Design And Application Of Metal-Organic Framework-based Cathode Catalyst For Lithium-Air Battery

With the rapid development of social economy and technology,the traditional non-renewable energy has been unable to meet the needs of human beings.The harm to the environment caused by fossil resources is huge.Therefore,the green energy storage equipment and energy converter develop rapidly.Due to the high energy density of lithium-air batteries,scholars have conducted a lot of research work in this field,hoping that lithium-air batteries can replace lithium-ion batteries.Oxygen reduction/oxygen evolution reaction（ORR/OER）are the core reaction paths of lithium-air batteries.However,the problems of poor cycle stability,overpotential and low discharge specific capacity caused by unsatisfied ORR/OER reaction kinetics hinder the practical application.In order to solve the above problems,we carried out related research focus on the design and structure optimization of cathode electrocatalyst for lithium-air batteries.In this paper,the catalysts with high catalytic activity were prepared by combining the carbon materials calcinated from the metal-organic framework ZIF-67 with other components and adjusting the structure.These catalysts were applied to batteries and tested the performance.The design idea,characterization and testing methods presented in this paper provide a novel idea for the design of air battery catalyst.The specific contents are as follows:（1）Co（NO₃）₂·6H₂O using as the cobalt source reacted with dimethylimidazole to form ZIF-67 by self-assemble,and then ZIF-67 was calcined at high temperature to obtain Co and N co-doped carbon materials（Co-N/C）.Through the analysis of the composition and structure,the Co-N/C is proved to be N-doped graded porous material.The calcination promotes the formation of the metal-N active center.Co-N/C shows good ORR/OER catalytic activity but its stability needs to be improved when it is applied to lithium-air battery cathode catalyst.（2）In order to further improve the catalytic activity of cobalt-based materials,especially the cycling stability,Co-N/C@C-MoS₂composites were synthesized by growing MoS₂nanosheets on the carbon.MoS₂can not only capture Li⁺dissolved in the electrolyte in time to avoid the side reactions,but also act as a protective layer to encapsulate the Co-N_xactive site.At the same time,in the growth process of MoS₂nanosheets,Mo and N coordinate to form the active site of Mo-N.When Co-N/C@C-MoS₂composites were used as cathode catalyst,the specific discharge capacity of the battery is up to 21197 mA h g^-1at the current density of 100 mA g^-1.The battery works steadily for 332 cycles with the limited specific capacity of 1000mA h g^-1and the 500 mA g^-1current density.Compared with Co-N/C,the catalytic activity and stability were significantly improved.（3）In order to improve the reaction rate and give full play to the catalytic performance,the Co-N/C was calcined in air and combined with CeO₂to form Co/CNT@CeO₂.During air calcination,partial Co transformed into Co₃O₄to form Co/Co₃O₄heterojunction.In addition,part of the amorphous carbon turned into CNTs during calcination which encapsulate the Co-based active sites.The catalytic activity was improved by enhancing the adsorption capacity of O₂and introducing more adjustable vacancies through recombining with CeO₂.When Co/CNT@CeO₂composites were used as cathode catalys,the battery shows excellent rate performance and cycle stability.At the current density of 100 mA g^-1,the specific capacity reaches 22016 mA h g^-1.The battery works steadily for 249 cycles with the limited specific capacity of 1000 mA h g^-1and the 500 mA g^-1current density.