Preparation And Modification Of High-nickel Cathode Materials For Lithium-ion Battery

High nickel materials for lithium-ion batteries are favoured by researchers because of their high mass energy densities and bulk energy densities.However,the poor cycling performances and low safety of high-nickel cathode materials severely limit their large-scale application.Ion doping and surface coating are the main methods to enhance the overall performance of high nickel cathode materials.Moreover,the calcination technology also plays an active role for overall performances of high-nickel cathode materials.In this thesis,high-nickel low cobalt and high-nickel cobalt-free materials are investigated sourced from common LiNi_0.82Co_0.12Mn_0.06O₂ and LiNi_0.6Mn_0.4O₂ materials.The synthesis process is also investigated in detail.The main points are as follows:（1）The electrode material LiNi_0.82Co_0.12Mn_0.06O₂ is obtained by high temperature calcination at 775°C,and the ratio of Li₂CO₃ and Li OH is also regulated during the synthesis process.In addition,the influence of calcination method on performances is investigated.The results show that the excellent cycling performance of high nickel ternary cathode materials is due to the presence of Li₂CO₃,however their multiplicative performance and initial coulomb efficiency are slightly weakened.When the ratio of Li₂CO₃ is 10%,the capacity retention can attain 93.1%after 100 cycles.Besides,the presence of CO₂improves the cycling performance of LiNi_0.82Co_0.12Mn_0.06O₂ in the calcination process,and the discharge specific capacity at 8 C（132.1 m Ah/g）is higher than that of the base material（113.8 m Ah/g）.（2）Nd ion doped high-nickel ternary cathode materials are prepared by high temperature solid phase method and the doping ratios are adjusted as 0.5%,1%and 1.5%.The structural characteristics and electrochemical properties of synthesized materials are obtained by corresponding tests.The results show that the 0.5%Nd ion doping improves the cycling performance of material（from 68.6%to 89.4%after 100 cycles）.The increase of Nd ion doping content has less effect on the first discharge specific capacity,but it reduces the cycling capacity of materials.This is possibly because the electrode material produces an impurity phase when the doping amount is larger than 0.5%.（3）The cobalt-free high-nickel cathode material LiNi_0.6Mn_0.4O₂（NM64）is prepared by hydrothermal and co-precipitation methods,respectively,and the co-precipitation method is determined to be the preferred method for the preparation of NM64.The electrochemical properties of NM64-C,prepared by co-precipitation method（NM64-C）,are optimised using a Ce ion doping strategy as a substrate.The larger bond energy of Ce-O bond can effectively improve the structural stability of material during the charge-discharge process.In addition,the introduction of larger radius Ce³⁺can expand the NM64-C lattice,thus improving the multiplicative performance of material.The experimental results display that the capacity retention rate of 0.02 Ce ion-doped NM64-C material is 79.5%after one hundred cycles at 0.3C,which is evidently higher than that of the base material NM64-C（69.8%）.