Thermal Runaway
Thermal runaway is a self-reinforcing feedback loop in which rising temperature triggers reactions that release more heat, accelerating until failure. In lithium-ion batteries it can cause fires or explosions when a cell is overheated, overcharged, or damaged. Larger cells are more prone to it because heat generation scales with volume while cooling scales with surface area. Mitigations include cell chemistry, thermal management, and failure isolation.
Thermal runaway is a positive feedback process in which an increase in temperature drives reactions that release still more heat, which raises the temperature further, in a self-accelerating loop that ends in device failure. It is a central safety concern for the Lithium-Ion Battery, where the flammable organic electrolyte can decompose rapidly when a cell is overheated, overcharged, defective, or physically damaged. Once it begins, a sealed cell can vent or, if safety vents are overwhelmed, explode violently, and the heat can propagate from one cell to its neighbors. A key driver is geometry, governed by the Square-Cube Law: heat production scales with volume (proportional to r cubed) while the surface available for heat dissipation scales only with area (proportional to r squared). Larger cells therefore have a worse ratio and are more prone to runaway, which is one reason batteries are built from many small cells rather than one large mass of energetic material. Mitigation operates on several fronts: safer cell chemistry (for example lithium iron phosphate cathodes or lithium titanate anodes and less flammable electrolytes), good thermal management to keep heat resistance low, circuit protection such as thermal fuses and current limiters, and physical failure isolation so a single runaway cell can be contained and bypassed rather than igniting a whole pack. These design choices are exactly why even very large storage products are packs of small, individually managed cells.