UC Berkeley

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.