Study On Preparation And Electrochemical Properties Of Manganese-based Cathode Materials

In widely used electrode materials,transition metal oxides have high ionic characteristics and potential due to their strong oxygen metal bonds.Manganese-based compounds are not only abundant in resources,but also inexpensive in price.They also have variable composition,complex structure and peculiar functions.KMn8O16 which is the Cryptomelane-type has a natural 2*2 tunnel structure with a diameter of about 0.46 nm.The existence of K+ in the tunnels can not only maintain the stability of the structure,but also improve the diffusion rate of Li+.The three-dimensional tunnel type Na0.44MnO2 has the MnO6 octahedron which can support each other,so in the impaction and release process of Na+,Na0.44MnO2 has a relatively stable cycle performance.Based on two problems of manganese ion dissolving and Jahn-Teller distortion in the charge and discharge process of manganese-based cathode materials,the purpose of this paper is to maintain the structure’s stability,improve the initial capacity and the electrochemical performance of the materials by a simple preparation method and modification process.The main research contents of this paper are as follows:(1)The Cryptomelane-type KMn8O16 cathode material was synthesized by a simple hydrothermal method.The effect of different hydrothermal time on structure and morphology of the material was studied by designing a controlled experiment.The results of the test show that KMn8O16 cathode material has pure phase only when the hydrothermal time is prolonged to 24 h,and the crystallinity of the material has also increased after calcination.Within the voltage range of 1.5～4.2 V,the first discharge specific capacity of KMnsO16 at a current density of 50 mA/g is 199.69 mAh/g,and after 50 cycles,its capacity retention rate is 78%.(2)Na0.44MnO2 nanorods were prepared by hydrothermal method with high concentration of NaOH solution.The results of the test show that under the same hydrothermal time,Na-bimessite is completely transformed into Na0.44MnO2 pure phase when the hydrothermal temperature reaches 200 ℃.With the same hydrothermal temperature,the crystallinity of Na0.44MnO2 nanorods becomes better gradually with the increase of hydrothermal time.The average diameter of the nanorods is between 100～200 nm and the length is about 2-5 μm.In the voltage range of 2.0～4.0 V,the initial discharge specific capacity of Na0.44MnO2 at a current density of 50 mA/g is 110.7 mAh/g,and the capacity retention rate is 75%after 100 cycles.Na0.44MnO2 which was synthesized at a hydrothermal time of 16 h also has the highest capacity retention rate under the high current density.Calcination can also improve the electrochemical performance of Na0.44MnO2.(3)Combining with graphene by a ball-milling treatment,the conductivity and capacity retention rate of Na0.44MnO2 can be improved,as well as its electrochemical performance.In the voltage range of 2.0～4.0 V,the initial discharge specific capacity of Na0.44MnO2/graphene is 106.9 mAh/g at a current density of 50 mA/g.After 100 cycles,the residual specific capacity is 91.8 mAh/g,and the capacity retention rate is 86%.What’s more,the cycle performance of Na0.44MnO2/graphene is higher than Na0.44MnO2.When the current density reaches 500 or 1000 mA/g,the corresponding initial discharge specific capacity of Na0.44MnO2/graphene is 82.4%or 72.1%,relative to 50 mA/g.And the capacity retention rate is far greater than that before.