How is the non-self-igniting power battery made?
A few days ago, CCTV's "Today's Statement" column reported a spontaneous combustion accident on Samsung Note 4 in 2017, which caused a 4-year-old girl to burn her face. Samsung's mobile phones were even banned from being carried on airplanes due to spontaneous combustion problems.
If the spontaneous combustion of a 3,500 mAh mobile phone battery may cause you to be injured, then starting at 16kWh, the consequences of spontaneous combustion of pure electric vehicles with a maximum of over 80kWh will be even more terrible.
However, Tesla’s battery accident seems to have not been interrupted. A suspected fire accident of the Tesla Model S battery was also found in Hong Kong earlier. The vehicle landed in September 2015.
Looking back at the recent accidents, the models were basically the first-generation Model S that was put on the market in 2013-2015, and the battery life was more than 4-6 years.
The "first burn" of Model S appeared in October 2013-when a Model S was driving, the chassis struck a sharp object. Then the vehicle issued an alarm and the owner abandoned the vehicle and fled. After 20 minutes, the vehicle began to burn, Model S The frame was burned.
In fact, "First Burn" vaguely revealed the terrible consequences of spontaneous combustion of such large-capacity lithium batteries, and the underlying reason lies in the fast charging and quick release of lithium batteries, which not only causes great damage to the battery, but also affects the thermal management of the battery. The requirements are very high, and Model S perfectly corresponds to the above two points.
Battery safety is a major prerequisite for us to enjoy the convenient life brought about by electrification. In order to ensure the safety of electric vehicle batteries, no matter the country, battery manufacturers or car manufacturers have done a lot of work for this.
What kinds of power batteries are in use today, and how the country, OEMs, and power battery manufacturers ensure the battery safety of electric vehicles? this life.
Power battery today
After years of development, pure electric vehicles and hybrid vehicles ushered in a full-blown explosion in 2018. The response in the power battery market is the continuous increase in power battery shipments.
How is the non-self-igniting power battery made?
Power battery shipments in the first 10 months of 2018 have surpassed that of 2017, with a year-on-year growth of more than 84%, and the total installed power reached 56.89GWh.
With the continuous launch of new energy models from old OEMs in 2019 and the delivery of new power car companies, this number is expected to continue to grow in 2019.
At present, the main batteries used in new energy vehicles on the market are the most widely used ternary lithium batteries, safe and stable lithium iron phosphate batteries, and Toyota's exclusive nickel-metal hydride batteries.
Comparing electric vehicles before 2017, it can be found that the energy density of power batteries has risen from 103.3Wh/kg to 142.4Wh/kg, and the country has set a target of 300kWh/kg by 2020. The fundamental reason for such a huge increase in the energy density of power batteries lies in the wide application of ternary lithium batteries.
Vehicles using ternary lithium power batteries include model 3, Corolla e+, BYD Yuan EV, and many other mainstream new energy models.
How is the non-self-igniting power battery made?
The advantage of ternary lithium lies in its high energy density. At present, the most advanced Tesla and Panasonic batteries can reach close to 300kWh/kg, while CATL and BYD can currently reach 200kWh/kg. At present, ternary lithium battery materials still have a lot of room for improvement. . However, the safety performance and battery cycle are not as good as lithium iron phosphate batteries, and they are banned by the state from being used on passenger vehicles.
The market share second only to ternary lithium is lithium iron phosphate batteries. Because of their outstanding safety performance, they are mainly used in commercial vehicles. At present, the electric buses that run on the streets mainly use lithium iron phosphate batteries.
Compared with ternary lithium batteries, electrolyte volatilization occurs at 200 degrees Celsius, which is prone to spontaneous combustion. Lithium iron phosphate batteries will have this problem only at 800 degrees Celsius. However, BYD, which has the highest battery density at present, can only reach 150kWh/h. The BYD Dynasty series, which used lithium iron phosphate batteries, has also switched to ternary lithium batteries.
Now that the energy density of lithium iron phosphate batteries is close to the theoretical limit, there is not much room for improvement. Moreover, the capacity will be reduced by less than 20% after charging 100 times below -10 degrees, and it is basically difficult to use in cold environments.
As for Toyota's exclusive nickel-metal hydride batteries, although the safety and reliability have been tested for many years, no battery safety accidents have occurred after so many years of use. However, Toyota has set up too many patent barriers in this regard, making it difficult for other manufacturers to use.
The cycle times of Ni-MH batteries are very low, and only low charge and low discharge cycles are possible. Toyota Prius keeps the battery at 40% to 60% capacity. In addition, the energy density is even lower than that of lithium iron phosphate batteries, so it cannot be used in hybrid models and pure electric models. Toyota's hybrid models and pure electric models also use ternary lithium batteries.
Relying on the extensive market share of ternary lithium batteries and lithium iron phosphate batteries, CATL’s 2018 shipments surpassed those of Panasonic, which relied on Tesla and Toyota and other pure-electric hybrid models, and BYD, which mainly supplies its own models. Aspiring to be the champion of shipments, with a market share of 41.3% in the domestic market.
However, in terms of energy density and cost, they are still at a disadvantage compared with Panasonic, LG and other Japanese and Korean batteries. Whether the current market can be maintained after reducing subsidies is still a question mark. Of course, as a partner of BMW in battery, I believe that CATL has enough strength to develop products with lower prices and better products.
How do lithium-ion batteries burn
Well, after talking about the classification of power batteries and the past and present, now let's talk about the lithium battery with the largest market share, why it is so easy to catch fire.
The source of lithium battery fire is thermal runaway.
The main reasons for the overheating and spontaneous combustion of lithium batteries are internal and external. The internal cause is mainly the aging of the battery, and the external causes are mainly: puncture, collision, short circuit, external overheating, and high-power discharge and overcharge.
Lithium batteries consist of a positive electrode, a negative electrode, and a separator that only allows lithium ions to pass through. The battery emits heat during operation. When the temperature is increased to a certain temperature, the diaphragm will thermally close, preventing lithium ions from passing through, isolating the positive and negative electrodes of the battery, stopping the reaction, and preventing the battery from overheating.
However, the diaphragm will rupture after a certain temperature and lose its protective effect. When external heat causes the diaphragm to rupture, or physical damage such as puncture or collision, or even the lithium ion crystal formed by the aging negative electrode punctures the diaphragm, the diaphragm will not be able to isolate the positive and negative electrodes, and an internal short circuit will occur in the battery.
Because of the internal short circuit, the battery has a large-area contact between the positive and negative electrodes and reacts violently, releasing a lot of heat, and this process continues to intensify, and the temperature continues to rise.
The electrolyte used in lithium batteries is not stable at high temperatures. In addition to volatilization at high temperatures, the formation of gas will cause the battery to expand and rupture, which intensifies the internal short circuit. After reaching a certain temperature, a series of decomposition reactions will occur, and a large amount of Heat, these heat will cause the reaction to further intensify, and ultimately produce the self-heating effect.
When a lithium battery has an internal short circuit due to various reasons, the heat released may cause a chain reaction of the remaining battery, which will eventually lead to a large area of thermal runaway.
The electrolyte used in lithium batteries is a volatile and flammable organic solvent, which can be ignited under thermal runaway. What finally appeared was just like in several Model S spontaneous combustion accidents. A large amount of smoke was suddenly emitted, and the fire was ignited in a short period of time, and it was difficult to extinguish the fire.
National mandatory standards ensure safety
Since there are problems with lithium batteries, in order to ensure the safe use of lithium batteries in passenger vehicles, the state has established two sets of strict mandatory standards for passenger car batteries and storage batteries, including system countries, with 16 and 10 safety test items respectively. All tests must be passed at the same time, and electric vehicles that meet the two national standards can be marketed to meet consumers.
All the tests are performed under the condition that the battery is fully charged. Several of the tests are more violent. The director will talk about it in detail and let everyone feel the strictness of this standard.
The acupuncture test is to use a steel needle with a diameter of 6-8mm to puncture vertically at a speed of 25mm/s and penetrate at least three batteries, and the steel needle stays in the battery. Observe for one hour without explosion, combustion, or fire.
The heating test is to increase to 130 degrees at a rate of 5 degrees Celsius per minute and keep it for 30 minutes. After stopping the heating, observe for one hour that no explosion, combustion, or fire can occur.
The temperature cycle test is to adjust the temperature according to the temperature and duration of the above table, cycle 5 times, and observe for one hour after that, but there is still no explosion, combustion, or fire.
There is also an external fire test. A fuel oil basin larger than the battery system is used. The battery is directly exposed to 50 cm above the brazier. The flame burns the battery directly for 70 seconds, and then the cover plate is added for 60 seconds or directly. Continue to burn for 60 seconds. If the battery has a flame after leaving the fire source, it will take less than 2 minutes to extinguish. Observe for 2 hours, there should be no explosion, combustion, or fire.
In fact, after these strict standard tests, the probability of spontaneous ignition of electric vehicle power batteries is not higher than that of fuel vehicles. For pure electric vehicles or hybrid vehicles produced and sold by powerful OEMs, everyone can rest assured in terms of safety. .
Continuously improving safety performance
In addition to the safety performance stipulated by the national mandatory standards of the battery itself, in order to ensure the safety of the power battery of the vehicle, there are many other equipment to ensure its safety.
For example, after Tesla was burned by a punctured battery in 2013, Tesla redesigned the external protection device of the battery.
The use of aluminum alloy and titanium materials to create a deflection "shield" can not only protect against frontal impacts, but also deflect some splashing or punctured objects, which greatly reduces the probability of the battery being punctured and impacted from the outside.
Another important device to avoid battery overheating is the power management BMS algorithm of the power system. An effective power management algorithm can effectively avoid the occurrence of overcharging. Because the battery power cannot be directly detected, it can only be estimated by current and voltage. When the power management strategy is wrong due to weather and other reasons, it is easy to cause overcharging.
Overcharge causes the positive electrode of the battery to dissolve, the electrolyte is oxidized and decomposed, the battery heats up and swells and ruptures, and finally catches fire.
Now different teams all over the world are studying more advanced and effective power management algorithms. An excellent power management algorithm can not only detect battery overcharge in time to avoid overheating, but also recognize whether an internal short circuit occurs, issue warnings to vehicle personnel, and guide personnel to quickly escape.
It can even reduce the temperature of the internal short circuit part through the active heat dissipation system, and finally realize the temperature control before the thermal runaway.
Of course, another way is to use an active temperature control strategy, using a liquid-cooled circulation system to wrap the battery pack. It can not only avoid overcharging and overdischarging caused by the battery temperature being too high or too low, but also keeping the battery in a suitable temperature range, keeping the battery charging at the best temperature, and achieving the best fast charging effect.
The traditional lithium battery diaphragm uses a single polyethylene or polypropylene, and the diaphragm will be damaged when the temperature exceeds 135 degrees, and there is a danger of spontaneous combustion. The new battery uses a polypropylene-polyethylene-polypropylene composite diaphragm, which can still maintain the blocking function of the diaphragm at higher temperatures.
In addition, the electrolyte in traditional batteries decomposes at high temperatures, generating a large amount of gas and heat, and thermal runaway occurs. By adding phosphate ester flame retardant to the electrolyte, the reaction can be effectively interrupted and the combustion reaction can be organized.
There are many more of these different measures, and they are constantly improving based on user feedback and test results. The safety of electric vehicles will not lag behind that of fuel vehicles because of changes to the power system.
As the future development direction, there are many different companies and different technical teams that are constantly contributing to the safety performance of electric vehicles. The current safety of fuel vehicles has also been summarized and improved in different accidents. In the future, as electric vehicles appear more widely in our lives, the safety of electric vehicles will surely be further improved.
Director has something to say
The safety of lithium batteries for electric vehicles is not low, and it is improving step by step.
As a new type of vehicle, consumers have no reason to ask for higher standards for electric vehicles than fuel vehicles. At the same time, we should look at electric vehicles in a developmental perspective, instead of blindly criticizing them with a conservative perspective.
Some people say that the worst car he can think of is a domestic pure electric car. All I can say about this is that when the automobile industry started, there was no belief that cars could replace horse-drawn carriages.
Tesla has not performed very well in terms of safety due to reasons such as being too aggressive. The more than 7000 18650 batteries loaded with Model S are simply a nightmare for the power management system. But we can't deny electric vehicles because of this. From the current market, electric vehicle battery safety technology has far exceeded these 18650 battery packs.
The decline in new energy subsidies in 2019 is bad news for the new energy vehicle industry, because the price advantage of fuel vehicles is no longer obvious. But from another perspective, it can also promote new energy vehicles.
In the past, many companies that lived on subsidies could only be eliminated by the market, and the rest were companies with sufficient R&D capabilities, production capabilities, and manufacturing capabilities. For the safety of electric vehicles, excluding these electric vehicle companies that have transformed from "Old Tou Le" can effectively improve the average level of safety of domestic pure electric vehicles.
