Thermal Runaway: Self-Heating Reaction Chains & Key Triggers
Thermal runaway in lithium-ion battery cells represents a self-sustaining positive feedback loop driven by exothermic reaction chains. As internal temperatures rise, secondary exothermic reactions are triggered, releasing additional heat that elevates temperatures further and activates more severe chemical responses. If left unchecked, this escalating self-heating phenomenon inevitably leads to catastrophic failure, including intense fires or explosions.
This hazardous process stems from three primary abuse modes: mechanical abuse (e.g., nail penetration, severe impact, or crushing), electrical abuse (such as overcharging, over-discharging, or external short circuits), and thermal abuse (exposure to high ambient temperatures or cooling system failures). Among these, internal short circuits remain the most subtle and dangerous failure mode, as local defects often bypass standard detection until uncontrollable heat generation has already begun.
Three-Stage Degradation Pathway in Lithium-Ion Cells
The progression toward thermal runaway unfolds across three distinct temperature thresholds. The initial phase begins around 90–120°C with the breakdown of the Solid Electrolyte Interphase (SEI) layer. This decomposition releases heat and exposes the reactive anode surface directly to the organic electrolyte, accelerating baseline thermal instability within the cell structure.
In the second stage (120–200°C), the exposed anode reacts vigorously with the electrolyte while the separator begins to melt (polyethylene separators typically collapse near 130°C). This breakdown leads to widespread internal short circuits and a rapid increase in internal resistance.
Past 200°C, the third stage takes over: cathode materials decompose and release oxygen, triggering violent combustion reactions with the electrolyte. Transient heat output during this critical phase can instantly reach the kilowatt level.
BMS Early Warning & System-Level Safety Strategies
To mitigate these risks, Battery Management Systems (BMS) rely on real-time monitoring of specific characteristic parameters. Core detection metrics include sudden voltage drops, gas generation pressure, and steep temperature increases. Identifying these early indicators provides vital lead time to issue safety alerts before thermal propagation spreads throughout the module.