LiCoO₂材料的结构、性能及锂离子电池制造技术的研究

【作者】 郭永兴；

【导师】 李新海；

【作者基本信息】 中南大学 ， 冶金物理化学， 2002， 硕士

【摘要】 锂离子电池与传统的二次电池如铅酸电池、Ni／Cd电池、Ni／MH电池等相比，在比功率、能量密度及充放电性能方面有着明显的优势。而且，锂离子电池还有着循环寿命长、自放电率低、“绿色”环保等优点，目前已广泛应用于小型用电器中，并正积极向空间技术、国防工业、电动汽车、UPS等领域发展。锂离子电池技术的关键在于嵌入式电极材料的成功开发。本文详细介绍了锂离子电池正极材料，特别是LiCoO₂材料的发展现状；并对锂离子电池的技术现状和发展趋势作了总结。运用理论电化学的基本原理，分析了LiCoO₂材料对锂离子电池性能影响的机理。在此基础上对锂离子电池的制造技术和性能进行了研究，并对车用动力型锂离子电池的制造与性能进行了初步研究。 采用XRD、BET、SEM、激光粒径分析等方法，分析了四种商品化LiCoO₂材料的晶体结构、物理化学性能以及形貌特征；并采用电化学测试方法研究了LiCoO₂电极反应的动力学特征。重点比较了不同厂家生产的LiCoO₂的晶体结构特征，以XRD图中（003）峰和（104）峰的强度比值I₀₀₃／I₁₀₄来表征LiCoO₂晶体的层状结构。采用大幅度恒电位阶跃技术测定了四种LiCoO₂样品中Li⁺离子的扩散系数，有着较大的I₀₀₃／I₁₀₄比值的LC2#LiCoO₂样品，Li⁺离子在其中的扩散系数最大。表明，I₀₀₃／I₁₀₄的值越大，LiCoO₂晶体的层状结构越完整。对Li⁺离子在LiCoO₂电极中的扩散系数进行测量时，提出了LiCoO₂球形电极的扩散模型，运用Fick第二定律推导出了Li⁺离子在LiCoO₂电极中的扩散系数D_Li+的表达式。 采用极小过电位下（5mv）的线性极化，使电极反应处于电化学控制，测定了LiCoO₂电极的交换电流密度。发现：未经充放电活化的LiCoO₂电极交换电流密度很小，LiCoO₂晶体没有电化学活性，其结构呈现出半导体的性质；随着少量Li的脱嵌，LiCoO₂电极的交换电流密度迅速升高，LiCoO₂晶体呈现出了导体的性质。随着Li的进一步脱嵌，由于LiCoO₂中Li⁺离子浓度的下降，电极的交换电流密度又出现下降趋势。 应用交流阻抗技术对LiCoO₂电极反应过程中电极界面变化进行了研究，测得了不同电位下LiCoO₂电极的交流阻抗Nyquist图谱。典型的LiCoO₂电极的Nyquist图谱由三部分组成：高频部分不随电极电位变化的半径较小的半圆，是由电极表面生成稳定的Li₂CO₃覆盖膜引起的。中频部分的半圆为电化学极化阻抗；低频部分是由扩散控制引起的与实轴成45°角的直线。由Nyquist图求得的LiCoO₂电极的交换电流密度与线性极化测得的值一致。 对四种LiCoO₂材料的充放电性能进行了比较研究，结果表明具有良好的动力学特性，尤其是扩散系数较大的LC2#LiCoO₂充放电性能和循环性能优于其它样品。对四种样品LiCoO₂制得的实验电池的电化学性能的测试也验证了上述结果。 对锂离子电池制造过程中的关键工序：正极成型工艺、负极成型工艺以及电解液体 中南大学硕士学位论文 摘要 h 系地选择进行了研究。提出了正负极浆料配制过程中的常规方法和高温配料方法，以适 应于不同的生产环境；对正负极浆料的涂布过程，提出了有望取代目前成本高、生产环 境不友好、有一定毒性的N’MP＋PVDF粘接剂体系的水溶性粘接剂体系。水溶性粘接剂 制得的试验电池测试的结果证明了这一观点；经过实验筛选指出lmol·L匕iliFwC＋ ＊＊C＊：1体积比)的电解液有利于改善电池的循环性能。 研究了理离子电池的充放电性能、放电电压特性和循环性能，结果表明：理离子电 池的大电流性能，尤其是大电流下电池的电压特性明显变差，一般使用时最好不超过ZC 倍率对电池进行充放电。利用本研究工艺制得的理离子电池有着非常优良的循环性能， 本文测得，063048型试验电池循环500次容量仅下降了5．05％。 研究了理离子电池的贮存性能，指出：锣离子电池在长期贮存时，不应以高于60％ 的荷电态进行贮存，最佳贮存荷电态为40％。测量了理离子电池在贮存过程中的电压变 化，结果表明：理离子电池自放电一般发生在贮存初期，特别是前5天电池的电压变化 很明显。在此基础上，提出了利用电池第2－5天的电压下降值的大小快速筛选自放电异 常的电池。 初步研究了车用动力型理离子电池的制造工艺、充放电性能和循环性能，计算了实 验电池的质量比能量和体积比能量。对车用动力型锤离子电池提出了设计思路和原则工 艺，由此制得的20An动力型理离子电池的充放电性能和循环性能都较为理想，0．SC倍 率充放电工作性能较好，接近甚至超过国外报道的C／3倍率下的数据。本研究20Ah动 力电池的质量比能量接近国外先进水平，但体积比能量较差，还需进一步改进。更多还原

【Abstract】 Compared with traditional secondary batteries, e.g., Pb-PbO2 battery, Ni-Cd battery, Ni-MH battery, etc., Lithium-Ion Battery (LIB) shows greater advantages at the aspect of rate capability, energy density, and charge-discharge performance. Moreover, LIB also shows the advantages of long cycling life, low self-discharge rate, and amity to the environment. Now LIB has been broadly used in portable electrics, and actively used in space technology, national defence industry, electric vehicles (EV), UPS, etc. As we know, the key to the commercialization of LIB is the successful development of the intercalated electrode materials. This thesis introduced the cathode materials of LIB hi detail, especially in the development status of LiCoO2, and a summary of the LIB development about technology status and development tendency was also given. The effect mechanism of LiCoOi on the performance of the LIB was discussed from the viewpoint of the primary principle of electrochemistry. Furthermore, the fabrication technique and performance of LIB were studied, and LIB for EV was also studied primarily.The crystal structure, physicochemical properties, and morphology of four commercial LiCoO2 specimens were studied by using modem analytic techniques, e.g., XRD, BET, SEM and laser particle size analysis. Various electrochemical methods were used to study the dynamic characteristic of the LiCoO2 electrode reaction. In the process of comparing crystal structure of different LiCoO2 specimens, the ratio of the intensity of diffraction peak (003) and (104), IOOS/IIM, were used. Li+ ion diffusion coefficient was measured using step-potential method (300mV), and it can be found that LC2# LiCoO2 specimen with higher ratio of I003/I104 shows the largest Li+ ion diffusion coefficient. This means that the higher ratio of I003/I104> the better crystal structure the LiCoOi material should show. Based on the proposed LiCoCh sphere electrode diffusion model, and the expression of Li+ diffusion coefficient -DU+ was deduced by using the second law of Pick.Line sweep was used to measure the exchange current density (i? of the LiCoO2 electrode at lower overpotential (5mV) when the LiCoO2 electrode was electrochemically polarized. It was found that the i?was very small when the electrode was not active, and LiCoO2 material showed a feature of semiconductor; but the i?was increased sharply after small scale of Li deintercalation from electrode, at this moment the electrode exhibited the feature of conductor. With the further deintercalation of Li from the LiCoO2 electrode, i?decreased partially because of the density of Li+ in the LiCoO2 electrode falling down.The interface phenomena of LiCoO2 electrode was studied by AC impedance method. From the Nyquist spectrum of LiCoO2 electrode, we found that the spectrum was made up of three part: the highest frequency part was a small radius circle, which was generated by the Li2CO3 thin film on the surface of the LiCoO2 electrode and the radius did not change withthe potential change; the medium frequency part was a large radius circle, which was generated by the electrochemistry polarization and the radius changed with the electrode potential change; and the low frequency was a line generated by the Li+ slow diffusion in the LiCoOi electrode, which has an angle of 45?from the real axis. From the Nyquist spectrum the i?was also calculated, and the value consisted with the above.The charge-discharge performance of the LiCoOa electrode was also studied. From the result we can see that the LiCoOi material, which has good kinetics behavior, especially has high Li+ diffusion coefficient, possesses good charge-discharge performance and long cycling life.The fabrication technique of the LIB was studied in detail. The process flow and some key procedure such as: the fabrication of cathode and anode, and the selection of the electrolyte were studied also. In order to adapt different circumstance, we introduced normal blending and high temperature blending. For improving the adhesive performa更多还原