|M.Sc Student||Luder Daniel Isaac|
|Subject||Room-Temperature Ionic Liquid (RTIL) based Magnesium-Air|
|Department||Department of Energy||Supervisor||Professor Yair Ein-Eli|
|Full Thesis text|
In the quest to develop new energy storage systems for power grids and transport, a promising solution lies in the development of a Magnesium-air battery, where the two reactants are magnesium and ambient oxygen. The cell has high theoretical energy density (6450 W*h/kg) due to the fact that the cathodic reactant in the cell, ambient oxygen, is not stored in the battery; Therefore, theoretical capacity and energy density are governed by the amount of magnesium alone. Past systems developed with an aqueous electrolyte were problematic for reasons including anode passivation, corrosion and hydrogen evolution.
In this work a new magnesium-air system is developed with an innovative electrolyte based on room-temperature ionic liquids (RTIL’s), a relatively new liquid class composed of 100% ions with special properties.
A suitable electrolyte was studied and analyzed for the prospective cell. The main suitable electrolytes were based on a mixture of an RTIL and ethers with dissolved Grignard reagents. Studies included electrochemical experiments with use of a potentiostat where currents and overpotentials for the anodic reaction were observed. In the course of research it was found that amongst various possibilities, a combination of the RTIL [1-Ethyl-2,3-dimethylimidazoluim bis(trifluoromethylsulfonyl)imide] and Tetrahydrofuran with dissolved Grignard reagent (EtMgBr) was more advantageous in terms of the electrochemical window and the ability to reversibly dissolve and deposit magnesium.
In a separate effort, an electrolyte was created in which the Grignard reagent was synthesized in-situ electrochemically inside the RTIL-ether solvent instead of being added externally.
Once a suitable electrolyte was found, a magnesium-air cell was assembled with a magnesium anode and a carbon based air-cathode. Cell discharge experiments were conducted while investigating the dependence of cell capacity on various variables in the system. Cell capacity was calculated at 400-600 [mA*h] per gram of active carbon. Additionally, the mechanism failure of the cell was investigated and was found mainly to be a result of air-cathode deactivation occurring due to discharge-product accumulation on the air cathode pores. Electrolyte deterioration also plays a part in leading to cell failure. Increasing the stability of the magnesium active species led to an increase in cell capacity.
Cell charging does not occur in the investigated cell type due to the inability to decompose discharge products and evolve oxygen at the air cathode.