Explore the Ternary Iron Lithium: a wonderful combination but a magical magic

As the "lifeblood" of new energy vehicles, power batteries do not affect the overall development of new energy vehicles in terms of quality, supply and cost. Among the vehicle companies, Weilai Automobile has always been at the forefront of the industry's exploration of batteries, and the ternary iron lithium battery that was loaded and delivered in November last year is the product of this exploration.

At a communication meeting in September last year, Zeng Shizhe, vice president of WEILAI battery system, revealed that with the expansion of the application of lithium iron phosphate market, when the 100kWh ternary lithium battery was released, the NIO battery team had completed the research and development of the 68kWh lithium iron phosphate battery pack, but there was a certain gap in the low temperature performance and SOC estimation of lithium iron phosphate compared to ternary lithium, and after the team reported, chairman Li Bin denied the plan, because 'WEILAI is a car company that serves users, and the launch of the battery cannot be explained to the user'. To this end, Weilai automotive battery research and development team to find a new direction, lasted more than 1 year, the team developed a ternary iron lithium battery pack composed of ternary lithium battery cells + lithium iron phosphate battery cell mixed, which also makes Weilai become the industry's first ternary iron lithium technology mass production of car companies.

In the power battery category, ternary lithium batteries and lithium iron phosphate batteries are the two most common types on the market at present, before the arrival of Ternary iron lithium batteries, they are clear and distinct, do not "disturb" each other, but their own physical characteristics make them have strengths, there are also shortcomings, because of this, Weilai will combine them in an orderly manner, mixed into a new battery pack type. So what kind of performance will this wonderful combination bring?

As we all know, the advantages of lithium iron phosphate batteries are good safety performance, long cycle life, and the cost is lower than that of ternary lithium batteries, while the disadvantages are poor low temperature performance, poor SOC measurement accuracy, and energy density is not as good as ternary lithium batteries. NIO's ternary iron lithium battery, by using the way of mixing ternary lithium and lithium iron phosphate, complements the strengths and complements each other's advantages, so that compared with the old 70kWh battery pack, the battery pack with a capacity of 75kWh can increase the comprehensive cruising range of about 35km compared to the old battery pack.

Not long ago, some media tested the actual performance of WEILAI EC6 equipped with a 75kWh ternary iron lithium battery in low temperatures, compared with the Tesla Model Y equipped with lithium iron phosphate batteries and the BMW iX3 using ternary lithium batteries tested together, ec6 equipped with ternary iron lithium batteries has excellent performance in a low temperature environment, whether it is driving at high speed in cold cars and charging cold cars.

Mixing ternary lithium batteries and lithium iron phosphate batteries in the same battery pack in series looks a bit "brain hole", but the actual test effect has proved the advantages of this. So, how does Weilai's ternary iron lithium battery solve the poor low temperature performance of lithium iron phosphate batteries, inaccurate SOC estimation, and electronic control problems caused by battery mixing?

Just now we mentioned that under low temperature conditions, the performance of lithium iron phosphate batteries will be inferior to ternary lithium batteries, so the batteries of two different material systems of lithium iron phosphate batteries and ternary lithium batteries are loaded into the same battery pack, the first thing to face is that the cold resistance of the two batteries is different. If the temperature of the individual cells is lower than the critical temperature, the discharge performance of the entire battery pack will be poor, which will not only affect the endurance of the vehicle, but also affect the safety of the battery.

In order to keep the same temperature change between the two batteries in the low temperature environment as much as possible, NIO ensures the temperature balance and thermal insulation performance inside the battery pack through structural optimization and arrangement of two hardware management methods of thermal insulation.

Common sense of life tells us that whether the vehicle is driving or standing, the four corners of the battery pack are the places most affected by the external temperature, so Weilai has placed a ternary lithium battery with better cold resistance in this part of the area, and the other areas are filled by lithium iron phosphate batteries, which is equivalent to giving a layer of "quilt" to the lithium iron phosphate battery that is not frozen.

Of course, the optimization of the cell structure can only passively reduce the impact of lithium iron phosphate batteries on the low temperature environment, and if they want to better perform, they also need a layer of "electric blanket". In the extremely low temperature environment, the NIO ternary iron lithium battery pack will heat the battery cell uniformly through radiant active heat compensation to ensure that the working temperature of the entire battery pack takes into account the energy consumption at the same time.

In addition, NIO ternary iron lithium batteries have also made achievements in the thermal management software level. For example, based on WEILAI's big data analysis and dual-system control algorithm, it can make full use of the low-temperature performance advantages of ternary lithium batteries and the precise control of two kinds of cells to improve the low-temperature energy efficiency of the entire battery system. The outer layer is wrapped with a "quilt", and there is also an "electric blanket" inside to continue to heat up, as well as the software blessing at the big data level, which finally reduces the low temperature endurance loss of the entire battery pack compared with the lithium iron phosphate battery pack by 25%.

The second problem that Weilai wants to solve, which also plagues many car owners, is the problem that the SOC estimation of lithium iron phosphate batteries is inaccurate. Before we can figure out NIO's solution, we need to understand several concepts, one is what SOC is, and the other is why we need to estimate SOC.

SOC is the abbreviation of the English word State of Charge, which means the state of charge of the battery, as the name suggests, it refers to the available state of the remaining charge in the battery, generally expressed as a percentage, we can also simply understand it as battery capacity.

So why estimate soc? In the fuel vehicle, the remaining capacity of a tank of gasoline can be measured by simple physical methods, and then through the analysis of fuel consumption in the vehicle system, the remaining mileage of the vehicle can be known, which is simple, linear and easy to calculate. In pure electric vehicles, the SOC of the battery cannot be estimated by a simple physical way like the fuel tank of the fuel vehicle, and if the SOC cannot be accurately estimated, the system will not be able to accurately calculate the charging time and mileage of the vehicle, which is very easy to cause the overcharge or over-discharge problem of the battery, which will cause safety hazards. Therefore, accurate SOC estimation of electric vehicles is necessary.

At present, the mainstream battery SOC estimation method is the ampere integration method and the OCV-SOC open circuit voltage correspondence method, about this part of us first press the table, we only need to know two things, one is that OCV and SOC have a one-to-one correspondence, we can accurately know how much the current SOC is by measuring the voltage OCV; the second is that the combination of these two methods allows us to estimate the battery SOC more accurately.

The ideal is very full, but the reality is very bone. Although we can estimate the battery SOC by the ampere integration method and the OCV-SOC method, the characteristics of the lithium iron phosphate battery itself cause the OCV-SOC curve to be very flat in the 10%-90% range, which also means that the SOC estimation accuracy is very low and cannot accurately reflect the discharge of the battery.

If the SOC of the entire battery system is estimated according to the SOC of the lithium iron phosphate battery, it may lead to inaccurate mileage and sudden reduction of power. However, fortunately, the physical characteristics of ternary lithium batteries are opposite to lithium iron phosphate batteries, and their SOC curves are relatively linear, which makes it easier to estimate, which is why electric vehicles equipped with ternary lithium batteries are stronger than lithium iron phosphate battery electric vehicles in the estimation of mileage.

After understanding these principles, Weilai's ternary iron lithium battery to solve the inaccurate estimation method is well understood. Each cell in the ternary iron lithium battery is connected in series, and the dual system power estimation method newly developed by Weilai maps the SOC upper and lower limits of the ternary lithium battery to the SOC range of the entire battery pack, that is to say, the "ruler" for measuring the entire battery pack SOC is no longer a larger number of lithium iron phosphate batteries, but a more accurate ternary lithium battery, so that the power estimation error is reduced to less than 3%. In addition, NIO's SOC estimation technology can also dynamically adjust the battery SOC range according to the degree of attenuation and decay rate of the battery, so that the SOC of the ternary lithium battery can always accurately reflect the charging state of the battery system.

In this way, the large number of lithium iron phosphate batteries can effectively reduce the cost of the entire battery pack, and at the same time, ternary lithium batteries can also borrow their own characteristics to help the entire system complete the accurate estimation of the battery pack SOC, which can be described as killing two birds with one stone.

The ingenious "mix-and-match" style of ternary iron lithium is the best embodiment of Weilai's continuous improvement of battery technology. With the continuous upgrading of battery technology, coupled with the mature power exchange method, the user experience has been improved, and weilai automobile has erected a battery technology barrier.