Once a cell passes its critical trigger point, thermal runaway is driven by a cascade of exothermic reactions rather than a single event. A key step in that cascade is cathode decomposition: at elevated temperature, a delithiated cathode breaks down and releases oxygen. Because oxygen is fundamental to combustion, this release is a major factor in how severe a thermal runaway event becomes, not just whether it starts.
This creates a failure pathway distinct from a conventional internal short circuit: chemical crosstalk. Rather than the anode and cathode failing because a physical short circuit connects them, oxygen released from the decomposing cathode migrates and reacts directly with the anode, generating significant additional heat. This can propagate a reaction even where separator integrity is intact and no internal short circuit has occurred.
Cathode chemistry has a direct bearing on this risk: different cathode materials decompose and release oxygen at different temperatures and rates, which is one of the reasons chemistry choice (see NMC vs LFP) is treated as a first-order safety variable rather than a secondary one.
Related: Physics of Thermal Runaway, Triggers of Thermal Runaway, Thermal Propagation, Solid Electrolyte Interphase
See also: Lithium Nickel Manganese Cobalt Oxides, Lithium Iron Phosphate, Cathode Electrolyte Interphase