Preparation And Study Of High Performance Layered Oxide Cathode For Lithium-ion Batteries

Lithium-ion batteries,a representative of new energy sources,have been widely used in different fields such as portable devices（such as mobile phones,digital cameras）and electric vehicles since they were successfully commercialized in the 1990.Although this new energy system has been a great success,the current lithium-ion batteries still do not meet the main requirement of industry production and human life.Therefore,the development of higher performance lithium-ion batteries is currently the primary goal of the scientific research community and industry.It is well known that the common cathode materials include layered oxide cathode Li Ni_xCo_yMn_zO₂,Olivine Li Fe PO₄and Spinel Li Mn₂O₄.Among these cathode,layered oxide cathodes Li Ni_xCo_yMn_zO₂are considered as the most promising candidates cathode materials for the next generation of high-performance lithium-ion batteries.High-nickel Li Ni_xMn_yCo_zO₂（NMC）have the advantages of high specific capacity,high average voltage and good magnification performance,which has gradually become the most concerned component among many NMC components.However,there are also problems with high-nickel NMC,the most notable of which is the problem of poor thermal stability and cyclic stability.These two problems must be solved before it can be successfully commercialized.In this paper,according to the problems mentioned above,a series of Li Ni_xMn_yCo_zO₂（NMC）are selected as the basic materials to systematically study the discrepancies of structure and electrochemical performance among NMC with different components.The mechanism behind these differences are systematically investigated and summarized,which is helpful to the development of high-performance NMC cathodes.In this work,the key problems including the thermal stability,the causes of the formation of Li/Ni in the structure,the influence of Li/Ni disorder on electrochemistry and the dissolution of transition metal ions are studied systematically.In the systematic research of thermal stability of ternary materials,through the combination of in-situ heating X-ray diffraction,Thermogravimetric Analyzer and first principle calculation,the structural unit determining the thermal stability of NMC is innovatively put forward,and the influence of different elements composition and Li/Ni disorder on thermal stability is further revealed.Then the formation mechanism of Li/Ni disorder in NMC is comprehensively understood.Through the combination of neutron diffraction and high energy XRD,the internal driving force of Li/Ni disorder is proposed for the first time by combining the magnetic frustration within transition metal layer and the super-exchange interaction between the layers.Then,the effect of Li/Ni disorder on electrochemical properties was further studied,and the effects of different elements in the structure of layered materials,as well as the subsequent electrochemical properties and the stability of structural oxygen were analyzed from the point of view of structural and electrochemical properties,using the comparison between two elements layered cathode and three elements layered cathode.In addition,the relationship between transition metal dissolution and structural stability of transition metal oxide cathode materials is studied in detail,and the inevitable connection between transition metal dissolution and irreversible phase change is revealed by the state of art characterizations（including various characterization of synchrotron radiation,neutron diffraction,scanning transmission microscope for spherical differential correction）.In general,through the comparison of these structures properties and electrochemical performance of NMC and the understanding of the mechanism behind,we have a deeper and more comprehensive basic understanding of NMC.These mechanisms can be used as a theoretical basis for the design of the next generation of high-performance cathode materials to deliver better battery performance.