Lithium-ion batteries are the most common chemical energy storage power source in our lives, and their safety is our eternal concern. In order to improve the safety of lithium-ion batteries, battery control circuits (BMS) have been added to control battery charging and discharging, and to prevent the safety risks caused by over-charging and over-discharging of lithium-ion batteries.
In the design of lithium-ion battery structure, three-layer composite diaphragm and ceramic coated diaphragm are used to improve the safety of lithium-ion battery under high temperature conditions.
However, there is still a type of security risk that even if it is a comprehensive security design, it is still difficult to avoid. This is the thermal runaway of lithium-ion batteries caused by mechanical abuse. For example, lithium-ion batteries suffer from external mechanical stress, causing the battery to be deformed or pierced. Short-circuit between positive and negative electrodes, the power of the entire lithium-ion battery is released in a short time through the short-circuit point, which will generate extremely high temperature at the short-circuit point, causing the active material of the positive electrode to decompose, releasing the strong free oxygen of the oxidizing machine and further oxidizing. The electrolyte, a large amount of heat, eventually leads to thermal runaway of the lithium-ion battery, causing fire and explosion.
More seriously, if the thermal runaway occurs on a battery in the battery pack, the high temperature released by the thermal runaway battery will cause thermal runaway to spread inside the battery pack, causing serious consequences.
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Therefore, how to avoid the thermal runaway of lithium-ion batteries and how to suppress the thermal runaway inside the battery pack has become the focus of attention. For example, in the previous article, we introduced a kind of filling inside the battery pack for The phase change composite PCC that propagates in the thermally runaway battery pack is suppressed.
Today, I want to introduce you to a thermal runaway inhibitor added to the battery. The main function of this material is to release the lithium ion battery in time when it is mechanically abused, thus suppressing the thermal runaway. development of.
Studies have shown that by adding 4% inhibitor to the battery, the maximum temperature of the battery in the puncture experiment can be reduced by 50%, and the material has little effect on the cycle performance of the battery. The research results were recently performed by the University of California, San Diego. Yang Shi et al. published in the JPS journal.
In general, some traditional electrolyte flame retardants can increase the safety of lithium-ion batteries, but they can seriously reduce the cycle performance of lithium-ion batteries. In order to solve this problem, Yang Shi et al. The thermal runaway inhibitor DBA (dibenzylamine) is encapsulated in a capsule structure and placed inside the battery. When the battery receives external mechanical pressure, the capsule structure is destroyed and a thermal runaway inhibitor is released, thereby shortening the time. Internal inhibition of thermal runaway.
In the test, the LIR-2450 button battery was used for testing. In the acupuncture test, the temperature of the experimental group with DBA added was 40 degrees Celsius, while the temperature of the control test battery reached 75 degrees Celsius, which was added by adding DBA. In the thermal runaway caused by mechanical abuse, the temperature rise of the battery is reduced by about 50%.
It can be found from calculation that the heat released by the experimental battery in the thermal runaway is 0.15Wh, while the control battery releases 0.23Wh of heat in the thermal runaway, and the battery is in thermal runaway by adding 4% DBA to the battery. Battery heat generation is reduced by about 1/3.
In the extrusion test, Yang Shi loaded the battery core weight 5% DBA into the aluminum plastic film bag and loaded it into the battery. When the battery was subjected to extrusion deformation, the aluminum plastic film broke to release the DBA to the electrolyte. Among them, the test results show that by adding DBA inhibitor to the battery, the battery temperature rise is reduced by about 50% during the thermal runaway caused by the extrusion, which is consistent with the result of the needling test.
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In order to reveal the working principle of DBA in lithium ion batteries, Yang Shi also studied the reactivity between DBA and the positive and negative electrodes. It was found that DBA can react with the fully charged positive electrode to form a solid-electrolyte film on the surface of the positive electrode. The impedance of the charge exchange is increased.
The ionic conductivity test of the electrolyte found that the addition of DBA caused the conductivity of the electrolyte to drop drastically. The conductivity of the pure electrolyte was 9.23 mS/cm, and the electrolyte was added after adding 5% and 10% of the DBA. The resistivity of the liquid dropped to 7.59 mS/cm and 6.38 mS/cm. The number of migration of Li+ in the electrolyte was tested. In the control group, the migration number of Li+ was 0.48, while the electrolyte of the experimental group to which 5% DBA was added, the migration number of Li+ was only 0.23.
It can be seen from the above analysis that by adding DBA to the electrolyte, the charge exchange resistance is increased, the ionic conductivity of the electrolyte is decreased, and the Li+ migration number is decreased. In general, the migration of Li+ between the positive and negative electrodes is suppressed. Thereby reducing heat generation.
YangShi work provides a new idea for the prevention of thermal runaway of lithium-ion batteries, especially by sealing DBA in aluminum plastic film, which can release DBA into the electrolyte in time when mechanical abuse occurs, by increasing the charge of the positive electrode. Exchange impedance, reduce the ionic conductivity of the electrolyte and reduce the number of Li+ migration, so as to reduce the short-circuit current. Studies have shown that 4% of the addition can reduce the thermal runaway temperature by about 50%. Another advantage of DBA's aluminum-plastic film packaging is that under normal conditions, DBA will not be released into the electrolyte, so it will not affect the cycle performance of the battery.
References: Exothermicbehaviors of mechanically abused lithium-ion batteries with dibenzylamine, Journal of Power Source, 326 (2016), Yang Shi, et. al
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