| In this thesis, a cost-effective solution combustion method based on the citric acid-nitrate process with an inexpensive iron compound (III) as one of raw materials is used to synthesize nanosized LiFeBO3/C composites. The dependence of the structure and electrochemical performance of LiFeBO3/C on the preparation parameters is explored.1) To better understand phase formation of LiFeBO3, various molar ratios of iron nitrate to citric acid were tried. The XRD results show that for the sample with a ratio of1.5, the XRD pattern of the product can be successfully indexed as a monoclinic structure with a space group of C2/c, in accordance with the typical structure of LiFeBO3. The electrochemical tests indicate that the sample with the ratio of1.5shows the optimum discharge capacity only111.5mAh/g at0.05C rate. To further increase the electrical conductivity, glucose (C6H12O6·H2O) was employed as a carbon source. It is found that not only the amount of glucose incorporated, but also the addition way influences phase formation, particle size and morphology of resulted LiFeBO3. Ball mixing glucose with the product from the auto combustion is the most effective way in improving electrode performance. The LiFeBO3/C with about3.5wt%carbon has a very high discharge capacity of about201.9mAh/g at0.05C rate and still a capacity of145.9mAh/g at a2C rate.2) To explore the effect of different carbon sources on phase formation and electrochemical performance of nanosized LiFeBO3/C composites, a cost-effective solution combustion method based on the citric acid-nitrate process with an inexpensive iron compound (Ⅲ) as one of raw materials is used, and then coated the samples with a carbon layer from different carbon sources by a low temperature calcinations, such as citric acid, ascorbic acid, glucose, sucros. The LiFeBO3/C nanocomposites obtained with ascorbic acid as the carbon source show a higher discharge capacity, excellent cycle stability and rate performance. Correlated with the molecule structure, it is believed that the ascorbic acid with more oxygenous groups would benefit their uniform adsorption on the electrodes and then produce a better electron-conductive carbon layer after calcinations. At the same time, due to the strong reduction property of ascorbic acid, the silight impurity phase of ferric iron on the surface of LiFeBO3/C composites is fully reduced, which can weaken the charge transfer resistance and accelerate the lithium ion intercalation/deintercalation, resulting in facility to reach equilibrium state in the process of charging and discharging.3) To improve the energy density of lithium-ion batteries, we increase active material of LiFeBO3up to94.87wt%in a LiFeBO3based bulk electrode. A solution combustion method similar to the one above was employed. The difference is that the co-incopration of ascorbic acid and citric acid into the precursors during the preparation of LiFeBO3/C bulk electrodes. The strength of this method is that citric acid remnants could be further decomposed to make the LiFeBO3/C bulk electrodes a porous structure. Meanwhile the decoposition of ascorbic acid is triggered by a low temperature calcinations to encapsulate LiFeBO3particle. Furthermore, the acetylene black introduced into the LiFeBO3/C bulk electrodes forms a three-dimensional conductive network as well. The combined effect of the above processes would greatly enhance the electrical conductivity and Li ion diffusion rate. The LiFeBO3/C bulk electrodes are shown to be porous, conductive, metal current collector-and polymeric binder-free. The bulk electrodes thus prepared exhibit excellent electrochemical performance with ultrahigh volumetric capacity. |
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