Electrolyte Formulations for Challenging Conditions

The ever-increasing push towards intermittent, renewable resources including wind, solar, and hydrothermal sources over fossil fuels requires the electrification of the transportation sector. Key in this process are technologies to store energy so that it is readily available for use. Currently, Li-ion batteries (LiB) are state-of-the-art in terms of such large scale uses, making them one of the most important and ubiquitous lines of research in the 21st Century. The general architecture of a battery has remained largely unchanged since the introduction of the technology in 1990, utilizing intercalation electrode materials which allow for the migration of Li+ through an electrolyte to compensate for the movement of negatively charged electrons through the external circuit. Therefore, energy is stored in a battery via electrochemical reactions, which may be leveraged to do work. To increase the efficacy of LiB technology, a commonly targeted metric is the increase of energy density of the cell, which is often defined as the theoretical energy content in a battery gravimetrically (by mass) or volumetrically (by physical bulk). This can be leveraged by choosing new electrode materials, increasing the cutoff voltage to drive more active material, or by employing thicker electrodes. All three of these methods, while valid, stress the electrolyte in different ways. Often, since the electrodes are treated as the determinants of battery performance, there is comparatively much less research into tailoring an electrolyte formulation for a specific application. For instance, the conventional commercial electrolytes available today based on a LiPF6 salt dissolved in a mixture of cyclic and linear carbonates is designed for usage around 4 V vs Li+/Li. However, often researchers employ conventional electrolyte in cells exceeding 4.3 V, where many carbonates undergo mass oxidative decomposition. This effect can lead to the thermal failure of the cell by exothermic reaction and generate environmental hazards such as hydrofluoric acid that is created by co-decomposition between a carbonate and the PF6- anion. There is an increasingly vast library of high voltage electrode materials, there must be new electrolytes for those couples. This thesis focuses on examining several attractive battery setups that have various degrees of practicality at time of writing and conveys several lines of research into novel electrolytes for these systems. Solid State Batteries (SSBs) are a burgeoning topic of research. They employ solid ionic conductors to facilitate mass transport while maintaining high contact pressure with vulnerable electrodes. By means of controlling the synthesis of a solid electrolyte material, it may be possible endow said material with specific properties based on particle size, shape, and crystal phase. First described is the synthesis of an attractive solid ionic conductor, Li3xLa2/3-x▢1/3-xTiO3 (LLTO) by means of a solvothermal reaction. This method led to the unique outcome of producing the cubic polymorph at low temperature, where it is metastable. The study further shows reaction conditions may be altered to influence the physical and chemical properties of the particles. Based on this architecture, the foundation is laid to not only tailor these particles for direct usage in solid state batteries, but also provides a platform for investigating novel electrolyte materials by solvothermal means. The following two chapters investigate the importance of solvent, passivating agent, and non-solvating dilutant in the development of novel liquid electrolytes for electrochemical cells based on a LiNi0.8Mn0.1Co0.1O2 (NMC-811) cathode coupled to a graphite anode meant to operate to a high cutoff voltage and mass loading. In Chapter 4, dilutant molecules were explored for enabling oxidatively stable electrolyte formulations for high mass loading electrodes. Fluorobenzene was found to outperform fluorinated ether molecules due to their enhanced chemical stability and physical properties. In Chapter 5, the importance of solvent and co-solvent was explored between various oxidatively stable sulfone solvents paired with various fluorinated carbonate co-solvents. It was found that the choice of sulfone was determinant for the overall performance of the cells and exemplified the compatibility between ethyl methyl sulfone and fluorinated carbonates. Additionally, trifluoro-propylene carbonate was identified as a uniquely suited co-solvent in these systems, leading to lower resistance buildup and capacity loss. Therefore, it was revealed that both the choice of passivation agent and major solvent are not independent of one another and may be optimized for cell performance. Overall, the association of these dynamics suggested new avenues for enhancing cell stability at higher cutoff voltages and mass loadings, respectively, and may have implications for currently commercial LiB technologies.