Microemulsion electrolytes for electrochemical energy storage devices

Abstract

The demand for electrical energy storage technology is growing at a rapid pace. The current market leader is the lithium ion battery which has found use in many applications from stationary, grid level storage to transportation and smaller applications like mobile phones. Supercapacitors are an up-and-coming alternative energy storage technology that compliments lithium ion batteries in many ways. Where lithium ion batteries have a high energy density, delivering energy for a long time, supercapacitors have a high power density which means they are capable of being charged and discharged within seconds. Both of these technologies rely on organic electrolytes that are toxic and flammable. Replacing these organic electrolytes with alternatives would go a long way to improving their sustainability and lessening the environmental impact. Using a water based electrolyte is an attractive option however water is limited by its electrochemical window. Above a voltage of 1.23 V it is thermodynamically favourable for water to be split into hydrogen and oxygen gasses. This voltage restriction limits the energy density of water based batteries and supercapacitors making them economically non-viable. The research in this thesis was investigating the use of microemulsions as an alternative class of electrolytes in ion batteries and supercapacitors. Microemulsions are a thermodynamically stable mixture of two immiscible phases. Despite being mostly made up of water, microemulsions can have a greatly extended electrochemical window, as much as 5 V. This means that they could have all of the benefits of aqueous and organic electrolytes in one solvent. Microemsulions have never been applied to supercapacitors or ion batteries before. They were found to retain their enhanced electrochemical stability and still allow for the devices to perform. In almost all aspects the devices presented within out-perform any other water based electrolyte in the literature and in many cases show comparable performance to organic electrolytes.