Fire Tests on E-vehicle Battery Cells and Packs

<b>Objective:</b> The purpose of this study was to investigate the effects of abuse conditions, including realistic crash scenarios, on Li ion battery systems in E-vehicles in order to develop safe practices and priorities when responding to accidents involving E-vehicles.<b>Method:</b> External fire tests using a single burning item equipment were performed on commercial Li ion battery cells and battery packs for electric vehicle (E-vehicle) application. The 2 most common battery cell technologies were tested: Lithium iron phosphate (LFP) and mixed transition metal oxide (lithium nickel manganese cobalt oxide, NMC) cathodes against graphite anodes, respectively. The cell types investigated were “pouch” cells, with similar physical dimensions, but the NMC cells have double the electric capacity of the LFP cells due to the higher energy density of the NMC chemistry, 7 and 14 Ah, respectively.Heat release rate (HRR) data and concentrations of toxic gases were acquired by oxygen consumption calorimetry and Fourier transform infrared spectroscopy (FTIR), respectively.<b>Results:</b> The test results indicate that the state of charge (SOC) affects the HRR as well as the amount of toxic hydrogen fluoride (HF) gas formed during combustion. A larger number of cells increases the amount of HF formed per cell. There are significant differences in response to the fire exposure between the NMC and LFP cells in this study. The LFP cells generate a lot more HF per cell, but the overall reactivity of the NMC cells is higher. However, the total energy released by both batteries during combustion was independent of SOC, which indicates that the electric energy content of the test object contributes to the activation energy of the thermal and heat release process, whereas the chemical energy stored in the materials is the main source of thermal energy in the batteries.<b>Conclusions:</b> The results imply that it is difficult to draw conclusions about higher order system behavior with respect to HF emissions based on data from tests on single cells or small assemblies of cells. This applies to energy release rates as well. The present data show that mass and shielding effects between cells in multicell assemblies affect the propagation of a thermal event.

<b>研究目的：</b>本研究旨在探究包括真实碰撞场景在内的滥用工况对电动汽车锂离子电池系统的影响，以期为电动汽车涉险事故的应急处置制定安全操作规范与优先级准则。<b>研究方法：</b>采用单燃烧件试验设备，对商用锂离子电池单体及电动汽车（E-vehicle）用电池包开展外部火烧试验。本次测试涵盖当前两种主流电池单体技术：分别为以石墨为负极的磷酸铁锂（Lithium iron phosphate, LFP）正极电池，以及以石墨为负极的混合过渡金属氧化物（锂镍锰钴氧化物, NMC, lithium nickel manganese cobalt oxide）正极电池。本次研究的测试单体均为软包（pouch）电池，物理尺寸相近；由于NMC化学体系的能量密度更高，其额定容量为14 Ah，是LFP单体（7 Ah）的两倍。研究分别通过耗氧热分析法采集热释放速率（Heat release rate, HRR）数据，通过傅里叶变换红外光谱（Fourier transform infrared spectroscopy, FTIR）检测有毒气体浓度。<b>测试结果：</b>本次测试结果表明，荷电状态（State of Charge, SOC）会影响热释放速率以及燃烧过程中生成的有毒氟化氢（HF）气体生成量。单体数量越多，单颗电池产生的氟化氢总量越高。本研究中，NMC与LFP电池在火烧暴露下的响应存在显著差异：LFP单体单颗产生的氟化氢更多，但NMC电池的整体反应活性更高。不过，两种电池在燃烧过程中释放的总能量与荷电状态无关，这说明测试对象的电能含量会影响热过程与热释放过程的活化能，而电池材料中储存的化学能才是热能的主要来源。<b>研究结论：</b>研究结果显示，仅基于单颗电池或小型电池组的测试数据，难以推断多电芯系统在氟化氢排放方面的行为规律，该结论同样适用于热释放速率。本次实验数据表明，多电芯组件内电池单体间的质量效应与屏蔽效应会影响热失控事件的传播。