Rational Design And Catalytic Performance Investigation In Nb₂C MXene And Mo-based Two-dimensional Materials For The Cathode Of Lithium-Oxygen Battery

The problems of shortage of modern fossil energy and environmental pollution are becoming more and more serious.China has formulated a major strategy of carbon neutrality,so the exploration and development of sustainable clean energy has attracted people’s attention.Among various energy storage and conversion systems,the theoretical energy density of rechargeable Li-O2 batteries is as high as 3457 Wh kg-1,which is 10 times higher than that of traditional lithium-ion batteries,thus attracting people’s interest in researching lithium-oxygen batteries.However,the widespread use of lithium-oxygen batteries is still limited by many factors,such as poor cycle efficiency,high overpotential,poor cycle stability,and poor rate capability.The main factor limiting the performance advantages of Li-O2 batteries is the slow kinetics of the oxygen reduction and oxygen evolution reactions on the cathode.During the discharge process,the surface of the positive electrode is gradually coated with lithium peroxide(Li2O2)as an insulating discharge product,which reduces the conductivity of the electrode system and increases the decomposition energy barrier of the surface product,which eventually leads to an excessively high overpotential and poor cycle performance,In addition,during the discharge/charge process,the generated reactive oxygen species-superoxide radicals attack the lithium anode,electrolyte,and cathode materials,resulting in the generation of side reactions on the electrodes and the formation of various by-products.The development of high-efficiency cathode catalysts is one of the important strategies to solve the performance problems of Li-O2 batteries.Efficient catalysts can accelerate the cathode reaction and control the nucleation and growth kinetics,morphology,crystallinity,and chemical composition of the discharge products.Lithium superoxide(LiO2)is considered to be an important intermediate during the oxygen reduction reaction and oxygen evolution reaction,which greatly affects the stability of the catalyst and the composition of the discharge products.The formation of discharge products at the interface of cathode and electrolyte can be via surface or solution pathways with LiO2 species.Furthermore,the morphology of the discharge products formed with the assistance of the cathode catalyst is closely related to the cycling stability.This thesis mainly explores the relationship between the surface composition,electronic structure and catalytic performance of two-dimensional metal carbides(MXene),and the coordination structure and chemical environment in the material affect the electronic properties of two-dimensional materials and the catalytic ability of lithium-oxygen battery cathodes.Therefore,starting from the crystal structure of the element,the structure-activity relationship of coordination characteristicselectronic structure-catalytic ability was established.The main research contents of this thesis are as follows:(1)Nb2C MXene ultrathin nanosheets with a uniform O-terminated surface were prepared by etching-stripping-annealing under the guidance of Density functional theory calculations,and their electrochemical performance as a cathode material for lithium-oxygen batteries was investigated,and their response to discharge was investigated.The catalysis of the generation and decomposition of the product,the morphology change and composition change of the discharge product lithium peroxide during the charge and discharge process,and the important influence of the surface characteristics of the catalyst material on the charge and discharge process and cycle capacity;(2)Basic analysis of Mo-based MXenes and chalcogenides(including typical eight structures of Mo2C MXene and Mo2N MXene and their oxygen-terminated surfaces,MoS2 and MoSe2 and their 1T,2H phases)by density functional theory calculations.The crystal structure,electron distribution characteristics,free energy change of cathode process,phase diagram of products,and electronic structure change during surface adsorption were calculated and analyzed in depth.The relationship between the coordination structure of Mo as the central transition metal atom,the formation of the surface properties,electronic structures of 2D materials and the performance descriptors of the cathode reaction process was analyzed,and the key crystal structure and electronic structure characteristics of ultra-thin 2D materials as cathode catalysts were discussed.Thereby we established the structure-activity relationship among properties of two-dimensional materials,electronic structure and catalytic ability.The MXene material processing strategy proposed in this topic and its use as a cathode catalyst for lithium-oxygen batteries have obtained satisfactory cycle life and specific capacity performance,which provides a new idea for designing the cathode structure of MXene materials;the common catalytic characteristics of two-dimensional materials were analyzed by taking Mo-based two-dimensional materials as calculation samples,and the corresponding structureactivity relationship was established,which provided a model and guidance for the design of lithium-oxygen cathode catalysts.The research significance of this paper is as follows:(1)the important influence of MXene surface groups on the intrinsic electronic structure and electrochemical cycle performance of cathode materials is proposed,which will provide theoretical basis and framework for the application of MXene surface engineering and posttreatment strategies in electrochemical energy storage systems;(2)the key crystal structure and electronic structure characteristics of ultra-thin two-dimensional materials as lithium-oxygen cathode catalytic materials are analyzed,so as to establish the structure-activity relationship of two-dimensional materials coordination characteristics-electronic structure-catalytic ability.A linear relationship between the parameter k composed of the calculated reaction free energy,and the total overpotential is formed,which has good statistical significance in high-throughput screening and optimization of cathode materials.