Electrochemical Characteristics of PEO-Based Ternary Polymer Electrolyte for Lithium-Oxygen Batteries

The biggest advantage of metal-air batteries is their high theoretical energy density. Furthermore, they use oxygen from nature and exhibit eco-friendly characteristics. Lithium-air battery is recognized as a potential candidate for the application in EV. In particular, lithium-air battery shows a similar theoretical energy density (11,140 wh/kg) to that of gasoline (13,000 wh/kg). However, its performance has still been limited by problems such as the stability of lithium metal. In practical applications for electric vehicles, air containing moisture should be used. The exclusion of water from air is difficult using a conventional filter, hence lithium metal can react vigorously with the water from moisture and make the battery unsafe for use. Thus, the protection of Li-metal from water is the critical problem for rechargeable Li-air batteries. The other prominent problem for rechargeable lithium-air batteries is the lithium dendrite formation during repeated charge and discharge cycles; the dendritic Li-metal causes an internal short circuit. To overcome this problem, the key component of aqueous type lithium-air batteries is the water stable LISICON type lithium conducting solid electrolyte, since lithium metal reacts severely with water. However, the LISICON type solid electrolytes are not stable in contact with Li-metal. Thus, interlayers are needed between the solid electrolyte and Li-metal, which must be chemically stable in contact with Li-metal during operation and should have high lithium ion conductivity. PEO-based polymer electrolytes are best candidates as an interlayer. However, conductivity of PEO-based electrolytes is still much lower compared to that of liquid and inorganic solid electrolytes. The PEO-LiTFSI electrolyte has the ionic conductivity of the order 10^-6 S×cm^-1 at room temperature. In order to use PEO-LiTFSI as an interlayer, its ionic conductivity needs to be improved. In the present study, an ionic liquid has been added to PEO-based polymer electrolyte to increase its ionic conductivity.                         

The ionically conducting polymer membrane was prepared by casting technique. Microstructure and morphology of surface were obtained by FE-SEM and optical microscope. The samples were obtained by optimizing the ratio of PEO, Li salt and ionic liquid. The ac impedance of symmetric cells were measured in frequency range of 1 Hz ~ 1 MHz. The stability test indicates that the PEO based polymer electrolyte improves the stability between the Li-metal and solid electrolyte with improved conductivity. The ac impedance and charge discharge performance measurements for Li-O₂ cell employing the polymer and LAGP solid electrolyte layers for the protected lithium electrode were also carried out. The Li-O₂ cell with the developed polymer electrolyte exhibited a favorable discharge-charge performance.