First-principles Study Of The Initial Oxygen Reduction Reaction On The CeO₂ Surfaces As Cathode Catalyst For Lithium-oxygen Batteries

To cope with the significant greenhouse effect caused by the global emission of CO₂,a transition of key energy sources from fossil fuel to the renewable enery is urgently required.Since the output of renewable energy is unstable,in the development of renewable energy technologies,high energy density batteries play critical roles.With their theoretically specific energy density comparable to that of gasoline,lithium-oxygen batteries?LOBs?have received extensive attention,although there are still many challenges to be solved,such as large overpotential and poor cyclability.As electro-catalyst at the cathode of lithium-oxygen batteries,CeO₂ has explored good performance in both oxygen reduction reaction and oxygen evolution reaction,especially for the beginning process of discharging.In order to get the clear picture of the initial oxygen reduction reaction,in this work,the oxygen adsorption,lithium adsorption and initial oxygen reduction reaction?ORR?at stoichiometric and reduced CeO₂?111?surfaces are systematically investigated by using the density functional theory?DFT?calculations.Due to the CeO₂'s feature that lattice oxygen are easily to escape,3 different CeO₂surface?Stoichiometric ST,Surface oxygen vacancy SOV and Subsurface oxygen vacancy SSOV?have been built in this study.The adsorption of oxygen molecule and lithium atom on these surfaces have been investigated also.The results show that the oxygen adsorption on ST and SSOV surfaces are weak adsorption,while it is strong adsorption on SOV surface;Lithium adsorption are strong on all these surfaces,and lead to a nearest or next-nearest Ce⁴⁺be reduced to be Ce³⁺.It is also shown that the most strong adsorption are accompanied with the Ce³⁺located at the next-nearest of adsorption site.What's more,the density of states?DOS?has proven the reduction of Ce⁴⁺also.According to the different oxygen and lithium adsorption on these surfaces,the possible path,intermediates and precursors of initial ORR have been investigated also.The results show that SOV CeO₂?111?exhibited the best catalyze ability.A Li₃O₂ precursor is the most probable initial reaction structure for catalyze the discharging process.The most possible reaction should be:At the beginning of discharging,a lot of Li₂O₂ and Li₃O₂clusters are generated on the SOV CeO₂?111?surfaces.Due to the special structure of Li₃O₂and conductive nature of the cathode,after all the surface vacancies have been filled by Li₂O₂ or Li₃O₂ clusters,the Li₂O₂ crystal prefers to grow on the top and edge of these clusters until all the cathode surfaces have been covered.This could explain the key reason why CeO₂ firstly formed on the surfaces of CeO₂ nanoparticles,instead of on N-RGO in experments.