1. Flame retardant of electrolyte
Electrolyte flame retardants are a very effective way to reduce the risk of thermal runaway of batteries, but these flame retardants often have a serious impact on the electrochemical performance of lithium ion batteries, so it is difficult to use in practice. In order to solve this problem, of the university of California, San Diego, YuQiao team [1] with the method of capsule packaging will flame retardant DbA (dibenzyl amine) stored in the interior of the micro capsule, scattered in the electrolyte, in normal times will not impact on the performance of lithium ion batteries appeared, but when the cells from being destroyed by external force such as extrusion, The flame retardants in these capsules are then released, poisoning the battery and causing it to fail, thereby alerting it to thermal runaway. In 2018, YuQiao’s team [2] utilized the above technology again, using ethylene glycol and ethylenediamine as flame retardants, which were encapsulated and inserted into the lithium ion battery, resulting in a 70% drop in the maximum temperature of the lithium ion battery during the pin pin test, significantly reducing the risk of thermal control of the lithium ion battery.
The methods mentioned above are self-destructing, which means that once the flame retardant is used, the whole lithium-ion battery will be destroyed. However, AtsuoYamada’s team at the university of Tokyo in Japan [3] developed a flame retardant electrolyte that will not affect the performance of lithium-ion batteries. In this electrolyte, a high concentration of NaN(SO2F)2(NaFSA)orLiN(SO2F)2(LiFSA) was used as lithium salt, and a common flame retardant trimethyl phosphate TMP was added to the electrolyte, which significantly improved the thermal stability of lithium ion battery. What’s more, the addition of flame retardant did not affect the cycle performance of lithium ion battery. The electrolyte can be used for more than 1000 cycles (1200 C/5 cycles, 95% capacity retention).
The flame retardant characteristics of lithium ion batteries through additives is one of the ways to alert lithium ion batteries to heat out of control. Some people also find a new way to try to alert the occurrence of short circuit in lithium ion batteries caused by external forces from the root, so as to achieve the purpose of removing the bottom and completely eliminate the occurrence of heat out of control. In view of the possible violent impact of power lithium ion batteries in use, GabrielM.Veith from Oak Ridge National Laboratory in the United States designed an electrolyte with shear thickening properties [4]. This electrolyte utilizes the properties of non-Newtonian fluids. In normal state, the electrolyte is liquid. However, when confronted with a sudden impact, it will present a solid state, become extremely strong, and even can achieve the effect of bulletproof. From the root, it alerts the risk of thermal runaway caused by short circuit in the battery when the power lithium ion battery colliders.
2. Battery structure
Next, let’s look at how to put the brakes on thermal runaway from the level of battery cells. At present, the problem of thermal runaway has been considered in the structural design of lithium ion batteries. For example, there is usually a pressure relief valve in the top cover of 18650 battery, which can timely release the excessive pressure inside the battery when thermal runaway. Secondly, there will be positive temperature coefficient material PTC in the battery cover. When the thermal runaway temperature rises, the resistance of PTC material will increase significantly to reduce the current and reduce the heat generation. In addition, in the design of the structure of the single battery should also consider the anti-short-circuit design between the positive and negative poles, alert because of misoperation, metal residues and other factors resulting in battery short circuit, causing safety accidents.
When second design in batteries, must use the more secure the diaphragm, such as automatic closed pore of three-layer composite at high temperature the diaphragm, but in recent years, with the improving of the battery energy density, thin diaphragm under the trend of three-layer composite diaphragm has gradually become obsolete, replaced by the ceramic coating of the diaphragm, ceramic coating to the diaphragm support purposes, reduce the contraction of the diaphragm at high temperatures, Improve the thermal stability of lithium ion battery and reduce the risk of thermal runaway of lithium ion battery.
3. Battery pack thermal safety design
In use, lithium ion batteries are often composed of dozens, hundreds or even thousands of batteries through series and parallel connection. For example, the battery pack of Tesla ModelS consists of more than 7,000 18650 batteries. If one of the batteries loses thermal control, it may spread in the battery pack and cause serious consequences. For example, in January 2013, a Japanese company’s Boeing 787 lithium ion battery caught fire in Boston, the United States. According to the investigation of the National Transportation Safety Board, a 75Ah square lithium ion battery in the battery pack caused thermal runaway of adjacent batteries. After the incident, Boeing required that all battery packs be equipped with new measures to prevent uncontrolled thermal spread.
In order to prevent thermal runaway from spreading inside lithium ion batteries, AllcellTechnology developed a thermal runaway isolation material PCC for lithium ion batteries based on phase change materials [5]. PCC material filled between monomer lithium ion battery, in the case of the normal work of the lithium ion battery pack, battery pack in the heat can be passed through the PCC material quickly to the outside of the battery pack, when thermal runaway in lithium ion batteries, the PCC material by its internal paraffin wax melting absorb a lot of heat, prevent the battery temperature rise further, Thus alert to heat out of control in the battery pack internal diffusion. In the pinprick test, the thermal runaway of one battery in a battery pack consisting of 4 and 10 strings of 18650 battery packs without the use of PCC material eventually caused the thermal runaway of 20 batteries in the battery pack, while the thermal runaway of one battery in the battery pack made of PCC material did not cause the thermal runaway of other battery packs.
Post time: Feb-25-2022