| Supercritical water is a medium between liquid water and water vapor. The physical and chemical properties of supercritical water such as such as hydrogen bond, solubility, viscosity, diffusion coefficient, and dielectric constant vary dramatically depending on the state variables (pressure, temperature, and density). These properties lead to fast reaction rate and crystal nucleation rate and produce materials with high degree of crystallinity, small grain size and narrow size distribution. In addition, a homogenous reducing or oxidizing atmosphere can be obtained by introducing oxidative or reductive gases. Supercritical hydrothermal synthesis method has been successfully applied in preparation of ultrafine powder materials.Lithium ion batteries are widely used in portable microelectronic products such as cellar phones, notebook computers, and digital cameras, and they have also received attention as potential power sources for electric and hybrid vehicles. The properity of the cathode materials is a key factor on the actual properties of Li-ion batteries. The lithium iron phosphate (LiFePO4) was shown to be an excellent candidate for the cathode for its unique advantages of inexpensive, nontoxic, environmentally benign, high specific discharge capacity, and recycle performance. But, it has some defect:low conductivity of electron and migration rate of Li+, low tap density, Fe (II) easy to be oxidized. This can be improved by better synthetic methods and material modification. Lithium manganese silicate (Li2MnSi04) is environmentally friendly and cheap and has a similar structure compared to LiFePO4. Its working voltage is4.0V, theoretical discharge capacity is330mAh·g-1. Li2MnSiO4is a very promising cathode material for lithium-ion batteries. However, there is a little research about it. The reported actual discharge capacity is also relative low. Li2MnSiO4is usually synthesized by Mn salt solution, but, phase composition, product structures and physical and chemical properties vary obviously, with the different silicon materials and manganese materials.In this study, LiFePO4cathode material and Mn/Si complex materials has been synthesized in supercritical water. The effect of preparation conditions on particle size, crystallinity and morphology of LiFePO4were investigated. The mechanisms were discussed. Both of reduction effect of reducing agents and carbon coating effect of carbon precursors were investigated. In addition, Mn/Si complexes were synthesized by different Mn precursor solutions and Si materials under supercritical water conditions. Primary conclusions are as followed:(1) The optimum conditions to synthesize LiFePO4using batch tube reactor in supercritical water were gotten. Electrochemical test was conducted under this condition. LiFePO4powders with high degree of crystallinity and regular morphology were obtained under the conditions of temperature of400℃, pressure of30MPa, resistance time of120s, precursor solution concentration of0.5mol·L-1, and n (Li):n (Fe).:n (P) as3:1:1. The initial charge and discharge specific capacity of the LiFePO4synthesized under optimal conditions reaches125.0mAh·g-1and118.9mAh·g-1respectively at0.1C rate with coulombic emciency of95.12%.(2) Sucrose and multi-walled carbon nanotubes were used as carbon precursor, the ascorbic acid and citric acid were used as reducing agent. Both the effects of contents and precursors on the formation of LiFePO4cathode material were studied. Both of ascorbic acid and citric acid demonstrate good reduction effect, and the ascorbic acid is a better candidate as the reducing agent for LiFePO4. The best addition content is5wt%. The multi-walled carbon nanotubes reveal good carbon coating effect and the best addition is5wt%, while the coating effect of sucrose is relative poor, for the amorphous carbon impair the crystallinity of LiFePO4. The initial charge and discharge specific capacity of the LiFePO4synthesized under optimal conditions added with5wt%multi-walled carbon nanotubes and ascorbic acid reaches134.0mAh·g-1and mAh·g-1respectively at0.1C rate with coulombic emciency of91.79%.(3) Reactions of various manganese salt (Mn(NO3)2, MnCl2, MnSO4and Mn(Ac)2) and various silicon sources (silica sand, silica sol, and tetraethyl orthosilicate) have been investigated in supercritical water using a tube reactor. It is found that the anion of manganese salt has a significant effect on the composition of products. The inorganic manganese salt with the oxyacid radical which is easy to be decomposed can produce MnO2/SiO2and Mn2O3/SiO2. The organic manganese salt with ligands like Mn(Ac)2can react with various types of silicon to generate Mn2SiO4. The hydroxyl groups on the surface of SiO2from different silicon sources enhance the reactivity of SiO2. |
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