Thermal runaway in lithium-ion batteries represents a critical safety concern, particularly due to the generation of flammable and toxic gases during the failure process. The phenomenon occurs when exothermic reactions within the cell lead to uncontrolled temperature increases, triggering further decomposition of materials and ultimately resulting in gas production, pressure buildup, and potential cell rupture. Understanding the chemical pathways of gas generation, the mechanics of venting, and the associated flammability risks is essential for improving battery safety.
The primary sources of gas generation during thermal runaway are the decomposition of the electrolyte and the breakdown of electrode materials. Lithium-ion batteries typically use organic carbonate-based electrolytes, such as ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). When exposed to high temperatures, these solvents undergo thermal decomposition, producing a mixture of gases including carbon monoxide (CO), carbon dioxide (CO₂), hydrogen (H₂), and various hydrocarbons like methane (CH₄) and ethylene (C₂H₄). The specific composition of the gas mixture depends on the electrolyte formulation and the temperature profile during thermal runaway. For example, at temperatures above 200°C, EC decomposes to form CO₂ and C₂H₄, while DMC and EMC produce CO and CH₄. The presence of lithium salts, such as LiPF₆, further complicates the reaction pathways, as the salt can decompose to generate PF₅, which reacts with trace moisture to form HF and additional gaseous byproducts.
In addition to electrolyte decomposition, electrode materials contribute significantly to gas evolution. The anode, typically composed of graphite with lithium intercalation, reacts with the electrolyte at elevated temperatures, forming solid electrolyte interphase (SEI) decomposition products and releasing gases such as H₂ and light hydrocarbons. The cathode, often a lithium metal oxide like LiNiₓMnₓCoₓO₂ (NMC) or LiCoO₂ (LCO), undergoes oxygen release at high temperatures, which can react with the electrolyte or anode materials to produce CO, CO₂, and other combustion products. The oxygen release from layered oxide cathodes is particularly hazardous, as it can sustain combustion reactions and accelerate thermal runaway.
As these gases accumulate within the sealed battery cell, internal pressure rises rapidly. Experimental studies have measured gas production rates ranging from 0.5 to 5 mL/Ah per second under thermal runaway conditions, depending on cell chemistry and failure mode. For a typical 50 Ah electric vehicle battery cell, this translates to gas volumes between 25 and 250 mL/s, creating immense pressure within milliseconds. The pressure buildup can exceed the mechanical limits of the cell casing, leading to venting or rupture.
Battery cells incorporate several safety mechanisms to mitigate the risks of catastrophic failure. Burst discs, or pressure relief vents, are designed to open at a predetermined pressure threshold, allowing controlled release of gases. Weak seams in the cell casing provide another venting pathway, intentionally failing at lower pressures to direct gas expulsion away from sensitive components. The venting process is critical in preventing cell explosion, but it introduces new hazards due to the flammable nature of the released gases. Gas compositions during venting typically consist of 20-40% CO, 10-30% CO₂, 10-20% H₂, and 5-15% hydrocarbons, with the remainder including trace compounds like HF and PF₃. These mixtures are highly combustible, with lower flammability limits (LFL) often below 5% concentration in air.
The flammability of vented gases plays a significant role in propagating thermal runaway, especially in multi-cell battery packs. When one cell vents, the expelled gases can ignite due to hot surfaces or sparks, creating a flame jet that heats adjacent cells and triggers cascading failures. Experimental data from large-scale tests show that vented gases can ignite within 50-200 ms after release, with flame temperatures exceeding 1000°C. The combustion of these gases not only accelerates heat transfer to neighboring cells but also generates additional thermal radiation that exacerbates failure propagation.
Gas generation and venting behavior vary depending on the trigger mechanism for thermal runaway. Overheating, internal short circuits, and overcharging produce distinct gas compositions and flow rates. For example, internal short circuits generate rapid gas evolution due to localized joule heating, often resulting in higher proportions of H₂ and CO. Overcharge-induced failures, in contrast, produce more CO₂ due to electrolyte oxidation at the cathode. These differences influence the severity of venting events and the resulting safety risks.
Efforts to improve battery safety focus on reducing gas generation and enhancing venting mechanisms. Advanced electrolytes with improved thermal stability, such as fluorinated carbonates or ionic liquids, can decrease the production of flammable gases. Additives like flame retardants (e.g., phosphates or hydrides) further suppress combustion risks. Cell design innovations, including multiple venting stages and directional venting paths, help manage gas release more effectively. Computational models of gas dynamics within battery systems also aid in predicting venting behavior and optimizing safety features.
The study of gas generation and venting remains an active area of research, with standardized testing protocols like UL 9540A and IEC 62619 providing methodologies for evaluating gas-related hazards. Quantitative data from these tests inform safety standards and engineering practices, ensuring that battery systems incorporate adequate protections against thermal runaway. Continued advancements in materials science and engineering will further mitigate the risks associated with gas generation, enabling safer and more reliable energy storage solutions.