Synthesis And Electrochemical Property Of Graphene-based Ternary Cathode Materials

Lithium ion batteries are widely used to consumptional electronic products,new energy vehicles,energy storage grid and so on.It has been indispensable in our daily life and economic development as energy products.So it is very necessary to research and develop the lithium-ion batterydue to rapid development of mobile phones and new energy vehicles.However,cathode material is key factor for performance of lithium ion battery.Thanks to the higher energy density,relatively low production costs,ternary lithium ion batteries are becoming research hot point.Therefore,the ternary cathode material was carried out in our thesis.Since the graphene was discovered in 2004,attention has been paid by many researchers because of its excellent performance such as ideal electric conductivity.The graphene can not only be used in the anode material,but it can also be used in cathode material.It can improve reverse charge-discharge capacity and rate capability of cathode materials.In this thesis,LiNi_1/3Co_1/3Mn_1/3O₂,LiNi_1/2Co_1/3Mn_1/4O₂,LiNi_0.7Co_0.2Mn_0.25O₂,LiNi_1/4Co_1/2Mn_1/4O₂,LiNi_0.1Co_0.6Mn_0.25O₂ were synthesized by sol-gel method.XRD,SEM,constant current charge and discharge test,rate capacity test,cyclic test,cyclic voltammetry and electrochemical impedance spectroscopy were used to study the structure andelectrochemical performanceof these materials.The XRD results show that all of the five materials have a layered structure.The electrochemical tests show that LiNi_0.7Co_0.2Mn_0.25O₂ has higher reversible capacity and stable cycling performance,and the first discharge capacity is up to 142mAh/g at 0.1C.Then LiNi_0.7Co_0.2Mn_0.25O₂ was synthesized at 700 ?,800 ? and 900 ?,and the effects of different synthesis temperatures on the material were investigated.The experimental results indicate that the material synthesized at 700 ? has the best performance.After 30 cycles,the capacity retention rate is 90%.With the increase of synthesis temperature,the electrochemical performance of the cathode material is worse.It can affect the migration of lithium-ion process and result in performance degradation that higher synthesis temperatures increase the particle size of materials andmake particle distribution uneven.LiNi_0.7Co_0.2Mn_0.25O₂ synthesized at 700 ? was compounded with 1%,2% and 3% graphene by ball milling respectively,and the cathode materials that are added by 1%,2% and 3% graphene directly were compared.At the current density of 0.1C,first discharge capacity in the voltage range of 2.0V to 4.5V reaches 166mAh/g for the composite material of the 2% graphene after the ball milled,which is nearly 24mAh/g higher than that of the pure cathode material,and the rate capability is also improved.After the best amount of graphene determined,the further processing by heat treatment under nitrogen is the closer bonding of the graphene to the cathode material.The treatment temperatures were 200?,300? and 400?,respectively.The experimental results show that the initial charge and discharge of the material after heat treatment has not been improved,but a little drop.The reason may be that the graphene and the cathode material after heat treatment cause the layer structure of cathode material to suffer from a certain degree of damage,so that the electrochemical performance decrease.However,the electrochemical impedance test shows that the charge transfer resistance of the composites at 300? after 50 cycles was 70.68m?,which was lower than the resistance value of any material after 50 cycles.It is indicated that the heat treatment could improve the combination of graphene and cathode material,which enhances the electronic conductivity of composite materials.