Electrochemical performance degradation and safety of lithium-ion batteries containing defects induced by collision
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摘要: 不可避免的碰撞会导致电动汽车锂离子电池出现缺陷，为了确认碰撞后的缺陷电池能否继续使用，重点研究了缺陷电池的机械性能、电化学性能、安全边界及衰退机理。首先，使用不同的压头通过准静态加载和落锤冲击对选用的电池样品分别预制了3种典型的缺陷，即压痕、50%偏置压缩缺陷和平板压缩缺陷。随后，分别通过准静态平板压缩和充电/放电循环评估其机械和电化学响应。结果发现，缺陷电池的机械性能显著下降，包括内部短路位移、短路载荷和能量吸收能力下降。相较于无缺陷电池，缺陷电池还表现出明显的电化学性能退化，包括更严重的容量衰退。此外，通过拆解电池解释了其降解机制，基于隔膜厚度提出了电池的机械失效标准。最后，讨论了预制缺陷时的加载速度和缺陷类型对缺陷电池性能的影响。预制缺陷时的加载速度越高，缺陷电池的性能退化越严重，这与惯性效应有关。不同类型的缺陷会导致隔膜厚度和石墨脱粘的变化，从而造成电池性能不同程度的退化。Abstract: Unavoidable electric vehicle collisions can cause defects in lithium-ion batteries, and whether defective batteries after minor collisions can continue to be used is still unknown. In this work, we focus on the mechanical performance and electrochemical performance of defective batteries, safety boundaries, and its failure mechanism. Firstly, three typical defects , namely indentation, 50%-offset compression defect and plate-compression defect, were prepared by quasi-static loading and drop-hammer impact with different indenters. The defective batteries did not exhibit voltage drops or temperature increases, indicating that no internal short circuits occurred. Subsequently, their mechanical and electrochemical responses were evaluated through quasi-static plate compression at a loading rate of 1 mm/min and 1C charge/discharge cycling, respectively. It was found that the defective batteries exhibited significant deterioration in mechanical performance, including earlier onset of internal short circuit, reduced short circuit force, and decreased energy absorption capacity. Defective batteries also exhibited significant electrochemical performance degradation, with greater capacity loss during cycling compared to new batteries. Further, its degradation mechanism was explained through disassembling the cells. The separator of the defective batteries exhibited significant thinning, making it more prone to rupture under secondary loading. Therefore, the mechanical failure criterion of the batteries was proposed based on the separator thickness. After 500 cycles, graphite delamination was observed in the defective batteries, whereas the defective batteries without cycling only exhibited cracking. Therefore, the degradation of electrochemical performance in defective batteries was caused by the combined effects of initial defects and cyclic aging stress. The effects of loading speed and defect type on the performance of defective cells were also discussed. Defective batteries subjected to higher loading rates exhibit greater performance degradation, which is related to inertia effects. Different types of defects lead to variations in separator thickness and graphite delamination, resulting in different levels of degradation. Results are instructive for the study of safety identification and treatment of defective lithium-ion batteries.
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天然气泄漏爆炸事故是油气储运过程中备受关注的问题，气体爆炸导致输气管道撕裂使事故后果更加严重。因此，对可燃气体在受限和非受限空间内的燃烧以及爆炸规律的研究就显得非常重要。周凯元等[1]通过管道内丙烷/空气的预混气体爆燃实验，研究了管道直径、点火能量以及障碍物等因素对爆燃波火焰阵面传播的影响规律。林伯泉等[2-3]也分析了瓦斯爆炸过程中障碍物对火焰传播的加速机理及其对爆炸过程中的激波诱导作用。陈先锋等[4]研究了瓦斯爆炸火焰的动力学行为及其对火焰阵面结构的影响规律。丁以斌等[5-6]通过实验研究了不同样式的平面障碍物和立体结构障碍物对于火焰传播规律的影响。然而，对于密闭输气管道中传播的爆炸波会由于阻火器等连接元件的作用产生较强的反射波，而以往关于该种反射波对预混气体爆炸火焰与压力波传播规律的影响机理的研究并不多。反射波对火焰阵面传播规律的影响，往往与反射波强度以及反射波与火焰相互作用的位置相关[7]。此外，内载爆炸波作用下输气管道管壁的动力学响应及其破坏规律目前研究也不够深入，亟需加强该方面的研究。基于长输管道的安全设计和安全运营，本文中开展末端闭口(闭口端)和末端开口(开口端)工况下甲烷/空气混合气体的燃爆实验，通过对火焰速度、爆炸压力和管壁环向应变的测量，探讨末端反射激波对气体反应及管道响应的影响，以期为后续研究提供一定参考。
1. 实验
1.1 实验装置
实验装置主要由配气系统、抽真空系统、点火系统和数据采集系统构成，如图 1所示。配气系统包括空压机、40 L体积分数为99.9%的甲烷储气瓶和预混气体储罐，实验时按照实验要求配置所需不同组分的预混气体。主体实验管道为316型不锈钢钢管，内径125 mm，外径136 mm，壁厚5.5 mm，总长12 m，设计最大可承受内压为5 MPa。点火系统采用EPT-6点火能量试验台，点火能量可调，最大点火能量1 000 mJ。
1.2 传感器布置
为研究管道内气体爆炸的火焰和压力传播规律以及管道的动态响应，分别在管道上布设光电传感器、压力传感器和应变传感器进行实验测量。传感器的布置如图 2所示，自点火端开始，共布置10个光电传感器，6个压力传感器和2个应变传感器，如表 1所示，L为距离点火端距离。由于管道内爆炸波压力较低(预计初始压力约0.2 MPa)，因此产生的应变较小，采用半导体应变片来监测管壁的环向应变，该半导体应变片灵敏度约为普通电阻式应变计的55倍，可以监测更小范围内的动态应变信号。
表 1 管道上传感器布置Table 1. Arrangement of sensors on the blast tube编号 传感器类型 L/m S1 光电 1.0 S2 光电 1.5 S3 光电 2.5 S4 光电 3.5 S5 光电 4.5 S6 光电 5.5 S7 光电 6.5 S8 光电 8.0 S9 光电 9.0 S10 光电 10.0 S11 压力 1.0 S12 压力 2.5 S13 压力 4.5 S14 压力 6.5 S15 压力 8.0 S16 压力 10.0 S17 应变 6.5 S18 应变 8.0 1.3 实验条件
实验在常温常压下进行，实验中配置的甲烷的体积分数为10.2%，点火能量为1 000 mJ。为研究反射波对管道内预混气体爆炸过程与管道动态响应的影响，开展末端闭口和末端开口2种工况的实验。为使管道内产生较强的前驱冲击波从而获得较大的管道加载效应，在点火端放置一组由6片阻塞率为60%的圆环形钢片串联而成的加速障碍物，环形钢片间距为15 cm，障碍物前端距离点火电极25 cm。
2. 实验结果
2.1 管道内压力
图 3(a)~(b)所示为甲烷体积分数为10.2%时，末端闭口和末端开口2种工况下的管道内各测点的压力时程曲线。从图中可以看出，经过障碍物的激励加速后(0.25~1.00 m)，激波的上升沿逐渐变得较为陡峭(S11~S13段)，距离点火端1.0 m处，爆炸激波的峰值压力约为0.3 MPa，在激波向下游传播的过程中，峰值压力逐渐降低。对于闭口端实验，爆炸激波到达末端后，在盲板的固壁反射作用下产生反射激波，反射激波自管道末端向点火端传播，并与当地压力波叠加产生更高的压力峰值，如图 3(a)所示。对于开口端实验，由于管道末端直接连通大气，因此在爆炸激波到达末端时，会向管道点火端反射回稀疏波，稀疏波自末端向点火端传播，并与当地压力叠加后产生负压，如图 3(b)所示。
图 3 不同工况下管道内各点压力时程曲线Figure 3. Pressure histories from different test points in experimental tubes2.2 管道应变
图 4(a)~(b)分别为末端闭口和末端开口工况下距点火端6.5 m处管壁的应变时程曲线，由图中知，闭口工况下，管壁的动态响应过程非常复杂，管壁应变时程曲线清晰地反映了激波在前后管端的来回反射形成的压力叠加对管道的加载作用。当爆炸激波在管道内来回反射时，管道内的压力会反复叠加，导致管壁周期性地膨胀与收缩。该应变信号主要分为2个部分，首先由激波引起的初始动态应变，其后随着反射激波的往返作用，应变曲线出现较长时间的震荡信号。对于开口端实验，爆炸激波首先导致管壁产生1个环向的冲击应变，其后由于惯性作用，出现收缩现象，但最大应变远小于闭口端实验时产生的应变最大。
3. 实验结果讨论与分析
3.1 反射波对火焰传播规律的影响
图 5所示为闭口端实验测得的4个典型位置的压力和光电信号对比图。由图知，随着气体爆炸向管道下游的传播，火焰与压力信号之间时差逐渐增大，即激波逐渐与火焰阵面分离。当激波传播到管道末端时，在盲板处产生反射，反射激波为压缩波并由管道的末端向点火端传播。当末端反射激波与燃烧反应区相遇时，对应时刻的光电信号出现1个阶跃峰值，如图 5(a)~(c)红线框内部分(约0.03 s处)所示，即在反射激波的作用下，此处火焰亮度增加，然而由于无法确定此时气体是否燃烧完全，火焰亮度的增大有可能是反射激波增大了波阵面后方燃烧区预混气体的扰动，因此对当地气体燃烧起到了正激励的作用；另一种情况是，如果此时气体已经完全燃烧，则此时只是反向激波对火焰厚度方向的压缩作用导致的亮度增大。而在管道后段(S8~S10段)，由光电信号幅值较低，火焰亮度下降，光电信号的变化反映了明显的火焰淬熄，然后又复燃的现象。林柏泉等[7]研究表明，当一维受限空间内反射激波与在火焰内部与反应区相遇时，对火焰的传播速度并无明显影响，但可能造成火焰内部的分离现象，而从图 5(c)~(d)可知，火焰阵面与反射激波相遇在S8和S10之间，因此分析认为S8所测火焰的熄灭与复燃应该是由反射波的气体伴流作用导致的火焰分离现象。对于图 5(d)中的对比信号(S10与S16)，首次末端反射激波通过测点时，火焰阵面尚未传播到该区域，反射激波对火焰传播无影响，此后的火焰内部也有压力作用下火焰亮度增大以及火焰的熄灭与复燃现象，但S10处气体反应已处于反射波流场中，由于缺乏更多的探测手段，此时是否是残留可燃气体的作用导致S10信号的突变目前无法详细解释。
图 5 末端闭口工况下典型位置处光电与压力信号对比Figure 5. Pressure and flame signals at typical positions in close-ended tube图 6所示为末端开口实验测得的4个典型位置的压力与光电信号对比图，由于末端开口，初始激波到达末端后产生的反射波为稀疏波并向点火端传播，稀疏波的到达使得测点处压力迅速下降直至出现负压区，此外稀疏波引起的伴流方向与火焰传播方向相同，会加速火焰传播，但同时会拉长火焰厚度，因此会使得火焰亮度下降，如图 6框内部分中所示，在稀疏波作用区，当地压力降低，对应的光电信号也呈现出迅速下降的趋势。
图 6 末端开口工况下典型位置处光电与压力信号对比Figure 6. Pressure and flame signals at typical positions in open-ended tube3.2 反射波对管内压力波传播与管壁应变的影响
为分析内部气体爆炸过程中管道的响应规律，选取第1组应变传感器所测应变信号进行分析，并将其与同一位置处所测压力信号进行对比，如图 7所示。图 7(a)、(b)分别为闭口端和开口端实验距离点火端6.5 m处压力和应变信号的对比图。
图 7 末端反射激波对管道内压力波传播与管壁应变的影响Figure 7. Effect of the reflected shock wave on the pressure and strain in the tube由图 7(a)可知，在管道末端闭口条件下，管壁的环向应变主要有2个部分构成：首先，在爆炸产生的前驱激波作用下，管道呈现环向膨胀状态，即图中框内部分；其次，由于压力激波在管道前端和末端来回反射，管道内压力水平逐次升高，会对管壁实现逐次的加载，产生较大的环向应变，应变信号与压力信号呈现出较好的一致性。此后相当一段时间内，激波在来回反射的过程中逐渐衰减，管道内压下降，管壁应变也随之逐渐趋于初始状态。即对于末端闭口空间内的管道气体爆炸实验，管壁环向应变的最大值是由激波在管道内来会反射逐次加载产生的。末端开口时，由图 7(b)可知，管壁产生的应变主要由前驱激波引起，当管道内压力在端部稀疏波的作用下迅速降为负压直至压力归零的过程中，管壁应变也随之迅速降低，即开口端实验所产生的最大应变是由激波引起的。
4. 结论
(1) 密闭管道内气体爆炸时，末端反射激波与火焰相交时，反射激波提高了火焰传播区域的预混气体反应剧烈程度，反射激波作用下火焰亮度增加。
(2) 密闭管道内气体爆炸时，末端反射激波作用下相应地出现当地火焰亮度增大现象，而前端反射波则有可能引起内部火焰分离而导致测量信号的熄灭与复燃现象。
(3) 管道末端闭口工况下，管壁的最大环向应变是由激波在管道两端产生的来回反射叠加所引起的，应变较大，管壁的环向应变时程关系与该处压力时程关系具有良好的一致性；而末端开口时，管壁的应变主要由前驱波引起，最大应变比末端闭口工况下的应变小。
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图 1 无缺陷锂离子电池的放充电电压和电流时间历程曲线
Figure 1. Variation of voltage and current of a non-defective lithium-ion battery with time during discharging and charging
图 2 采用准静态压缩设备在锂离子电池样品上制造缺陷
Figure 2. Preparation of defects on lithium-ion batteries by using a quasi-static compression machine
图 3 采用落锤测试设备在电池样品上制造压痕缺陷
Figure 3. Preparation of indentation defect on a lithium-ion battery by using a drop-weight test machine
图 6 含不同深度压痕缺陷的锂离子电池的准静态平板压缩实验结果
Figure 6. Quasi-static plate compression experimental results of lithium-ion batteries containing different depth indentation defects prefabricated by quasi-static compression
图 7 含不同深度压痕缺陷的锂离子电池的充放电循环实验结果
Figure 7. Charge-discharge cyclic experimental results of lithium-ion batteries with different depth indentation defects
图 8 不同加载速度下预制了压痕缺陷的锂离子电池样品在准静态平压下的力-电-热耦合响应
Figure 8. Force-electric-thermal response of lithium-ion battery samples containing indentation defects pre-fabricated at different loading velocities under quasi-static flat compression
图 9 在不同加载速度下预制了深度为3 mm的压痕缺陷的锂离子电池样品在准静态平压下的关键力学参数
Figure 9. Critical mechanical parameters of lithium-ion battery samples containing 3-mm-depth indentation defect pre-fabricated at different loading velocities under quasi-static flat compression
图 10 5 mm缺陷深度下，不同缺陷类型电池样品在准静态平压下的力-电-热耦合响应
Figure 10. Force-electric-thermal response of lithium-ion battery samples containing different-type 5-mm-depth defects pre-fabricated at the loading velocity of 1 mm/min under quasi-static flat compression
图 12 电池缺陷部分的隔膜厚度和短路应变随缺陷类型的变化
Figure 12. Separator film thickness and short-circuit strain of defective parts of batteries varing with defect type
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