First-Principles Study On Transition-Metal Carbides As Cathode Catalysts In Li-O₂ Batteries

Li-O2 batteries have theoretical energy density of 11400 Wh/kg and actual usable energy density of 1700 Wh/kg,which is comparable to gasoline and much higher than other power systems.It will be a potential energy storage device in the future.However,the practical application of Li-O2 batteries still faces many problems,such as high charge/discharge overpotential,poor rate capability,poor cycle performance,and electrolyte decomposition,etc.The main reason is that the rate of charge and discharge kinetics is slow.Finding highly active catalysts is an effective way to solve the slow kinetic problems of Li-O2 batteries.The dynamic process of batteries involves oxidation and reduction reactions under electrochemical catalysis,and there are many other influencing factors.It is inefficient and difficult to find efficient catalysts only by the experimental method of trial and error.Theoretical calculation is an indispensable method to understand the mechanism of electrocatalytic activity and the intrinsic influencing factors.Transition-metal carbides are investigated based on the first-principles calculation method and starting from the intrinsic thermodynamic reactions of oxidation-reduction involved in the charge and discharge process of Li-O2 batteries.A cathode electrochemical interface catalytic model that combines the coupling of electrons,ions,interfaces,and electrochemical reactionstheoretical is constructed.The charging and discharging reaction path of Li-O2 battery is simulated.The change of reaction free energy is calculated.The overpotentials are calculated to quantitatively describe and evaluate the catalytic performance of the battery.The catalytic activity mechanism and intrinsic influencing factors are stuied.The high-efficiency strategies for the design of cathode catalysts for Li-O2 batteries are explored.The material states researched include doping effects,surface modification,two-dimensional material terminated surfaces,and series of bulk materials of 3d-transition-metal carbides and early transition-metal carbides.The main research contents and conclusions are as follows:(1)Exploring the effect of TiC and its surface doping on catalytic activity.TiC is selected as cathode catalytic material of Li-O2 batteries that show high catalytic activity experimentally for theoretical research.Doping functionally with with B,N,Al,Si,and P and introducing more surface active sites on TiC.The result shows B doping improves the ORR catalytic activity of TiC,and its ORR overpotential is 0.49 V(TiC is 0.69 V).Considering the difference of electronegativity and atomic radius of doped atoms,the surface-doped TiC is divided into B,N and P doped TiC and Al,Si doped TiC.It is found that Al,Si doped TiC as positive catalysts for Li-O2 batteries have adverse effects.ORR and OER overpotentials have a negative linear relationship with(Li2O)2/Li2O2/LiO2 adsorption energy,respectively.ORR overpotential has a negative correlation with Li+ desorption energy,while OER overpotential has a positive linear relationship with O2 desorption energy,and its functional relationship is y=0.748x+0.005.(2)Exploring the effect of surface modification on catalytic activity of TiC.Referring the experimental results of TiC as a cathode material for Li-O2 batteries,an oxidized TiC(100)-O100%surface is constructed,in which O atoms combines with a C atom and two Ti atoms forming a three pyramid surface-modified structure on TiC(100)surface with a binding energy of-3.10 eV.The fully covered O layer is an electron-withdrawing layer with an average charge of-0.94 e.Oxidized TiC(100)-O100%surface has lower ORR,OER and total overpotentials than those of pure TiC(100)surface,respectively,showing better catalytic activity.Theoretically confirmed the conclusion of experimental studies on oxide layer upon TiC surface improves the performance of Li-O2 batteries.The mechanism is that the O layer promotes the transfer of electrons from LixO2(x=2,1,and 0)molecules to the substrate,and helps shorten the length the O-O bond,both of which accelerate the formation of O2.(3)Exploring the influence of two-dimensional TiC MXenes and its terminated surface on catalytic activity.Two-dimensional TiC MXenes:Ti2C and Ti3C2 are studied theoretically,which have an O-terminated surface and a larger specific surface area during the experimental preparation,according to the effect of surface oxide layer on the catalytic activity of TiC.The adsorption of H,O,F and OH groups on the surface of Ti2C,Ti3C2 MXenes shows that O atom has the strongest adsorption capacity,followed by F,OH and H.The catalytic trend is:Ti2CO2>Ti2CF2>Ti2C(OH)2>Ti2C;Ti3C202>Ti3C2F2>Ti3C2(OH)2>Ti3C2,which is consistent with the trend of adsorption energy.Among them,O-terminated Ti2C(Ti2CO2)has the best catalytic activity,with the lowest ORR and OER overpotentials of 0.10 and 0.16 V.respectively.And Ti-3d orbits near its Fermi level are fully polarized,resulting in the strong oxidation performance of oxidizing O22-.(4)Exploring the intrinsic properties of transition metal carbides that affect catalytic activity.Combined with the conclusions obtained from the case study of TiC system,the catalytic activities of 3d-transition-metal carbides(ScC,TiC,VC,CrC,MnC,FeC,CoC and NiC)and early transition-metal carbides(ScC,TiC,VC,ZrC,NbC,TaC,MoC and WC)of rock salt structure are studied systematically.Among them,TiC shows the best catalytic activity,and its ORR and OER overpotentials are 1.19 and 1.88 V,with moderate O binding energy,the lowest Li+ and O2 desorption energy.It is found that ORR overpotential and the adsorption energy of Li and LiO2,and OER overpotential and the adsorption energy of LiO2 have positive linear relationships.ORR overpotential has a positive linear relationship with surface energy.The ORR overpotential has a negative correlation with Li+desorption energy,while OER overpotential has a negative linear relationship with the desorption energy of O2,and its functional relationship is y=0.758x+0.021.In addition,the catalytic activity reaches its maximum when Fermi level happens to be in the pseudogap,corresponding to TiC.(5)Proposing design strategy insights of cathode catalysts for Li-O2 batteries is proposed.A comprehensive insight is proposed from the perspective of micro-mechanism of the rate-determining step,material intrinsic properties,and the electronic structure.The design of high-activity cathode catalysts for Li-O2 batteries should consider the following factors:high conductivity of materials,strong electron-withdrawing substrate;weaker reactive intermediate adsorption energy,higher material surface energy,moderate O atom binding energy,weaker O2 desorption energy;higher average d-band orbital energy,etc.,in order to provide useful theoretical references for efficient screening and development of cathode catalysts of Li-O2 batteries.