Optimization Design On Transition Metal-based Cathode Catalysts For Lithium Air Batteries

With the development of society,secondary batteries have been widely used in more and more fields.And at the same time,the demand for secondary batteries with high energy density has become more and more strong.Among several new secondary battery systems that are currently under research,lithium air batteries have attracted wide attention due to their high theoretical energy density of 11400 Wh kg^-1.If the practical application of lithium air batteries can be realized,a large number of industries that need to use high-capacity batteries will benefit greatly.However,some key problems still exist in current lithium air batteries,among which the problems on the cathode are most prominent.The fundamental reason why the cathode becomes a soft rib is that the kinetics of the cathode reaction are too slow,which leads to a series of problems such as low energy efficiency,short cycle life and poor rate performance.To solve the above problems,the most convenient and effective way is to improve the ORR and OER kinetics by using a suitable cathode catalyst,which can effectively improve the power density,specific capacity,efficiency,and life of batteries.In this dissertation,from the perspective of microstructure design and optimization of chemical composition,the following studies around the transition metal-based cathode catalysts were carried out:（1）From the viewpoint of exploring the effect of chemical composition on catalytic performance,a series of Cu_xCo_3-xO₄ nanorods were synthesized using hydrothermal methods with different reactant concentrations.The obtained oxides were used as catalysts for lithium air batteries,and examined from the perspective of polarization and cycling of the batteries.The results show that all electrodes using Cu_xCo_3-xO₄ exhibited higher specific capacity and lower overpotential than electrodes using pure Co₃O₄ and pure KB,indicating that doping of Cu²⁺did indeed improve catalytic activity.At the same time,those with higher Cu²⁺doping concentrations exhibted higher specific capacities and longer cycle life.The first-principles calculations of the surface charge reaction of Cu_xCo_3-xO₄ show that more Cu²⁺in Cu_xCo_3-xO₄ promotes the decomposition of Li₂O₂ during charging,thereby improving the battery performance.By controlling the doping concentration,the charge transfer ability of the material can be changed and its catalytic activity can be influenced.（2）From the viewpoint of using non-noble metal doping to enhance catalytic activity,the large specific surface area advantage of graphene is combined with the positive effect of iron doping.The nitrogen-doped graphene decorated with uniformly loaded Fe/Fe₃N/Fe₄N nanoparticles is synthesized by using an inexpensive and convenient method.The resulting product has a typical three-dimensional porous structure,a large specific surface area and an evenly distributed insert.When applied to lithium-air batteries,the material exhibits a significantly increased specific capacity and a markedly reduced overpotential,and is capable of over 100 stable cycles under limited capacity cycling.Through the analysis of the discharge products and the characterization of the material structure,we believe that the reason for the significant increase in catalytic activity lies in the synergistic effect exerted by the effective combination of the active dopant nanoparticles and the porous substrate.（3）From the viewpoint of compounding different material,a carbon network material with a three-dimensional structure was prepared and used as a carrier to load Co₉S₈ nanosheets,thereby improving its electrical conductivity and electrochemical performance.Observations by scanning electron microscopy and transmission electron microscopy confirmed that the two components were effectively combined.In addition,the three-dimensional carbon network was found to be also affected by the sulfur source during the recombination process,and thus a certain degree of sulfur doping was obtained.When applied to lithium air batteries,the batteries exhibit good rate performance and stable cycling performance.This composite of Co₉S₈ and three-dimensional carbon network materials can solve the original defects,and thus have a significant positive effect on the improvement of catalytic performance.（4）From the viewpoint of simultaneously utilizing the electron conductivity of the transition metal and the catalytic activity of the transition metal oxide,a triple composite structure material of rGO/Co/Co₃O₄ was constructed.By comparing the results of XRD characterization of different products at different stages,it was confirmed that the resulting product is a triple composite structure with rGO,Co and Co₃O₄ from the inside to the outside.When this material is used in a lithium air battery,the battery exhibits good rate performance.At the same time,the overcharge potential of the battery during the long-term cycle can remain stable.By comparison,it was confirmed that the three different components in the composite material played their respective roles in the catalytic process.