Detailed discussion: The

Definition and harm of thermal runaway

Thermal runaway is an intense reaction that occurs in lithium-ion batteries under certain conditions, characterized by a rapid increase in temperature accompanied by combustion or explosion. This phenomenon can not only lead to complete battery failure, but also cause fires, posing a serious threat to personal safety and property. According to statistics from the Hong Kong Fire Department, there were 37 fire accidents caused by thermal runaway in lithium-ion batteries in 2022, an increase of 15% compared to the previous year, suggesting that the problem is becoming more serious.

The harm of thermal runaway is mainly reflected in three levels: First, the high temperature generated by the chemical reaction inside the battery can exceed 800°C, which is enough to melt the metal. Second, the decomposition of the electrolyte releases large amounts of flammable gases (such as hydrogen and methane), which are more prone to explosion when exposed to oxygen. Finally, toxic gases from combustion, such as hydrogen fluoride, can cause permanent damage to the human respiratory system. This also explains whyThermal runaway protection is listed as a top priority topic.

Thermal Runaway Trigger

Mechanical damage: puncture, crushing, impact

When lithium-ion batteries are physically damaged, their internal structural integrity is compromised. For example, an electric vehicle collision can cause a hole in the cell separator, causing a direct short circuit between the positive and negative electrodes. Experimental data from the Hong Kong Polytechnic University shows that a 3mm diameter steel needle can cause thermal runaway within 0.3 seconds of penetrating an 18650 battery. These types of injuries often accompany them(Lithium battery welding tip), insufficient solder joint strength, leading to lug breakage, etc.

 

 

• Crash Test Criteria:The International Electrotechnical Commission (IEC) stipulates that batteries must withstand a pressure of 13kN without igniting
• Impact Protection:The battery of the power tool must pass the 1.2m free fall test

Abuse of electricity: overcharge, over-discharge, external short circuit

Overcharging (above 4.2V) disintegrates the cathode material structure and releases reactive oxygen species, while overcharging (below 2.5V) dissolves the copper current collector. A case study by the Hong Kong Electrical and Mechanical Services Bureau pointed out that 80% of power bank fire accidents were caused by inferior charging ICs failing to block overcharging. At this time(Lithium battery detection technology) is very important, such as early detection of electrode degradation by electrochemical impedance spectroscopy (EIS).

 

Types of abuse	Rate of temperature rise	Standard trigger time
過充(120%SOC)	10°C/sec	5-15 minutes
External short circuit (	50°C/sec	30-60 seconds

Thermal abuse: external heating, internal heat generation

When the ambient temperature exceeds 60°C, the SEI membrane begins to decompose and react with the electrolyte to release heat. Tesla's thermal diffusion experiment proved that a single cell thermal runaway can ignite an entire battery pack in less than 15 seconds. This will be highlightedThe importance of medium heat shields, such as ceramic-coated separators that can withstand high temperatures of 200°C.リチウムイオン電池 検査技術

Chain reaction of thermal runaway

SEI film decomposition, electrolyte decomposition, separator melting

When the temperature reaches 90-120°C, the solid electrolyte interface (SEI) film on the anode surface decomposes, and the active lithium reacts with the electrolyte (usually LiPF6+ carbonate) to produce gases such as ethane. At this time, the closed-cell temperature of the separator (mainly made of PE/PP) (about 135°C) is an important line of defense. Japan manufacturer'sUltrasound imaging has shown that problems with uneven septal thickness can be detected in advance.

The positive electrode material is broken down, and the negative electrode reacts with the electrolyte

When the temperature exceeds 180°C, the common NMC cathode material decomposes and releases oxygen, which undergoes a violent oxidation reaction with organic solvents. At this time, it can release 1500 J of heat per 1 gram of active substance, and the thermal backflow between the graphite anode and the electrolyte can reach as much as 2000 J / g. This explains why you should use a power batteryEnsures heat resistance of lug connections.

Flammable gases are generated, the temperature rises sharply, and the combustion explodes

In confined spaces, the internal pressure can exceed 2 MPa due to gases generated by the decomposition of the electrolyte. According to test data from the Hong Kong Science and Technology Park, a 50Ah pouch battery emits a flame 3 meters long when it gets out of control. At this time, if there is no pressure relief valve design (core elements), the battery burns in bursts.

Thermal runaway monitoring and early warning

Temperature Monitoring: Thermocouples, Fiber Optic Sensors

Distributed thermocouple networks can track the temperature gradient of the battery pack in real time, but the response speed is limited (about 1-2 seconds). The fiber Bragg grating (FBG) sensor developed by the University of Hong Kong can reduce the detection delay to 0.1 seconds and achieve an accuracy of 0.5°C ±. This type of technology is integratedIntelligent BMS system.

Gas Monitoring: Flammable Gas Sensor

Early thermal runaway releases characteristic gases (CO, HF, etc.), and when the gas concentration reaches 50 ppm, the electrochemical sensor from the Japan manufacturer can trigger an alarm. It is worth noting thatCross-sensitivity is considered, such as ethanol vapors, which can cause false alarms.

Voltage and current monitoring: anomaly change detection

Increased mutations in DC internal resistance (DCR) are a precursor to thermal runaway. CATL's patented technology shows that 0.1 Hz AC impedance analysis can predict risk 30 minutes in advance, which is more preventive than traditional voltage monitoring (only 2-3 minutes in advance).

Safety protection measures for thermal runaway

Battery Level: PTC, CID, Pressure Relief Valve

A positive temperature coefficient (PTC) component can self-disconnect the circuit in the event of an overcurrent. Current interrupters (CIDs) use air pressure to mechanically cut the ear. After the Samsung Note 7 incident, the pressure relief valve design standard was raised from 0.8MPa to 1.2MPa, which is reflectedEvolve.電池製造安全対策

Module level: fireproof material, heat dissipation design

Ceramic fiber firemats can delay heat diffusion for up to 15 minutes, while phase change materials (such as paraffin/graphene composites) can absorb 200 J/g of latent heat. These technologies need to be adjustedThe hot aisle design of the heat path avoids localized heat build-up.リチウムイオン 電池 溶接 コツ

System Level: Thermal Management System, Fire Protection System

The liquid cold plate with thermally conductive silicone can control the temperature difference within ±2°C. Hong Kong MTR's battery pack is also equipped with an automatic perfluorohexanone automatic fire extinguishing system with zero ODP value, meeting environmental requirements.

Importance and future prospects of thermal runaway research.

While solid electrolytes are seen as a fundamental solution, mass production costs remain a barrier. In the short term,In addition to AI prediction models,The structural reinforcement process continues to improve safety. The Hong Kong Science and Technology Commission has allocated HK$200 million to support related research, with the goal of reducing the rate of thermal runaway accidents by 40% by 2025.