Thermal runaway propagation is a safety concept related to the high energy density of modern lithium-ion batteries. It describes the sequential failure of adjacent battery cells, where heat from one failing cell triggers an uncontrolled reaction in its neighbors. This chain reaction, often called a cascading failure, poses a significant risk in large battery systems, such as electric vehicles and grid-scale energy storage. Understanding propagation is fundamental to engineering safer, more reliable battery packs.
The Initial Event of Thermal Runaway
The entire sequence begins with a single battery cell entering thermal runaway, a self-accelerating cycle where heat generation exceeds heat dissipation. This initial failure can be triggered by internal or external factors that destabilize the cell’s internal structure. Internal causes often involve manufacturing defects, metallic contaminants, or the uncontrolled growth of lithium dendrites, which can pierce the separator.
A separator failure, where the polymer layer isolating the positive cathode and negative anode melts (typically 130°C to 150°C), results in an internal short circuit. When the electrodes make contact, a sudden, localized current flow generates intense heat (Joule heating). This temperature spike initiates a series of highly exothermic chemical reactions, including the decomposition of the solid-electrolyte interphase (SEI) layer and the breakdown of the cathode material, which releases oxygen and further accelerates the temperature rise.
External triggers, such as physical damage, excessive external heat exposure, or electrical abuse like overcharging, can also push a cell past its thermal stability threshold. Regardless of the trigger, the temperature inside the failing cell rapidly skyrockets, often reaching hundreds of degrees Celsius in seconds. This causes the cell casing to rupture and vent its contents, setting the stage for thermal propagation to the rest of the battery module.
The Mechanism of Cell to Cell Spread
Once the initial cell enters thermal runaway, the subsequent cell-to-cell spread is governed by mechanisms of heat transfer to adjacent, healthy cells. This phenomenon, known as thermal coupling, is concerning in densely packed battery modules designed for high energy output. The primary method of heat transfer between directly touching cells is thermal conduction through the shared structural components, such as module casings and electrode tabs.
The intense heat and material ejection from the failing cell also facilitate heat transfer through convection and radiation. Convection occurs as the extremely hot gases and ejected matter—including molten material and flammable electrolyte vapor—are violently vented from the ruptured cell. This hot plume of material impinges directly upon the surface of neighboring cells, rapidly elevating their temperature.
Radiation heat transfer also plays a significant role, especially when the receiving cells are not in direct physical contact with the initial failing cell. The high surface temperature of the runaway cell radiates energy outward, preheating the surrounding cells and pushing them toward their thermal runaway temperature threshold. This combination of conduction, convection, and radiation creates a thermal cascade that sustains the propagation sequence until all cells in the module or pack are consumed.
Safety Hazards and Toxic Byproducts
The consequences of thermal runaway propagation extend beyond the immediate fire hazard, involving the release of toxic and flammable byproducts. As cell components decompose at high temperatures, they release a mixture of gases that pose severe risks to personnel and the environment. Among the emissions are large volumes of highly flammable gases, including hydrogen, methane, and carbon monoxide.
The accumulation of these flammable gases within an enclosed battery pack can lead to a secondary gas explosion, intensifying the event and making the resulting fire difficult to control. Furthermore, the decomposition of the electrolyte, which often contains fluorinated salts like lithium hexafluorophosphate (LiPF6), produces hydrofluoric acid (HF) gas. HF is highly corrosive and toxic, posing a serious inhalation and contact hazard to emergency responders.
The fire is difficult to extinguish because the thermal event generates its own oxygen supply from the decomposing cathode material, allowing combustion to continue without external atmospheric oxygen. Studies show that carbon monoxide concentrations released can reach levels exceeding 12,800 parts per million (ppm), which is rapidly fatal to humans. This combination of fire, explosion risk, and highly toxic gas generation necessitates specialized safety protocols and equipment.
Engineering Strategies for Containment
The primary goal of modern battery pack engineering is to interrupt the thermal cascade and prevent cell-to-cell propagation once a single cell failure occurs. Engineers employ a layered approach that includes both passive and active design measures to mitigate this risk. Passive measures focus on physically isolating cells and increasing the thermal resistance between them.
This includes incorporating thermal barriers, often made of materials like mica, ceramic, or intumescent polymers, placed between individual cells or cell groups. These barriers are designed to absorb heat and maintain structural integrity at elevated temperatures, significantly slowing conductive heat transfer. Structural designs also incorporate adequate physical spacing between cells to allow for heat dissipation and reduce direct thermal coupling.
Active measures involve systems that rapidly manage heat and pressure once a thermal event is detected. Advanced cooling systems, such as liquid cooling loops, are instrumental in drawing heat away from adjacent cells to keep them below their runaway temperature threshold. Pressure relief and venting systems are carefully designed to manage the hot, flammable gases released by the failing cell. These vents direct the hot gases and ejected matter away from neighboring cells, preventing convective heating and flame impingement from triggering the next cell.