Controlled Preparation,Modification And Performance Study Of High Nickel Ternary Lithium-Ion Batteries

The cathode material plays a pivotal role in the performance of lithium-ion batteries,and the nickel-rich ternary cathode material(Li Ni_xCo_yMn_1-x-yO₂)has attracted significant attention from researchers because of its high energy density.However,its low ionic conductivity and small lithium-ion diffusion coefficient leads to unsatisfactory cycling performance and rate capability,there are also problems such as harmful side reactions between electrolyte and cathode material.In order to improve the above disadvantages,many modification methods have been tried,Among varies modification methods,coating has attracted much attention as an effective way to improve cycling performance and rate capability of the material.The main studies carried out in this thesis is as follows:Li Ni_0.8Co_0.1Mn_0.1O₂（abbreviated as NCM811,the same below）,a high nickel ternary layered cathode material,is modified from the coating point of view.Firstly,NCM811coated with niobium pentoxide was prepared,and then further treated with carbon coated.The effect of single and double coating of niobium carbon on the performance of the battery was compared and the characterization revealed that the coating treatment did not change the layered structure of the material,while reducing the degree of electrode polarization and irreversible electrochemical reaction.Then,the influence of titanium-carbon single and double coating on NCM811 was further investigated.In this thesis,three groups of concentration gradient coating amount were set,and after high temperature sintering,titanium coated NCM811 material was obtained.Samples with 2%coating concentration were selected for further carbon coating treatment,and the influence of carbon coating on the layered structure and electrochemical performance of the material was studied.After morphological and structural characterization,it was found that the titanium element was uniformly coated on the surface of NCM811 material and did not affect the internal crystal structure of the original material;After the rate capability test,it was found that the cyclic capacity retention rate of 1%and 2%titanium dioxide coated samples was better than that of the original sample at 0.5 C,1 C and 2 C high rate.The 1 C cycling performance test results show that the titanium-carbon double-coated sample has a superior cycling capacity retention rate:the capacity after 100 cycles is 183.12 m Ah/g,which is 6.18 m Ah/g higher than the original sample;the cyclic capacity retention was 98.07%,which was 2.54%higher than that of the original sample.Based on the results of the above concentration gradient research,in order to make the coating more uniform and dense,titanium element obtained by hydrolysis of tetrabutyl titanate was used for the monolayer coating of the material.The results showed that this experimental group improved the first discharge specific capacity by 16.2 m Ah/g and the first discharge efficiency by 3.59%compared with the titanium dioxide coated group.In addition,in order to better study the AC impedance of the battery,this thesis analyzes three charging schemes for lithium-ion batteries in the AC impedance test preliminaries:constant-current charging method,constant-current-constant-voltage charging method and three-stage constant-current-constant-voltage charging method,then simulates the three-stage constant-current-constant-voltage charging method using MATLAB/simulink software.The charging time,the fluctuation range of battery voltage and the steady-state quality of the batteries were compared among the three charging schemes,and it was concluded that the steady-state quality of the battery under the three-stage constant-current-constant-voltage charging method is superior,and its voltage fluctuation range is only 0.2 m V within 300 s,which was significantly reduced compared with that of the constant-current-constant-voltage charging method.