Coating Modification Of Ni-rich LiNi_0.8Co_0.1Mn_0.1O₂ Cathode Materials

Ni-rich layered Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）cathode material are recognized to have the advantages of large reversible specific capacity,low cost and wide operating voltage range.However,the electrochemical performance of NCM811 gradually deteriorated so that its wide application.This is mainly due to that NCM811 often suffers from cation mixing,irreversible phase transitions,oxygen release,transition metal dissolution,micro-crack,etc.Surface coating is considered as one of the most simple and effective modification methods,which can stabilize the physical and chemical properties of the material surface,improve the migration of Li⁺,reduce the interfacial impedance and inhibit the interfacial side reaction.In this paper,three kinds of coatings were prepared on the surface of NCM811 by different preparation methods.In addition,the key factors and mechanism of coating modification to improve its electrochemical performance were studied in depth.In this study,a polysiloxane（EPS）coating containing ethoxy functional group was formed on the surface of NCM811 particles by in situ hydrolysis and condensation reaction of ethyl orthosilicate（TEOS）,and its electrochemical properties and coating modification mechanism were studied.The results show that the sample with EPS coating content of 3%has the best electrochemical performance.Accordingly,excellent capacity retention of 95.6%（100 cycles,1C）and rate performance（118.2 m A h/g,5C）which is higher than that of uncoated sample（108.6 m Ah/g）were obtained.Then,the electrochemical properties were investigated in detail by XRD,SEM,TEM,XPS,CV and EIS.The results displayed that a good barrier buffer layer was formed on the surface of the coated sample.And the micro-cracks of primary-like particles were effectively suppressed under coating and the cyclic stability of NCM811 was improved.Secondly,by adjusting the amount of lithium（Li OH）,the process of Si O₂-Li₂Si O₃coating formed by solid state reaction of lithium residue and EPS（mole ratio of 3%）on NCM811was studied,and its influence mechanism on the electrochemical performance of NCM811was discussed.The results show that the modified NCM811 exhibits excellent cycling and rate performance when the lithium exceeds 9%.The specific capacity at 0.05 C was 222.4m Ah/g,the capacity retention rate was 92.0%after 100 cycles at 1 C,and the specific capacity at 10 C was 131.1 m Ah/g,which was much higher than that of the uncoated sample（82.2 m Ah/g）.Then,the electrochemical properties were investigated in detail by XRD,SEM,TEM,XPS,CV and EIS.The results displayed that the EPS reacts with Li OH in the process of high temperature heat treatment to generate Si O₂-Li₂Si O₃coating,reducing the surface residual lithium,which not only slows down the surface side reaction,but also improves the diffusion rate of Li⁺,greatly improving the discharge capacity and rate performance.EPS coating layer was formed by in-situ hydrolysis of TEOS on the surface of precursor Ni_0.8CO_0.1Mn_0.1（OH）₂.NCM811 coated with Li₂Si O₃ was prepared by high temperature calcination（LSO）,and PPy was coated on the surface of LSO to obtain LSO-PPy double-coated sample.The results show that the sample has excellent electrochemical performance when the content of PPy is 1%.The first coulomb efficiency reached 87.27%,the capacity retention reached 93.7%after 100 cycles at 1 C,and the specific capacity at 10 C was 147.3m Ah/g.Then,the electrochemical properties were investigated in detail by XRD,SEM,TEM,XPS,CV and EIS.The result found that the existence of double coating can effectively reduce the irreversible phase transition and electrochemical polarization of the material,and can increase the migration of Li⁺and electronic conductivity,which contributes to the improvement of cycling and rate performance.