Thermal runaway in lithium-ion batteries is a complex exothermic process initiated by the decomposition of battery materials when exposed to elevated temperatures or mechanical/electrical abuse. The severity and progression of thermal runaway are heavily influenced by the choice of cathode, anode, electrolyte, and separator materials, each contributing differently to the heat generation and propagation mechanisms.

Cathode materials play a critical role in thermal stability due to their oxygen release tendencies and reactivity with electrolytes. Lithium cobalt oxide (LCO) cathodes exhibit poor thermal stability, with decomposition beginning around 150–180°C, releasing oxygen that reacts exothermically with the electrolyte. Differential scanning calorimetry (DSC) measurements show sharp exothermic peaks above 200°C, with heat generation exceeding 1000 J/g. Nickel-manganese-cobalt (NMC) cathodes, depending on nickel content, demonstrate varying stability. High-nickel NMC (e.g., NMC811) decomposes at lower temperatures (180–220°C) compared to lower-nickel formulations (NMC622, NMC532), which exhibit onset temperatures near 220–250°C. Lithium iron phosphate (LFP), in contrast, shows superior thermal stability with decomposition occurring above 300°C and minimal oxygen release, resulting in significantly lower exothermic heat output (around 200–400 J/g). Accelerating rate calorimetry (ARC) tests confirm that LFP-based cells exhibit slower temperature rise rates and lower peak temperatures during thermal runaway compared to NMC or LCO.

Anode materials also contribute significantly to thermal runaway. Graphite anodes react with electrolytes at elevated temperatures, forming a solid-electrolyte interphase (SEI) that decomposes around 80–120°C, releasing flammable gases. The lithiated graphite further reacts with the electrolyte above 200°C, contributing additional heat. Silicon anodes, while offering higher capacity, introduce greater thermal instability due to their larger volume changes and higher reactivity. Silicon’s SEI is less stable, decomposing at lower temperatures (60–100°C), and its reaction with the electrolyte generates more heat than graphite. DSC data indicate that silicon-containing anodes can produce exothermic peaks 20–30% larger than graphite under identical conditions.

Electrolytes are a major contributor to thermal runaway due to their flammability and reactivity with electrode materials. Conventional liquid electrolytes, typically composed of lithium salts (LiPF6) in organic carbonates (EC, DMC, EMC), decompose at 60–80°C, releasing combustible gases (CO, CO2, HF) and reacting violently with cathodes. Polymer electrolytes, such as polyethylene oxide (PEO)-based systems, exhibit higher thermal stability, with decomposition onset above 200°C, reducing gas evolution and flame propagation. However, their lower ionic conductivity at room temperature limits widespread adoption. ARC tests show that cells with liquid electrolytes reach thermal runaway faster, with temperature rise rates exceeding 10°C/min, while polymer-based cells exhibit slower heating rates below 5°C/min.

Separators, though passive components, influence thermal runaway by melting or shrinking at high temperatures, leading to internal short circuits. Polyethylene (PE) and polypropylene (PP) separators melt between 130–165°C, while ceramic-coated separators extend shutdown temperatures to 200°C or higher. Advanced separators with thermal shutdown properties can delay thermal runaway but do not prevent it if other materials have already begun decomposing.

Material decomposition thresholds and their exothermic contributions can be summarized as follows:

| Material | Decomposition Onset (°C) | Exothermic Heat (J/g) | Key Reactions |
|-------------------|--------------------------|-----------------------|----------------------------------------|
| LCO Cathode | 150–180 | 1000–1200 | Oxygen release, electrolyte oxidation |
| NMC811 Cathode | 180–220 | 800–1000 | Oxygen release, layered structure collapse |
| LFP Cathode | >300 | 200–400 | Minimal oxygen release, stable structure |
| Graphite Anode | 80–120 (SEI), >200 | 400–600 | SEI decomposition, electrolyte reduction |
| Silicon Anode | 60–100 (SEI), >180 | 600–800 | Unstable SEI, violent electrolyte reactions |
| Liquid Electrolyte| 60–80 | 300–500 | Solvent decomposition, gas generation |
| Polymer Electrolyte| >200 | 100–300 | Slower decomposition, reduced gas release |

The interplay between these materials determines the overall thermal runaway behavior. High-energy cathodes like NMC and LCO paired with silicon anodes and liquid electrolytes create a highly reactive system prone to rapid temperature escalation. In contrast, LFP cathodes with graphite anodes and polymer electrolytes exhibit delayed and less severe thermal runaway. Mitigation strategies often focus on material substitutions, coatings, and additives to raise decomposition thresholds and reduce exothermic heat.

Understanding these material-specific behaviors is essential for designing safer battery systems, particularly in applications where thermal runaway risks must be minimized, such as electric vehicles and grid storage. Advanced characterization techniques like DSC and ARC provide critical data to evaluate new materials and formulations before deployment.