Rechargeable Aqueous Lithium-Air Batteries

Abstract

The lithium-air battery is a promising power source for electric vehicles because of its high theoretical specific energy density. Two types of the lithium-air battery are proposed, namely, non-aqueous and aqueous systems [1]. The theoretical specific energy densities of the non-aqueous and aqueous systems are 3458 and 1910 Wh kg-1, respectively. However there are many changeable research drawbacks that must still be addressed. At present, non-aqueous lithium air battery is mainly focused, but this system has severe problems such as lithium corrosion by water, high polarization during charge process, etc. The aqueous lithium-air system could avoid these problems by using water stable lithium electrode [2] and aqueous electrolyte. In this study, we demonstrate the cell performance of the aqueous lithium-air system. The laminate type cell was tested at room temperature. The cell was consisted of Li/interlayer/water stable lithium conducting solid electrolyte/aqueous electrolyte/air electrode. As the interlayer, a non-aqueous electrolyte such as Li(FSO2)2N in glyme [3] was used, because the water stable lithium conducting solid electrolyte of Li1+xAlxTi2-x(PO4)3 (LATP) is unstable in contact with lithium. The LATP plate of ca. 0.15 mm in thickness was supplied by Ohara Inc. Japan. The platinum catalyst coated carbon sheet for a large size cell and the composite electrode of carbon black (KB) supported MnO2 catalyst for a small size cell were used as the air electrodes, respectively. The cells were operated under open air. The impedance profiles of the test cells with different interlayer electrolytes of glyme were examined. The profiles showed two semi-circles. The high frequency semi-circle showed no dependence on the character of the interlayers, but the low frequency semi-circle showed dependence on it. The high frequency semi-circle corresponds to the grain boundary resistance of the LATP plate and the second semi-circle to the interlayer resistance between lithium and interlayer electrolyte. The lowest cell resistance of ca. 270 Ω cm2 at room temperature was observed for the Li/DME-LiTFSI/LTAP/saturated LiCl aqueous solution/C cell. A large size cell of 6.2 Ah was successfully discharged at 50 °C with 2 mA cm-2 for 150 h. The initial cell voltage of 3.0 V was gradually decreased to 2.7 V after 150 h polarization. A cycle performance of the small size cell was examined at room temperature. For a single cycle, the charge-discharge test was performed with 2 mA cm-2 for 4 h. The discharge and charge voltages were not changed significantly for 100 cycle and 400 h. In conclusion, the aqueous lithium air battery was operated with high current density at room temperature under open air. These results suggest that aqueous lithium-air rechargeable batteries have a possibility for further development with high specific energy and power density.