Surface Coating Design And Performance Of Nickel-rich Ternary-type Cathode Materials For Lithium-ion Batteries

Nickel-rich ternary materials are identified as the preferred cathode for lithium-ion batteries（LIBs）with a specific energy density of300 Wh Kg^-1,which has attracted extensive attention from academia and industry.However,the large-scale application of Nickel-rich ternary materials is limited by unstable crystal structure,excessive residual lithium and poor safety performance.In this paper,the most potential cathodes of Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）and Li Ni_0.83Co_0.11Mn_0.06O₂（NCM83）are chosen as the research objects.On the one hand,Y（PO₃）₃ is used to repair the storage degraded NCM811 materials.On the other hand,surface modification is adopted to reduce the residual lithium for NCM83 cathodes,so that to improve the electrochemical performance.Therefore,this work is important to promote the development of high specific energy lithium batteries.The specific works are as follows:（1）The evolution of surface structure and electrochemical properties of NCM811 are systematically studied after air-exposure.It is found that Li⁺ions in the bulk of the material can gradually migrate to the surface and react with H₂O and CO₂ to form the inactive Li₂CO₃/Li OH.Meanwhile,the partially layered structure can gradually transform into Ni O rock salt phase,which seriously deteriorates the electrochemical properties of the materials.Therefore,the yttrium metaphosphate（Y（PO₃）₃）was developed as a repair agent to reactivate the decayed material.The results show that the repaired materials can not only recover the original electrochemical properties,but also construct the Li₃PO₄-YPO₄-Y（PO₃）₃ multifunctional repair layer on the surface of the secondary particles.The optimized material shows excellent cyclic and thermal stability.The capacity retention rate is 92.3%after 100 cycles at 1 C,which is much higher than the bare sample（63.8%）.And the temperature of the maximum heat generation is increased from 217.4°C to 224.1°C.This work will provide an efficient repair technology to restore the degraded Nickel-rich cathode materials.（2）The stable Li₄Mn₅O₁₂ is successfully coated on the surface of NCM83 by a wet coating method.And the impact of Li₄Mn₅O₁₂ coating layer on the structure and properties of NCM83 material are systematically investigated.It is found that uniform and compact spinel Li₄Mn₅O₁₂coatings are formed in situ by the reaction between manganese acetylacetonate(C₁₀H₁₄Mn O₄)and residual lithium on the surface of the material.This method not only converts the harmful residual lithium on the NCM83 into electrochemically active Li₄Mn₅O₁₂,but also protects the active material from corrosion by electrolyte.The capacity retention rate of NCM83@LMO-2 cathode is 97%even after 100 cycles at 1C,which is much higher than the bare sample（73.9%）.And the specific discharge capacity of NCM83@LMO-2 can still reach 156.6 m Ah g^-1 at 5 C.Moreover,the discharge capacity（0.5 C）of NCM83@LMO-2 is 209.6m Ah g^-1,and the capacity retention rate is 83.7%after 60 cycles under the wide voltage window（2.7-4.6 V）.In addition,the heat release of NCM83@LMO-2 is 687.0 J g^-1,which is much lower than the pristine sample(909.8 J g^-1).Overall,the Li₄Mn₅O₁₂ coatings can significantly improve the electrochemical performances and thermal stability of Ni-rich cathode materials.