The Structural Evolution And Performance Improvement Of Ni-rich Layered Oxides

The lithium Ni-rich layered oxide is one of the most important cathode materials for the Li-ion batteries of the electric vehicles due to its high energy density and environmental friendliness.However,the structural and thermal stability of the Ni-rich materials decreases with increasing Ni content during cycling.The commercial applications of these materials are hindered with the irreversible structural and performance degradation due to the Li/Ni mixing,gas release and microcracks.On the basis of the background hereinbefore,this thesis investigated the evolution of the electronic and crystalline structures and the electrochemical performances of LiNi O₂,clarified the interplays between its cation mixing,anionic redox and the structural degradation and improved the comprehension of the mechanism of structural degradation in LiNi O₂.The universality of these interplays was studied in the Ni-rich oxide cathode materials.As an application of these findings,Nb doping was conducted to inhibit the cation mixing,enhance the structural stability and improve the electrochemical performance of LiNi O₂ on the basis of the above understandings.The38)structured LiNi O₂ with high and low intrinsic Li/Ni cation mixings（HCM and LCM）were prepared and their structural evolutions were characterized in the bulk and near the surface.In combination of the density functional theory calculations and experimental characterizations,the Li/Ni anti-siting was found thermodynamically inevitable in LiNi O₂.The Ni migration started at rather low charge potentials,became aggravated with the increasing Li vacancies and O defects,and was ended up with a rock-salt-like surface.Moreover,the Ni migration was found self-restricted to some extent because the presence of the Ni ions in the Li layer increased the migration energy barrier of the neighboring Ni ions.In addition,the oxygen redox occurred in LiNi O₂due to the intrinsic and extrinsic cation mixing that can results in Li-O-Li and/or Li-O-Vac _TM(Vac _TM is for the transition metal（TM）vacancy)configuration（s）.These findings clarified the interplays between the cation mixing,oxygen defect and structural degradation of LiNi O₂ and was helpful for innovatively designing high-capacity Ni-rich materials.The oxygen redox reaction and the structural reversibility of the deeply delithiated LiNi_0.83Co_0.12Mn_0.05O₂ were studied by the synchrotron X-ray absorption spectroscopy,scanning transmission electron microscopy and density functional theory calculations.The oxygen redox took place upon delithiation in this intrinsically Li-O-Li-free material due to the cation mixing.The I4₁ structure was recognized and its formation was attributed to the migration of the transition metals at the deep delithiation states,which deepened the understanding on the routes of the transformation from the layered to the rock-salt structures.Nb was homogeneously doped in LiNi O₂ by a wet chemistry method.Characterizations by the X-ray diffraction,scanning transmission electron microscopy and the synchrotron X-ray absorption spectroscopy showed that the Nb doping reduced the cation mixing in LiNi O₂.The Nb doping improved the specific discharge capacity and the coulomb efficiency of LiNi O₂ in the first cycle,increased its capacity retention from 67.0%to 86.7%in 100 cycles and enhanced its rate performance.The crystalline and electronic structural characterizations of the materials after prolonged cycling revealed that Nb doping boosted the structural stability by inhibiting the Ni migration.