The research and application of power batteries for electric driven road vehicles have gone through a development process from lead-acid batteries, nickel hydrogen batteries to lithium batteries, while lithium batteries have also evolved from metallic lithium to lithium compounds to current lithium-ion batteries.
Lithium ion power batteries for electric vehicles mainly include lithium iron phosphate battery, lithium cobalt phosphate battery and lithium manganate battery, which are characterized by relatively high energy density. Among them, lithium iron phosphate battery is widely used in domestic pure electric vehicles due to its relatively good safety.
From a structural perspective, lithium-ion power batteries are mainly composed of battery cells, battery packs, or battery systems. Due to the active presence of lithium ions on metal surfaces in lithium-ion batteries, there may be safety and stability issues in the development, use, and testing of lithium-ion batteries, which need to be prevented and eliminated.
2.Safety Level Description
Regardless of whether the sample is a battery cell, battery pack, or battery system, regardless of the triggering factor, fire or explosion will be triggered in the battery cell process during vehicle installation or laboratory testing. IEC62660 provides a standardized description of the test results of lithium-ion battery cells for electric drive road vehicles and serves as a typical term for the test conclusions.
(1) No results
There is no change in appearance.
Appearance changes or deformations, including swelling. The internal pressure of the battery is caused by an increase in gas pressure due to the reaction and escape of gas inside the battery. Deformation can be detected through appearance dimensions. Excessive air pressure will break the shell and cause explosion. For safety reasons, lithium-ion batteries are designed with one-way explosion-proof relief valves.
Electrolyte overflows from the vent, vent, or explosion-proof relief valve, and there is a mist.
Electrolyte overflows from structural joints such as the casing, seals, and/or terminals outside the ventilation port.
Smoke is emitted from the vent, usually as vaporized electrolyte.
Due to internal or external reasons, the battery cell container is mechanically damaged, resulting in exposure or overflow of internal materials, but no material is sprayed, including smoke.
The battery cells emit flames or have signs of combustion.
The battery cell container has ruptured and the main components have been violently opened.
3.Causes of Danger
The battery cells of lithium-ion batteries are the main components that cause ignition and explosion during use and testing, and the reasons for their hazards can be summarized as follows:
3.1 Short Circuit
Short circuit between battery poles, mainly caused by faults or damages in the external structure, usually caused by mechanical or physical reasons. Laboratories generally use short-circuit test electricity of mΩ level (IEC62660: 5 mΩ. UL1642 and UN38.3: 100 mΩ)
Conduct a timed external short circuit test for resistance. Some experiments are not even time limited, and the intensity of their reactions in the laboratory can be imagined.
Except for the physical short circuit between the electrode ends (pieces) in the battery body. Lithium ion batteries may also be affected by polymer separators
Rupture causes a short circuit. Under laboratory conditions, internal short circuits are usually generated during various overload tests, environmental tests, and life cycle tests.
The mainstream diaphragm thickness used in power lithium-ion batteries is generally above 30μm. Although it is 16-20μm times higher than general purpose lithium-ion batteries. The diaphragm of m is much thicker, but after all, it is very thin.
The diaphragm is so fragile. After being damaged by mechanical forces such as external mechanical forces and thermal deformation, it will directly lead to internal short circuits. In addition, overheating can also cause damage to the diaphragm, causing internal short circuits.
In addition, defects in the raw materials of the diaphragm may cause minor damage to the diaphragm during the production process. These reasons can all cause local temperature rise in power lithium-ion batteries under small short circuit conditions. Moreover, these small short circuits will gradually expand during use or experimental loading, forming influential internal short circuits.
3.2 Temperature Rise
The temperature rise of a battery is defined as the difference between the internal temperature of the battery and the ambient temperature. The above safety factors are accompanied by a process of temperature rise. Therefore, the power lithium-ion battery separators used for electrically driven road vehicles should have the physical performance of “automatic shutdown protection” to improve the safety performance of electric vehicle use.
3.3 Direct Mechanical Damage
During mechanical environment testing and abnormal operation (abuse) testing, it can directly cause damage to the battery cell or cause performance degradation after damaging the internal structure of the battery cell. Direct combustion or explosion in mechanical shock test is also recorded. After the vibration test, the internal structure of the battery cell is damaged, and there is also a possibility of delayed ignition or explosion hazards.
The temperature environment test and humidity environment test undoubtedly deteriorate the conditions of the loading test. Due to conducting tests in the test chamber, safety issues and secondary disasters have become important elements that need to be prevented.
4.Test Main Points
1) In laboratory tests, fire and explosion are the two situations that need to be carefully guarded. The first few results may cause pollution or secondary harm, and may not necessarily result in destructive direct harm, but they should also be recorded in pictures and text as much as possible.
2) The overload generated in various experiments is one of the main reasons for laboratory safety issues. The sample battery generates heat due to various overload reasons, which leads to the melting of the diaphragm, resulting in short circuit heating and electrolyte vaporization and explosion.
Obviously, temperature and the rate of temperature rise during the period are measurable factors. The measurement of temperature elements will be repeated in all stages of the experiment. As long as appropriate temperature sampling, measurement, recording, and comparison are arranged in the experimental nodes, the deterioration degree of the samples can be detected and evaluated early, and prevention measures can be taken accordingly. For abnormal work (abusive temperature tests), appropriate preventive measures should be taken.
3) Mechanical testing environment test. Mechanical shock and vibration are the effects of driving environment conditions on battery samples during simulated loading and actual use, and the severity level of the test is not high.
Even if the safety impact is caused by direct testing, the reaction is generally mild or delayed. However, mechanical tests that do not work properly (abuse) may have severe reactions that can directly affect the laboratory environment.
After the sample has undergone mechanical testing, temperature sampling, measurement, and inspection should also be continuously arranged to anticipate and prevent the occurrence of adverse conditions, in response to potential lagging reactions.
4) There are also tests with severe temperature reactions and short circuit tests with abnormal operation (abuse).
5) When the temperature of the battery sample increases sharply, it should be noted that all external controls of the battery pack/system have failed at this time.