| In 2005,Nyten etc. obtained the synthesis of cathode material Li2 Fe Si O4 for the first time Li2 Fe Si O4 has two lithium-ion, and theory of specific capacity(331 mah/g) is equivalent to the specific capacity of anode materials commonly used. So it has the very good application prospect. The raw resource for Li2 Fe Si O4 elements are also more common in the anode material, so the price can be low. Ferric metasilicate lithium and lithium iron phosphate containing Si-O covalent bond is similar to the P-O key contained by lithium iron phosphate system, which has excellent thermal stability and chemical stability, but iron lithium silicate had a disadvantage of low electronic conductivity, which restrict its high rate charge and discharge performance. At Present the method to improve the electrical performance of lithium iron silicate materials contain, the morphology structure optimization and doping methods to improve the electrical performance of lithium iron silicate materials. This article will state the innovation and improvement in the preparation process, optimize the structure and morphology of crystal materials, and adding the conductive carbon black to improve Li2 Fe Si O4 positive-ion conductivity, so as to improve the electrochemical properties of materials. The main work of the thesis are as follows:1.The co-precipitation process for lithium iron silicate materials has not yet been reported, the results showed that materials with rules of the sample morphology and particle unifomity. Based on the co-precipitation,it puts forward the experiment technology and method. and sample is obtained by roasting Li2 Fe Si O4 materials, the technological process is simple, the preparation of the material Li2 Fe Si O4 uniform shape and uniform particle size small, compared with other methods reported at present is a big advantage. The process that silicon dioxide is obtained by co-precipitation inalkaline oxidation environment, solved the alkaline oxidation of ferrous iron ion precipitation and the problem of easy oxidation by lithium salt, and the addition of carbon source were Li2 Fe Si O4 precursor materials, preparation of precursor materials under the process is simple in process, material mixing uniform characteristics characteristics of structure, and studied the synthesis of the precursor of lithium iron silicate preparation technology, the results have shown that the crystal structure of the lithium iron silicate materials is orthogonal crystal system, belongs to space group(Pmn21); The diffraction peak of Lithium iron silicate anode composite prepared in800 ℃ is consistent with the results reported in literatures, and no obvious impurity peak, It turns out that we successfully obtain the pure phase Li2 Fe Si O4 material by this method and Its SEM figure shows that the size of uniform particle is small. In addition the electrochemical performance is excellent.2.Based on co-precipitation preparation technology, by different roasting time(4h,6h,8h,10h) preparation, studying the formation process of crystal growth and morphology. By XRD, EDS and SEM characterization of the structure and morphology can be invested. This thesis mainly studied the process system reaction and the process of crystal growth mechanism, and discussed the crystal morphology of the relationship between structure and performance. Test results show that the 4h in 800 ℃ the morphology of materials is the class ball class, and the material of the lithium silicate impurity peak is more, which show the generated lithium silicate did not react completely, the shape of morphology of the material is spherical, of the sample is almost same with Li2 Fe Si O4 materials in the materials and Li2 Fe Si O4 the main peak of the sample, but there is a small amount of impurity peak, the morphology of 8h material is peanut shape, sample is pure Li2 Fe Si O4 material at this time Rules of the morphology of pure material, the morphology of material is unified, and the size of particle is small,which ranges form 500 nm to 600 nm.the sample of 10 h generated lithium iron silicate material in the connection occurs between grains, the growth of peanut shape of lithium iron silicate gradually became a coralloid lithium iron silicate materials.Studying materials with different roasting time of crystal growth are based on spherical silicon dioxide particles, spherical lithium silicate materials generated class firstly, then another class of spherical lithium silicate material growth for spherical iron lithiumsilicate material, spherical iron lithium silicate will occur under high temperature and long time connection form between peanut shape crystal growth, if time will have a further extension the sample will grow as coralloid lithium iron silicate material, the electrochemical test results show that the samples in the roasting conditions for 8h have a largest lithium ion diffusion concentration: 1.24×10-13,there is an obvious charge and discharge platform, for the first time charging capacity of 164.9 m Ah/g. Initial discharge capacity of 154.2 m Ah/g, and in the rate of 0.2 C conditions have higher discharge capacity, cycle capacity almost has no attenuation, after 20 cycles of 129.9m Ah/g.3.The Li2 Fe Si O4 was modified by carbon coating. Effects of Vanadium substitution at different sites on the structure of Li2 Fe Si O4/C are examined by X-ray diffraction(XRD),and scanning electron microscopy(SEM).uniform lithium iron silicate with nanoscale conductive carbon black homogeneously dispersing in the clearance.as a result of the join nanoscale conductive carbon black(20-40nm) which particle size is small, there is a reunion phenomenon, but the overall distribution is more uniform. And studying the influence of the addition of conductive carbon black on the electrical performance, research has shown that carbon content is 4%wt under the condition of synthesis of Li2 Fe Si O4/C has the best electrochemical performance, The results indicate that the Li2 Fe Si O4/C sample prepared by co-precipitation method as a lithium source exhibits a good crystallinity with little impurities. The size of Li2 Fe Si O4/C particles were approximately 500-600 nm,The electronic conductivity of Li2 Fe Si O4 was about2.21×10-13 m2s-1,50 cycles capacity retention rate is above 85%. It can be ascribed to the small size of the nanocomposite to reduce the diffusion distance of Li+ in Li2 Fe Si O4/C;the large specific surface area to increase the effective contact area between the nanocomposite Li2 Fe Si O4/C and electrolyte; carbon-decoration for improving the electronic conductivity of Li2 Fe Si O4; the texture with mesoporous can be infiltrated by the electrolyte in favor of Li+migration in the electrode. |
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