Study On Surface Modification And Lithium Storage Performance Of Nickel-rich Ternary Cathode Materials

Ni-rich layered cathode materials LiNi_xCo_yMn_1-x-yO₂?x+y+z=1,0.5<x<1,NCM?are the most promising cathode materials for lithium ion batteries,due to its high specific capacity,low cost and environmental friendliness.However,the commercial application process of Ni-rich layered cathode materials are hindered by capacity decay and low safety due to the structural and thermal instability.In this paper,the surface modification and lithium storage performance of nickel-rich ternary cathode materials are studied to improve the rate performance and cycle stability.LiNi_0.8Co_0.1Mn_0.1O₂?NCM811?is selected as the research object to explore the preparation process of NCM811.Meanwhile,the influence mechanism of Sn doping and Ag coating modification technology on the structure and electrochemical performance of NCM811 is studied.Finally,the NCM811material with excellent performance is obtained.The main contents and results of this paper are as follows:?1?Spherical NCM811 cathode material is successfully prepared by co-precipitation method and then high temperature lithiation process.By investigating the influence of lithium type,lithium content and sintering atmosphere on the structure and electrochemical property of NCM811,it is determined that LiOH with 5%excess and oxygen atmosphere are the best conditions of the process.The prepared NCM811 has a good layered structure with the initial discharge specific capacity of 181.1 mAh·g^-1,and the capacity retention rate after circulating 90 cycles at 1 C(1 C=200 mA·g^-1)current density is 84.94%.?2?The NCM811 material is further doped with SnO₂ as the Sn source.And the effect of Sn doping on the structure and electrochemical performance of NCM811 are also investigated.The Sn doped NCM811 has the enlarged interlayer spacing,reduces degree of Li⁺/Ni²⁺mixing,forms Li₂SnO₃ crystal phase surface layer with reduced electrode polarization,smaller impedance and increases Li⁺diffusion coefficient.Therefore,the Sn-doped NCM811 material exhibits enhanced initial discharge specific capacity,rate performance and cycle stability.The 2 wt%Sn-doped NCM811 material shows the most outstanding electrochemical performance,which initial discharge specific capacity at 0.1C current density is 186.5 mAh·g^-1,when current density increases to 10 C,the discharge specific capacity is 113.4 mAh·g^-1,and the capacity retention rate after circulating 90cycles at 1 C current density is 87.90%.?3?The 2 wt%Sn-doped NCM811 material is further doped with AgNO3 as the Ag source.And the effect of Sn doping and Ag coating co-modification on the structure and electrochemical performance of NCM811 is also investigated.The co-modified NCM811has no significant change interlayer spacing,and reduces cation mixing,and Ag⁺forms amorphous surface layer with reduced electrode polarization,smaller impedance and increased Li⁺diffusion coefficient.Therefore,the electrochemical properties of co-modified material further improved.The 2 wt%Ag-coated NCM811 material shows the most outstanding electrochemical performance,which initial discharge specific capacity at 0.1 C current density is 191.1 mAh·g^-1,when current density increases to 10 C,the discharge specific capacity is 121.5 mAh·g^-1.And the capacity retention rate after circulating 90 cycles at 1 C current density is 90.50%.