Theoretical Study On Two-Dimensional Carbon-Rich Transition Metal Carbides As Potential Anode Materials For Lithium/Sodium Ion Batteries

With the rapid development of social economy,the problems of energy waste,environmental pollution and energy transition development are increasingly highlighted.Exploring new non-polluting and renewable energy sources has become a new window to break the bottleneck and urge development.Lithium/Sodium ion batteries(L/SIBs),as a new type of energy storage devices,have been widely used in various fields such as portable electronic devices,electric vehicles,and aviation/aerospace equipments with the advantages of moderate energy density,long life,fast rechargeability,and environmental friendliness.With the sprouting of high-tech new industries such as electric vehicles and new energy generation,the performance of batteries in terms of energy density,cycle life,safety,etc.has ushered in new challenges.The application of two-dimensional(2D)materials in L/SIBs anode materials brings unique advantages such as fast electrochemical reaction and abundant active sites,and makes it rapidly transform into one of the hot materials for new high-performance batteries.2D transition metal carbides,nitrides and carbonitrides(MXenes)are unique materials with intrinsic metallic properties.Extensive experimental and theoretical works have confirmed that MXenes exhibit high electrical conductivity,excellent cycling stability and low diffusion potential barriers as anode materials for L/SIBs.However,the low-capacity of MXenes severely limits the increase of battery energy density.This is mainly because the surface of MXenes is completely covered by transition metal(TM)atoms,and the functionalization of the TM surface during synthesis and application leads to structural complexity and performance degradation.Given the moderate attraction between carbon(C)and metal ions,and the high carbon content of most commonly used electrode materials,enhancing the conductivity,this paper proposes a strategy to design C-rich component 2D transition metal carbides to enhance the performance of anode materials.In recent years,a large number of theoretically predicted 2D materials have been synthesized experimentally,which fully demonstrate the significance of theoretical calculations to guide the development of new materials.This paper focuses on exploring the structural features,physical properties of the carbon-rich components of transition metal carbides and their relevant properties as anode materials for L/SIBs based on the first-principles structure search approaches.The main research contents and conclusions are as follows.A theoretical exploration of the relationship between carbon content in 2D transition metal tantalum carbon compounds and anode material properties.Based on the first-principles swarm structural structure search approach,we investigate the structures and stabilities of 2D Ta_xC_y(x=1 and y=1-4,or x=2 and y=1).Besides reproducing the reported 2D Ta C,TaC₂ and Ta₂C are found to be stable,and have high thermal stabilities.Metallic Ta₂C,Ta C and TaC₂provide good electronic conductivity.The lithium ion(Li⁺)diffusion energy barrier becomes progressively smaller with the increasing carbon content in the compounds,which indicates that increasing carbon content is beneficial to enhance the rate capacity of the anode.It is worth emphasizing that,unlike Ta₂C,the C atoms of the carbon-rich components Ta C and TaC₂ are exposed on the structural surface.They can directly adsorb Li⁺without surface functionalization.Among them,the structural integrity of TaC₂ monolayer is well preserved after adsorption of two layers of Li⁺.The resultant theoretical capacity(523m A h g^-1),diffusion energy barrier(0.16 e V)and open circuit voltage(OCV=0.28 V)of TaC₂ monolayer are much better than those of commercial graphite electrodes.This mainly stems from the fact that TaC₂ contains carbon dimers(C₂)exposed on the surface providing more active sites for Li⁺adsorption and more suitable Li⁺migration paths.Migrating the carbon-rich idea to Ti-C system,it is found that TiC₃ shows excellent performance as anode material for sodium ion batteries.Ti₃C₂,as the first synthesized 2D transition metal carbides,is the star material in the MXenes family and has been successfully used in metal ion batteries.In this thesis,a metallic TiC₃monolayer anode material with not only remarkably high storage capacity of 1278 m A h g^-1 but also low diffusion potential barrier(0.18 e V)and OCV(0.18 V)was found by variable component structure prediction.Its excellent performance can be mainly attributed to the presence of unusual n-biphenyl unit,which provided a large adsorption area,strong adsorption capacity and fast ion transport pathway for Na⁺.More intriguingly,the surface functionalization not only enhances the capacity of TiC₃,but also has no significant effect on its Na⁺transport.This is in contrast with metal-rich MXenes.Moreover,even in the bulk-stacking limit,TiC₃ still exhibits better anode material performance than 2D metal-rich Ti₃C₂ and Ti₂C.Compared with the reported Ti₂C and Ti₃C₂,TiC₃ is not only thermodynamically stable but also has high cohesion energy,which mainly stems from multibonding coexistence(i.e.,covalent,ionic,and metallic bonds).Attempts and explorations of carbon-rich component ideas in other transition metal carbides.Transition metal elements have rich d-electron configurations,which provide great scope for exploration in the study of different component carbons.Through extensive structural searches(carbides of V,Nb,Mo and W),stable VC₄ and WC₄ monolayers were found.The structure is characterized by graphene nanoribbons composed of C atoms alternating with transition metal zigzag chains in the 2D plane,thus presenting more graphite-like atoms on the 2D surface,implying that more active sites are available.The study of its sodium adsorption behavior reveals that it exhibits a stronger attraction to individual Na⁺,demonstrating a larger theoretical capacity(1354 m A h g^-1).On the other hand,the strong attraction hinders Na⁺migration and VC₄ exhibits only a moderate Na⁺diffusion capacity(0.32 e V).The relatively low theoretical capacity of WC₄compared to VC₄ mainly stems from its large molecular mass.In summary,the design of anode materials for metal ion batteries requires a comprehensive consideration of both transition metal elemental properties and C chemical components,and it is hoped that the research in this thesis will provide an effective strategy for the development of high-performance anode materials.