Preparation And Electrochemical Mechanism Of Layered Oxide Cathode Materials For Lithium Ion Batteries

Developing a new generation of highly energetic,environment friendly,economic friendly and recyclable LIB has become emergency as the nation raise the policy on the Ev.Among the LIBs,the cathode material,the existing product of which hardly meet the demand,is the key to enhance the capacity,cost and security of the LIB and the NCM and Li-rich has become the cutting-edge product.However,these materials still have some key problems.For high-nickel materials,such as serious Li/Ni mixing,serious interfacial reactions between electrolyte and active materials,etc.;for lithium rich materials,such as short cycle life,serious phase transition and low initial coulombic efficiency are the most serious problems.In response to the above problems,a mass of research has been done in recent years.The normal solutions are coating or doping modification.So far,the reasons and mechanisms of these phenomena have not been fully comprehend,and theoretical knowledge is still scarce.There is still a long way to go to find strategies to figure out these problems.This thesis explores the influence of lithium-nickel mixing and lithium vacancy factors on the structure and performance of high-nickel materials from the perspective of synthetic methods;and we also designed a simultaneous optimization strategy on the surface/bulk to improve the electricchemical performance of Li-rich by drawing lesson from the work about Ni-rich layered cathode.The main work is summarized as follows:1.A total of four different high-nickel ternary layered materials in the two major systems（NCM523 and NCM811）were synthesized by hydrothermal synthesis and sol-gel method.They are named NCM523-NJ,NCM523-SR,NCM811-NJ,and NCM811-SR respectively.Four samples were compared and analyzed in pairs.The electrochemical performance of NCM523-NJ and NCM523-SR is not much different,while the two samples of NCM811 are quite different.This is mainly due to the following points:（1）The post-added Li source in the hydrothermal synthesis method caused the violent mixing of Li/Ni,resulting in the formation of a spinel-like structure;（2）The hydrothermal method would make part of the Ni ions in the material are more likely to enter the Li layer,inhibiting the H2-H3 phase transition process,which is more obvious in high-nickel materials.But at the same time,the structure can be stabilized to a certain extent;（3）The hydrothermal method will increase the resistance of the material and decrease the capacity.Finally,we proved that different synthesis methods will have a greater impact on the generation of Li vacancies and Li/Ni mixing in the layered cathode.2.The solvothermal method assisted molten salt method was used for the first time to co-modify the titanium-based surface integration layer and bulk phase doping in the lithium-rich manganese-based layered cathode material.Electrochemical tests at a charge-discharge rate of1 C(200 m A g^-1),after 300 cycles,the discharge capacity of the Ti-150 sample is still as high as 235.0 m Ah g^-1.Even go through 500 complete charge-discharge processes,the specific discharge capacity of Ti-150 samples can still maintain 200 m Ah g-1.The average discharge medium voltage attenuation is also very tardily,reaching 0.72 m V/cycle（30-500 cycle）.This is caused by the following several parts:（1）The Ti-based surface integrated layer inhibits the interface reaction between the cathode material and the electrolyte;at the same time,it can strengthen the lattice structure;（2）the strong Ti-O bond formed by Ti doping slows down the migration of Mn and Ni ions;（3）Ti-based surface integrated layer slow down the activation of Li₂Mn O₃ phase and the evolution of the material from layered to spinel structure.In summary,the Ti-based integrated layer we constructed can greatly improve the cycle life of lithium-rich cathode material.