22 Years' Battery Customization

Why does Tesla choose Panasonic 18650 lithium battery?

Dec 18, 2018   Pageview:991

[Editor's note] The author of this article is Frei Liu, ceramic materials, lithium battery materials, power batteries, Dr. THU. Currently work at Tsinghua University Energy Internet Innovation Institute, and be mainly responsible for energy storage technology development.

 

Why does Tesla use 18650 lithium battery?

 

As an engineer working in a research institute, I finally have the opportunity to say a few words in my field.

 

First of all, to correct a concept, "lithium battery" is the abbreviation of " lithium ion battery " that we generally say , and some people say that "ferroelectric" is actually a lithium-ion battery using lithium iron phosphate as a positive electrode material, which is "lithium battery".

 

Let me start with a simple version of the popular version:

 

Tesla uses the Panasonic 18650 lithium battery, with NCA as the positive electrode, and designed a complex battery management system to ensure and improve the efficiency and safety of the battery as much as possible. As for absolute safety, this cannot be answered. If you want to say spontaneous combustion, I would also like to say that the gasoline car also spontaneously ignites in the summer.

 

For pure electric vehicles (regardless of plug-in hybrids and pure mixing, people can rely on gasoline to open), what is our most tangled? Mileage anxiety is not far away, because the energy density of the battery can be stored too low. After the vehicle battery is grouped, it usually has an energy density of 100~150Wh/kg. The value of gasoline is about 10000...so even if you It’s not a problem to solve a problem with a car battery like a turtle. Everyone squirts electric cars every day, every day, charging, running, not far, halfway, no electricity, what to do, all energy density is too low.

 

The biggest shortcoming of battery technology is that the energy density is too low, which is far behind Moore's Law. Even if their energy density is not high enough, the key is far from practical.

 

So why not use lithium iron phosphate battery, I want to say, the main reason is that the capacity (Capacity unit is Ah), and energy (Energy, that is, the capacity of Ah multiplied by the voltage, get Wh) is low (the capacity of lithium iron phosphate is lower than ternary, the voltage is still low, only 3.4V, so the energy that is multiplied is even lower).

 

The actual battery packs for automobiles are combined in series and parallel. It is necessary to increase the voltage by series connection. At this time, the voltage of a single cell and the capacity consistency between different batteries are very important. It is not rigorous to say that the capacity is low.

 

My job is to research and develop an upgraded product of lithium iron phosphate in a domestic research, and also look at some other materials, lithium batteries and electric cars, so answer it here.

 

To compare several cathode materials, we must introduce this graph, which is the five important performance criteria:

 

Power, Life, Cost, Safety, and Energy

 

Comparative materials were NMC/NCA ternary material/NCA, LCO lithium cobaltate, LFP lithium iron phosphate, LMO lithium manganate. NCA and NCM are similar, and they are close relatives in the material, so they are classified as one class here.

 

LCO=LiCoO2, layered, NMC=LiNixMnyCozO2, layered, NCA=LiNi1-y-zCoyAlzO2, layered, LMO=LiMn2O4spinel, LFP=LiFePO4olivine

 

From this figure, we can see:

 

Energy is the lowest (tragedy, low capacity is one aspect) The low voltage of 3.4V is the problem. The reverse example is lithium nickel manganate spinel, voltage 4.7V). Due to space limitations, it is not here to charge and discharge the curve.

 

Power is not low at all. (In the research institute, the self-made pilot-grade lithium iron phosphate, 5C can achieve 130mAh / g drops (of course PHOSTECH can also ...). Carbon + nanomaterial material rate performance Still very powerful!

 

Life and safety are the best, which is mainly due to the combination of polyanion PO43- in the material, which makes the oxygen bond better and the reactivity with the electrolyte is low, unlike the ternary material. It is more prone to some phenomena such as oxygen bubbling. Lifetime is generally considered to be >4000 cycles.

 

Cost of lithium iron phosphate is not bad, cost is second only to LMO lithium manganate material (this thing, air burning, manganese) The source is cheap), the second is competitive. The raw material of lithium iron phosphate, lithium iron phosphate is relatively cheap, but it takes some cost to make nano powder, heat treatment must be carried out under an inert atmosphere, various process requirements, resulting in the material The cost (about 10W/t made in China) is not as low as LMO (6~7W/t), but LCO (more expensive) is cheaper than NMC (13W/t).

 

Reason: Cobalt is more expensive than nickel (China Cobalt-poor, there is no), nickel than manganese To you, what material, what cost.

 

Then compare and analyze the following NCM/NCA materials.

 

Energy is the most advantageous (electric cars want to run far, this is the most important). In addition, with the development of high-nickel NCM materials, the energy density of this material can be further improved.

 

(In fact, it is enough. For pure electric vehicles, energy is more important than power characteristics for Toyota Prius. Hybrid cars, power characteristics are more important, but the premise is that energy cannot be too frustrating)

 

life is not bad. In the past, ternary materials may have a life expectancy of about 1,000 times, but in recent years, with the progress of research and development, the life of the material can reach 2000 weeks (it seems that the standard can still maintain 80% or how much, cannot remember) This is already very impressive, such as your electric car, one charge a day, 365 times a year, 2000 times enough for you for 6 years, many people are planning to change cars at this time.

 

The cost is a bit high (first acknowledge this).

 

After all, some nickel-cobalt metal is used, the cost is high, but this material is at least cheaper than LCO lithium cobaltate, so in the future, in the field of consumer electronics, replacing LCO materials is still promising.

 

Poor safety, especially with respect to lithium iron phosphate, NCM/NCA materials will take oxygen when charging, and the possibility of accidents in use is higher than that of LFP materials. The safety of ternary material batteries has always been problematic.

 

But here, there is not only a positive electrode material in the battery, but we can also alleviate this problem by adjusting the electrolyte composition, diaphragm optimization (ceramic diaphragm Shenma) and optimizing the battery control system (cooling, safety protection). Although the safety of NCM/NCA materials has always been a problem, there is still room for improvement and solutions. If I say that I can completely solve the security problem, I think it is all hooligans. The battery control system is as safe as possible and cannot guarantee 100% safety.

 

Which is more secure?

 

So compare these two materials here, the most important energy density is NCM/NCA wins, the life NCM/NCA is not bad, the safety NCM/NCA is worse, but not serious, the power is not too important, both Yes, the cost is higher on NCM/NCA.

 

However, it is noted here that the cost of NCM/NCA is high, which is the cost per unit mass (rmb/kg).

 

Considering the high NCM/NCA energy density per unit mass (Wh/kg), if the cost per unit of energy is converted (rmb/Wh), NCM/NCA is more advantageous, so it is calculated from the cost of the unit material. The result may be more deceptive.

 

Speaking of this, I can't help but vomit: you are too expensive for electric vehicles every day, because the battery is used more, because the cost of unit energy (rmb/Wh) is too high, it is really difficult to drop, we

 

I am going to drink the northwest wind. Ah, ah ah == Another point is that the lithium iron phosphate discharge platform is too flat, it is not good to do battery control system, NCM/NCA is better.

 

However, as a lithium iron phosphate research dog, I have to say a few words for our materials: in the pursuit of safety, rate of occasion, lithium iron phosphate battery still has an advantage, especially please screaming electric cars every day, People with unsafe batteries vote for us lithium iron phosphate.

 

In addition, lithium iron phosphate does not need to use nickel and cobalt, which is helpful for national resource security (well, in fact, it is not used too much, but this industry is also expensive once it is used);

 

and, lithium iron phosphate material After all, life expectancy is still the best, so in some occasions, it has advantages (such as some batteries are cheap, but the life may be less than half of lithium iron phosphate, the cost is not the same after the use of time, I only Don't tell you that it is LMO), everyone must learn to settle accounts.

 

In cases where the energy density requirements are not particularly high, lithium iron phosphate is still advantageous, such as new areas such as energy storage.

 

So, to put it bluntly, in order to make pure electric vehicles run farther, batteries with higher energy density are more popular.

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