Study On The Surface Modification Of LiNi_0.6Co_0.2Mn_0.2O₂ Material By Two Functional Compounds

With the development rapidly of society,lithium ion batteries?LIBs?have been widely used in various fields,such as portable electronic products,hybrid electric vehicles,aerospace and military fields.This is because LIBs have the advantages of environmental friendliness,higher specific energy,smaller size and lighter weight.The specific energy and cycle life of traditional LIBs have been unable to meet the needs of people.Therefore,nickel-rich materials with low cost,high energy density and environmental protection have become one of the research hotspots.However,nickel-rich materials have the following shortcomings: firstly,with the increase of Ni2+ in the material,the trend of cation?Li/Ni?mixing increases,which will cause the transformation of layered structure to disordered structure and rock salt phase,resulting in irreversible phase transition;Secondly,the increasing nickel makes materials easier react with water and carbon dioxide in the air,causing the destruction of the surface structure;Thirdly,Ni4+ formed in the process of charging has a strong reducing property.In order to maintain charge balance,there will be gas release,and it is easy to react with electrolyte.So,in order to overcome the above problems,the researchers adopt a series of modifications.The common modification methods include surface coating and/or ion doping,gradient materials and electrolyte modification and so on.The surface coating and/or doping has drawn much attention because of low cost,simple operation.Many researchers have confirmed that the surface modification can inhibit the reactions between nickel-rich materials and water and carbon dioxide in the air,the surface coating can also inhibit the side reaction between the material and electrolyte enhancing the structure and thermal stability;In addition,in order to inhibit the side reactions between materials and electrolyte,some researchers added additives to the electrolyte,which inhibited the occurrence of side reactions between activity materials and electrolyte to a certain extent.Therefore,in order to solve the shortage of Ni-rich layered materials(LiNi_0.6Co_0.2Mn_0.2O₂),two kinds of materials with different structures and properties,La₂Zr₂O₇ and Li₂SnO₃,were selected for modification based on the structure and properties of the materials and the factors such as diffusion channels,structure matching and conductivity.The electrochemical properties of the two modified materials were improved.The coating materials inhibited the occurrence of interfacial side reactions and the growth of CEI film,reduced the exposure of nickel-rich materials in the air,and improved the structural stability of the materials.The main works of the paper mainly includes the following parts:1.Li₂SnO₃,as an ionic conduction material,is used to modify nickel-rich layered materials.Li₂SnO₃ surface modified Ni-rich material was prepared by sol-gel method.By electrochemical test of different Li₂SnO₃ coating amount,the optimum electrochemical performance is 0.5 wt.%.Physical characterization XRD,SEM and TEM suggested the surface coating layer didn't change the bulk structure,and electrochemical data indicates the Li₂SnO₃ surface modified material's cycling capacity and rate performance have improved obviously.The mechanism of the modified layer was studied by electrochemical aging experiment.The results showed that the Li₂SnO₃ coating layer can protect the material from the corrosion of the electrolyte,inhibit the side reaction and the growth of the CEI film and show a good structural stability.2.Ni-rich material NCM622 was prepared by sol-gel method,and then La₂Zr₂O₇ was coated on the surface of NCM622.The SEM,TEM and XRD confirmed that the coating layer didn't influence on the structure and morphology of the material.The electrochemical test results show that the cyclic stability,structural stability and thermal stability have been significantly improved after modification.In order to further explore the mechanism of La₂Zr₂O₇ coating,electrochemical aging experiments were carried out.We can found that the surface modified by La₂Zr₂O₇ generated less CEI films and less transition metal ions dissolved showing better structural stability from SEM and XRD data.