Design And Electrocatalytic Performance Of Transition Metal Oxides And Carbides Catalysts For Li-O₂ Batteries

Li-O2 batteries have become the most promsing new energy source due to their extremely high theoretical energy density,environmental friendliness,and recyclability.They are expected to replace the lithium-ion batteries which is currently used in portable electronic devices and are expected to be powered as a major energy system for booming electric vehicles(EV)and hybrid electric vehicles(HEV).However,there are still certain problems affecting the practical application of Li-O2 batteries such as low specific capacity,high overpotential,and poor cycle performance.For the Li-O2 batteries,the improvement of its performance mainly depends on the selection and optimization of the oxygen cathode catalyst.An efficient catalyst needs to meet the following conditions:(1)A large specific surface area,to ensure that the electrolyte is in full contact with the cathode catalyst material and to provide sufficient active sites.(2)Significantly reduce the over-potential during charging and discharging,which would help to improve the Coulomb efficiency.(3)Good conductivity,to ensure the transmission of electrons.In this paper,two different systems,CeO2/Co3O4 NWs grown on nickel foam and two-dimensional layered graphene-like Ti3C2 MXene materials,are constructed from two different perspectives as cathode catalysts for Li-O2 batteries.We hope that through the design and optimisition of advanced cathode catalyst,better electrochemical performance of Li-O2 batteries could be obtained.In this dissertation,the synthesis of CeO2/Co3O4 NWs grown on nickel foam and the electrochemical performance of lithium-oxygen batteries were tested.Co3O4 has been widely used as a cathode catalyst for lithium-oxygen batteries due to its highly efficient dual-functional catalytic activity and excellent electrochemical stability.The cerium ions in CeO2 can be quickly converted between oxidized and reduced states(Ce3+ and Ce4+).Some Ce4+ ions are easily reduced to Ce3+ ions and oxygen vacancies,and transferred to the surface of the crystal to become active sites,thereby adsorbing superoxide.Free radicals and reduced activation energy for electrochemical reactions.Therefore,we designed a cathode material with Co3O4 NWs as the matrix and composite CeO2 as the initial nucleation/decomposition site of the discharge product Li2O2,so that the lithium-oxygen battery has a controlled discharge/charge product evolution path.This project uses cerium nitrate hexahydrate,cobalt nitrate hexahydrate,and urea as reactants.The CeO2/CO3O4 organic precursor is grown in situ on nickel foam by hydrothermal method,and CeO2/CO3O4 composite nano is prepared by further heat treatment.Li-O2 battery and directly used as a carbon-free and binder-free positive electrode for lithium-oxygen batteries.The CeO2/CO3O4 composite nanowire positive electrode exhibits good electrocatalytic performance in lithium-oxygen batteries,effectively reduces the overpotential during the charge and discharge process,and has a long cycle life.The discharge capacity of the CeO2/CO3O4 composite nanowire cathode has been greatly improved compared to other carbon-free electrodes,and it has steadily cycled more than 500 times under the high current density of 500 mA g-1 and the cut-off capacity of 500 mA h g-1.At the same time,we used SEM,XRD,TEM,DEMS,and ex-situ XPS methods to systematically analyze the generation and decomposition process of the discharge products of Li-O2 batteries.The electrochemical performance and the composition of CeO2/CO3O4 nanowires were studied.The relationship between appearance and structure provides new ideas for designing transition metal oxide cathodes for lithium-oxygen batteries.In addition,the thesis introduces the research of Ti3C2 MXene as the cathode catalyst for Li-O2 batteries.MXene,as an advanced layered material with graphene-like structure,has been applied in the fields of electrocatalysis and energy storage,such as supercapacitor,solar cell,Li-ion batteries and so on.Meanwhile,there have been studies predicting that MXene materials have adsorption properties for discharge products,therefore effectively reduce the overpotential of electrochemical reactions.Therefore,we tried to synthesize Ti3C2 MXene material and use it as a cathode catalyst for Li-O2 batteries.We analyzed different factors affecting the electrocatalytic activity of Ti3C2 MXene and screened out the most suitable conditions for applying Ti3C2 MXene cathode catalyst in Li-O2 batteries,in which excellent capacity and cycle performance were obtained.We used Ti3AlC2 as raw materials to prepare F/M-Ti3C2 MXene material by lithium-ion intercalation etching and ultrasonic stripping,which were coupled with different electrolyte systems which have different reaction principles.The electrolyte system with the highest activity was selected,in which the capacity,rate and cycle performance of the Li-O2 battery were tested.In the maintime,the influence of micromorphology,structural composition,specific surface and other factors on the electrocatalytic performance of the Ti3C2 MXene cathode catalyst was studied.Moreover,SEM,XRD,TEM,DEMS,and ex-situ XPS are used to further study the electrichemical reaction mechanism.The information about the overpotential,path,rate,and number of gain/loss of the electron in electrocatalytic reaction and investigation of the evolution of discharge products in the cycling could help us further optimize the electrochemical performance of Li-O2 batteries and promote the practical application of new MXene material catalysts in the field of Li-O2 batteries.