Ionic Liquid Based Electrolytes for High-Temperature Lithium-Ion Batteries

Licentiate thesis, 2015

Today, lithium-ion batteries (LIBs) are ubiquitous in mobile phones, laptops, and other portable devices. The research community strives to further improve the LIB to increase electric driving distance and efficiency of both hybrid electric vehicles (HEVs) and fully electric vehicles (EVs). 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. All components of the LIB must be thermally stable to deliver the targeted performance and life-time. The electrolytes of conventional LIB 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 temperatures. In this thesis one possible alternative is investigated in the form of ionic liquid (IL) based electrolytes. ILs often exhibit properties advantageous for electrolytes: low vapour pressures, high thermal stabilities, low flammabilities, and high ionic conductivities. The physico-chemical properties of several IL based electrolytes have been assessed and furthermore a detailed characterization of several commercial sources of an often used electrolyte Li-salt has been performed.