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

"As front - line creators of automotive products, engineers are the people who know automotive products and technologies best. 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 real 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 irritability 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 affect 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, they may not have enough time to evenly embed into the negative electrode, but form lithium dendrites on the surface of the negative electrode. When lithium dendrites accumulate to a certain extent, they may affect battery life, 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.

From 36Kr's communication with battery industry insiders, the probability of the above - mentioned problems occurring will start to increase when an electric vehicle has been used 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 - discussed Xiaomi YU7 is only equipped with CATL's Kirin battery in the Max version priced at 329,900 yuan. Xiaomi officially said that this battery supports 5.2C ultra - fast charging. In the standard version priced at 253,500 yuan and the Pro version priced at 279,900 yuan of Xiaomi YU7, the charging rate of the battery is within 3C.

Another example is Leapmotor, which is jokingly called the "half - price Li Auto". It did not follow Li Auto in using ultra - fast charging batteries when choosing power batteries.

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

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

"Interviewee: A, with more than a decade of battery experience in leading automobile companies"

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

A: Ultra - fast charging will definitely affect 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 used by 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 the test conditions among different manufacturers, so the measured cycle life will also vary. The lower one may be 700 cycles, and some may be 1000 cycles, and there are even higher ones.

This is because some manufacturers stop the test when the battery decays to 70%, so the measured cycle life will definitely be longer; while some manufacturers have higher standards and stop when the decay reaches 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 battery under the ultra - fast charging system can definitely meet the 3 - year, 150,000 - kilometer warranty.

36Kr Auto: What is the principle behind the fact that ultra - fast charging technology shortens battery life?

A: Ultra - fast charging technology affects battery life mainly because of lithium plating, the expansion during the charging and discharging process will damage the battery's micro - 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 enough time to evenly embed into the negative electrode, 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 formed, it will consume the active substances in the battery, and ultra - fast charging accelerates the rupture of the SEI film.

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

A: The impact on ternary lithium batteries will be greater. Because the ternary lithium material has a layered structure, compared with the olivine - type lithium iron phosphate material, ternary lithium is more likely to be damaged. Under high - voltage conditions, the side reactions of ternary lithium batteries will also increase. Moreover, lithium iron phosphate batteries originally have a long life, 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 south?

A: The impact of temperature is very small. Power batteries are protected by electromagnetic waves. They will heat the battery in low - temperature environments and cool it in high - temperature environments. However, the external environment temperature cannot be too high or too low, such as reaching 40 or 50 degrees Celsius or dropping to 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, has a layered structure. To make a fast - charging battery, first, the inter - layer spacing of graphite needs to be expanded, which is equivalent to expanding the channels for lithium ions to migrate; then, the particles need to be made smaller. Because the larger the particles, the longer the channels for lithium ion migration will be. Making the particles smaller is equivalent to shortening the migration path of lithium ions.

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 as the number of small particles increases, the specific surface area will increase, and the side reactions will be more intense.

As a result, the raw materials of 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 applied outside the graphite. The carbon coating layer can provide additional electron - transport paths and reduce the impedance of the reaction. Coating the negative electrode can also protect it from having too many negative reactions with the electrolyte, while improving the stability of the negative electrode and suppressing 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 - firing process should be adopted, and graphite, carbon, graphene, etc. should be used for coating. The principle is similar to that of the negative electrode material, which is to shorten the migration path of lithium ions and reduce the impedance when lithium ions are embedded.

The changes at the electrolyte level mainly involve 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 a new type of lithium, VC (Vinylene Carbonate), or FEC (Fluoroethylene Carbonate), and some are synthesized 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 is also required.

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.

In terms of structure, the main consideration is the battery's heat - dissipation and current - carrying capacity. There are poles on the battery cover. Originally, there was only one, but now it has become double - poles, which will increase the heat - dissipation channels of the battery; originally, it was a single tab, but now it has become a full tab, which will improve the current - carrying rate of the battery.

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 cooling capacity of the whole vehicle, the air - conditioning system, etc., to improve the heat - exchange coefficient between the battery and the whole vehicle.

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

In addition, as the fast - charging voltage and current increase, the corresponding wiring harness needs to be thickened, and all high - voltage relays need to be replaced; the BMS algorithm also needs to be adjusted. Since fast - charging technology increases the current, the algorithm needs 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 complete set of systematic changes.

"Interviewee: Q, with many years of battery experience in leading automobile companies"

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

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

At the battery - 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.

However, for consumers, charging under strong cooling conditions with water - cooling turned on 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 - exchange efficiency of the battery pack to meet the cooling requirements. The main methods include increasing the liquid - cooling area, using multi - sided liquid - cooling, and optimizing the flow - channel design. As mentioned before, replacing the single - pole with multi - poles is a means to reduce the heating power, not to improve the heat - exchange efficiency. As for how high the specific heat - exchange 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 plating, 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, which had nothing to do with lithium plating. 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 plating cannot be prevented by the separator. Once lithium plating occurs, it will become more and more serious over time, and no matter how thick the separator is, it will be useless.

36Kr Auto: Why are current ultra - fast charging systems limited to a certain power range?

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 be, which is 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 below 10%. However, on the one hand, according to user data, users generally start charging in the 10% -