Thermal runaway in lithium-ion batteries represents a critical safety concern characterized by uncontrolled temperature increases that can lead to catastrophic failure. The process involves complex heat generation and propagation mechanisms that depend on multiple factors, including battery chemistry, design, and operating conditions. Understanding these mechanisms is essential for developing effective mitigation strategies.

Heat generation during thermal runaway originates from three primary sources: Joule heating, exothermic chemical reactions, and internal short circuits. Joule heating occurs due to the internal resistance of the battery components, particularly during high-current operations or fault conditions. The heat generated is proportional to the square of the current and the internal resistance. Exothermic chemical reactions contribute significantly to heat buildup, especially as temperatures rise beyond safe operating limits. These reactions include electrolyte decomposition, anode-electrolyte reactions, and cathode decomposition. For instance, the breakdown of lithium cobalt oxide cathodes releases oxygen, which further accelerates electrolyte combustion. Internal short circuits, caused by dendrite growth, separator failure, or mechanical damage, create localized high-current pathways that generate intense heat rapidly.

Heat propagation within a lithium-ion cell occurs through conduction, convection, and radiation. Conduction is the dominant mechanism within the cell, as heat transfers through solid components such as electrodes, current collectors, and the separator. The thermal conductivity of these materials plays a crucial role in determining how quickly heat spreads. For example, aluminum current collectors conduct heat more efficiently than polymeric separators. Convection becomes relevant when gases or liquids are present, such as during electrolyte vaporization or in liquid-cooled systems. However, in sealed cells, convection is limited until venting occurs. Radiation contributes minimally under normal conditions but becomes significant at extremely high temperatures during thermal runaway.

Cell design significantly influences heat distribution and propagation. Cylindrical cells, with their metal casing, provide better mechanical strength and heat dissipation along the radial direction compared to pouch cells. However, their rigid structure can lead to pressure buildup and explosive failure if not properly vented. Prismatic cells offer a compromise between mechanical stability and thermal management, often incorporating cooling plates to enhance heat dissipation. Pouch cells, with their thin, flexible packaging, have poor heat conduction across their large surface areas but can be designed with integrated cooling systems to mitigate thermal runaway risks. The absence of a metal casing in pouch cells makes them more susceptible to external heat sources and mechanical damage.

Thermal imaging studies have provided valuable insights into heat propagation during thermal runaway. High-speed infrared cameras capture temperature gradients across battery surfaces, revealing hot spots and propagation patterns. Research shows that thermal runaway initiates at localized defects or weak points, such as electrode edges or regions with separator damage. The heat then spreads unevenly, depending on internal cell architecture and material properties. For instance, in large-format cells, heat propagation can exhibit wave-like behavior, moving from one electrode region to another due to sequential chemical reactions.

Modeling approaches are critical for predicting thermal runaway behavior and designing safer batteries. Electrochemical-thermal coupling models simulate the interaction between heat generation and chemical reactions, accounting for parameters such as reaction kinetics and heat capacities. Finite element analysis discretizes the battery into small elements to compute temperature distributions under various failure scenarios. Multiscale simulations bridge molecular-level reactions with macroscopic heat transfer, providing a comprehensive understanding of thermal runaway initiation and propagation. Machine learning techniques are increasingly used to analyze large datasets from thermal tests, identifying patterns that improve predictive accuracy.

Mitigation strategies focus on interrupting heat transfer pathways to prevent or delay thermal runaway. Passive methods include incorporating thermally stable materials such as ceramic-coated separators or flame-retardant additives in the electrolyte. These materials slow heat propagation by increasing thermal resistance or suppressing exothermic reactions. Active strategies involve battery management systems that monitor temperature and current, disconnecting the battery when abnormal conditions are detected. Thermal barriers, such as phase-change materials or heat-absorbing layers, can be integrated into cell designs to absorb and redistribute heat. Cell-level safety features, including pressure vents and current interrupt devices, provide additional protection by relieving internal pressure and cutting off electrical pathways during overheating.

The interplay between heat generation and propagation dictates the severity of thermal runaway events. High-energy-density cells, while desirable for performance, often exhibit more violent thermal runaway due to greater stored chemical energy. Advances in materials science and thermal management continue to improve safety, but challenges remain in balancing energy density, cost, and reliability. Future research aims to develop inherently stable chemistries and smarter thermal management systems that can predict and prevent thermal runaway before it occurs.

Understanding these mechanisms is crucial for engineers and researchers working on next-generation battery systems. By addressing heat generation sources and propagation pathways, safer and more reliable lithium-ion batteries can be developed for applications ranging from electric vehicles to grid storage. The integration of experimental data, advanced modeling, and innovative materials will drive progress in mitigating thermal runaway risks while maintaining high performance.