Thermal runaway (TR) in lithium-ion batteries is a complex event. By studying the relevant dynamics, Feng, Xuning, et al. from Tsinghua University, China, has provided valuable insights into potential methods to manage & mitigate these risks. Research elaborates on two key pathways: the 'IN' path, denoting the internal cell processes leading to thermal runaway, & the 'OUT' path, representing the observable external consequences.
The IN path:
- It refers to the internal physical & chemical processes leading to TR.
- Initial stage (T1) is when the Solid Electrolyte Interphase starts to decompose.
- Progression from T1 to T2 is driven by SEI decomposition & regeneration in lithium-ion cells with a graphite anode.
- T2 stage, compared to a "detonator," is triggered by an internal short circuit caused by separator collapse, oxygen release from the LiNixCoyMnzO2 cathode, or lithium plating on the anode surface due to improper charging.
- Concept of a "TR-trident" is introduced, referring to these three causes of the T2 stage.
- Redox reactions between T2 & T3 generate a large amount of gases, which play a significant role in the TR process.
The OUT path:
- OUT path summarizes externally observable phenomena during TR tests, such as swelling, rupture, smoke, fire, & explosion.
- Cell rupture occurs when the inner pressure surpasses the preset open pressure of the vent valve due to gas accumulation from gasifying carbonate solvents & side reactions.
- Smoke color during TR tests indicates the internal conditions of the cell. Black smoke indicates temperatures surpassing the aluminum collector's melting point (660°C) & white or grey smoke indicates vaporization of the electrolyte.
Thermal Runaway Mitigation:
- Mitigation strategies aim to increase T1 & T2 temperatures while decreasing T3 & maximum heat release rate.
- Chemical, mechanical, electrical, & thermal prevention strategies should be employed to inhibit the triggering process.
- Techniques to achieve these include the use of more stable SEI through electrolyte additives, improving the thermal stability of the anode, & employing the "self-poisoned" technique that neutralizes the oxidant & reductant for milder reactions during TR.
- At the OUT path level, focus on breaking the fire triangle by designing a proper vent valve & diluting flammable gases into an extreme lean zone using inert gases.
- Flame retardants can be added to the electrolyte or replaced with non-flammable electrolytes.
Conclusion:
- Framework provides a useful tool for understanding & mitigating TR, the actual sequence of failure events can be more complex.
- Consider the impact of mitigation strategies on the performance & cost of LIBs.
- TR mitigation at the system level becomes increasingly important for the safe utilization of LIBs.