Ionic liquid and coordination complex electrolytes for economically viable aluminum and magnesium metal batteries
Keywords: Chemistry.

This thesis describes, generally, the conception and development of the aluminum-graphite battery using chloroaluminate ionic liquid and ionic liquid analog electrolytes, with a transition to magnesium metal batteries also utilizing the tetrachloroaluminate anion in a magnesium aluminum chloride complex in ethereal solvent electrolyte system. Reversible aluminum deposition is generally achieved from liquid electrolytes that can support the Al2Cl7- anion, and was established in imidazolium-based ionic liquids in the 1982 by Wilkes. With no vapor pressure and large electrochemical stability windows, the concept of ionic liquids in general presents a scaffold for developing the ideal battery electrolyte, as they are non-flammable and have the potential to provide for highly efficient redox processes at both electrodes in the battery. Unfortunately, they tend to be rather expensive due to the use of synthetic organic cations. The majority of the work done here describes the development of ionic liquid analog electrolytes based on the asymmetric cleavage of Al2Cl6 by urea ($0.50/kg) to generate an ionic system. Furthermore, the invention and characterization of ionic liquid analogs of drastically decreased viscosity and increased ionic conductivity, using N-alkyl derivatives of urea, is presented. Additionally, more dense and even aluminum metal deposit morphologies were observed using ionic liquid analogs derived from N-alkylated derivatives of urea, making for significantly improved systems from which to deposit aluminum. Graphite has long been established as capable of forming intercalation compounds, particularly with the lithium cation upon chemical/electrochemical reduction, and is used as such for the anode of the common lithium-ion battery. An amphoteric material, graphite has become the subject of immense study for hosting both cationic and anionic species through reduction and oxidation, respectively, in the formation of lamellar intercalation compounds for use as the active material in battery electrodes. In this work, graphite is shown to highly reversibly intercalate/de-intercalate AlCl4- anions, which were derived from ionic liquids capable of reversible aluminum deposition, allowing for the development of a high-rate aluminum metal-graphite battery. Ionic liquid analog electrolytes based on urea and its N-alkylated derivatives were also incorporated successfully, with high efficiency redox reactions occurring at both electrodes, providing for an economically viable technology that (when estimated at scale) is cheaper than lead-acid batteries, which account for two-thirds of a 30 billion dollar battery industry, today. Magnesium has been shown to be able to be reversibly deposited with high efficiency from magnesium aluminum chloride complex (in ethereal solvent) electrolytes, which also contain AlCl4- anions. In theory, a battery similar in nature to the aluminum metal-graphite system should therefore be conceivable using a magnesium anode. However, despite the Lewis acid nature of the components of the electrolyte (AlCl3, MgCl2), it has been shown that in order to reversibly deposit magnesium, electrochemical conditioning of the electrolyte must take place, during which time Al3+ is removed from the electrolyte, which generates free Cl- that cannot be reversibly intercalated into the graphite used here, requiring another type of host. For this battery system, it is shown that Ag metal can act as a high capacity, highly reversible "host" for the Cl- ion when anodized to form AgCl, which can be used in conjunction with the magnesium metal anode to produce another highly efficient, dual-ion type cell. However, unlike in the case of the aluminum-based cell using ionic liquids, globular dendritic growth (not quite identical to the issues plaguing lithium metal based batteries) prevents a long cycle life, with short circuiting of the battery inevitable with the production of a high surface area magnesium deposit during recharging of the battery. Here we discuss in detail the optimization and characterization of these different electrolyte systems and the electrochemical mechanisms involved in the respective redox processes at the positive and negative electrodes during battery operation. Future avenues for improvement of these systems are discussed, and viability from an industry perspective is considered