Study On The Micro-nano Structure And Electrochemical Propoties Of Carbon-coated Manganese-based Vanadates

In recent years,China’s dependence on fossil energy has increased,which has led to a series of environmental problems.The development of new energy requires excellent energy storage devices.Lithium-ion batteries,as the main functional devices of current electronic devices,have been widely used in production and daily life.As an important part of lithium-ion batteries,anode materials have great strategic significance in improving their energy density and cycle performance.Transition metal vanadate has the characteristics of abundant reserves and high theoretical capacity,and has become a research hotspot of anode materials.In this thesis,manganese vanadate is selected as the research target.However,manganese vanadate still has problems such as low electronic conductivity and large volume expansion during the cycling,which leads to slow progress and poor electrochemical performance.Based on the microstructure design and composite modification of manganese vanadate,the materials with different micromorphologies were prepared by atmospheric pressure microwave assisted liquid phase method.XRD,SEM,TEM,XPS,BET were used as characterization means,analysis of its composition,micro-morphology and micro-nano structure;conducting electrochemical performance test,exploring the relationship between material structure and properties,and studying the method and root cause of electrochemical performance improvement of manganese-based vanadate.The Mn₂V₂O₇ multiple-layer precursor and MnV₂O₆·2H₂O double-layer sheet precursor were prepared by atmospheric pressure microwave assisted liquid phase method by adjusting the ratio of Mn and V,reaction conditions and pH value.Secondly,the mechanism of two-piece formation is proposed by time slice experiment and control variable method.The battery was assembled for the above two materials and evaluated for electrochemical performance.The Mn₂V₂O₇multiple-layer sheet material can circulate over 730 cycles at a high current of 2 A g^-1 and maintain a specific capacity of 566 mA h g^-1,representing excellent cycle performance and rate performance,while MnV₂O₆·2H₂O double-layer sheet performance poor.Studies have shown that the double-layer morphology can be preserved during the cycle,while the double-layer structure is fragmented during the lithium ion intercalation process.Therefore,the carbon coating process was designed in this study.The MnV₂O₆·2H₂O double-layer sheet precursor was coated with carbon and then calcined at high temperature to synthesize MnV₂O₄C hollow sheet.The obtained hollow sheet was subjected to morphology and structural characterization,and the pore and specific surface area of the surface were measured.In addition,the MnV₂O₆·2H₂O double-layer sheet precursor was reduced by a hydrogen reduction calcination process to obtain MnV₂O₄ nanoparticles.Electrochemical performance tests were performed on the above two materials.The MnV₂O₄C hollow sheet exhibits excellent high current cycle performance and rate performance.At 5A g^-1current,it can be circulated for more than 1000 cycles,maintaining a specific capacity of 527.3 mA h g^-1.At currents 1,2,5 and 10 A g^-1,the corresponding capacities are760.2,602.2,395.3 and 146.6 mA h g^-1,respectively.And then when the current density returns to 1 A g^-1,the average capacity is reverted to 723.5 mA h g^-1,the capacity retention rate was as high as 95.2%.The material has a large specific surface area(39.98 m² g^-1)and reconstitutes a stable piece during the cycle,and because of the coating of the carbon layer,the piece does not stick,resulting in more surface and active sites.The article analyzes its capacity increase and maintains good performance by means of different sweeping CV,electrochemical impedance spectroscopy,SEM and other methods.