Thermal propagation, the rapid spread of heat within a lithium-ion battery, can lead to hazardous conditions such as thermal runaway & cell fire. It is crucial to comprehend the causes & effects of thermal propagation to develop effective strategies for mitigating these risks.
Let's explore the various factors contributing to thermal propagation, including mechanical stress, operating stress, & thermal stress, as well as the potential consequences & safety implications.
Mechanical Stress & its Influence:
Mechanical stress refers to the forces & impacts exerted on a battery system during various stages, including manufacturing, assembly, & usage. These stresses can lead to the initiation or exacerbation of thermal propagation. For example, when an external force, such as an accidental impact, is applied to a battery cell, it can cause internal damage, resulting in an internal short circuit. This can lead to separator melting & the subsequent release of flammable materials, increasing the risk of cell fire.
Operating Stress & its Impact:
Operating stress encompasses the forces & conditions experienced by a battery during its regular usage. Factors such as state of charge (SOC), current intensity (Imax), & ageing can significantly influence thermal propagation. The SOC range of a battery (SOCmin to SOCmax) & exceeding the maximum current (Imax) can induce internal heating, which may trigger exothermal reactions within the cell. These reactions generate heat & gas formation, leading to an increase in cell temperature & potentially resulting in thermal runaway.
Thermal Stress and its Consequences:
Thermal stress occurs when a battery is subjected to temperature extremes, either excessively high or low. Elevated temperatures can accelerate the ageing process of the battery, compromising its performance and stability. When exposed to high temperatures, the heat dissipation of the battery may decrease, leading to an accumulation of heat within the cell. This accumulated heat can cause material and gas ejection, cell opening, or even explosion if the pressure inside the cell exceeds its structural limits.
Separator Failure and Electrochemical Reactions:
The separator within a battery is a critical component that prevents direct contact between the positive and negative electrodes. However, mechanical stress, internal/external short circuits, or plating/dendrite formation can compromise the integrity of the separator, and subsequent electrochemical reactions. These reactions generate heat and gas, further contributing to thermal propagation and the associated risks.
Thermal runaway is a serious safety hazard that can be caused by a number of factors, both internal and external. It is important to take steps to prevent thermal runaway, such as using safe operating procedures and monitoring battery health and condition.
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