Investigation On Green Preparation And The Enhancement Cyclability Of Lithium-rich Manganese-based Ternary Cathode Materials

Lithium-rich manganese-based ternary cathode materials x Li₂Mn O₃·（1-x）Li MO₂（M=Ni,Co,Mn,etc.）due to its high discharge specific capacity(>250 m Ah·g^-1)and discharge voltage（>3.8 V）are considered as one of the best candidate cathode materials for next-generation high energy density batteries.However,the existing preparation process is relatively long with a lot of water loss,and these materials suffer from sever fading during the cycle process and low rate capability,which severely hinder its application in lithium-ion batteries.In this study,we used acetate as transition metal salt,and Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode material was prepared by spray-drying and subsequent annealing treatment method.And then,both of the carbon-coated and ammonia heat treatment and surface modification with transition elements were used to improve the electrochemical performance of cathode material.XRD,SEM,TEM,XPS,galvanostatic charge-discharge,cyclic voltammetry,electrochemical impedance spectroscopy tests were also conducted to study the mechanism of improving its electrochemical performance.（1）The precursor slurry synthesis conditions of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ by spray-drying were investigated.We studied the effects of the different surfactants and their amounts on the structure and electrochemical performance of the cathode material.It was found that when using0.6 wt.%hexadecyl trimethyl ammonium bromide,the cathode material has the optimal layered structure and excellent electrochemical performance.Subsequently,a one-step solvothermal method was successfully developed to prepare lithium-rich manganese-based cathode materials.At the same time,we studied the hydrothermal temperature in slurry synthesis and found that the appropriate hydrothermal temperature would make the layered structure of the cathode material more orderly.At the temperature T=70℃,the cathode material achieved the best electrochemical performance.It shows an ultra-high specific capacity of 291.9 m Ah·g^-1 at 0.1C with the capacity retention of 93%after 300 cycles at 1 C.（2）In view of the shortcomings of lithium-rich manganese-based cathode materials,such as insufficient cycle performance and low rate capacity,carbon-coated and ammonia heat treatment were used to modify the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode material surface.The galvanostatic charge-discharge test results show that the materials when the carbon coating amount is 0.5 wt.%,and subsequent in argon at 400°C for 1 h and then in ammonia at 400°C for 1 h has the best electrochemical performance.The maximum discharge capacity reached273.5 m Ah·g^-1 at 0.1 C,and the discharge specific capacity was still 196.1 m Ah·g^-1 after 300cycles at 1 C with the capacity retention rate reached 96%,while the unmodified cathode material only has 70%.Combining material characterization methods such as XRD,TEM,EIS,CV,we have determined the mechanism of this modification strategy to improve the electrochemical performance of the cathode material:the carbon coating process can optimize the layered structure of the cathode material,and the outermost layer of the cathode material after c-coated and ammonia heat treatment carbon layer,which can significantly increase the electron transfer rate during charge and discharge.At the same time,the lower degree of cation mixing can greatly reduce the damage of Li⁺de-intercalation to the material structure.（3）Through the simple suspension evaporation method,transition metal elements（Mo,V,Zr,Nb）were successfully modified on the surface of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂.The galvanostatic charge-discharge test results show that when Mo=3%,the specific discharge capacity,cycle performance,and rate performance of the cathode materials have been greatly improved.Combining with the SEM and TEM characterization results of the materials before and after modification,we found that surface modification using Mo can form a Li₂Mo₄ nano-layer with a thickness of about 1 nm on the surface of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ which could improve the capacity retention rate of the material in the long cycle.And capacity retention of the Mo-doped cathode materials is 82%after 300 cycles at 1 C.Combining with the XRD and XPS characterization results,we believed that Mo⁶⁺has entered the surface layer of lithium-rich cathode materials,which would widen the interlayer distance of Li,and improved the lithium-ion diffusion coefficient.Combining with the CV curve,we found that the surface modification of Mo can increase the content of Ni²⁺/Ni⁴⁺redox couples and activate the Li₂Mn O₃ component in advance,thereby greatly improving the discharge capacity and rate performance of the cathode material.