There is no consensus on the safety of ultra-fast battery charging | A letter from an engineer

“As frontline creators of automotive products, engineers are the ones who know the most about automotive products and technologies. Whether a product is good, how advanced the technology is, and whether the materials used are genuine. In 36Kr's "Engineer's Letter" column, you can see a more realistic side.”

During peak travel hours, the long queues in front of charging piles at highway service areas, the charging time often exceeding 30 minutes, and the boredom and irritation during the charging process. For quite some time, these seemed to be the entire experience of charging pure electric vehicles.

Automobile manufacturers and battery manufacturers have noticed this pain point of users. Previously, Li Auto tried to prove to consumers that the MEGA was indeed worth 500,000 yuan through "5C charging". After "5C charging" was successively adopted by brands such as XPeng, Zeekr, Voyah, and Xingjiyuan, new technical terms such as "megawatt flash charging" and "super e-platform" have emerged. Even CATL has increased the charging rate of batteries to 12C after BYD.

Companies aim to tell consumers a beautiful story of "charging experience comparable to refueling" through a series of new technologies.

However, at the consumer end, some users are rather cautious about ultra-fast charging batteries. Their concerns about ultra-fast charging batteries often focus on: whether ultra-fast charging technologies such as 5C, 6C, and even 12C will have an impact on battery life; whether the safety of batteries under the ultra-fast charging system is still trustworthy.

36Kr found in communication with battery engineers that ultra-fast charging technology does shorten battery life.

However, while battery manufacturers are trying to increase the charging speed, they have also made a series of corresponding improvements to the battery's materials, structure, etc., so that the impact of ultra-fast charging on battery life is controlled within an acceptable range. At least within the 8-year, 150,000-kilometer warranty period, the battery can be used normally.

As for the safety of batteries under the ultra-fast charging system, there is no consensus yet.

The process of battery charging is the process in which lithium ions leave the positive electrode and embed into the negative electrode. During the ultra-fast charging process, the migration speed of lithium ions is very fast. Therefore, it is very likely that they do not have time to embed into the negative electrode evenly, but form lithium dendrites on the surface of the negative electrode. When lithium dendrites accumulate to a certain extent, they may affect battery life and capacity, and even cause more serious problems, such as piercing the separator and causing a short circuit in the battery cell.

Moreover, there is a breathing effect during the charging and discharging process of power batteries. The battery volume is large when fully charged and small when empty. Under the ultra-fast charging system, this breathing effect is more obvious. As the number of charging and discharging cycles increases, the battery volume will expand, thus accelerating the aging of the battery. Aging batteries are prone to produce gas, causing the battery to bulge, and safety hazards will follow.

In addition, in order to better meet the needs of time-sensitive users, some automobile manufacturers are jointly expanding the ultra-fast charging power range with battery manufacturers, which undoubtedly puts forward higher requirements for the thermal management capabilities of battery manufacturers.

However, not all battery manufacturers can accurately recognize their own ability boundaries, which will undoubtedly bring more uncertainties to the safety of batteries under the ultra-fast charging system.

Judging from 36Kr's communication with battery industry insiders, the probability of the above problems occurring will start to increase when an electric vehicle has been in use for more than 2 - 3 years.

Based on these considerations, some automobile manufacturers are not in a hurry to follow up on ultra-fast charging technology.

For example, the recently hotly debated Xiaomi YU7 only equips the Max version priced at 329,900 yuan with CATL's Kirin battery. Xiaomi officially claims that this battery supports 5.2C ultra-fast charging. While in the standard version priced at 253,500 yuan and the Pro version priced at 279,900 yuan of the Xiaomi YU7, the charging rate of the battery is within 3C.

Another example is Leapmotor, which is jokingly called the "half-price Li Auto". In the selection of power batteries, it did not follow Li Auto in using ultra-fast charging batteries.

In addition to safety issues, ultra-fast charging technology will also make consumers pay more for charging.

Currently, batteries that support ultra-fast charging above 5C generally require strong water-cooled heat dissipation. However, when charging under strong cooling conditions with water cooling turned on, more electricity will inevitably be consumed. Therefore, users need to spend more money on charging.

“Interviewee: A, with more than a decade of battery experience in leading automobile manufacturers”

36Kr Auto: Since this year, the industry has launched 10C and 12C ultra-fast charging technologies. Will this have an impact on battery life? To what extent will it be affected?

A: Ultra-fast charging will definitely have an impact on battery life, but this impact is within a controllable and acceptable range. Because when battery manufacturers develop ultra-fast charging technology, they will consider the design life of the whole vehicle. So even if the battery life decays, it can definitely meet the 8-year, 150,000-kilometer battery warranty.

The method for manufacturers to test the impact of ultra-fast charging on battery life is to perform ultra-fast charging on the battery when its health is 100%, stop when the battery decays to 75%, and then calculate the number of cycles the battery has gone through in this process.

Of course, there will be some slight differences in test conditions among different manufacturers, so the measured cycle life will also vary. The lower one may be 700 cycles, some may be 1000 cycles, and there are also higher ones.

This is because some manufacturers stop when the battery decays to 70%, so the measured cycle life will definitely be longer; while some manufacturers have higher standards and stop when it decays to 80%, so the measured life will definitely be shorter. Currently, the mainstream practice in the industry is to stop when the battery decays to 75%.

According to the cruising range designed by current battery manufacturers, one cycle is several hundred kilometers. So the batteries under the ultra-fast charging system can definitely meet the 3-year, 150,000-kilometer warranty.

36Kr Auto: What is the principle behind the shortening of battery life by ultra-fast charging technology?

A: Ultra-fast charging technology affects battery life mainly because of lithium deposition, the expansion during the charging and discharging process will damage the battery's microscopic structure, and the rupture and regeneration of the SEI film will consume the active substances in the battery.

During the ultra-fast charging process, the migration speed of lithium ions is very fast. After leaving the positive electrode, they may not have time to embed into the negative electrode evenly, which will form lithium dendrites on the surface of the negative electrode.

Moreover, there is a breathing effect during the charging and discharging process of power batteries. The battery volume is large when fully charged and small when empty. Under the ultra-fast charging system, this breathing effect is more obvious. As the number of battery cycles increases, the battery volume will also expand, accelerating the aging of the battery. The aging of the battery is specifically manifested as the thickening of the SEI film and the increase of internal resistance.

When the battery is charged and discharged for the first time, the electrolyte reacts with the battery's negative electrode, forming an SEI film on the surface of the negative electrode. Over time, the SEI film will rupture, and then it will regenerate. Each time the SEI film is produced, it will consume the active substances in the battery, and ultra-fast charging accelerates the rupture of the SEI film.

36Kr Auto: Will the impact of ultra-fast charging on the cycle life of batteries with different material systems vary?

A: The impact on ternary lithium batteries will be greater. Because the ternary lithium material is layered, compared with the olivine-type lithium iron phosphate material, ternary lithium is more easily damaged. Under high-pressure conditions, the side reactions of ternary lithium batteries will also increase. Moreover, the life of lithium iron phosphate batteries is originally long, so they are less affected by ultra-fast charging.

36Kr Auto: Will the impact of using ultra-fast charging technology on battery life vary under different environmental temperatures in the north and the south?

A: The impact of temperature is very small. Power batteries are all protected by electromagnetic waves. They will heat the battery in low temperatures and cool it in high temperatures. However, the external environment cannot be too high or too low, such as as high as 40 or 50 degrees Celsius or as low as minus 30 degrees Celsius. If the temperature is too high, the electrochemical reaction of the power battery will be too intense, and the consumption of lithium ions will increase; if the temperature is too low, the electrolyte will be too viscous, and the electrochemical reaction inside the power battery will be very difficult. However, we rarely encounter such extreme climate conditions.

36Kr Auto: What corresponding adjustments will battery manufacturers make to battery materials when developing ultra-fast charging technology?

A: Let's start with the battery's negative electrode material. The commonly used negative electrode material, artificial graphite, is layered. To make a fast-charging battery, first, the interlayer spacing of graphite needs to be expanded, which is equivalent to expanding the channel for lithium ions to migrate; then, the particles need to be made smaller. Because the larger the particles, the longer the channel for lithium ions to migrate. Making the particles smaller is equivalent to shortening the path for lithium ions to migrate.

However, the negative electrode material also needs to balance energy density and charging rate, so it is necessary to mix large and small particles. If there are too many small particles, the battery's storage performance will decline, and with more small particles, the specific surface area will increase, and the side reactions will be more intense.

As a result, the raw materials for the battery's negative electrode will definitely change. For example, if the original raw material was needle coke, it now needs to be replaced with petroleum coke. Because needle coke is in a long strip shape, while petroleum coke is granular and round, which can shorten the migration path of lithium ions.

Moreover, a soft carbon coating needs to be done on the outside of the graphite. The carbon coating layer can provide an additional electron transmission path and reduce the impedance of the reaction. And coating the outside of the negative electrode can also protect the negative electrode, prevent it from having too many negative reactions with the electrolyte, and at the same time improve the stability of the negative electrode and suppress its expansion during the charging and discharging process.

The positive electrode material of the battery also needs to balance energy density and charging rate, so it also needs to mix large and small particles. In addition, a double-sintering process needs to be adopted, and coating with graphite, carbon, graphene, etc. is done. The principle is similar to that of the negative electrode material, all aiming to shorten the migration path of lithium ions and reduce the impedance when lithium ions are embedded.

The change at the electrolyte level is nothing more than the change of electrolyte additives. The main purpose is to increase the ion migration coefficient and control the formation of the SEI film. This additive may be new-type lithium, VC (Vinylene Carbonate), or FEC (Fluoroethylene Carbonate), and there are also some synthesized ones developed by manufacturers themselves.

36Kr Auto: In addition to materials, what changes will there be in the battery's manufacturing process, structural design, etc.?

A: In terms of process, the most core thing is to make the electrode coating thinner, thereby shortening the transmission path of lithium ions. For a charging rate above 6C, the coating thickness needs to be reduced by 1/3. At the same time, a layered coating process needs to be used.

In addition, as the current of the fast-charging battery increases, the copper foil of the negative electrode also needs to be made thicker accordingly, and the tabs need to be made into full tabs.

At the structural level, the main consideration is the battery's heat dissipation and overcurrent capacity. There are poles on the battery cover. Originally, there was only one, but now it has become double poles, which will increase the battery's heat dissipation channels; originally, it was a single tab, but now it needs to be changed to a full tab, which will increase the battery's overcurrent rate.

By the way, at the battery pack level, the biggest difficulty in fast charging is heat dissipation. The cooling area inside the battery pack needs to be increased, and the flow channel design needs to be optimized. It also needs to be combined with the vehicle's cooling capacity, air conditioning, etc., and the heat transfer coefficient between the battery and the vehicle needs to be improved.

Especially at the position of the poles, because it is the channel for heat dissipation, which means that all the heat is concentrated on the poles, so the thermal management needs to be strengthened.

In addition, as the fast-charging voltage and current increase, the corresponding wiring harnesses need to be thickened, and all high-voltage relays need to be replaced; the BMS algorithm also needs to be adjusted. The increase in current due to fast-charging technology requires the algorithm to be more accurate in controlling the accuracy and time of current and voltage. For example, the voltage acquisition accuracy of the lithium iron phosphate series needs to be within 3%; the power and heat dissipation of the charging pile also need to keep up. So the changes brought about by fast-charging batteries are a whole set of systematic changes.

“Interviewee: Q, with many years of battery experience in leading automobile manufacturers”

36Kr Auto: Is the safety of batteries under the ultra-fast charging system still trustworthy?

Q: The safety of batteries is divided into active safety and passive safety. Active safety refers to the thermal diffusion control effect of the entire battery pack with the help of water cooling; passive safety refers to achieving thermal diffusion control only relying on the battery's own design after turning off the water cooling.

At the cell level, at least below 6C and within 1 - 2 years, there will not be too much risk for ultra-fast charging batteries.

Currently, almost all ultra-fast charging batteries on the market can only achieve active safety, that is, they must rely on water cooling. Otherwise, the temperature during charging will rise rapidly. This puts forward relatively high requirements for the reliability design of the battery's thermal management system.

But for consumers, charging under strong cooling conditions after turning on the water cooling will definitely consume more electricity, so consumers will spend more money.

36Kr Auto: Are there any strict thermal management requirements for power batteries under the ultra-fast charging system?

Q: It is necessary to improve the heat transfer efficiency of the battery pack to meet the cooling demand. The main methods are to increase the liquid cooling area, use multi-sided liquid cooling, and optimize the flow channel design, etc. As mentioned before, replacing the single pole with multiple poles is a means to reduce the heating power and does not improve the heat transfer efficiency. As for how high the specific heat transfer efficiency needs to be, it depends on the battery pack and the fast-charging system.

36Kr Auto: Now the separators of power batteries are getting thinner and thinner. Will this increase the safety hazards of batteries under the ultra-fast charging system?

Q: The main cause of safety accidents in ultra-fast charging batteries is lithium deposition, which has little to do with the thickness of the separator. In the early days, the thickness of the separator was defined as twice the size of the burrs and had nothing to do with lithium deposition. Later, it was found that it didn't need to be so thick, so the thickness of the separator has gradually decreased. Now the separator mainly uses a base film + coating, such as glue or ceramic coating, which can effectively reduce the severity of the failure after an internal short circuit in the power battery. So, although it is thin, its safety performance is not bad.

Moreover, lithium deposition cannot be prevented by the separator. Because once lithium deposition occurs, it will become more and more serious over time, and no matter how thick the separator is, it will be useless.

36Kr Auto: Currently, the ultra-fast charging system is limited to a certain power range. Why is this?

Q: Currently, the mainstream ultra-fast charging SoC range in the industry is 30% - 80%, or 10% - 80%.

According to the characteristics of the battery, 30% - 80% is the comfortable charging range of the battery. The higher the SoC, the lower the charging rate will definitely be. This is relatively easy to understand. For example, if a parking lot has 100 parking spaces, when there are not many cars at the beginning, I can park my car anywhere. But when 80% of the parking spaces are full, it may take me some time to find a parking space.

It is actually possible to charge quickly below 30%, or even 10%. On the one hand, from the user data, users generally start charging in the 10% - 30% SoC range, so manufacturers generally define 30% as the starting point of ultra-fast charging; on the other hand, the wider the ultra-fast charging range is set, the more