Optimizationanl Preparation And Surface Microstructure Controlling Of High-stability Ternary High Nickel Cathode Material

The high nickel ternary material LiNi_xCo_yMn_zO₂（x+y+z=1,x≥0.6）have the advantages of high specific capacity,environmental friendliness and low cost,but they also have problems such as cycle stability and poor rate performance.In order to improve the above-mentioned shortcomings of the material for meeting higher commercial requirements,it is necessary to optimize the preparation process,modify the surface of the material and further improve their electrochemical performance.In this dissertation,a gradient mixing lithium process is used to synthesize nickel-rich ternary cathode materials with different lithium ratios.The effects of the amounts of lithium on the crystal structure,particle morphology,particle size distribution,valence state change,phase transition evolution and electrochemical performance of the material with relating mechanism are studied.After the structure and morphology characterization and electrochemical performance test on samples with different lithium content,the optimal lithium content for the nickel-rich ternary cathode material is obtained.Then TiNb₂O₇ is used to modify the surface of the nickel-rich ternary cathode material with the best lithium ratio.This dissertation studies the influence of different coating amounts and synthesis temperatures on the morphology,crystal structure,electrochemical performance and other aspects of the coated nickel-rich ternary material.It especially focuses on the ion intercalation from the coating layer,the reversibility of the phase transition for the material.And the change of the microstructure,the electrochemical performance under high voltage and high temperature,and the molecular stability after TiNb₂O₇coating.The main findings are as follows:（1）The Li/TMs molar ratios in the process of gradient mixing of lithium are set as 1/1,1.03/1,1.06/1,1.09/1 and 1.12/1.The high temperature solid phase method（sintering temperature is 750°C）is applied to obtain samples with different lithium ratios.Compared with other samples with different lithium ratios,when the ratio of Li/TMs=1.06,the nickel-rich ternary cathode material has the smallest Li⁺/Ni²⁺cation mixing ratio,the most stable TMO₆structure,the largest lithium layer spacing,the longest Li-O bonds,the shortest TM-O and O-O bonds.In addition,the particle sizes distribution of its primary particles is more uniform,and the formed secondary particles after agglomeration are the most compact and tight.At the same time,it delivers the best electrochemical performance:the first discharge capacity（0.1C）and the coulombic efficiency are 209.1 m Ah g^-1 and 85.3%,respectively.The discharge capacity and the capacity retention rate after cycles（1C,100 times）are 168.9 m Ah g^-1 and 91.7%,respectively.When the rate is 10C,the discharge capacity is 140.5 m Ah g^-1.Its excellent performances are mainly attributed to its good crystalline structure and uniform and dense particles,and therefore it can still maintain the integrity of most of the spherical particles after 100 cycles of 1C.（2）Sol-gel combined with high-temperature-solid-phase method is applied to explore the effect of TiNb₂O₇ coating on the structure and electrochemical performance of Li_1.06Ni_0.88Co_0.06Mn_0.06O₂.The influences of different coating amounts and sintering temperatures for TiNb2O7 on the material are studied separately.The XRD result of the sample with 1wt%coating amount sintered at 700°C shows that its lattice parameters are slightly larger than those of other samples.The Ni/Li mixing ratio is smaller than that for other samples.SEM results show that TiNb₂O₇ are uniformly coated on the sample particles.The results of electrochemical tests（3.0-4.3V,25°C）show that the Li_1.06Ni_0.88Co_0.06Mn_0.06O₂ sample coated with 1wt%TiNb₂O₇ sintered at 700°C has the best electrochemical performance.And after the first charging-discharging（0.1C）,the discharge specific capacity and coulombic efficiency are206.9 m Ah g^-1 and 86.2%,respectively.The capacity retention values for the sample after 100cycles at 0.2C and 200 cycles at 1C are 98.5%and 87.9%,respectively,and the discharge capacity of the sample at 10C is 162.3 m Ah g^-1.Under the same condition,the capacity retention values of the uncoated sample are only 95.5%（0.2C）and 59.8%（1C）,and its discharge capacity at 10C is only 145.0 m Ah g^-1.DFT calculations combined with TEM,XPS and FIB-TEM tests show that TiNb₂O₇ is successfully coated on the surface of the material,and partial Ti⁴⁺is embedded in the material matrix but Nb⁵⁺is not.Cyclic voltammetry,AC impedance analysis and lithium ion diffusion coefficient show that TiNb₂O₇ coating effectively suppresses the polarization of the cathode material and accelerates lithium ion diffusion.SEM,FIB-SEM,TEM and in-situ XRD test results show that the coated sample maintains a layered structure and complete spherical particles after a long time cycle process（3.0-4.3V,25°C,1C,200 times）.The XPS technology is used to detect and analyze by-products on the interface of the samples after cycles.The results show that TiNb₂O₇ coating on the nickel-rich ternary cathode material can effectively reduce the side reactions at the interface.The capacity retention rate increases from54.1%to 77.8%under high voltage（3.0-4.5 V,25°C）after 200 cycles at 1C,and from 74.83%to 88.83%after 100 cycles at 1C under high temperature（3.0-4.3 V,60°C）.Finally,DFT is used to calculate ELF,CDDS,oxygen release energy and PDOS,and it is found that TiNb₂O₇ coating can effectively improve the molecular stability of nickel-rich ternary materials.