Ionic Liquid-Based Electrolyte for Lithium-Air Battery

The Li-air battery has received much interest in the last few years as the global energy demand is growing and the availability of fossil energies becomes limited. At present, the specific energy of Li-ions battery is limited to 150 W.h/kg whereas the new approach of Li-air battery offers a theoretical specific energy superior to 1000 W.h/kg [1]. This value is comparable to the specific energy of gasoline for automotive application [2]. For this reason, the main application for Li-air battery is transportation, with the possibility in the next few years to develop an electric car with a same autonomy as gas powered car but cheaper and more environmentally friendly. However, this new technology will remain a research topic for at least the next few years, due to the low cyclability and the wide gap between practical and theoretical energy density.

The project STABLE (“STable high-capacity lithium-Air Batteries with Long cycle life for Electric cars” FP7-NMP 314508) aims to conceive, from innovative materials, a full electrical vehicle with the best performances and the lowest cost possible. The objective is to obtain a battery with a specific capacity higher than 2000 mA.h/g with a cyclability of 100-150 cycles. In this context, a multidisciplinary work team is employing his expertise in material synthesis, characterization and cell assembly to achieve the following tasks: (i) nanostructuration of the Li-anode by electrodeposition, (ii) fabrication of nanostructured carbon cathode and (iii) preparation of ionic liquid (IL)-based electrolyte.

Hydrophobic ionic liquid-based electrolytes have been designed by metathesis of new room temperature ionic liquids (RTILs) and electrochemically tested in split flat cell. The high voltage stability of RTILs (up to 5V) enables a better cyclability of the battery while the capacity stays reasonable (up to 1000 mA.h/g) compared to conventional Li-ions organic electrolytes like the ethylene carbonate (EC)/diethyl carbonate solution (DEC) (Figure 1). Selection of additives and other combinations of ionic liquids have been made in order to reach capacity values up to 4200 mA.h/g in case of Tetraethylene Glycol Dimethyl Ether (TEGDME) which also shows a higher cyclability (up to 10 cycles). Moreover, the RTILs are stable at higher temperatures (> 300˚C), non-flammable and, therefore, more suitable for electrical vehicles.

 [1] Christensen J. et al., J. Electrochem. Soc. 159 (2012) R1-R30

[2] Girishkumar G. et al., J. Phys. Chem. Lett. 1 (2010) 2193-2203

Figure 1: Specific capacities of Li-air batteries using different organic and ionic liquid-based electrolyte for the first three discharges.