Studies On Synthesis, The Control Of Microstructure And Electrochemical Properties Of LiCoO₂ As Cathode Material For Lithium-Ion Battery

Cathode material is one of the most important factors that affect the electrochemical properties (dischargeable capacities, cycle life, etc) of Lithium-Ion battery. In this thesis, we have synthesized LiCoO2 by the reaction using Li2CO3 and Co3O4 as precursors. The phase transitions, microstructure variations and the sintering process in this reaction was intensively studied and the preparation method of LiCoO2 was optimized to improve its electrochemical performance.Investigations on the phase transitions, crystal structure and microstructure changes in synthesis reaction of LiCoO2 indicate thatLi2O-Li2CO3 low-temperature eutectic containing Li2CO3 formed at 200℃and then LiyCo3-xO4 was created by the diffusion into the inner of Co3O4 crystallite. By the increasing of temperature, LiyCo3-xO4 was further changed into LiCoO2 and LixCo1-xO, and then LiCoO2 formed by the reaction of LixCo1-xO with Li2O. The reaction that produces LiCoO2 is almostcompletely done at 700℃, although some remaining Li2O and Li2CO3present as a partially amorphous film dispersed on the surface of LiCoO2, which can accelerate the growth of LiCoO2 crystallites. However, whentemperature is higher than 850℃, some of LiCoO2 begin to decompose intothe solid solution phase of LiyCo2-yO2 and LixCO1-xO, and damaged its own crystal structure.Microstructures of LiCoO2 gradually change in the synthesis reaction. As the increasing of calcinating temperature, crystallite size of Co3O4 is reduced, while that of LiCoO2 increased and furthermore, there is less defects in the crystal structure. The maximum crystallite size of LiCoO2 canbe obtained at 850 ℃. When the temperature is higher than 700 ℃, constant of a-axis is gradually reduced while that of c-axis increased with the rising of temperature. However, when the temperature is lower than 700 ℃, thoseof a-axis and c-axis keep constant. Microstructures of LiCoO2 such as crystallite size, particle size and aggregate have some succession with those of precursor Co3O4.Through the synthesis of LiCoO2 by using non-water dispersant and two-step roasting, influences of microstructures of precursor, the roasting temperature and the reaction time on the electrochemical properties of LiCoO2 were studied. It indicated that Co3O4 with nano-crystalline have very good electrochemical property, and can shorten solid reaction time. Theprecursor 00304 with nano-crystalline was prepared at 50 ~ 90℃ andpH=9 - 11 using [Co(NH3)6]3+ and Co2+ ions. Effects of the temperature oncrystallites, including the crystallite sizes and their distributions, the shape and the crystallinity were investigated. The mechanism for nucleation and growth of the crystallites were also discussed.