Study On Modification Strategy Of Manganese Oxide Cathode Materials For Lithium-oxygen Batteries

With the progress of science and technology and the development of the times,issues such as the shortage of non renewable resources and environmental pollution have become increasingly serious worldwide.It is necessary to develop and improve new energy storage technologies.Lithium-oxygen batteries(LOBs)have attracted much attention of researchers due to its high theoretical specific capacity close to fuel,and is considered to be the most outstanding one in the post lithium ion battery system.However,the development of LOBs still faces scientific problems such as high charge and discharge overpotential,poor cycle stability and rate performance,and the root cause of the above development bottleneck is the slow kinetics of the cathode reaction of LOBs.On the one hand,the slow electrode reaction kinetics lead to the accumulation of lithium peroxide(Li2O2),which as the discharge product with insulating properties that cannot be completely decomposed during cycling and passivates the active site.On the other hand,the limited electrode reaction kinetics directly lead to high charging overpotential,so that the electro lyte and electrode materials decompose or side reactions at high potentials,and by-products accumulate for a long time,eventually leading to battery failure.Therefore,the construction of efficient bifunctional anode catalyst has far-reaching significance for improving the rate performance,long cycle performance and charge discharge reversibility of LOBs.In this paper,manganese based oxides are selected as the research object,to construct and improve the cathode catalytic materials for LOBs,and to study the influence of the valence state of metal ions and doping strategies on the catalytic activity of materials.The main research contents are as follows:(1)The Mn3O4@CNT and MnO@CNT samples were synthesized by simple room temperature redox plus annealing under a protective atmosphere,and the setting gradient test determined the annealing temperature conditions for stable generation of Mn3O4 and MnO particles on CNT.During the charging and discharging process of LOBs,Mn3O4 with mixed valence states undergoes inter-valence state transition between metal ions,which provides additional electrons for the generation and decomposition of Li2O2,thereby promoting ORR and OER processes,and finally reducing the charge-discharge overpotential,which can be proved by the analysis of the fine spectrum of manganese XPS.Mn3O4@CNT exhibits excellent electrochemical performance when used as a positive electrode of LOBs.Mn3O4@CNT-based batteries exhibit good rate performance at current densities of 200,400,800 and 1000 mA g-1 and limited capacity of 1000 mAh g-1.Excellent cycle stability at 200 mA g-1 current density(stable cycle more than 150 cycles).The specific capacity of the first discharge is up to 16895 mAh g-1.Most importantly,under the conditions of 1000 mAh g-1 limited specific capacity and 200 mA g-1 current density,Mn3O4@CNT-based batteries exhibit a lower overpotential(0.76V)than MnO@CNT.(2)Two dimensional lamellar MnCoCrO,MnCrO and MnCoO catalysts were synthesized by sol-gel and annealing methods.The first principle calculation shows that the(100)crystal faces of MnCoCrO and MnCrO have different adsorption capacities for the discharge product intermediate LiO2.Because of the relatively high intermediate adsorption capacity of the tri metal spinel,Li2O2 can be attached to the electrode material through the surface mechanism and finally closely adhere to the electrode surface in a circular hollow shape.This unique Li2O2 can achieve close contact with the active site,which is conducive to establishing a low impedance Li2O2/active site interface,improving the reversibility of the generation and decomposition of Li2O2,improving the electrode reaction kinetics,and ultimately improving the cycle stability.MnCoCrO,MnCrO and MnCoO were used as positive electrodes of LOBs.The results showed that MnCoCrO cathode had better electrochemical performance.Specifically,the MnCoCrO cathode displays a high initial discharge capacity of 13600 mAh g-1 at a current density of 200 mA g-1,has a low overpotential of 1.40 V at a fixed capacity of 1000 mAh g-1 and a current density of 400 mA g-1,and can stably cycle for more than 200 cycles at 400 mA g-1.The cycle life of MnCoCrO-battery is significantly higher than that of MnCrO(130 cycles)and MnCoO(100 cycles)batteries.