INVESTIGATION OF LI-ION BATTERY ELECTROLYTE FOR HIGH VOLTAGE AND LOW TEMPERATURE CONDITIONS
Developing energy storage technologies play a key role in adoption of intermittent renewable energy sources and expansion of electromobility as a way of overcoming challenges related to climate change. To meet the demands of such diverse applications, Lithium-ion battery (LIB) technology, one of the most reliable battery technologies today, have to be able to work at a wide range of operating conditions. This dissertation attempts to understand the role and the failure mechanisms of the liquid electrolyte in a Lithium-ion battery when operated at high voltage and low temperature conditions.
LIB operated at high operating potentials suffer from rapid capacity fade and several components of the cells participate in this capacity fade, electrolyte being one of them. The electrolyte can undergo oxidative decomposition reactions on the cathode surface at high operating potentials (> 4.3 V vs Li/ Li+) because the cathode potential exceeds the electrochemical stability window of the electrolyte. The oxidative decomposition of the electrolyte has been reported to result in the generation of a complicated mixture of products, many of these species are acidic in nature. These acidic species along with transition metals from the cathode participate in crossover reactions that degrade the anode solid electrolyte interphase promoting capacity fade. In this dissertation chapter 2 and 3 focuses on exploring the correlation between the concentration of these crossover species in the electrolyte and capacity fade in spinel and layered cathode materials, respectively, cycled to high potentials.
Lithium ion battery cycling performance is also strongly dependent upon the operating temperatures. In carbonate-based electrolytes, cycling performance is drastically decreased at temperatures below zero degrees Celsius because of reduced lithium ion transport properties and high charge transfer polarization at the electrode- electrolyte interphase. By the introduction of novel solvents that can lower the viscosity thereby improve the Li ion conductivity in the bulk electrolyte and additives that are able to form a more favorable SEI that would reduce the charge transfer impedance, low temperature performance of lithium ion batteries can be improved. In chapter 4 ethylene carbonate free, ester-based electrolyte formulations are investigated for improved low temperature performance of NCM 622 and graphite cells. Lithium tetrafluoroborate (LiBF4), lithium difluoro(oxalato)borate (LiDFOB) lithium salts were explored as alternative lithium salts in this study.