| According to the national relevant industry plan,in 2025,the sales volume of new energy vehicles will reach 20 % of the sales volume of new vehicles.This puts higher demands on lithium-ion secondary batteries,the main power source of new energy vehicles.However,in the composition of various parts of the lithium-ion secondary battery,the cathode material is still a shortcoming.Due to lower actual capacity(150 m A h g-1),relatively expensive price and environmental pollution,the first commercialized Li Co O2 does not apply to the field of new energy vehicles.Therefore,it is urgent to develop a cathode material with high energy density,good cycle stability and low cost.Among so many cathode materials,high nickel ternary cathode materials have received widespread attention,due to their higher specific capacity,discharge voltage and low price.However,the co-precipitation method,as the mainstream preparation method of high nickel cathode materials,still has several problems such as high environmental pollution control cost,long synthesis cycle,and insufficient element ratio accuracy.In addition,high-nickel ternary cathode materials have serious problems such as Li+/Ni2+ mixing,unstable surface structure,cracks on grain crystal planes and residual alkali on the surface,which seriously affect the actual capacity and cycle life of the lithium-ion secondary batteries.Therefore,this article aims at the Li Ni0.8Co0.1Mn0.1O2 cathode material(hereinafter referred to as NCM811),which is one of the high nickel ternary cathode materials.Through literature research,based on understanding the relevant research progress at home and abroad,researches have been carried out in the aspects of the preparation process,morphology modification and surface modification.The main contents are as follows:(1)First of all,this article studies the preparation process of NCM811.One is the orthogonal design experiment on the three key process parameters of the spray drying method.Through the range analysis method,it is concluded that the air outlet temperature has the greatest influence on the yield.Furthermore,it demonstrated that the most suitable process parameters are: fan frequency 50 Hz,peristaltic pump speed4 rpm,air outlet temperature 200 ℃;Second,the influence of precursor solution concentration on powder yield and precursor morphology was studied,and it was found that under the same preparation conditions,as the concentration of the precursor solution continues to increase,the lower the powder yield,the better the sphericity and uniformity of the precursor;The third is to discuss the process flow and compare the order before and after mixing lithium as well as the effects of primary sintering and secondary sintering on the material structure.It is found that the solid-phase mixed lithium and secondary sintering methods have a greater positive effect on the ordering and cation mixing of layered materials.In addition,this thesis uses high-temperature hydrothermal method to prepare nano-sheet NCM811 cathode material,analyzes and compares the influence of different hydrothermal temperature on the morphology and electrochemical performance of NCM811.The sample has a more complete stamen morphology under the hydrothermal temperature of 180 ℃.And its magnification performance and cycle performance are the best.In the range of 2.8-4.3 V and 0.1 C current density,it has an initial capacity of 186 m A h g-1.After 80 cycles,it still has a capacity of 161 m A h g-1.(2)Secondly,aiming at the problems of low-concentration precursors such as poor spherical shape and easy sintering,this article uses NH4+ to modify them,comparing the influence of different NH4+ concentrations on the morphology of the precursors,and studying the surface morphology,phase structure and electrochemical performance of the 0,0.25,0.5 M sample.After testing,the NH4+modified sample has good sphericity.The 0.5 M NH4+ modified sample has an initial capacity of 138 m A h g-1 at a current density of 5 C.The capacity retention rate is at79.2 % after 140 cycles at 1 C.Compared with the sample before modification,the0.25,0.5 M sample increase 18.5 %.(3)At last,this paper has launched the study of the Li2 Ti O3 coated NCM811(LTO@NCM)core-shell structure design and electrochemistry.In this paper,a process method for in-situ coating of electrode material Li2 Ti O3 is designed,which utilizes the substitution reaction between the hydroxyl functional groups of polydopamine and tetrabutyl titanate(TBOT),and then builds the LTO@NCM coreshell structure through secondary sintering.The Li2 Ti O3 shell material effectively alleviates the problem of instability on the surface of NCM811,and at the same time,constructs a 3D channel for Li+ transmission.After testing,the capacity retention rate of LTO@NCM composite material can reach 99.7 % after cycling at 0.2 C and 75 cycles.After cycling at 2 C and 240 cycles,the capacity retention rate is 92.5 %.At10 C current density,the initial capacity is 117.4 m A h g-1.In summary,this article compares the spray drying and high-temperature hydrothermal methods for preparing NCM811 cathode materials.The best process is obtained through analysis,and then the influence of different NH4 + concentrations on the morphology of the NCM811 precursor and the electrical properties of the cathode material is explored.The influence of chemical properties has been found to be able to effectively improve the sphericity and electrochemical properties of the material after modification by NH4+.Then,the strategy of in-situ coating with Li2 Ti O3 to construct the core-shell structure effectively alleviated the problems of high-nickel ternary materials and achieved excellent electrochemical performance. |
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