Preparation Of Porous Cobalt Cobalt Iron Non-carbon Positive Electrode And Its Application In Lithium Air Battery

Lithium-air?Li-air?batteries have recently drawn considerable attention due to their ultrahigh specific energy.However,Li-air battery is far from practical application because its large charge and discharge overvoltages led to poor cycle performance and rate performance.All of these are closely related to the passivation of oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?processes in the air cathode.Optimizing the structure of air electrode combined with stable catalyst is an effective way to promote the ORR and OER processes of Li-air battery.In this article,to avoid the drawbacks of carbon cathodes,ferrum cobaltate?FeCo₂O₄?sheets were in situ grown on the Ni-foam to construct hierarchical macroporous/mesoporous carbon-free FeCo₂O₄@Ni cathodes for Li-air batteries.The macropores provided storage space for discharge products Li2O2 and diffusion channels for O2,while mesopores on FeCo₂O₄ sheets supplied adequate catalytic active sites for ORR/OER.Then noble metal gold?Au?were used to modify FeCo₂O₄@Ni and prepared a new type of Au/FeCo₂O₄@Ni air electrodes,to further reduce the charge and discharge overvoltages and promote cycle performances of Li-air cells.The performances of the two kinds of carbon-free cathodes are shown as follows.?1?Carbon-free FeCo₂O₄@Ni cathodes were prepared by hydrothermal method and heat treatment,then morphology and structure of hierarchical porous FeCo₂O₄ were adjusted by varying hydrothermal solvent and calcination temperature?300-400 ??.The physical and chemical properties of the materials were characterized by SEM,BET,XRD etc,and 2032 coin-type cells were used for assembling the Li-air batteries consisting of FeCo₂O₄@Ni air cathodes,following determined by a variety of electrochemical performance tests.The results showed that ethylene glycol could reduce the stack of FeCo₂O₄ sheets and thus prevent the agglomeration phenomenon;the FeCo₂O₄@Ni air cathode after 350 ? calcination had a larger specific surface area and maximum pore volume,as well as good crystallization.And it presented the maximum discharge capacity and the optimal discharge and charge voltage plateaus.Therefore,the hydrothermal solvent containing ethylene glycol and calcination temperature of 350 ? were confirmed as the optimum conditions for the preparation of hierarchical macroporous/mesoporous FeCo₂O₄@Ni,and subsequent FeCo₂O₄@Ni were prepared under the same conditions.?2?The application of carbon-free FeCo₂O₄@Ni cathodes in the Li-air batteries was researched deeply.The three-dimensional macroporous/mesoporous structure of FeCo₂O₄@Ni and the oxygen vacancies on FeCo₂O₄ sheets were further confirmed by TEM and XPS tests.After comparing the electrochemical properties between the carbon-free FeCo₂O₄@Ni cathode and the carbon-based KB/FeCo₂O₄/Ni cathode,it was found that the overvoltages of the former was lower than that of the latter during charge and discharge,and cycle times of the former were more than that of the latter.All of these fully demonstrated that FeCo₂O₄@Ni played a role in promoting the processes of ORR and OER.By characterization of SEM and XRD,it was found that discharge products on the FeCo₂O₄@Ni electrode were mainly amorphous low-dimensional Li2O2.And amorphous Li2O2 could be easily decomposed in the charging process by comparing the crystalline Li2O2.Although the Fe Co₂O₄@Ni relieved the passivation of air electrode to some extent,the discharge voltages of cells dropped quickly during the circulation due to the poor conductivity of the carbon-free FeCo₂O₄@Ni cathode,and its electrical conductivity is needed to be further improved.?3?FeCo₂O₄@Ni was immersed in chloroauric acid solution to form novel composite Au/FeCo₂O₄@Ni air electrode.The successful adding of Au and the retention of FeCo₂O₄ structure were confirmed by the characterizations of SEM and XRD as well as XPS.In addition,the adding of noble metal Au could improve the conductivity of the air electrode,so as to stabilize the electrode discharge voltage.Au/FeCo₂O₄@Ni electrode significantly reduced the overvoltages of the Li-air cells and improved the cycle performance during the later period of circulation.In summary,two types of carbon-free cathodes with FeCo₂O₄ as main catalysts were synthesized successfully.Their catalytic mechanism and electrochemical performance in the Li-air battery were analysed systematically.It is confirmed that compared with the corresponding carbon-based cathode,both the carbon-free cathodes effectively lowered the charge and discharge overvoltages and improve the cycle performance of the battery.