Pure, Hybrid and Polymerized Ionic Liquid Based Electrolytes For High Temperature Lithium-Ion Battery Application

Today, lithium-ion batteries (LIBs) are ubiquitous in mobile phones, laptops, and other portable devices. Additionally, LIBs are becoming more and more popular for powering hybrid and electric vehicles. The research community strives to further improve the LIBs to increase electric driving distance and efficiency of both hybrid and fully electric vehicles. Conventional LIBs need to be strictly temperature controlled, most often cooled, to ca. 30°C, to ensure an acceptable and predictable life-time. Increasing the thermal stability and hence making possible operating temperatures of up to ca. 100°C would enable a merging of the cooling systems of the LIB and the power electronics – resulting in an overall reduced system complexity, saved mass, and a higher energy efficiency.<br /><br />All components of the LIB must be thermally stable to deliver the targeted performance and life-time. The electrolytes of conventional LIBs all contain organic solvents and lithium salts, the former flammable with high vapour pressures and the latter meta-stable at room temperature and unstable at temperatures above 60°C. Thus more stable solvents and salts are needed to improve the inherent safety of the electrolyte – especially if aiming at elevated operating temperature applications.<br /><br />In this thesis procedures to investigate electrolytes for viability in HT-LIBs are demonstrated by investigating novel high-temperature LIB electrolyte alternatives primarily in the form of pure, hybrid and polymerized ionic liquid based systems. For several of these, physico-chemical properties such as viscosity, thermal stability, flammability and electrochemical stability window have been assessed and correlated with molecular level interactions, and furthermore a detailed characterization of several commercial sources of an often used electrolyte Li-salt has been performed