Thermal runaway is a serious safety issue that affects lithium-ion batteries, which are widely used in various applications such as electric vehicles, smartphones, laptops, and grid storage. Thermal runaway is a phenomenon in which the lithium-ion cell enters an uncontrollable, self-heating state that can result in extremely high temperatures, violent cell venting, smoke and fire12.
Thermal runaway can be caused by internal or external factors that trigger a positive feedback loop of heat generation and accumulation within the cell. Some of the possible causes are:
- Internal short circuit: This occurs when the separator, which is a thin layer of material that prevents the contact between the positive and negative electrodes, is damaged or degraded due to manufacturing defects, mechanical abuse, or aging. An internal short circuit can create a local hot spot that initiates thermal runaway123.
- Overcharge: This occurs when the cell is charged beyond its maximum voltage limit, which can be due to incompatibility between the cell and the charger, or a poorly designed battery management system (BMS). Overcharging can cause the breakdown of the electrolyte, the formation of metallic lithium, and the increase of internal pressure, all of which can lead to thermal runaway12.
- Overdischarge: This occurs when the cell is discharged below its minimum voltage limit, which can be due to excessive load, self-discharge, or a faulty BMS. Overdischarging can cause the formation of copper shunts, the dissolution of the cathode material, and the degradation of the separator, all of which can increase the risk of thermal runaway2.
- External short circuit: This occurs when the cell terminals are accidentally connected by a conductive material, such as a metal object or a liquid. An external short circuit can cause a large current to flow through the cell, generating heat and possibly igniting the flammable components of the cell2.
- High- and low-temperature environments: This occurs when the cell is exposed to temperatures outside its safe operating range, which can be due to environmental conditions, improper storage, or inadequate thermal management. High temperatures can accelerate the chemical reactions and the degradation processes within the cell, while low temperatures can increase the internal resistance and the viscosity of the electrolyte, both of which can contribute to thermal runaway2.
Thermal runaway can have severe consequences for the safety of the users, the devices, and the surroundings. Therefore, it is important to design and test the lithium-ion batteries with proper safety measures, such as thermal fuses, venting mechanisms, flame retardants, and thermal isolation2. Additionally, it is advisable to follow the manufacturer’s instructions and precautions when using, charging, storing, and disposing of the lithium-ion batteries.
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