Preparation Of Hollow Structure Catalyst And Its Performance In Lithium Air Battery

In recent years,new generation energy storage device such as lithium sulfur,zinc air,aluminum air and lithium air batteries have emerged to be developed for alleviate the energy crisis and environmental problems that caused by excessive use of fossil fuels.Compared with lithium-sulfur,zinc-air,and aluminum-air batteries,lithium-air batteries show higher specific energy(³600 Wh kg^-1).However,the kinetics of the oxygen reduction reaction?ORR?and the oxygen evolution reaction?OER?occurring in the positive electrode of the air are slow,which seriously hinder its operation in electric practical applications in vehicles due to large charge-discharge over-potential,low-rate performance,and poor cycle life.In order to overcome the above problems,a number of solutions have been explored,including the design of a bifunctional cathode catalyst,improved electrolytes,and adjustment of the morphology or composition of the discharge products.Carbon materials?CNTs,rGO,KB?exhibit higher ORR activity,but their OER properties are generally poor,and they are also easy to be oxidized,causing some side reactions at high voltage.Transition metal oxides?MnO₂,Co₃O₄,NiCo₂O₄,etc.?have been extensively studied due to their low cost and high efficiency.The OER activity of metal oxides is generally higher than that of carbon materials.For metal oxide catalysts,the surface atomic arrangement and valence distribution play an important role in the electrocatalytic activity and electrochemical performance of lithium air batteries.In this thesis,a hollow structure cathode catalyst was constructed.Based on the advantages of carbon materials and Co₃O₄,the electrocatalytic properties of the materials were improved by hetero atom doping,synthesis of heterostructures and oxygen vacancies.?1?TEOS is hydrolyzed to form SiO₂ as hard template,and the surface of the SiO₂ was coated with dopamine to form SiO₂@PDA.After high temperature carbonization and stencil etching to prepare nitrogen-doped hollow carbon spheres?N-HC?.The FE-SEM test showed that the N-HC was a dispersed sphere with a diameter of²50 nm.The carbon layer of N-HC was observed to be²0 nm by TEM.XRD and Raman spectroscopy confirmed that dopamine was carbonized to form graphitized carbon.The specific surface area of N-HC was about 880 m² g^-1.XPS analysis showed that the N element was indeed doped into the carbon layer.Compared with solid nitrogen-doped carbon spheres?NC?and nitrogen-doped carbon-coated SiO₂?SiO₂@NC?,N-HC exhibited superior electrochemical performance:The discharge specific capacity was²5000 mAh g^-1 at a current density of 300 mA g^-1 and cut-off voltage to 2V;The battery operated 36 cycles with a limiting capacity of 500 mAh g^-1 and a current density of 300 mA g^-1.?2?Selecting N-HC as the substrate and transition metal CoO-Co₃O₄heterostructure anchored on the surface to synthesize N-HC@CoO-Co₃O₄.XRD confirmed that CoO,Co₃O₄,N-HC existed simultaneously in N-HC@CoO-Co₃O₄.It exhibited extremely high discharge specific capacity when used as a a lithium air battery air positive electrode.The discharge specific capacity was²4000 mAh g^-1 at a current density of 300 mA g^-1 and cut-off voltage to 2V.It operated 112 cycles with a limiting capacity of 500 mAh g^-1 and a current density of 300 mA g^-1,showing good cycle performance.Compared with N-HC,the performance of N-HC@CoO-Co₃O₄was significantly improved,mainly due to the synergistic effect of the hollow structure of N-HC and the heterostructure of CoO-Co₃O₄.?3?Inspired by the heterostructure catalyst,a certain proportion of glycerol and isopropanol were used as hydrothermally solvents,cobalt nitrate was used as the cobalt source,and the hollow structure Co₃O₄ was synthesized.The oxygen vacancy was synthesized on the surface of Co₃O₄ by strong reducing agent NaBH₄.The morphology and micro structure of R-Co₃O₄ was observed by FE-SEM and TEM.The X-ray,Raman,XPS were used to analysis of R-Co₃O₄ and confirmed that existence the oxygen vacancies on the surface.Electrochemical tests showed that the increase of oxygen vacancies and Co²⁺content improved the electrocatalytic properties of the materials.The discharge products of the R-Co₃O₄ electrode were studied.XRD confirmed that the discharge product was Li₂O₂.The morphology of the discharge products was nano-granular,and the discharge products disappeared after charging,which proved that the catalytic reversibility of R-Co₃O₄ was good.The XPS analysis of the R-Co₃O₄ electrode at different charge and discharge stages revealed that the discharge product was first deposited on the Co²⁺ site.