【作者】 何岩；

【导师】 吴莉莉； 褚卫国；

【作者基本信息】 山东大学 ， 材料物理与化学， 2013， 硕士

【摘要】 本文利用柠檬酸-硝酸盐溶液燃烧法,选择聚阴离子化合物铁系正极材料硼酸铁锂(LiFeBO3)作为研究对象,针对其存在的电导率不高、倍率性能欠佳、纯度难控制等问题有针对性地开展研究。1)通过探索硝酸铁和柠檬酸加入比例对纯相LiFeBO3合成的影响可知,当硝酸铁和柠檬酸加入比例为1.5时,所制备样品1.5-LiFeBO3的X射线衍射峰均能够与单斜晶系C2/c空间群硼酸铁锂相标准衍射峰位相吻合,证明制备出纯相硼酸铁锂相。然而仅经溶液燃烧合成法制备的样品1.5-LiFeBO3,在0.05C倍率下放电比容量最大仅为111.5mAh/g,为进一步改善硼酸铁锂正极材料的电化学性能,我们选取葡萄糖(C6H12O6·H2O)作为有机碳源用于硼酸铁锂正极材料的碳包覆改性处理。不同葡萄糖加入方式直接影响最终合成样品的纯度、颗粒尺寸、形貌等,进而决定LiFeBO3/C复合材料的电化学性能。探索研究发现将硼酸铁锂前驱体经预热燃烧后粉体与葡萄糖共同进行球磨处理,而后结合低温煅烧制备的LiFeBO3/C复合材料展现了优异的电化学活性——较高的放电比容量、卓越的倍率性能以及循环稳定性能。2)我们利用柠檬酸-硝酸盐溶液燃烧法合成硼酸铁锂／碳复合材料的技术路线,探索柠檬酸、抗坏血酸、葡萄糖、蔗糖四种碳源对LiFeBO3/C复合材料合成及电化学性能的影响。研究结果显示,选用抗坏血酸作为有机碳源对硼酸铁锂进行表面碳包覆改性处理得到的样品展示了较高的放电比容量、卓越的循环稳定性和倍率性能。我们推测选用抗坏血酸作为有机碳源对硼酸铁锂进行表面碳包覆改性处理得到的样品电化学性能较优的原因主要是由于抗坏血酸中具有不同氧基团,使其更容易吸附在硼酸铁锂纳米颗粒表面,从而使热解产生的碳与硼酸铁锂颗粒具有更好的接触,进而提高硼酸铁锂正极材料电子导电性能；其次,抗坏血酸凭借其较强还原性能,可使硼酸铁锂正极材料表面轻微三价铁杂相被充分还原,进而减弱硼酸铁锂正极材料充放电过程中的电荷转移电阻,加快锂离子在其内部脱出嵌入过程,使LiFeBO3/C复合材料体系在充放电过程中更易达到平衡状态,从而促使LiFeBO3/C复合材料放电比容量增加,倍率性能和循环稳定性能得到改善。3)为进一步制备具有更高能量密度的锂离子电池电极,我们提出了LiFeBO3/C块状电极。选用柠檬酸-硝酸盐溶液燃烧合成法,结合低温煅烧抗坏血酸有机碳源进行碳包覆改性处理的技术路线,我们探索了LiFeBO3/C块状电极的制备与电化学性能研究。在LiFeBO3/C块状电极制备过程中,由于柠檬酸预热处理时未能反应完全,致使后续低温煅烧阶段,柠檬酸进一步分解引起LiFeBO3/C块状电极内部形成多孔结构,同时抗坏血酸热解生成的碳均匀包覆在硼酸铁锂颗粒表面,以及乙炔黑在LiFeBO3/C块状电极内部提供一个相互关联的三维孔导电碳网络,共同增强了离子和电子在LiFeBO3/C块状电极中的运输能力。我们所制备的LiFeBO3/C块状电极具有较高的电子导电性和多孔状结构,且在传统电极薄膜涂布过程中必不可少的绝缘聚合物粘结剂、金属基集流体等也不需要添加,同时更展现出卓越的电化学性能。更多还原

【Abstract】 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.更多还原