Surface Coating Modification And Electrochemical Performance Study Of High Nickel Ternary Cathode Materials

At this stage,driven by the economies of scale of new energy vehicles,the power battery market is developing more and more rapidly,and the requirements for power batteries are more stringent.Among them,lithium-ion battery has the advantages of high energy density and environmental friendliness.It can be applied to portable electronic devices and hybrid electric vehicles.The specific capacity and structural stability will directly affect the comprehensive performance of lithium-ion batteries.The high nickel ternary cathode material Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）has better theoretical specific capacity and energy density than other cathode materials.However,NCM811 is subject to surface structure reconstruction,Li-Ni mixing,interfacial side reactions and bulk phase structure changes during charge and discharge,which affect the electrochemical performance of the battery.Therefore,this article aims to modify the surface coating of NCM811 cathode material,so as to change the material composition and structure of the cathode material for the purpose of achieving higher electrochemical performance.In this paper,NCM811 was selected as the active material of cathode material.NCM811@Co₃O₄,NCM811@c PAN and NCM811@c PAN@ZIF-8 with different coating layers were prepared by simple wet chemical and heat treatment method,which were used as lithium-ion battery cathode materials to explore their electrochemical properties,The main study contents are as follows:（1）Using NCM811 as the main body and Co（NO₃）₂·6（H₂O）as the cobalt source,a uniform and continuous nano-scale Co₃O₄ coating was formed on the surface of NCM811by PVP-assisted and wet chemical methods.The effects of Co（NO₃）₂·6（H₂O）and PVP content on the composition phases,microscopic morphology of NCM811 and electrochemical properties at different cut-off voltages were studied.The results showed that the electrochemical performance of NCM811@Co₃O₄ cathode material at different cut-off voltages showed a trend of increasing and then decreasing with the increase of Co₃O₄ content.When the Co₃O₄ content was 1 wt（4）,the NCM811@Co₃O₄ cathode material has an initial discharge specific capacity of 182.50 m Ah·g^-1 at 0.1 C and a specific discharge capacity of 151.10 m Ah·g^-1 at 2 C at a voltage window of 2.8-4.3 V.The initial discharge specific capacity of 205.30 m Ah·g^-1 at 0.1 C,the capacity retention rate after 100 cycles was 87.15（4）at 1C at the high voltage window of 2.8-4.6 V,both of which showed better electrochemical performance than NCM811.（2）Using NCM811 as the main body and polyacrylonitrile（PAN）as the precursor of the coating material.Uniform nanoscale c PAN coatings were formed on the surface of NCM811 by heat treatment and wet chemistry.The effects of heat treatment temperature and c PAN content on the physical phase composition,microscopic morphology and electrochemical properties of NCM811 were investigated.The results show that when the heat treatment temperature was 400°C and the c PAN content was 2 wt%,the NCM811@c PAN has an initial discharge specific capacity of 208.60 m Ah·g^-1 at 0.1 C and a discharge specific capacity of 179.30 m Ah·g^-1 at 2 C,the discharge specific capacity after 100 cycles at 1 C was 154.40 m Ah·g^-1,and the capacity retention rate was 90.50%.The formed c PAN coating not only reduces the direct contact between NCM811 and the electrolyte but also increases the e^-transport rate.（3）Using NCM811 as the main body and NCM811@c PAN as the template,ZIF-8with high ionic conductivity was coated on the surface of NCM811@c PAN by wet chemical method as the cathode material.The coated ZIF-8 has a large surface area and high porosity,and the sufficient air space can alleviate the volume change in the process of Li⁺deintercalation,and the existence of c PAN and ZIF-8 double coating can provide channels for the diffusion of Li⁺and e^-during charging and discharging,which promotes the transport rate of Li⁺and e^-.The results show that the electrochemical performance of the c PAN and ZIF-8 double-coated cathode materials is better than that of the c PAN single-coated material than that of NCM811.When the ZIF-8 content was 1.0 wt（4）,the NCM811@ZIF-8 cathode material has an initial capacity of 180.20 m Ah·g^-1 at 0.1 C and the specific discharge capacity of 155.53 m Ah·g^-1 at 2 C.When the coating content was1 wt（4）-c PAN and 1.0 wt（4）-ZIF-8,the NCM811@c PAN@ZIF-8 cathode material has an initial capacity was 209.66 m Ah·g^-1 at 0.1 C,and the coulomb efficiency was 97.27（4）,and the discharge specific capacity was 158.80 m Ah·g^-1,and the capacity retention rate was 88.69（4）after 100 cycles at 1 C.