Synthesis And Modification Of Ni-rich Cathode Material LiNi_xCo_yMn_1-x-yO₂（x≥0.88） For Lithium-ion Batteries

Layered Ni-rich oxides are promising cathode materials for lithium-ion batteries（LIBs）for electric vehicles due to their high energy-density and low price.However,these materials suffer from poor cycling stability and inferior rate capability due to the poor structural stability and the interfacial side reactions with electrolyte.Enhancing the energy/power density and structural stability,as well as lowering the cost for LIBs are essential topics for the sustainable development of new energy industry.Therefore,this thesis focuses on the bulk and surficial composition of layered Ni-rich cathode materials(Li Ni_xCo_yMn_1-x-yO₂,x≥0.88),proposing some strategies including process optimization,element doping and surface modifications with heterogeneous phases to improve their structural stability,electrochemical reversibility and reaction kinetics.In this case,their cycling performance and rate capability can be enhanced.In addition,this thesis explores the way to reducing the raw material cost of Ni-rich cathode materials by recycling spent LIBs.The main researches and results of the thesis are as follows.（1）For the issue of the fast capacity fading and lowered rate capability originating from the internal crack and the degradation of layered structure,Ni_0.88Co_0.09Mn_0.03（OH）₂ precursor and the tungsten-doped counterpart are synthesized by co-precipitation,and then made into cathode materials（pristine and tungsten-doped one）by high-temperature lithiation.The effects of doping amount on the structure and electrochemical properties are also studied.The results show that the 0.3 mol%tungsten-doped sample exhibits the best electrochemical performance.It delivers an initial discharge capacity of 200 m Ah/g at 0.1 C,and an initial discharge capacity of 181.2 m Ah/g at 1 C.It has 150.1 m Ah/g at an ultrahigh current density of 20 C;after 300 cycles at 20 C,it still has a discharge capacity of 97.0m Ah/g with a capacity retention of 64.2%.It significantly outperforms the pristine one both at room-temperature and high-temperature（45℃）.The results demonstrate that the tungsten doping is beneficial to electrochemical kinetics and structural stability during cycling,hence significantly improving the rate capability.（2）For the lithium residues on the surface and the side reactions with electrolyte,Li Ni_0.88Co_0.06Mn_0.03Al_0.03O₂ is synthesized by a solid-state method,and then coated with lithium boron oxide.The effect of surface coating on the electrochemical performance and reaction kinetics is studied.The physicochemical characterizations reveal that the coating layer can eliminate the lithium residues on the surface of the cathode material and improve the surficial stability.The sample with 0.2 wt%coating agent shows the best electrochemical performance.Its capacity retention after300 times of charge-discharge cycles at 1 C under room-temperature is as high as 87.5%.This value is also as high as 97.2%after 120 cycles at 1 C under 50℃.The in-depth study on the modifying mechanisms shows that the lithium boron oxides coating layer can improve the structural and surficial stability,and suppress the impedance growth during cycling.（3）For the issues of poor structural stability,the lithium residues on the surface and the side reactions with electrolyte,a dual-modification combining Zr-doping and lithium boron oxides coating is applied to Li Ni_0.88Co_0.06Mn_0.06O₂ which is synthesized by a solid-state method.This strategy is used to improve the structural stability and surficial stability simultaneously.The results show that the dual-modification does not change the crystal structure and morphology of the cathode material,but has positive effect on its electrochemical performance.Besides,the dual-modification is superior to the doping modification.The dual-modified sample shows a capacity retention of 92.1%after 200 charge-discharge cycles at 1 C,which outperforms the Zr-doped sample（87.1%）and the pristine one（66.6%）.Besides,the dual-modified sample exhibits the best rate capability.The study on the mechanisms of dual-modification shows that this strategy can improve not only the structural stability,but also the surface stability of the cathode by suppressing the side reactions with electrolyte.（4）For the increasing price of the raw materials of layered cathode materials and the resource shortage of lithium,cobalt and nickel,spent LIBs are treated by a“reduction smelting-oxygen-assistant leaching-directional impurity deposition-co-precipitation-new cathode synthesis”route without dismantling and regenerated into high value-added Li Ni_0.92Co_0.03Mn_0.05O₂ cathode material.The as-prepared cathode material shows an initial discharge capacity of 231.8 m Ah/g at 0.1 C with a coulombic efficiency of 90.05%.It shows an initial discharge capacity of213.4 m Ah/g at 0.5 C and good cycling performance.After 300 cycles at0.5 C,it still has a discharge capacity of 135.1 m Ah/g,with a capacity retention of 63.3%.More importantly,the regenerated cathode material also has excellent rate capability.It shows a discharge capacity of 178.3m Ah/g at 5 C.（5）For the deteriorated cycling stability and rate performance caused by the poor structural stability,metal dissolution and impedance rising,Li₂Al Zr（PO₄）₃ is coated onto the surface of the regenerated Li Ni_0.92Co_0.03Mn_0.05O₂ cathode material by a wet chemical route to improve the reaction kinetics.Electrochemical characterizations show that the sample with 1 wt%coating agent exhibits the best cycling performance and outstanding rate capability.It delivers 144.2 m Ah/g after 400 cycles at 0.5C,and a reversible capacity of 181.5 m Ah/g at 5 C rate.The study on the mechanisms of the surface modification demonstrates that the NASICON-type Li₂Al Zr（PO₄）₃ coating layer can suppress the side reactions between the cathode and electrolyte;meanwhile,it can enhance the Li⁺diffusion kinetics at the interface.