Research On Hydrothermal Synthesis Of α-FeOOH And α-LiFeO₂

The cathode materials play a key role to further improve battery performance and reduce the cost in the lithium-ion battery applications. Therefore, preparing the high performance, low cost of novelty cathode material of lithium ion batteries become the main interesting of the lithium-ion batteries industry.LiCoO2, as the mainstream in the present market, displays many favorable capabalities including high capacity, stable discharge curve, excellent cycle life and high thermal stability. But its high cost and toxicity of the Co limited its wide use. It is the general trend that LiCoO2 will be replaced on the view of economic and environmental. Lithium intercalation compounds with the compositionα-LiFeO2 are potentially very attractive cathodes for rechargeable lithium batteries because of its high theoretieal capacity, good cycle performance, low cost, safety and low harm to environmentAs the raw material,α-FeOOH is the key factor of preparingα-LiFeO2. In this thesis,α-FeOOH andα-LiFeO2 are prepared by hydrothermal method. The optimum processing are obtained from the effect of different preparing conditions. The micro-structures and morphologies of these materials have been investigated by XRD, SEM, FT-IR, EDS and TG. In addition, the effects of CTAB, SDS and SDBS on hydrothermal synthesis ofα-LiFeO2 are also studied. The main contents and results are as follows:Good-crystallineα-FeOOH nanosticks are synthesized by hydrothermal treatment of Fe(NO3)3·9H2O and KOH. XRD pattern and SEM images showed that the products are orthorhombic structure with diameter of 80nm and length up to 1μm. By studying the influence of the factors on the concentration of mineralizer, initial concentration of Fe(NO3)3·9H2O, temperature, reaction time and filling factor, the optimal preparing conditions are got as follows:mineralizer concentration is 0.1 mol·L-1, the concentration of Fe3+ is 0.1 mol·L-1 and the filling factors is 80% at the temperature of 100℃for 8 h. The result showed that:the high concentration of mineralization, Fe(NO3)3·9H2O initial concentration and reaction temperature are not conducive to a pure-phaseα-FeOOH formation in the study of the experimental conditions. The reaction time and filling factor have little effects on the phase composition of the products, only affected the crystallinity and micro structure of the crystal.α-LiFeO2 have been prepared by means of hydrothermal method in this work. The lithium ferrite have been obtained fromα-FeOOH (goethite) as iron source and LiOH·H2O as lithium source. The morphology of products is polyhedral, and perfect cleavage appeared in the products. Its diameter is about 2μm, a narrow particle size distribution range. By studying the Li/Fe in molar ratio, the initial concentration ofα-FeOOH, reaction temperature, reaction time and filling factors, the optimal preparing conditions are got as follows:the molar ratio of Li/Fe is 30:1, the initial concentration of a-FeOOH is 0.5 mol-L-1 and the filling factors is 80% at the temperature of 210℃for 6 h. The results showed that:a·LiFeO2 is obtained at the LiOH·H2O-rich region.α-Fe2O3 and Li0.5Fe2.5O4 are obtainable by gradually reducing the content of LiOH·H2O. Single phase a-LiFeO2 is obtained for Li/Fe≥30:1 at high hydrothermal temperature (and pressure). The system does not react while Cα-FeOOH> 0.1 mol·L-1 and T=160℃. Similarly, in the study of the experimental conditions, the reaction time and filling degree are almost no effects on the phase composition of products, only affected the crystallinity and the degree micro-morphology of the crystal.Heat treatment of the as-preparedα-FeOOH andα-LiFeO2 in the above at 20-800℃. The results showed that phase transformation of a-FeOOH occurred between 240℃and 300℃, which is discribed as 2α-FeOOH→α-Fe2O3+H2O. While no weight loss and crystal form changed in the process ofα-LiFeO2 treatment. This indicates that theα-LiFeO2 obtained by hydrothermal methode have good stability.By studying at the effects of SAA on the phase composition and morphology of α-LiFeO2 which is synthesized by hydrothermal method, the results showed that:no effects on the phase compositions, but larger effects on micro-shape appearance ofα-LiFeO2 when added surface-active agent to the reaction system. In the experimental concentration range, the CTAB plays a major role in controlling the particle size ofα-LiFeO2, while the SDS and SDBS not only have the ability to reducing the particle size, but also affect the microstructure of the powders. With the increase of SDBS, the samples showed three kinds of morphology, such as polyhedron, layered and spherical, and the average size dropped to about 0.3μm.