Electrochemical Performance Of Graphene And Perovskite For Li-Air Battery

In order to solve the serious problem of energy shortage in the future,human has to search for renewable energy to replace conventional fossil.In the past couple of years,Li-O2 battery has drawn a great research interest because of its extremely high theoretical energy density(e.g.,11400 Wh Kg-1).However,Li-O2 batteries suffer some issues such as low coulomb efficiency,poor rate performance,poor cycle performance and so on,and the slow kinetics of oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)cause excessive overpotentials,which affect specific capacity and cycle life.In this paper,we prepare an air electrode with graphene as the carbon carrier and La0.5Sr0.5CoO3-?(LSCO)as the catalyst.The LSCO and modified-LSCO were studied for their influence on the electrochemical performance of air electoedes,and the reaction mechanism of Li-O2 batteries was also studied.In this work,we had synthesized the graphite oxide(GO)by modified Hummers method followed with preparing the microwave expanded grapheme(MEGO)by microwave expansion and simultaneous reduction of GO.The structures and morphologies of GO and MEGO were characterized by XRD,Raman and SEM.The results reveal that GO was reduced to the plicated layer of MEGO nanosheet through microwave method.The electrochemical testing results concluded that under the conditions of voltage range from 2.0 to 4.5 V and current density of 200 mAh g-1,the discharge capacity of pure MEGO electrode was 20793 mAh g-1 with the coulombic efficiency of 75%and the charge-discharge voltage difference of 1.66 V.At the current density of 200 mAh g-1 with the limited capacity of 1000 mAh g-1,the cycle life of pure MEGO electrode were 22 cycles.Meanwhile,we used the nitrate as the source of metal ions and urea as a mineralizer to obtain the homogeneous hydroxide,which was formed by the combine of hydroxyl from gradually decomposed urea and metal ions during the microwave heating process.Then the hydroxide was annealed at 800 ? in an argon atmosphere to obtain the LSCO.The structures and morphologies of LSCO was characterized by TG,XRD,and SEM.The results reveal that the pure and single phase LSCO with the microstructure of 150 nm diameter nano-particles could be obtained by calcining the LSCO precursor at 800 ?.LSCO/MEGO electrode was prepared by mixing MEGO,LSCO and binder with the ratio of 6:3:1.The electrochemical testing results concluded that under the conditions of voltage range from 2.0 to 4.5 V and current density of 200 mAh g-1,the discharge capacity of LSCO/MEGO electrode was 24354 mAh g-1 with the coulombic efficiency of 80%and the charge-discharge voltage difference of 1.39 V.At the current density of 200 mAh g-1 with the limited capacity of 1000 mAh g-1,the cycle life of pure MEGO electrode were 33 cycles.Compared with pure MEGO electrode,LSCO/MEGO electrode exhibits excellent charge-discharge performance and cycling capacity,which was attributed to the addition of the catalyst LSCO to improve the ORR and OER reaction efficiency of the electrode.The XRD and alternating-current impedance results under different status during charge-discharge process have indicated that the main reaction of Li-O2 batteries during discharge process was the formation of Li2O2 while the decomposition of Li2O2 became the main reaction during charge process.It is the fact that the conductivity of electrode decreased because of the accumulation of Li2O2 on the electrode surface during discharge and it would increase again due to the decomposition of Li2O2 during charge.Howevre,the unstable characteristic of Co would cause the promotion of cycle life was not so satisfying.It was necessary to modify the LCSO for improved performance.In order to further improve the electrocatalytic performance of LSCO,the ions of Co site(B site)were partially replaced with 10%of Fe or Ni to obtain La0.5Sr0.5Co0.9Fe0.1O3-?(LSCFO)and La0.5Sr0.5Co0.9Ni0.1O3-?(LSCNO).The preparation method is similar to that of LSCO.The XRD and SEM test results show that:10%of Fe or Ni doped LSCO showed a single-phase material with the uniform nanoparticles of 50-100 nm,but it was a tendency to agglomeration for LSCFO.The electrochemical test results show that under the current density of 200 mA g-1 with the voltage range from 2.0 to 4.5 V,the discharge capacity of LSCFO/MEGO electrode was 25344 mAh g-1 with the coulombic efficiency of 81%and the charge-discharge voltage difference of 1.37 V.At the current density of 200 mAh g-1 with the limited capacity of 1000 mAh g-1,the cycle life of LSCFO/MEGO electrode was 35 cycles.Compared with LSCO/MEGO electrode,the electrochemical performance of LSCFO/MEGO electrode did not increase much,which was attributed to that the catalytic activity of Fe was not completely effective.The electrochemical test results show that under the current density of 200 mA g-1 with the voltage range from 2.0 to 4.5 V,the discharge capacity of LSCNO/MEGO electrode was 27900 mAh g-1 with the coulombic efficiency of 84%and the charge-discharge voltage difference of 1.32 V.At the current density of 200 mAh g-1 with the limited capacity of 1000 mAh g'1,the cycle life of LSCFO/MEGO electrode was 43 cycles.Compared with LSCO/MEGO electroed,the electrochemical performance of LSCNO/MEGO electrode was obviously improved,which was mainly due to the increase of oxygen vacancy causing by the doped Ni would lead to the improve of ORR and OER catalytic activity.