Preparation And Characterization Of Graphene-like Doping Nickel-cobalt-aluminium Ternary Cathode Materials

So far,with the popularity of electric vehicles?EVs?,the use rate of lithium-ion batteries has been increasing rapidly.When pure LiNiO2 cathode material is applied to the positive electrode of lithium ion battery,its electrochemical performance,as well as the storage performance,is quite poor.Nevertheless,the doping of a certain proportion of Co and Al elements in its structure can help to adress these issues.It is worth noting that almost all cathode materials rely on Ni and Co transition metals,which are electroactive elements in the layered structure of cathode materials,such as Li?Ni,Mn,Co?O2 and Li?Ni,Co,Al?O2.Compared with the commercial LiCoO2 cathode materials,the nickel-rich cathode materials have certain advantages.The costs are lower,and both the energy density and discharge specific capacity are higher.Additionally it is more environmentally friendly.However,there are still a few deficiencies existing in the nickel-rich cathode materials.Typically,as the Li⁺radius?0.76??and Ni²⁺radius?0.69??are similar,ion mixing can be generated during the material preparation and cyclic charge and discharge cycles,which would further affect the electrochemical performance of the materials.Current methods for modifying nickel-rich materials are generally doping elements and surface coatings.In this thesis,the method of doping graphene is used to modify LiNi0.8Co0.15Al0.05O2?NCA?The specific preparation methods are spray drying method and electrospinning method.The spray drying method was firstly used to obtain a precursor of the material,which was then calcined at a high temperature to obtain a graphene-doped NCA.The specific steps are as follows:?1?taking a certain amount of metal salt and citric acid according to a certain molar ratio;?2?adding a certain proportion of laboratory-made low polyacrylonitrile?LPAN?for doping,and configuring to stir the solution;?3?obtaining a precursor of NCA material after spray drying;?4?calcining the two stages to obtain the final product.Herein the temperature and duration of pre-burning the obtained precursor after spray drying are further explored,and the best calcination conditions is determined as pre-calcinated at 450°C for 5 h.The effects of different temperatures and durations on the structure and properties of the materials at high temperature calcination stage are also investigated.The optimum calcination condition is discovered to be calcined at 750°C for 11 h.proportion of citric acid added in the solution configuration is next investigated,and it is found that the optimal addition ratio is 1:2.Then the effects of adding different proportions of LPAN on the structure and properties of NCA are studied and discussed.By analyzing the XRD,XPS and Raman spectroscopy results of all samples,it is found that the doped LPAN can form a graphene-like structure when calcined at high temperature.The graphene-like structure is inserted into the NCA layered structure,which discreses the lithium-nickel mixing phenomenon of the NCA material during the crystallization process,makes the material structure stable,and also enhances the material conductivity.Furthermore,by analyzing the electrochemical properties of the material,it is found that the electrochemical performance of the sample doped with 20%LPAN has been improved compared with the pristine sample.Besides,the first discharge specific capacity can reach 227.9 mA h g^-1,and the capacity retention rate is 93.59%after 200 cycles.After investigating the use of spray drying to prepare graphene-doped NCA cathode materials,we also successfully prepare the graphene-doped NCA using electrospinning.This thesis explores the effects of electrospinning using different polymers on the structure and electrochemical properties of the prepared materials.By comprehensively analyzing the XRD,Raman and XPS results of all samples,it is also found that the doped LPAN is electrospun in the material.After the wire is calcined,a graphene-like structure is formed.The graphene structure of the sample obtained by using PVP as the polymer spinning is the most obvious.The Li⁺/Ni²⁺mixing degree is the lowest,and combining the electrochemical performance and capacitance contribution rate under the conditions of 0.1 C and 5 C magnification,the sample has achieved the best overall performance.To sum up,the graphene-doped NCA cathode materials with excellent performance can be successfully prepared by spray drying method and electrospinning method.The synthesis process is relatively simple,the cost of the raw materials is quite low,and there is no synthesis process.Therefore,the resaech in this thesis can provide an effective approach and direction for doping modified NCA.