钛酸锂基锂离子电池析气机理研究

At present, a carbonaceous anode is generally adopted in commercial lithium-ion batteries. A novel anode material of lithium titanate will find a specific application in fields such as hybrid electric vehicles, wind-light-electricity grids, and intelligent electric grids, because of its zero strain during the charge-discharge process, high safety, and long cycle life. But it should be noted that excess gases will be produced when manufacturing, operating, and storing lithium-ion batteries using a lithium titanate anode, thus leading to case distortion and gas evolution. As a result, the performance of such lithium-ion batteries deteriorates, which greatly limits their practical applications. The related researches have rarely been reported up till now. In this project, we intend to found a rapid evaluation method of gas evolution in lithium-ion batteries using a lithium titanate anode based on the internal pressure at first, and then to systematically investigate the gas evolution law of subjecting the lithium-ion batteries to formation, cycling, and storing, which includes the effects of the formation method, the charging and discharging anode potentials, the cathode active materials' properties, the ration of cathode capacity to anode capacity, the cycling method, the temperatures, the electrolytes, and the impurities in lithium titanate on the gas evolution, with the aid of many characterization methods such as GC-MS, XPS, FE-SEM, HR-TEM,EDS,XRD, ICP, and FTIR. Our study focus will be put on the reactions occurring at the anode/electrolyte interface, especially on the properties of the passivation film formed on the anode surface. The nature and related mechanisms of the gas evolution in the lithium-ion batteries using the lithium titanate anode will be systematically analyzed, and consequently some schemes such as adding electrolyte additives and surface coating for suppressing the gas evolution will be put forward. The research results of this project will be benefit to the research and application of lithium titanate and its related lithium-ion batteries.

目前商用锂离子电池大多使用碳负极。零应变的钛酸锂是比碳更安全、寿命更长的负极材料，在混合电动汽车和风/光发电并网、智能电网等领域有独特的应用前景。但是该系列电池在生产、使用及储存过程中会发生气胀，导致外壳变形、电池向外析气、电池性能急剧下降，这是制约其实际应用的最大障碍。本课题采用内压法来定量检测电池的析气量，并建立电池气胀的快速评价方法；综合运用GC-MS、XPS、 FE-SEM、HR-TEM、EDS、XRD、ICP、FTIR 和三电极检测等手段，系统考察电池在化成、循环和储存过程中气胀的规律，包括化成制度、负极充放电电位、正极活性材料的种类、正负极容量比、循环制度、温度、电解液组成、钛酸锂杂质含量等多种因素的影响；重点研究负极/电解液界面的反应，特别关注负极表面钝化膜的性质；系统分析钛酸锂基锂离子电池析气的本质及相关机制，从而为抑制电池气胀奠定必要的基础。本课题的研究结果将能促进该系列电池及相关材料的研发与应用。

零应变的钛酸锂（Li4Ti5O12，LTO）负极活性材料具有高安全性、长寿命、工作温度宽及快速充放电特性，在混合电动汽车及储能电池领域具有明显优势，但该系列电池在生产、使用及储存过程中会发生气胀，导致外壳变形、电池向外析气、电池性能急剧下降，这是制约其实际应用的最大障碍。本项目首先针对圆柱18650型锂离子电池，设计并制作了可实时监测电池内压的测试装置，明确了钛酸锂电池在不同阶段的产气规律：电池充电时内压下降、放电时内压升高，低SOC储存时更易产气，电池产气率随放电倍率增加而增加，电池在循环过程中持续产气并最终达到稳定值；分析了钛酸锂粉末在不同粒径、比表面积、电解液、温度和电位储存后的气胀特性，初步探讨可能导致钛酸锂电池产气现象的影响因素；重点研究了负极/电解液界面特性，与传统观念认为电解液不会在高电位的LTO电极表面还原分解成膜不同，LTO电极在电池充放电过程中表面会形成钝化膜，该钝化膜随循环进行逐渐致密；膜组分在多次循环后会逐渐由可溶性的有机沉积物逐渐转化为不可溶的羧酸盐和碳酸盐。据此，提出了一种钛酸锂电池可能的产气机理：钛酸锂的产气主要来自于有机溶剂的分解，其中以环状有机物EC及PC的开环反应生成CO2为主；链状有机溶剂在Ti4+的催化作用下脱氢、脱小分子基团生成小分子有机气体；在低电位下，Ti3+的催化作用下会进行少量气体复合反应；表面钝化膜的形成过程伴随上述产气反应的进行，但均匀致密化后会抑制后续界面产气副反应的发生。在此基础上，通过活性物质的改性、电芯设计优化、电解液优选等改进措施制备了低气胀体系的钛酸锂电池，为该体系电池的产业化应用提供了一定的理论与基础。