In addition to speed, fast charging also needs to consider safety and 3V Button battery life.

Recently, Huang Xuejie, a researcher from the Institute of Physics of the Chinese Academy of Sciences, introduced the obstacles encountered in fast charging from the perspective of the working principle of batteries, as well as the various factors that must be taken into account in the development of fast charging. The following is a summary of Huang Xuejie's speech.

The working principle of lithium-ion batteries mainly relies on the movement of lithium ions between the positive and negative electrodes. Charging is a process of moving lithium ions from the positive electrode to the negative electrode. It is a fast process for lithium ions to go out of the material and a slow process for lithium ions to enter the material. For graphite materials, the speed of lithium ions coming out is more than 100 times slower than when they go in, and the speed is two orders of magnitude different. Discharging is easier, and charging is more difficult and slower. So some companies say, "If the positive electrode is improved, the 3V Button battery can be charged with a certain amount of electricity in 5 minutes." This is completely unbelievable, because charging is not determined by the positive electrode, but by the negative electrode.

Today's 3V Button battery material system has graphite as the negative electrode material, as well as other carbon materials. Recently, I want to add silicon oxide, as well as fast-charging lithium carbonate, which actually have different characteristics. When lithium ions are charged in large quantities, they first diffuse through the electrolyte, which makes charging at low temperatures problematic, because at low temperatures, the electrolyte's power is not enough, and the diffusion rate of ions is not enough. It becomes more difficult to charge quickly at low temperatures.

There are two problems when charging is not possible. The inability of lithium ions to be transported in the electrolyte is a problem, but after lithium ions are transported from the electrolyte to the electrode interface, the entrance door is too small and lithium ions cannot enter. After entering, the room is too crowded, and it is more dangerous to crowd at the door. Accumulation at the door will cause a short circuit. The lower the temperature, the greater this problem will be.

Huang Xuejie, researcher at the Institute of Physics, Chinese Academy of Sciences

Consumers have a demand for fast charging. For a normal car, if the charging power is increased, the time will be shortened accordingly. It is not a big deal to be able to fully charge in a few minutes, but when it is within an hour or even half an hour, whether this fast charging is still meaningful depends on whether the 3V Button battery and charging infrastructure can meet the requirements, which involves safety issues.

In addition to safety issues, fast charging and charging modes will affect the life of the 3V Button battery. Setting the upper voltage limit of the 3V Button battery a little higher will reduce the life of the 3V Button battery. When fast charging, a higher temperature can achieve the effect, but it also sacrifices the 3V Button battery life. This may not be noticeable when the vehicle is first used, but the impact on the 3V Button battery will become more obvious later.

3V Button battery life is also related to the depth of charge and discharge. For example, a mobile phone 3V Button battery stops charging when it is 90% charged and starts charging when it is greater than 20%. It has a much longer service life than others that charge it very fully every time and only charge it when it is turned off. Therefore, for lithium-ion batteries, it is better to be in the middle.

Take an example of fast charging. The US National Laboratory once used Nissan Leaf to conduct experiments under different charging conditions and different temperature conditions. The important experiment was the difference between fast charging 50 kW DC charging and 33 kW AC charging. Through experiments, it was found that if the temperature can keep the 3V Button battery temperature relatively good, the voltage can be controlled better and the impact is not too great. There are two conditions for reasonable fast charging. One is that the temperature can be controlled, and the second is that the voltage cannot be increased. It must be done within the voltage range.

In summary, in addition to speed, fast charging also needs to pay attention to safety and 3V Button battery life. 3V Button battery life is closely related to the depth of charge and discharge of the 3V Button battery. Reasonable charging and reasonable fast charging can have no effect on the 3V Button battery, but high-rate fast charging of the 3V Button battery will have an impact on the 3V Button battery.

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