Studies On The Synthesis And Surface Modiifcation Of Lithium-rich Cathode Materials

This synthesis and surface modification of a lithium-rich cathode materialLi1.2Mn0.54Ni0.13Co0.13O2were studied in this thesis. The mechanism for itselectrochemical performance improvement by PPy surface coating was alsoinvestigated.Li1.2Mn0.54Ni0.13Co0.13O2was prepared by various methods, including sol-gel,co-precipitation, Pechini, quaternary eutectic Li-Ni-Mn-Co acetate process and sprydrying methods. The prepared cathode materials were investigated by X-raydiffraction （XRD）, inductively coupled plasma-atomic emission spectrometry（ICP-AES）, scanning electron microscope （SEM）, X-ray photoelectron spectroscopy（XPS）, and galvanostatic testing. It was shown that most of the XRD diffractions canbe indexed to the R-3m structure. Traces of a superstructure were observed between20°and25°. The main valence of Mn, Co and Ni were determined to be+4,+3,+2,respectively. It was found that the sample prepared by the Pechini method had betterelectrochemical properties （reversible capacity, initial coulomb efficiency andcapacity retention） than that by the other methods. The mechanism ofcharge-discharge cycling of Li_1.2Mn_0.54Co_0.13Ni_0.13O₂was studied by ex-situ X-rayabsorption near edge structure （XANES）. The results displayed that the valance stateof Mn hardly changed while that of Ni and Co changed drastically in the initial cycle.It was also found that the oxidation of Co3+to Co4+took place before the Ni2+to Ni4+oxidation.A composite is prepared by coating Li_1.2Mn_0.54Co_0.13Ni_0.13O₂with polypyrrole（PPy）. No significant structural differences were observed between the pristinesample and PPy-coated sample. SEM, TEM, and TG-DSC were employed tocharacterize the morphology and content of PPy of the Li_1.2Mn_0.54Co_0.13Ni_0.13O₂.Galvanostatic test revealed that the initial coulomb efficiency increases from78.3%（pristine） to88.9%（PPy-coated） while the capacity retention in30cycles increasesfrom69.3%（pristine） to86.1%（PPy-coated）. The mechanism of the PPy-coatedmaterial was investigated by CV, HRTEM, FT-IR, EIS, etc.. It was found that the PPy coating improves the initial coulombic efficiency and the cycling stability of thematerial by suppressing the electrolyte decomposition and oxygen vacancyelimination at high potentials. Furthermore, the presence of the solid electrolyteinterphase （SEI） is confirmed on the PPy-coated sample. Li2CO3and ROCO2Li weresupposed to be the main components of the SEI layer on the PPy-coatedLi_1.2Mn_0.54Co_0.13Ni_0.13O₂.