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Reasons for ignition of power lithium-ion battery packs

The essence of causing a lithium-ion battery pack to catch fire is that the heat inside the battery is not released according to the design intention, causing the ignition point of internal and external combustibles to ignite. The main causes of this are external short circuits, external high temperatures, and internal short circuits.

1. Internal short circuit: Due to the abuse of batteries, such as dendrites caused by overcharging and discharging, impurities and dust during battery production, it will deteriorate and form piercing membranes, resulting in micro short circuits. The release of electrical energy leads to temperature rise, and the material chemical reaction caused by temperature rise expands the short circuit path, forming a larger short circuit current. This cumulative and mutually reinforcing damage leads to thermal runaway.

2. External short circuit: Taking electric vehicles as an example. The probability of danger occurring during actual vehicle operation is very low. Firstly, the entire vehicle system is equipped with fuse and battery management system BMS. Secondly, the battery can withstand short-term high current shocks. In extreme cases, when the short circuit point crosses the vehicle fuse and the BMS fails, a prolonged external short circuit generally leads to the burning of weak connection points in the circuit, rarely causing thermal runaway events in the battery. Nowadays, many PACK companies have adopted the method of adding fuses in the circuit, which can more effectively prevent the harm caused by external short circuits.

3. External high temperature: Due to the structural characteristics of lithium-ion batteries, the SEI membrane, electrolyte, EC, etc. will undergo decomposition reactions at high temperatures. The decomposition products of the electrolyte will also react with the positive and negative electrodes, and the battery cell separator will melt and decompose. Multiple reactions will cause a large amount of heat to occur. The melting of the diaphragm caused an internal short circuit, and the release of electrical energy increased the production of heat. The cumulative and mutually reinforcing destructive effects result in the rupture of the explosion-proof film of the battery cell, the ejection of electrolyte, and the occurrence of combustion and fire.

Analysis of the causes of ignition in lithium-ion batteries

As a source of energy for pure electric vehicles, the important reason for lithium-ion battery pack catching fire is the thermal runaway caused by battery overheating, which is most likely to occur during battery charging and discharging processes. Due to the inherent internal resistance of lithium-ion batteries, they generate a certain amount of heat while outputting electrical energy to power pure electric vehicles, causing their temperature to increase. When their temperature exceeds their normal operating temperature range, it will damage the entire battery's lifespan and safety. In pure electric vehicles, the power lithium battery system is composed of multiple individual power lithium battery cells. During operation, a large amount of heat accumulates in the narrow battery box. If the heat cannot be quickly dissipated in time, high temperatures can affect the lifespan of the power lithium battery and even cause thermal runaway, leading to accidents such as fires and explosions.

Ways to solve thermal runaway

▲ Improvement of cooling methods

The thermal management system is responsible for controlling temperature and ensuring that the battery is always at a reasonable operating temperature. Usually, the thermal management system is controlled by the vehicle controller. When the temperature of the battery pack is abnormal, timely heat dissipation or heating is carried out through the air conditioning system to ensure the safety and lifespan of the battery. The heat dissipation methods of batteries are divided into four categories based on their different thermal conductivity and media: air cooling (air cooling), liquid cooling (water cooling), phase change materials (solid), and combined cooling (air cooling/water cooling+solid cooling).

Internal improvement refers to the modification of the material structure inside the battery cell, in order to make the battery have better heat resistance and heat dissipation performance. In terms of current research hotspots, developing solid-state electrolytes; Structural modification of positive and negative electrodes; One of the mainstream methods to improve the thermal performance of batteries from within is to introduce safer diaphragm materials.

How to deal with lithium-ion battery fires?

After analyzing the causes of battery fires, let's take a look at Tesla's recommended methods when lithium-ion batteries need to be extinguished:

1. If a small fire occurs and the flames do not spread to the high-voltage battery section, carbon dioxide or ABC dry powder fire extinguishers can be used to extinguish the fire.

2. When thoroughly inspecting a fire, do not come into contact with any high-voltage components and always use insulated tools for inspection.

3. If a high-voltage battery is bent, twisted, or damaged in a fire, it will either become unsightly or suspect a problem with the battery. So the amount of water used for firefighting should not be too small, and there should be sufficient water for firefighting.

4. It may take 24 hours for the battery to catch fire and be completely extinguished. By using a thermal imaging camera, it is possible to ensure that the high-voltage battery is completely cooled before the end of the accident. If there is no thermal imaging camera, it is necessary to monitor whether the battery will reignite. Smoking indicates that the battery is still very hot, and monitoring should be maintained until at least one hour after the battery no longer smokes.

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