Preparation And Electrochemical Properties Of MnO_x And MnO_x Based Composites

Manganese has the advantages of abundant storage,environmentally friendly and low price.Manganese has a unique position in the periodic table as a transition metal with five unpaired electrons,and thus has the most oxidation states,including the highest oxidation state（VII）in the entire periodic table.At the same time,MnO_x has an ultra-high theoretical specific capacity(the theoretical specific capacity of MnO₂ is 1370 F g^-1)and a high voltage window.Therefore,it has been widely used in the field of supercapacitor and lithium ion battery.However,due to the poor conductivity and great volume change during the charge/discharge process,manganese oxide delivers poor rate performance and cycle stability.In this paper,the electrical conductivity and electrochemical performance of MnO_x electrode material were optimized by means of grain refinement,heteroatomic doping,conductive carbon composite and heterostructure construction.The microstructure of the material was systematically characterized by scanning electron microscopy,transmission electron microscopy,X-ray diffraction analysis and nitrogen adsorption/desorption.The electrochemical properties of the material were tested in detail by cyclic voltammetry,constant current charge-discharge and electrochemical impedance tests.The main work is as follows:According to the idea of heteroatom doping and grain refinement,the ultra-small manganese dioxide nanoflake doped with Na was prepared.By using the transmission electron microscopy,X-ray diffraction,thermogravimetric analysis and X-ray photoelectron spectroscopy,we found there is a large number of Na distributing in manganese dioxide.Na doping can reduce the average valence of manganese and increase the interlayer water content of MnO₂.Then,the as-prepared MnO₂ was applied as supercapacitor electrode and carried out in a series of electrochemical performance tests.It was found that Na doping is beneficial to improve the electron transport and ion transport rate of MnO₂ electrode materials,thus improving the conductivity,rate performance and cycle stability.Subsequently,in order to further study the energy storage mechanism of Na-doped MnO₂,the microstructure characterization and electrochemical performance test of Mno O₂ prepared by different drying methods were carried out,and the influence of interlayer water on the electrochemical performance of Na-doped MnO₂ was further explored.Moreover,K,Ni and Co doped MnO₂were prepared,and the effects of different cationic doping on the microstructure and electrochemical properties of MnO₂ were further investigated.Co-doped MnO₂ exhibits folded nanosheets in microstructure and demonstrates good electrochemical performance.We use the Na-doped MnO₂ synthesized above as raw material,assembling with graphene and calcining to construct a dense graphene/Manganese oxide composite electrode material with"sandwich"structure.It was observed by scanning electron microscope and transmission electron microscope that the manganese oxide particles were evenly dispersed among the graphene sheets,and the particle size was concentrated at about 100-200 nm.Subsequently,the composite material was systematically characterized,the electrochemical performance of lithium-ion battery was tested,and the energy storage mechanism was discussed.The prepared composite showed good rate performance and cycling stability.The graphene nanosheets act as a conductive frame and MnO nanoparticles as pillars support the structure of the carbon skeleton.This kind of structure not only has porosity and stable mechanical properties of conductive network,but also has a number of fast lithium-ion transport channel.In the process of charging and discharging,the presence of graphene can alleviate the volume change of manganese oxide,and maintain the cycle stability of the electrode.Subsequently,a series of comparative experiments on the proportion of graphene in the preparation process and the selection of calcination temperature were carried out and discussed.MnO₂/Fe₂O₃ and MnO₂/Sn O₂ heteromaterials were prepared by cation replacement method using the Na-doped MnO₂ synthesized above as raw materials according to the performance optimization strategy of grain refinement and defect engineering.The microstructure was characterized by transmission electron microscopy,X-ray diffraction analysis and X-ray photoelectron spectroscopy.It is observed that the prepared materials are aggregates of ultra-small nanosheets and have a large number of pore structures,in which MnO₂and Fe₂O₃ are evenly distributed,and there are a large number of defect structures between the lattice interfaces of the two materials.After heterostructure construction,the specific surface area of the material has been greatly improved.Subsequently,two heterogeneous materials were applied to the anode of lithium-ion battery and a series of electrochemical performance tests were carried out.The rich pore structure and defect structure of MnO₂/Fe₂O₃heteromaterial can provide a fast transmission channel for the insertion and release of lithium ions in the charging and discharging process,and increase the effective specific surface area of the material.MnO₂/Fe₂O₃ heteromaterial shows good rate performance and cycle stability.Finally,the effects of different parameters on the microstructure and electrochemical properties of MnO₂/Fe₂O₃ heteromaterial during the synthesis were further discussed.It was found that increasing the amount of Fe²⁺and the reaction time can improve the crystallinity of the heterogeneous materials,and then improve the rate performance of the materials.