Thermal Conductivity Testing of Power Battery Materials for Electric Vehicles

Thermal conductivity testing of power battery materials is a critical element of electric vehicle thermal management, verifying that thermal interface materials, gap fillers, heat spreaders, and insulation components can dissipate heat efficiently across the battery pack. Accredited third-party laboratories perform these measurements in accordance with ISO, ASTM, IEC, GB, and UL standards, delivering the traceable thermal property data required for material selection, pack design validation, and safety compliance. This article explains the typical test samples, testing items, measurement methods, and engineering applications of thermal conductivity testing for EV power battery materials.

Technical Overview

Thermal management is one of the most decisive factors governing the performance, service life, and safety of lithium-ion power batteries in electric vehicles. Lithium-ion cells perform best within a narrow temperature window, generally between 15 °C and 40 °C, and temperature gradients within a pack accelerate uneven aging, reduce usable capacity, and in extreme cases can contribute to thermal runaway. Because the heat generated during fast charging and high-power discharge must be transferred rapidly from the cell surfaces to the cooling system, every material in the heat transfer path, including thermal interface materials, gap fillers, heat spreaders, potting compounds, and coolants, must be characterized with accurate and traceable thermal property data.

Thermal conductivity testing of power battery materials quantifies how efficiently a material conducts heat, expressed as the thermal conductivity coefficient λ in W/(m·K). A complete characterization normally also covers thermal diffusivity, specific heat capacity, thermal impedance, and thermal contact resistance, because these parameters jointly determine real-world heat transfer behavior in modules and packs. Due to the strong anisotropy of materials such as graphite sheets and the significant temperature dependence of polymer-based interface materials, professional testing must evaluate both in-plane and through-plane directions across the full operating temperature range of the vehicle, commonly from -40 °C to 150 °C.

Accredited third-party laboratories conduct these measurements in accordance with standards published by ISO, ASTM, IEC, GB, and UL. The resulting data supports cell and pack level thermal simulation, cooling system design, material qualification, incoming quality inspection, and compliance demonstration for battery safety regulations such as GB 38031 and UL 2580.

Testing Samples

A wide variety of materials used throughout the cell, module, and pack are submitted for thermal conductivity characterization. The most common sample categories are listed below.

Thermal Interface Materials (TIMs)

Thermal greases, gels, films, and thermally conductive adhesives are applied between cylindrical, prismatic, or pouch cells and cooling plates to minimize interfacial air gaps. These silicone or non-silicone materials are typically filled with alumina, aluminum nitride, or boron nitride particles and exhibit thermal conductivity values ranging from approximately 0.5 to 6 W/(m·K).

Thermal Gap Fillers and Thermal Pads

Cured silicone or non-silicone pads and two-component gap fillers bridge the tolerance gaps between cells and cold plates while providing electrical insulation. Their compressibility and thickness significantly influence the measured thermal impedance, so testing is often performed at defined clamping pressures representative of pack assembly conditions.

Graphite Sheets and Heat Spreaders

Natural and synthetic graphite films are widely used for lateral heat spreading. These materials are strongly anisotropic, with in-plane thermal conductivity values that can reach several hundred W/(m·K) while the through-plane value is typically only 5 to 20 W/(m·K), which makes directional testing essential.

Phase Change Materials

Paraffin-based composite phase change materials absorb transient heat spikes during high-power operation. Characterization covers thermal conductivity, latent heat, and specific heat capacity across the phase transition temperature range.

Insulation and Thermal Barrier Materials

Mica sheets, ceramic fiber boards, aerogel blankets, and other fire-resistant barriers are placed between cells to delay or prevent cell-to-cell thermal runaway propagation. These materials are intentionally low in thermal conductivity, commonly between 0.01 and 0.2 W/(m·K), and require highly sensitive low-conductivity measurement techniques.

Encapsulation, Potting, and Structural Materials

Epoxy, polyurethane, and silicone potting compounds, thermally conductive structural adhesives, battery housing aluminum alloys, steels, and sheet molding compound enclosure materials are tested to validate their role in the heat rejection path and in pack-level heat dissipation.

Coolants and Thermal Fluids

Water–ethylene glycol mixtures, refrigerants, and dielectric immersion cooling fluids are characterized for thermal conductivity, specific heat capacity, and thermal diffusivity as functions of temperature and concentration, since these properties directly determine the heat carrying capacity of the cooling loop.

Testing Items

Depending on the material type and the engineering requirement, thermal characterization of power battery materials typically includes the following test items.

• Thermal conductivity (λ), in W/(m·K), measured in through-plane and in-plane directions for anisotropic materials
• Thermal diffusivity (α), in mm²/s, describing the rate of heat propagation through the material
• Specific heat capacity (cp), in J/(g·K), determined by differential scanning calorimetry
• Volumetric heat capacity, derived from density and specific heat capacity
• Thermal impedance and apparent thermal conductivity of interface materials at defined thickness and clamping pressure
• Thermal contact resistance at material interfaces under simulated assembly loads
• Temperature-dependent property curves from -40 °C to 150 °C, or wider ranges where required
• Thermal stability and repeatability assessment after thermal cycling or aging exposure

Testing Methods

Measurement techniques are divided into steady-state methods, which establish a constant temperature gradient, and transient methods, which analyze the time-dependent thermal response. Method selection depends on the conductivity range, geometry, and anisotropy of the sample, and each technique is anchored to recognized ISO, ASTM, or GB standards.

Laser Flash Analysis (LFA)

The laser flash method, standardized in ASTM E1461, ISO 22007-4, and GB/T 22588, applies a short laser pulse to one face of a disc-shaped specimen and records the temperature rise on the opposite face with an infrared detector. The measurement yields thermal diffusivity, and thermal conductivity is calculated from λ = α · ρ · cp using independently measured density and specific heat capacity. The technique covers materials from 0.1 to more than 1000 W/(m·K) across a very wide temperature range and is particularly suitable for graphite sheets, metals, ceramics, and filled polymer plaques.

Transient Plane Source (Hot Disk) Method

Specified in ISO 22007-2 and GB/T 32064, the transient plane source method uses a double-spiral nickel sensor placed between two pieces of the sample. A controlled electrical heating pulse generates a three-dimensional temperature field, and the resistance change of the sensor allows simultaneous calculation of thermal conductivity and diffusivity. The method covers 0.005 to 500 W/(m·K) and offers dedicated measurement modes for anisotropic materials, making it well suited to gap fillers, elastomer pads, potting compounds, and insulation materials.

Guarded Heater Method for Interface Materials (ASTM D5470)

ASTM D5470 is the industry reference method for thermal interface materials used in electronics and battery cooling. A meter bar stack applies a controlled heat flow through the material under a defined compression load, and the measured total thermal impedance is separated into intrinsic material resistance and contact resistance by testing at multiple thicknesses. This method directly reproduces the installed condition of a TIM between a cell and a cold plate and provides the thermal impedance data required by pack designers.

Transient Hot Wire Method

The transient hot wire method, applied to fluids under ASTM D7896 and to solids and refractory materials under ISO 8894, measures the temperature response of a thin heated wire immersed in the sample. It is the preferred technique for coolants, oils, and immersion dielectric fluids because it suppresses natural convection effects through rapid measurement, providing accurate liquid thermal conductivity as a function of temperature.

Heat Flow Meter and Guarded Hot Plate Methods

For low-conductivity insulation and thermal barrier materials, the heat flow meter method (ASTM C518, ISO 8301, GB/T 10295) and the guarded hot plate method (ASTM C177, ISO 8302, GB/T 10294) provide precise steady-state measurements in the range of roughly 0.002 to 2 W/(m·K). These methods are essential for mica sheets, aerogel blankets, and enclosure insulation used in thermal runaway mitigation structures.

Guarded Comparative Longitudinal Heat Flow Method

ASTM E1225 establishes one-dimensional heat flow through a sample sandwiched between reference meter bars of known conductivity. It is suitable for moderately conductive solids such as filled polymers, composites, and ceramics in the approximate range of 0.2 to 200 W/(m·K), including potting resins and structural components.

Specific Heat Capacity by Differential Scanning Calorimetry

Specific heat capacity is determined by DSC in accordance with ASTM E1269, ISO 11357-4, and GB/T 19466. This value is a mandatory input for converting laser flash diffusivity results into thermal conductivity and is itself required for transient thermal simulation of battery modules.

Applications

Thermal conductivity test data for power battery materials are applied at every stage of the electric vehicle battery value chain, from material development to pack certification. Typical applications include the following.

• Selection and qualification of thermal interface materials, gap fillers, and heat spreaders during cell-to-pack and module design
• Generation of accurate material property inputs for cell and pack level thermal simulation and computational fluid dynamics models
• Cooling system sizing and validation for liquid cooling plates, immersion cooling systems, and refrigerant direct cooling architectures
• Verification of thermal runaway propagation mitigation measures in support of GB 38031 and UL 2580 safety demonstrations
• Incoming material inspection and supplier datasheet verification for mass production quality control
• Research and development of highly filled polymers, graphite composites, and phase change material formulations
• Comparative benchmarking of candidate materials under identical temperature and pressure conditions

FAQ

Q1: What thermal conductivity values are typical for materials used in EV battery packs?

A: Thermal greases and gap fillers generally range from 1 to 6 W/(m·K), advanced filled pads can exceed 10 W/(m·K), graphite sheets reach several hundred W/(m·K) in-plane but only 5 to 20 W/(m·K) through-plane, and thermal barrier insulation materials are deliberately low at approximately 0.01 to 0.2 W/(m·K).

Q2: Which method should be used for thermal interface materials?

A: ASTM D5470 is preferred for interface materials because it measures total thermal impedance and contact resistance under realistic clamping pressure. Laser flash analysis or the transient plane source method is recommended when the intrinsic conductivity of the bulk material itself is required.

Q3: What sample size and preparation are required?

A: Requirements depend on the method. Laser flash analysis typically uses flat discs of 10 to 12.7 mm diameter, transient plane source testing requires two plane-parallel pieces of defined dimensions, and ASTM D5470 uses samples matching the meter bar cross-section at several thicknesses. Samples must be dry, flat, and free of defects, and laboratories issue specific sampling instructions before testing.

Q4: Can testing be performed across the vehicle operating temperature range?

A: Yes. Standard temperature sweeps from -40 °C to 150 °C are routinely performed, and laser flash instruments can cover substantially wider ranges. Temperature-dependent data are strongly recommended because polymer-based interface materials can change conductivity by 20 percent or more across this range.

Q5: What is the difference between thermal conductivity and thermal diffusivity?

A: Thermal conductivity quantifies the steady-state heat flow through a material, while thermal diffusivity describes how quickly a temperature disturbance propagates and governs transient response during fast charging. The two are linked by the relationship λ = α · ρ · cp, where ρ is density and cp is specific heat capacity.

Q6: How should a testing provider be selected?

A: Projects should be placed with accredited third-party laboratories operating under an ISO/IEC 17025 quality management system, with validated instrumentation, documented method uncertainty, and experience in battery materials, so that results are traceable and accepted by vehicle manufacturers and regulatory authorities.