Where should the hydrogen leak alarm be installed?

                        1. Chemical structure damage        During the thermal runaway of the battery, a series of violent chemical reactions occur inside the battery. In lithium-ion batteries, for example, high temperatures cause a change in the crystal structure of the positive electrode material. Like the common lithium nickel cobalt manganese (NCM) ternary cathode material, at the thermal runaway temperature, the lithium ions in the lattice will lose the original ordered arrangement, resulting in a decline in the performance of the cathode material. This structural damage is irreversible and will greatly reduce the capacity and performance of the battery.        At the same time, the negative electrode material of the battery will also be affected. In the case of the negative electrode of graphite, high temperatures may rupture the solid electrolyte interface (SEI) film on the graphite surface. The SEI film originally played a role in protecting the negative electrode and controlling the transmission of lithium ions, but the rupture will lead to direct contact between the negative electrode and the electrolyte, further causing side reactions and accelerating the aging of the battery.        2. Decomposition and loss of electrolyte        Thermal runaway will cause a large amount of decomposition of the electrolyte. The electrolyte is the medium of ion transport in the battery and is essential for the normal operation of the battery. At high temperatures, the organic solvent in the electrolyte (such as vinyl carbonate, dimethyl carbonate, etc.) will decompose. These decomposition reactions not only consume the electrolyte, but also produce gas, resulting in increased pressure inside the battery.        After the decomposition of electrolyte, its composition changes and the ion transport ability decreases. For example, decomposition products may form a passivation film on the surface of the electrode, preventing the insertion and removal of lithium ions, thus reducing the efficiency of the battery's charge and discharge. With the continuous loss of electrolyte and the deterioration of performance, the cycle life of the battery will be significantly shortened.        3. The occurrence and aggravation of internal short circuit        During the thermal runaway process, the diaphragm inside the battery may melt and shrink due to high temperature. The main function of the diaphragm is to prevent direct contact between the positive and negative electrodes of the battery, and once the diaphragm is damaged, it will lead to a short circuit of the positive and negative electrodes. For example, in the initial phase of thermal runaway, local defects in the diaphragm may cause a slight short circuit, generating a small amount of heat. With the development of thermal runaway, short circuits will become more and more serious.        Short circuit will cause the battery in the charge and discharge process of abnormal large current, according to Joule's law (Q = I²Rt, Q is heat, I is current, R is resistance, t is time), large current will generate a lot of heat, further accelerate the damage of the battery, so that the battery life is sharply reduced. This vicious cycle will make the battery quickly lose its original function.        4. Shedding and loss of active substances        Drastic temperature changes and internal pressure changes can cause the active material on the battery electrode to fall off. For example, at high temperatures, particles of the positive electrode material may fall off the collector. These shed active substances will accumulate inside the battery and can no longer participate in electrochemical reactions.        For lithium-ion batteries, the loss of active substances directly affects the capacity of the battery. As the active material continues to fall off, the available capacity of the battery will gradually decrease. Moreover, the shed material may block the channel inside the battery, affecting the flow of electrolyte and ion transport, further reducing the performance and life of the battery.

        The detection of gas concentration in the early stage of battery pack by battery thermal runaway monitoring system is an important means to prevent battery thermal runaway accidents.        First，The principle of gas generation is related to thermal runaway        1. Hydrogen (H₂)        During the operation of the battery, especially in lithium-ion batteries, hydrogen gas is produced when the electrolyte inside the battery breaks down. For example, in the case of overcharging or internal short circuit of the battery, the positive and negative electrode materials of the battery may react abnormally with the electrolyte. For lithium-metal batteries, lithium reacts with organic solvents in the electrolyte to produce hydrogen. When the hydrogen concentration begins to rise, this is often a serious side reaction occurring inside the battery, which may be a prelude to thermal runaway. Because the production of hydrogen is usually accompanied by a large amount of heat release, and hydrogen itself is a flammable and explosive gas, its accumulation will increase the risk of battery explosion.        2. Carbon Dioxide (CO₂)        The production of carbon dioxide is also related to chemical reactions inside the battery. When the negative electrode material of the battery (such as graphite) is under high temperature or abnormal electrochemical reaction, it may react with some components in the electrolyte to produce carbon dioxide. In addition, during the thermal runaway process of the battery, the combustion of organic components such as the battery housing material or the diaphragm inside the battery will also produce carbon dioxide. If the concentration of carbon dioxide is increased, it indicates that the chemical reaction inside the battery has gone beyond the normal range, possibly due to the battery overheating or internal short circuit, which is an important signal that the battery thermal runaway is developing.        3. Carbon Monoxide (CO)        Carbon monoxide is usually produced by incomplete combustion of organic components inside batteries or some complex chemical reaction. For example, carbon monoxide may be produced when the battery separator breaks down with heat or when the organic solvent in the electrolyte breaks down at high temperatures. Carbon monoxide is a toxic gas, and its appearance means that there has been a more serious thermal runaway inside the battery, which may be caused by the battery being in a high temperature environment for a long time or suffering a serious external impact.        Second, the principle of gas detection technology        1. Electrochemical sensor        Electrochemical sensor is one of the common techniques for detecting gas concentration. For hydrogen detection, it uses hydrogen to undergo oxidation reaction on the electrode surface of the electrochemical sensor to generate a current signal. According to Faraday's law, the current generated is proportional to the concentration of hydrogen. This sensor has high sensitivity and good selectivity, and can send alarms at low hydrogen concentration.        For the detection of carbon monoxide and carbon dioxide, electrochemical sensors are also based on their electrochemical reactions at the electrode surface. The carbon monoxide is oxidized on the sensor's working electrode, and the carbon dioxide reacts with the electrolyte in the sensor to produce the corresponding electrical signal, thus achieving accurate measurement of the concentration of these two gases.        2. Infrared absorption spectroscopy        Infrared absorption spectroscopy is based on the absorption characteristics of different gas molecules to specific wavelengths of infrared light. Carbon dioxide and carbon monoxide have characteristic absorption peaks in infrared band. For example, carbon dioxide has a strong absorption peak near 4.26μm, and carbon monoxide has an absorption peak around 4.6μm. By emitting infrared light and detecting the change in light intensity after being absorbed by the gas, the concentration of the gas can be calculated. This technology has the advantage of high precision, non-contact, rapid and accurate measurement of gas concentrations, and can detect multiple gases simultaneously.        3. Semiconductor gas sensor        Semiconductor gas sensors use the principle that certain metal oxide semiconductors (such as SnO₂, ZnO, etc.) change their electrical properties (such as resistance) after adsorbing gas molecules. For hydrogen detection, when hydrogen molecules are adsorbed on the semiconductor surface, the resistance of the semiconductor will decrease. By measuring the change in resistance, the concentration of hydrogen can be determined. For carbon monoxide and carbon dioxide, there are similar detection mechanisms based on the change of electrical properties caused by the interaction of semiconductor materials with gases, but their selectivity is relatively weak, and other techniques are needed to improve the detection accuracy of specific gases.        Third，Early warning and control strategy based on gas concentration detection        1. Set the alarm threshold        According to the battery type, capacity, working environment and other factors, set different gas concentration warning thresholds. For example, for hydrogen concentration, when a certain volume fraction (such as 0.1-0.5%) is reached, the system will issue a level one warning, indicating that the battery may have potential safety hazards. For carbon monoxide and carbon dioxide, thresholds are also set depending on the degree to which they are associated with thermal runaway of the battery. When the concentration of carbon monoxide reaches a certain level (such as 50ppm to 100ppm) or the concentration of carbon dioxide exceeds a certain range (such as 1%-2%), the system will determine that the safety condition of the battery is deteriorating.        2. Hierarchical response measures        When the gas concentration exceeds the alarm threshold, the system takes corresponding response measures. In the first warning stage, simple ventilation measures may be initiated to expel the air containing a high concentration of gas from the battery compartment, while reducing the battery's charge and discharge power to see if the battery status can return to normal.        If gas concentrations continue to rise, reaching a level two alert or higher, the system will take more aggressive measures. For example, emergency cut off the battery's charge and discharge circuit, start the cooling system, and even notify the relevant personnel for emergency evacuation to avoid serious consequences caused by the battery's thermal runaway, such as fire or explosion.

                                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.