【作者】 吴凯；

【导师】 杨军；

【作者基本信息】 上海交通大学 ， 应用化学， 2014， 博士

【摘要】 电动汽车与大规模储能等战略新兴产业的大力发展，迫切需要开发高功率、高安全以及长寿命的动力锂离子电池。相比其它负极而言，Li4Ti5O12（LTO）被称为“零应变材料”,具有高安全性、长寿命及快速充放电等特性。因而，以LTO为负极的锂动力电池在电动汽车以及要求高安全性、高稳定性和长寿命周期的应用领域具有独特的优势。目前，阻碍LTO基锂动力电池商业化的最主要障碍就是在高温应用条件下的胀气问题。针对该问题，本论文采用纳米LTO为负极，镍钴锰三元材料（LiNi1/3Co1/3Mn1/3O2, NCM）为正极，制备LTO/NCM软包装电池，对其电化学性能、产气行为、产气的原因进行了分析，最后对LTO电池高温胀气问题的解决方案进行了初步的探讨。主要研究结果如下：1、制作了5.5Ah的HEV用高功率LTO/NCM软包电池并对其电化学性能进行评价。结果表明，在室温下电池表现出高功率、长寿命、快充快放等优异的电化学性能以及安全性能，可以给出约2500W kg-1的峰值放电功率，20C高倍率充电和放电容量分别可以达到1C容量的92%和94%；循环寿命超长，在10C/10C的高倍率充放电条件下，循环5000次后容量保持高于89%。但是，在高温下，LTO电池由于存在严重的胀气现象，电池界面被严重破坏，电池在循环过程中容量衰减非常快。研究发现H2是LTO电池高温贮存所产气体中的主要成分。2、采用EIS研究了LTO电极在不同电位下首次嵌脱锂过程中的电化学行为。结果表明，电池在首次嵌锂过程中,按频率由高至低阻抗谱呈现三个半圆/圆弧及一条斜线，应分别归因于肖特基接触、材料电子特性、电荷传递及固态扩散过程。电池在首次脱锂过程中，发现一个重要现象，即电化学阻抗谱的低频弧分裂出两个相邻的半圆弧，这可能是由于LTO电池的胀气所造成。另外，通过等效电路模拟EIS详细分析了LTO电极在首次嵌脱锂过程中的动力学参数。在上述研究结果的基础上，提出了一个锂离子在LTO电极中传输的嵌入脱出的新模型，此模型能够更好地解释LTO电极中锂离子的嵌入脱出机理。该结果也一定程度上说明了LTO电池的胀气现象是一种必然的电化学行为。3、系统研究了80℃/120小时高温加速试验条件下，LTO/NCM电池产气的可能原因和关键因素。研究表明，未化成的LTO/NCM电池在高温下储存基本不胀气。说明与水分含量相关的化学催化反应对电池胀气贡献较小，在高温贮存试验后水分对胀气的贡献率只有15%；相比之下，经过化成然后重新密封的LTO电池在不同荷电状态下体积膨胀均在97%左右。说明LTO电池胀气与电池的荷电状态关系不大。进一步研究发现LTO电池高温胀气的主要原因是由于在LTO表面不能有效形成SEI膜，致使碳酸酯类溶剂与LTO电极表面直接接触，从而发生还原分解反应所致。分析得知碳酸酯溶剂结构不同，其经历的分解路径也有所不同。最后给出了两类典型的碳酸酯溶剂环状碳酸酯和链状碳酸酯的分解机理。4、在对LTO电池的高温胀气行为，胀气原因分析研究的基础上，从电解液配方、导电剂选择、LTO颗粒尺寸控制以及化成工艺等方面着手对LTO电池胀气问题的解决方案进行了初步的探讨。在电解液方面，添加一定量的四甘醇-二甲醚（TEGDME）之后，电池胀气得到明显改善，电池的体积膨胀从未加入TEGDME的53%下降到17%~32%之间。在导电剂选择方面，发现用石墨化的导电碳KS-6替换无定形的Super-P能够明显降低LTO/NCM电池的胀气。在LTO材料方面，发现材料的比表面积对LTO电池产气有比较大的影响，降低比表面积有利于减小电池高温存储时的膨胀量。在化成制度方面，发现在85℃条件下，2.8V恒压化成8小时是最佳条件。更多还原

【Abstract】 The strategic emerging industries of electric vehicles (EVs) andlarge-scale energy storage etc. are vigorously developing, which needhigh-power, high-safety and long-life lithium-ion battery urgently.Compared with other anode materials, Li4Ti5O12(LTO) is called “zerostrain” material and possesses high safety, long cycle life and fastcharging-dischargeing characteristics. Thus, the LTO based lithium batteryis a promising candidate for application in EVs and other heavy-loadapplications which require high safety, high stability and long-cycle life.Currently, the main obstacle to commercialization of LTO-based lithiumbattery is the swelling problem under elevated temperatures. Aim to thisproblem, the LTO/NCM pouch cells were preparaed using nano LTO asanode and nickel-cobalt-manganese ternary materials (LiNi1/3Co1/3Mn1/3O2,NCM) as cathode in this work, and its electrochemical properties, swellingbehaviors, and swelling reasons were systematically investigated. Finally,the solutions on suppressing gas generation of LTO based battery under elevated temperatures were proposed preliminarily. The main findings andconclusions are as follows:1) The5.5Ah LTO/NCM pouch cells were prepared and evaluated forpotential hybrid electric vehicle (HEV) application. The as-prepared LTObatteries showed excellent safety and electrochemical performance (such ashigh power, long-cycle life and fast charging-dischargeing characteristics) atroom temperature. They delivered a peak discharge power density of ca.2500W kg-1, and featured a high C-rate charge-discharge performance (94and92%of discharge and charge capacity at20C, respectively) and a prolongedcycle life (89%capacity retention after5,000cycles at10C/10Ccharge/discharge rate). However, the severe capacity decay was observed atelevated temperatures because of loose (worse) interfaces caused by gasgeneration. It was found that H2was the dominant gas component.2) Electrochemical impedance spectroscopy (EIS) was used toinvestigate the initial Li-ion insertion and extraction in LTO anode at differentpotentials. In the first Li-ion insertion process, as the frequency decreased, theEIS spectra exhibited three semicircles and a slightly inclined line, whichcorrespond to the Schottky contact, the electronic properties of the material,the charge transfer step and solid state diffusion process respectively. In thefirst Li-ion extraction process, a significant phenomenon was observed that the low-frequency semicircle of the EIS spectrum split into two jointsemicircles, which may be attributed to gas generation in LTO cells. Inaddition, the kinetic parameters obtained from EIS fitting data of LTOelectrode in the first charge-discharge cycle have been analyzed in detail.Based on these results, a new model was proposed to interpret the mechanismof Li-ion insertion/extraction reaction. These results could confirm that thegas generation in LTO cells is an inevitable electrochemical behavior.3, The possible reasons and key factors on gas generation of LTO cellswere investigated by the accelerating measurement baking at80°C for120h.It is found that the LTO cell before formation didn’t generate gas duringbaking at elevated temperatures, which means that the chemically catalyticreaction related to moisture makes a minor contribution to cell swelling (only15%). By contrast, the swelling ratio of the charged LTO cells(re-sealed afterformation) is kept at~97%regardless of states of charge, which indicates thatthe LTO swelling has little relationship with the battery’s state of charge.Further investigation shows that the main reason causing the LTO batteryswelling at high temperature is the reductive decomposition reactions ofcarbonates solvents on the LTO electrode surface due to no SEI film formedeffectively. The carbonate solvents undergo different decomposing pathwaysaccording to their structures. Finally, the decomposition mechanisms of the two types of carbonate solvents are proposed based on the experiment results.4) Based on the investigation on LTO cell swelling behavior andmechanisms, the solutions of LTO cell swelling problem are preliminary putforward from the aspects of electrolyte formula, electron conductor selection，LTO particle size control and formation process. In respect of electrolyte，thecell swelling has been significantly improved after adding TEGDME as acosolvent to the electrolyte and the volume expansion is reduced to17～32%from53%before adding TEGDME. In respect of carbon conductor, it isfound that replacement Super-P （amorphous carbon black） byKS-6(graphitized carbon) can reduce the swelling of LTO/NCM batterysignificantly. In respect of LTO anode, it is found that the specific surface areaof the LTO has relatively strong impact on cell swelling and reducing thespecific surface area is benefit to suppressing gas production of LTO cell atelevated temperature. As for the formation process, it was found that85oC,2.8V and8hours are the optimized conditions.更多还原