Lithium-ion batteries have become a typical example of new energy because of their low cost, high performance, high power, green environment and many other advantages. They are widely used in 3C digital products, mobile power supplies and electric vehicles, etc. With lithium-ion batteries With continuous promotion, the safety of lithium-ion batteries is getting more and more attention. Due to the technical problems of the battery itself or improper use, lithium-ion batteries may explode and cause fire accidents. Especially in recent years, electric cars have been used. The demand for lithium-ion batteries in the main electric vehicle market has been increasing. During the development of high-power lithium-ion battery systems, battery safety issues have attracted widespread attention, and existing problems urgently need further resolution.
Lithium-ion battery thermal runaway process
In the case of fires caused by thermal runaway of batteries in recent years, the heat generation rate of the battery is much higher than the heat dissipation rate, and a large amount of heat is accumulated without being emitted in time. Essentially, the heat “Out of control” is a cyclic process of positive energy feedback: An elevated temperature will cause the system to become hot, and the system will heat up and the temperature will increase, which in turn will make the system hotter.
Lithium-ion battery thermal runaway process diagram
Stage 1: Thermal runaway inside the battery
The battery is safe at 80~90°C. When the temperature rises to 90~120°C, the SEI film begins to decompose, releasing heat and the temperature increases. However, when the temperature reaches 120~130°C, the protective layer SEI film is destroyed. The negative electrode reacts with the solvent and the binder. As the temperature rises, the membrane melts and closes. After the temperature continues to rise above 150°C, the internal electrolyte begins to decompose, continuing to release heat, further heating the battery.
Phase 2: Battery Drum Phase
When the battery temperature reaches above 200°C, the positive electrode material decomposes, releasing a large amount of heat and gas, and continuously heating. 250-350°C intercalation lithium negative electrode begins to react with the electrolyte.
Stage 3: Battery Thermal Runaway, Explosion Failure Phase
In the course of the reaction, the oxygen produced by the reaction between the electrolyte and the positive electrode reacts violently and further causes the battery to run out of heat.
Lithium-ion battery thermal runaway causes
In fact, the probability of an internal battery short circuit appearing in an electronic product is one ten-millionth, which means that the safety of a single battery used in ordinary life is relatively high. However, in an electric vehicle, the battery pack of an electric vehicle needs With thousands of batteries, the probability of thermal runaway increases from one ten-millionth to one-thousandth. Also, once the battery of an electric vehicle is in danger, the consequences will be very serious. It is particularly important to study the causes of thermal runaway of batteries. .
1 production process
Battery production process
1 cathode material
The safety performance of the positive electrode material mainly includes overcharge safety and thermal stability. In the oxidation state, the positive electrode material undergoes an exothermic decomposition reaction and releases oxygen gas.
2 anode material
Although the anode material is relatively stable, the carbon negative electrode in the lithium intercalation state will react with the electrolyte first at high temperature.
3 electrolyte
The electrolyte contains inorganic conductive agents and organic solvents, and the flammable nature of organic solvents will have a certain impact on the safety performance of the battery.
4 production process
The production process of the battery is very complicated. Even if it is strictly controlled, metal impurities or burrs in the production process cannot be completely avoided. If impurities, burrs or dendrites occur inside the battery, the electrical conductivity increases and the temperature rises due to amplification and deterioration. The heat generated by the reaction and discharge heating is continuously accumulated, which may eventually cause the battery to lose thermal control.
2 use process
1 Battery overcharge triggers thermal runaway
The battery itself has overshoot protection, but when the overshoot protection fails, the battery will continue to charge and the battery will overheat and trigger thermal runaway. With the continuous use of the battery, the aging of the battery is gradually serious. The consistency of the battery pack is getting worse and worse. At this time, if the battery is overcharged, it may easily cause thermal safety problems. Therefore, it should be used for safe charging at any time.
2 Battery overheating triggers thermal runaway
When an electric vehicle is driving in real traffic conditions, when the electric vehicle maintains a high speed or meets an extreme operating condition, it must continue a large current discharge. At this time, the internal temperature of the battery begins to slowly increase. When the battery heat is accumulated in a large amount, if it is not timely Limiting the discharge current, it is very likely to cause thermal runaway of the power battery.
3 mechanical trigger thermal runaway
Power battery packs suffer from impact deformation, battery pack internal battery short circuit, and other damage to the battery pack may cause the thermal runaway of the battery.
Precautions for proper use of lithium-ion batteries
1 Safely use the battery to prevent the battery from catching fire. Explosion and other safety accidents need to be purchased by the regular manufacturer and the safety battery.
2 Keep the battery in good ventilation and heat dissipation environment, and ensure that the battery temperature during operation does not exceed the internal electrochemical reaction temperature of the battery. If you find that the battery temperature is too high, you must find ways to cool the battery or suspend the battery if necessary. To ensure safety.
3 When charging, use the charging equipment that is matched with the battery and charge it according to regulations to prevent the battery from overcharging. Once overcharge, stop charging immediately.
4 To ensure the correct use of the battery Do not hit the battery with a sharp or heavy object, you can protect the battery outside the battery under the conditions of good ventilation, heat dissipation, etc. to prevent external damage to the battery and lead to safety accidents.
