Characterizing Ion Correlation and Transport in Next-Generation Lithium Battery Electrolytes

Li-ion batteries are ubiquitous in present society in applications ranging from portable electronics and wearable to electric vehicles to grid energy storage. Given the maturity of the technology, lithium-ion batteries are likely to play an increasingly larger role in renewable energy storage as the global economy is decarbonized. Despite their ubiquity, improvements in Li-ion battery performance are still sought, with enhanced charging rates, increased energy density, reduced safety risks all being essential areas of ongoing development. The electrolyte has a significant impact on all of these areas of performance, with the rate of ion transport through the electrolyte limiting the battery charge and discharge rate as well as overall battery efficiency. Electrolyte properties also have a significant impact on the stability of electrode interfaces and long term cyclability. Building fundamental understanding of structure-property-performance relationships in battery materials is essential to developing design heuristics that will allow rapid development across all battery components. In this work, I contribute to this task by developing an understanding of the role of electrolyte composition and structure on ion transport properties in three classes of next-generation lithium electrolytes./nIn this thesis, I develop and apply spectroscopic and electrochemical methodologies to rigorously characterize ion transport in concentrated liquid electrolyte systems. Using these measurements and the Onsager transport framework, I develop molecular level insight as to the origins of these bulk transport properties. In chapter 2 of this dissertation, I also examine the social implications of lithium-ion battery technology development. In chapter 3, I examine existing electrochemical methodology used for transport characterization. Development of Li+-containing electrolytes with improved transport properties requires reliable, reproducible, and ideally low volume techniques to rigorously understand ion-transport with varying composition. I apply a potentiostatic polarization-based transport characterization approach to liquid electrolyte systems in an attempt to fully measure all transport coefficients (conductivity, total salt diffusion coefficient, thermodynamic factor and transference number) for the model system of LiPF6 in an ethylene carbonate - ethyl methyl carbonate (EC:EMC) mixture. Using systematic timescale and statistical analyses, I find that transport coefficients measured using potentiostatic polarization of Li-Li symmetric cells exhibit strong correlation to Li electrode interfacial resistance, indicating that such methods are probing both bulk and interfacial phenomena. As a result, I find that these commonly-used methods do not readily result in reliable liquid electrolyte transport coefficients./nIn chapter 4, I demonstrate alternate transport characterization techniques that can overcome the reliability issues of lithium polarization techniques discussed in chapter 3, and apply these to non-aqueous polyelectrolyte solutions (PESs). PESs have been proposed as high conductivity, high lithium transference number (t+) electrolytes where the majority of the ionic current is carried by the electrochemically active Li-ion. While PESs are intuitively appealing because anchoring the anion to a polymer backbone selectively slows down anionic motion and therefore increases t+, increasing the anion charge will act as a competing effect, decreasing t+. I directly measure ion mobilities in a model non-aqueous polyelectrolyte solution using electrophoretic Nuclear Magnetic Resonance Spectroscopy (eNMR) to probe these competing effects. While previous studies that rely on ideal assumptions predict that PESs will have higher t+ than monomeric solutions, I demonstrate that below the entanglement limit, both conductivity and t+ decrease with increasing degree of polymerization. I find that distinct anion-anion correlation through the polymer backbone and cation-anion correlation through ion aggregates is responsible for this reduction in t+ in PESs, leading me to conclude that short-chained polyelectrolyte solutions are not viable high t+ electrolytes./nIn chapter 5, I use eNMR and electrochemical techniques to fully characterize transport in two classes of next-generation electrolytes: high concentration electrolytes (HCEs) and localized high concentration electrolytes (LHCEs) where a non-solvating diluent is added to HCEs. In both HCEs and LHCEs lithium salts are present at a similar mol fraction as solvents such that there is insufficient solvent to fully fill the Li+ primary solvation shell. The vastly different solvation environments in these systems is likely to result in different transport mechanisms than conventional concentrated electrolytes. I find that increasing viscosity alone cannot explain sharp drops in conductivity and ion self-diffusion observed as salt concentration increases. I find that t+ increases with increasing salt concentration and that an exceptional transference number of 0.52 is achieved in a 1.1:1 Li:solvent HCE due to fast ligand exchange and positively correlated cation motion- phenomena indicative of a concerted hopping lithium conduction mechanism. On the other hand, I find that similar salt:solvent molar ratio LHCEs have significantly lower t+, mainly due to an increase in anti-correlated cation-cation motion, indicating that diluents are likely interrupting the cation-hopping mechanism./nThe results of this thesis point to the importance of understanding molecular level ion-ion interaction when designing electrolytes with improved transport properties. The methodology combining eNMR methods and insights from the Onsager transport framework described herein have broad applicability to the design and study of liquid and polymer electrolytes.

Advisor: McCloskey, Bryan