Investigations of Ionic Liquid Electrolytes for Li-Air Batteries

Ionic liquids are molten salts at room temperature that possess many unique properties1. There has been continued increase of interest in ionic liquids because their properties make them attractive for many applications including electrocatalysts, electrodeposition, biosensors, and electrolytes for ionic and electronic devices, including batteries and capacitors 2. Their low volatility, high ionic conductivity, and large electrochemical window make them ideal candidates for next generation electrolytes for primary and secondary lithium batteries. The lithium-O2 battery, with a theoretical specific energy of 5200 Wh/Kg3, ten times as high as Li-ion technology, faces many challenges in development before it can become a practically viable power source. The role of electrolyte is crucial in this system and in particular it is necessary to understand reversible O2 electrochemistry in the electrolyte. Therefore, we have been investigating a series of advanced non-aqueous electrolytes for the Li-O2 battery in our Laboratory at Northeastern University Center for Renewable Energy Technology. Our recent studies on the influence of non-aqueous solvents on the electrochemistry of oxygen in the rechargeable lithium-air battery showed that the solvent and the supporting electrolyte cations in the electrolyte act in concert to influence the nature of reduction products and the rechargeability 4.;In the first part of this thesis, Nuclear Magnetic Resonance (NMR) spectroscopy was employed to determine the relative Lewis acidity of ionic salt cations including room temperature ionic liquids, and the Lewis basicity of organic solvents. The Varian 400MHz and 500MHz NMR instrument at Northeastern was used for conducting 13C NMR and 7Li NMR chemical shift studies respectively. The 13C NMR provided a linear plot of chemical shift (ppm) versus the Lewis acidity of cations of the selected salts including ionic liquids dissolved in the non-aqueous organic electrolyte, propylene carbonate. From this a Lewis acidity scale for Li salt cations and two ionic liquids has been developed. The 7Li NMR study resulted in no correlation between the chemical shifts and Gutmann's donor number, a measure of the Lewis basicity, of various solvents selected. The challenges of 7Li NMR technique including solvent effect and choosing a proper reference material are presented and several experimental approaches taken in attempt to overcome those challenges are discussed. 13C NMR longitudinal relaxation time measurement by inversion recovery method was further conducted with Varian 500 MHz NMR and its preliminary results are reported.;In the second investigation we synthesized and fully characterized 1-ethyl-3-methyl-imidazolium (EMITFSI) Ionic Liquid and studied the electrochemistry of O2 on this electrolyte5. In this thesis, the synthesis of 1-butyl-1-methyl-pyrrolidinium bis-(triflouromethanesulfonyl) imide (PYR14TFSI) ionic liquid 6 and the discharge profile of the Lithium-air pouch cell utilizing these ionic liquids as electrolytes are introduced. The battery discharge capacities obtained in these lithium air pouch cells will also be discussed.;Acknowledgement: Partial financial support for this work was provided by US Army CERDEC through contract ARMY-PT GTS-S-09-1-057.;References 1. Wasserscheid. P.; Welton, T., Ionic Liquids in Synthesis. Wiley-VCH 2. Zhou, Z.B.; Matsumoto, H.; Tatsumi, K., Cyclic Quaternary Ammonium Ionic Liquids with Perfluoroalkyltrifluoroborates: Synthesis, Characterization, and Properties. Chem. Eur. J. 2006, 12, 2196 - 2212 3. K. M. Abraham and Ziping Jiang, A Polymer Electrolyte-Based Rechargeable Lithium/Oxygen Battery, J. Electrochem. Soc., 143, 1(1996) T 4. Laoire, C.O.; Mukerjee, S.; Abraham, K.M., Influence of Nonaqueous Solvents on the Electrochemistry of Oxygen in the Rechargeable Lithium-Air Battery. J. Phys. Chem.C. 2010, 114, 9178-9186 5. Allen, C.J. et al., Oxygen Electrode Rechargeability in an Ionic Liquid for the Li-Air Battery. J. Phys. Chem. Lett. 2011, 2 (19), 2420-2424 6. Appetecchi, G.B. et al., Synthesis of Hydrophobic Ionic Liquids for Electrochemical Applications. J.Electrochem.Soc. 2006, 153, A1685-1691.