First-principles Study Of TiO₂ And CeO₂ As A Cathode Catalyst For Lithium-oxygen Batteries

Lithium-oxygen batteries?LOB?have attracted wide attention from researchers because of its high specific energy,but their commercialization process still faces some key problems to be solved.One of the most important issues for non-aqueous LOB is that the slow electrochemical kinetics of the positive electrode which results in poor rate capability,poor rate efficiency,poor cycle stability and so on.A new efficient catalyst is an effective way to solve this problem.In this paper,the transition metal elements were doped into TiO₂ and CeO₂ as catalytic materials for LOB.The objective is to explore the intrinsic factors of the doping modification affecting the catalytic performance.TiO₂ and CeO₂ with high performance for redox reaction were widely used in the field of industrial catalysis.In this work,through the establishment of TiO₂?101?and CeO₂?111?models,the path of surface reaction products is studied and the overpotentials during charging and discharging are calculated,which was used to compare the catalytic performance.The material is then doped with transition metal elements.The essential factors of doping-induced catalysis were investigated by calculating the changes in various properties of the system before and after doping.The general idea of this paper is that TiO₂ was first selected as the research object,and initially obtain the factors of affecting the catalytic performance,checking with the published experimental data and then study CeO₂ to further verify and improve the conclusions.TiO₂ is used as a cathode catalyst for lithium-oxygen batteries.It is found that the surface before and after doping can form structure with nearly diamond of Li₂O₂.The overpotentials during charging and discharging were used to compare the catalytic performance before and after doping.It can be concluded that the Mn doping into TiO₂?101?has a reducing effect on the overpotential during charge and discharge.The intrinsic factor of the doping-promoting catalytic reaction is the distribution d-state orbital of the doping atom and its average energy.From the distribution of the d-state orbital,the modality of the d-state orbit of the doping atom at the Fermi level induces the p-state orbital of O.The d-state orbital of the doped atom and the p-state orbital of the oxygen atom work together to form a plurality of new peaks at the Fermi level of the total density of states of Mn_xTi_1-xO₂?101?,improving the electron transport performance.From the average energy of the d-state orbital,it is found that the average energy of the d-state orbital of the doped atom Mn is higher than that of the Ti atom.The calculation of the oxygen vacancy formation near the Mn-doped atom can be found to be lower than that of the undoped region,which also confirm our results.The presence of oxygen vacancy defect also provides more active sites for oxygen reduction of the cathodic catalytic reaction during the discharge process and facilitates the adsorption and reduction of oxygen.Next,we chose CeO₂ as the cathode catalyst for LOB to further enrich and verify the conclusions obtained from TiO₂.By doping Rh,Pt,Ag into the CeO₂ and calculating the overpotential of the battery under each system,it can be obtained that Rh,Pt,Ag doping into CeO₂?111?has a positive effect on the overpotential of the battery.Through further research,it is found that the essential factor of the system is due to the distribution and average energy of the d-state orbital of the doped atoms,which is consistent with the conclusions of TiO₂.From the energy of the d-state orbital,if the doping atom's average energy of d-state orbital is higher,the overpotential of the battery is lower.The higher the average energy of the d-state orbital of the doped atoms,the stronger the p-state orbital excitation of the oxygen atoms near the doping sites,and the doping atoms will form active sites on the catalyst surface.From the distribution of the d-state orbital,we can get the same conclusion as that of TiO₂.Finally,by performing reverse calibration with Zn and Cu doping into CeO₂,the following conclusions can be summarized:two main determinants of the transition metal element doping TiO₂/CeO₂ to improve the catalytic performance:1)the doping atom forms a new induced peak at the Fermi level;2)under the condition of 1)the higher the d-state orbital energy of the doping atoms,the better the catalytic performance.Theoretical research is expected to provide deeper insights into the exploration of better catalysts.