Cylindrical lithium-ion batteries are widely used in power tools, energy storage systems, electric vehicles, and portable devices. Their safety performance directly affects equipment operation and service life. When the heat generated inside a battery cannot be released effectively or abnormal internal chemical reactions occur, thermal runaway may develop. In severe cases, this can lead to high-temperature gas venting, fire, or even an explosion. For battery manufacturers and pack assemblers, understanding the causes, progression, and prevention of thermal runaway helps improve cell selection and battery pack safety design while reducing the risk of accidents.
Main Internal Causes of Thermal Runaway in Cylindrical Batteries
Abnormal changes in a battery’s internal materials or structure may trigger continuous exothermic reactions. Common causes include the following.
- Internal Short Circuits: The separator isolates the positive and negative electrodes to prevent direct contact. If the separator is damaged, penetrated by metal particles, or affected by manufacturing defects, an internal short circuit may occur. The localized heat generated by the short circuit can further damage the cell structure and intensify internal reactions.
- Side Reactions Between Electrodes and Electrolyte: As battery temperature rises, the protective layer on the anode surface, the electrolyte, and the cathode materials may undergo decomposition reactions that release heat. Some reactions also generate gas, increasing the internal pressure of the cell.
- Cell Aging and Material Defects: Long-term charging and discharging can cause changes in electrode materials, increased internal resistance, and capacity degradation. Manufacturing defects or internal contamination can also increase the likelihood of abnormal heating during operation.
These internal abnormalities may interact with one another, allowing localized temperature increases to develop into sustained heat generation.
How Can External Operating Conditions Trigger Thermal Runaway?
Improper charging and discharging parameters, excessive operating temperatures, and mechanical damage can all affect the internal condition of a battery and increase safety risks.
- Overcharging and Overdischarging: Charging voltage that exceeds the permitted range may cause lithium plating on the anode, changes in cathode material structure, and electrolyte side reactions. Excessive discharging can damage the electrode structure and increase the risk of abnormalities during subsequent charging.
- Excessive Current: When a battery continuously handles charging or discharging currents beyond its rated capability, the heat generated by internal resistance increases. If heat dissipation is insufficient, the temperature may continue to rise and accelerate internal side reactions.
- High Temperatures and Mechanical Damage: Prolonged exposure to high temperatures accelerates battery material aging. Dropping, crushing, or puncturing a cell may damage its casing or internal separator, increasing the risk of a short circuit.
Controlling charging and discharging parameters, operating temperature, and mechanical impact is essential for reducing the risk of thermal runaway caused by external factors.
How Thermal Runaway Develops in Cylindrical Batteries?
Thermal runaway generally begins with a localized abnormality and may gradually develop into an intense exothermic reaction as the temperature and internal pressure change.
- Localized Temperature Rise: An internal short circuit, overcharging, or external heating may cause an abnormal increase in local temperature, triggering side reactions in the electrode materials and electrolyte. At this stage, the cell may show no obvious external changes, even though abnormal conditions have already developed internally.
- Gas Generation and Pressure Increase: As side reactions continue, some internal materials decompose and generate gas, causing the pressure inside the cell to rise. Some cylindrical batteries are equipped with safety pressure-relief structures that release pressure when specific design conditions are reached. However, pressure relief cannot prevent all thermal runaway reactions.
- Intense Heat Generation and Gas Venting: When the internal temperature reaches a dangerous level, exothermic reactions may accelerate rapidly, accompanied by high-temperature gas venting, smoke, or fire. If adjacent cells are exposed to excessive heat, thermal propagation may occur, causing other cells in the battery pack to develop abnormalities.
The chemical composition, state of charge, and structural design of different cells affect the rate at which thermal runaway develops. Cell safety cannot be assessed based on external appearance alone.
How to Reduce the Risk of Thermal Runaway in Cylindrical Batteries?
Before assembly, inspect cell appearance, voltage, and internal resistance to identify and remove abnormal cells. Ensure that cells within the same battery pack have consistent specifications and performance. Configure overcharge, overdischarge, overcurrent, and temperature protection according to cell parameters, and set appropriate protection thresholds. Temperature sensors should be positioned where they can accurately reflect actual temperature changes. Maintain suitable spacing between cells and design effective heat dissipation channels. Proper insulation, fixation, and vibration protection are also necessary to prevent compression or loose connections.
Thermal runaway in cylindrical batteries is rarely caused by a single fault. It results from the combined effects of internal material reactions, operating conditions, and structural design. During battery pack manufacturing and assembly, attention should be given to cell consistency, charging and discharging protection, temperature monitoring, and heat dissipation layout rather than relying on a single protective device. If a battery shows abnormal heating, smoke, swelling, or an unusual odor during use, stop operating the equipment and move away from the hazardous area. Do not continue charging or attempt to disassemble the battery. Effective preventive measures and emergency response procedures can help reduce the risk of thermal runaway and its propagation.
