Co系Fe系锂离子电池正极材料的制备改性及表征

【作者】 金翼；

【导师】 陈春华；

【作者基本信息】 中国科学技术大学 ， 材料学， 2008， 博士

【摘要】 经过近20年的发展，锂离子电池已经成为全世界范围内广泛使用的一种高能量密度、高容量密度、长循环寿命的储能装置，并在通讯产品、数码产品等领域得到了大规模工业化应用。但是，随着人们对于锂离子电池在安全性能、使用寿命、材料环保等方面提出了更高的要求，现有的工业化应用材料（正极LiCoO₂，负极石墨及有机液态电解质）已经难以满足。因此，目前锂离子电池的研究重点在于如何通过改性提高现有材料体系的综合性能，并同时开发新型材料以满足未来锂离子电池的发展需求。众所周知，正极材料是锂离子电池中最关键的材料，因为它是锂离子的唯一来源，而且很大程度上决定了电池的整体成本、安全性能及使用寿命等。本博士论文的主体正是通过改性现有Co系正极材料体系（LiCoO₂）及合成下一代Fe系锂离子动力电池正极材料体系（LiFePO₄）来探索提高锂离子电池正极关键材料综合性能的途径和方法。在论文的第一章中，作者综述了电化学电源及锂离子电池的发展史，锂离子电池的结构、工作原理，锂离子电池常见电极及电解质材料以及锂离子电池正极材料的常见合成、改性方法。在第二章中，主要介绍了本论文中的实验方法和仪器，详细介绍了实验用的扣式电池的制备过程，以及常用的结构学、形态学、电学及电化学测试手段。在第三章中，作者以共沉淀法成功制备了Si掺杂的LiCoO₂系列样品，并发现掺杂量为1％的样品LiCo_0．99Si_0．01O₂表现出了最佳的电化学性能，在2．8-4．2V的充放电区间内，其首次放电比容量达到137mAh／g，经过50次循环容量保持率为100％，而其经过25次循环后的3．6V平台效率仍然高达97％。第四章则着重研究了四种不同的方法对钴酸锂商品粉进行了表面修饰处理。实验结果表明，以沉淀法制备纳米磷酸锂前驱体后再与钴酸锂固相机械混合并在不同温度下二次热处理的方法是简易可行而行之有效的表面修饰改性方法。经过450℃的二次热处理后，钴酸锂的电化学性能得到了明显的提高，尤其是在2．8-4．5V的较宽电压区间内的容量保持率和长期循环后的3．6V平台效率。其中，450℃热处理的样品表现出了最佳的电化学性能。通过交流阻抗谱和扫描循环伏安法的研究表明，磷酸锂的表面修饰并没有能够抑制钴酸锂放电过程中的相转变，而只是能有效抑制钴酸锂在长期循环过程中的表面钝化膜生长以及由此带来的电荷交换阻抗的增加。在第五章中，作者研究了不同有机前驱物作为碳源对于氧化铝电学性能提高的影响。在优化出PVDF作为碳源后，对PVDF残碳包覆磷酸铁锂的包覆比例、合成温度等条件进行了优化，得到了最佳PVDF添加量为30％，而最佳合成温度为710℃，在此条件下合成的LiFePO₄／C在0．2C倍率下初始放电比容量高达140mAh／g以上，而1-2C倍率下比容量达到120mAh／g以上，经过30次循环后容量保持率为100％。论文的第六章首先合成了β、γ-Li₂ZrCuO₄，并研究了两种同质异构体的电学及电化学性能及电化学插锂反应机理。研究结果表明，Li可以在1．5V以下插入Li₂ZrCuO₄结构中与其发生反应，而首次放电过程中Li插入使Li₂ZrCuO₄分解为Li₂O、Cu₂O和ZrO₂三相。在后续充放电过程中，分解后的三相复合物中只有Cu₂O能作为活性物质与Li发生可逆氧化还原反应。而γ-Li₂ZrCuO₄为纯电子导体，并呈现出典型半导体特性，其在800℃以上出现相转变而逐渐变为β相。在133-1273K的温度范围内的平均活化能为14．4kJ／mol。第七章中，作者对锂化的Boltorn^?超支化聚合物作为锂离子电池凝胶型聚合物电解质进行了制备和表征的探索性研究。研究结果表明，LH20样品在室温下的电导率能达到5．9×10^-6S／cm，而在120℃下电导率高达1．8×10^-4S／cm，其电化学稳定窗口为0-5V。因此，锂化超支化聚酯有着良好的全固态锂离子电池应用前景。最后，在第八章中，作者对本论文的创新和不足之处进行了总结并对未来的研究工作提出了展望。更多还原

【Abstract】 In the last 20 years, lithium-ion batteries have been widely commercialized in industry for their relatively high and particularly reversible specific capacity, high power density and long cyclic life characteristics. They are successfully applied in various communication appliances and digital devices. However, with the requirements of higher battery safety, longer cycle life and lower pollutions, the present cell chemistry （LiCoO₂ as active cathode material, graphite as active anode material, and liquid electrolyte） can hardly meet these needs. Recently, the focus of research on lithium-ion battery is to improve the performance of present industrialized materials and search for new materials. It is well-known that the cathode material is the most important among all of the components in a battery, because it is the only source of lithium-ions and it to a large extent decides the cost, safety characteristic and the cycle life. Therefore, in this thesis, several modification approaches have been employed to improve the performance of Co-based cathode material （LiCoO₂）. Furthermore, a systematic study on the solid-state reaction synthesis and up-scale production of the safer Fe-based material （LiFePO₄） is conducted. An exploration on a potential new polymer electrolyte is also attempted.In Chapter 1, a general introduction is given on following aspects: the development and status of traditional electrochemical power sources and lithium-ion batteries, the structure and working mechanism of lithium-ion battery, the common cathode materials and methods to synthesize and modify cathode materials reported in literature.In Chapter 2, the author mainly introduces the experimental processes and equipment used in the project of this thesis. A detailed description on the process to making a coin cell is presented. The structural, electrical and electrochemical analysis methods are also summarized.In Chapter 3, Si-doped LiCoO₂ powders are synthesized through a co-precipitation method. The optimal composition is found to be LiCo_0.99Si_0.01O₂. In the voltage region from 2.8V to 4.2V, its initial discharge specific capacity is 137mAh/g and after 50 cycles its capacity retention is even as high as 100%. The 3.6V-plateau efficiency of the optimal sample is also improved compared with the pristine LiCoO₂ and reaches 97% after 25 cycles.Chapter 4 presents four methods that are employed to modify the surface of a commercial LiCoO₂ powder. Through a solid-state reaction process, the cathode material LiCoO₂ can be surface modified by a Li₃PO₄ nano-powder. The electrochemical performance of LiCoO₂, especially its capacity retention in a wide voltage window from 2.8 to 4.5V, can be significantly improved. After heat-treated at the optimal temperature of 450℃, 1wt%-Li₃PO₄ modified LiCoO₂ can deliver a reversible capacity of about 164mAh/g at 1C rate during 50 cycles. A phosphorus-containing surface layer is believed to act as a separation layer to suppress the reactions between the Co⁴⁺ and electrolyte.In Chapter 5, with an Al₂O₃ powder as a model material for carbon-coating, the author investigates the variations of residual carbon from four different organic precursors and obtains that the best precursor among them is PVDF. Then the amount of PVDF as a carbon precursor in the synthesis of C-coated LiFePO₄ （LiFePO₄/C） is investigated. Under the optimal conditions that the amount of PVDF in the starting material is 30% and the sintering temperature is 710℃, the initial discharge capacity of synthesized LiFePO₄/C is above 140mAh/g at 0.2 C rate and the specific capacity even stabilizes at 120mAh/g after 30 cycles at 1-2 C rate.In Chapter 6, the author synthesizes a lithium-containing complex metal oxideβ,γ-Li₂CuZrO₄ with a double rock-salt structure. It may electrochemically react with lithium at potentials below 1.5V vs. Li/Li⁺. The final reaction products are Li₂O, Cu and ZrO₂. As the results of the electrical analysis, theγ-phase is found to be a pure electronic semiconductor with a rather high conductivity and it transforms intoβ-phase at a temperature over 800℃. The activation energy ofγ-Li₂CuZrO₄ is 14.4kJ/mol in the temperature region of 133-1273K.In Chapter 7, lithiated hyperbranched polymers LHn （n=20, 30 and 40） are synthesized from the reactions between the commercial hyperbranched polyester Boltorn^? Hn and lithium metal under an inert atmosphere. Structural analysis and conductivity measurements indicate that their conductivity is up to 5.9×10^-6 S/cm at room temperature and 1.8×10^-4 S/cm at 120℃. LH20 is stable in the voltage window from 0 to 5 V versus Li. This lithiated polyester material is very promising for future all-solid-state lithium-ion batteries.Finally, in Chapter 8, the author gives an overview on the originality work and the deficiency in this thesis. Some prospects and suggestions of the possible future research directions are pointed out.更多还原