Study On Surface Coating Modification Of Ni-rich Cathode Material LiNi_0.8Co_0.1Mn_0.1O₂

With the development of new energy vehicles,researchers are paying more and more attention to lithium-ion batteries with high energy density,high power,and high safety performance.Compared with the graphite anode with a specific capacity of more than 350m Ah/g,the anode capacity of a lithium-ion battery is the main factor limiting its energy density.Among many cathode materials,the layered high nickel ternary material Li Ni_0.8Co_0.1Mn_0.1O₂has an actual specific capacity exceeding 200 m Ah/g.However,due to the high nickel content of Li Ni_0.8Co_0.1Mn_0.1O₂,the mixing of Li/Ni cations is serious,the lithium residue on the particle surface is high,the capacity retention rate is low,the power is low,the thermal stability is poor,and the safety performance is low.It severely restricts the large-scale commercial application of Li Ni_0.8Co_0.1Mn_0.1O₂.In order to improve the cycle stability and safety performance of Li Ni_0.8Co_0.1Mn_0.1O₂,this paper adopts the research strategy of surface modification,graphite-like carbon nitride（g-C₃N₄）,poly 3,4-ethylenedioxythiophene（PEDOT）and Polyaniline（PANI）are coated on the surface of positive electrode particles or cathode pole pieces by different methods to improve the electrochemical cycle stability of Li Ni_0.8Co_0.1Mn_0.1O₂ positive electrode material.The specific research content is as follows:（1）g-C₃N₄ coated Li Ni_0.8Co_0.1Mn_0.1O₂ was prepared by wet method and heat treatment.XRD,SEM,TEM and FT-IR tests show that g-C₃N₄ can be coated on the surface of the positive electrode and has no effect on the crystal structure of the positive electrode material.After 100cycles（0.2C）of 2wt.%g-C₃N₄ coated Li Ni_0.8Co_0.1Mn_0.1O₂,the capacity retention rate was98.37%,while the pristine material was 90.18%.When rate was 1C,the capacity retention rate increased from 72%of the pristine material to 85.90%.After 100 cycles（1C rate）at 55°C,the capacity retention rate of the material modified by 2wt.%g-C₃N₄ was 80.78%,while pristine material was 61.55%.The results show that Li Ni_0.8Co_0.1Mn_0.1O₂ coated with 2wt.%g-C₃N₄ has higher cycle stability and thermal stability than the pristine material.（2）In the three-electrode system,the cyclic voltammetry method or potential polymerization method can cycles between-0.2～1.3 V,and electrochemical polymerization was used to prepare PEDOT-coated Li Ni_0.8Co_0.1Mn_0.1O₂ positive pole pieces.SEM,TEM and FT-IR tests show that PEDOT is not only successfully coated on the surface of the positive pole piece,but also exists in the gap which inside of the pole piece.Among them,the PEDOT-coated pole piece synthesized after 20 cycles of cyclic voltammetry scanning has the best rate performance and cycle performance.After 100 cycles（1C rate）,the discharge specific capacity of PEDOT-CV20 dropped from 187.50 m Ah/g to 171.25 m Ah/g,and the capacity retention rate was 87.28%,which was higher than the 72.74%capacity retention rate of the pristine material.The PEDOT coating can enhance the electronic conductivity on the electrode surface,and effectively inhibit the side reaction of the positive electrode active material and the electrolyte,avoid the elution of transition metal ions,reduce the polarization of battery,and improve the cycle life of the pole piece.（3）In the three-electrode system,the cyclic voltammetry method or potential polymerization method can cycles between-0.2～1 V,and electrochemical polymerization was used to synthesize PANI-coated Li Ni_0.8Co_0.1Mn_0.1O₂ positive pole pieces.SEM,TEM and FT-IR proved that PANI not only successfully polymerized on the surface of the positive electrode particles,and but also exists in gap which inside of the pole piece.Among them,the PANI-coated positive electrode sheet synthesized by cyclic voltammetry scan 3 times has the best rate performance and cycle performance.Its discharge capacity in high rate is higher than pristine material.After cycled 100 times at 1C rate,The specific discharge capacity of positive electrode coated with PANI-CV3 decreased from 197.78 m Ah/g to 176.31 m Ah/g,the capacity retention rate was 89.14%,and the capacity retention rate of the pristine electrode was only 71.72%.Therefore,the PANI coating can improve the rate performance and cycle stability of the positive pole piece.This is because the PANI coating has higher electronic conductivity,can increase rate of electrochemical reaction of the electrde,and effectively avoid the corrosion of the active material by the electrolyte.Thereby improving the stability and safety of the positive pole piece.