Doping And Coating Modification Of Nickel-rich Ternary Cathode Materials For Lithium-ion Batteries

Nickel-rich ternary cathode Li Ni_xCo_yMn_1-x-yO₂（0.6≤x<1）has the advantages of high energy density and long cycle life,and occupies an important position in the power battery market.However,the traditional polycrystalline nickel-rich ternary cathode suffers from poor structural stability and rapid capacity decay,and the single-crystallization strategy can effectively solve the above problems.However,the large size of single crystal particles increases the transport pathways of lithium ions,resulting in the poor rate performance.Doping metal ion and coating fast ion conductor are two effective modifications to improve the rate performance and cycling stability of single crystal cathode materials,but there are problems of uneven doping and inter-diffusion of transition metal ions during high temperature when coating fast ion conductors.In view of the above problems,in this thesis,the high nickel single crystal cathode was studied from the perspective of modification technology and electrochemical performance,providing a reference for the modification and application of single crystal cathode.A novel double hydroxyl alcoholic precipitation method with ethylene glycol as complexing agent was used to form organometallic salt precursors of La,Ti,Zr and Ta with three-dimensional network structure on the surface of single-crystal Li Ni_0.8Co_0.1Mn_0.1O₂（SC-NCM811）cathode particles,and different temperatures were used for heat treatment to obtain uniform metal oxide coated or metal ion doped SC-NCM811 cathodes.The effects of different metal oxide coatings and metal ion doping on the microscopic morphology,phase structure and electrochemical properties of SC-NCM811 cathodes were compared laterally,and the results showed that the cells assembled with Ti and Ta doped SC-NCM811 had better rate performance and cycle stability.Using fast ionic conductor（LATP）as coating layer can further improve the rate performance of single crystal cathodes.However,the phase of LATP is formed under high temperature,whereas high temperature during coating can lead to the transition metal diffusion.Accordingly,in this context,boron oxide was used as fusing agent to sinter fast ionic conductor LATP under low temperature（600°C）,and LATP was uniformly coated on the surface of SC-NCM811 cathode particles by sol-gel method.In the voltage range of 2.7-4.3 V and at the rate of 0.5 C,the modified single crystal sample with the best performance could retain the capacity of 157.2 m Ah g^-1 after 150 cycles with the retention rate of 85.02%.At the rate as high as 10 C,the discharge specific capacity could still reach 144.8 m Ah g^-1.And the charge transfer impedance（R）was significantly reduced,and the lithium-ion diffusion coefficient(D⁺_Li)was effectively increased.