Research On Lanthanum Cobalt Perovskite Oxide As Cathode Catalyst For Lithium Air Battery

With the increasing threat from environmental pollution and greenhouse effect,searching for new clean and rechargeable energy has been our research urgency.Making Electric Vehicles?EVs?to replace conventional fuel vehicles is an effective way to release the environmental problems.Lithium air battery,as a new type of energy storage device,with high energy density,is considered to be the most promising device in the future Electric Vehicles field.Among four types of lithium air batteries,non-aqueous system has been the research hotspot due to its high energy density,high reversibility and simple structure.However,the insoluble products will passivate the porous electrode and hinder the reaction.Therefore,to develop a bi-functional catalyst for both oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?is the key to solve the above problems.With remarkably lower cost,high conductivity and adequate catalytic performance,perovskite-type ABO3 oxides have been considered as the most promising alternatives to noble metals in various catalytic reactions.Our research will focus on the LaxSr1-xCoO3 series of perovskite oxides,which are used as cathode catalysts in nonaqueous lithium air batteries?LABs?.This paper discusses about three aspects,including the selection of catalysts,the discussion about catalytic mechanism and further optimization of these catalysts.?1?The selection of perovskite catalysts.The perovskite oxide La_0.6Sr0.4CoO3?LSC?together with Fe and Mn doped materials La_0.6Sr_0.4Co_0.2Fe0.8O3?LSCF?and La_0.6Sr_0.4Co_0.2Fe0.8O3?LSCM?are prepared by sol-gel method and applied as catalysts for LABs.The cycle performances are evaluated by two test molds and the best catalyst La_0.6Sr_0.4Co_0.2Fe0.8O3 is selected.At the same time,we also select other two kinds of perovskite catalysts La_0.6Sr_0.4Co_0.2Fe0.8O3 and LaO.8Sr0.2Co0.8Ni0.2O3 through the same method.?2?The discussion about catalytic mechanism.First,the effects of Mn and Fe doped La_0.6Sr0.4CoO3 on the performance of LABs are analyzed in depth,the optimization mechanism of two kinds of element doping is proposed and the advantages of Fe doping are highlighted.Both of the two elements can cause lattice distortion and increase the particle transport channels involved in the redox reaction.Among them,the Mn doped LSC can produce excess electrons and increase the conductivity of the crystal.However,Fe doping will lead to the coexistence of trivalent and divalent Fe,which tends to increase the oxygen vacancies in the crystal and form the redox pair.Moreover,Fe doped LSC can cause lower sintering temperature and refining grain.And then the reaction mechanism of LSCF applid in LABs is studied from the point of view of product morphology.When LABs are tested in oxygen atmosphere,LSCF catalyst can increase the nucleation sites,provide more oxygen transport channels and reshape the morphology of discharge products.On the surface of LSCF-KB electrode,the Li₂O₂ products show tiny leaf-like morphology with small size and high specific surface area,which can improve the conductivity of Li₂O₂ and promote the decomposition of Li₂O₂,resulting in weaker polarization and better performances of LABs.When LABs are tested in ambient air,LSCF can enhance the formation of the main discharge product Li₂O₂ and reduce the formation of by-products Li2CO3 and LiOH.And LSCF can also promote the decomposition of the products in the charging process.?2?Further optimization of these catalysts.In order to achieve better catalytic effect,three different material optimization methods are applied for the three selected catalysts based on the above mechanism analysis results.First,a composite catalyst consist of perovskite nanoparticles La0.8Sr0.2Co0.8Fe0.2O3 and nano layered nitrogen doped reduced graphene oxide?N-rGO?was prepared by hydrothermal treatment.The results show that the LSCF@N-rGO composite can be used as a bifunctional cathode material for LABs.The morphology analysis shows that the unique flower-like Li₂O₂ appears on the surface of LSCF@N-rGO composite cathode.LSCF can not only improve the formation of flower-like Li₂O₂ with higher specific surface area but also can be used as their centers which can promote their decomposition,resulting in weaker polarization and better performances of LABs.Then,based on the lower sintering temperature of La_0.6Sr_0.4Co_0.2Fe0.8O3,3D-LSCF nano powders with three-dimensional porous structure are synthesized by polystyrene microspheres templates method.When applied as cathode catalyst of LABs,3D-LSCF catalyst can significantly improve the discharge-discharge capacity,cycle stability,coulombic efficiency and reduce the overpotential.And the catalytic performance of 3D-LSCF is much better than LSCF obtained by sol-gel method.B site cations doping of ABO3 has an important influence on the catalyst oxygen vacancy concentration,which is crucial for LABs.Due to the small size difference between nickel and cobalt ions,it is easier to adjust the cation ratio of them in LaO.8Sr0.2Co0.8Ni0.2O3 crystal structure.Thus,three nano catalysts LaO.8Sr0.2Co0.8Ni0.2O3,LaO.8Sr0.2Co0.4Ni_0.6O₃ and LaO.8Sr0.2Co0.4Ni_0.6O₃ are prepared by sol-gel method.All of them have uniform nano pore structures.The pores and oxygen vacancies of LaO.8Sr0.2Co0.4Ni_0.6O₃ are the most abundant.These three catalysts all can improve the capacity and cycle stability of LABs and the catalytic effect of LaO.8Sr0.2Co0.4Ni_0.6O₃ is the best.