High Transference Number Polymer-Based Electrolytes for Lithium Batteries

The composition of modern electrolytes is key to the performance of lithium ion batteries. State-of-the-art electrolytes are based on lithium hexafluorophosphate (LiPF6) dissolved in a liquid carbonate solvent with stabilizing additives, which provide a sufficient combination of conductivity and stability towards the highly reactive electrode components. This electrolyte composition has been developed, and continues to evolve, to meet materials design and engineering requirements for high-performance energy storage, but work remains to enable the next generation of high energy density, fast charging batteries. While there are still challenges in electrode formulations and cell management, this dissertation focuses on an important remaining problem involving the electrolyte: concentration polarization as a result of the low lithium transference number (t+) of the electrolyte. t+ characterizes the relative motion of cations to anions within an electric field and is unity for an electrolyte where only lithium ions are mobile, and zero for the opposite case where only anions migrate. The standard liquid electrolyte discussed above has a transference number below 0.5, indicating the bulky anions move faster than lithium ions as a result of the large solvation shell of lithium ions. This high anion motion allows concentration gradients to form within a cell, limiting energy density and charge rates. In this dissertation, polymers are utilized in an effort to create higher transference number electrolytes by attaching the anion to the polymer backbone. This method has suffered from the key drawback of low conductivity for many years, and thus a primary concern of each section herein is improving electrolyte conductivity.To study this class of electrolytes, initially a new polymer was synthesized based on polysulfone (PSF) condensation chemistry. This polymer allows incorporation of ion conducting poly(ethylene glycol) (PEG) segments, and ion containing sulfonate groups. This synthesis was an extension of existing sulfonated polysulfone and polysulfone-co-poly(ethylene glycol) polymers, but had never been combined into a single polymer before. This polymer, though not an ideal homogenous, low dispersity polymer, allows a wide range of compositions to be formed that could then be used in a variety of electrolytes.In the first section, the wide accessible composition window of sulfonated PSF-co-PEG is employed to study the fundamentals of ion conduction in dry polymer electrolytes that have appended ions. Conductivity as a function of both PEG and sulfonate content is studied, demonstrating a tradeoff between ion content and segmental motion of the polymer backbone. This tradeoff has been observed in the past and typically in the literature is analyzed through the Vogel-Tammann-Fulcher (VTF) equation, a modified Arrhenius equation originally developed for polymer viscosity but also applied to conductivity. Here it is shown that careful fitting of this equation to conductivity data is crucial to interpret the results, and that a correlation may exist between the equation prefactor and activation energy. These parameters are usually fit to decouple the effects of ion content, related to the prefactor, and segmental motion, related to the activation energy. This correlation was found to exist in other polymer systems and implies that this equation does not necessarily decouple these effects, complicating any analysis based on it. Further, this correlation implies that decreasing the activation energy will also decrease the prefactor, significantly limiting potential design changes to improve conductivity. Blending of a short chain PEG to the dry polymer system is found to break the correlation, further motivating this common conductivity enhancing technique. Following this discovery, this dissertation transitions to liquid state polyelectrolyte solutions utilizing the same sulfonated PSF-co-PEG polymer. Here the polymers are dissolved in a solvent such that lithium motion may be completely disconnected from polymer segmental motion. These polymer solutions were only recently suggested for battery application, with most prior polyelectrolyte work confined to water. The work here represents the first efforts to transition polyelectrolyte solutions into battery-relevant carbonate solvents. Comparison is first made between a highly polar solvent, dimethylsulfoxide (DMSO) and a carbonate blend solvent. It is shown through NMR characterization of peak width and diffusion measurements that the lithium does not dissociate from the sulfonate group in the carbonate blend solvent even though the polymer is fully dissolved and the dielectric constant of the carbonate blend is the same as DMSO. This demonstrates that new theories which do not solely utilize the dielectric constant to dictate ion interactions in solution will be necessary to predict polyelectrolyte behavior in these nonaqueous solvents. A further challenge in polyelectrolyte solution design for battery applications is that the vast majority of polyelectrolyte literature focuses mainly on the behavior of the polymer, particularly from a structure perspective. Design of an electrolyte must primarily take transport into account, and for a battery the primary interest is in fact the counterion transport. Existing theory must therefore be understood in a new light to inform rational design of future electrolytes. Here, a fundamental study of transport in polyelectrolyte solutions with multiple different molecular weight polymers and as a function of solvent quality is undertaken. Fully sulfonated polysulfone without PEG is employed here because it is soluble in both DMSO and water, where DMSO represents a good solvent for the backbone and ions, while water is only a good solvent for the ions. It is demonstrated that many of the fundamental theories of polyelectrolyte solutions hold for this previously unstudied system, despite the relatively short chains. By comparing the diffusion of counterions and solvent with the same data for solutions of the monomer alone, the effect of the polymeric anion can be determined. It is found that the presence of a good solvent for the backbone causes an additional slowing of the solvent and lithium in DMSO, as opposed to water. This is despite much higher viscosity in the water systems. From this, several recommendations for polyelectrolyte solution design are made. Taking inspiration from the current state of the art electrolyte, the final work contained herein discusses the use of additives to improve ion dissociation and conductivity in the carbonate blend solvent used previously. It is shown that crown ethers, and particularly 15-crown-5, are capable of achieving an order of magnitude increase in solution conductivity with the sulfonated PSF-co-PEG previously employed. This conductivity is shown to be sufficient to fabricate a full battery with commercial lithium iron phosphate and graphite electrodes. With the optimized electrolyte, nearly 90% of the theoretical capacity is achieved, three times as high as without additives, demonstrating the potential of these new electrolytes

Ion Transport and the True Transference Number in Nonaqueous Polyelectrolyte Solutions for Lithium Ion Batteries.

Nonaqueous polyelectrolyte solutions have been recently proposed as high Li+ transference number electrolytes for lithium ion batteries. However, the atomistic phenomena governing ion diffusion and migration in polyelectrolytes are poorly understood, particularly in nonaqueous solvents. Here, the structural and transport properties of a model polyelectrolyte solution, poly(allyl glycidyl ether-lithium sulfonate) in dimethyl sulfoxide, are studied using all-atom molecular dynamics simulations. We find that the static structural analysis of Li+ ion pairing is insufficient to fully explain the overall conductivity trend, necessitating a dynamic analysis of the diffusion mechanism, in which we observe a shift from largely vehicular transport to more structural diffusion as the Li+ concentration increases. Furthermore, we demonstrate that despite the significantly higher diffusion coefficient of the lithium ion, the negatively charged polyion is responsible for the majority of the solution conductivity at all concentrations, corresponding to Li+ transference numbers much lower than previously estimated experimentally. We quantify the ion-ion correlations unique to polyelectrolyte systems that are responsible for this surprising behavior. These results highlight the need to reconsider the approximations typically made for transport in polyelectrolyte solutions

High Transference Number Polymer-Based Electrolytes for Lithium Batteries

The composition of modern electrolytes is key to the performance of lithium ion batteries. State-of-the-art electrolytes are based on lithium hexafluorophosphate (LiPF6) dissolved in a liquid carbonate solvent with stabilizing additives, which provide a sufficient combination of conductivity and stability towards the highly reactive electrode components. This electrolyte composition has been developed, and continues to evolve, to meet materials design and engineering requirements for high-performance energy storage, but work remains to enable the next generation of high energy density, fast charging batteries. While there are still challenges in electrode formulations and cell management, this dissertation focuses on an important remaining problem involving the electrolyte: concentration polarization as a result of the low lithium transference number (t+) of the electrolyte. t+ characterizes the relative motion of cations to anions within an electric field and is unity for an electrolyte where only lithium ions are mobile, and zero for the opposite case where only anions migrate. The standard liquid electrolyte discussed above has a transference number below 0.5, indicating the bulky anions move faster than lithium ions as a result of the large solvation shell of lithium ions. This high anion motion allows concentration gradients to form within a cell, limiting energy density and charge rates. In this dissertation, polymers are utilized in an effort to create higher transference number electrolytes by attaching the anion to the polymer backbone. This method has suffered from the key drawback of low conductivity for many years, and thus a primary concern of each section herein is improving electrolyte conductivity.To study this class of electrolytes, initially a new polymer was synthesized based on polysulfone (PSF) condensation chemistry. This polymer allows incorporation of ion conducting poly(ethylene glycol) (PEG) segments, and ion containing sulfonate groups. This synthesis was an extension of existing sulfonated polysulfone and polysulfone-co-poly(ethylene glycol) polymers, but had never been combined into a single polymer before. This polymer, though not an ideal homogenous, low dispersity polymer, allows a wide range of compositions to be formed that could then be used in a variety of electrolytes.In the first section, the wide accessible composition window of sulfonated PSF-co-PEG is employed to study the fundamentals of ion conduction in dry polymer electrolytes that have appended ions. Conductivity as a function of both PEG and sulfonate content is studied, demonstrating a tradeoff between ion content and segmental motion of the polymer backbone. This tradeoff has been observed in the past and typically in the literature is analyzed through the Vogel-Tammann-Fulcher (VTF) equation, a modified Arrhenius equation originally developed for polymer viscosity but also applied to conductivity. Here it is shown that careful fitting of this equation to conductivity data is crucial to interpret the results, and that a correlation may exist between the equation prefactor and activation energy. These parameters are usually fit to decouple the effects of ion content, related to the prefactor, and segmental motion, related to the activation energy. This correlation was found to exist in other polymer systems and implies that this equation does not necessarily decouple these effects, complicating any analysis based on it. Further, this correlation implies that decreasing the activation energy will also decrease the prefactor, significantly limiting potential design changes to improve conductivity. Blending of a short chain PEG to the dry polymer system is found to break the correlation, further motivating this common conductivity enhancing technique. Following this discovery, this dissertation transitions to liquid state polyelectrolyte solutions utilizing the same sulfonated PSF-co-PEG polymer. Here the polymers are dissolved in a solvent such that lithium motion may be completely disconnected from polymer segmental motion. These polymer solutions were only recently suggested for battery application, with most prior polyelectrolyte work confined to water. The work here represents the first efforts to transition polyelectrolyte solutions into battery-relevant carbonate solvents. Comparison is first made between a highly polar solvent, dimethylsulfoxide (DMSO) and a carbonate blend solvent. It is shown through NMR characterization of peak width and diffusion measurements that the lithium does not dissociate from the sulfonate group in the carbonate blend solvent even though the polymer is fully dissolved and the dielectric constant of the carbonate blend is the same as DMSO. This demonstrates that new theories which do not solely utilize the dielectric constant to dictate ion interactions in solution will be necessary to predict polyelectrolyte behavior in these nonaqueous solvents. A further challenge in polyelectrolyte solution design for battery applications is that the vast majority of polyelectrolyte literature focuses mainly on the behavior of the polymer, particularly from a structure perspective. Design of an electrolyte must primarily take transport into account, and for a battery the primary interest is in fact the counterion transport. Existing theory must therefore be understood in a new light to inform rational design of future electrolytes. Here, a fundamental study of transport in polyelectrolyte solutions with multiple different molecular weight polymers and as a function of solvent quality is undertaken. Fully sulfonated polysulfone without PEG is employed here because it is soluble in both DMSO and water, where DMSO represents a good solvent for the backbone and ions, while water is only a good solvent for the ions. It is demonstrated that many of the fundamental theories of polyelectrolyte solutions hold for this previously unstudied system, despite the relatively short chains. By comparing the diffusion of counterions and solvent with the same data for solutions of the monomer alone, the effect of the polymeric anion can be determined. It is found that the presence of a good solvent for the backbone causes an additional slowing of the solvent and lithium in DMSO, as opposed to water. This is despite much higher viscosity in the water systems. From this, several recommendations for polyelectrolyte solution design are made. Taking inspiration from the current state of the art electrolyte, the final work contained herein discusses the use of additives to improve ion dissociation and conductivity in the carbonate blend solvent used previously. It is shown that crown ethers, and particularly 15-crown-5, are capable of achieving an order of magnitude increase in solution conductivity with the sulfonated PSF-co-PEG previously employed. This conductivity is shown to be sufficient to fabricate a full battery with commercial lithium iron phosphate and graphite electrodes. With the optimized electrolyte, nearly 90% of the theoretical capacity is achieved, three times as high as without additives, demonstrating the potential of these new electrolytes

The Compensation Effect in the Vogel–Tammann–Fulcher (VTF) Equation for Polymer-Based Electrolytes

Single-ion conducting polymer electrolytes have been proposed to significantly enhance lithium ion battery performance by eliminating concentration gradients within the cell. Such electrolytes have universally suffered from poor conductivity at low to moderate temperatures. In an attempt to improve conductivity, numerous studies have sought to better understand the fundamental interplay of ion content and segmental motion, with typical analyses relying on a fit of temperature-dependent conductivity data using the Vogel–Tammann–Fulcher (VTF) equation to assist in separating these effects. In this study, we leverage the large accessible composition window of a newly synthesized, single ion conducting polysulfone–poly­(ethylene glycol) (PSf-<i>co</i>-PEG) miscible random copolymer to more completely understand the interrelationship of glass transition temperature, ion content, and the polymer’s Li⁺ conductivity. It is demonstrated here that choice of fitting procedure and Vogel temperature plays a crucial role in the observed trends, and importantly, after optimization of the data fitting procedure, a strong positive correlation was observed between the VTF equation prefactor and apparent activation energy for polymers in this electrolyte class. This relationship, known as the compensation effect (among other names) for the related Arrhenius-type behavior of activated processes such as chemical kinetics and diffusion, is shown here to exist in several other polymer electrolyte classes. Given conductivity’s inverse exponential dependence on the apparent activation energy, maximum conductivity within an electrolyte class is achieved in samples where the activation energy is small. For a system in which the compensation effect exists, decreasing activation energy also decreases the prefactor, highlighting the limiting nature of the compensation effect and the importance of escaping from it. Blending of small molecules is shown to break the apparent trend within the PSf-<i>co</i>-PEG system, suggesting a clear route to high transference number, high conductivity electrolytes

Promising Routes to a High Li⁺ Transference Number Electrolyte for Lithium Ion Batteries

The continued search for routes to improve the power and energy density of lithium ion batteries for electric vehicles and consumer electronics has resulted in significant innovation in all cell components, particularly in electrode materials design. In this Review, we highlight an often less noted route to improving energy density: increasing the Li⁺ transference number of the electrolyte. Turning to Newman’s original lithium ion battery models, we demonstrate that electrolytes with modestly higher Li⁺ transference numbers compared to traditional carbonate-based liquid electrolytes would allow higher power densities and enable faster charging (e.g., >2C), even if their conductivity was substantially lower than that of conventional electrolytes. Most current research in high transference number electrolytes (HTNEs) focuses on ceramic electrolytes, polymer electrolytes, and ionomer membranes filled with nonaqueous solvents. We highlight a number of the challenges limiting current HTNE systems and suggest additional work on promising new HTNE systems, such as “solvent-in-salt” electrolytes, perfluorinated solvent electrolytes, nonaqueous polyelectrolyte solutions, and solutions containing anion-decorated nanoparticles

Electrochemically mediated carbon dioxide separation with quinone chemistry in salt-concentrated aqueous media

© 2020, The Author(s). Carbon capture is essential for mitigating carbon dioxide emissions. Compared to conventional chemical scrubbing, electrochemically mediated carbon capture utilizing redox-active sorbents such as quinones is emerging as a more versatile and economical alternative. However, the practicality of such systems is hindered by the requirement of toxic, flammable organic electrolytes or often costly ionic liquids. Herein, we demonstrate that rationally designed aqueous electrolytes with high salt concentration can effectively resolve the incompatibility between aqueous environments and quinone electrochemistry for carbon capture, eliminating the safety, toxicity, and at least partially the cost concerns in previous studies. Salt-concentrated aqueous media also offer distinct advantages including extended electrochemical window, high carbon dioxide activity, significantly reduced evaporative loss and material dissolution, and importantly, greatly suppressed competing reactions including under simulated flue gas. Correspondingly, we achieve continuous carbon capture-release operations with outstanding capacity, stability, efficiency and electrokinetics, advancing electrochemical carbon separation further towards practical applications

Giant secondary grain growth in Cu films on sapphire

Single crystal metal films on insulating substrates are attractive for microelectronics and other applications, but they are difficult to achieve on macroscopic length scales. The conventional approach to obtaining such films is epitaxial growth at high temperature using slow deposition in ultrahigh vacuum conditions. Here we describe a different approach that is both simpler to implement and produces superior results: sputter deposition at modest temperatures followed by annealing to induce secondary grain growth. We show that polycrystalline as-deposited Cu on α-Al2O3(0001) can be transformed into Cu(111) with centimeter-sized grains. Employing optical microscopy, x-ray diffraction, and electron backscatter diffraction to characterize the films before and after annealing, we find a particular as-deposited grain structure that promotes the growth of giant grains upon annealing. To demonstrate one potential application of such films, we grow graphene by chemical vapor deposition on wafers of annealed Cu and obtain epitaxial graphene grains of 0.2 mm diameter

Electrochemical and Molecular Assessment of Quinones as CO2-Binding Redox Molecules for Carbon Capture

The complexation and decomplexation of CO2 with a series of quinones of different basicity during electrochemical cycling in dimethylformamide solutions were studied systematically by cyclic voltammetry. In the absence of CO2, all quinones exhibited two well-separated reduction waves. For weakly complexing quinones, a positive shift in the second reduction wave was observed in the presence of CO2, corresponding to the dianion quinone-CO2 complex formation. The peak position and peak height of the first re-duction wave were unchanged, indicating no formation of complexes between the semiquinones and CO2. The relative heights of both reduction waves remained constant. In the case of strongly complexing quinones, the second reduction wave disappeared while the peak height of the first reduction wave approximately doubled, indicating that the two electrons transferred simultaneously at this potential. The observed voltammograms were rationalized through several equilibrium arguments. Both weakly and strongly complexing quinones underwent either stepwise or concerted mechanisms of oxidation and CO2 dissociation depending on the sweep rate in the cyclic voltammetric experiments. Relative to stepwise oxidation, the concerted process requires a more positive electrode potential to remove the electron from the carbonate complexes to release CO2 and regenerate the quinone. For weakly complexing quinones, the stepwise process corresponds to oxidation of the uncomplexed dianion and accompanying equilibrium shift, while for strongly complexing quinones the stepwise process would correspond to the oxidation of mono(carbonate) dianion to the complexed semiquinone and accompanying equilibrium shift. This study provides a mechanistic interpretation of the interactions that lead to the formation of quinone-CO2 complexes required for the potential development of an energy efficient electrochemical separation process and discusses important considerations for practical implementation of CO2 capture in the presence of oxygen with lower vapor pressure solvents

Enhanced Forward Osmosis Desalination with a Hybrid Ionic Liquid/Hydrogel Thermoresponsive Draw Agent System.

Forward osmosis (FO) has emerged as a new technology for desalination and exhibits potentials for applications where reverse osmosis is incapable or uneconomical for treating streams with high salinity or fouling propensity. However, most of current draw agents in FO are salts and difficult to be recycled cost- and energy-effectively. In this work, we demonstrate a new and facile approach to efficiently recover water from the FO process with enhanced water purity by using a binary ion liquid/hydrogel system. The hybrid ion liquid/hydrogel draw solution system demonstrated in this work synergistically leverages the thermoresponsive properties of both the ionic liquid (IL) and hydrogel to improve the overall FO performance. Our findings corroborate that the hydrogel mitigates the water flux decline of the IL as the draw agent and provide a ready route to contiguously and effectively regenerate water from the FO process. Such a route allows for an efficient recovery of water from the draw solute/water mixture with enhanced water purity, compared with conventional thermal treating of lower critical solution temperature IL draw solute/water. Furthermore, hydrogels can be used in a continuous and readily recyclable process to recover water without heating the entire draw solute/water mixture. Our design principles open the door to use low-grade/waste heat or solar energy to regenerate draw agents and potentially reduce energy in the FO process considerably