Tram Fires: New Battery National Standards Can't Prevent Them | A Letter from an Engineer

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When a car crashes into a highway median at a speed of 100 kilometers per hour, what follows is an interior temperature of 400 degrees Celsius, windows that melt within 30 seconds, and a vehicle that is left with only a metal frame. Escaping, or rather escaping within a few seconds, is of course a rather difficult thing under such conditions.

By the end of last year, the penetration rate of new energy vehicles in China had exceeded 50%, but the burning battery packs still seem to be an inexpressible pain for new energy vehicles. Can power batteries avoid catching fire? Can passengers have more escape time after a fire? A series of battery fire cases have put these questions in a more prominent position, and relevant departments have once again raised the technical standards for power batteries.

In March this year, the "Safety Requirements for Traction Batteries for Electric Vehicles" (GB38031 - 2025) was promulgated. Compared with the GB/T38031 - 2020 standard promulgated in 2020 and implemented in 2021, one of the main revisions of the new battery national standard is to change the technical requirements for the thermal diffusion test from the previous "providing a thermal event alarm signal 5 minutes before fire or explosion" to the current "no fire, no explosion (still requiring an alarm), and the smoke does not cause harm to passengers".

Moreover, compared with the current test methods that induce thermal runaway through needle penetration and external heating, the new national standard also adds an internal heating test to simulate more severe working conditions.

However, please note that even if the requirements of the new national standard of "no fire, no explosion" are met, it does not mean that there will definitely be no fire or explosion during actual vehicle use. After communicating with several engineers, 36Kr learned that the experimental conditions cannot cover all complex vehicle - using situations. Currently, there is no technology that can completely eliminate power battery fires, and the initiative for driving safety still lies in the hands of the driver.

Interviewee: A, with many years of experience in the battery division of a vehicle manufacturer

36Kr Auto: As early as a year ago, most car companies' requirement for batteries was to achieve NTP (No Thermal Propagation design at the battery pack level). Why haven't battery fire cases been eliminated? Does this mean that just achieving NTP at the pack level is far from enough?

Q: NTP at the pack level is safety under specific test conditions. Most companies can also prevent thermal runaway through a series of measures such as water cooling, exhaust, and power cut - off after a single cell experiences thermal runaway under normal conditions. However, it is impossible to cover all extreme situations.

Xiaomi's case last time is an example. When the car crashed into a concrete pier at such a high speed, the protection failed. Just like wearing a seatbelt does not guarantee survival in a crash.

Another case I saw before is that a vehicle was driving on the sidewalk. Because the floor tiles were loose, when the vehicle drove over, a whole tile flipped up and inserted into the battery pack. This was a battery pack produced by a leading battery manufacturer and it also passed the test standards.

36Kr Auto: There is a view that the system safety of the battery is more important than the safety of the cells. Is it meaningful to require that a single cell does not catch fire or explode in the new national standard?

Q: The intrinsic safety of a single cell is meaningful for the safety of the entire battery pack. The safer the single cell is intrinsically, the easier it is for the entire battery pack to meet the national standard for thermal safety.

Interviewee: Q, engaged in cell R & D, with 6 years of work experience at a leading battery supplier

36Kr Auto: To achieve non - flammable and non - explosive cells, what optimizations can battery manufacturers make in terms of technology, materials, etc.?

Q: A cell is composed of four main materials, namely the cathode, anode, electrolyte, and separator.

The materials for the cathode and anode are basically fixed, which is determined by the electrochemical principle of lithium - ion batteries. Currently, the main cathode materials are in the form of lithium oxides such as NMC (Nickel - Cobalt - Manganese Oxide), LFP (Lithium Iron Phosphate), NCA (Nickel - Cobalt - Aluminum Oxide), and LCO (Lithium Cobalt Oxide). The anode material is mainly artificial graphite, but there may be some fine - tuning of auxiliary materials during doping or feeding. Unless the material system changes, such as all - solid - state or silicon anodes which are generally used for carriers with high capacity, and sodium - ion batteries. The separator material can theoretically be upgraded.

Currently, the separators are generally made of PP (Polypropylene) or PE (Polyethylene) as the base film and then coated with ceramic or PVDF (Polyvinylidene Fluoride). However, if it can be replaced with a material like PI (Polyimide), which cannot be burned even with fire. The clothes firefighters wear contain PI. In this way, it is difficult for the electrolyte to pierce the separator and cause a short - circuit. Currently, short - circuits generally occur when the electrolyte comes into contact with water vapor and LiPF6 (Lithium Hexafluorophosphate) turns into HF (Hydrofluoric Acid), which is more corrosive than sulfuric acid. However, the problem is that the cost of PI is very high, and its commercialization may be difficult.

The electrolyte is the most important part of the cell, so the electrolyte formula generally has a relatively high density. Moreover, its formula is very complex. Generally, each type or project of battery has an independent electrolyte formula, but generally, the electrolyte contains flame - retardant components.

Coating may be relatively important in terms of technology because there are two relatively important processes in lithium - ion battery production. One is coating, and the other is winding for cylindrical batteries or lamination for square aluminum - shell batteries.

The surface density, uniformity, tolerance, and flatness of the coating are all very important. It is also necessary to check the thickness of the electrode sheets at the head, tail, middle, and on both sides through CD full - point inspection. When winding or laminating, attention should be paid to avoid misalignment or folding.

If some defective electrode sheets continue to be processed, it will definitely be unsafe. Therefore, monitoring should be carried out during production, and some inferior electrode sheets should be eliminated during the production process, which will also improve safety performance.

36Kr Auto: The new national standard has also added a bottom impact test and a safety test after fast - charging cycles. How should battery manufacturers meet these new requirements?

Q: Let's start with the bottom impact test. It's a bit like an advanced needle - penetration experiment.

In this regard, the safety of blade batteries may be a bit poor. The structural design of its battery pack is actually not very resistant to impact, which is related to its manufacturing process.

The lamination method of blade batteries is like a sandwich, with a separator sandwiched between the cathode and anode, and the battery case is still made of a relatively soft aluminum shell. The effect produced by this lamination process is not as resistant to needle penetration as Tesla's fully wound structure. Moreover, Tesla's batteries are all filled with glue.

However, the blade structure, by making the battery into a long strip and stacking it on the chassis, can make more full use of space, allowing more batteries to be installed, improving the cruising range, and the overall constant voltage. The advantage of the aluminum shell is that aluminum is lighter, which can reduce the weight of the battery pack. The disadvantage is that the shell is fragile and not resistant to impact.

Of course, there must be a certain difference between the simulated effect and the actual vehicle - using scenario. In the needle - penetration experiment, only one cell may be pierced, so it will not explode. In actual vehicle use, an oblique collision may occur, squeezing several cells, and it may explode.

Material optimization can also be used to improve safety. The bottom of the battery is the bottom guard plate, which is currently generally made of materials such as profiles and sheet metal, combined with some special coatings. Above the bottom guard plate is the cold plate, and the cells are glued to the cold plate. The buffer gap between the bottom guard plate and the cold plate can be filled with materials such as foam. One way to improve safety is to coat the bottom guard plate with a bullet - proof coating.

Most batteries can currently pass the safety test after fast - charging cycles. Even from 0 SOC (Battery State of Charge is 0) to 100 SOC (Battery State of Charge is 100), 15 minutes is a 4C rate. Moreover, now it is only in the range of 20% - 80%, which is equivalent to a charging rate of not less than 3C. So it is not difficult to pass this test.

Moreover, the test does not define the capacity of a single cell. Some single cells are 280 amp - hours, and some are more than 300 amp - hours. Since the capacity is not defined, some low - capacity cells can be used to pass the test. The larger the capacity of a single cell, the more difficult it is to pass the fast - charging test.

36Kr Auto: What will be the future of ternary lithium cells after the new national standard is introduced? Compared with lithium iron phosphate cells, it should be more difficult for ternary lithium cells to meet the new national standard. So what technical means can be used to overcome these difficulties?

Q: This is related to the microscopic structure of the materials. LFP (Lithium Iron Phosphate) has an olivine - like structure, unlike NMC (Nickel - Cobalt - Manganese Oxide), which has a layered structure formed by the co - polymerization of three elements. The diffusion coefficient of the layered structure is significantly better than that of the olivine structure, so the probability of fire and explosion is higher.

Due to the characteristics of the material itself, currently, no one can make ternary lithium vehicles extremely safe. There are also enterprises like CATL that use AI technology to optimize the overall electrochemical formula, but it seems that no one can guarantee that ternary lithium batteries can achieve non - flammable and non - explosive performance.

Interviewee: H, with many years of experience as a battery pack supplier

36Kr Auto: What impact might the new national standard have on some battery material suppliers? Will it cause a reshuffle among the upstream suppliers of battery manufacturers?

Q: The impact brought by the new national standard is more targeted at battery manufacturers. The impact on the upstream of the battery industry chain may not be that great because many battery manufacturers originally implemented relatively high standards. For example, when an upstream supplier conducts pre - welding debugging, there are SAIC's welding standards and CATL's welding standards. Their leaders require using the stricter standard. In the actual application process, the requirements may sometimes be tightened. Tightening means implementing more stringent requirements than the general standards. For example, if the general requirement for size deviation is ±8mm, in the actual production process, to ensure product quality, it may be implemented according to ±6mm.

36Kr Auto: What potential rules in the battery industry in the past might the new national standard restrict?

Q: To restrict some potential rules in the industry, it may rely more on the punishment mechanism. Just raising the protection standards, many acts of cutting corners may still occur because the punishment is not severe enough.