Thermal conductivity testing of power batteries is a critical step in evaluating heat dissipation performance, validating thermal management designs, and ensuring the safety and service life of lithium-ion battery systems for electric vehicles and energy storage. By measuring the thermal conductivity, diffusivity, and specific heat capacity of cell materials, complete cells, and thermal interface components, manufacturers obtain the reliable data needed for simulation, material selection, and quality control. This article explains the sample types, testing items, standard test methods, and typical applications involved in power battery thermal conductivity testing.
Technical Overview
Thermal conductivity, usually denoted by the symbol λ (or k) and expressed in W/(m·K), quantifies the ability of a material to conduct heat. In power batteries, heat is continuously generated by internal resistance and electrochemical reactions during charging and discharging. If this heat is not conducted away efficiently, temperature gradients develop inside the cell, leading to non-uniform aging, localized lithium plating, accelerated capacity fade, and, in extreme cases, thermal runaway.
A distinctive feature of lithium-ion cells is their strongly anisotropic structure. Because cells are built from stacked or wound layers of electrode foils, separators, and electrolyte, the in-plane conductivity (parallel to the layers, dominated by the aluminum and copper current collectors) can reach tens of W/(m·K), while the through-plane conductivity (perpendicular to the layers) is typically below 2 W/(m·K). Accurate directional measurement of these properties is therefore essential for cell-level and pack-level thermal modeling and cooling system design.
Power battery thermal conductivity testing is generally performed by accredited third-party laboratories operating under ISO/IEC 17025 quality management systems, using internationally recognized methods published by ISO, IEC, ASTM, EN, GB, UL, and JIS standards organizations.
Testing Samples
Separator Films
Polyethylene (PE), polypropylene (PP), trilayer, and ceramic-coated separators, typically 10–30 µm thick. Because such thin, porous films are difficult to measure individually, laboratories usually test laminated multi-layer stacks and report an effective through-plane conductivity. Both dry and electrolyte-wetted conditions can be evaluated, since wetting significantly changes the effective value.
Electrode Sheets and Coatings
Cathode coatings such as NCM or LFP on aluminum foil, and anode coatings such as graphite or silicon-based composites on copper foil. Both through-plane and in-plane values are required: the through-plane direction is governed by the porous coating, while the in-plane direction is dominated by the metal collectors.
Liquid, Gel, and Solid Electrolytes
Carbonate-based liquid electrolytes, gel polymer electrolytes, and oxide or sulfide solid-state electrolytes. Liquid samples must be tested in sealed cells using transient methods with short measurement times to suppress convection effects.
Complete Battery Cells
Pouch, prismatic, and cylindrical cells can be characterized at their actual dimensions to obtain effective in-plane and through-plane conductivities. These effective values are directly usable in pack-level thermal simulation and cooling design.
Thermal Interface Materials
Gap fillers, thermal pads, greases, phase-change sheets, and thermally conductive adhesives installed between cells, busbars, and cooling plates. Their bulk conductivity and contact resistance under realistic clamping pressure are key evaluation targets.
Insulation, Packaging, and Cooling Components
Mica sheets, aerogel blankets, ceramic fiber boards, foam insulation, cell housings, potting compounds, and aluminum alloy cold-plate materials used in battery packs and modules.
Testing Items
Typical test parameters reported for power battery materials and components include:
- Through-plane (cross-plane) thermal conductivity, W/(m·K)
- In-plane (lateral) thermal conductivity, W/(m·K)
- Thermal diffusivity, mm²/s
- Specific heat capacity, J/(g·K)
- Volumetric heat capacity, J/(m³·K)
- Thermal resistance and thermal impedance for interface materials, K·cm²/W
- Temperature-dependent property curves over ranges such as −40 °C to 200 °C or higher
Testing Methods
Laser Flash Method (LFA)
Conducted according to ASTM E1461 and ISO 22007-4, a short laser pulse heats the front face of a disc-shaped sample, typically 10–12.7 mm in diameter and 1–3 mm thick, while an infrared detector records the rear-face temperature rise to determine thermal diffusivity. Thermal conductivity is then calculated as λ = α × ρ × cp, where density is obtained from geometry and mass, and specific heat capacity is measured in the same run or separately by DSC per ASTM E1269. The method covers a wide temperature range, requires only small samples, and is the preferred technique for electrode coatings, separator stacks, ceramics, and solids.
Transient Plane Source (TPS) Method
Specified in ISO 22007-2, a double-spiral sensor is sandwiched between two pieces of the sample and acts simultaneously as heater and resistance thermometer. The method suits solids, powders, and viscous materials, and dedicated anisotropic modules provide in-plane and through-plane values in a single measurement. It is widely used for complete cells, elastomers, gap fillers, and composites.
Heat Flow Meter Method
Based on ASTM C518, ISO 8301, and the corresponding GB/T standards, this steady-state comparative method uses calibrated heat flux transducers. It is best suited to low-conductivity insulation such as aerogel blankets, mica sheets, foams, and cellular materials with thicknesses of roughly 10–100 mm.
Guarded Hot Plate Method
Described in ASTM C177, ISO 8302, and GB/T 10294, this is the absolute reference method for thermal resistance and conductivity of insulation materials, offering the highest accuracy for low-conductivity samples.
Guarded Heat Flow Meter Method
Following ASTM E1530, this steady-state technique covers moderate conductivity ranges, typically 0.1–40 W/(m·K), and is commonly applied to polymers, rubbers, and composite plates in disc form.
Transient Hot-Wire and Line-Source Methods
A thin heated wire immersed in a sealed liquid cell measures the temperature response to determine conductivity, diffusivity, and volumetric heat capacity of liquid electrolytes and thermal greases. Short test durations minimize convection. Relevant standards include ASTM D7896 for liquids and the transient line-source procedures of ASTM D5334 and ASTM D5930 for other material classes.
ASTM D5470 for Thermal Interface Materials
This steady-state method measures thermal impedance as a function of thickness and applied clamping pressure, separating bulk conductivity from contact resistance. It is essential for qualifying gap fillers and pads installed between cells and liquid cooling plates.
Applications
Thermal conductivity data generated by accredited third-party laboratories directly supports the design, validation, and certification of power battery systems:
- Cell- and pack-level thermal modeling, providing verified inputs for CFD and FEA simulation and equivalent RC thermal networks
- Cooling system design, including cold-plate sizing, coolant flow optimization, and thermal interface material selection
- Fast-charging development, enabling accurate prediction of temperature rise and avoidance of lithium plating
- Thermal runaway propagation studies and the selection of barrier materials such as mica and aerogel
- Material qualification, incoming inspection, and supplier comparison for separators, coatings, electrolytes, and interface materials
- Regulatory and customer documentation demonstrating compliance with ISO, IEC, ASTM, EN, GB, UL, and JIS requirements
FAQ
What are typical thermal conductivity values for power battery materials?
Indicative ranges include: organic liquid electrolytes about 0.1–0.5 W/(m·K); dry PE/PP separators roughly 0.15–0.35 W/(m·K) through-plane; electrode coatings about 0.5–2 W/(m·K); complete cells commonly 0.2–1.5 W/(m·K) through-plane and 10–30 W/(m·K) in-plane; gap fillers 1–8 W/(m·K); and aerogel insulation about 0.02 W/(m·K). Actual values depend on formulation, porosity, wetting state, state of charge, and temperature, which is why measurement is necessary.
How is the anisotropy of battery cells handled in testing?
Laboratories address anisotropy by testing samples cut in orthogonal orientations with the laser flash method, by using the anisotropic module of the transient plane source method, or by preparing stacked samples for through-plane measurement. Both directional values should be reported for simulation work.
Can complete battery cells be tested, or only materials?
Complete pouch, prismatic, and cylindrical cells can be tested directly, mainly with the transient plane source method or custom steady-state fixtures. The results are effective values that include the influence of internal layers, electrolyte, and casing, and are directly applicable to pack design.
What sample preparation is required?
Requirements depend on the method: laser flash testing needs flat, parallel-faced discs with an opaque coating; heat flow meter testing requires larger flat plates; liquid electrolytes are sealed in specialized cells; and wet separator or electrode samples require controlled wetting and conditioning. Detailed preparation instructions are provided by the testing laboratory before submission.
What temperature range and measurement uncertainty can be expected?
Most methods cover −40 °C to 200 °C or 300 °C, and laser flash systems can extend to 1000 °C or beyond. Typical measurement uncertainty is about 3–7% depending on the material, conductivity level, and method, and accredited laboratories report uncertainties in accordance with ISO/IEC 17025.
How do I select the appropriate test method?
Method selection depends on the sample type, expected conductivity range, geometry, anisotropy requirements, and target temperature. As a general guide, laser flash is used for solids and coatings, transient plane source for cells and composites, heat flow meter methods for insulation, transient hot-wire for liquids, and ASTM D5470 for interface materials. Accredited third-party laboratories can recommend the optimal combination for a specific development or certification project.
