Thermal runaway is the most serious safety accident in the use of lithium-ion batteries. Thermal runaway is often caused by lithium-ion batteries undergoing squeezing, puncture, or high-temperature simmering and the like. The diaphragm is destroyed and the positive and negative electrodes are short-circuited, or the battery is externally short-circuited. , Caused a large amount of heat inside the lithium-ion battery to accumulate within a short time, causing decomposition of the positive and negative active materials and electrolyte, resulting in lithium-ion battery fire and explosion, a serious threat to the user's life and property safety. Therefore in lithium-ion battery In the safety test, lithium ion batteries are generally required to pass overcharge, overdischarge, short-circuit and squeeze, and acupuncture. However, as the power battery's energy density and battery capacity continue to increase, the batteries are increasingly undergoing acupuncture experiments. Difficulty, therefore, in the “Safety Requirements for Lithium-ion Power Battery for Electric Vehicles” issued by the Ministry of Industry and Information, it is stipulated that the acupuncture experiment is not executed temporarily. However, the new version requires that the acupuncture experiment is not executed temporarily, and whether or not the recovery will be resumed is unknown. A large-capacity, high-energy-density power battery has successfully passed the acupuncture experiment and will surely compete. Achieved a significant advantage. Today we'll talk about those lithium-ion battery technology to install the thermal runaway 'brake' is.
1. Electrolyte flame retardant
The electrolyte flame retardant is a very effective method to reduce the risk of thermal runaway of batteries, but these flame retardants tend to have a serious impact on the electrochemical performance of lithium-ion batteries and are therefore difficult to apply in practice. To solve this problem Problem, Yu Qiao's team at the University of California, San Diego, uses a capsule encapsulation method to store the flame retardant DBA (dibenzylamine) inside the microcapsules and disperse it in the electrolyte. The electrical properties of the lithium ion battery will not be normal. Affected, but when the battery is damaged by external forces such as extrusion, the flame retardants in these capsules will be released. The 'poisoning' of the battery causes the failure of the battery, thus avoiding the occurrence of thermal runaway. In 2018, the Yu Qiao team used it again. The above technologies, using ethylene glycol and ethylene diamine as flame retardants, are incorporated into the lithium ion battery after encapsulation so that the maximum temperature of the lithium ion battery in the acupuncture experiment is reduced by 70%, which significantly reduces the thermal runaway of the lithium ion battery. risk.
The above-mentioned methods are self-destructive, that is, once the flame retardant is working, the entire lithium-ion battery will be scrapped, and the Atsuo Yamada team at the University of Tokyo developed a method that does not affect the lithium-ion battery. Performance of a flame-retardant electrolyte using a high concentration of NaN(SO2F)2(NaFSA) or LiN(SO2F)2(LiFSA) as a lithium salt, to which is added a common flame retardant trimethyl phosphate TMP significantly improves the thermal stability of lithium-ion batteries. What is more, the addition of flame retardants does not affect the cycling performance of lithium-ion batteries. The batteries using this electrolyte can stably cycle more than 1,000 times (C/5 cycle). 1200 times, capacity retention rate 95%).
The use of additives to make lithium-ion batteries have flame-retardant properties is one of the ways to avoid thermal runaway of lithium-ion batteries. Some people also find new ways to try to avoid the occurrence of internal short-circuits in lithium-ion batteries caused by external forces, so as to achieve the goal of drastic reduction of the bottom of the battery. To completely eliminate the occurrence of thermal runaway. Concerning the situation that the power battery may be exposed to violent impact during use, Gabriel M. Veith of the Oak Ridge National Laboratory designed an electrolyte with shear thickening properties. The characteristics of non-Newtonian fluids, in the normal state, the electrolyte presents a liquid state, but in the event of a sudden impact, it will be in a solid state, become very strong, and even be able to achieve bullet-proof effect, from the root to avoid 2. The risk of thermal runaway due to a battery short circuit in the event of a battery collision.
2. Battery structure
Take us to look at how to brake the thermal runaway from the battery cell level. At present, lithium ion batteries are considered in the structural design of the problem of thermal runaway, such as in the cover of the 18650 battery will generally have The pressure valve can release the excessive pressure inside the battery in time of thermal runaway. Next, the positive temperature coefficient material PTC in the top cover of the battery will increase the resistance of the PTC material when the thermal runaway temperature rises to reduce the current reduction. Heat production. In addition, in the design of single cell battery structure, it is also necessary to consider the design of short-circuit prevention between the positive and negative electrodes to avoid external short-circuit of the battery due to erroneous operation, metal extraneous matter and other factors, and cause safety accidents.
Second, in the design of the cell, it is necessary to use a more secure separator, such as a three-layer composite separator that automatically closes the pores at high temperatures. However, in recent years, as the energy density of the battery continues to increase, the three-layer composite separator has become a trend toward the thinning of the separator. Gradually eliminated, replaced by the ceramic coating membrane, ceramic coating can play a supporting role in the diaphragm, reduce the shrinkage of the diaphragm at high temperatures, improve the thermal stability of lithium-ion batteries, reduce the risk of thermal runaway lithium-ion batteries.
3 battery thermal safety design
Power batteries are often composed of dozens, hundreds or even thousands of batteries in series and in parallel. For example, Tesla's Model S battery pack consists of as many as 7000 or more 18650 batteries. One of the batteries, which is subject to thermal runaway, may spread within the battery pack, causing serious consequences. For example, in January 2013, a Japanese airline of the Boeing 787 passenger aircraft lithium-ion battery fired in Boston, the United States, according to The investigation of the National Transportation Safety Board of the United States was due to a thermal runaway of a 75Ah square lithium-ion battery in the battery pack that caused the thermal runaway of neighboring batteries. After this incident, Boeing Company demanded an increase in all battery packs. Measures to prevent thermal runaway diffusion.
In order to avoid thermal runaway inside the lithium-ion battery, Allcell Technology of the United States has developed a phase change material based lithium-ion battery thermal runaway insulation material PCC. PCC material is filled between the single lithium-ion battery, in the lithium-ion battery pack Under normal working conditions, the heat generated by the battery pack can be rapidly transferred to the outside of the battery pack through the PCC material. When the lithium ion battery suffers thermal runaway, the PCC material can melt through the internal paraffin material to absorb a large amount of heat, preventing the battery temperature from furthering. Rising to avoid thermal runaway inside the battery pack. In the acupuncture experiment, a battery pack of 4 and 10 strings consisting of 18650 cells, without using PCC material, a thermal runaway of one battery eventually triggered the battery pack 20 Only the battery was out of control, and in the battery pack with PCC material, one battery thermal runaway did not cause thermal runaway of other battery packs.
Lithium-ion battery thermal runaway is the lithium ion battery safety accident that we least want to see and try to avoid. To improve the safety of lithium-ion batteries, to avoid the occurrence of thermal runaway from the battery formulation design, structural design and thermal management of the battery pack On a multi-pronged approach to jointly improve the thermal stability of lithium-ion batteries and reduce the possibility of thermal runaway.
