Factors affecting Li2O2 Formation in Li-O2 Batteries

Abstract

As the market for electronic devices and electric vehicles (EVs) has increased significantly, the real demand for users has also increased. In particular, the batteries that supply power to electronic devices and EVs are a key factor and can greatly affect the performance of the device. But compared to the speed of development of other components, the enhancement of battery development is lagging behind. The battery, which is adopted in most electronic devices and EVs, is a lithium-ion battery, which has a large energy density compared to conventional batteries and has made great achievements in the development of small electronic devices. However, as the system and structure design of the lithium-ion battery has been optimized, the performance of the lithium-ion battery has begun to be braked. Nonetheless, there is a continuing need for improved performance for users. To solve this problem, the next generation lithium ion battery is spotlighted. Among them, lithium air battery has a extremely high energy density and are emerging as potential candidates to replace lithium-ion battery. Lithium air battery uses eco-friendly carbon materials as cathode materials rather than heavy metals of conventional cathode, and the reaction of the cathode takes place through oxygen and lithium in the air. Oxygen in the air participates in the direct reaction to form Li2O2 during discharge, and Li2O2 is decomposed during charging. The lithium air battery having these properties, is advantageous in that it is lighter and larger energy density than conventional Li-ion batteries. However, very low charge/discharge energy efficiency, slow rate capability, and poor lifetime have hampered the commercialization of lithium air battery. All of the above problems are related to the discharge product Li2O2. During the discharging process, oxygen in the air reacts with lithium ions in the electrolyte to form Li2O2 (lithium peroxide). This Li2O2 is an insulator, which is difficult to decompose when charged, thereby reducing the energy efficiency and increasing the irreversibility of the lithium air battery. Therefore, it is very important to control the formation process of Li2O2, which is a discharge product, so that all Li2O2 can be decomposed well. Therefore, in this research, we investigated the factors affecting the formation and structure of Li2O2, the discharge product, and studied the conditions under which Li2O2 decomposition can be easily done. In Chapter 1, we investigated the variation of operating temperature of lithium air battery. The factors affecting the charging/discharging behavior were investigated. The viscosity of the electrolyte solvent, the diffusion coefficient of lithium ion, and the morphology of the discharge product, Li2O2, were investigated. Cyclic voltammetry analysis showed that there were other factors as well as the viscosity of the solvent and the diffusion coefficient of the electrolyte. And change of the morphology of Li2O2 was investigated by TEM (Transmission Electron Microscope) analysis. In Chapter 2, the pore size of the carbon material as the cathode material was controlled to investigate the change of the lithium air battery according to the pore size. The effect of the pore size of the cathode was investigated and the change in the morphology of Li2O2 produced during discharging was confirmed. Also, the mechanism of Li2O2 formation during discharge when cathode pores exist is presented. In Chapter 3, we studied the effect of catalyst size on the cathode. The difference of the electrochemical performance of lithium air battery was investigated according to the size of Ag catalyst and the change of position and structure of discharging product was analyzed in order to investigate the cause of the performance differences. We have studied the factors affecting the structural change of Li2O2, the discharge product of lithium air battery, and propose the optimal conditions of Li2O2 decomposition efficiently.