What is the process of thermal runaway of the battery?

        1. Stage of inducing factors

        Overcharge: Overcharge is one of the common causes of battery thermal runaway. When the battery charging voltage exceeds its rated voltage, too much electrical energy is forced into the battery. For example, in lithium-ion batteries, the normal charge cutoff voltage is generally around 4.2V. If the charging system fails or the charging control is not proper, resulting in a continuous rise in voltage, the positive electrode material structure inside the battery may change. For example, when lithium nickel cobalt manganese oxide (NCM) cathode material is overcharged, lithium ions will be excessively removed, resulting in irreversible changes in the crystal structure of the positive electrode material. At the same time, overcharging will also cause the decomposition of the electrolyte, generating a lot of heat, which is the starting point of the thermal runaway process.

        Internal short circuit: The internal short circuit of the battery may be caused by impurities in the battery production process, battery diaphragm damage and other reasons. For example, in the battery assembly process, if there are metal particles mixed between the positive and negative electrodes, it may cause a short circuit. When the internal short circuit occurs, the positive and negative electrodes of the battery are in direct contact, and the current will increase sharply in a short time. According to Joule's law (Q = I²Rt, where Q is heat, I is current, R is resistance, and t is time), because the short circuit current I is large, a large amount of heat will be generated locally, which will trigger a rapid rise in battery temperature.

        High temperature environment: When the battery is in a high temperature environment for a long time, the chemical reaction rate inside the battery will be accelerated. For example, in the summer high temperature weather, if the battery cooling system of the electric vehicle fails, the ambient temperature of the battery may exceed its safe operating temperature range (the general safe operating temperature of lithium-ion batteries is -20 ℃ -60 ℃). High temperature will enhance the activity of the electrolyte inside the battery, causing its decomposition reaction to occur more easily, and the performance of the positive and negative electrode materials will also be affected, increasing the risk of thermal runaway.

        2. Initial thermal runaway (self-heating stage)

        Once the above inducible factors cause heat to be generated inside the battery, the battery enters the self-heating stage. At this stage, the chemical reactions inside the battery begin to accelerate. For lithium-ion batteries, for example, the decomposition reaction of the electrolyte intensifies as the temperature increases. The heat generated by the decomposition reaction further increases the temperature of the battery, creating a positive feedback loop. At this time, the battery temperature may gradually rise from the normal operating temperature, such as from about 30 ° C to 60-80 ° C. At the same time, the battery may begin to release small amounts of gases, such as hydrogen, carbon dioxide, etc. These gases are produced due to the decomposition of the electrolyte and the reaction between the positive and negative electrode materials and the electrolyte. At this point, the battery thermal runaway monitoring system can issue an early warning if it can detect changes in temperature and gas concentration.

        3. Thermal runaway metaphase (thermal runaway trigger stage)

        As the temperature continues to rise, when a certain critical temperature is reached (the critical temperature is different for different battery types, generally around 80℃ -120 ℃), a series of violent chemical reactions will occur inside the battery, marking the formal trigger of thermal runaway. For example, in lithium-ion batteries, the cathode material may undergo a violent REDOX reaction at this time, releasing a large amount of heat. At the same time, the diaphragm inside the battery will melt or shrink due to high temperature, resulting in a further worsening of the short circuit between the positive and negative electrodes. The pressure inside the battery will also rise sharply, because the large amount of gas generated cannot be discharged in time. At this stage, the temperature of the battery will rise rapidly, possibly from about 80 ° C to several hundred degrees in a few minutes. A large amount of hot gases, including flammable toxic gases such as carbon monoxide and hydrogen, will be expelled from the battery. These gases, if exposed to a source of fire or accumulated in an enclosed space, pose a risk of explosion or poisoning.

        4. Thermal runaway stage (violent reaction and destruction stage)

        At the late stage of thermal runaway, the chemical reaction inside the battery reaches its most intense level. The battery case can rupture or explode due to the high pressure inside. For example, in some lithium-ion power battery packs, if the thermal runaway is not controlled in time, the housing of the battery module may be blown open, and the material inside the battery will be ejected. At this point, the combustion reaction can spread throughout the battery pack, starting a larger fire. The positive and negative electrode materials inside the battery will undergo various complex chemical reactions at high temperatures, such as combustion and decomposition. These reactions will not only release more heat, but also produce a large amount of harmful gases, causing serious damage to the surrounding environment and equipment. The entire thermal runaway process from the initial inducement factor to the final violent reaction and destruction can take anywhere from a few minutes to tens of minutes, depending on the type of battery, capacity, initial induction conditions and other factors.