Preparation And Modification Of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ As Cathode Material For Lithium Ion Batteries

High energy density is crucial for the application of lithium ion battery in portable electronic products and electric vehicles.It is necessary to develop high performance cathode materials for lithium ion batteries.Compared with other cathode materials,lithium-rich cathode material xLi₂MnO₃??1-x?LiMO₂?M=Mn,Ni,Co etc.0?x?1?with a high actual discharge specific capacity of exceeding 250 mAh/g.However,severe capacity fading and voltage dropping still exist in the cycling process,which limit its practical application.Therefore,it is necessary to further improve the performance of lithium-rich cathode materials.In this work,Li-rich material Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ was synthesized via a sol-gel method and optimized the parameters during calcining process.Doping and coating modification of the materials were investigated.Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ was synthesized via a sol-gel method,the crystal structure and morphology of the material were characterized and the electrochemical performances of the material were investigated.According to the experimental results,the optimal condition is that the material was calcined at 900?for 12 h and the amount of lithium excess 5%with the best properties.At this point,the material with ordered layered structure and low degree of Li⁺and Ni²⁺mixing,the particles were shown irregular polyhedral morphology and homogeneous distribution.The initial discharge capacity of the material was 264.4 mAh/g at 0.1 C current density.After 50 cycles,the specific capacity was 213.1 mAh/g and corresponding capacity retention rate was 80.6%.To improve the electrochemical properties of the material,the material was modified by Cr doping,F doping and Cr&F co-doping.Cr doping can increase the lattice parameters of the material,facilitate the diffusion of Li⁺,and improve the reactivity of the material.When the doping amount of Cr was 0.04,the initial discharge specific capacity of the material was 280.7mAh/g at 0.1 C rate,and the average discharge specific capacity was 130.4 mAh/g at 5 C rate.F doping can make the crystal structure of the material more integrity and improve the cycling stability significantly.When the doping amount of F was 0.03,the capacity retention rate was94.32%at 0.2 C rate after 50 cycles.The Cr and F co-doped sample with well layered structure and high crystallinity.The electrochemical properties of the co-doped material were greatly improved.The initial discharge specific capacity of the co-doped material was 261.8 mAh/g at 0.2 C rate,the capacity retention rate was 94.76%after 50 cycles,which much higher than that of undoped sample?78.84%?.The discharge mean voltage of the co-doped sample dropped 0.2945 V after 50 cycles,which was evidently lower than that of the undoped sample?0.4396 V?.Certain amount of Cr and F doping can stabilize the crystal structure of the material,improve the electrochemical reversibility of the electrode,reduce the electrochemical polarization in the delithiation-lithiation process and improve the cycling stability and rate performance.FePO₄ was coated on the surface of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂,the experimental results show that FePO₄ layer was uniformly coated on the surface of the material,and small amount of coating did not change the layered structure of the material.5%FePO₄ coated material with the best performance.The initial charge and discharge specific capacity were 324.4 mAh/g and 280.4 mAh/g at 0.1 current density,corresponding initial coulomb efficiency was 86.4%,which was remarkably improved compared with the uncoated material?75.6%?.The discharge specific capacity was 134.8 mAh/g at 5 C rate,higher than 95.3 mAh/g of the undoped material.FePO₄ coating can reduce the oxygen loss during the activation of Li₂MnO₃ phase,thus improving the initial coulomb efficiency.Furthermore,FePO₄ coating can effectively prevent the contact between active material and electrolyte on the surface of electrode,reduce the dissolution of metal ions,stabilize the electrode interface,and increase the electrochemical performances.