Understanding the Thermal Runaway stages is essential for creating a theoretical model and testing it during lithium ion battery development. 12 steps of thermal runaway are described comprehensively by Manh-Kien Tran et. al. in their technical paper, 'A Review of Lithium-Ion Battery Thermal Runaway Modeling and Diagnosis Approaches'.
1. Metal Ion Dissolution:
The first sign of trouble is often the dissolution of metal ions, which lasts until around 90°C. Heat disrupts the stable crystal structure of the cathode materials, causing metal ions to migrate into the electrolyte.
2. Solid Electrolyte Interface (SEI) Film Decomposition:
Around the 90–180 °C mark, the SEI layer starts to decompose, releasing more heat and causing further dissolution of metal ions. The SEI is a passive film on the surface of the anode formed during the initial charging cycles and plays a crucial role in stabilizing the battery's operation.
3. Reaction between the Lithium and Electrolyte:
The lithiated carbon (anode material) starts to react with the electrolyte. This reaction is exothermic and releases heat, further accelerating the temperature rise and triggering a chain reaction of degradation processes.
4. Separator Melting:
Around 130-225°C, the separator, a critical component preventing contact between the anode and cathode, starts to melt.
5. Micro Inner Short Circuit:
Small internal short circuits may form as the cathode and anode come into direct contact, causing a further temperature rise.
6. Safety Venting:
As the internal pressure rises due to gas formation from electrolyte decomposition, the safety vent opens, typically between 160-280°C, to slow the runaway process.
7. Separator Break Up:
The separator may completely break up at around 160-250°C which leads to large-scale internal short circuits within the battery, causing a rapid surge in temperature.
8. Large Scale Inner Short Circuit:
The large-scale inner short circuit results in a massive release of energy, accelerating the temperature rise and leading to the decomposition of the cathode material and electrolyte, usually at temperatures above 200°C.
9. Cathode Material Decomposition:
At temp. above 200°C, cathode materials start to decompose, releasing oxygen, which reacts with the organic electrolyte, leading to combustion if the temperature is high enough.
10. Electrolyte Decomposition:
Beyond the 200-230°C range, the electrolyte begins to decompose, releasing gases that can increase the internal pressure.
11. Reaction of Graphite Anode with Binder:
At extremely high temperatures, the graphite anode can react with the binder material (which holds the active material onto the current collector), further increasing the temperature.
12. Combustion of Electrolyte:
Finally, the volatile gases from the electrolyte can ignite, resulting in combustion, which is a catastrophic stage of thermal runaway, with temperatures often exceeding 300°C.
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