| Aprotic lithium-oxygen(Li-O2)batteries(LOBs)have emerged as promising alternative candidate for large-scale energy storage applications and practical electric vehicle by virtue of low cost,favorable environmental-friendness and high theoretical energy density.In 1996,the concept of secondary lithium-air battery was firstly proposed.Subsequently,lithium-air battery has raised extensive attention in the realm of energy storage over decades.However,there are still many key scientific issues remaining unresolved in the way to achieve large-scale commercial applications of lithium-oxygen batteries:including low activity of the electrocatalyst,large polarization,and poor cyclic stability.These problems can be attributed to the slow kinetics of oxygen reduction(ORR)and oxygen evolution(OER)reactions in lithium-air batteries during cycling.The introduction of high-efficient and stable electrocatalysts can significantly increase the rate of oxygen reduction and oxygen evolution reactions,thereby reducing the overpotential during cycling and improving the overall performance of LOB.Therefore,the preparation of cathode materials with high catalytic activity and structural stability is the key to the practical implementation of lithium-oxygen batteries.Transition metal elements(such as Co and Fe)have a variety of valence states,and the combination of two or more of which can form a large variety of oxides,beneficial for the regulation of electrochemical performance.According to this,Co/Fe-based oxides was utilized as the base material,and developed Co/Fe-based multimetal oxides through structural design and surface control,as a cathode catalyst in lithium-oxygen batteries.Besides,theoretical calculations were incorporated to danalyse the catalytic mechanism of the prepared catalysts in ORR and OER in details.The research content of this paper is as follows:(1)We prepared FeCo2O4/CeO2 composite with nano-heterojunction structure by simply constructing a metal-organic framework.FeCo2O4/CeO2 composite has a porous hexagonal three-dimensional structure.Furthermore,nano-scale heterojunction regions are formed between the CeO2 and FeCo2O4 interface,which exposes more ORR and OER active sites,and provide a convenient channel for fast electrons transmission and full infiltration of the electrolyte.When FeCo2O4/CeO2 composite served as the cathode of lithium-oxygen battery,the LOBs exhibit higher charge-discharge specific capacity and cycle stability.We attribute the high electrochemical performance of the composite material to the high catalytic activity of the spinel oxide FeCo2O4 and the high oxygen reaction efficiency brought by the CeO2 with aboundant oxygen vacancy.At the same time,the heterojunction structure and synergy between FeCo2O4 and CeO2 further improve the ORR/OER reaction kinetics in lithium-oxygen batteries.(2)We used the one-step solvothermal method to successfully synthesize trimetallic CoFeCe oxide composite with amorphous/crystalline heterostructure on carbon paper,which was used as a binder-free integrated positive electrode for lithium-oxygen batteries.We found that the CeO2-CoO heterostructure formed during the preparation process of trimetallic oxide has a small lattice mismatch,which is beneficial to reduce the energy barrier of charge transfer during the catalytic reaction.At the same time,the special structure of the coexistence of amorphous and crystalline can expose more active sites.Therefore,this composite electrode has high electrochemical activity in lithium-oxygen batteries,showing very low overpotential(0.95 V),high discharge specific capacity(12280 mAh·g-1)and excellent cycle stability(2900 h).Through density functional theory(DFT)calculations,we found that there is a strong interaction and synergistic effect between Fe2O3,CoO and CeO2,which can significantly enhance the adsorption of composite to LiO2,and promote the corresponding decomposition of Li2O2 with an amorphous sheet structure.(3)Replace Ce element with Ni to prepare self-supporting CoFeNi oxide electrode.The integrated positive electrode has a rich fold structure and uneven nano-particle morphology,which is beneficial to the rapid transmission of active materials and can ensure the complete penetration of the electrolyte and the full storage of Li2O2.Besides,there is a strong interaction among the Co-based,Fe-based and Ni-based oxides,including the regulation of surface electrons and stable synergistic effects.Moreover,the introduction of highly conductive nickel oxide(NiO)has a prominent role in promoting the complete decomposition of the discharge product Li2O2.As a result,trimetallic CoFeNi oxide-based lithium-oxygen batteries could deliver lower charge/discharge overpotential(1.25 V),high discharge specific capacity(8170 mAh·g-1),excellent rate performance and cycle stability(2000 h).(4)With Mo as the third element,we prepared a trimetallic CoFeMo oxide with amorphous and uniform dispersion structure.The trimetallic CoMnMo oxide was prepared with Mn instead of Fe.By comparing the differences in the structure and electrochemical properties between CoMnMo oxide and CoFeMo oxide,we discussed the influence of the addition of different third metal elements on trimetallic oxide in detail.TEM and XPS results show that the presence of Fe can effectively increase the oxygen defects and vacancies of the composite and optimize the composite’s oxygen reaction(ORR/OER)activity.Besides,the introduction of Mn could strengthen the chemical coupling effect between the oxide components,increases the relative content of Co2+,and exhibits higher catalytic activity.Through the electrochemical performance analysis of the lithium-oxygen battery,it is further confirmed that the catalytic performance of the CoMnMo electrode is better than that of the CoFeMo electrode,and the CoMnMo-based battery exhibits a higher full charge-discharge specific capacity,excellent cycle stability and high rate performance.The investigation on the structure of Co/Fe-based trimetallic oxides and their applicataion in LOBs indicate that the introduction of the third metal element can effectively adjust the electronic structure,thereby regulating the adsorption and desorptionof the intermediates on the surface of catalyst,essential in improving the electrocatalytic performance of trimetallic oxides. |
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