Synthesis And Electrochemical Properties Of ZIF-67 Derived Cobalt-based Materials For Li-O~2 Batteries

The sluggish oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)kinetics on the air electrodes leading to the low capacity,poor efficiency and short cycle life of Li-O2 batteries.Therefore,the air-breathing cathode has been identified as the key factor that affects the overall performances of Li-O2 batteries.Materials which derivatizing from metal-organic frameworks(MOFs)usually show more functionalities comparing with traditional materials.In this thesis,we synthesised of the metal-organic framework material ZIF-67 controllably,prepared the nitrogen-doped and carbon-coated cobalt-based materials Co@NC from pyrolysis ZIF-67 precursor under inert atmosphere.CoS2@NC and Co1-xS@NC were prepared by in-situ vulcanization,and the microstructure and electrochemical properties of the above materials were studied.ZIF-67 precursor was synthesized by homogeneous precipitation,and the synthesis was controlled by controlling the concentration of the reactants and the ratio of the metal center Co to the organic ligand 2-methylimidazole.The results showed that the size of ZIF-67 crystal decreasing and the the degree of irregulartion increasing when increasing the reactants concentration and changing the ratio of Co to organic ligand changes from 1:4 to 1:32.When Co2+concentration was 0.05 M and the ratio of Co to the organic ligand was 1:4,as-prepared ZIF-67 precursor was monodisperse and it had regular dodecahedron morphology and ultra-high specific surface area of 1143.3 m2 g-1.The in-situ thermogravimetric-differentiall-infrared analysis of the above ZIF-67 precursor in an inert atmosphere revealed that the organic ligands within the structure began to decompose and release aromatic hydrocarbon gas at 460?.Co2+ began to be reduced to metal Co significantly at 600? and the size of Co particles increasing with the rising of temperature.The organic ligands stopped decomposing at 1000?.Pyrolyzing the ZIF-67 under the 900?,the pyrolysis product Co@NC-900 had large surface area of 259.7 m2 g-1,high degree of graphitization,suitible N element content(N-5&N-Q,74.5%)and the best electrochemical performance compared to other Co@NC materials.When using it as air electrode for lithium air batteries,its discharge capacity was 3110 mAhg-1 at current density of 0.1 mA cm-2,limiting the capacity to 1000 mAh g-1,it can cycle more than 70 times.Using sublimed sulfur as the sulfur source to prepare the new type of cobalt sulfide CoS2@NC and Co1-xS@NC by in-situ vulcanization method which was calcination Co@NC-900 at 300?600? under the Ar flow.It can prepare the CoS2@NC below the 500?and when increasing the temperature above 500?,Co1-xS@NC was gained.The electrochemical performance of all cobalt sulfide was better than Co@NC-900.the COS2@NC-400 obtained at 400? had the specific of discharge capacity of 7059 mAh g-1 at current density of 0.1 mA cm'2,and can cycle more than 80 times uder the limition capacity of 1000 mAh g-1.