| LiFePO4has been considered as the most promising cathode candidate for the next generation lithium-ion battery, because of its inherent merits including stable operating voltage (3.5V), well performances on thermal stability and cycle stability, environmentally friendly, low cost and so forth. However, the development of LiFePO4was limited by its drawbacks-low electronic conductivity and poor lithium ion diffusivity. A lot of efforts have been devoted to overcome such obstacles. Preparation of LiFePO4nanoparticles was found to be a good method. The capability of LiFePO4nanoparticles depends on their structure, shape and size, which can be affected by employing different synthetic techniques or reaction conditions. Thus, it is necessary to study the controllable preparation, the nucleation and growth mechanism of the LiFePO4nanoparticles. In this work, uniform LiFePO4nanoparticles were obtained by a solvothermal method. In comparation to other methods, it had many advantages, such as, simple operation, mild reaction conditions, obtaining well crystalline LiFePO4nanoparticles without calcinations and so on. To optimize the synthesis of LiFePO4nanoparticles, the effects of reaction time, reaction temperature, molar ratio among the reactants (Li+:Fe2+:PO43+), the concentration of reactants were investigated in detail. Furthermore, LiFePO4nanoparticles with different sizes were synthesized with addition of heterologous seed crystals under otherwise the same reaction conditions. Finally, the formation mechanism of LiFePO4nanomaterials by the solvothermal method and how heterologous seed crystals worked were explored.The main research results of this paper can be briefly described as follows:(1) Uniform LiFePO4nanoparticles were obtained by the solvothermal method. Results from X-ray diffraction (XRD) indicated that the prepared LiFePO4nanoparticles were orthorhombic phase with space group Pnma. Transmission electron microscopy (TEM) characterization demonstrated that LiFePO4were flake like diamond nanoparticles, the length of the particles dispersed in170～200nm, and the width were80-100nm.(2) Using different characterization methods, the optimum reaction conditions for synthesis of LiFePO4nanoparticles were concluded as followed:using ethylene glycol as the solvent, lithium hydroxide, ferrous sulfate and phosphoric acid as starting materials in a molar ratio (Li+: Fe2+:PO43-) of2.7:1.0:1.0, aging at180℃for12h. Besides, the molarity of reactants was0.10mol/L.(3) Using the solvothermal method under the same reaction conditions, LiFePO4nanoparticles with different sizes were obtained with addition of heterologous seed crystals-Fe3(PO4)2. The addition of heterologous seed crystals made no difference on the structure and crystal form of the synthesized LiFePO4nanoparticles, which were proved by measurements of XRD and FTIR. Results from experiments indicated that size of the obtained LiFePO4nanoparticles were reduced obviously with addition of only0.5%heterologous seed crystals. With the increasing amount of heterologous seed crystals, size of the produced LiFePO4nanoparticles decreased. The observed phenomenon demonstrated that size controlled LiFePO4nanoparticles could be synthesized effectively by addition of heterologous seed crystals. Moreover the addition of heterologous seed crystals was observed to accelerate the extent of reaction during the same process.(4) The possible formation mechanism of LiFePO4nanoparticles prepared by the solvothermal method is deduced in this paper. The reaction process can be divided into three steps-(Ⅰ) prior to nucleation.(Ⅱ) nucleation step and (Ⅲ) growth step. In step (Ⅰ), the potential barrier is so large that it is hard for the reactants in solution to form LiFePO4directly. In order to overcome this barrier, ferrous phosphate is firstly generated, and then it dissolves slowly. Meanwhile, the concentration of LiFePO4monomer increase, and ultra fine Fe3(PO4)2crystals also exist in the solvent; In step(Ⅱ), LiFePO4crystal nuclei is formed by combination of LiFePO4monomers in a very short time, which lead to the decrease of the the concentration of LiFePO4monomers, thus causing dissolution of Fe3(PO4)2, and fading away of Fe3(PO4)2; In step (Ⅲ), the possibility of LiFePO4monomers deposition on the surface of the nuclei is larger than that of renucleation, so the LiFePO4crystal nuclei become larger and larger. Finally, uniform LiFePO4nanoparticles are obtained. When the heterologous seed crystals are added into the solvent, the nucleation step will probably happen earlier and last longer, producing much more nuclei in the solvent. In the growth step, the LiFePO4monomers for each nuclei will be less, thus we can get LiFePO4nanoparticles with smaller size than before. |
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