The Application Of Synergistic Effect By Single-Atom Catalysts And Redox Mediator In Lithium-Oxygen Batteries

Lithium-Oxygen(Li-O2)batteries utilize lithium as anode and oxygen as active material of cathode.The theoretical energy density is as high as 3456 Wh/kg,which is the electrode materials combination with the highest theoretical energy density in the currently known electrochemical systems.However,lithium-oxygen batteries suffer problems such as high overpotential,low energy efficiency,poor cycling stability of the batteries,and low actual discharge capacity.In this thesis,we explore efficient single-atom catalysts to promote the kinetic process of lithium-oxygen batteries,reduce the reaction overpotential,and promote the formation of lithium peroxide in solution phase.The synergistic effects of redox mediator and single-atom catalytic materials in lithium-oxygen batteries are further explored.The main research contents are:1.The exploration of single-atom cobalt anchored in porous N-doped hollow carbon spheres(CoSAs-NHCS)as cathode catalyst for lithium-oxygen batteries.The hollow carbon spheres were prepared by the silica template method,and the single-atom cobalt anchored in porous N-doped hollow carbon spheres cathode materials were successfully prepared through ammonia reduction,freeze-drying and other treatments.As the catalytic active sites of the reaction,the single-atom Co can effectively catalyze the charge-discharge reaction of lithium-oxygen batteries.Compared with the batteries utilizing Super P(SP)as cathode under the test conditions of 500 mA/g and 1000 mAh/g,the overpotential of the reaction is reduced from 1.39 V to 0.74 V.The morphology of the cathode after discharge is observed,and it is found that the discharge product is in the form of toroid morphology,indicating that the CoSAs-NHCS material can promote the formation of lithium peroxide in the solution phase.Under the test conditions of 200 mA/g and 400 mAh/g,the CoSAs-NHCS | Li batteries can stably cycle for 60 cycles.2.The study of the synergistic effect of the redox mediator LiBr and single-atom cobalt anchored in porous N-doped hollow carbon spheres materials in lithium-oxygen batteries.The single-atom catalysts can only catalyze the decomposition of lithium peroxide directly deposited on their surface,and it is difficult to catalyze the decomposition of lithium peroxide that is not in direct contact with them,resulting in a decrease in the cycling stability of batteries.LiBr is added as redox mediator in the charging process.During the charging process,Br-is oxidized to Br2 and Br3-,and reacts with lithium peroxide to further promote the decomposition of lithium peroxide.At the same time,the CoSAs-NHCS cathode has a certain adsorption effect on Br2 and Br3-,which can slow down the shuttle effect of Br2 and Br3-.Under the test conditions of 500 mA/g and 1000 mAh/g,the redox mediator LiBr and the single-atom cobalt anchored in porous N-doped hollow carbon spheres materials cooperate to further reduce the overpotential of the lithium-oxygen batteries to 0.69 V.Under the test conditions of 200 mA/g and 400 mAh/g,the batteries can stably cycle for 250 cycles.