Study Of Reaction Mechanism On Boron-and Nitrogen-doped Graphene As Cathode Materials For Lithium-oxygen Batteries

Secondary rechargeable nonaqueous lithium-oxygen battery(Li-O2)has attracted wide attention because of its high theoretical energy density,which is an ideal energy storage device for the electric vehicles.However,the development of Li-O2 battery is still in its initial stage,and there are many problems to be solved.Of those,the slow dynamic process of Li-O2 battery cathode is one of the problems to be faced.This problem involves two basic reactions,namely oxygen reduction reaction(ORR)and oxygen evolution reaction(OER).In order to address this problem,we must have a clear understanding of the reaction mechanism and therefore the ORR and OER of Li-O2 battery have been studied in this paper.First,we studied the reaction mechanism of ORR on the nitrogen-doped graphene by the first-principles method.Results show that upon the doping of nitrogen,the adsorption energy of oxygen molecules are obviously enhanced,and the 0-0 bond length becomes longer,indicating that N-doped graphene can promote the ORR.An analysis of Bader charge shows that both the doped nitrogen atom and the adsorbed oxygen molecule obtain charges from the carbon atom,and N-doped graphene can make oxygen molecule obtain more charges.Comparing with different nitrogen doping,it is found that 3.13 at%is the best nitrogen doping concentration.Secondly,the structure and electronic properties of small cluster(Li2O2)nm+(n=1,m=0,1 and n=2,m=0,1,2)were studied systematically by the first-principles method,and the reaction energies of various dissociation processes were also calculated Results show that the ground state structure of(Li2O2)1 is a low spin state,while the ground state structure of(Li2O2)2 is a high spin state.The analysis of bond length,molecular orbital and projected density of states shows that the interaction of 0-0 is stronger in cationic clusters than that in neutral clusters.Instead,the interaction of Li-0 is weaker in cationic clusters than that in neutral clusters.These indicate that Li2O2 cationic clusters are easier to be decomposed.In addition,the calculated reaction energies show that small Li2O2 clusters prefer two-step dissociation rather than one-step dissociation.Finally,the dissociation pathway of(Li2O2)2 is predicted:(Li2O2)2→Li2O2→(Li2O2)+→LiO2→O2.Finally,the dissociation process of the neutral molecule and monovalent cation[(Li2O2)0,+]on the intrinsic graphene and boron-doped graphene was studied systematically from the reaction thermodynamics and kinetics by the first-principles calculation method.Results show that(Li2O2)0,+ likes to two-step dissociation on the intrinsic graphene and boron-doped graphene with lithium superoxide as the intermediate product.Boron-doped graphene can enhance the dissociation of(Li2O2)0,+.The dissociation of(Li2O2)+ is easier than(Li2O2)0.Oxygen evolution process is the rate-determining step for(Li2O2+)G system,while lithium removal process are the rate-determining step for(Li2O20)G,(Li2O20)BG and(Li2O2+)BG systems.