Thermal runaway in lithium-ion batteries represents a critical safety concern, driven by a cascade of exothermic electrochemical reactions that lead to uncontrolled temperature rise, gas generation, and potential cell rupture or fire. The process begins when a battery is subjected to abusive conditions such as overheating, overcharging, mechanical damage, or internal short circuits. The sequence of reactions depends on the cell chemistry, state of charge, and temperature, but generally follows a predictable pattern involving solid-electrolyte interphase decomposition, electrolyte oxidation, cathode breakdown, and anode-electrolyte interactions.
The first stage of thermal runaway involves the decomposition of the solid-electrolyte interphase (SEI) layer, a passivating film that forms on the anode surface during normal operation. The SEI consists primarily of lithium salts, such as lithium carbonate and lithium alkyl carbonates, which are stable under normal operating temperatures but begin to break down at around 80-120°C. This decomposition is mildly exothermic and releases flammable hydrocarbon gases. Once the SEI layer is compromised, the exposed lithium-intercalated graphite anode reacts directly with the electrolyte, typically composed of organic carbonates like ethylene carbonate or dimethyl carbonate. These reactions generate heat and additional gases, including hydrogen, methane, and ethylene, further elevating the cell temperature.
As temperatures rise beyond 150°C, the electrolyte begins to undergo bulk oxidation and decomposition. The organic solvents break down into smaller hydrocarbon fragments, carbon dioxide, and carbon monoxide, accompanied by significant heat release. The rate of electrolyte decomposition accelerates with temperature, creating a positive feedback loop where heat generation promotes further reactions. In cells with high nickel content cathodes, such as NMC (lithium nickel manganese cobalt oxide) or LCO (lithium cobalt oxide), the electrolyte decomposition can be particularly violent due to the catalytic effect of transition metal ions released during cathode breakdown.
Cathode decomposition occurs at higher temperatures, typically between 180-250°C, depending on the specific chemistry. Layered oxide cathodes like NMC and LCO undergo exothermic oxygen release as the lattice structure collapses. This released oxygen reacts with the electrolyte and decomposition products, creating additional heat and sometimes leading to combustion. In contrast, lithium iron phosphate (LFP) cathodes exhibit greater thermal stability due to the strong phosphorus-oxygen bonds, which resist oxygen release until much higher temperatures, around 300°C. This makes LFP cells less prone to violent thermal runaway compared to NMC or LCO.
The anode continues to contribute to heat generation through reactions with the electrolyte and, at sufficiently high temperatures, with the binder materials like polyvinylidene fluoride (PVDF). These reactions can produce toxic fluorine-containing compounds and further elevate the temperature. If the cell reaches temperatures above 300°C, the aluminum current collector may begin to melt, leading to internal short circuits that dump the remaining energy in an uncontrolled manner.
Gas evolution plays a critical role in thermal runaway progression. The combined reactions produce a mixture of gases that increase internal pressure, potentially leading to venting or cell rupture. The exact gas composition depends on the electrolyte formulation and electrode materials but commonly includes carbon dioxide, carbon monoxide, hydrogen, and various hydrocarbons. In some cases, the vented gases can ignite if they mix with oxygen and encounter an ignition source.
State of charge significantly influences thermal runaway severity. Cells at higher states of charge contain more reactive lithium in the anode and more oxidized cathode material, both of which contribute to more violent reactions. For example, an NMC cell at 100% state of charge may reach peak temperatures exceeding 800°C during thermal runaway, while the same cell at 50% state of charge might peak at 400°C. The difference arises from the greater availability of lithium and higher oxidation state of transition metals in fully charged cathodes.
Cell chemistry also determines thermal runaway characteristics. NMC cells tend to exhibit more severe thermal runaway than LFP due to their lower onset temperatures and greater oxygen release. LCO cells, while less common in large-format applications, are particularly hazardous due to cobalt's catalytic effects on electrolyte decomposition. Experimental studies using accelerating rate calorimetry have quantified these differences, showing that NMC cells can generate heat at rates exceeding 1000 W/kg during runaway, while LFP cells typically remain below 200 W/kg.
Several experimental techniques have been employed to study thermal runaway mechanisms. Differential scanning calorimetry reveals the temperatures and enthalpies of individual decomposition reactions. Accelerating rate calorimetry provides data on overall heat generation rates under adiabatic conditions. Mass spectrometry coupled with thermal analysis identifies gaseous decomposition products. These methods collectively demonstrate how the interplay between materials chemistry and environmental conditions dictates thermal runaway behavior.
Mitigation strategies focus on interrupting the reaction cascade. Thermal barriers can slow heat propagation between cells in a pack. Flame-retardant additives in the electrolyte reduce flammability but may compromise cell performance. Advanced separators with shutdown functionality block ion flow at elevated temperatures. Battery management systems monitor temperature and voltage to prevent abusive conditions. Material innovations, such as thermally stable cathodes or ceramic-coated separators, aim to raise the onset temperatures of key reactions.
Understanding the electrochemical reactions during thermal runaway enables better battery design and safety protocols. By identifying the critical temperature thresholds and reaction sequences for different chemistries, researchers can develop more robust systems while manufacturers can implement appropriate safeguards. Continued study of these mechanisms remains essential as lithium-ion batteries push into higher energy density applications where thermal stability becomes increasingly challenging to maintain.