Research On Nano Electrocatalysts For The Oxygen Reduction Reaction For Organic Electrolyte Based Lithium-Air Battery

As a novel "Semi-fuel cells", lithium air(oxygen) battery has been attracted considerable attention and regarded as a promising chemical power device. Lithium is used as a negative electrode, and the oxygen in the air as the positive electrode active material. The oxygen from the outside goes through the gas diffusion electrode into the battery and reaches the interfaces among the gas, liquid and solid three-phase interfaces. And the electrode reaction occurs and releases the electricity energy. Although there has been some progress in the electrolyte, carbon materials and catalysts, but the discharge performance of the battery, the cycle performance, polarization properties have not been well improved. Catalyst is mainly used to improve the kinetics of the charge-discharge reaction and the reaction to reduce the activation energy of the reaction. The catalyst can be added to improve the polarization of the battery and reduce the overpotential. Therefore, the research work in this thesis is to systematically study the efficient and stable catalytic activity of lithium air battery cathode catalyst. Nano silver supported nano manganese dioxide core-shell composites, nano manganese dioxide loaded nanotubes core-shell composites and nano transition metals loaded nanotubes core-shell composites are mainly investigated. The effects of the reaction conditions on the formation mechanism, morphology and electrochemical properties are studied.First, one dimensional bar structure of a-MnO2and β-MnO2with good dispersion characteristics were successfully prepared by hydrothermal method in presure of PVP as a surfactant and KMnO4(NH4)2S2O8、 MnSO4·H2O as the reactants. The hydrothermal temperature and time are controlled at160℃and220℃,12h and24h. The surface of a-MnO2and P-MnO2are successfully modified though in situ composite technology. The amount of silver can effectively controlled by the concentration and dropping rate of silver ammonia solution. Ag/a-MnO2is in favor of the oxygen reduction reaction and has better charge-discharge cycle performance. LiTFSI/TEGDME helps to improve the reversible cycling formance of the battery, while EC/DEC/LiTFSI is self-decomposed during discharge. The charge-discharge capacity and cycle performance of surface modified composite is much better than that of pure MnO2.Second, the surface of MWCNTs were treated by PDDA, overcoming the traditional acid treatment leaving for structural defects MWCNTs and improve the dispersion and wettability in water. Transition metal oxides MnO2was successfully modificated on the surface of MWCNTs. High specific surface area of the catalyst (MnO2/MWCNTs) is conducive to the oxygen reduction reaction. The first discharge capacity is801mAh·g-1(electrode) and the charge capacity is798mAh·g-1(ectrode).The discharge platform voltage is2.90V and charge platform voltage is3.70V. The coulomble efficiency is99.7%.Third, CoMn2O4(CoFe2O4) nanostructures were successfully modified on the surface of MWCNTs by hydrothermal synthesis method under precise controlling of Co, Mn(Fe) atomic ratio, temperature and reaction time. It avoids the disadvantages of co-precipitation method. The specific surface area of CoMn2O4/MWCNTs is1825.63m2·g-1and the pore volume is2.81cm3·g-1.It belongs to typical of mesoporous structure and it can effectively provide convenient transport channel for the transmission of oxygen. It effectively improve the electrical properties and catalytic of air electrode. Two distinct oxidation peaks around3.26V and3.80V were observed, corresponding to the generation of Li2O and Li2O2.Due to strong coupling between CoFe2O4and MWCNTs, the surface electron is more easily converted into free electrons. A distinct oxidation peaks appeared around3.62V, corresponding to the generation of Li2O and Li2O2.