The Preparation Of Graphene/NCM-cathode Composites And Its Application In Lithium-ion Batteries

Lithium-ion batteries have the advantages of high energy density,long cycle life,safety,environmental protection,and no memory effect,making them the most ideal energy storage devices.With the country’s strong support for new energy vehicles,higher requirements are placed on the performance of lithium-ion batteries.The performance of lithium ion batteries is mainly affected by the structure of the electrode materials.Among them,the development and design of positive electrode materials are particularly important,therefore it is of great significance to the research and development of positive electrode materials.The ternary cathode material(Li N i_1-x-yCox Mny O₂),which combines the advantages of Li Co O₂,Li N i O₂,and Li Mn O₂（Li Mn₂O₄）at the same time,has become a hot spot of research.The most studied is Li N i_0.5Co_0.2Mn_0.3O₂（NCM）,because of its high Ni content,it has higher specific capacity and lower cost,but due to problems such as poor conductivity,poor cycling performance,and se vere capacity attenuation,etc,hindered its development.The most effective method is doping and cladding.Graphene is the thinnest carbon material in the world.It has excellent electrical conductivity,stable chemical properties,and hard mechanical properties.It can be used as a protective layer to cover the surface of NCM.Thin film coating improves the conductivity of NCM and improves electrochemical performance.The main research contents of this paper are as follows:1.First explore the coating method for graphene-coated NCM.The effects of liquid nitrogen cooling extraction method and ball milling method on the structure and electrochemical performance of graphene@NCM were investigated.The results show that the liquid nitrogen extraction method will cause changes in the crystal structure of NCM,and ball milling will destroy the sphe rical agglomerate structure of NCM,which will affect its electrochemical performance.It is not good to get a perfect structure of graphene@NCM coating.2.In this paper,the graphene-oxide@NCM structure was prepared by using the interface effect of graphene oxide in different solvents,and the graphene@NCM structure was obtained after reduction.The results showed that graphene could be clearly seen on the surface of NCM particles by scanning electron microscopy（SEM）.The electrochemical performance was tested and found that the initial discharge capacity of the uncoated NCM particles was 171.3mAhg^-1 at the charge and discharge rate of 0.5C,and the first charge-discharge efficiency was 83.5%.The first discharge capacity of the graphene@NCM samples was found that the 137.8mAhg^-1,the first charge-discharge efficiency was 78.43%,and the first charge-discharge performance after graphene coating was decreased.After 500 cycles,the discharge capacities of uncoated NCM and graphene@NCM were 81.4 and 120.3mAhg^-1,respectively;and the cycle stability was47.52%and 87.3%,respectively.Compared with uncoated NCM,graphite the discharge capacity of graphene@NCM increased by 38.9mAhg^-1,the cycle stability increased by39.78%,and the cycle stability of NCM was obviously improved.Even after 1000 cycles,the discharge capacity of graphene@NCM could reach 90.6mAhg^-1,and the cycle stability was 65.75%.On this basis,we built an oxygen bridge between the graphene and the encapsulated ternary material by adding sodium citrate as a surfactant to increase the ion conductivity of the material and thus improve the overall electrochemical performance of the material.Scanning electron microscopy（SEM）and transmission electron microscopy（TEM）tests showed that more tightly after the SC was added,and the thickness of the coating layer was reduced,which facilitates lithium ion transmission,and the thickness of the coating decreased.After the electrochemical performance test,charging and discharging at 0.5C rate,the first discharge capacity of graphene@NCM sample after adding SC was 145.7mAhg^-1,the first charge-discharge efficiency was 81.7%,and the first charge-discharge performance was low.After 500 cycles,the dischar ge capacity was142.9mAhg^-1 and the cycle stability was 98.08%.When the cycle is 1000 times,the discharge capacity can reach 122.2mAhg^-1,the cycle stability is 83.87%,and the cycle stability can be obviously improved.In addition,in order to verify the versatility of this method,we also performed graphene coating on other types of ternary materials.As a result,it was found that graphene can still be coated uniformly and ultra-thinly.