# Battery Firebreak: full text Site: https://batteryfirebreak.com/ Licence: CC BY 4.0 Machine-readable data: https://batteryfirebreak.com/api/v1/index.json # Materials ## Silica aerogel blanket / pad (fibre-reinforced) Silica aerogel trapped in a glass- or ceramic-fibre mat. Its nanoporous structure suppresses gas conduction, giving the lowest thermal conductivity of the common barrier materials at room temperature and a clear advantage at a few hundred °C. Most battery grades are encapsulated in a film or faced with mica to control dust and add electrical and abrasion resistance. - undefined: 0.015–0.025 [typical] - undefined: ≈0.035–0.05 at 400 °C; ≈0.05–0.09 at 500–600 °C [typical] - undefined: ≈650 °C continuous (unshielded); short exposures higher when faced [typical] - undefined: ≈70–300 kg/m³ [typical] - undefined: 1–5 mm (often 2–3 mm) [typical] - undefined: Compressible ≈10–30 %, partial rebound [typical] - undefined: Insulating; breakdown typically ≥3.5 kV per pad when encapsulated [typical] ## Mica sheet / board (muscovite, phlogopite) Mica paper bonded with silicone resin into flexible sheet or rigid board. It insulates heat about ten times worse than aerogel but is very hard to burn through, resists the abrasive jet of hot particles from a venting cell, and keeps high dielectric strength at temperature. It is the standard material for pack lids, busbar shields and the facing layer of composite barriers. - undefined: ≈0.2–0.6 (through-plane) [typical] - undefined: Stays ≈0.2–0.6 [typical] - undefined: Muscovite ≈500–600 °C; phlogopite ≈700–800 °C continuous; higher short-term [typical] - undefined: ≈1,800–2,350 kg/m³ (bonded sheet) [typical] - undefined: 0.2–2 mm [typical] - undefined: Essentially rigid [typical] - undefined: Excellent; typically ≥20 kV/mm at room temperature [typical] ## Ceramic-fibre / bio-soluble fibre paper Paper made from alumina-silica refractory fibres or alkaline-earth silicate (bio-soluble) fibres with a small organic binder. It survives far higher temperatures than aerogel at low cost, but insulates less well at the temperatures that matter for propagation. - undefined: ≈0.035–0.05 [typical] - undefined: ≈0.06–0.12 at 400 °C; ≈0.08–0.16 at 600 °C [typical] - undefined: Fibre rated ≈1,100–1,400 °C [typical] - undefined: ≈140–290 kg/m³ [typical] - undefined: 0.8–6 mm [typical] - undefined: Some initial compression; limited rebound [typical] - undefined: Good when dry; binder burns out at ≈250–450 °C [typical] ## Intumescent / endothermic barrier sheet Active materials. Intumescent sheets swell into an insulating char when heated, filling gaps and adding thickness. Endothermic sheets absorb heat by releasing chemically bound water. Both buy time rather than offering a fixed thermal resistance. - undefined: ≈0.05–0.3 (before activation) [typical] - undefined: Not comparable: works by expanding or absorbing heat [typical] - undefined: Activates ≈150–200 °C; char stable to ≈1,000 °C [typical] - undefined: ≈300–1,500 kg/m³ [typical] - undefined: 0.5–3 mm between cells [typical] - undefined: Varies; expansion is irreversible [typical] - undefined: Depends on formulation: graphite-based chars become conductive [typical] ## Silicone foam / compression pad Closed-cell or syntactic silicone foams used mainly to manage cell swelling and hold stack pressure. Fire-rated and ceramifying grades add some thermal-runaway protection, often in combination with a mica or aerogel layer. - undefined: ≈0.06–0.15 [typical] - undefined: Polymer pyrolyses above ≈350 °C; ceramifying grades form a silica char [typical] - undefined: ≈200 °C continuous; fire-blocking grades survive short flame exposure [typical] - undefined: ≈190–800 kg/m³ [typical] - undefined: 1.5–6 mm [typical] - undefined: Best reversible compression and low compression set [typical] - undefined: Good [typical] ## Phase-change material (PCM) plate Plates that absorb heat while melting, used to smooth temperature peaks in normal operation. They are not fire barriers: organic PCMs are flammable and graphite-filled composites conduct heat toward neighbouring cells once the latent heat is used up. - undefined: ≈0.2 (paraffin) to 3–15 (graphite composite) [typical] - undefined: Not applicable: decomposes above ≈200–300 °C [typical] - undefined: Melts at ≈25–70 °C; organic types unstable above ≈200 °C [typical] - undefined: ≈760–1,700 kg/m³ [typical] - undefined: 2–6 mm [typical] - undefined: Rigid [typical] - undefined: Paraffin insulates; graphite composites conduct [typical] ## Composite barrier (aerogel or fibre core + mica / film facing) Laminates that put a low-conductivity core (aerogel or fibre) between tougher facings (mica, ceramic paper, polyimide or PET film). This is the dominant format in current production because it covers all four barrier jobs: insulation, compliance, dielectric strength and burn-through resistance. - undefined: ≈0.015–0.03 (aerogel core) [typical] - undefined: ≈0.04–0.06 at 500 °C (aerogel core); higher for fibre core [typical] - undefined: Facing survives short exposures ≈1,000 °C+ [typical] - undefined: ≈150–300 kg/m³ [typical] - undefined: 0.5–6 mm (often 1–3 mm) [typical] - undefined: Core compresses ≈10–30 % [typical] - undefined: Set by the mica or polymer facing [typical] # GB 38031-2025: Safety requirements for power batteries for electric vehicles (China) > GB 38031-2025, China's mandatory EV traction-battery safety standard, has been in force since 1 July 2026. It replaces the 5-minute warning rule with a 'no fire, no explosion' thermal-propagation requirement and adds bottom-impact and post-fast-charge tests. Source: https://batteryfirebreak.com/standards/gb-38031-2025/ Updated: 2026-11-15 Publisher: Battery Firebreak. Licence: CC BY 4.0. GB 38031-2025 is China's mandatory safety standard for electric-vehicle traction batteries. It replaced GB 38031-2020 and has applied to new vehicle type approvals since 1 July 2026. The biggest change is in the thermal propagation test: the battery pack must not catch fire or explode after a cell goes into thermal runaway, and smoke must not harm occupants. The old rule only required a warning 5 minutes before fire or explosion. ## Key facts | Item | Detail | |---|---| | Full title (Chinese) | 电动汽车用动力蓄电池安全要求 | | Replaces | GB 38031-2020 | | Published | 2025 (sources give different dates between March and May 2025; check the official notice) | | New type approvals | Requirements apply from 1 July 2026 | | Existing approved types | Reported compliance deadline of 1 July 2027 (confirm against the official notice) | | Scope | Lithium-ion and nickel-metal-hydride traction batteries for electric vehicles: cells, packs and systems | | Test count | 7 cell-level tests and 17 pack/system-level tests, as reported by Xinhua | ## What changed from GB 38031-2020 1. Thermal propagation: from "warn" to "no fire, no explosion". The 2020 edition required an alarm at least 5 minutes before fire or explosion, so that occupants could get out. The 2025 edition requires that the pack does not catch fire or explode after thermal runaway of a cell. An alarm is still required, and smoke must not harm occupants. Some industry articles report an observation period of 2 hours and a temperature limit at monitoring points. We have not confirmed these figures against the official text, so check the standard itself. 2. New bottom-impact test, representing damage from road debris or grounding. 3. New post-fast-charge safety test. After a series of fast-charge cycles (reported as 300), the battery must pass an external short-circuit test without fire or explosion. ## What it means for barrier and fire-protection materials - Barriers now carry the pass/fail result. Under the old rule, a pack could pass if it warned in time. Under the new rule, propagation inside the pack must be stopped or contained, which raises demand for cell-to-cell barriers, pack-lid fire shields and venting paths. - Designs must cope with the worst case. The test can trigger the cell that is hardest to protect, so barrier thickness and material choice are set by the worst position in the pack, not the average. - Ageing matters. Because fast-charge cycling is now part of the test sequence, barrier materials that lose thickness or crack under repeated cell swelling are a risk. - Exporters are affected too. Chinese cell and pack makers supplying global OEMs often use one design across markets, so this rule is shaping barrier design well beyond China. ## Sources - Xinhua via Beijing News, "电动汽车电池新国标7月落地，安全门槛再升级" (29 June 2026): bjnews.com.cn - China Securities Journal (3 July 2026): cs.com.cn - MPR China Certification, "New battery certification standard GB38031-2025 replaces previous regulation": china-certification.com - CarNewsChina (27 December 2025): carnewschina.com - Official text: National Public Service Platform for Standards Information, std.samr.gov.cn (search "GB 38031-2025") --- # UL 9540A: Test method for evaluating thermal runaway fire propagation in battery energy storage systems > UL 9540A is the test method that shows how a battery energy storage system behaves when a cell goes into thermal runaway, at cell, module, unit and installation level. The 6th edition, published on 13 March 2026, adds large-scale fire testing. Source: https://batteryfirebreak.com/standards/ul-9540a/ Updated: 2026-11-16 Publisher: Battery Firebreak. Licence: CC BY 4.0. UL 9540A is a test method, not a pass/fail standard. It forces cells into thermal runaway and measures what happens at up to four levels: cell, module, unit and installation. The data (gas volume and composition, heat release, flame spread, whether propagation stops) are used by fire codes such as NFPA 855 and by fire authorities to decide spacing, ventilation, explosion control and suppression. The current 6th edition was published on 13 March 2026 and adds large-scale fire testing (LSFT) at installation level. ## How the four levels work | Level | What is tested | Main outputs | |---|---|---| | Cell | Single cell forced into runaway, usually by external heating | Runaway onset temperature; vent gas volume and composition (including hydrogen); lower flammability limit of the gas | | Module | One module with one cell (or more) forced into runaway | Whether runaway propagates within the module; heat and gas release | | Unit | Full unit (e.g. rack or cabinet) with an initiating module, next to target units | Propagation to adjacent units; surface temperatures; flaming outside the unit | | Installation | Unit in a representative installation, with and without fire protection | Effectiveness of suppression and other installation-level protection | Testing moves to the next level only if the previous one shows a hazard. For example, if cell-level gas is not flammable, or module-level runaway does not propagate, later levels can be reduced. ## What changed in the 6th edition (2026) The 6th edition was published on 13 March 2026. Its main addition is large-scale fire testing (LSFT): a test of whether a fire in one energy storage unit spreads to adjacent units and nearby structures, and how ventilation and gas release affect fire behaviour. Earlier editions did not explicitly include LSFT. Sources link the change to NFPA 855; check the edition of NFPA 855 adopted in your jurisdiction. Commentators describe UL 9540A differently: Intertek calls it a data-generating test method rather than a certification standard, while other comments describe LSFT as part of the certification process. Rely on the official text for that question. ## Earlier change: the 5th edition (2025) The 5th edition (March 2025) added clearer criteria for cell-to-cell propagation, high-temperature test methods for different chemistries and protocols for applications such as rooftop and open-garage installations, according to UL Solutions. ## UL 9540A vs UL 9540 - UL 9540 is the safety standard for energy storage systems and equipment. A product is listed to it. - UL 9540A is the test method for thermal-runaway fire propagation. Its data are often required as part of UL 9540 certification and by fire codes for installation approval. ## What it means for barrier materials Module-level results decide whether a system needs extra installation-level protection. A barrier design that stops propagation at module level can greatly simplify site requirements such as spacing and suppression. That makes cell-to-cell and module-to-module barriers a direct cost lever for BESS integrators, not just a safety extra. ## Sources - Energy-Storage.news, "UL9540A: New edition of key BESS fire safety standard 'establishes new precedent'" (2026): energy-storage.news - Intertek, "Understanding the 2026 update to UL 9540A" (23 March 2026): intertek.com - UL Solutions, large-scale fire testing for BESS (April 2026): ul.com - UL Solutions, "UL Solutions Enhances Battery Energy Storage System Safety Test Methods…" (April 2025): ul.com - UL 9540A 2025 updates (presentation, hosted by Santa Fe County): santafecountynm.gov - Solar Power World (April 2025): solarpowerworldonline.com - Official standard: UL Standards & Engagement (search "9540A") --- # IEC 62619: Safety requirements for secondary lithium cells and batteries for use in industrial applications > IEC 62619:2022 sets safety tests for industrial lithium batteries, including stationary storage. Clause 7.3.3 requires a propagation test: a single-cell thermal runaway must not result in a fire of the battery system. Source: https://batteryfirebreak.com/standards/iec-62619/ Updated: 2026-11-15 Publisher: Battery Firebreak. Licence: CC BY 4.0. IEC 62619 is the main international safety standard for industrial lithium-ion batteries, including stationary energy storage, forklifts and similar uses. The current edition is 2.0 (2022). Its propagation test (clause 7.3.3) forces one cell into thermal runaway and checks that this does not lead to a fire of the battery system. Fire from the trigger cell itself is accepted. ## Key facts | Item | Detail | |---|---| | Current edition | IEC 62619:2022 (edition 2.0) | | Regional adoption | EN IEC 62619:2022 in Europe; used as a reference for battery approvals in many other markets | | Propagation test | Clause 7.3.3 (battery system level) | | Trigger | Should represent an internal short circuit as realistically as possible. Informative annexes describe laser irradiation and other methods | | Pass criterion | No fire of the battery system (outside the enclosure, or outside an area defined by the manufacturer) | ## What it means for barrier materials Because the trigger cell is allowed to burn, the job of the barrier is to keep neighbouring cells below their runaway onset and to stop flame from leaving the enclosure. Barrier and enclosure design, including vent paths, therefore decide the result. ## Sources - IEC webstore preview, IEC 62619:2022: webstore.ansi.org (preview PDF) - CETECOM advanced, "Propagation tests for lithium-ion batteries": cetecomadvanced.com - Battery Design, IEC 62619 overview: batterydesign.net - Official standard: IEC webstore --- # UN 38.3: Transport testing of lithium batteries (UN Manual of Tests and Criteria, section 38.3) > UN 38.3 is the set of eight transport tests (T.1–T.8) that lithium cells and batteries must pass before they can be shipped. It does not include a thermal propagation test. Source: https://batteryfirebreak.com/standards/un-38-3/ Updated: 2026-11-15 Publisher: Battery Firebreak. Licence: CC BY 4.0. UN 38.3 is the transport-safety test series every lithium cell and battery design must pass before it can be shipped. It has eight tests (T.1–T.8). It does not include a thermal runaway propagation test, so passing UN 38.3 says nothing about how a pack behaves when one cell fails. ## The eight tests | Test | Name | Applies to | |---|---|---| | T.1 | Altitude simulation | Cells and batteries | | T.2 | Thermal test (temperature cycling) | Cells and batteries | | T.3 | Vibration | Cells and batteries | | T.4 | Shock | Cells and batteries | | T.5 | External short circuit | Cells and batteries | | T.6 | Impact / crush | Cells | | T.7 | Overcharge | Rechargeable batteries | | T.8 | Forced discharge | Cells | Manufacturers and distributors must also be able to provide a UN 38.3 test summary for the battery design. ## What it means for barrier materials Barrier materials do not affect UN 38.3 directly, but the battery must be retested if its design changes in ways the regulations treat as significant. Changing barrier materials inside a battery during development should be planned with transport testing in mind. ## Sources - UNECE, Manual of Tests and Criteria, section 38.3 (dangerous goods pages): unece.org --- # AS/NZS 5139:2019: Electrical installations: safety of battery systems for use with power conversion equipment > AS/NZS 5139:2019 sets installation requirements for battery systems connected to inverters in Australia and New Zealand, including where lithium batteries may and may not be located. Source: https://batteryfirebreak.com/standards/as-nzs-5139/ Updated: 2026-11-15 Publisher: Battery Firebreak. Licence: CC BY 4.0. AS/NZS 5139:2019 is the Australian and New Zealand standard for installing battery systems that work with inverters, such as home solar batteries. It mainly controls where and how a battery may be installed: location restrictions, clearances, barriers to habitable rooms, signage and isolation. It is an installation standard, not a product test standard; battery products are assessed separately. ## What it covers - Location rules for battery systems, including places where they must not be installed, such as escape routes and certain living areas. - Separation and barrier requirements between a battery and habitable rooms. - Ventilation, clearances, labelling and isolation requirements. - A risk-based approach that depends on the battery chemistry and its hazards. ## Why it matters for barrier materials Where a battery is installed close to a habitable room, the wall or barrier between them must meet the standard's requirements. Products that pass stricter product-level fire tests can give installers more options. This makes Australian home-battery makers and installers a potential audience for barrier and fire-protection materials. ## Status AS/NZS 5139 has been under review. Before relying on any detail on this page, check the current edition and any amendments on the Standards Australia store. ## Sources - Standards Australia store, AS/NZS 5139:2019: store.standards.org.au (search "5139") - Clean Energy Council, battery installation guidance: cleanenergycouncil.org.au --- # Aerogel vs mica vs ceramic fibre: choosing a cell-to-cell thermal barrier > A practical comparison of aerogel, mica and ceramic-fibre barriers for lithium-ion packs: insulation at temperature, compression, dielectric strength, burn-through resistance and cost, with a selection checklist. Source: https://batteryfirebreak.com/guides/aerogel-vs-mica-vs-ceramic-fibre/ Updated: 2026-11-15 Publisher: Battery Firebreak. Licence: CC BY 4.0. Choose aerogel (usually encapsulated) when the main problem is heat conduction between closely packed prismatic or pouch cells and you need the thinnest barrier. Choose mica where the barrier faces flame, particle jets or high voltage, for example pack lids, busbar shields and vent paths. Choose ceramic-fibre paper for low-cost, high-temperature insulation where some extra thickness is acceptable. For cell-to-cell positions in high-energy packs, composites that combine these materials are now the norm. ## Side-by-side | Property | Silica aerogel pad | Mica sheet | Ceramic-fibre paper | |---|---|---|---| | Thermal conductivity, ~25 °C | ≈0.015–0.025 W/(m·K) | ≈0.2–0.6 W/(m·K) | ≈0.035–0.05 W/(m·K) | | Thermal conductivity, ~400–600 °C | ≈0.035–0.09 | ≈0.2–0.6 | ≈0.06–0.16 | | Temperature limit | Sinters above ≈650–800 °C unless shielded | Very high; resists burn-through | Very high (fibre rating ≈1,100 °C+) | | Compression / swelling | Compressible, partial rebound | Rigid | Limited rebound | | Dielectric strength | Moderate (depends on encapsulation) | Excellent | Moderate when dry | | Particle and flame erosion | Weak if unfaced | Excellent | Moderate | | Relative cost per m² | High | Low | Low | Values are typical ranges. See the [materials data page](/materials/) for details and sources. ## Five questions that decide the choice 1. What is the cell format? Prismatic and pouch cells have large flat faces in direct contact, so conduction dominates and thin, low-conductivity layers such as aerogel pay off. Cylindrical cells touch along lines, so potting compounds, foams and module-level shields often matter more than flat barriers. 2. How much does the stack swell over life? If the barrier also has to take up cell swelling, it needs a compressible core (aerogel or foam) and a known compression-set behaviour. A rigid mica sheet will not do this alone. 3. Where does the vent jet go? If the hot jet from a venting cell can hit the barrier, it needs a tough facing, such as mica or ceramic paper, or the core will be eroded within seconds. 4. What voltage does the barrier have to withstand? In high-voltage packs, check the dielectric withstand of the whole barrier, ideally after heat exposure, not just the room-temperature value. 5. Which test do you have to pass? Under China's [GB 38031-2025](/standards/gb-38031-2025/), the pack must not catch fire or explode after a cell goes into runaway. For stationary storage, [UL 9540A](/standards/ul-9540a/) data is used to decide installation requirements. The trigger method and observation time in your target test set how much margin you need. ## Common mistakes - Comparing room-temperature conductivity only. What matters is performance at several hundred °C, and the materials rank differently there. - Ignoring compression. A barrier that is 3 mm thick when fitted may be 2 mm after years of swelling, with a matching loss of insulation. - Treating the barrier as the whole solution. Busbars, cooling plates and shared vent channels can carry heat and flame around the best barrier. Design the heat and gas paths as a system. - Skipping ageing before the propagation test. Test the barrier in the state it will be in after ageing, not only when new. ## Related - [Thermal runaway propagation explained](/guides/thermal-runaway-propagation-explained/) - [Materials data](/materials/) - [Standards tracker](/standards/) ## Common questions ### Is aerogel better than mica for thermal runaway protection? For blocking heat conduction between cells, yes: aerogel's thermal conductivity is roughly ten times lower. For resisting flame, particle jets and high voltage, mica is better. That is why many production barriers laminate an aerogel core with mica facings. ### How thick should a cell-to-cell barrier be? Typical production barriers are 1–3 mm for aerogel or composite pads between prismatic or pouch cells, and 0.2–1 mm for mica sheets. The required thickness depends on cell chemistry, capacity, the trigger method in your target standard and how much compression the barrier sees over life, so it has to be confirmed by a propagation test. ### Can I use a phase-change material as a thermal runaway barrier? Not on its own. PCMs absorb heat only while melting, at a few tens of °C, and organic PCMs are flammable. They help with normal temperature management, not with stopping propagation. --- # Cell swelling and cell-to-cell barriers: what the compression data tell you > How cell growth loads a thermal barrier between prismatic or pouch cells, with compression and thermal-conductivity data from published datasheets and a method for checking a barrier from beginning to end of life. Source: https://batteryfirebreak.com/guides/cell-swelling-and-barrier-compression/ Updated: 2026-11-16 Publisher: Battery Firebreak. Licence: CC BY 4.0. A thermal barrier between prismatic or pouch cells is part of the mechanical stack, so it is compressed as the cells grow. Published datasheets for products marketed for cell-to-cell use show compression resistance differing by an order of magnitude: about 25–35 kPa versus 295–340 kPa at 20 % deflection for two papers in one product family, and roughly 1,000 kPa at 10 % strain for a rigid insulation board. What protects the neighbouring cell is the barrier in its compressed, aged state, so selection has to combine the compression curve, high-temperature conductivity and the stack's displacement budget, and be confirmed by a propagation test. ## How cells load the barrier Prismatic and pouch cells change thickness in two ways. Reversible "breathing" follows state of charge and temperature as the electrodes expand and contract. Irreversible growth accumulates over life from ageing processes such as interphase growth and gas generation. In most modules, end plates with straps or tie rods limit the stack's length, so cell growth is shared between the cells themselves, the barriers and pads, the frame and fasteners, and any spring elements or intentional gaps in the design. Two requirements pull against each other. The barrier must stay thick enough to insulate when a neighbouring cell fails. The stack pressure must stay within the window the cells need for long life, without overloading the end plates. The pressure window is specific to each cell design and should come from the cell supplier. Cell format also matters: pouch cells need the pressure spread evenly over the face, while the rigid can of a prismatic cell tends to bow outward at the centre, so the barrier is loaded unevenly and local pressure can be well above the average. ## Published compression and conductivity data | Product | Compression | Thermal conductivity | Density | Source | |---|---|---|---|---| | Morgan Superwool EST C30 paper | 25–35 kPa at 20 % deflection; 380–470 kPa at 60 % | 0.06 (200 °C), 0.08 (400 °C), 0.11 (600 °C), 0.15 (800 °C) W/(m·K) | 240–310 kg/m³ | [Morgan datasheet](https://www.morganthermalceramics.com/media/j3gmxyki/est-compression-paper_eng.pdf), p. 2 | | Morgan Superwool EST C310 paper | 295–340 kPa at 20 % deflection; 1,700–1,970 kPa at 60 % | 0.06 (200 °C), 0.08 (400 °C), 0.10 (600 °C), 0.13 (800 °C) W/(m·K) | 400–510 kg/m³ | same datasheet, p. 2 | | Rogers ProCell TB1000 board | Typical curve: about 1,000 kPa at 10 % strain and about 2,000 kPa at 17 % strain (read from the published graph) | 0.029 (25 °C), 0.032 (400 °C), 0.035 (600 °C), 0.037 (800 °C) W/(m·K), ISO 8301 | 350–450 kg/m³ | [Rogers datasheet](https://www.rogerscorp.com/-/media/project/rogerscorp/documents/elastomeric-material-solutions/procell-ev-firewall/english/data-sheets/180-506-procell-tb1000-thermal-insulation-board-data-sheet.pdf), pp. 1–2 | Morgan's datasheet lists both papers for pouch and prismatic cell-to-cell protection, chosen by the required compression characteristics. At 20 % deflection, EST C310 resists about ten times more strongly than EST C30, and both stiffen steeply as deflection increases: the resistance of EST C30 rises more than tenfold between 20 % and 60 % deflection as the fibre network densifies. The Rogers board's published curve reaches about 2,000 kPa at only 17 % strain, and its reported conductivity at test temperatures from 25 to 800 °C is 0.029–0.037 W/(m·K), roughly a quarter to a third of the ceramic-fibre papers' values at 600–800 °C. Read together, these figures suggest a material that changes thickness little under load, so a design relying on it to take up large cell growth would need checking; the datasheet table gives the compression range for finished parts as "based on design". Measured conductivity at temperature is not the same as retained barrier performance in a fire, which depends also on dimensional stability and integrity. These are datasheet values obtained by different methods (Morgan lists ASTM C201 / C177 for conductivity; Rogers lists ISO 8301 and, for compression, ASTM D1056), specimen conditions and cycle histories. They are not a ranking of pack-level fire or propagation performance. For fibre papers in particular, permanent set on the first compression cycle means a first-cycle curve may not represent cyclic or aged behaviour. Conductivity is measured on uncompressed material. ## What the datasheets usually do not tell you - **Stress relaxation.** In a clamped stack the barrier is held at a fixed or slowly increasing strain. What matters is how much its force decays over time at temperature, because the stack pressure falls with it. - **Compression set and recovery.** When cells contract at low state of charge or low temperature, a barrier with high compression set recovers little, and depending on preload and frame compliance the stack can lose contact pressure. - **Thermal resistance at the compressed thickness.** Heat flow through the barrier scales with its conductance, conductivity divided by thickness. Compression reduces thickness and also changes conductivity, in a way that depends on the material: densifying a porous insulator can lower radiative transfer but raise solid conduction. Contact resistance and gas gaps also matter. Ask for conductivity at the end-of-life thickness or, better, a transient or full propagation result. - **Electrical insulation.** Dielectric strength at temperature, volume and surface resistivity, and tolerance to pinholes and cut edges, measured on the compressed and aged part. - **Condition after ageing.** Repeated breathing can crack rigid laminates or shed fibres from unencapsulated papers. Either damage can reduce dielectric isolation between cell cans and make the barrier easier for vent jets and particles to erode. ## A method for specifying a barrier in a swelling stack 1. **Build the displacement budget.** For each barrier, deflection at end of life is roughly the assembly preload deflection, plus the irreversible cell growth and maximum reversible breathing it has to take up, minus what the frame, fasteners and other compliant layers absorb. Include manufacturing and thickness tolerances, cell-to-cell variation and thermal expansion, and check local peaks where prismatic cans bow, not just the average. Cell growth data come from the cell supplier. 2. **Check the operating points.** Locate the beginning-of-life, low-state-of-charge strain and the end-of-life, high-state-of-charge strain on the barrier's compression curve. Then correct the forces using stress-relaxation data measured at the relevant temperature and dwell time, since relaxation cannot be read from a static curve. Barrier stress is not the same as cell-face pressure, which can be uneven; both should stay inside the window the cells require. 3. **Consider a layered barrier.** If one material cannot meet both the displacement budget and the thermal requirement, combine a thin, stiff insulating layer with a compliant layer that takes up growth. Define what each layer must do during a fire and for how long; a compliant layer may be designed to decompose, while the insulating layer usually has to keep its thickness and integrity. 4. **Check the failure case.** During thermal runaway the failing cell can swell before it vents, loading the barrier at the same time as it heats. The barrier must not shrink, crack or extrude out of the gap under combined temperature and pressure, and must stay in place after neighbouring cells vent. Vent gas, flame and hot particles, edge bypass paths and decomposing adhesives can dominate the outcome even when bulk conductivity is low. 5. **Test in the aged, compressed state.** Regulations do not normally specify barrier ageing. For example, the test added to China's [GB 38031-2025](/standards/gb-38031-2025/) after fast-charge cycling is an external short-circuit test, separate from the thermal-propagation (热扩散) test; check the official text for the exact clauses. Running propagation tests on barriers that have seen representative compression, relaxation and temperature is therefore part of the pack developer's own validation, and a result from one pack design does not transfer automatically to another. ## Related - [Aerogel vs mica vs ceramic fibre: choosing a cell-to-cell barrier](/guides/aerogel-vs-mica-vs-ceramic-fibre/) - [Thermal runaway propagation explained](/guides/thermal-runaway-propagation-explained/) - [Materials data with verified product values](/materials/) ## Common questions ### Why does cell swelling matter for a thermal barrier? A barrier between prismatic or pouch cells sits in the mechanical load path of the cell stack. As the cells grow over life, the barrier is compressed. That changes the stack pressure on the cells and reduces the barrier's thickness, and therefore its thermal resistance, at the time a thermal runaway might occur. ### What compression data should I ask a barrier supplier for? The compression-force deflection curve with its test method and cycle number, compressive stress relaxation at temperature, compression set, and thermal conductivity or a propagation result at the compressed thickness expected at end of life. A single stress value at one deflection is not enough. ### Is a stiffer barrier better? Not in general. A stiff barrier keeps its thickness but turns cell growth into stack pressure and end-plate load. A compliant barrier keeps pressure low but loses thickness. Many packs combine a thin, stiff insulating layer with a compliant layer that takes up cell growth. --- # Thermal runaway propagation explained: how one cell ignites the next, and how barriers stop it > How lithium-ion thermal runaway starts, the three routes by which it spreads from cell to cell, and the four jobs a cell-to-cell thermal barrier has to do. Source: https://batteryfirebreak.com/guides/thermal-runaway-propagation-explained/ Updated: 2026-11-15 Publisher: Battery Firebreak. Licence: CC BY 4.0. Thermal propagation happens when a cell in thermal runaway heats its neighbours past their own runaway onset temperature. Heat reaches them by conduction through contact surfaces and busbars, by hot vent gas and flame, and by ejected particles. A cell-to-cell barrier must keep the neighbouring cell cool enough for long enough, while surviving compression, flame and high voltage. ## How thermal runaway starts in one cell A lithium-ion cell goes into thermal runaway when the heat produced by internal chemical reactions exceeds the heat it can lose, and the rising temperature speeds the reactions further. The usual sequence, as summarised by Feng et al. (2018), is: 1. SEI decomposition at relatively low temperatures (roughly the 80–120 °C range). The passivating film on the anode breaks down and the anode starts reacting with the electrolyte. 2. Separator melting and shrinkage. Polyethylene separators soften at around 130 °C and polypropylene ones at higher temperatures. When the separator fails, a large internal short circuit can form. 3. Cathode decomposition and oxygen release. Layered oxide cathodes such as NMC release oxygen at elevated temperature, which reacts with the electrolyte and anode. This is the step that drives the steep temperature rise. 4. Venting. Pressure builds up and the cell vents hot, flammable, toxic gas. Hydrogen, carbon monoxide, carbon dioxide and hydrocarbons are typical. The vent stream often carries burning electrolyte and solid particles. Triggers include overcharge, internal short circuits from manufacturing defects or dendrites, mechanical damage, and external heating. ## Three routes from one cell to the next | Route | What happens | What blocks it | |---|---|---| | Conduction | Heat flows through the cell faces that touch the barrier, and through busbars, cooling plates and the module frame | Low through-thickness thermal conductivity at high temperature; thermal breaks in busbars and cooling plates | | Vent gas and flame | Hot gas and burning electrolyte flow over neighbouring cells and can ignite inside the pack | Directed venting channels, flame-resistant covers, gas-tight separation between cells and modules | | Particle ejection | Hot, electrically conductive particles hit nearby surfaces, erode thin barriers and can cause arcing | Barriers with high mechanical and erosion resistance; mica and ceramic facings | In practice, engineers who design only for conduction are often surprised in testing by hot gas and particles. ## The four jobs of a cell-to-cell barrier 1. Thermal insulation at high temperature. What matters is the thermal conductivity at several hundred °C, not the room-temperature value quoted on page one of a datasheet. A good barrier keeps the far face of the barrier below the neighbouring cell's runaway onset temperature for the required time. 2. Mechanical compliance. Cells swell during cycling and ageing. A barrier between prismatic or pouch cells has to accept that compression over life without losing thickness, or thermal performance, permanently. 3. Electrical insulation. In high-voltage packs the barrier often also has to provide dielectric insulation between cells, including after it has been exposed to flame. 4. Fire and erosion resistance. The barrier itself must not burn, melt through or crumble when hit by flame and particles. No single material is best at all four, which is why many commercial barriers are laminates, such as an aerogel core with mica or ceramic facings. See the [materials data page](/materials/) for typical values. ## What standards ask for Most modern standards no longer only ask whether a single cell fails safely. They also require a propagation test: one cell is forced into thermal runaway and the response of the whole module, pack or system is observed. Examples include China's [GB 38031-2025](/standards/gb-38031-2025/) for EV traction batteries and [UL 9540A](/standards/ul-9540a/) for energy storage. The exact trigger methods, observation times and pass criteria differ; each standards page sets them out. ## References - Feng X. et al., "Thermal runaway mechanism of lithium ion battery for electric vehicles: A review", *Energy Storage Materials* 10 (2018) 246–267. doi:10.1016/j.ensm.2017.05.013 - Wang Q. et al., "Thermal runaway caused fire and explosion of lithium ion battery", *Journal of Power Sources* 208 (2012) 210–224. doi:10.1016/j.jpowsour.2012.02.038 ## Common questions ### What is the difference between thermal runaway and thermal propagation? Thermal runaway is the self-heating failure of a single cell. Thermal propagation is the spread of that failure to neighbouring cells, modules or racks. Barriers cannot prevent the first; they are there to stop the second. ### Can a thermal barrier stop a cell from going into thermal runaway? No. Barriers slow heat transfer to the neighbouring cells so that those cells stay below their own runaway onset temperature. The failing cell itself will still vent and heat up. ### Why do LFP and NMC cells need different barrier designs? NMC and other nickel-rich cells usually reach higher peak temperatures and release heat faster than LFP cells, so they demand barriers with lower high-temperature thermal conductivity and better resistance to flame and particle jets. LFP cells release large volumes of flammable gas, so gas management matters as much as heat blocking. --- # Glossary - Thermal runaway (热失控): Self-accelerating failure of a single cell: internal heat generation exceeds heat loss, the temperature rises, and the reactions speed up further until the cell vents and may burn. - Thermal propagation (热扩散): The spread of thermal runaway from the first failing cell to neighbouring cells, modules or racks. Barriers, spacing and venting design aim to stop it. - Thermal barrier (cell-to-cell barrier) (隔热层（电芯间隔热垫）): A layer placed between cells to slow heat transfer from a failing cell to its neighbours. Often also has to absorb cell swelling and provide electrical insulation. - Thermal interface material (TIM) (导热界面材料（TIM）): A material designed to conduct heat, for example from cells to a cooling plate. It does the opposite job from a thermal barrier, and the two are often confused. - SEI (solid electrolyte interphase) (SEI 膜（固体电解质界面膜）): A thin passivating film on the anode surface. Its decomposition is usually the first exothermic step in thermal runaway. - Venting (排气（泄压）): Release of gas from a cell when internal pressure exceeds the strength of the vent or casing. Vent gas from lithium-ion cells is hot, flammable and toxic. - Ejecta (喷射物): Hot solid and liquid particles expelled with vent gas. They can erode barriers and create conductive paths. - Silica aerogel (二氧化硅气凝胶): A highly porous solid of silica nanoparticles. It has very low thermal conductivity. For battery use it is usually reinforced with fibres to form a blanket, felt or paper. - Mica (云母): A family of layered silicate minerals (mainly muscovite and phlogopite) made into paper, sheet or rigid board with a binder. Valued for dielectric strength and flame and particle resistance. - Ceramic fibre paper (陶瓷纤维纸): A flexible paper of alumina-silica or similar refractory fibres with a small amount of binder. Used for high-temperature insulation. - Intumescent material (膨胀型材料): A material that swells into an insulating char when heated, increasing its thickness and insulating effect during a fire. - Endothermic material (吸热型材料): A material that absorbs heat by decomposing or releasing water when heated, delaying the temperature rise of the protected surface. - Phase-change material (PCM) (相变材料（PCM）): A material that absorbs heat as it melts. It is used to buffer temperature peaks; on its own it is not a high-temperature fire barrier. - Propagation test (热扩散试验): A test in which one cell is deliberately forced into thermal runaway and the response of the module, pack or system is observed. Pass criteria vary by standard. - Trigger method (触发方式): How the initiating cell is forced into thermal runaway in a test, for example a heater, nail penetration or overcharge. - BESS / ESS (BESS / ESS（储能系统）): Battery energy storage system / energy storage system. Stationary installations, from home batteries to grid-scale containers. - Cell-to-pack (CTP) (CTP（无模组电池包）): A pack design that places cells directly in the pack without intermediate modules. It raises energy density but removes module walls that used to slow propagation. - LFP / NMC (LFP / NMC（磷酸铁锂 / 三元）): Lithium iron phosphate and lithium nickel manganese cobalt oxide cathode chemistries. NMC cells usually reach higher peak temperatures in thermal runaway; LFP cells still vent large volumes of flammable gas. - Lower flammability limit (LFL) (爆炸下限（LFL）): The lowest concentration of a flammable gas in air that can ignite. It is used to size ventilation and explosion protection for battery rooms and enclosures.