Research On Structural Regulation And Electrochemical Performance Of Nickel-rich Cathode Material For Lithium-ion Batteries

Secondary batteries are efficient and convenient energy storage and conversion devices.In recent years,the new energy vehicle industry has developed rapidly,and the demand for lithium-ion batteries has ushered in an explosive increase.Nickel-rich cathode materials become the current research hotspot due to their high energy density.However,Nickel-rich cathode materials still have problems such as poor bulk phase and interface stability,and increasing the charge cut-off voltage will accelerate the failure of the material,which need to be solved urgently.This subject is based on high-energy density and low-cost nickel-rich cathode materials,firstly starting from interface and structure modification,respectively.Design and synthesis surface-coated,bulk-doped nickel-rich cathode materials to improve its electrochemical performance.Morever,the development of cobalt-free nickel-rich materials provides a new way for the development of low cost and high energy density nickel-rich materials in the future.The specific contents are as follows:1.Aiming at the problem of poor stability of the interface and crystal structure of NCM-811,a dual-modification method of Ti element bulk doping and Co oxide surface modification was used to obtain the NCM-811（TCo-NCM）cathode.On the one hand,the introduction of high-valence Ti ions can effectively reduce the mixing of lithium and nickel,expand the interlayer spacing,and promote the rapid transport of Li⁺;On the other hand,the spinel-phase Co oxide coating layer not only has high voltage stability,but also isolates the active material from the electrolyte,improving the interfacial stability of the material.The two work together to suppress the irreversible phase transition in the electrochemical process,slow down the shrinkage and expansion of the lattice,and effectively reduce the generation of micro-cracks in the particle during the cycle,improve the surface and bluk stability during the electrochemical processes.2.In order to simplify the two-step process of surface coating and bulk doping,a one-step Gd element modification strategy is proposed to achieve integrated.By increasing the sintering temperature and longer sintering holding timetime,a mixed coating layer of Gd₂O₃&LiGdO₂ is formed on the surface of the active material,which isolates the electrolyte and reduces the amount of residual alkali on the surface of the active material,and the coating layer with excellent Li⁺conductivity,builds a Li⁺fast diffusion channel.Moreover,the Gd ion concentration gradient doping is formed inward,enhances the reversibility of the phase transition during electrochemical cycling.3.In view of the high price and supply chain instability of Co source in the NCM-811material,Using Mn and Mg to replace Co,LiNi_0.95Mn_0.05O₂（NiMn）,LiNi_0.95Mg_0.05O₂（NiMg）cobalt-free materials were synthesized,and their structures and electrochemical properties were compared with those of LiNi_0.95Co_0.05O₂（NiCo）.The cycling and rate performances of the three materials show a trend of NiCo>NiMg>NiMn.The structures of the materials were further analyzed,and it was found that the size effect and coulomb effect of doping ions and Li⁺in the transition metal layer will increase the Jahn-Teller distortion of Ni³⁺,where the lattice distortion of NiMn material is the largest,its electrochemical performance is also the worst.Among the three materials,the cobalt-free NiMg material has a similar cycle performance to the cobalt-containing NiCo material and has potential application value.Aiming at the problem of poor rate performance of NiMg cobalt-free materials,a modification strategy for surface lithium-ions conductor LiAlO₂ coating was proposed.The existence of the uniform coating layer reduces the residual lithium on the surface,constructs a stable electrode-electrolyte interface,suppresses the particle fragmentation problem caused by lattice shrinkage and expansion during cycling,and builds a fast diffusion channel for Li⁺.The LiAlO₂ coating modification strategy makes up for the insufficiency of NiMg materials in rate capability and improves the cycle performance,providing a feasible and effective solution for the development of cobalt-free materials.