As we all know, at present, electric vehicle batteries mainly have two kinds of ternary lithium batteries and lithium iron phosphate batteries, and car companies will choose one of them to load the car. However, just recently, Weilai released a ternary lithium battery pack that mixes the ternary lithium formula with the lithium iron phosphate formula.
The battery pack has a capacity of 75kWh, replacing the original 70kWh ternary lithium battery, and plans to start delivering the corresponding model to users in November this year, and launch a power exchange system, and the price and BaaS price will remain unchanged. The new battery pack has a larger capacity that can increase the NEDC range of many NIO models by about 35km.
However, the new battery that does not increase the price has not made Weilai receive applause, but has attracted many user questions in the Weilai APP community. In the eyes of many people, lithium iron phosphate battery is the technology of the previous generation of electric vehicles, and now it is also used in low-end models below 300,000, and should not appear on the high-end car of Weilai.
In my opinion, such a view is too superficial. You know, the battery is not as good as the hot pot dip can be mixed at will, the chemical principle of ternary lithium and lithium iron phosphate batteries is completely different, charge and discharge characteristics, heat production dynamics, BMS algorithm is completely different, so that the two work together is very difficult.
Since research and development is difficult, users will also question why Weilai still "knows that there are tigers in the mountains and prefers to go to the tiger mountains"? This is because ternary iron lithium batteries have three major benefits and broad application prospects. More importantly, compared with other car companies, Weilai is more qualified to explore hybrid battery technology.
Three advantages of hybrid batteries
1, improve security
BYD blade battery needle prick test, lithium iron phosphate battery than ternary lithium battery safer characteristics have been widely known. Laboratory data show that the spontaneous combustion temperature of the ternary lithium battery is 200 ° C, that is, once the temperature reaches 200 ° C, the material of the ternary battery will begin to melt and spontaneously combust until it causes an explosion. The auto-ignition temperature of lithium iron phosphate batteries is 800 °C. In layman's terms, ternary lithium batteries are like gunpowder, and lithium iron phosphate batteries are like coal, and CTP processes like BYD blade batteries and Ningde era have been able to improve the safety of lithium iron phosphate batteries to the level of needle prick.
Although NIO's ternary iron lithium battery still has a ternary lithium battery module, the lithium iron phosphate module can still reduce the risk of the entire battery pack being pricked and destroyed. For consumers, this is a real guarantee of safety.
2, reduce costs
Recently, cobalt-free batteries have become the direction of research and development of many car companies, in ternary lithium batteries, but the cobalt component is a layered structure of stable materials, which can improve the safety factor. At the same time, cobalt can also reduce the cationic mixing, which facilitates the deep discharge of the material, thereby increasing the capacity of the battery cell.
However, the reason why car companies want to abandon cobalt is because the price of this thing has soared in recent years, resulting in the high cost of power batteries. A few years ago, the price of cobalt did not exceed 180 yuan / kg. But since the fourth quarter of 2017, each kilogram of cobalt has risen to about 420 yuan. In 2018, the price of cobalt rose to more than 650 yuan per kilogram. At present, the price of cobalt is already very expensive, accounting for about 10% of the cost of power batteries. What is more severe is that the proven cobalt reserves on the earth are only 700 tons, and the ternary lithium battery pack of a new energy vehicle may contain more than 10kg of cobalt, if the recovery and reduction of cobalt consumption are not considered, by 2026, there will be a state of "no cobalt available".
Therefore, "de-cobaltation" is bound to become the main trend of future battery technology, and lithium iron phosphate is a mature cobalt-free battery technology, and the amount of precious metals used makes it much lower than that of ternary lithium batteries. Replacing lithium iron phosphate batteries or replacing the departmental modules in battery packs with lithium iron phosphate can significantly reduce the cost of car companies, and consumers can also enjoy lower car purchase prices.
The proposed battery mixing scheme is not only Weilai, CATL also said when it released the sodium-ion battery two months ago that the AB scheme will be used in the battery structure, that is, the sodium-ion battery and the lithium-ion battery are mixed and shared. This could also reduce battery costs, as lithium is also an expensive material, and sodium is too large to be used up on Earth.
3, reduce the attenuation speed
The attenuation rate of lithium iron phosphate batteries is much slower than that of ternary lithium batteries, in other words, lithium iron phosphate batteries have a longer cycle life. Experimental data show that if the remaining capacity / initial capacity = 80% as the end of the test, the current lithium iron phosphate battery laboratory 1C cycle life of more than 3500 times, some up to 5000 times, while the ternary lithium battery laboratory 1C cycle life of about 2500 times.
The reason why the cycle life of ternary lithium batteries is short is because its voltage range is too exhausted, and frequent full charge will cause the growth rate of negative lithium dendrites to grow too fast.
The nominal voltage of the general ternary lithium battery is about 3.7V, the working voltage range is as wide as 3.0-4.2V, after changing to the lithium iron phosphate formula, the nominal voltage is reduced to 3.2V, the working voltage range is between 3.0-3.3V, and the termination voltage of the charge is only about 3.6V.
If the voltage of this mixed battery is marked as 3.6V of lithium iron phosphate, ternary lithium does not need to exhaust the voltage, and the growth rate of lithium dendrites is slower, and the life is longer.
Musk recently praised the advantages of lithium iron phosphate batteries: "The use experience of lithium iron batteries is the same as that of ternary batteries, I prefer to use lithium iron batteries, which can charge to 100%, while nickel can only be charged to 90%. ”
However, Musk can only believe the second half of this sentence. There is a big gap between the experience of lithium iron phosphate battery and ternary lithium battery, because it has shortcomings such as difficulty in measuring SOC power and poor anti-low temperature performance. Zeng Shizhe, vice president of NIO battery system, also revealed that NIO had developed a 68 KWH lithium iron phosphate battery pack, but it was directly rejected by THE NIO executives, because the user experience was not good.
So, how does Weilai's ternary iron lithium battery pack solve these problems? The answer is: ruler, fire forest and CTP.
Ruler, fire forest and CTP
1, to the three yuan as a ruler
Although the SOC of lithium iron phosphate is difficult to measure, it is very simple in ternary lithium batteries. So Weilai thought of mapping the upper and lower limits of the ternary lithium SOC to achieve accurate SOC estimation of the entire battery pack. Zeng Shizhe said: "Once doing an experiment, we suddenly thought that we can make a ruler of ternary lithium batteries, which can solve the problem of inaccurate estimates of lithium iron phosphate SOC. ”
Above is the NIO Ternary Iron Lithium Battery SOC estimation schematic diagram, from the leftmost column chart can be seen, the upper and lower limits of the SOC of the Ternary battery are set at about 90% and 10%, and the lithium iron phosphate SOC is all exhausted. 10-90% is the optimal and most linear range of the voltage slope of the ternary lithium battery, in order to correct the voltage slope of the lithium iron phosphate battery "middle flat, steep at both ends", thereby correcting the entire range of the entire lithium iron battery pack. Such an approach can be described in one word : wonderful.
In addition, NIO has also built high-power DCDC components into the battery pack to improve the accuracy of SOC calculation. DCDC is the power battery pack high voltage into a constant 12V or 14V, 24V low voltage, both to power the whole vehicle electrical appliances, but also to assist the battery charging equipment, DCDC converter in pure electric vehicle function is equivalent to the generator and regulator in the traditional fuel vehicle function.
Most of the DCDC of electric vehicles is independent of the battery pack, generally placed under the trunk, and after the vehicle is turned off and the power is cut off, the DCDC stops working. However, when the power is cut off, the battery capacity will continue to decrease, and the power will run secretly. If the vehicle is parked for a long time, the power drop will be very obvious. When started again, the car is prone to SOC jumping when the power will be exhausted, which in turn will cause the vehicle to show that it has a certain endurance, but suddenly lies down.
The built-in DCDC allows the battery pack to not be powered down when the whole vehicle is powered down, and the battery pack can still be self-tested when the vehicle is parked. With the help of the ruler principle and the built-in DCDC, the power estimation accuracy of the Ternary Iron Lithium Battery Pack is consistent with the Ternary Lithium Battery, and the error is reduced to less than 3%.
In addition, the ternary lithium as a ruler can also improve the service life of the battery pack, just said, our lithium iron phosphate battery cycle life is longer than the ternary cell, Weilai by dissatisfied with the full discharge of ternary cells, but also improve the life of the ternary cells, the two SOC life alignment. Finally, "in the first year of use, the battery attenuation of the ternary iron lithium standard battery pack (75kWh) will be slightly higher than that of the original ternary lithium 70kWh battery pack; in the medium and long term, the degree of attenuation of the two is the same, and the overall is better than the traditional lithium iron phosphate battery." ”
2, new thermal management system
In THE NIO TER LITHIUM battery pack, the ternary lithium module and the lithium iron phosphate module are not the layout of each side, but the ternary lithium is located in the four corners (the total capacity is about 7 kWh), and the lithium iron phosphate is located in the center. This is because the ternary lithium battery heats up quickly, and it is placed in the four corners with better heat dissipation, which can make the temperature rise of the entire battery pack more balanced. At low temperatures, the outer ternary module can also better insulate the internal lithium iron phosphate module.
In addition, NIO has also redesigned the thermal management system with full thermal path physical barrier, which WEILAI calls "fireproof forest".
In the fireproof forest, the main frame of the battery pack is arranged with a number of black soft insulation materials to reduce the heat exchange with the outside world, and these arrangement points are scientifically measured by Weilai. Zeng Shizhe said: "The heat dissipation of the battery is like a bottle leaking water, and you need to know where the leak point is." ”
Inside the battery shell, NIO has also arranged a yellow insulation material to further reduce heat loss at low temperatures. In addition, Weilai also has a fault on the shell, also to reduce heat conduction.
These are passive thermal management, and in terms of active thermal management, NIO designed a more uniform heating cell, which ultimately achieved a 60% increase in temperature uniformity and a 40% increase in the minimum temperature of the battery.
A more uniform active temperature control can control the standing temperature of the entire battery pack at about -5 °C. The selection of -5 °C is also the result of Weilai's repeated calculation experiments. "It's too high, and the energy consumed when heated is not cost-effective; falling below this point, the loss will be very large." In Inner Mongolia and Northeast China, as long as this point is stabilized, the overall endurance improvement is very amazing. Zeng Shizhe revealed. According to Weilai's calculations, the active temperature control system can reduce the low-temperature endurance loss of the ternary iron lithium battery pack under low temperature conditions, compared with the low temperature loss of the lithium iron phosphate battery pack.
Just now we also mentioned that the charge and discharge thermal properties and heat production dynamics of ternary lithium batteries and lithium iron phosphate batteries are completely different, and active and passive thermal management is not enough for Weilai to unify the two batteries. Therefore, at the software level, NIO has developed a dual-system control algorithm.
Weilai said: "The whole set of passive, active, including algorithms, big data and other aspects of the data, spent seven or eight months debugging, and the whole set of code (the amount of code) is already ten times that of the traditional SOC algorithm." ”
This part of the algorithm Weilai does not explain in detail, but in my opinion, the algorithm is the most difficult place for soC calculation and thermal management of the entire battery pack. This also means that the battery pack design is changing from heavy hardware to heavy software, and mastering the core algorithm of the battery pack may be the core area of future car companies competing with each other.
3, CTP process
Nio's ternary iron lithium battery pack is provided by CATL and uses CTP process for structural optimization.
Zeng Shizhe said that Weilai actually used a blade battery to do experiments, the overall capacity can be increased, but the weight will also be improved, and the CTP process in the Ningde era is more suitable for mixed installation schemes. From the data point of view, the manufacturing and assembly of ternary iron lithium battery packs is simplified by 10%, the volume utilization rate is increased by 5%, and the energy density is increased by 14%, reaching 142Wh/kg.
Is BaaS fertile ground for new battery technologies?
Li Bin revealed on an earnings call in February that NIO's BaaS service selection rate exceeded 55%. That is to say, more than half of Weilai's car owners choose not to buy out the battery, but to use the service of changing or charging the car, which provides unique conditions for Weilai to promote new battery technology.
Imagine if other car companies launch new technologies like mixed batteries, they can only sell new cars, while Nio can quickly replace the old batteries on the market with new batteries through the power exchange mode. For the promotion of new technologies, it can achieve marginal utility faster, reduce costs, and achieve profitability.
Therefore, the power exchange mode seems to have a huge investment in funds, but it is very conducive to the promotion of new battery technology, presumably this is also the way weilai has thought of when planning the power exchange mode in the early days.
Weilai's hybrid battery technology is really a wonderful new technology, and this is based on THE BASIS service foundation and strong research and development strength of Weilai's characteristics, especially the strength of software research and development. This is also worth learning from other car companies and thinking about how to jump out of the inherent thinking and innovate in a more efficient way in the research and development and promotion of technology.