Thermal Conductivity Testing of Phase Change Materials for Power Batteries: Methods, Standards, and

Thermal conductivity testing of phase change materials (PCMs) for power batteries is essential for evaluating how effectively these materials absorb, store, and dissipate heat within electric vehicle battery modules. Accurate measurement of thermal conductivity, diffusivity, and specific heat capacity helps battery manufacturers select and validate PCM-based thermal management solutions that control cell temperature, suppress temperature gradients, and delay thermal runaway propagation. This article provides a comprehensive technical overview of sample types, testing items, measurement methods, applicable standards, and industry applications for power battery phase change material thermal conductivity evaluation.

Technical Overview

Phase change materials have become a key enabler of passive thermal management in lithium-ion power battery systems. By absorbing latent heat during melting and releasing it during solidification, PCMs buffer temperature spikes caused by high-rate discharge, fast charging, and ambient thermal loads. However, most pure phase change materials—particularly paraffin waxes—exhibit inherently low thermal conductivity, typically in the range of 0.15 to 0.25 W/(m·K). This limitation restricts heat penetration depth into the PCM body and reduces the effective utilization of its latent heat storage capacity.

To overcome this drawback, battery thermal engineers formulate composite PCMs by blending paraffin or other phase change substrates with high-conductivity fillers such as expanded graphite, graphene, carbon nanotubes, metal foams, or boron nitride. These composites can achieve thermal conductivity values from 1 W/(m·K) up to 16 W/(m·K) or higher, depending on filler loading and microstructure. Because thermal conductivity directly governs the transient thermal response of the battery module, it must be quantified through reliable, standardized laboratory measurement before a PCM can be qualified for production use.

Thermal conductivity testing of power battery phase change materials is typically performed by accredited third-party laboratories or in-house characterization centers using internationally recognized methods, including the transient plane source technique defined in ISO 22007-2, the laser flash method defined in ASTM E1461, and guarded heat flow meter techniques defined in ASTM E1530 and ASTM C518. Supporting measurements of specific heat capacity and phase change enthalpy are conducted in accordance with ASTM E1269 and ISO 11357. Together, these tests deliver a complete thermophysical property profile that supports thermal simulation, module design, and supplier qualification.

Testing Samples

Paraffin and Expanded Graphite Composite PCMs

Paraffin/expanded graphite composites are the most widely used shape-stabilized PCMs in power battery applications. The expanded graphite network provides structural support, prevents leakage of molten paraffin, and dramatically enhances thermal conductivity. Samples are typically supplied as plates, blocks, or discs with filler mass fractions between 5% and 30%. For transient plane source testing, flat-faced disc specimens with parallel surfaces are preferred, while laser flash testing requires thin circular discs, commonly 10 to 12.7 mm in diameter and 1 to 3 mm thick.

Shape-Stabilized PCM Sheets and Films

Flexible PCM sheets, films, and mats are installed between battery cells as thermal buffering layers. These products often combine a paraffin or fatty acid substrate with polymer binders and thermally conductive fillers. Specimens must be cut to the dimensions specified by the selected test method, with surface flatness and thickness uniformity tightly controlled, since air gaps between the sample and sensor can introduce significant measurement error.

Inorganic and Hydrated Salt PCMs

Salt hydrate PCMs, such as sodium sulfate decahydrate or calcium chloride hexahyrate blends, offer higher volumetric latent heat and sharper phase transitions. These materials may present subcooling, phase separation, and corrosion issues that affect measurement stability. Test specimens must be homogenized and conditioned before testing, and sealed containers are often required for liquid-state measurements.

PCM-Integrated Battery Components

Encapsulated PCM modules, heat sink plates impregnated with PCM, aerogel-PCM hybrid pads, and PCM-based thermal interface pads used in battery packs may also be evaluated. For such finished components, testing focuses on the effective through-plane and in-plane thermal conductivity, which reflects the influence of encapsulation shells, adhesives, and laminate layers on overall heat transfer performance.

Testing Items

A comprehensive thermophysical characterization program for power battery phase change materials typically includes the following parameters:

• Thermal conductivity in the solid state (below phase change temperature), expressed in W/(m·K)
• Thermal conductivity in the liquid or molten state (above phase change temperature)
• Thermal diffusivity, expressed in mm²/s, measured across the operating temperature range
• Specific heat capacity as a function of temperature, expressed in J/(g·K)
• Latent heat of fusion and phase change enthalpy, expressed in J/g
• Phase change onset, peak, and conclusion temperatures, expressed in °C
• Material density at solid and liquid states, required for converting diffusivity to conductivity
• Anisotropic conductivity characterization for graphite-based composites with oriented filler structures
• Thermal cycling stability, verifying conductivity and enthalpy retention after repeated melting and solidification

Among these items, thermal conductivity is the core deliverable, while thermal diffusivity, specific heat capacity, and density serve both as direct inputs to battery thermal models and as intermediate quantities used to calculate conductivity through indirect methods.

Testing Methods

Transient Plane Source Method (ISO 22007-2)

The transient plane source (TPS) technique, often referred to as the hot disk method, uses a double-spiral nickel sensor sandwiched between two pieces of the sample. A controlled electrical heating pulse raises the sensor temperature, and the transient temperature response is recorded and fitted to a heat conduction model to extract thermal conductivity and diffusivity simultaneously. The method covers a measurement range from approximately 0.005 to 500 W/(m·K), accommodates both solid and liquid samples, and requires minimal specimen preparation. It is particularly suitable for battery PCM composites because testing can be performed below and above the phase change temperature to capture the conductivity transition between solid and liquid states.

Laser Flash Method (ASTM E1461)

The laser flash method measures thermal diffusivity by applying a short energy pulse to the front face of a thin disc specimen and recording the temperature rise on the rear face with an infrared detector. Thermal conductivity is then calculated using the equation λ = α × ρ × Cp, where α is thermal diffusivity, ρ is density, and Cp is specific heat capacity. The method offers rapid measurement, excellent repeatability, and a wide temperature range from room temperature to high temperature, making it ideal for mapping diffusivity across the complete battery operating window. Accurate density and specific heat data, typically obtained by DSC and density measurement, are essential for reliable conductivity calculation.

Guarded Heat Flow Meter Method (ASTM E1530)

The guarded heat flow meter establishes steady-state one-dimensional heat flow through a disc specimen clamped between two temperature-controlled plates. The heat flux is measured by calibrated transducers, and thermal conductivity is derived from Fourier's law. This method is well suited for moderate-conductivity PCM sheets and composite plates in the range of approximately 0.1 to 20 W/(m·K). Testing above the melting point requires leakage-proof encapsulation of the specimen to prevent molten PCM from contaminating the instrument interfaces.

Heat Flow Meter Method (ASTM C518)

For thicker PCM boards, insulation-integrated PCM panels, and low-conductivity PCM formulations, the heat flow meter method provides steady-state conductivity measurement in accordance with ASTM C518. Two calibrated heat flux transducers monitor heat flow through the specimen placed between hot and cold plates, enabling accurate results for materials with conductivity below about 2 W/(m·K).

Differential Scanning Calorimetry (ASTM E1269 and ISO 11357)

While DSC does not measure conductivity directly, it supplies the specific heat capacity values required for laser flash calculations and delivers the phase change temperature and latent heat data that define the effective operating window of the PCM. DSC testing is performed on small-mass specimens, typically 5 to 15 mg, under nitrogen purge, with heating and cooling rates commonly set at 5 or 10 °C/min to resolve the phase transition envelope.

Applications

Thermal conductivity test data for battery phase change materials support a broad set of engineering and commercial activities across the new energy vehicle value chain:

• Cell-to-cell temperature uniformity control in EV battery modules, where PCM layers limit maximum temperature spread to within 3 to 5 °C
• Peak temperature suppression during DC fast charging, where latent heat absorption buffers transient heat generation from high C-rate charging
• Thermal runaway propagation mitigation, where high-conductivity PCM composites combined with insulating barriers delay heat transfer between adjacent cells
• Battery pack thermal simulation and modeling, where measured conductivity, diffusivity, and enthalpy data calibrate finite element and lumped-parameter thermal models
• Supplier qualification and incoming material inspection for PCM sheet and composite manufacturers
• Product certification support and export documentation for battery system manufacturers supplying global OEMs
• Research and development of next-generation form-stable PCMs, including graphene-enhanced and metal-foam-impregnated formulations
• Energy storage system (ESS) container thermal management, where PCM modules dampen ambient temperature swings for stationary battery racks

FAQ

Why is thermal conductivity so important for battery phase change materials?

Low-conductivity PCMs only absorb heat in a thin layer near the heat source, leaving the bulk of the latent heat unused. Enhancing and verifying thermal conductivity ensures that heat penetrates the entire PCM volume, maximizes latent heat utilization, and keeps cell temperatures within the optimal 20 to 40 °C window during demanding operating conditions.

Should thermal conductivity be tested in both solid and liquid states?

Yes. PCMs operate across their phase transition, and conductivity can change significantly after melting due to filler network relaxation and convection effects. Testing both below and above the phase change temperature provides designers with the full property profile needed for transient thermal simulation of real-world battery duty cycles.

How is thermal conductivity obtained from laser flash testing?

Laser flash testing directly yields thermal diffusivity. Conductivity is calculated by multiplying diffusivity by density and specific heat capacity, each measured separately. This is why a complete test package typically combines LFA diffusivity measurement, DSC specific heat measurement, and density determination.

What sample preparation is required for accurate results?

Specimens must have flat, parallel surfaces with uniform thickness, free of pores, cracks, and surface contamination. For anisotropic expanded graphite composites, the orientation of testing relative to the compression direction must be documented, since in-plane and through-plane conductivity can differ by a factor of several times.

How does thermal cycling affect measured conductivity?

Repeated melting and solidification can degrade the conductive filler network, cause filler agglomeration, or induce leakage in poorly stabilized formulations. Conducting conductivity verification after 100 to 1,000 thermal cycles confirms long-term reliability of the PCM over the battery pack service life.

Which method should be selected for a given PCM product?

Transient plane source testing is generally the most versatile choice for battery PCMs because it handles both states and a wide conductivity range. Laser flash is preferred when temperature-dependent diffusivity mapping is required, while guarded heat flow meter testing suits steady-state evaluation of sheet and plate products. Accredited third-party laboratories can recommend the optimal method based on material form, expected conductivity range, and target temperature window.