Thermal runaway propagation in lithium-ion batteries represents a critical safety challenge, particularly in large-scale energy storage systems and electric vehicles where multiple cells are densely packed. The phenomenon occurs when heat generated within a failing cell triggers adjacent cells to fail, creating a domino effect that can lead to catastrophic system failure. Understanding the propagation mechanisms requires examining the exothermic chain reaction at the cell level before analyzing how thermal energy transfers between cells in a module or pack.
The initiation phase begins with localized overheating within a single cell, typically caused by internal short circuits, overcharging, mechanical damage, or external heating. As temperature rises, the anode reacts with the electrolyte at around 90-120°C, forming a solid electrolyte interphase decomposition layer. This exothermic reaction generates additional heat, accelerating temperature rise. Between 120-150°C, the polyethylene separator melts, causing further internal short circuits. The increased current flow through these shorts produces more Joule heating, pushing temperatures higher.
At approximately 180-200°C, the cathode material begins decomposing, releasing oxygen. For lithium nickel manganese cobalt oxide (NMC) cathodes, this decomposition occurs in distinct stages as the layered structure collapses. The liberated oxygen reacts with organic electrolyte solvents, creating intense exothermic reactions that can rapidly elevate temperatures beyond 600°C. Lithium iron phosphate (LFP) cathodes exhibit more stable thermal behavior due to stronger phosphorus-oxygen bonds, typically requiring higher temperatures for oxygen release.
The propagation phase occurs when heat from the failing cell transfers to neighboring cells through three primary pathways: conduction through metallic interconnects, convection via hot gases, and radiation from flames or hot surfaces. Conduction often serves as the dominant propagation mechanism in tightly packed modules, where aluminum or copper busbars efficiently transfer heat. The rate of propagation depends on several factors including cell spacing, thermal conductivity of materials, and state of charge. Cells at higher states of charge contain more reactive materials and propagate failures faster.
Module design significantly influences propagation characteristics. In prismatic cell configurations, the large flat surfaces promote heat transfer across adjacent cells. Cylindrical cells arranged in compact arrays may develop hot spots where thermal energy concentrates between cells. Pouch cells present unique challenges as their thin form factor allows rapid temperature rise but provides limited thermal mass to absorb heat. The aluminum laminate casing offers minimal heat resistance compared to steel cylindrical cans.
Several high-profile failure incidents demonstrate real-world propagation scenarios. In one grid storage system failure, a single 60 Ah pouch cell undergoing thermal runaway heated adjacent cells to critical temperatures within 45 seconds. The entire 2 MWh rack became involved within 5 minutes as convection currents carried hot gases upward through the cabinet. An electric vehicle battery pack incident showed propagation occurring along the high-voltage busbars at approximately 15 cm per minute, with flames emerging from vented cells accelerating the process.
Mitigation strategies focus on interrupting propagation pathways through engineered solutions. Thermal barriers made from aerogels or phase change materials can be inserted between cells to block conductive heat transfer. These materials typically exhibit thermal conductivities below 0.05 W/m·K while maintaining electrical insulation. Cell spacing represents a fundamental design parameter, with gaps of 3-5 mm often sufficient to slow propagation in standard conditions. However, this must be balanced against energy density requirements.
Active cooling systems can remove heat from propagating modules, though their effectiveness depends on response time and thermal mass. Some designs incorporate intumescent materials that expand when heated, creating insulating char layers between cells. Ventilation channels direct hot gases away from adjacent cells, while flame arrestors prevent combustion products from spreading. Battery management systems contribute by detecting early warning signs like temperature gradients or pressure changes, enabling preemptive isolation of failing sections.
Material selection plays a crucial role in propagation resistance. Ceramic-coated separators maintain integrity at higher temperatures than standard polyolefin separators, delaying internal short circuits. Electrolyte additives that increase thermal stability or produce flame-retardant compounds when decomposed can slow the exothermic reactions. Cathode materials with higher thermal decomposition temperatures, such as LFP or thermally-stabilized NMC formulations, provide additional propagation resistance.
System-level design approaches include compartmentalization strategies where modules are physically separated by firewalls. Some automotive packs implement sacrificial thermal fuses that disconnect modules upon detecting excessive heat. The emerging practice of embedding fiber-optic temperature sensors throughout packs enables real-time monitoring of propagation fronts, allowing for targeted countermeasures.
Understanding thermal runaway propagation requires consideration of the entire energy transfer chain from molecular-scale reactions to system-level heat flow. Effective mitigation combines material science innovations with intelligent pack design and thermal management. As energy densities continue increasing and pack configurations become more compact, preventing propagation remains a key challenge for battery safety engineering. Ongoing research focuses on developing inherently propagation-resistant systems through materials that self-extinguish or absorb thermal energy without transferring it to adjacent cells.