Meaning
Thermal runaway in lithium-ion batteries containing transition metal oxide cathodes represents a severe hazard due to the release of stored chemical energy. The intense reaction, referred to as high nickel NMC combustion, occurs when the cathode lattice releases oxygen at elevated temperatures. It creates self-sustaining fires that require specialized mitigation systems within the battery pack enclosure.
Chemical Process
Oxygen release occurs when the delithiated cathode becomes structurally unstable under high thermal loads. In cells with nickel content above eighty percent, this decomposition starts at lower temperatures, leading to high nickel NMC combustion. The liberated oxygen immediately reacts with the flammable organic solvents of the liquid electrolyte.
This rapid oxidation generates heat faster than the cell can dissipate it, leading to the destruction of the cell casing and the expulsion of hot gases and molten materials. Consequently, the transition from local overheating to full combustion happens in a matter of seconds, posing a severe challenge to the pack structural materials.
Thermal Output
Energy release during these combustion events escalates with the state of charge of the cell. Calorimetry tests measure the maximum heat release rate during high nickel NMC combustion, which is crucial for sizing the gas vents and thermal barriers in the pack. This testing guides the selection of fire-resistant aerogels and phase-change materials that can block the thermal wave from reaching adjacent cells.
Protection Strategy
Sourcing teams select cell designs with ceramic-coated separators and advanced flame-retardant additives in the electrolyte to suppress early-stage thermal events. These preventive features delay the onset of high nickel NMC combustion and increase the time available for safety systems to respond. Packs constructed with these advanced materials achieve higher safety ratings in regulatory crash and puncture tests.
