[Creative Cultivation Program] How to tame the "hot-tempered" lithium battery?

Lithium batteries can be said to be a major project of modern civilization. Counting how many things with lithium batteries are around you will give you a deeper understanding of this. But on the other hand, when lithium batteries are unleashed and uncontrolled, they immediately become demons. Burning, explosions, and poisonous gases are simply terrifying. From time to time, we always hear news about accidents caused by lithium batteries. For example, an electric bike that was originally riding on the road suddenly burst into flames in the next second, turning a tool that carries us back to a safe harbor into a life-threatening weapon. There are also accidental explosions of electric bikes in elevators, accidental fires caused by electric bikes while charging, and so on.

So why do lithium batteries burn so easily, and how can we prevent them from burning? Let's talk about it today.

In fact, lithium is a very active metal. How active is it? It is so active that it will burn anything it touches, because it will react violently with water to produce hydrogen, and hydrogen is not a fuel that is easy to burn. So once it spontaneously ignites, it is like pouring fuel on the fire, and it will burn more and more vigorously until it burns out. This is why even though Edison actually invented the lithium battery more than 100 years ago, we still had to wait until the 1990s to really use it. Because before that, humans simply could not tame this beast.

How did they tame it later? Actually, they didn't tame it. Scientists suddenly realized it. Although lithium metal is very powerful, I can separate you and turn you into independent lithium ions. Can you still be so arrogant? Sure enough, you are not.

So when we talk about lithium batteries now, we are actually referring to lithium-ion batteries. There is no metallic lithium in this type of battery. The positive electrode material is usually a lithium-containing metal compound, and the negative electrode material is usually graphite. The two are connected by the electrolyte, but blocked by the diaphragm. Both connection and blocking are important for the following reasons:

We can imagine the positive and negative electrodes as a hotel built specifically for lithium ions. When the battery is charging, the lithium ions check out of the hotel at the positive electrode, pass through the electrolyte, and check in at the negative electrode, but they do not plan to carry their luggage with them, because that would add to their burden. So this luggage, which is actually an electron released by it, will go from the external circuit to the negative electrode.

During discharge, the process happens in reverse: the lithium ions residing at the negative electrode check out and return to the positive electrode through the electrolyte alone, while their luggage, the electrons, have to go through the external circuit again and follow them back to the starting point.

This process forms an electric current, which helps us drive the motor and light up the phone. It looks harmonious, right? These lithium ions that go back and forth seem to turn very slowly and pose no threat at all. Yes. They pose no threat largely because the diaphragm blocks them.

Don't underestimate this material. Although it does not increase the power, it is the core of lithium batteries. Because it isolates the positive and negative poles during the charging and discharging process of lithium batteries, avoids the occurrence of short circuits, prevents the generation of abnormal heat, and prevents lithium ions from returning to their violent nature and causing combustion or even explosion. It can be said that the diaphragm is an explosion-proof wall in lithium batteries.

At this point you can probably guess the unavoidable cause of dangerous lithium battery problems: overheating.

Although the designers of lithium batteries have tried their best to improve safety, external or internal reasons such as overcharging, short circuit, collision, etc. may still cause the battery to overheat, burn and explode. For example, when too many lithium ions are overcharged from the positive electrode to the negative electrode, but there is no room for them, they can only gather outside the hotel. Such aggregation or deposition will cause the continuous growth of lithium dendrites, which will then puncture the diaphragm, causing a local short circuit, releasing a large amount of heat and causing danger. Another example is a short circuit, which is more direct. If it is used improperly or the circuit is modified privately, it will cause the battery to heat up abnormally after a short circuit. Another example is that the collision will destroy the internal structure of the lithium battery under the action of external force, especially the barrier function of the diaphragm, which will also cause a short circuit and heat rise, and then burn.

Another point that is actually quite important is that the different positive electrode materials used will also affect the battery's tolerance to high temperatures.

Generally speaking, in the molecular formula of lithium compounds, the higher the lithium content, the lower the temperature required for the decomposition reaction to occur, that is, the combustion. For example, ternary lithium materials will decompose at 200 degrees Celsius, while lithium iron phosphate will decompose at around 800 degrees Celsius. In comparison, the latter is safer than the former. However, the contradiction is that the former contains more lithium, so the energy density of the battery is higher, and the battery life of the same weight is much stronger. It can be seen that there is really no perfect thing.

To ensure safety, on the one hand, we need to understand the common sense of the above batteries, and on the other hand, we need to strengthen our own common sense of safe use.

Author: Gan Shudong

Reviewer: Liu Xinhua