Everyone has seen the news about the explosion of lithium ion batteries and is very interested. Today we will talk about why lithium ion batteries explode. First of all, we need to know the working principle of lithium battery and under what conditions it may explode.

 

1、 Why the Lithium Ion Battery Explodes

 

 

 

1. Overcharging leads to excessive lithium release and insufficient cathode capacity. Lithium generated during charging cannot be inserted into the interlayer structure of negative electrode graphite, and metal lithium will be formed on the surface of negative electrode. For a long time, lithium molecules have grown reticular crystal structures from the negative surface layer to lithium ion batteries. This lithium metal crystal will pass through the diaphragm, causing a positive and negative short circuit fault. Sometimes, the rechargeable battery will explode before the short-circuit fault occurs, because raw materials such as lithium battery electrolyte will destroy the steam, causing the battery sleeve or pressure valve to break, so that carbon dioxide reacts with lithium atoms deposited on the negative electrode surface layer, and then explode

 

 

 

2. Current limitation is also required when charging the battery. When the current is too high, the lithium ion battery cannot catch up with the upper structure and will gather on the surface of the positive material. This lithium-ion battery will produce lithium-ion crystals on the surface of raw materials, just like overcharging, which will lead to dangerous factors

 

 

 

Therefore, in addition to the lithium battery, there is also a lithium battery protection board in the lithium battery pack, commonly known as BMS (battery management system), and the Chinese name is rechargeable battery intelligent management system. It can detect the lithium power supply under interference according to the working voltage, current and temperature information of the lithium power supply

 

 

 

Whether it is a pure electric vehicle or an electric vehicle, lithium ion safety accidents are not new. In the old national standard period, the application of lithium ion in electric vehicle industry was small, and safety accidents occurred frequently. The key reason is that the lithium battery is unstable, the packaging and processing technology is not mature, and little is known about the application habits of lithium ion. With the increase of lithium ion usage in the national standard period, if some countermeasures are not taken, there is a great possibility of safety accidents

 

 

 

 

 

2、 Safety measures to avoid explosion of lithium ion battery

 

 

 

1. Lithium metal will form on the negative electrode surface

 

 

 

At present, China's lithium ion manufacturing level is still lower than that of South Korea. Due to Japan's consistent accuracy, the lithium ion magnetic core itself has certain safety risks. If only the BMS board with pure hardware configuration is maintained, such electronic components may fail. Once BMS fails, it will inevitably lead to safety production accidents

 

 

 

The solution is to add a remote control transmission data control module to complete the data interconnection of all important electrical equipment components, including lithium ion battery charging data information. According to the company's data information monitoring and management platform, the battery failure mode, early warning users, convenient service items and safety risks are predicted and in the embryonic stage

 

 

 

2. Improve the structure of lithium battery to improve safety

 

 

 

Compared with pure electric vehicles, the application of lithium ion in electric vehicles is more extreme. Poor shockproof performance of the whole vehicle; Complex real-time road conditions; The vehicle is usually placed outdoors, and the rechargeable battery will be exposed immediately. The self-designed tailing design scheme adopts sealed, breathable, dry and waterproof gas structure, and silicone rubber with high heat transfer coefficient is added inside. In addition, it also ensures that all lithium-ion batteries are in the middle of the average temperature to ensure the consistency of lithium-ion batteries, thus improving the service life of the entire battery

 

 

 

3. Two way handshake identification circuit to ensure that the battery is not abused

 

 

 

The electric vehicle is based on the direct connection and discharge scheme of the sales market, which is convenient for users to use the battery. If lithium is used as the carrier of lead-acid batteries, that is to say, lead is sold in the lithium market; Or as a treasure house for trial operation, it can be used for the cycle test of lead-acid batteries; Maintenance personnel will continue to use the lead-acid battery control panel or charging head to replace the lithium electric vehicle control panel and charging head. This is a bad habit of lead-acid batteries. Once lithium is used in lead-acid battery vehicles, the lead-acid battery control panel has the function of preventing battery charging and does not meet the low-voltage protection point of lithium battery, which will cause safety risks

 

 

 

The corresponding measure is to propose a dual wave identification power supply circuit between the lithium-ion battery board and the control board, before the BMS board and the charging head. If the lithium-ion battery does not match, the charging control circuit will be closed

 

 

 

4. PGR motor avoids reverse charging of lithium battery

 

 

 

Resistance charging is a good interest point of lead-acid batteries, but it has become a safety risk point of lithium-ion batteries

 

 

 

What is the harm of lithium battery reverse charging? The basic principle of lithium ion battery charging is introduced in detail, that is, the whole sliding process of lithium ion battery from the normal stage, from the diaphragm and the electrolyte of lithium ion battery to the high-purity graphite with negative hole structure. If the transfer speed of the lithium ion battery is not fast enough and the total transfer amount is too high, it will stay on the surface layer of the negative electrode and produce lithium dendrites. The main factors affecting the total transfer amount and transfer rate of lithium ion batteries are operating temperature and current

 

 

 

Although all lithium-ion vehicles have eliminated the single vehicle effect, there is still a reverse charging current during the whole downhill process. When the rated power of the motor exceeds the rated speed ratio, the working voltage of the self induced electromotive force will be higher than the battery voltage, and the kinetic energy will flow back to the rechargeable battery. The faster the vehicle descends, the higher the magnetic induction electromotive force and the greater the reverse current

 

 

 

Especially when the maximum speed of the national standard electric vehicle is limited to 25kmh, the sliding speed of the downhill section will exceed 25kmh, and the reverse current is large. Especially in cold winter, the conductivity of lithium battery electrolyte and the transmissivity of lithium ion battery are reduced, the lithium dendrite of negative metal material is more likely to change, the diaphragm is more likely to produce, the slight penetration of battery volume is reduced, and the battery life decreases faster. In particular, there is a large area of short circuit heating, and eventually uncontrolled explosion and combustion.