Thermal conductivity testing of insulation materials for new energy electrolyte systems is a critical step in validating the safety and reliability of lithium-ion batteries, energy storage modules, and electric vehicle battery packs. This article presents a comprehensive experimental overview covering sample types, measured parameters, internationally recognized test methods, and practical applications of thermal conductivity data for battery insulation materials. By understanding these laboratory procedures, battery engineers and material developers can select insulation solutions that effectively delay or prevent thermal runaway propagation while remaining stable under long-term electrolyte exposure.
Technical Overview
In new energy systems such as electric vehicles and grid-scale energy storage stations, lithium-ion battery electrolytes typically consist of flammable organic carbonate solvents. When a single cell enters thermal runaway, the heat released can propagate to neighboring cells, triggering a cascading failure. Low thermal conductivity insulation materials installed between cells serve as engineered thermal barriers that slow heat transfer and extend the escape and response time available during a failure event.
The thermal conductivity experiment described here is designed to quantitatively characterize how effectively these insulation materials resist heat flow, both in their as-received condition and after simulated service environments, including exposure to battery electrolyte. Accredited third-party laboratories perform these tests in accordance with ISO, ASTM, EN, GB, UL, and IEC standards to provide material suppliers, cell manufacturers, and pack integrators with comparable, traceable data.
The primary objectives of the experiment are to determine the effective thermal conductivity of candidate insulation materials, evaluate thermal performance across representative operating temperatures, assess anisotropic heat transfer behavior, and quantify any degradation in insulation performance caused by electrolyte absorption or thermal aging.
Testing Samples
Silica Aerogel Insulation Blankets
Silica aerogel blankets, composed of a fiber reinforcement matrix impregnated with nanoporous silica aerogel, are among the most widely used cell-to-cell barrier materials. Their initial thermal conductivity typically falls between 0.018 and 0.025 W/(m·K) at room temperature. Test specimens are cut to the dimensions required by the selected method, typically 300 mm × 300 mm for plate methods, and conditioned to a stable mass and moisture state before measurement.
Microporous and Ceramic Fiber Boards
Microporous insulation boards achieve thermal conductivities below that of still air through opacified pore structures, generally 0.020 to 0.035 W/(m·K) at moderate temperatures. Ceramic fiber boards, with higher density and service temperature capability, range from 0.05 to 0.15 W/(m·K) depending on density and mean temperature. These materials are commonly tested at elevated mean temperatures up to several hundred degrees Celsius to simulate thermal runaway conditions.
Mica-Based Insulation Sheets
Muscovite and phlogopite mica sheets provide excellent dielectric strength and high-temperature stability, with thermal conductivity values generally between 0.2 and 0.4 W/(m·K). Although their conductivity is higher than aerogel materials, mica sheets are frequently specified as fire-protection layers on busbars, covers, and module walls, and their directional layer structure requires careful attention to measurement orientation.
Silicone Foam and Porous Elastomer Pads
Silicone foam pads combine compression resilience with thermal insulation, typically exhibiting thermal conductivity between 0.05 and 0.10 W/(m·K). They serve dual roles as compression pads and thermal barriers in prismatic and pouch cell stacks. Because their effective conductivity depends strongly on compression ratio and residual thickness, specimens are tested at controlled compression loads representative of the actual pack stack pressure.
Polyimide and Polymer Foam Materials
Polyimide foams and other engineered polymer insulations, with thermal conductivity values typically between 0.03 and 0.06 W/(m·K), are used where weight reduction and flame resistance are priorities. These materials may be tested for electrolyte compatibility because polymer matrices can absorb carbonate solvents and plasticize over time.
Testing Items
The experimental program characterizes the following parameters for each specimen group:
- Thermal conductivity at ambient and elevated mean temperatures, expressed in W/(m·K)
- Thermal diffusivity, in mm²/s, required for transient thermal simulation of battery packs
- Specific heat capacity, in J/(kg·K), measured separately or derived during analysis
- Thermal resistance per unit area as a function of material thickness
- Through-thickness versus in-plane (anisotropic) thermal conductivity for fiber-reinforced and layered materials
- Thermal conductivity after electrolyte immersion aging, to quantify performance loss from solvent uptake
- Dimensional stability and mass change following immersion and thermal exposure
- Thermal conductivity retention after thermal cycling between low and high temperature extremes
Testing Methods
Guarded Hot Plate Method
The guarded hot plate method, performed in accordance with ASTM C177, ISO 8302, and GB/T 10294, is the absolute reference technique for thermal conductivity measurement. A horizontally oriented central metering section is sandwiched between two specimens, and guard heaters eliminate lateral heat losses. This method offers the lowest measurement uncertainty, often better than ±2 percent, and is suitable for mean temperatures from below ambient up to several hundred degrees Celsius.
Heat Flow Meter Method
The heat flow meter method, based on ASTM C518, ISO 8301, and GB/T 10295, is a comparative technique calibrated against certified reference materials. It is faster than the guarded hot plate method and well suited to routine quality control of blanket and board products at mean temperatures between roughly 0 °C and 100 °C. Typical specimen size is 300 mm × 300 mm, with thickness measured under a standard compressive load.
Laser Flash Analysis
Laser flash analysis, conducted per ASTM E1461 and ISO 22007-4, measures thermal diffusivity by applying a short laser pulse to one face of a small disc specimen, typically 12.7 mm in diameter, and recording the temperature response on the opposite face. When combined with independently measured density and specific heat capacity, thermal conductivity is calculated. The method accommodates a wide temperature range and is appropriate for dense materials such as mica sheets and ceramic components.
Transient Plane Source Method
The transient plane source method, standardized in ISO 22007-2, uses a spiral heater sensor clamped between two pieces of the test material. A controlled electrical pulse heats the sensor while its resistance change records the transient temperature response, from which thermal conductivity, diffusivity, and volumetric heat capacity are extracted simultaneously. This technique handles small, irregular, and compressible specimens and can measure in different directions to characterize anisotropy.
Electrolyte Immersion Conditioning
To simulate worst-case electrolyte leakage or spillage inside a battery pack, specimens are immersed in representative carbonate-based electrolyte, such as lithium hexafluorophosphate salt dissolved in ethylene carbonate and dimethyl carbonate solvents, under controlled temperature in an inert dry atmosphere. After defined exposure intervals, specimens are removed, surface liquid is blotted, and thermal conductivity is remeasured. Mass uptake and dimensional change are recorded to correlate solvent absorption with insulation performance degradation.
Applications
Thermal conductivity test data directly support battery safety design and regulatory compliance in several areas:
- Cell-to-cell thermal barrier design in prismatic, cylindrical, and pouch battery modules, where low-conductivity pads delay propagation of a single-cell thermal runaway event
- Thermal runaway propagation demonstration tests required by regulations and standards such as GB 38031, UL 2580, UL 9540A, IEC 62660, and IEC 62619
- Energy storage container and cabinet insulation, where barrier materials protect enclosure structures during failure events
- Thermal simulation and modeling of battery packs, where accurate conductivity, diffusivity, and specific heat inputs determine prediction fidelity
- Material qualification and incoming inspection, comparing production batches against validated reference values
- Comparative selection studies between aerogel, microporous, mica, and foam solutions for specific pack architectures and space envelopes
In addition to barrier applications, insulation characterization contributes to thermal interface design, where controlled conductivity materials balance heat dissipation during normal operation against heat blocking during abnormal events.
FAQ
What thermal conductivity value is considered effective for cell-to-cell insulation?
For inter-cell thermal barriers, materials with thermal conductivity below approximately 0.05 W/(m·K) are generally preferred. Silica aerogel blankets and microporous boards, in the range of 0.018 to 0.035 W/(m·K), are widely adopted because they combine low conductivity with high-temperature stability and thin installed profiles.
Which test method should be selected for a specific material?
The choice depends on material type, thickness, temperature range, and required accuracy. The guarded hot plate method provides the highest accuracy for thick, low-conductivity insulation. Heat flow meter instruments suit routine testing of blankets and boards near ambient temperature. Laser flash analysis is preferred for dense, thin, or high-temperature samples, while the transient plane source method offers flexibility for small, soft, or anisotropic specimens.
Does electrolyte exposure degrade insulation performance?
Yes. Electrolyte liquid absorbed into the pore structure of an insulation material replaces low-conductivity air with a higher-conductivity liquid, measurably increasing effective thermal conductivity. Immersion aging experiments quantify this increase and reveal whether the degradation stabilizes or progresses with continued exposure, information essential for worst-case safety analysis.
What specimen dimensions and conditioning are required?
Specimen dimensions follow the selected method, commonly 300 mm × 300 mm for heat flow meter tests, metering-area plates for guarded hot plate tests, and 12.7 mm discs for laser flash analysis. Specimens are conditioned at standard laboratory temperature and humidity until mass stabilization, and thickness is measured under the applied load specified by the relevant standard.
At which temperatures should conductivity be measured?
A typical test matrix includes mean temperatures of 25 °C for room temperature benchmarking, plus elevated points such as 100 °C, 200 °C, and 300 °C or higher to reflect the temperature excursion during thermal runaway. Low-temperature points may be added for cold-climate energy storage applications.
How are measurement uncertainty and repeatability handled?
Accredited third-party laboratories report measurement uncertainty budgets and typically test replicate specimens. Guarded hot plate results may achieve uncertainty near ±2 percent, while heat flow meter and transient methods generally fall within ±3 to 5 percent, which is sufficient for material comparison and modeling input.
