The Art of Cycling – Polymer Electrolytes at Extreme Conditions

Sammanfattning: With the rapid development of batteries for applications like electric vehicles and energy storage devices, it is essential to design and develop batteries with improved safety, long cycle life, and high energy density. To achieve this goal, the development and improvement of solid-state batteries, containing solid polymer electrolytes, is a promising solution. The interest in polymer electrolytes is primarily owed to their proposed compatibility with high temperatures and reactive electrodes, such as metallic lithium, and their ability to withstand higher temperatures than traditional liquid electrolytes. Cycling polymer electrolytes at high temperature and with high-voltage cathodes, such as lithium-nickel-manganese-cobalt-oxide (NMC) involves a combination of high chemical, electrochemical, and mechanical stability, as well as the understanding of how to achieve these properties.This thesis provides an overview of some challenges and possibilities of cycling batteries with polymer electrolytes at high temperatures and with high-voltage cathodes. With a focus on the stability of the polymer electrolyte, the effect of changing the polymer host material, the electrolyte salt, and the introduction of additives for enhanced mechanical stability or electrochemical stability, were all evaluated by both standard techniques and techniques developed for polymer electrolytes. Long-term cycling at high temperature was achieved for a poly(ε-caprolactone-co-trimethylene carbonate) (PCL-PTMC) electrolyte by crosslinking additives that increase the mechanical stability of the polymer electrolyte; however, the cycling with high-voltage cathodes also required a high electrochemical stability of the polymer electrolyte. With the techniques developed herein, such as cut-off increase cell cycling, the electrochemical stability of PCL-PTMC was evaluated. By introducing zwitterionic additives to PCL-PTMC, the cycling performance with NMC was enhanced and the enhancement proved to stem from prevention of electrolyte salt decomposition. Finally, by changing the electrolyte salt, it was found that cycling with NMC was possible for PCL-PTMC below its oxidative degradation potential, as long as the electrolyte had an ionic conductivity that was high enough. By utilizing additives, the long-term stability and electrochemical stability toward NMC was also improved. Overall, cycling solid polymer electrolytes at high temperatures and with high-voltage cathodes presents a unique set of challenges, which require that the electrochemical stability of the electrolyte is accurately described, and that the following properties are high: ionic conductivity, electrochemical and mechanical stability; all of which can be improved by utilizing additives in the polymer electrolyte.