摘 要

近年来，随着新能源汽车等领域的兴起，人们对于高容量、高性能的锂电池需求越来越迫切。目前市场上的主流锂二次电池主要还是锂离子电池而非锂金属电池，这是因为锂离子电池的安全性能要比锂金属电池好得多。但是锂离子电池的技术已经很难再取得较大的突破，且锂离子电池的储能效果远不如锂金属电池，对于锂金属电池的研究迫在眉睫。锂金属电池难以真正实现应用主要还是由于锂金属负极上不可避免的枝晶生长。锂枝晶在形成后一方面会破坏电池内部结构，另一方面会与电解质反应，造成电池容量的衰减，同时反应还会放出大量的热量，造成电池的热失控和促使电池内部产生热应力。

本文从锂电池枝晶相变后温度升高会导致电池产生热应力这一方面出发，利用有限元方法模拟固态电解质锂电池在温度升高后的内部热应力分布，具体的研究工作如下：

(1)假设电池最初工作在环境温度20℃下，然后将温度分别增高10℃、20℃和30℃，观察电池内部的应力分布，发现在温度升高30℃后，电池内部的热应力已经相当大，特别是电池外壳上的应力已经达到材料的屈服极限；

(2)比较了不同电解质对电池内部的应力分布的影响，发现由于电解质材料的不同，其他条件相同时电池内的最大等效应力差异明显，因此开发新的电解质材料对于改进锂电池性能至关重要；
(3)观察了电池升温后的变形，发现在温度升高30℃后，电池的变形已经十分严重，主要变形区域是集中在枝晶与极板连接处，同时电池总体膨胀也十分明显，这对电池的正常工作有很大影响。

关键字：锂电池、固态电解质、枝晶、热应力

ABSTRACT

In recent years, with the rise of new energy vehicles and other fields, people's demand for high-capacity, high-performance lithium batteries is more and more urgent. At present, the mainstream secondary lithium battery in the market is mainly lithium-ion battery rather than lithium-metal battery, because the safety performance of lithium-ion battery is much better than that of lithium-metal battery. However, the technology of lithium-ion battery has been difficult to achieve greater breakthroughs, and the energy storage effect of lithium-ion battery is far less than that of lithium-metal battery, so the research on lithium-metal battery is imminent. It is difficult to realize the application of lithium metal battery because of the inevitable dendrite growth on lithium metal anode. On the one hand, the formation of lithium dendrite will destroy the internal structure of the battery, on the other hand, it will react with the electrolyte, resulting in the attenuation of the battery capacity, and at the same time, the reaction will release a lot of heat, resulting in the thermal runaway of the battery and the generation of thermal stress inside the battery.

In this paper, the thermal stress distribution in solid electrolyte lithium battery is simulated by the finite element method from the point of view that the temperature rise after dendrite transformation will lead to thermal stress. The specific research work is as follows:

(1) It is assumed that the battery works at the ambient temperature of 20 ℃, and then the temperature is increased by 10 ℃, 20 ℃ and 30 ℃, respectively. The stress distribution inside the battery is observed. It is found that after the temperature is increased by 30 ℃, the thermal stress inside the battery has been quite large, especially the stress on the battery shell has reached the yield limit of the material;

(2) The effect of different electrolytes on the stress distribution in the battery was compared. It was found that the maximum equivalent stress in the battery with the same other conditions was significantly different due to the different electrolyte materials. Therefore, the development of new electrolyte materials is very important for improving the performance of lithium battery;

(3) The deformation of the battery after heating up was observed. It was found that the deformation of the battery was very serious when the temperature increased 30 ℃, the main deformation area was concentrated at the junction of dendrite and electrode plate, and the overall expansion of the battery was also very obvious, which had a great impact on the normal operation of the battery.

Key words: lithium battery, solid electrolyte, dendrite, thermal stress

目录

第一章 绪论 1

1.1研究背景、目的及意义 1

1.2国内外研究现状 1

1.2.1锂枝晶的形成与实验观测 1

1.2.2对锂电池固态电解质力学性能的研究 2

1.2.3运用有限元方法对锂电池的研究 2

1.3研究内容及目标 3

第二章 热传导与热应力问题 4

2.1热传导问题的基本概念和基本方程 4

2.2瞬态热传导问题的有限元一般格式： 5

2.3热应力的计算 6

2.3.1热应力的产生 6

2.3.2弹性热应力问题的有限元方程 6

2.4本章小结 7

第三章 有限元模型的建立 8

3.1软件介绍 8

3.2有限元分析的步骤 8

3.3模型的尺寸和材料的选择 9

3.3.1模型的尺寸 9

3.3.2材料的选择 10

3.5边界条件的施加 10

3.6网格的划分 11

3.7本章小结 12

第四章 模型计算分析 13

4.1温度升高程度不同对电池中应力分布的影响 13

4.1.1电解质材料为石榴石形固态电解质Li_6.24La₃Zr₂Al_0.24O_11.98 13

4.1.2电解质材料为Li_0.35La_0.55TiO₃ 15

4.1.3电解质材料为硫代磷酸盐Li₂S-P₂O₅ 17

4.1.4本节小结 19

4.2电解质材料对电池内部各部分最大应力的影响 19

4.3电池升温后的变形 21

4.4本章小结 23

第五章 结论与展望 25

5.1总结 25

5.2展望 26

参考文献 27

致谢 28

第一章 绪论

1.1研究背景、目的及意义

锂金属由于其质量轻、能量比大和还原势低的特性而成为高能电池负极体系的首选。目前记载的最早的锂电池是由Gilbert N. Lewis在1912年提出并研究的，它的工作原理与普通的干电池一样，是通过负极金属锂的氧化腐蚀来产生电能的。但是由于这种电池有很多弊端，例如电流密度低、不可循环、热失控、短路和锂枝晶问题，锂金属电池并没有得到大规模应运，而是逐渐被更加安全的锂离子电池而取代。

关于锂电池的失效机制，Aurbach^[2]和Wakihara^[3]等人进行了详细的论述。对于锂金属电池，主要是因为反应产生的锂离子和电子在负极表面结合沉积，这些沉积的锂单质会呈枝晶化生长。一方面锂枝晶会与电解质反应，造成电解质的不可逆损失，加快电池老化，同时反应放出大量的热量，容易造成电池的热失控；另一方面，枝晶在电解质内的生长存在刺穿电池和造成电池短路的风险。对于锂离子电池，它以碳或石墨代替了锂金属作为负极，有效避免了锂金属在负极表面的沉积。但是实验证明，在过充等情况下，枝晶仍有可能会生成，对电池安全造成威胁。