the nature of the ion clusters and found that the clusters had a characteristic size of 0.6 nm.

The final section of this dissertation focuses on the effect of salt concentration on the polymer chain dynamics of SEO/LiTFSI. Neutron spin echo (NSE) spectroscopy was used to measure the segmental dynamics in the Rouse regime (t≤10 ns) and the polymer chain dynamics in the reptation regime (t≥50 ns). In both regimes, the dynamics slow down as salt concentration increases. This is quantified through an increase in monomer friction seen at low times and a decrease in tube diameter seen at longer times. The NSE results were used to calculate a normalized longest molecular relaxation time and it was found that block copolymers will flow more slowly with increasing salt concentration. This is important in a battery operating at high current wherein the diffusion of salt ions in one direction must induce diffusion of polymer chains in the opposite direction.

Materials with nanostructured conducting domains are essential for a wide range of applications related to alternative energy. Active materials in battery and fuel cell electrodes such as LiFePO4, graphite, and platinum, are either electronic or ionic insulators. Nanoscale electron- and ion-conducting domains are necessary for enabling redox reactions in these materials. For example, a traditional porous lithium battery electrode consists of a redox-active material, carbon black for electronic conduction, and non-conductive binder that holds the particles in place. The pores are backfilled filled with organic electrolyte for ionic conduction. In some cases such as LiFePO4, electronic and ionic conductivities are so low that the active materials must be in nanoparticle form, and addressing such particles requires the transport of both kinds of charges to occur on nanometer length scales. Materials such as block copolymers can self-assemble and form co-continuous nanoscale domains. In this study, poly(3-alkylthiophene)-block-poly(ethylene oxide) (P3AT-PEO) copolymers are used to conducts both electronic and ionic charges. P3AT-PEO block copolymer molecules self-assemble on the nanometer length scale to yield P3AT-domains that conduct electronic charges and PEO-domains that conduct ionic charges. We propose to create a unique battery electrode where the LiFePO4 active material is dispersed in a nanostructured P3AT-PEO block copolymer, which functions simultaneously as the conductor of lithium ions and electronic charge, as well as the binder material in the electrode.

The first phase of this dissertation work involved the synthesis of P3AT-PEO block copolymers. Regioregular P3ATs were synthesized using the Grignard metathesis (GRIM) polymerization method where in-situ end-group functionalization was employed to obtain ethynyl-functionalized P3ATs. Azide-functionalized PEOs were obtained through end-group modification of PEO monomethyl ether. Ethynyl-functionalized P3ATs and azide-functionalized P3AT-PEO were coupled using 1,3-dipolar cycloaddition "click" reaction to obtain P3AT-PEO block copolymer. In particular, poly(3-hexylthiophene)-block-poly(ethylene oxide) (P3HT-b-PEO) copolymers and a poly(3-ethylhexylthiophene)-block-poly(ethylene oxide) (P3EHT-b-PEO) copolymer were synthesized in this study.

Next, the morphology of the P3AT-b-PEO copolymer was characterized using small angle X-ray scattering (SAXS). The morphologies of P3HT-b-PEO copolymers, where the P3HT block is the major component, are dominated by nanofibrils due to the crystallization of P3HT. In contrast, the nearly symmetric P3HT-b-PEO copolymers self-assemble into a lamellar phase. In addition, we show that P3EHT-b-PEO chains self-assemble to produce traditional nanoscale morphologies such as lamellae and gyroid in the melt-state. The segregation strength between the two blocks is controlled through the addition of lithium bis(trifluromethanesulfonyl) imide (LiTFSI). Our approach enables estimation of the "effective" Flory-Huggins interaction parameter, $chi_eff, using the random phase approximation (RPA). The $chi_eff trends with salt concentration suggest that the TFSI anion preferentially segregates into the P3EHT phase while Li⁺ remains in the PEO phase. For the salt-free sample, the gyroid morphology, obtained in the melt-state, is transformed into lamellae when the P3EHT block is crystallized. This is due to the "breaking out" of the crystalline phase. At high salt concentrations, P3EHT-b-PEO has a lamellar morphology in both melt and crystalline states (confined crystallization).

We present the first reported study on the relationship between morphology and electronic/ionic charge transport of P3HT-b-PEO/LiTFSI mixtures. Using ac impedance spectroscopy, we show that P3HT-b-PEO/LiTFSI mixtures can conduct electronic and ionic charges simultaneously. At 90 °C, the electronic conductivity of P3HT-b-PEO/LiTFSI mixtures ranged from 10^-8 to 10^-5 S/cm depending on the volume fraction of P3HT. The decoupled ionic conductivity is around ~10^-4 S/cm. It was shown that LiTFSI partitions between P3HT and PEO microphases. In particular, LiTFSI only partitions between the microphases when the PEO block molecular weight is 2 kg/mol while we observe no partitioning when the PEO block molecular weight of 4.2 kg/mol. It thus appears that the chemical potential of LiTFSI in PEO is a function of the PEO block molecular weight. We propose that the higher chemical potential of LiTFSI for P3HT-b-PEO copolymers with PEO molecular weight of 2 kg/mol drives the LiTFSI into the P3HT rich microphase.

The electronic conductivity can be further increased by electrochemically chemically doping the P3HT chains with LiTFSI. Therefore, we quantified the electronic conductivity P3HT-b-PEO copolymers electrochemically oxidized with LiTFSI. We use a novel solid-state three-terminal electrochemical cell that enables simultaneous conductivity measurements and control over electrochemical doping of P3HT. At low oxidation levels, the electronic conductivity increases from 10^-8 S/cm to 10^-4 S/cm. At high oxidation levels, the electronic conductivity approaches 10^-2 S/cm. These values match or exceed the ionic conductivity, which is important for enabling redox reactions in a battery as they involve equal moles of lithium ions and electronic charges.

A lithium metal battery was assembled where the positive electrode consisted of P3HT-b-PEO conductive binder and LiFePO₄ active material. We were able to cycle batteries and obtain capacities approaching the theoretical limit of LiFePO₄. Importantly, P3HT is electroactive within the voltage window of a charge/discharge cycle. The electronic conductivity of the P3HT-b-PEO copolymer binder is in the 10^-4 to 10^-2 S/cm range over most of the potential window of the charge/discharge cycle. This allows for efficient electronic conduction needed for the successful cycling of the batteries. However, at the end of the discharge cycle, the electronic conductivity decreases sharply to 10^-7 S/cm, which means the "conductive" binder is now electronically insulating. The ability of our conductive binder to switch between electronically conducting and insulating states in the positive electrode provides an unprecedented route for automatic overdischarge protection in batteries.

Smaller, lighter, more powerful batteries will help drive the next generation of mobile technologies with energy from renewable sources. Battery chemistries incorporating a lithium metal anode offer high theoretical specific energies and energy densities, but no current electrolyte has yet been shown to be adequate, despite considerable effort.

Lithium metal electrodes suffer from irreversibility during battery cycling which diminishes their utility. First, lithium metal is highly reactive and tends to consume electrolyte with irreversible chemical reactions at electrode/electrolyte interfaces. Second, lithium metal tends to plate and strip in a nonplanar fashion, resulting in lithium protrusions that may span the electrolyte and short-circuit the cell. This is the so-called “dendrite problem”, named for the branched, filamentous lithium structures first observed on lithium metal after cycling in liquid electrolytes. Theoretical models suggest that planar deposition of lithium can be encouraged by using a rigid electrolyte, wherein compressive forces on the electrode discourage the growth of protrusions. However, the non-negligible displacement of the electrode/electrolyte interface as the lithium electrode is charged and discharged places additional constraints on the electrolyte: if the electrolyte cannot maintain good electrochemical contact with the electrode, it is unsuitable. Thus, it is desirable for an electrolyte to have some solid-like properties (e.g. resistance to lithium protrusion growth) and some liquid-like properties (e.g. able to accommodate of volume change). The viability of next generation lithium and beyond-lithium battery technologies hinges on the development of electrolytes with improved performance.

A suitable electrolyte must promote reversible lithium metal electrodeposition and stripping, and also conduct ions at a sufficiently high rate. Nanostructured electrolytes have shown promise for this purpose: such materials contain conducting domains for ion transport and rigid domains to promote stable lithium deposition. This dissertation focuses on model nanostructured block copolymer electrolytes comprising polystyrene-block-poly(ethylene oxide) (PS-b-PEO, or SEO) copolymers mixed with a lithium salt. By changing total chain length and the ratio of PS to PEO, one can obtain electrolytes with a variety of self-assembled morphologies and a range of mechanical properties. The salt is preferentially located in the PEO-rich lamellae. We focus on those electrolytes that self-assemble into lamellar morphologies, with alternating layers of ion-conducting PEO and mechanically reinforcing PS. Such composite materials have ion transport properties that are not easily predicted from the ion transport properties of the conducing domain only.

The design of new solid electrolyte materials is currently limited by incomplete fundamental understanding of the relationship between the initial properties of the electrode and electrolyte (e.g. mechanical, ion transport), and the resulting morphology of electrodeposited lithium after cycling. To overcome this challenge, this dissertation includes studies of both fundamental ion transport and lithium morphology in the same electrolyte. The morphology of electrodeposited lithium is affected by many factors, and lithium electrodeposition through different electrolytes has largely been studied qualitatively. We study lithium deposition by analyzing three-dimensional (3D) position space images of lithium deposition using synchrotron hard X-ray tomography. 3D renderings provide an opportunity to quantitatively correlate various factors such as experimental current density, interelectrode distance, and electrode shape and understand how lithium protrusion morphology evolves over time under conditions of plating and stripping. Ion transport through an electrolyte and lithium electrodeposition are inextricable. We also study lithium ion transport through a series of lamellar SEO electrolytes.

Chapter 1 provides an introduction to topics of interest to this dissertation, including lithium metal batteries and associated challenges, electrolytes for lithium batteries, nonplanar electrodeposition of lithium metal, and information about the imaging technique X-ray tomography. Current density, properly normalized, is perhaps the key variable in the electrodeposition of lithium. Chapter 2 is the first attempt at quantifying the effect of current density on the geometry and density of dendrites and other protrusions during electrodeposition through a solid polymer electrolyte. Lithium protrusions in cells with SEO electrolytes have been observe to nucleate on impurity particles in the existing lithium metal electrode. In Chapter 3, an “electrochemical filtering” treatment was performed on lithium symmetric cells in order to reduce the concentration of impurity particles near the electrode-electrolyte interface, and cells were cycled to determine the effects of the treatment on lifetime. In Chapter 4 we demonstrate that globules are preferentially stripped compared to a planar electrode in our system, which incorporates a nanostructured block copolymer electrolyte. First, we show and quantify the healing of lithium metal anodes during stripping; second, we also view protrusions from a moving reference frame attached to the electrode/electrolyte interface. Chapter 5 aims to quantitatively analyze the plating and stripping of a lithium protrusion in a lithium metal symmetric cell over a series of time steps based on X-ray tomography imaging. Current density at the positive electrode, current density at the negative electrode, and interelectrode distance were quantified and mapped as a function of position. Correlation functions are calculated to reveal the relationships between these three microscopic measured parameters. These results shed light on the interactions between parallel electrodes separated by a battery-relevant distance. Chapter 6 deals with ion transport in a series of twelve SEO block copolymer electrolytes, all with nominally lamellar morphology. These electrolytes include those through which lithium electrodeposition was studied in Chapters 2, 3, 4, and 5. Ionic conductivity, salt diffusion coefficient, current fraction, interfacial impedance, and limiting current density were measured in these electrolytes and compared to those of homopolymer PEO. Scanning electron microscopy was used to characterize the self-assembled morphology of these electrolytes at different salt concentrations. We compare measured limiting current density in the twelve nanostructured electrolytes with predictions based on the effective-medium theory of Sax and Ottino and Newman’s concentrated solution theory.

To make progress towards commercially viable Li metal batteries, a deep, fundamental understanding of lithium deposition that simultaneously considers chemical, mechanical, and electrochemical factors will be crucial. The objective of this work is to increase our understanding of these interactions in order to inform models and materials design to enable next-generation electrochemical energy storage.