The Study And Modification Of La_0.6Sr_0.4CoO_3-? Perovskite Cathode Catalyst For Li-O₂ Batteries

As the energy crisis and environmental issue are getting worse and worse,searching new clean and reproducible power source to satisfy the requirement of social development has become a significant issue.Li-air battery is considered as a potential substitute for gasoline to become the new power of vehicle due to its ultrahigh theoretical specific energy(11140 Wh kg^-1).However,the reaction efficiency of cathode for Li-air battery is unsatisfactory and it suffers some issues such as high overpotential,low practical capacity,low cycle life and so on.Recent years,researchers try to find a suitable catalyst to solve the mentioned problems,but they have failed to found a perfect catalyst to overcome all the disadvantages of Li-air batteries.There are many kinds of catalyst for Li-air battery and the La_0.6Sr_0.4CoO_3-??LSC?was used as the perovskite catalyst in this paper.The LSC and doped-LSC were studied for their influences on the catalytic mechanism of cathode reactions.The reaction mechanism of catalyst was aslo studied.The catalyst material was prepared by sol-gel method in this work.The details of technology were given as follows:the stoichiometric ratio of various nitrate were homogeneous mixed and dried to obtain the dry gel.Then the dry gel was burned at500°C for 15 min followed by being calcined in muffle furnace at 900°C for 6 h with air atmosphere.Subsequently,LSC nanopowders can be obtained.Ni-doped or Mn-doped LSC samples were produced by the same synthetic process.The prepared LSC and doped-LSC samples were measured by X-Ray Diffraction?XRD?,Thermogravimetric-Differential Scanning Calorimeter?TG-DSC?,Scanning Electron Microscope?SEM?,X-ray Photoelectron Spectroscopy?XPS?and Brunauer-Emmett-Teller?BET?.XRD results showed the pure LSC could be abtained by sol-gel method.If the doping content was no more than 10%,the samples displayed the pure phase and those samples with more than 10%doping content contained some oxide phase.SEM showed the surface of LSC samples displayed a porous structure with the average grain size of 100 nm.The grain size was hardly change with doping.XPS results revealed that the adsorbed oxygen of LSC samples on the surface increased with the Ni doping.BET results demonstrated the specific surface area,average pore size and pore volume of LSC samples were 4.59 m2g^-1,12.36 nm,and 0.0142 cm3g^-1 respectively.The ratio and homogeneity of constituents on Li-O₂ battery cathode would influence the electrode's performance.The preparation method of cathode used in this paper was given as follows.Before the cathode ink was coated on Ni mesh to obtain the cathode of Li-O₂ battery,the catalyst,carbon supporter and binder were homogeneous mix by ultrasonic agitation for several hours.The mass ratio of catalyst:carbon:binder was adjust to 27:63:10.For fabricating a battery,the bottom current collector,Lithium wafers,one layer of glass fibre,cathode mesh and the top current collector were stacked layer by layer.Then they were placed in battery testing mould.1 M solution of LiTFSI?bis?trifluoromethane?sulfonimide lithium salt?in Tetramethylene sulfone was used as the electrolyte.The electrochemical performance testing of fabricated Li-O₂ batteries would be carried out.The test objects included La_0.6Sr_0.4Co_1-xM_xO_3-?/Super P?M=Ni,Mn x=0,0.5,0.1?and pure Super P electrode.The test items contained Linear Sweep/Cyclic Voltammetry Curve,Charge-Discharge Performance,Cycling Performance and Alternating-Current Impedance.The efficiency of oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?for the electrode loaded with catalyst have been improved.Compared with the pure Super P electroded,the La_0.6Sr_0.4CoO_3-?/Super P cathode possessed higher initial discharge capacity(4420 mAh g^-1)and longer cycling life?25 times?.Both the current density and discharge depth can affect cycling performance,especially the latter.Because the increase of current density and discharge depth would lead to the charge transfer resistance of electrode increased faster and conductivity of electrode decreased faster.The rate capability of electrode was associated with the catalytic rate.The main valence state of Ni in doping samples was+2,so it belonged to the low state doping compared with Co and make the oxygen vacancy of material increase leading to the improve of ORR and OER catalytic activity.The research results indicated that the catalytic activity and initial discharge capacities of electrodes were enhanced in the following order:pure Super P<La_0.6Sr_0.4CoO_3-?<La_0.6Sr_0.4Co_0.95Ni_0.05O_3-?<La_0.6Sr_0.4Co_0.9Ni_0.1O_3-?.The rate capability of La_0.6Sr_0.4Co_0.9Ni_0.1O_3-?/Super P electrode was better than that of La_0.6Sr_0.4CoO_3-?/Super P electrode.In the same circulation condition,the cycle life of La_0.6Sr_0.4Co_0.9Ni_0.1O_3-?/Super P electrode?33 cycles?was higher than that of La_0.6Sr_0.4CoO_3-?/Super P?25 cycles?and pure Super P electrodes?13cycles?.The cycling performance of La_0.6Sr_0.4 Co_0.9Ni_0.1O_3-?/Super P electrode carried out under different conditions revealed that the cycle of La_0.6Sr_0.4 Co_0.9Ni_0.1O_3-?/Super P electrode maintained 33,14,25 and 8 times corresponding to different limiting conditions of 100-500?100-800?200-500 and 200-800(mA g^-1-mAh g^-1)respectively.Compared with La_0.6Sr_0.4CoO_3-?/Super P electrode,10%Ni doped electrode had an improved rate capability,a longer cycle life,a lower growing rate of overpotential and a better capacity retention ability.The cycle life of electrode depended on many factors,in which the stability of electrode during work was one of the significant factor.Due to the more stable of Mn compared with Co,the stability of material can be improved by doping Mn to make sure the high efficiency catalysis of catalyst during long time.Then the cycle performance of electrode could be enhanced.The research results revealed that the catalytic activity and initial discharge capacities of electrodes were enhance in the following order:pure Super P<La_0.6Sr_0.4CoO_3-?<La_0.6Sr_0.4Co_0.95Mn_0.05O_3-?<La_0.6Sr_0.4Co_0.9Mn_0.1O_3-?.Although the rate capability of La_0.6Sr_0.4Co_0.9Mn_0.1O_3-?/Super P electrode hardly changed,the cycling stability of La_0.6Sr_0.4Co_0.9Mn_0.1O_3-?/Super P electrode increased.In the same circulation condition,the cycle life of La_0.6Sr_0.4Co_0.9Mn_0.1O_3-?/Super P electrode?53 cycles?was higher than that of La_0.6Sr_0.4CoO_3-?/Super P?25 cycles?and pure Super P electrodes?13 cycles?.The cycling performance of La_0.6Sr_0.4 Co_0.9Mn_0.1O_3-?/Super P electrode carried out under different conditions revealed that the cycle of La_0.6Sr_0.4 Co_0.9Mn_0.1O_3-?/Super P electrode maintained 53,16,37 and 8 times corresponding to different limiting conditions of 100-500?100-800?200-500 and 200-800(mA g^-1-mAh g^-1)respectively.Compared with La_0.6Sr_0.4CoO_3-?/Super P electrode,10%Mn doped electrode had an improved cycle life,a lower growing rate of overpotential and a better capacity retention ability.LSC was a cathode catalyst candidate with good catalytic performance for Li-O₂batteries.The oxygen vacancy of materials was imporoved by doping Ni,so the rate capability was enhanced.In addition,the stability of materials increased by doping Mn,so the cycle performance was enhanced.