Research On C Coating And CoAl₂O₄ Coating Of Ternary Cathode Materials For Lithium-ion Batteries

Since LiNi_1/3Co_1/3Mn_1/3O₂?NCM111?ternary cathode material was successfully synthesized in 1999,it has attracted extensive attention of researchers because it combines the advantages of lithium cobalt oxide?LiCO₂?,lithium nickel oxide?LiNiO₂?and lithium manganate?LiMn₂O₄?.A good two-dimensional channel in the layered structure of?-NaFeO₂ contributes to the rapid de-embedding of Li⁺.It has good capacity characteristics?170-220 mAhg-1?,working voltage matching with the existing electrolyte?4.1V?.At the same time,the introduction of manganese reduces the overall price of materials and is more suitable for large-scale commercial production.However,the low lithium ion diffusion coefficient and electronic conductivity of ternary materials make the material's rate performance unsatisfactory,and the capacity retention rate is not high after many charges and discharges.These factors restrict the further development of ternary materials.At present,coating modification can effectively improve the cycling performance of materials,mechanical separation of cathode materials and electrolyte direct contact,alleviate the formation of solid electrolyte membrane?SEI?,thereby improving the electrochemical performance.In this paper,firstly,hollow-microsphere MnO₂ powder was prepared as manganese source to synthesize hollow structure LiNi_1/3Co_1/3Mn_1/3O₂.Sucrose was used as carbon source to coat ternary cathode material LiNi_1/3Co_1/3Mn_1/3O₂.It was found that the coating did not change the structure of the material's?-NaFeO₂.Carbon coating enhanced the conductivity of interparticle and surface electrons and reduced the polarization of the battery.As a result,the electrochemical performance of the material has been improved obviously,especially when the coating content is 0.5 wt%.Then,the influence of a spinel structure CoAl₂O₄ on the electrode material was discussed.Firstly,manganese dioxide spherical shell was successfully prepared using carbon sphere as template and potassium permanganate as manganese source.With this manganese dioxide,Li_1.2Ni_1/3Co_1/3Mn_1/3O₂ hollow microsphere cathode material was successfully synthesized.Different content of CoAl₂O₄?0-5 wt%?coating was prepared at high temperature,and the surface modification was studied.The effect of the surface modification on the structure and electrochemical performance of the cathode was evaluated.Microstructural analysis using X-ray diffraction,scanning electron microscopy,and transmission electron microscopy revealed that the CoAl₂O₄ coating did not affect the crystal structure of the electrode material and that the coating only appeared on the electrode surface.The effect of the CoAl₂O₄ coating on the electrochemical performance was evaluated by examining charge/discharge cycles,rate performance,and electrochemical impedance.The cathodes coated with CoAl₂O₄ exhibited improved discharge capacity and cyclic performance compared with those of the uncoated cathode.The electrode coated with 2.5 wt%CoAl₂O₄ exhibited the highest discharge capacity(202.5 mAhg^-1 vs.185 mAhg^-1 for the uncoated material)and highest capacity retention?84.8%for 50 cycles vs.78.6%for the uncoated material?.