Lithium-ion batteries are the backbone of modern electric vehicles (EVs) and energy storage systems, but their high energy density comes with severe safety risks. The primary threat is thermal runaway (TR), an uncontrollable, self-sustaining chain reaction where a battery’s heat generation vastly exceeds its ability to dissipate heat. During TR, internal temperatures can rapidly escalate to 900°C or even exceed 1000°C. This intense heat causes the battery cells to violently rupture and degrade, creating a cascading sequence of highly destructive and interrelated issues.
Massive Ejection of Materials and Gases
When a cell goes into thermal runaway, it acts like a pressure vessel blowing its top. Data highlights the staggering volume of material released during these events:
- Mass Ejection: A battery can emit approximately 66% of its total mass in the form of gas and solid particles. In one test of an 893g pouch cell, roughly 636g of material was forcefully ejected.
- Gas Volume and Pressure: Failing cells can generate gases at rates up to 3 litres per ampere-hour of cell capacity. This violent venting can create dangerous shockwaves and overpressures exceeding 200 kPa inside battery enclosures, which is enough force to blow heavy garage doors off their hinges. →Venting
Extreme Flammability and Toxicity Hazards
The vented gas is a volatile, toxic cloud composed of carbon dioxide, carbon monoxide, hydrogen, hydrocarbons, and vaporized electrolyte.
- Explosion Risk: Hydrogen (H2) makes up a significant portion of this gas and is incredibly volatile. It has a Lower Explosive Limit (LEL) of just 4.0% and a uniquely low Minimum Ignition Energy (MIE) of approximately 0.017 to 0.02 millijoules (mJ). This means even a microscopic, low-energy spark can detonate the accumulated vapor cloud.
- Chemical Toxicity: Beyond fire, the venting gas is highly toxic. An EV battery fire can release between 20 to 200 mg of corrosive hydrogen fluoride (HF) per watt-hour of nominal battery capacity, posing severe respiratory hazards to passengers and first responders.
The Threat of High-Voltage Arcing and Dielectric Breakdown
One of the most insidious problems triggered by thermal runaway is electrical arcing, which is essentially an uncontrolled electrical wildfire inside the battery pack.
- The Role of Particles: Surprisingly, the pure, filtered venting gas itself has high dielectric strength; it requires around 1850V to break down a 0.8 mm gap at 900°C, meaning the gas alone won’t easily arc at standard 400V or 800V EV levels. However, the solid particles ejected during TR completely change this. Experiments show that when vent particles are present, the critical breakdown voltage drops dramatically from 400V to roughly 120V.
- High Voltage Thresholds for Pure Gas: Experimental evaluations show that even at extreme temperatures like 900°C, a pure venting gas jet requires approximately 1850V to bridge a 0.8 mm physical clearance gap. Even when this threshold is adjusted downward to account for the lower work functions of battery materials like aluminum and copper, the required breakdown voltage remains above 1560V, which is far beyond standard traction battery voltages.
- The Role of Specific Gases (e.g., Hydrogen): Different gases have different Paschen’s Law coefficients, which dictate their breakdown voltages. Among the gases released, hydrogen (H2) provides the lowest breakdown voltage. However, worst-case approximations applying Paschen’s law to H2-rich mixtures in typical test setups (e.g., a 5 mm gap at 1.2 bar) still yield theoretical breakdown voltages of over 9000V.
- Testing with Artificial Venting Gas: Researchers have utilized breakdown voltage detectors filled with particle-free artificial venting gas mimicking a typical thermal runaway composition. These tests confirmed that while elevated temperatures (up to 900°C) do lower the breakdown voltage slightly compared to room-temperature air, the minimum required voltages still hover around 330V to 350V under very specific pressure-distance conditions, but would demand over 1800V to arc across a standard 0.8 mm safety clearance.
- Bridging the Gap: The breakdown distance required for an arc to jump increases by a factor of 50 when vent gas particles are present. Researchers sieving just 135.4g of ejected battery mass found 60 pieces of conductive copper larger than 5mm, which act as physical bridges that short-circuit electrode gaps.
- Destructive Power: Once an arc is initiated, the power is catastrophic. In a typical system with 396V and 55mΩ of resistance, the maximum power of the resulting arc can reach 712.8 kW. This massive energy instantly melts aluminum casings, destroys safety barriers, and ignites the flammable venting gases, accelerating cell-to-cell propagation. Alarmingly, data indicates that arc faults account for up to 60% of vehicle fires in new energy vehicles.
Design and Manufacturing Vulnerabilities
The physical constraints of modern battery design exacerbate these hazards.
- Tight Clearances: Standard 400V battery packs typically feature air clearances of only 2 to 5 mm and surface creepage distances of 5 to 15 mm.
- Tolerance Stack-Up: Normal manufacturing variances (e.g., ±0.25mm cell width tolerances, ±0.1mm busbar machining) can accumulate to reduce these critical safety clearances by 0.5 mm or more, pushing conductors dangerously close to arcing thresholds.
- Environmental Contamination: When a cell vents, the conductive, moisture-absorbing electrolyte residue coats the internal components of the battery pack. This instantly changes the internal environment from a clean “Pollution Degree 2” to a highly contaminated “Pollution Degree 3 or 4”. At these pollution levels, the existing physical spacing on the printed circuit boards (PCBs) and busbars becomes grossly insufficient, leading to surface tracking, short circuits, and inevitably, thermal propagation.
CATL Electrical to Thermal Isolation
To break this cascade of failure, CATL developed thermal-electrical separation technology (also referred to as gas-electric separation) as a core part of its NP 2.0 (No Thermal Propagation) safety framework. The guiding principle of this strategy is that heat and electricity must take entirely decoupled physical pathways. Meaning “vented gases take the heat/gas path, electricity takes the electrical path”.
This post has been built based on the support of and sponsorship from: illumion, Custom Power, Quarto Technical Services, Eatron Technologies and About:Energy
References
- Mulder, B.; Birke, K.P.; Obry, B.; Wigger, S.; Kozakov, R.; Smirnov, P.; Schein, J. Thermal Propagation Test Bench for the Study of the Paschen Curve and Lightning Arcs of Venting Gas. Batteries 2024, 10, 397. https://doi.org/10.3390/batteries10110397
- Chen, Y.; Li, Y.; Wang, J.; Lu, L.; Wang, H.; Li, M.; Xu, W.; Shi, C.; Li, C. Characterization of Breakdown Arcs Induced by Venting Particles Generated by Thermal Runaway of Large-Capacity Ternary Lithium-Ion Batteries. Electronics 2024, 13, 3168. https://doi.org/10.3390/electronics13163168
- Yue Zhang, Ping Ping, Xiantong Ren, Wei Gao, Depeng Kong, Xiaokang Yin, Characteristics and generation mechanism of ejecta-induced arc for lithium-ion battery during thermal runaway, eTransportation, Volume 24, 2025, 100429, ISSN 2590-1168, https://doi.org/10.1016/j.etran.2025.100429
- Rüdiger Beykirch, Sebastian Kox, A simulation methodology for detecting arc formation in EV batteries during thermal propagation, Advanced Battery Power, 2026