Structure Design,Preparation And Structure-activity Relationship Of High Nickel Cathode Materials

As an energy storage conversion device with good comprehensive performance,lithium-ion batteries（LIBs）are considered as a new generation of green high-energy batteries.Lithium-ion batteries have been widely used in micro electronic devices such as mobile phones,tablet computers and digital cameras due to the high discharge specific capacity,long cycle life,environmental friendliness and memoryless effect.However,the power density/energy density and safety of LIBs can not fully meet the needs of the development of electric vehicles and hybrid vehicles.The cathode material is an indispensable part of lithium-ion battery,closely related to the capacity and safety of lithium-ion battery.In order to achieve the large-scale energy storage,the development of cathode materials with excellent performance has become one of the research hotspots in the field of lithium ion batteries.Among the high nickel cathode materials for lithium ion batteries,LiNi_0.8Co_0.1Mn_0.1O₂ and LiNi_0.8Co_0.15Al_0.05O₂ are used as power cells in Tesla and BMW electric vehicles because of the high discharge specific capacity and good commercial value.However,there are still some problems in the two materials.The composition and content of transition metal elements in the two materials are different,playing different typical roles.Low Co content in high nickel LiNi_0.8Co_0.1Mn_0.1O₂ material leads to low electronic conductivity and poor rate performance.In addition,interface side reactions and surface residual lithium lead to poor cycle stability.Compared with NCM material,high nickel LiNi_0.8Co_0.15Al_0.05O₂material is more prone to experience the phase transition from layered phase to rock salt phase.The above problems of high nickel materials hinder their large-scale application,which need to be optimized and improved.The purpose of this paper is to develop and design high-performance high nickel cathode materials.Through the structural analysis and electrochemical performance analysis of the materials,the mechanism of element doping and core-shell structure on the modification of high nickel LiNi_0.8Co_0.1Mn_0.1O₂ and LiNi_0.8Co_0.15Al_0.05O₂ materials is deeply explored.（1）Aiming at the poor rate performance of LiNi_0.8Co_0.1Mn_0.1O₂ cathode material,Na⁺was chosed to introduce into the lithium layer.The larger radius of Na⁺expands the spacing of lithium ion layers effectively,by which we designed a high nickel cathode material assembled to lithium-ion battery with high rate performance.And the effect of Li sites doped large radius Na⁺on the internal structure and electrochemical properties of the material was systematically explored.The results show that Li site doping with large radius Na⁺increases the lithium layer spacing from 2.5810?to 2.6030?,provides a wider channel for Li⁺de-intercalation and reduces the diffusion barrier of Li⁺.Moreover,Na⁺doping reduces the polarization and voltage drop,enhances the reversibility of phase transition between H2-H3.Li_0.99Na_0.01Ni_0.8Co_0.1Mn_0.1O₂material shows excellent rate performance.At high current density of 5 C,the specific discharge capacity is up to 136 m Ah g^-1,which is25 m Ah g^-1 higher than the pristine sample.（2）In order to improve the poor cycle stability of LiNi_0.8Co_0.1Mn_0.1O₂ cathode material,Zr was selected as the doping element,and the effects of doped Zr⁴⁺on the internal structure and electrochemical behavior of LiNi_0.8Co_0.1Mn_0.1O₂ material were studied.The results reveal that the high valence Zr⁴⁺can adjust the valence of transition metal elements,reduce the content of highly active Ni³⁺on the surface and inhibit the interfacial side reaction between the material and the electrolyte.There is a strong binding energy between Zr and O.The doping of Zr⁴⁺not only regulates the Li/Ni mixing on the surface of the material to stabilize the internal structure of the material,but also optimize the thermal stability and cycle stability of the material.The discharge specific capacity for Li_0.97Zr_0.03Ni_0.8Co_0.1Mn_0.1O₂ material is 106 m Ah g^-1and the capacity retention is 71%after 500 cycles at 5 C.（3）In order to effectively solve the serious phase transition of LiNi_0.8Co_0.15Al_0.05O₂ material,La was selected as the doped element on the basis of the advantages of large radius and high valence elements.And the influence of La³⁺on the structure and electrochemical properties of LiNi_0.8Co_0.15Al_0.05O₂ cathode material has been deeply explored.La³⁺doping enhances the ability of transition metals to transfer electrons around oxygen atoms,improves the migration energy barrier of transition metal elements from transition metal octahedron to lithium octahedron,and inhibits the phase transition of materials from layered spinel phase to rock salt phase effectively.Thereinto,the anionic redox reaction turned out to be existed in non lithium rich materials.It is concluded through the first principle calculation that La³⁺doping enhances the ability of oxygen electron loss in the materials and the activity of anionic redox reaction,which makes up for the adverse effect of La as an inert element on the specific capacity of the first cycle discharge of the materials.The initial discharge specific capacity of the Li_0.99La_0.01Ni_0.8Co_0.15Al_0.05O₂ material(about 185 m Ah g^-1)is 20 m Ah g^-1 higher than that of the pristine material.（4）In order to further optimize the material structure and suppress the generation of microcracks,the core-shell structure cathode material with high nickel in the core and high cobalt on the surface was designed by using the characteristic reaction of dimethylglyoxime and Ni²⁺.The core-shell structure inhibits the adverse interface side reaction between electrolyte and material and the change of CEI on the surface of cathode material during the process of charge and discharge.What’s more,this distinctive structure improves the reversibility of phase transition between H2-H3 and shows excellent electrochemical properties.Unlike pristine material,the capacity retention rate of LiNi_0.8Co_0.15Al_0.05O₂（7%C₄H₈N₂O₂）material is increased from 45%to 73%after 200 cycles at high cut-off voltage 4.5 V（5 C）.Based on the analysis of neutron diffraction structure,it is concluded that the core-shell structure inhibits the Li/Ni mixing,improving the diffusion rate of lithium ions and rate performance.It is well known that the cathode particles will form micro-cracks during the cycle,and the electrolyte can penetrate the particles to severely collapse the material structure.The core-shell structure is confirmed to effectively inhibit the occurrence of this phenomenon.In order to verify the universal applicability of core-shell structure modification methods,an extension work is carried out on the basis of this work.The core-shell structure is applied to the O3 phase cathode material of sodium ion battery.The research shows that the core-shell structure also improves the structural stability of sodium ion battery.In order to further improve the rate performance of the material,Y³⁺doping was carried out on the basis of core-shell structure.The synergistic effects of core-shell structure and element doping on the material structure and electrochemical properties were fully discussed.The core-shell structure with high nickel in the core and high manganese on the surface combined with Y³⁺doping inhibit the phase transition,the change of cell parameters during the process of charge and discharge,and improve the structural stability of the material.Neutron diffraction results show that Y³⁺doping further expands the sodium layer spacing,increases the diffusion rate of Na⁺and rate performance.