Preparation And Electrochemical Performance Of Multi-Defect Integrated Lithium-Rich Manganese-Based Cathode Materials

Lithium-ion batteries have become an indispensable energy storage device in modern life,but with the development of society,lithium-ion batteries with the higher energy density are required.Nowadays,the limitation on the energy density of lithiumion batteries mainly comes from the cathode material.The lithium-rich manganesebased cathode material,xLi2MnO3·(1-x)LiMO2(M=Mn,Co,Ni),has a high specific capacity(>250 mA h g-1)and the high working voltage,which exhibits greatly enhanced energy density compared to traditional cathode materials.So it is deemed as one of the most promising cathode materials for next-generation lithium-ion batteries.Although the lithium-rich manganese-based cathode material has the advantage of high energy density,there are some problems such as low initial Coulombic efficiency,serious capacity and voltage decay,and poor rate capability,which hinder its practical application.The strategies such as doping,surface modification,and heterojunction composite have been used to overcome the above problems of lithium-rich manganesebased cathode materials.Among them,Na+doping has been proved to be one of the effective methods to improve the structure stability of lithium-rich manganese-based cathode materials.However,the single Na+ bulk doping or surface concentration gradient doping is inadequate to maintain structure stability of electrode materials during long-term cycling.In this context,Na-rich engineering is put forward in this thesis to prepare multiscale deficiency integrated lithium-rich manganese-based cathode materials.Through the synergistic effect of multiscale deficiencies,the surface and bulk stability of the cathode materials can be improved significantly.The main research contents are as follows:(1)Lithium-rich manganese-based cathode materials Li 1.2xNa2xMn0.54Ni0.13Co0.13O2 doped with different amount of Na+ ions were prepared by a simple sol-gel method.Electrochemical performance results show that the best sample is Li1.1Na0.2Mn0.54Ni0.13Co0.13O2(Na+0.1 LRM).The Na+0.1 LRM cathode electrode has surface coating,bulk doping and stacking fault,which can improve the stability of the bulk and surface structure of the material greatly,thus exhibiting excellent cycling performance and rate capability.After 400 cycles at 1 C(corresponding to a current density of 250 mA g-1),the specific capacity of 201 mA h g-1 is achieved,and the capacity retention rate is as high as 89.0%.The voltage is maintained at 3.13 V with a voltage retention as high as 89.6%.The improvement mechanism of the electrochemical performance of Na+0.1 LRM cathode was systematically characterized and analyzed by in-situ X-ray diffraction(XRD),transmission electron microscopy(TEM)and first-principles calculation(DFT).It is considered that the Na0.7MnO2.05 coating layer on the surface can effectively prevent the electrolyte from dissolving and corroding the cathode material.The stacking faults in the subsurface and the Na+doping in bulk effectively maintains the structure stability of materials through "pinning effect"and "pillar effect",respectively.(2)The Na+0.1 LRM was matched with commercial Li4Ti5O12 and graphite anode materials to assemble different lithium-ion full cells.The influence of the mass ratio of cathode and anode,the area ratio of the disk electrode,and the pre-lithiation on the electrochemical performance of the full cell were explored in detail.The research results show that appropriately improving the quality and the electrode area of the anode can increase the specific discharge capacity of the full cell in some extent,and the prelithiation of the Li4Ti5O12 anode can improve the cycle stability of the full cell.