1.3 Overcharge test
The overcharge of single battery shall be stopped after 1 I1 (A) current constant current charging until the voltage reaches 15 times of the charging termination voltage specified in the enterprise’s technical conditions or the charging time reaches 1h. Observe for 1h. During this process, the battery shall not explode or catch fire. The overcharge of module battery is also to stop charging after 1 I1 (A) current constant current charging until the voltage of any single battery reaches 15 times of the charging termination voltage specified in the enterprise’s technical conditions or the charging time reaches 1h, and observe for 1h. During this process, the battery shall not explode or catch fire.
At the initial stage of overcharge test, the metastable layer of the solid electrolyte boundary film (SEI) formed on the surface of carbon negative electrode first exothermic decomposition reaction. Continue to charge, the battery voltage continues to increase, and the battery temperature continues to rise. In addition to the reaction between the positive and negative electrodes and the electrolyte described herein, high voltage will also cause the decomposition of the electrolyte. Therefore, a large amount of gas will be generated inside the battery, and the battery will swell seriously (as shown in Figure 1). Continue charging. Under the action of high temperature and high pressure, a large amount of gas is sprayed from the inside of the battery to form thick smoke, as shown in Figure 2. When this happens, the linear carbonate in the electrolyte will be ignited by high temperature after tens of seconds, causing fire or even explosion, as shown in Figure 3.
Another kind of battery fire may occur during the module short-circuit test in which multiple batteries are connected in parallel and then connected in series. When the battery is seriously deformed due to gas production and inflation, the positive and negative lugs outside the battery are in contact with each other under the action of the connecting piece, resulting in short circuit and fire, as shown in Figure 4.
1.4 Crush test
The monomer crush shall be a half cylinder with a radius of 75mm (the length of the half cylinder is greater than the size of the extruded battery). The crush direction shall be perpendicular to the direction of the battery plate. Press the battery at the speed of (5 ± 1) mm / s to stop after one of the following conditions: the voltage reaches 0V or the deformation reaches 30% or the crush force reaches 200kn, and observe for 1h. In this project, the battery shall not explode or catch fire. The crush plate used for module crush is similar to the single crush. The crush direction is the same as the direction in which the battery module is most likely to be squeezed in the layout of the whole vehicle (if the direction most likely to be squeezed is not available, apply pressure perpendicular to the arrangement direction of the single battery). When the deformation of the battery module reaches 30% at (5 ± 1) mm / s or the crush force reaches a certain value, stop Keep for 10min and observe for 1h. During this process, the battery module shall not explode or catch fire.
There are two situations in which the crush causes the battery to lose control of heat: the crush pressure deforms the battery, and the internal diaphragm is broken. The reaction caused by the contact of the positive and negative plates in the battery is similar to that in the acupuncture test, resulting in the fire and explosion of the battery, as shown in Figure 5; The second case is similar to the short-circuit test. After the battery is deformed, the positive and negative lugs contact to form the external short-circuit phenomenon of the battery, and finally fire and explosion occur, as shown in Figure 6.
2.Safety test in GB /T314673-2015
GB / T314673-2015 standard is aimed at the safety requirements and test methods of lithium-ion power battery packs and systems for electric vehicles. There are 16 safety test items. The electrical performance safety test (over discharge protection, over charging protection, short circuit protection and over temperature protection) of power battery pack and system are all protective tests. That is, if the battery pack or system has protective actions such as relay disconnection and fuse fusing during the test, the test passes, and generally there will be no thermal runaway. Overall, the proportion of thermal runaway of lithium-ion power battery pack or system is small, which is mainly concentrated in the process of vibration and crush test.
2.1 Vibration test
Install the test object on the shaking table. The vibration test is carried out in three directions, starting from the z-axis, then the Y-axis and finally the x-axis. For test objects installed in other locations, the test time in each direction is 21h. During the test, monitor the status of the smallest monitoring unit inside the test object, such as voltage and temperature. After the vibration test, observe for 2h that the battery pack shall be free from leakage, shell rupture, fire or explosion. The insulation resistance after the test shall not be less than 100 Ω/V.
In the process of long-term vibration, the insulating sheet of the module battery is easy to fall off or wear out, and the positive and negative lug contacts or contacts with the battery pack shell to form a short circuit, resulting in thermal runaway of the battery, as shown in Figure 7. At the same time, during the vibration process, it is also found that the connecting part of the battery generates strong stress, and it is easy to be torn off at the pole ear with strong bright connection under the static state, as shown in Figure 8.
The vibration standard of GB / T314673-2015 is too strict compared with other standards, and many battery packs will have thermal runaway during vibration test. In the No. 1 amendment, the vibration standard is changed to 15 min sine wave vibration of the battery pack or system, and the vibration frequency increases from 7Hz to 50Hz and then returns to 7Hz. This cycle shall be repeated 12 times in 3 hours in the vertical direction of the installation position of the battery pack or system specified by the manufacturer. Run 1 standard cycle after vibration. After the test, observe for 1h under the ambient temperature of the test. Requirements: the battery pack or system shall be reliably connected and the structure shall be intact. The battery pack or system shall be free from leakage, shell rupture, fire or explosion; The insulation resistance after the test shall not be less than 100 Ω/V. After the implementation of the modification order, the thermal runaway of the battery pack rarely occurs. The vibration test standard of the battery pack shall be formulated according to the road spectrum of the electric vehicle running on the general road. It is inappropriate to be too strict or too loose. Therefore, it is the current focus to formulate and implement the battery pack vibration standard with correct parameters and perfect steps as soon as possible.
2.2 Battery Pack Crush test
The crush of the battery pack adopts a half cylinder with a radius of 75mm. The length of the half cylinder is greater than the height of the test object, but not more than 1m. Stop crush when the crush force reaches 200kn or the crush deformation reaches 30% of the overall size in the crush direction. Keep for 10min and observe for 1h. The battery pack shall be free from ignition, explosion and other phenomena.
During the battery pack crush test, it is found that the battery pack passing the crush test generally stops the test after the crush force reaches 200kn. If the shell strength of the battery pack is not enough and the deformation of the battery pack reaches 30%, a fire will generally occur (as shown in Figure 9). Because after the battery pack is deformed, the deformation of some batteries inside the battery pack will even exceed 80%. In this case, the monomer or module inside the battery pack will be thermally out of control.
In amendment No. 1, the crush force of the crush head is changed from 200 kn to 100 kn, and other standards remain unchanged. In the actual operation of the whole vehicle, the crush force after collision is not certain, and the deformation of the battery may be very huge. Therefore, many electric vehicles will catch fire in the event of collision accidents.
3.Conclusion
A series of reactions will occur in lithium-ion power battery due to individual reasons or under the condition of misuse and abuse, resulting in thermal runaway and battery fire and explosion. Correct parameters and standardized testing standards are important means to verify the safety of batteries. This paper introduces several representative tests in which battery cells, modules, battery packs and systems are prone to thermal runaway, and analyzes the causes and mechanism of thermal runaway. At present, the lithium-ion power battery is not perfect, and the safety problem is the primary problem to limit the new energy industry. However, with the popularization and application of high safety lithium-ion battery materials, the maturity of battery management technology and the improvement of inspection standards, lithium-ion batteries will play a great role in the future.