| In this paper, the low-heating solid-state reaction method has been systematically investigated for the use of synthesizing the cathode materials for lithium-ion batteries. The studied cathode materials include layered LiCoO2 and LiCo0.8Ni0.2O2, spinel LiMi2O4, and layered LiMnO2. The effects of the fabrication conditions on the crystal structure, grain size, micromorphology and electrochemical performance of these materials have been studied in depth. The reaction mechanism of the low-heating solid-state reaction method has also been investigated. In addition, a novel electrochemical method (RPG method) based on the concept of " Ratio of Potentio-Galvano-Charge Capacity" has been for the first time developed to determine the diffusion coefficient of lithium-ion within insertion-host materials on the basis of the spherical diffusion model. The paper includes six chapters, which are mainly given as follows.In the chapter 2, the spinel LiM2O4 cathode material has been studied. The precursors have been prepared from lithium hydroxide, manganese acetate and citric acid by the method of low-heating solid-state reaction with a molar ratio of Li+/Mn2+=1:2. The composition, microstructure, reaction mechanism and thermo-decomposing process of the precursors have been studied using element analysis, IR spectrum, MS spectrum and TG/DTA analysis. The results show that the composition of these precursors are identified to be LiMn2L(Ac)2 (or LtMn2C10H11O11), in which L represents citric acid radical and Ac is acetic acid radical. The sintering temperature and sintering time have remarkable effects on the microstructures of LiMn2O4 samples. Spinel LiMn2O4 begin to form at 350 with the sintering time of 4 hours because there are one Li+ and two Mn2+ in one precursor molecule. When the sintering temperature is larger than 750 , a specific sintering route is designed to avoid the formation of Li2MnO3 impurities. The samples which are obtained by sintering at 500 for 2 hours and then at 750 for 8-16 hours, have good electrochemical performances. The first charge/discharge capacity reaches 120 mAh.g-1; the capacity lost is about 15% after 50 cycles. The results also indicate that the samples with larger crystal cell, more smooth particle surface and larger particle size exhibit better electrochemical performances.In the chapter 3, the layered LiCoO2 and yCo0.8Ni0..2O2 cathode material have been studied. The precursors of LiCoO2 as the cathode material of Li-ion batteries are prepared by the method of the low-heating solid-state reaction using lithium hydroxide, cobalt acetate and oxalic acid as raw materials. The LiCoO2 samples are obtained by sintering theprecursors at different temperatures for 6hr. The structures and morphologies are characterized by the powder XRD and TEM techniques. Results show that the grain size of all samples is below 100nm,and the lattice parameters and grain sizes are dependent on the sintering temperature. Electrochemical tests show that the sample prepared at 700 (sintering temperature) exhibits a good cyclability. However, the capacity lose resulted from the polarization of electrodes cannot be negligible. The precursors of LiCo0.8Ni0.2O2 cathode material for lithium-ion batteries are prepared from lithium hydroxide, cobalt acetate, nickel acetate and oxalic acid by the method of low-heating solid-state reactioa The UCoagNioiQz samples are obtained by sintering the precursors at different temperatures for 12hr. Their structures and morphologies are studied by the powder XRD and SEM. techniques, and their physico-chemical parameters such as specific areas, particle size distributions and diffusion coefficients are measured by the BET method, laser scattering technique and galvanostatic intermittent titration technique (GITT), respectively. SEM photographs show that these samples are made up of the irregular porous granules, which are conglobated by many small spherical crystals. This is beneficial for the electrolyte diffusing into the granules of LiCo0.8Ni0.2O2 Electrochemical tests... |
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