Thermal Barriers in Battery Energy Storage: Refractory Materials Against Thermal Runaway

A thermal runaway begins in a single cell: an internal short circuit, mechanical damage, overcharging or a manufacturing defect raises the cell temperature to the point where the decomposition of the electrolyte becomes self-accelerating. Within seconds, several hundred degrees Celsius are reached, along with flammable gases (hydrogen, carbon monoxide, hydrocarbons) and a jet of hot particles. The cell itself cannot be extinguished in this state — the reaction runs until the stored energy is consumed.

What endangers the installation is not the single cell but propagation: the heat from the first cell warms the neighbouring cells until the threshold is exceeded there as well. Without barriers, the chain runs through the module, the rack and finally the container. Every stage that delays transmission by minutes buys the battery management system time to shut down, the gas venting system time to exhaust, and the fire brigade time to arrive.

• Cell level: temperatures at the cell surface above 600 °C, particle jet exceeding 1000 °C
• Module level: gas release of several hundred litres per kWh, pressure build-up inside the enclosure
• Rack and container level: fire load from plastics, cables and electronics is added
• Time window: propagation between adjacent cells without a barrier typically within minutes, with an effective barrier significantly delayed or interrupted

In Germany there is no single, definitive law for stationary battery energy storage — the requirements are assembled from product standards, test methods, insurers' guidelines and building regulations. For material selection inside the container, the decisive elements are above all the test methods by which resistance to propagation is demonstrated.

UL 9540A is the internationally established test method that examines the spread of a thermal runaway in stages at cell, module, unit and installation level. Many tender specifications and insurers now demand a UL 9540A report; the temperatures and times documented in it are the input values for designing the thermal barriers. NFPA 855, as the installation standard, governs separation distances, fire compartments, and suppression and venting concepts. In Europe, IEC 62933-5-2 describes the safety requirements for grid-connected electrochemical storage systems. The VdS 3103 guideline summarises the expectations of German property insurers regarding lithium-ion storage systems.

For the refractory materials themselves, the classifications under DIN EN 13501-1 (reaction to fire, Class A1 for non-combustible products) and the fire resistance tests under DIN EN 1363 and DIN EN 1364 apply. For fibre materials, occupational safety comes on top: alumino-silicate wool (ASW) falls under TRGS 558, bio-soluble AES wool does not — a point that almost always tips the balance in battery storage applications.

An effective barrier concept follows the structure of the storage system from the inside out. Each level has its own task, its own temperature load and therefore its own appropriate material. Anyone who installs the same material at every level either pays too much or provides too little protection at the critical point.

• Cell to cell: thin interlayers of 1 to 3 mm that delay heat transfer to the neighbouring cell — ceramic fibre paper in bio-soluble quality or microporous film laminates; what matters is low thickness combined with low thermal conductivity at high temperature
• Module to module: boards of 5 to 25 mm between the modules in the rack — microporous boards with the highest insulating performance per millimetre where installation space is tight; calcium silicate boards where compressive strength is also needed
• Rack to rack and rack to wall: partition walls and linings with fire resistance — calcium silicate or fibre boards in layers, joints staggered, connections sealed with fibre rope
• Container internal lining: wall and ceiling lining of bio-soluble fibre blanket or board that keeps the steel shell below the critical temperature and prevents fire spread to neighbouring containers

Then there are the penetrations: cables, cooling lines, busbars and ventilation openings pass through every level. A partition wall is only as good as its weakest penetration — here, fibre ropes, paper gaskets and fire collars become part of the concept, not accessories.

In an industrial furnace, temperature dictates the material. In battery storage, three additional requirements shape the selection more strongly: installation space, occupational safety and the customer's bill of materials.

Bio-soluble AES wool (SOLUT BW Blanket, BW Board, BW Paper) is exempt from classification as carcinogenic under Note Q of the CLP Regulation and does not fall under TRGS 558. For storage system manufacturers this means: no exposure categories during installation, no labelling obligation in the product, no SVHC notification under REACH Art. 33. With a classification temperature of 1200 °C it covers the temperatures occurring at wall and ceiling during a thermal runaway. Its practical continuous-service limit below roughly 900 °C is irrelevant here — the load case is an event lasting minutes, not continuous operation.

Microporous boards (SOLUT MP Board) offer the lowest thermal conductivity of all insulating materials — around 0,02 to 0,03 W/mK at 200 to 400 °C — and thus the greatest barrier performance per millimetre. That makes them the material of choice between modules, where every millimetre of installation space costs energy density. They are fibre-free, non-combustible (A1) and available faced with aluminium foil, glass cloth or PE film so that they do not release dust during handling.

Calcium silicate boards (SOLUT CAL Board) go where the barrier has to carry load: beneath racks, as partition walls with fixings, as bases for components. Compressive strengths of 13 to 27 N/mm² at a classification temperature of 1000 °C, fibre-free and workable with woodworking tools.

• Cell interlayers: SOLUT BW Paper 1–3 mm or microporous laminate
• Module separation: SOLUT MP Board 5–25 mm, faced
• Load-bearing partition walls and bases: SOLUT CAL Board 12,7–50 mm
• Container wall and ceiling lining: SOLUT BW Blanket 25–50 mm or BW Board
• Penetrations and joints: fibre rope and BW Paper, combined with approved penetration sealing systems at fire compartment boundaries

The following overview compares the four most important barrier materials — with the properties that really matter in battery storage: thermal conductivity in the event case, installation space requirement, mechanical load capacity and regulatory status.

The best material selection is worthless if the barrier has gaps. From our installations we know four recurring weak points: open butt joints between boards, unsealed penetrations, steel fixings that act as thermal bridges through the insulation, and insulation layers that are crushed during assembly and thereby lose their thickness and insulating performance.

• Stagger board joints in two layers or use tongue-and-groove joints; back single-layer butt joints with fibre rope
• Draw every penetration as its own detail: rope, paper or collar, with a sealing length at least equal to the board thickness
• Thermally decouple fixings — ceramic or stainless steel anchors with washers instead of continuous steel screws
• Check insulation thickness in the installed condition, not as delivered; document clamping and compression
• Assess facings and films on the side exposed to the event: they protect during handling but contribute nothing in a fire

Verification is carried out in two stages: the material properties come from the data sheets and the fire classifications, and the barrier performance of the system from a propagation test at module or unit level — sensibly in accordance with UL 9540A, because the results are then also usable for insurers and approval authorities. We supply the material data and support the manufacture of test specimens so that the tested system matches what is installed later.

Most delays in storage projects arise not during installation but before it — because the fundamentals are missing when quotations are to be compared. These twelve points should be clarified before the first enquiry goes out:

• UL 9540A reports from the cell/module supplier available at module and unit level? What temperatures and times were measured?
• Protection objective defined: prevent cell-to-cell propagation, delay module-to-module, or only separate container from container?
• Installation space budget set for each level (mm between cells, modules, racks)?
• Insurer's requirements obtained (VdS 3103, plus any specific conditions)?
• Reaction to fire of the materials specified as Class A1 to DIN EN 13501-1?
• Fibre-free bill of materials required — or is bio-soluble fibre (Note Q) permissible?
• Penetrations counted and a sealing detail defined for each type (cable, cooling line, busbar, ventilation)?
• Fixing concept planned without continuous thermal bridges?
• Compression of the insulation layers during assembly calculated — thickness in the installed condition?
• Test specimens for the propagation test scheduled (material, date, test institute)?
• Series production: cutting supplied as a kit or as individual parts? Tolerances defined?
• Dismantling and spare parts concept: how is a barrier reinstated after a maintenance intervention?

A thermal barrier does not work in normal operation — it waits. Which is exactly why it is easily damaged during maintenance and modification: boards are removed for access and reinstated incompletely, seals at penetrations are lost during cable replacement, insulation layers are crushed by retrofitted components. The maintenance documentation should therefore include a barrier plan that identifies every barrier, every penetration and every seal.

• After every intervention: visual inspection of the affected barrier against the barrier plan, photo on file
• Annually: spot checks of the penetration seals and the compression of the insulation layers
• After a thermal event: completely replace the affected barriers — bio-soluble wool and microporous boards also lose their properties after a temperature breakthrough
• On removal: separate materials according to their labelling — bio-soluble and fibre-free materials are not hazardous waste, while used ASW falls under waste code 170603*