Application Of Green Energy Materials In Lithium-ion Batteries

Because of the environmental pollution of NOx and PM2.5 released by the combustion of fossil fuel in automobile, developing the green energy power system is urgently required in the 21st century. Lithium-ion batteries have been widely considered for the application in automobile owing to its high energy density and high power density. Among the conventional cathode materials for Li-ion battery, layered LiCoO2 exhibits high costs owing to the scarcity and toxicity of Co and low security at high temperature and olivine LiFePO4 could hardly satisfy the demand for next generation battery due to its low energy density. Therefore, developing environmental friendly cathode material with high energy and power density has become the hot topic among researchers. This paper focused on developing environmental friendly cathode material for Lithium-ion batteries, designed and prepared a series of Li-rich layered cathode materials and examined their application in lithium-ion batteries. This paper mainly includes the following two parts:(1) Gradient surface Na+ doping with pinning effects in enhancing the dynamics of Li-rich cathode materialGradient surface Na+ doping was realized in molten NaCl flux based on the concentration diffusion of Na+ in thermodynamics at elevated temperature. We design that the pinning effect of Na+ in the Li+ slab could stabilize the structure and facilitate the diffusion of Li+ and finally enhance the kinetics of the material. Scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Aberration-corrected scanning transmission electron microscopy (STEM) technique were applied to explicitly examine the micro-morphology, crystal structure, surface components and atomic structure of the material. Moreover, it was tested as the cathode electrode material for lithium-ion batteries. High discharge capacity of 285 mAh g-1 with initial coulombic efficiency of 87% was obtained at room temperature at the current density of 25 mA g-1. Excellent rate performances of 265 mAh g-1 at 0.2C,236 mAh g-1 at 0.5C,215 mAh g-1 at 1C and 185 mAh g-1 at 2C were obtained by the material. After 100 cycles at 0.2C,233 mAh g-1 capacity was retained representing the superior cycling stability.(2) Synthesis and electrochemical performances of a high capacity Li-rich cathode material in molten KCl-Na2CO3 fluxLi-rich precursor with homogeneous morphology was prepared through co-precipitation method and pre-calcinated to oxide precursor. The oxide precursor was further mixed with stoic hio metrie amount of Li2CO3, Na2CO3 and KC1 at elevated temperature to obtain the Na+ doped material with high degree of structural defects. The material was studied in detail based on the technique of scanning electron microscope (SEM), X-ray diffraction (XRD) and electrochemical measurement and novel results were obtained in stabilizing the Li-rich material and enhancing its rate capacity. Superior electrochemical performance was exhibited by the material with discharge capacity of 270 mAh g-1 and initial coulombic efficiency of 84% at the current density of 0.1C (1C=250 mA g-1). High rate capacity of 252 mAh g-1 at 0.2C, 230 mAh g-1 at 0.5C,200mAh g-1 at 1C,180 mAh g-1 at 2C and 130 mAh g-1 at 4C. In addition, brilliant cycling stability was exhibited by the electrode with 92% capacity retention after 100 cycles at 0.5C.