Study On The Structure-effect Relationship Of Polycrystalline Nickel-rich Lithium-ion Battery Cathode Materials

In the context of the global search for alternative fossil fuel energy sources and"carbon peaking"and"carbon neutrality",lithium-ion battery technology is developing rapidly.Polycrystalline nickel-rich layered ternary cathode materials have attracted the attention of the industry with their excellent capacity characteristics and outstanding cycle stability.However,problems such as cation mixing,phase structure evolution and the low first charge/discharge capacity limit the further application of polycrystalline nickel-rich cathode materials in lithium-ion power batteries.In this paper,the causes of capacity degradation in nickel-rich Li Ni_1-x-yCo_xMn_yO₂（NCM）cell systems are explained from the perspective of the degradation of long-cycle performance induced by phase change in the bulk phase and the effect of polycrystalline grain boundary barriers on first-cycle capacity loss using various electron microscopy techniques,and the constitutive relationships of polycrystalline nickel-rich layered ternary cathode materials are constructed.1.In this work,high-angle annular dark field images（HAADF）and annular bright field images（ABF）combined with electron energy loss spectroscopy（EELS）analysis were used to confirm the oxygen release behavior of the NCM622 material during charging and discharging,as well as the bulk phase transition from the layered to the domain rock salt phase.The reversible strain distribution around the phase change was explored by peak pair analysis（PPA）to visualize the effect of kinetic constraints on lithium-ion transport at the（003）crystal plane on cycling stability and capacity retention.2.The potential distribution at the grain boundaries of NCM811 polycrystals before and after coating was investigated using electron holography.It was confirmed that the double coating of Al₂O₃at the surface and interface reduces the overall average internal potential and forms a’lithium trap’at the interface where some lithium ions are trapped,resulting in irreversible capacity degradation during the first cycle.The research in this paper clarifies the performance degradation of polycrystalline nickel-rich lithium-ion batteries and provides ideas for understanding the kinetic mechanism of lithium-ion transport,which also has important guiding significance for the selection of future cathode materials and the optimization of battery structures.Later,we will improve the structure of polycrystalline nickel-rich layered ternary cathode materials in three ways:increasing the nickel content to enhance the battery capacity,solid electrolytes are chosen to replace traditional liquid electrolytes to improve battery safety issues,and choosing a suitable surface interface cladding material to improve the structural stability and grain boundary ion transport performance of the electrode material.