Practice 32700 battery safety VS specific energy balance technique

　　What is the biggest shortcoming of electric vehicles? For consumers, there may be range anxiety, but for manufacturers, battery safety is their biggest concern.

　　Lithium batteries are not only the core components of electric vehicles, but also the primary cause of spontaneous combustion in electric vehicles. As of May this year, the national regulatory platform for new energy vehicles has discovered a total of 79 safety accidents. 58% of fires are caused by battery problems.

　　On October 7, 2019, the 3rd International Battery Safety Symposium was held in Beijing. The theme of the conference was "Making safer high-specific energy batteries for electric vehicles". In the context of the continuous improvement of the specific energy of power batteries, 32700 battery R&D designers from well-known universities and enterprises around the world discussed the mechanical-electrical-thermal inducements and prevention and control methods of battery thermal runaway, the occurrence mechanism and suppression methods of battery thermal runaway, Issues such as battery combustion and explosion characteristics and fire safety, battery system thermal runaway spread and thermal management.

　　According to Wang Fang, chief expert at the China Automotive Technology and Research Center, trends such as the continuous improvement of battery system energy density, new material systems, and larger batteries have brought huge challenges to battery safety. A team led by Ouyang Minggao, an academician of the Chinese Academy of Sciences, conducted in-depth research on lithium batteries and reduced the probability of thermal runaway and controlled heat diffusion through cell voltage monitoring, flammable gas early warning, improving electrolytes, and establishing firewalls. Car companies are also strengthening battery safety in aspects such as crash safety design, monitoring, and controlling heat spread.

　　On the left is high specific energy demand, on the right is safety - 32700 battery practitioners need to move forward while maintaining a balance on both sides. What “balancing skills” have they learned so far?

　　1

　　32700 battery safety evaluation:

　　Four major challenges

　　The demand for long driving range and fast charging of electric vehicles has brought about technological changes, and changes will bring challenges. Wang Fang, chief expert at the China Automotive Technology and Research Center, summarized it into four major challenges.

　　Wang Fang, chief expert of China Automotive Technology and Research Center

　　First, increased energy density brings stability challenges. The energy density of battery systems has increased year by year, from 90 Wh/kg in 2015 to more than 140 Wh/kg now, and the problem is obvious. "In 2016, 2017, and 2018, I tested domestic and foreign products that were relatively large in volume at that time, including batteries from Samsung and LG. With the increase in energy density, no matter how you improve the safety of your body, the stability of the battery is changing. Bad." Wang Fang said.

　　Second, the challenge of material system changes. Today's products pursue high specific energy, and batteries are changing from lithium iron phosphate to ternary systems, from ternary 333, to 532, and then to 811 systems. The disadvantages brought about by this change are that the thermal runaway time continues to advance, the oxygen release temperature of the cathode material gradually decreases, and the thermal stability of the battery material becomes worse and worse.

　　Third, the challenge of long driving range. To improve the driving range, in addition to changing the material system, it is necessary to stuff as many batteries as possible into a limited space. This will cause the battery to become larger and larger, and the aluminum foil and copper foil of the battery will inevitably be made thinner, and the separator will also Make it thin. However, the thinner the separator, the worse its puncture resistance will be, and the easier it will be to be punctured and cause the battery to short-circuit.

　　Fourth, there are safety challenges after battery degradation. Wang Fang pointed out that many of the accidents they counted occurred after more than 10,000 kilometers. This proves that the battery is a dynamically changing process.

　　This means that throughout the life cycle, the evaluation of battery availability, controllability and loss of control faces huge challenges. Battery testing and evaluation technology may be an evaluation project throughout the entire life cycle. Throughout the life cycle of the battery cell, safety will change with the decay of life. Under different cycle numbers, the internal state and external indicators of the battery core are also changing.

　　2

　　Thermal runaway:

　　Find out from three major reasons

　　The danger of batteries comes from thermal runaway. To deal with battery thermal runaway, we must first understand the mechanism and find the manifestation. Ouyang Minggao concluded that there are three reasons for battery thermal runaway, namely internal short circuit, oxygen release from the positive electrode, and lithium precipitation from the negative electrode.

　　Ouyang Minggao, academician of the Chinese Academy of Sciences

　　○Rely on BMS to detect internal short circuit

　　Internal short circuit is divided into slow change type and sudden change type. Ouyang Minggao said that for a slow-variation internal short circuit, the first step is a voltage drop, and then there is a temperature rise in the second step, and finally thermal runaway occurs. For slow-varying short circuits, fault diagnosis can be detected during the first process, the voltage drop stage, and can prevent further deterioration. For example, for series-connected battery packs, the first step is to analyze the voltage consistency. If the voltage of a certain battery drops, it means that the battery may have an internal short circuit. But if you still can't confirm it, add temperature detection.

　　To deal with sudden internal short circuits, such as a micro short circuit, you can rely on combustible gas sensors, which can provide thermal runaway warnings at least 3 minutes in advance. In other words, internal short circuit can be effectively detected through BMS.

　　○Improve the positive electrode and electrolyte to reduce oxygen release

　　There will still be thermal runaway without an internal short circuit. The separator collapses and material exchange occurs between the positive and negative electrodes, that is, the oxygen released from the positive electrode runs to the negative electrode, causing a violent reaction and triggering thermal runaway. To improve the materials, one is the cathode material and the other is the electrolyte. For example, Ouyang Minggao said that changing the cathode material from polycrystalline to single crystal can increase the temperature of oxygen release by 100 degrees. As for electrolytes, high-concentration electrolytes such as DMC (dimethyl carbonate) can be used.

　　In addition, there is much potential in electrolyte additives, high-concentration electrolytes, new electrolytes, etc.

　　○Charging control reduces lithium deposition

　　The most important factor affecting the safety of the battery throughout its life cycle is lithium precipitation. If there is no lithium precipitation and attenuation, the safety of the battery will not become worse. The more lithium is precipitated, the more exothermic it is. The precipitated lithium will directly react violently with the electrolyte, causing a large temperature rise, which can directly induce thermal runaway.

　　"The potential of the negative electrode is related to lithium evolution. As long as the overpotential of the negative electrode is controlled, it can be guaranteed that no lithium will evolve. Through this model, a charging curve without lithium evolution can be derived. We make the negative electrode potential never lower than zero, and we can get no The optimal charging curve for lithium evolution. We can use three electrodes to calibrate this curve and use this to create a charging algorithm." Ouyang Minggao said that they have cooperated with companies and can use this algorithm to completely eliminate lithium evolution. However, this is a calibration process, and the attenuation performance of the battery will change over time, so they also developed a feedback-free lithium elution control algorithm, that is, an observer is required to observe the overpotential of the negative electrode. It is actually a mathematical model.

　　○Control heat diffusion

　　In Ouyang Minggao's view, thermal runaway is generally regular. The characteristic of thermal runaway of parallel battery packs is that the first battery will short-circuit after thermal runaway, causing a voltage drop; thermal runaway of series batteries is a process of heat conduction; the third situation is that it spreads in an orderly manner at first, and then violently. Spread, which will lead to immediate explosion and combustion accidents.

　　Ouyang Minggao believes that heat insulation alone is not enough for batteries, and heat dissipation design is also needed. "Using firewall technology, heat insulation and heat dissipation work together to block heat transfer through heat insulation and take away energy through heat dissipation."

　　Another type of thermal runaway is an eruption. It can be seen from experiments that eruptions have three states: solid, liquid, and gaseous. The gaseous state in the middle is some combustible gas, which is fuel, and the solid state is some solid particles, which often form flames. Generally, particulate matter is collected, just like a traditional car, by passing the particulate matter through a filter; another method is to dilute combustible gas.

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