Thermal runaway in lithium-ion batteries is a critical safety concern, with propagation behavior heavily influenced by cell geometry. Pouch, prismatic, and cylindrical cells exhibit distinct thermal runaway characteristics due to differences in mechanical structure, heat dissipation pathways, and internal component arrangement. Understanding these variations is essential for designing safer battery systems, particularly in applications where thermal runaway could have catastrophic consequences.
Pouch cells, with their flexible, laminated aluminum packaging, lack the rigid external casing found in prismatic and cylindrical cells. This design allows for efficient heat dissipation across the large surface area but also means that thermal runaway can propagate more rapidly once initiated. The absence of a metal can results in less mechanical restraint on electrode expansion during thermal runaway, leading to faster gas venting and potential rupture. Studies have shown that pouch cells can reach peak temperatures exceeding 800°C during thermal runaway, with propagation speeds between adjacent cells in a module ranging from 1 to 5 seconds depending on the state of charge and cooling conditions. The flat, stacked electrode arrangement in pouch cells also creates more uniform heat distribution, which can accelerate propagation if thermal barriers are not integrated.
Prismatic cells, housed in rigid metal or hard plastic casings, offer better mechanical stability than pouch cells but can still experience rapid thermal runaway under certain conditions. The semi-rigid design allows for controlled venting, which can delay propagation by releasing pressure and hot gases in a directed manner. However, the larger format of prismatic cells means that internal heat generation during thermal runaway is more concentrated, leading to higher localized temperatures. Nail penetration tests on prismatic cells have demonstrated that thermal runaway can initiate at temperatures around 150°C to 200°C, with propagation through the cell occurring within 10 to 30 seconds. The thicker casing also retains heat, increasing the risk of cascading failure in tightly packed modules unless active cooling is employed.
Cylindrical cells, such as the common 18650 or 21700 formats, benefit from the structural integrity of their steel or aluminum casings. The circular cross-section distributes mechanical stress more evenly, making them more resistant to internal pressure buildup during thermal runaway. However, the compact winding of electrodes in cylindrical cells creates multiple points of thermal contact, which can facilitate faster heat transfer between layers. Research indicates that thermal runaway in cylindrical cells typically propagates from the point of initiation at speeds of 0.5 to 2 meters per second, with peak temperatures reaching 600°C to 700°C. The small diameter of cylindrical cells also limits the total energy released in a single event, but this advantage diminishes when cells are arranged in high-density packs without adequate spacing or thermal barriers.
Nail penetration testing reveals significant differences in thermal runaway behavior between these geometries. Pouch cells often exhibit violent rupture and ejection of materials due to rapid pressure buildup, while prismatic cells may show delayed venting with more sustained burning. Cylindrical cells tend to vent from predetermined weak points, such as the positive terminal or rupture discs, but can still experience projectile hazards if the casing fails catastrophically. Data from standardized tests show that pouch cells reach thermal runaway thresholds at lower nail penetration depths compared to cylindrical cells, while prismatic cells exhibit intermediate sensitivity.
Design innovations like bi-polar electrodes can mitigate thermal runaway risks across all cell geometries. In a bi-polar design, the current collector serves as both the anode for one cell and the cathode for the adjacent cell, reducing the number of interconnections and potential failure points. This configuration also improves thermal conductivity between cells, which can be a double-edged sword: while it helps distribute heat under normal operation, it may also facilitate thermal runaway propagation if not properly managed. Experimental results suggest that bi-polar electrode arrangements can reduce peak temperatures during thermal runaway by 15% to 20% compared to conventional designs, but the effectiveness varies by cell geometry. Pouch cells with bi-polar electrodes show the most significant improvement due to enhanced heat spreading, while cylindrical cells benefit less due to their inherent thermal isolation from neighboring cells.
Material choices also interact with cell geometry to influence thermal runaway. For example, the use of ceramic-coated separators can delay propagation in pouch and prismatic cells by providing additional thermal insulation between electrodes. In cylindrical cells, the same coatings may have less impact due to the tighter winding and greater mechanical pressure on the separator. Similarly, flame-retardant additives in the electrolyte can reduce the intensity of thermal runaway in all geometries, but their effectiveness is more pronounced in pouch cells where the electrolyte is more likely to pool and sustain combustion.
The orientation and packing density of cells within a module further affect thermal runaway propagation. Pouch cells stacked vertically may experience faster upward propagation due to convective heat transfer, while horizontally arranged prismatic cells can exhibit more uniform heat distribution. Cylindrical cells packed in hexagonal close arrangements create natural thermal channels that can either help dissipate heat or accelerate propagation, depending on the presence of cooling systems. Module designs that incorporate phase-change materials or heat-resistant barriers between cells can interrupt propagation pathways, but these solutions must be tailored to the specific cell geometry to be effective.
Advanced simulation tools have been used to model thermal runaway propagation in different cell geometries, providing insights into heat generation rates and gas flow patterns. These models confirm that pouch cells require more aggressive cooling strategies to prevent cascading failures, while cylindrical cells benefit from designs that minimize thermal coupling between individual units. Prismatic cells occupy a middle ground, where the optimal safety strategy depends on the trade-off between energy density and thermal management complexity.
In summary, cell geometry plays a decisive role in thermal runaway behavior, with each type presenting unique challenges and opportunities for mitigation. Pouch cells demand robust thermal barriers and advanced cooling due to their rapid propagation characteristics, prismatic cells require careful venting design to manage pressure buildup, and cylindrical cells need precise spacing and isolation to prevent cascading failures. Innovations like bi-polar electrodes and advanced materials can enhance safety across all geometries, but their implementation must be geometry-specific to achieve the desired results. As battery systems continue to evolve, understanding these geometric factors will remain critical for developing safer energy storage solutions.