High Performance All-Solid State Lithium-Air Batteries Employing Aluminum-Substituted Lithium Lanthanum Titanate Solid Electrolyte
Authors: Le Hang T.T. , Ngo Duc Tung , Park Chan-Jin 
Published: February 2020

Recently, lithium(Li)-air battery has attracted much attention due to its extremely high theoretical specific energy density as 11,680 Wh kg-1
Li, which can be even comparable to that of gasoline. Despite potential of Li-air battery, the commercialization of the battery is a great challenge, since it can suffer from plenty of problems majorly associated with the use of liquid electrolytes and metallic Li anode; leakage, flammability, electrochemical instability under reduced oxygen environment and uncontrollable dendritic growth of Li. In this study, to address these issues, all-solid state Li-air battery has been proposed. By using a stable solid electrolyte with proper Li+ conductivity, the risks of leakage, short-circuit and fire/explosion can be successfully addressed. Moreover, the solid electrolyte can act as a barrier to protect Li metal from harmful gases such as O2, CO2 and moisture diffusing from outside air. In this study, we used aluminum-substituted lithium lanthanum titanate (A-LLTO) ceramic as a main ceramic solid electrolyte for all-solid state Li-air batteries. Herein, A-LLTO ceramic with a nominal formula of (Li0.33La0.56)1.005Ti0.99Al0.01O3 was prepared using citrate-gel method. In particular, the air electrode was specially designed by engaging a thin layer solid electrolyte to form an integrated cathode structure. The integrated cathode consisted of a dense ~20-µm-thick A-LLTO solid electrolyte layer adhering to the 500-µm-thick pellet of porous cathode. Moreover, CoO catalyst particles were directly deposited on the surface of A-LLTO cathode backbone. Further, due to instability of A-LLTO solid electrolyte against Li metal anode, an interlayer of composite gel polymer electrolyte (CGPE) was inserted between A-LLTO and Li metal. The interlayer CGPE was stabilized by the addition of small amount of an ionic liquid. After cell assembly, the solid-state Li-air cell was cycled at the current density of 0.3 mA cm-2 in pure O2 gas atmosphere at 50 oC. Under the limited capacity mode of 500 mAh g-1, the cell exhibited the cycle life of 132 cycles. During the cycling process, the discharge potential of the cell almost remained. Meanwhile, the charge cell potential appeared to increase significantly. By using SEM, XPS and Raman spectra analysis, the formation of reaction products on the cathode surface after cycling test was identified. With the increase in cycle number of discharge-charge, the amount of reaction products was found to increase. This demonstrates the degradation of catalytic activity of CoO catalyst. Because Li2O2 possesses the low Li ionic conductivity of 4×10-19 S cm-1 and low electrical conductivity of 5×10-20 S cm-1, it hinders the transportation of electrons and Li+ ions in the cathode. Accordingly, the degree of polarization for the cell increased gradually with cycle. Although the solid state Li-air cell developed in this work could prevent the problems which can happen when liquid electrolyte is used, its life span was not still satisfactory. Therefore, the factors such as operation temperature, catalyst type, which can enhance the kinetics for the reaction process, needs to be investigated further.