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Electrolytes for magnesium-air batteries
thesisposted on 06.02.2017, 03:19 by Khoo, Timothy
Our world is in peril! Gaia the spirit of the Earth can no long stand the terrible destruction plaguing our planet. Environmental concerns coupled with our increasingly power hungry society have prompted considerable research into improving energy storage density, enhancing the efficiency of devices, and decreasing the environmental impact of our energy system. One technology with the potential to address these concerns is advanced metal–air batteries. Metal-air cells offer great possibilities for inexpensive, reliable energy storage with higher energy densities than other types of battery chemistries and have the potential to significantly reduce the pollution produced by our energy system, while also improving the performance of many of the products desired by consumers. In particular, the magnesium–air battery uses a low cost, light and abundant anode material that is also environmentally benign and relatively easy to handle. Electrochemically, magnesium has a high theoretical specific charge capacity (2205 A h/kg) and high theoretical energy density (3.8 A h/cm3), making it an excellent candidate as a metal battery anode. However, current technology has not yet developed a commercially viable battery, due to high overpotentials, related to the chemistry of the battery electrolytes, and low efficiency and shelf life, due to the high rates of corrosion of the magnesium. anode Understanding the interfacial properties that lead to improvements in stability and efficiency at the magnesium anode is likely to lead to advancements in this system. If improvements in stability and efficiency were adequately addressed, primary Mg/air cells could find uses as energy sources in important applications ranging from biocompatible devices, communications, and vehicle propulsion. Initially in this research, the use of aqueous electrolyte systems was explored, and several electrolytes were found to be largely unsuitable for a magnesium battery. Problems associated with these electrolytes include the fast build up of a thick, insulating salt film on the magnesium surface, or the very aggressive attack of the Magnesium anode from some of the more corrosive solutions. However, it was shown that variation of electrolyte composition allowed for the control of the morphology of the interface, leading to improved electrical efficiency of the battery. Using a unique electrochemical cell that allowed the magnesium to be closely monitored optically, a system containing a 12 M LiCl pH11 electrolyte was demonstrated to achieve a potential of -1.5 V vs Ag|AgACl at a discharge current of 4 mA/cm2 with an efficiency of 75%. Combining this electrolyte with 5.5 M MgCl2 pH11 improved the potential to -1.9 V vs Ag|AgCl, although there was a great reduction in efficiency, due to the increased hydration level of the solution increasing the localised corrosion of magnesium. Another approach was to utilise electrolytes based on ionic liquids, due to their non-volatility. Some ionic liquids have been shown to form passive films on magnesium surfaces. While this response, in an extreme case, would lead to high cell resistivity and hence impractical devices, controlling the interface through judicious choice of the IL anion or cation or both, can lead to desirable interfacial properties that result in high stability, low leakage currents and still have good cell performance under load. Therefore, this work also explored the interface that forms on magnesium in a series of ionic liquid mixtures when the Mg anode was polarised alvanostatically. Half-cell discharge experiments, EIS measurements and surface characterisation of the chemical nature of the magnesium surfaces were performed. The results showed that electrolytes based on the ionic liquid, trihexyl(tetradecyl)phosphonium chloride ([P6,6,6,14 ][Cl]), were capable of forming an amorphous gel-like interface on Mg that led to a level of passivation when the cell was at open circuit. It was appreciated that H2O played an important role in the formation of the protective film on the Mg surface and in the operation of the cell. A discharge rate of 1 mA/cm2 and cell voltage -1.6 V vs Ag|AgCl was achieved using an 8 wt% H2O solution in the [P6,6,6,14 ][Cl] IL. Impedance measurements also describe the film as ‘self-healing’ as the film was shown to break down during discharge, allowing for increased current densities, while recovering to a high resistance when the cell was stopped. The magnesium surface film was characterised and was found to be a highly hydrated complex gel based on the phosphonium cation, magnesium and chloride. FTIR also showed the film to contain magnesium hydroxide chloride species, which was also confirmed by XPS analysis. The film appeared to be in an amorphous gel-like state, which was also shown using XRD, and appearing to be considerably different from the salt-like state of the magnesium surface in aqueous solutions. A synthetic gel, based on the determined composition was prepared and demonstrated to sustain a 0.05 mA/cm2 galvanostatic discharge for at least 48 h. This supported conclusions regarding the conductive nature of the surface film. Since the surface film also showed IL cationic elements, it is clear that the nature of the ionic liquid species plays a vital role in the formation of the interfacial film. Water was required to form and sustain a stable gel, and the cell exhibited improved lifetime in a more humid environment. The increased water in the electrolyte [P6,6,6,14][Cl] was also shown to support the oxygen reduction reaction required for the cathodic process in a Mg–air battery and it was demonstrated that a magnesium–air cell using this electrolyte and a carbon air cathode provided a stable voltage of 1.6 V during a discharge current density of 50 microampere/cm2. The combined work in this thesis highlighted the importance of the magnesium/electrolyte interface on the performance of the battery. Chemical design of a beneficial interface was possible through manipulation of the electrolyte, in particular, by adding water to non-aqueous systems such as the [P6,6,6,14][Cl] ionic liquid. This represents a highly significant finding, especially since the role of water also improved the cathodic processes in the Mg– air battery. Clearly the nature of an ionic liquid electrolyte and its composition with respect to water will play an important role in developing a functional Mg/air battery.