Preparation And Performance Improvement Of High-Nickel Cobalt-Free Layered Cathode Materials For Lithium-Ion Batteries

The research and development of high-capacity cathode materials is crucial for the development of high energy density lithium-ion batteries.The industrialization of high nickel ternary materials such as Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）and Li Ni_0.8Co_0.15Al_0.05O₂（NCA）with high-capacity advantages has attracted widespread attention.However,due to the scarcity of cobalt resources and high cost,the research of high-nickel cobalt-free layered oxide cathode materials with high-capacity have more profound significance.To prepare high-nickel cobalt-free layered oxide cathode materials with high capacity,good rate performance and cycling performance,this paper focused on the preparation and Sn doping modification of Li Ni O₂ and Li Ni_0.9Mn_0.1O₂.The optimized single crystal preparation process of Li(Ni_0.9Mn_0.1)_0.98Sn_0.02O₂ material was also explored,and the conclusions obtained are as follows:（1）The research of preparation and Sn doping modification of Li Ni O₂indicate that the Li Ni O₂ material prepared under optimized conditions of lithium excess of 5%and calcination temperature of 700℃has a goodα-Na Fe O₂ layered structure and a capacity of up to 203.5 m Ah·g^-1（0.2C）.After optimizing the Sn doping amount,the Li Ni_0.98Sn_0.02O₂ material prepared under the same preparation conditions shows a significantly improved capacity,rate capability and structural stability.The capacity at0.2 C increases to 231.7 m Ah·g^-1,and the capacity at 5 C high rate reaches109 m Ah·g^-1.After 60 cycles at 0.5 C rate,the capacity can still reach 173.4m Ah·g^-1.The improvement in performance is attributed to the effective suppression of Li/Ni mixing by appropriate Sn doping,increasing the interlayer spacing and lithium-ion diffusion rate,and generating Li₂Sn O₃on the material surface that is conducive to lithium-ion transport and surface protection,reducing the interface impedance.First principles calculation analysis shows that Li/Ni mixing is the main factor leading to poor conductivity of Li Ni O₂ materials.Sn doping can suppress Li/Ni mixing,which is beneficial for improving the conductivity of Li Ni O₂materials and thus improving their charge-discharge performance.（2）In order to obtain better performance high-nickel cobalt-free layered oxide cathode materials,further research was conducted on the preparation of and Sn doping modification of Li Ni_0.9Mn_0.1O₂.The results show that the Li Ni_0.9Mn_0.1O₂ material prepared at 750℃has a high degree of cation order and good grain development,exhibiting significantly improved rate performance and cyclic stability compared to Li Ni O₂ and Li Ni_0.98Sn_0.02O₂ materials.Its specific capacity at 0.2 C and 5 C rate reaches191.5 m Ah·g^-1 and 164.2 m Ah·g^-1,respectively.After 25 cycles of charging and discharging at different rate,the specific capacity increases to 196.4m Ah·g^-1 while returning to 0.2 C.To further improve the electrochemical properties of the material and exert the synergistic effect of Sn doping,Li(Ni_0.9Mn_0.1)_0.98Sn_0.02O₂ material was prepared by optimizing the Sn doping amount.On the one hand,the doping of Sn elements into the lattice further suppresses Li/Ni mixing,increases interlayer spacing,and reduces interface impedance;On the other hand,the formation of a Li₂Sn O₃ coating layer on the surface of the material stabilizes the surface structure and effectively suppresses the increase in interfacial impedance during cycling.Sn doping increases the capacity of the material at a rate of 0.2 C to 199.5m Ah·g^-1,while at a high rate of 5 C,the capacity is still as high as 179.7m Ah·g^-1.After a total of 25 cycles of charge and discharge at different rates,the specific capacity increased to 205.3 m Ah·g^-1 while returning to 0.2 C,and after 200 cycles at a rate of 1 C,the capacity retention is as high as84.3%,much higher than 70.8%of undoped Li Ni_0.9Mn_0.1O₂.（3）The research of Li(Ni_0.9Mn_0.1)_0.98Sn_0.02O₂ single crystal prepared by pulse high temperature sintering method show that the single crystal material with primary grain size of micron can be prepared by adding 1min pulse high temperature sintering treatment to the traditional process of sintering at 750℃for 15 h.Lithium ratio and pulse temperature are the main factors affecting the performance of single crystal.After optimization,the single crystal Li(Ni_0.9Mn_0.1)_0.98Sn_0.02O₂ prepared with a lithium excess of 5%and a pulse temperature of 1020℃has good grain development,high cation order,and a capacity of 211.7 m Ah·g^-1 at a 0.2 C rate.Compared with the polycrystalline material,the single crystal shows a slightly lower capacity(143.6 m Ah·g^-1)at high rate of 5 C,but exhibits an improved cycling stability.After 30 cycles of charging and discharging at different rate,the capacity retention in the subsequent 70 cycles at 0.5 C is as high as 92.8%,higher than 91.8%of the polycrystalline Li(Ni_0.9Mn_0.1)_0.98Sn_0.02O₂ material.This is attributed to its highly crystallized layered structure and the reduction of electrolyte side reactions due to a decrease in specific surface area.