Study On Preparation And Modification Of Lithium Rich Manganese Based Cathode Materials

Lithium ion batteries（LIBs）,as the core energy supply component of portable intelligent communication equipment and commercial new energy electric vehicles（EVs）,have always been the focus of the industry.Based on safety,it is the fundamental goal of researchers to achieve high discharge power,high specific energy,long battery life,high voltage,high specific capacity,low operating cost,small pollution and portable LIBs to meet the diversified functions of electronic products.Compared with LIBs anode material,the performance of cathode material plays a decisive role in the electrochemical performance of LIBs.Although common commercial LIBs cathode materials have entered mature commercialization,their theoretical capacity still cannot meet the increasing performance requirements of EVs.Layered lithium-rich manganese-based oxide（LLMO）materials have attracted wide attention in the academic field in recent years due to their excellent working voltage platform（4.5V）,high specific capacity(theoretical capacity≥250m Ahg^-1)and good machinability.However,the commercialization process of LLMO materials has been limited by the problems of poor cyclic stability,serious voltage attenuation and poor rate performance caused by high capacity.Therefore,it is of great significance to further study on the optimization of the synthesis route and the modification of LLMO materials for promoting the development of new energy industry.In view of this,this paper will focus on improving the cyclic performance,the initial Coulombic efficiency（ICE）,the rate performance and the remittence of voltage attenuation of LLMO materials.Carrying out research on the synthesis route optimization,surface coating and modification treatment of LLMO materials.The research contents are briefly described as follows:（1）LLMO is prepared by co-precipitation method and sol-gel method respectively.By comparing the effects of different preparation methods on the yield,morphology,structure and electrochemical properties of the materials,the co-precipitation method is selected as the most suitable industrial preparation method.The Co-containing/Co-free LLMO materials are prepared by co-precipitation method for SEM characterization and electrochemical performance tests,and the Co-containing LLMO material is selected as the research subject.The effects of pH,reaction time and complexing agent concentration on the morphology of the precursor are investigated,and the preparation process of the precursor is optimized.The influences of temperature and time on the structure,morphology and electrochemical properties of the materials in the solid-phase process are investigated to optimize the most suitable solid-phase process.（2）By the comproportionation of MnSO₄ and KMnO₄,single-crystal MnO₂（SCMO） shells are grown in-situ on the surface of LLMO microspheres.SEM characterization shows that the SCMO shell has a composite structure of inner particles（p-SCMO）/surface sheets（s-SCMO）.The first discharge specific capacity of the best prepared sample can reach 238.2m Ahg^-1 at 1C current density.Due to the excellent structural stability and low Li⁺migration barrier,the single crystal phase has excellent cycling performance（200 cycling capacity retention rate of 70.6%）and rate performance in the full-cell test at 2C current density.In-situ XRD test shows that the structure of the coated material does not change due to the activation effect during the initial charge and discharge process,and the ICE（0.05C,93.0%）of the material is improved due to the excellent lithium storage properties of the SCMO.SEAD images of different cycles revealed that new single-crystal Li₂MnO₃ phase is formed during the cycling process.Based on first-principles calculations,the results fully support the fact that the single crystal Li₂MnO₃ shell has a lower Li⁺migration barrier,which improves the rate and cycling properties of the material.（3）The Li⁺is removed from the surface of the LLMO material by ion exchange with acid and LiClO.The etching XPS characterization indicats that the delithium depth is5nm and the lithium content of the lithium-poor layer was about 1/3 of untreated samples,which improved the ICE of the material（0.05C,95.4%）.HRTEM characterization shows that the acid-treated samples had spinel mosaics,which resulted in a 200 cycles voltage attenuation of 27.16m V.FIB-SEM test shows that the material after acid treatment has the morphology of hollow double shell,and FIB-EDS shows the doping of acid ions in the bulk phase of the material,which improves the overall stability of the material.First-principles calculations show that the doping of PO₄^3-increases the vacancy energy of transition metal and oxygen in the material,and in-situ electrochemical mass spectrometry（in-situ DEMS）confirms that the acid-treated samples has less oxygen loss during charge and discharge process.