【作者】 李景坤；

【导师】 马紫峰；

【作者基本信息】 上海交通大学 ， 化学工程与技术， 2011， 硕士

【摘要】 随着LiFePO₄正极材料的商业化,锂离子电池已经成为电动汽车和储能系统应用中重要的动力电源。然而,对于未来电动汽车应用而言,基于LiFePO₄正极材料的锂离子电池的能量密度和功率密度依然不能满足其要求。许多汽车制造商希望为下一代电动汽车开发出更高能量密度和功率密度的锂离子电池。尖晶石LiNi_0.5Mn_1.5O₄材料因其可在4.7 V左右高电位下工作,并有良好的循环特性,已经成为最具潜力的、可用于设计新型高功率密度锂离子电池的高电位正极材料。LiNi_0.5Mn_1.5O₄正极材料的电化学性能和电化学特征受其晶体结构影响很大,而其结构又与其制备过程有很大关系。要制备理想尖晶石结构的LiNi_0.5Mn_1.5O₄材料,一个最重要的步骤就是原材料的混合均匀性。喷雾干燥是一个出色的单元操作,可以从混合溶液中直接获得粉末。本论文尝试应用喷雾干燥或熔盐法制备前躯体,通过优化烧结程序,以期改进尖晶石型LiNi_0.5Mn_1.5O₄材料合成工艺。同时采用原位XRD研究其充放电过程结构相变,为LiNi_0.5Mn_1.5O₄正极材料的应用与制备工艺放大提供依据。首先,分别采用喷雾干燥辅助烧结法和熔盐法,通过改变不同的热处理条件制备LiNi_0.5Mn_1.5O₄正极材料,研究混料方式、热处理条件对材料结构与性能的影响。从工艺的复杂性、操作的可控性和产品质量的稳定性等方面来看,喷雾干燥辅助烧结法优于熔盐法。采用XRD、SEM和FT-IR等技术对所制备的LiNi_0.5Mn_1.5O₄材料的结构和表面形貌进行表征。可以发现,样品的平均晶粒尺寸都处于1²μm。从其FT-IR光谱中发现所制备的LiNi_0.5Mn_1.5O₄粉体材料有一对高频(⁶27 cm^-1)和低频(⁵00 cm^-1)振动峰,所制备的LiNi_0.5Mn_1.5O₄材料具有Fd3m空间群的立方相尖晶石型结构,并包含（111）晶面的四面体形单晶。第二,利用循环伏安、交流阻抗等手段对所制备的LiNi_0.5Mn_1.5O₄正极材料的电化学特征进行测量。在3.5⁵.0 V电压区间,以不同的充放电倍率,对所制备的LiNi_0.5Mn_1.5O₄正极材料的电化学性能进行测量。研究结果表明,所有的LiNi_0.5Mn_1.5O₄材料均在4.7 V区域展现出一个水平的充电电压平台,这是由Ni²⁺/Ni⁴⁺氧化还原电对造成的;另一个水平的充电电压平台出现在4.0 V区域,这是因存在Mn³⁺/Mn⁴⁺电对所致。LiNi_0.5Mn_1.5O₄正极材料电化学性能还受热处理条件影响。对于喷雾干燥辅助烧结法,经过喷雾干燥得到的前躯体在空气气氛中按照三种热处理条件下烧成,热处理条件分别是:样品（a）,升温至700℃烧结4 h然后升温至900℃烧结8 h;样品（b）,升温至700℃烧结6 h然后升温至900℃烧结6 h;样品（c）,升温至700℃烧结8 h然后升温至900℃烧结4 h。可以看到,在0.1 C充放电倍率下,样品（a）、（b）和（c）的首次放电容量分别达到132.0,131.7和129.2 mAh/g。在5 C充放电倍率下,样品（a）、（b）和（c）的首次放电容量分别达到109.1,106.0和104.4 mAh/g。经过300次循环后,其放电容量分别保持在93.9,95.7和95.2 mAh/g。为了深入地了解LiNi_0.5Mn_1.5O₄正极材料结构相变,利用同步辐射XRD技术,在线研究了LiNi_0.5Mn_1.5O₄正极材料在充放电过程中结构相变规律。从原位XRD谱图的连续性变化过程可以看出,在充电（脱锂）过程中,对应立方晶相的所有布拉格衍射峰向高角度方向位移。根据原位XRD谱图及充放电曲线特征研究发现,尖晶石型LiNi_0.5Mn_1.5O₄材料在充电过程中存在四个显著的相变。在开始充电阶段,其初始阶段表现出来的立方晶相我们称之为立方晶相-Ⅰ,随着充电电位增加,尖晶石型LiNi_0.5Mn_1.5O₄材料晶格常数减少结构发生收缩,表明有立方相结构相变存在。在4.78 V电压平台区域,所有的特征衍射峰突然出现一个意外的增强并停留在同样的位置,指明有第二立方晶相生成,称之为立方晶相-Ⅱ,出现两相共存区域。当充电电压达到4.81 V时,形成完全立方晶相-Ⅱ。当充电电位大于4.90 V时,（440）和（400）衍射峰分别发生分裂现象,LiNi_0.5Mn_1.5O₄正极材料从立方相转变成为四面体结构。在放电过程中,在4.7 V区域,四面体结构转变成立方晶相-II,而在4.5 V区域,立方晶相-II变回到立方晶相-I结构,而当放电电压低于4.1 V时,所有的特征衍射峰又位移到初始的低角度位置。LiNi_0.5Mn_1.5O₄正极材料在脱锂和嵌锂过程中结构相变过程是可逆的。更多还原

【Abstract】 Lithium ion battery has become a promising power sources for electric vehicle and energy storage system application since LiFePO₄ cathode material is close to commercialization. However, the energy density and power density of lithium ion batteries based on LiFePO₄ cathode materials is still lower than the requirement of the future electric vehicle application. Many automakers hope to develop much higher energy and/or power density lithium ion batteries for next generation electric vehicle. Because of its good electrochemical performance and high operating voltage around 4.7 V, LiNi_0.5Mn_1.5O₄ spinel is one of the most promising high voltage cathode materials to design novel lithium ion batteries with high power density. The electrochemical characteristics and performance of LiNi_0.5Mn_1.5O₄ spinel should be influenced significantly by its crystal structure, which is related to the synthesis process. One of the most important steps to prepare the ideal LiNi_0.5Mn_1.5O₄ spinel is how to mix the raw materials uniformly. Spray drying is an excellent unit operation which can obtain the powder from mixture solution directly. In this dissertation, we tried to apply spray drying or melt salt method to prepare the precursors, and to optimize annealing process. The in situ synchrotron X-ray diffraction technique was also used to study the phase transition of LiNi_0.5Mn_1.5O₄ spinel during cycling so as to scale up the synthesis routes of LiNi_0.5Mn_1.5O₄ spinel.Firstly, the LiNi_0.5Mn_1.5O₄ spinel materials were prepared using spray drying assisted annealing process and melt salt method with different heat treatment conditions, respectively. The spray drying process is prior to melt salt process from the comparison of process complex and stability of product quality. The crystal structures of the prepared LiNi_0.5Mn_1.5O₄ materials were characterized by X-ray diffraction （XRD）, scanning electron microscopy （SEM） and Fourier transformed infrared spectroscopy （FT-IR）. It has been found that the average particle size of the prepared LiNi_0.5Mn_1.5O₄ powders is estimated to be 1 to 2μm. All of the prepared LiNi_0.5Mn_1.5O₄ powders have couple of high vibration frequency (⁶27 cm^-1) and low vibration frequency (⁵00 cm^-1) in their FT-IR spectrum. XRD pattern indicates that the prepared LiNi_0.5Mn_1.5O₄ powders show phase-pure cubic spinel of Fd3m structure and are consist of single crystal of octahedral shape with （111） planes.Secondly, the electrochemical characteristics of the LiNi_0.5Mn_1.5O₄ spinel materials were measured using cyclic voltammetry （CV） and electrochemical impedance spectroscopy （EIS）. The electrochemical performances of the prepared LiNi_0.5Mn_1.5O₄ cathode materials were tested at different charge/discharge rates between the potential limit of 3.5– 5.0 V. It can be seen, all of the LiNi_0.5Mn_1.5O₄ spinel materials exhibited a flat voltage profile at around 4.7 V that is attributed to the Ni²⁺/Ni⁴⁺ redox couple and another flat voltage profile at around 4.0 V region that arises from the Mn³⁺/Mn⁴⁺ redox couple. The electrochemical performance of the prepared LiNi_0.5Mn_1.5O₄ cathode materials were affected by the heat treatment temperature. As for the spray drying assisted annealing process, three heat treatment conditions were set up. The precursor obtained from spray drying were calined at 700℃for 4 h and 900℃for 8 h in air （sample （a））, 700℃for 6 h and 900℃for 6 h in air （sample （b）） and 700℃for 8 h and 900℃for 4 h in air （sample （c））, respectively. It is found that the initial discharge capacity of the LiNi_0.5Mn_1.5O₄ spinel sample （a）, （b） and （c） with 0.1 C rate are 132.0, 131.7 and 129.2 mAh/g, respectively. The initial discharge capacity of the sample （a）, （b） and （c） with 5 C rate can also reach 109.1, 106.0 and 104.4 mAh/g, respectively, and their discharge capacity still keep at 93.9, 95.7 and 95.2 mAh/g after 300 cycles. In order to understand the structure transition of the LiNi_0.5Mn_1.5O₄ spinel cathode materials thoroughly, the in situ synchrotron X-ray diffraction technique was firstly carried out to study the online phase transition of LiNi_0.5Mn_1.5O₄ spinel during cycling. We found that all of the Bragg peaks refer to the cubic phase of LiNi_0.5Mn_1.5O₄ spinel were shifted to the higher angle as lithium ion extracted from the host materials. From the in situ XRD patterns and charge-discharge profile, it can be found that four phase transitions existed for LiNi_0.5Mn_1.5O₄ spinel during charge process. In the initial charge stage, LiNi_0.5Mn_1.5O₄ spinel with original cubic phases （cubic-I） was observed. The lattice constants of LiNi_0.5Mn_1.5O₄ spinel decreased with the increase of charge potential, indicating that the material experienced cubic phase transition and structure shrinkage. When the charge potential reached 4.78 V, while the X-ray diffraction peak stayed at the same position, the intensity of it was enhanced suddenly. Thus it can be inferred that two phases （cubic-I and cubic-II） co-existed in this region. However, when the voltage was higher than 4.81 V, only cubic-II remained. Each of the （400）, （440） and （511） peaks split into two peaks when the charge potential was higher than 4.90 V, indicating that the LiNi_0.5Mn_1.5O₄ spinel structure changed from cubic phase to tetragonal one. During the discharge process, the tetragonal phase transformed back to cubic-II structure in the 4.7 V region, and transformed from cubic-II to cubic-I structure in the 4.5 V region. All of the peaks shifted back to the lower angle when discharge state was lower than 4.1 V. It is a reversible phase transition for LiNi_0.5Mn_1.5O₄ cathode material during lithium ion intercalation process.更多还原