Manganese Oxides:Fabrication And Their Catalytic Mechanism For Lithium-Oxygen Batteries

Lithium-oxygen battery(LOB)takes lithium metal as an anode and oxygen as a cathode reactant.And its theoretical energy density can be comparable to that of fuel,which is expected to be one of the most potential options for new energy storage systems.However,the discharge-charge reactions of LOB are heterogeneous,and the main discharge product of Li2O2 in the organic electrolyte is insoluble and non-conductive,inducing the polarization and the termination of reaction.Above problems have severe influences on the electrochemical performance of LOB,such as energy conversion efficiency,discharge capacity and cycle life,which lead to that they cannot meet the requirements of practical applications.In order to further improve the electrochemical performance and promote its practical application of LOB,it is necessary to construct the cathode catalyst materials with low cost,high electrocatalytic activity and abundant pore structure.Transition-metal based material of manganese oxides has abundant reserves in earth and show a decent intrinsic catalytic activity.Moreover,manganese oxides with multiple oxidation states and crystal phase structures are highly designable.Hence,they show great application potential in the field of catalysis.Therefore,our researches base on the controllable modification of manganese oxides,focusing on construction of pore structure of catalyst material,regulation of catalytic active sites,and formation mechanism of discharge product.As a result,as-prepared manganese oxide cathodes for LOBs showed excellent electrochemical performances.Moreover,the relationship between morphology and electrochemical performances of manganese oxides was established,and the mechanisms of manganese oxide catalyzing the formation of discharge products with different morphologies and crystal phases were also studied.The main research contents are shown as follows:(1)Firstly,in view of regulating the external properties of the material(pore structure),polyhedral and porous Mn2O3(PP-Mn2O3)catalyst was fabricated via electrospinning method followed by calcination.The construction of pore structure of Mn2O3 was achieved,and the catalytic ability of PP-Mn2O3 for reversibly formation and decomposition of the discharge products was investigated.PP-Mn2O3 catalyst containing unpaired electrons can effectively reduce the reaction polarization of LOB,and the overpotential in the first cycle is reduced to～1.1 V.At the same time,the discharge capacity of LOB with mesoporous PP-Mn2O3 catalyst increases almost twice as high as that of Mn2O3 nanoparticles.Moreover,LOB with PP-Mn2O3 catalyst can keep more than 160 cycles at the current density of 200 mA g-1 with the limited capacity of 500 mAh g-1.(2)Secondly,the dual regulation of external properties(material size,morphology and structure)and intrinsic properties(crystal structure)of manganese oxide were successfully controlled by introducing different amounts of Ag+during the process of hydrothermal reaction.Moreover,the relationship between structural characteristics and electrochemical performances of manganese oxides containing silver was established.The research results show that Ag+can regulate the formation of more regular three-dimensional sea urchin manganese oxides,and construct Mn4+/Mn3+ reaction sites and more oxygen defect active sites.LOB with Ag-MnO2 and bimetallic oxide Ag2Mn8O16 catalysts show high discharge specific capacity and long-term lifespan.When the current density is 200 mA g-1 and the limited capacity is 500 mAh g-1,the positive electrodes of Ag-MnO2 and Ag2Mn8O16 can last 500 and 320 cycles,respectively.When the current density and limiting capacity are increased to 400 mA g-1 and 1000 mAh g-1,they still retain 170 and 133 cycles,respectively.(3)In order to regulate the formation of discharge products,a binder-free cathode(Ag/δ-MnO2@CP)composed of hydrangea-like δ-MnO2 with Ag nanoparticles(NPs)embedded in carbon paper(CP)was fabricated via one-step hydrothermal method.The discharge product was successfully converted from LiO2 to LiOH with higher electrochemical stability by Ag/δ-MnO2@CP,and assembled LOB could reversibly cycle under dry and certain humid oxygen.Ag/δ-MnO2@CP cathode can retain 867 cycles at the current density of 200 mA g-1 with the limited capacity of 500 mAh g-1 under dry oxygen atmosphere,and the overpotential is about 0.5 V in the early cycling.At the same time,Ag/δ-MnO2@CP cathode can catalyze water in the environment to participate in the reaction,so as to increase initial discharge specific capacity.Ag/δ-MnO2@CP cathode shows a long cycle life-span of 216 cycles under humid O2 atmosphere with a RH 2.5%,which pave the way for the possible application of LOB in practical environment.(4)Finally,based on the experimental results that electrocatalytic activity can be optimized via above modification strategies,the study is carried out around the essential characteristics and potential mechanism for improving the electrocatalytic performance of manganese oxides.The relationship between structural characteristics and electrochemical performances of manganese oxides containing silver was explored by combining experimental and density functional theory(DFT)methods,and the mechanisms of different discharge products catalyzed by the catalyst were further studied.Ag doping leads to the appearances of oxygen defects and more Mn3+ due to the stable formation of Ag-OMn bond.Experimental and DFT results show that they play key roles in improving the catalytic activity of manganese oxide and changing the adsorption of LiO2 on the catalytic active surface.In addition,the synergistic effect of Ag/δ-MnO2 composite catalyst and the formation mechanism of atypical discharge product LiOH was also discussed.In-situ Raman test and DFT results show that δ-MnO2 is the key factor in the catalytic formation of LiOH,and the introduction of Ag clusters is beneficial to improve the charge transfer,thus effectively enhance the catalytic activity.