Weilai Vice President Zeng Shizhe talks about ternary iron-lithium batteries: The solution is difficult to copy, and the most difficult part is the control algorithm

After Tesla and Xiaopeng Motors, NIO has also joined the ranks of lithium iron phosphate batteries. However, NIO does not intend to continue to endure the stubborn problems of lithium iron phosphate batteries.

Recently, NIO launched a 75-degree (kW·h) ternary iron-lithium hybrid battery pack to upgrade the company's 70-degree ternary lithium battery pack, and it is expected to start delivery in November.

The 70-degree battery pack is the first generation of battery packs launched by NIO. It was released at the first NIO Day in December 2017 and delivered in June 2018. It has been in service for more than three years. It is the basic battery pack for NIO cars and the most widely delivered battery pack by NIO. However, this battery is obviously difficult to meet the needs of today's users. Due to the available range and the low temperature in winter, it is not uncommon for the battery life to be reduced to more than 200 kilometers.

To carry out alternative upgrades to such basic configurations, NIO needs to act quickly and cautiously.

From the naming, we can see that NIO has put a lot of effort. There are two most commonly used technical solutions for power batteries, namely ternary lithium and lithium iron phosphate. Ternary iron lithium is relatively unfamiliar. What is its secret?

As the name suggests, the ternary lithium iron battery is a mix of lithium iron phosphate and ternary lithium batteries, extracting the advantages of both batteries and making the availability of lithium iron phosphate batteries reach the level of replacing conventional ternary lithium battery packs.

Lithium iron phosphate battery is a mature system. Its biggest advantage is that it does not contain cobalt metal, which has obvious cost advantages. Secondly, the internal structure of lithium iron phosphate battery is stable and can withstand high temperature and overcharge, so the probability of spontaneous combustion is relatively low. This is why Xiaopeng Motors and Tesla both use lithium iron phosphate batteries in their entry-level models.

However, the disadvantages of lithium iron phosphate batteries are also obvious. First, the estimation of SoC (remaining power) is inaccurate, making it difficult to provide users with an accurate reference for power consumption. The bigger problem is that in low-temperature environments, the electrochemical activity is low, which leads to a significant reduction in battery life and charging efficiency in winter. A concentrated feedback Tesla received last winter was that the battery life of lithium iron phosphate models in the north was seriously reduced. He Xiaopeng, CEO of Xpeng Motors, also admitted that "lithium iron phosphate batteries are more suitable for warm climates."

Zeng Shizhe, Vice President of Battery System at NIO, told 36Kr and other media that the ternary iron-lithium mixed battery pack is the solution the company has found to solve the above two problems. According to reports, NIO's ternary iron-lithium battery pack has a total of 118 cells, of which 104 are lithium iron phosphate cells. There are only 14 ternary lithium cells, distributed in the four corners of the battery pack.

Schematic diagram of the structure of NIO's ternary iron-lithium battery pack, from NIO's official website

Zeng Shizhe said that the estimation of lithium iron phosphate battery capacity is inaccurate, but this problem does not exist for ternary lithium batteries. The latter can have more than 100 opportunities to evaluate the battery capacity every day, so ternary batteries can be added to the lithium iron phosphate battery pack as a ruler to calibrate the battery capacity of the entire battery pack.

"The ternary battery can be used as a ruler to tell us the actual power level. It is like putting a coach inside it. The coach will tell me how much power is actually left in the SoC." Zeng Shizhe said that NIO's mixed iron-lithium battery has reduced the industry-recognized iron-lithium power estimation accuracy from 10% to within 3%.

In addition to solving the SoC power estimation problem of the iron-lithium battery, NIO has also solved the low-temperature attenuation problem of the iron-lithium battery by improving the structure and materials, including using new insulation materials, replacing the battery shell, and using heating film and other technologies. According to information provided by NIO officials, compared with the lithium iron phosphate battery pack, the low-temperature endurance loss of the ternary iron-lithium mixed battery pack is reduced by 25%.

The mixed installation of ternary batteries and iron-lithium batteries is not the first of its kind by NIO, and NIO’s ability to implement the project and achieve mass production mainly relies on its self-developed control algorithm.

Zeng Shizhe also confidently stated that NIO's battery pack design is difficult to copy because it contains a complex algorithm that is derived by combining the optimal capacity retention rate and the optimal power characteristics of the two batteries. "We have developed a very complex dual-system heating algorithm, which is implemented not only on the BMS side but also on the cloud."

The ternary iron-lithium mixed battery pack is a balance that NIO is striving to find between corporate costs and user experience. If this solution passes the market test, it will not only reflect NIO's engineering and R&D capabilities, but will also accelerate the popularization of iron-lithium batteries.

The following is a conversation between 36Kr and other media and Zeng Shizhe, Vice President of NIO Battery System, with some excerpts:

Q: How many people are there in NIO’s battery R&D team? Through the mixed CTP technology, NIO has accumulated a lot of R&D and engineering experience. Do you have any plans to design battery cells and go for OEM production in the future?

Zeng Shizhe: Currently, there are about a hundred people working on battery R&D. As for the battery cells, there is no relevant plan yet.

Q: From the 100-kWh CTP to the mixed-loading CTP technology, NIO was one of the first companies in the industry to adopt it. I would like to ask if you have accumulated any systematic experience in production and manufacturing?

Zeng Shizhe: I would also like to say that our supplier management is different this time. We have added many quality personnel because automotive battery packs are a relatively professional and fast-growing business. Many companies can now rely on this industry for their livelihoods. We have also developed many better suppliers under this background. Whether it is welding technology or quality control, it is actually much better than before.

In the past, when we were producing 100 kWh of electricity, we actually had limited manpower. In order to improve quality, we even hired a lot of quality personnel from outsourcing companies to be stationed on site. This time, we completely expanded our quality team and arranged on-site staff at each first- and second-level supplier.

Q: Regarding the distribution of ternary and iron-lithium battery cells, is the ternary battery at the edge of the battery pack? Is it a single battery cell or does it have its own ternary module?

Zeng Shizhe: We use the CTP design, which does not have the concept of modules. We determine the position of the ternary elements based on big data analysis. The purpose is to reduce the temperature difference of the entire battery to within three degrees. Under commonly used mild working conditions, it can be achieved within 1-2 degrees. Therefore, the number of ternary elements in each corner is different, two in the front and four in the back.

Question: I would like to ask that the discharge platforms and voltages of ternary lithium and lithium iron phosphate are different. Will connecting these two in series have any impact on the battery life?

Zeng Shizhe: Yes, different currents and different platforms do have an impact on battery life. Generally speaking, lithium iron phosphate has a relatively long life, which is why we have made capacity redundancy in ternary batteries, and combined with high-power DCDC, we have also made dynamic adjustments. When there is an impact, high-power DCDC can be used to make dynamic adjustments in the range, which will solve the problem, but the algorithm will be very complicated, and the team has also spent a lot of energy on solving this problem.

Q: I heard you say that this calibration method is different from the calibration method of ordinary ternary lithium batteries. If ordinary ternary lithium battery users want to replace the battery pack, do they need to recalibrate? Or what?

Zeng Shizhe: Our BMS software is already compatible with the initial calibration settings of multiple batteries, and each battery pack has its own unique ID (identity card). BMS will identify and correspond to the battery pack ID information, so there is no need to reset it. It can work adaptively after it is replaced. And now that the cloud platform is connected, the calibration settings can be updated in a targeted manner to improve the user experience.

Q: Some people have suggested that the industry should bring acupuncture back to the national normal standards. What do you think about this issue?

Zeng Shizhe: There have been solutions to the puncture of battery cells before. As for the test plan, the battery cell is punctured directly into the center of the large surface. However, this may not be the case in an actual accident. The battery cell may be punctured from different angles. Therefore, we have always believed that safety should be considered from the real environment. We should not do puncture for the sake of puncture or just to pass it. We think that if the entire exothermic peak of the battery cell can be stabilized, this problem can be solved.

Question: Are you also talking about the charging device issue? Now after our competitors replace the battery, the charging curve changes, resulting in a decrease in charging speed. Has this problem been solved?