Investigation of Reaction Mechanism in All-Solid-State Li-Air and Li-O2 Batteries

Abstract

Introduction The lithium-air batteries have attracted much more attentions because of its high theoretical energy density, which is over 3500 Wh kg-1. However the lithium-air batteries still have a lot of problems and the problem related with the decomposition of organic liquid electrolytes is the most serious and one of the main research topics in the present stage. Thus, we have developed all-solid-state lithium-air batteries using the inorganic solid electrolytes, which are more stable than the liquid electrolytes. The application of the solid electrolytes to the lithium-air batteries can solve not only the decomposition of electrolytes but also the safety problem and some other problems specific to the liquid such as leakage and volatilization. In this study, all-solid-state cells consisting of Li anode, Li1+xAlyGe2-y(PO4)3 (LAGP) solid electrolyte, and LAGP-carbon nanotube (CNT) composite cathode was operated in the air atmosphere (so-called Li-air cell) and pure oxygen atmosphere (so-called Li-O2 cell). Reaction mechanism during discharge-and charge was investigated. Experiments The Li / LAGP / LAGP-CNT cells were assembled by the following procedure. LAGP powders and pellets were prepared by the conventional solid-state method. CNT and LAGP powders were dispersed into the ethanol solution. A drop of the solution was put on the LAGP pellet and dried at room temperature. The air electrode deposited on the LAGP pellet was heated at 700 oC. After sintering, the air electrode combined with the solid electrolyte layer was obtained. On the other side of the LAGP pellet, a Li anode was contacted with the LAGP pellet by a thermal adhesion process and sealed with a plastic film. An Al mesh and a Cu mesh were used as a current collector for the cathode and the anode, respectively. The laminate-type cell was sandwiched by two plastic plates and held by a screw clamp. The Li-air cell was operated in the laboratory air atmosphere. The Li-O2 cell was put into the bottle connecting to the oxygen gas flow channel, and discharged and charged under a pure oxygen gas-flowing condition. Results and discussion The all-solid-state cells were discharged and charged by the constant current mode in the voltage range of 2.0 - 4.8 V. The Li-O2 cell showed a discharge plateau at 2.0 - 2.5 V and, the charge plateau gradually increased from 3.0 to 4.2 V. On the other hand, the Li-air cell showed a discharge plateau, which starts 3.0 V at a low current density, and the charge curve obviously showed two plateaus. In such cells, the air electrode after discharging and charging was analyzed by using XRD, XPS, Raman, IR, and TEM. From these measurements, it was suggested that Li2O2 was electrochemically produced after discharging in both air and oxygen atmosphere. Also Li2CO3 was produced by side-reactions. The reaction products in air atmosphere were the same as that in oxygen atmosphere. However, the elementary process seems to be different between air and oxygen. It was also found that the CNT was degraded during cycling, which would cause the capacity degradation.