Table of contents
Why thermal diffusivity is more than just a material parameter
Thermal diffusivity α describes how quickly a temperature disturbance spreads in a material. Via the relationship λ = α – ρ – cₚ, it is directly linked to the thermal conductivity and thus determines in lithium-ion cells whether locally generated heat – for example due to side reactions, current density nests or local overcharging – is dissipated quickly or builds up to a dangerous hot spot. Numerical 3D models of thermal runtime show that even moderate inhomogeneities of thermal diffusivity at electrode and separator level can lead to highly localized temperature peaks [Oehler et al., 2021; Cloos et al., 2024]. For cell architecture, this means that the distribution of thermal diffusivity across layer thickness, surface direction and transitions between layers is at least as important as the absolute value of an individual material.
An illustrative practical example is the combination of highly conductive current collectors with significantly less conductive active mass layers. If the diffusivity in the graphite coating is significantly lower than in the collector, a pronounced temperature gradient forms within the anode at high C rates, which favors local lithium plating and degradation [Gandert et al., 2025]. Conversely, selectively increased diffusivity or thermally conductive additives can mitigate temperature peaks at critical points – provided they are sensibly integrated into the overall design.
Graphite anodes: Anisotropy as opportunity and risk
Graphite anodes are thermally anisotropic: in-plane – along the layer plane – the thermal conductivity and thus the thermal diffusivity is significantly higher than through the layer thickness, which has a direct effect on the propagation of hot spots. Measurements on commercial NMC/graphite cells show that the effective diffusivity value of the anode coating is not determined solely by the graphite, but essentially by the binder, conductive soot, porosity and contact with the copper collector [Cloos et al., 2024; Oehler et al., 2021]. It follows from this: The microstructural design of the electrode layer – particle sizes, degree of filling, pore network – is a lever to control the heat propagation in a targeted manner without necessarily worsening the electrochemical performance.
Operando studies show that even mild local temperature increases in graphite composites can change the lithium behavior and lead to local Li leakage from LiₓC₆ phases or to underpotential plating [Wang et al., 2022; Alujjage et al., 2025]. In combination with limited thermal diffusivity, self-reinforcing hot spots arise: Increased temperature accelerates side reactions, these generate additional heat that remains trapped locally due to lack of fast diffusion. Thermal diffusivity of the anode is therefore not only a safety parameter, but also a degradation parameter that must be taken into account in fast charging strategies and service life models.
Separators: Thermal bottleneck with safety potential
Separators typically have a significantly lower thermal diffusivity than electrodes and current arresters and therefore often represent the thermal bottleneck in the cell cross-section. As a result, they can amplify temperature differences between the electrode sides; at the same time, modern separator concepts deliberately act as a “thermal fuse”, for example through targeted pore closure at defined temperatures. Current work on so-called smart thermal shutdown separators shows that the combination of low base diffusivity and specifically increased thermal conductivity through ceramic fillers – for example boron nitride (BN) – can mitigate local hot spots while maintaining electrochemical function during normal operation [Li et al., 2025; Liu et al., 2021].
It is crucial not to consider separators in isolation, but in combination with the anode, cathode and electrolyte. Studies indicate that the interplay of separator diffusivity, electrode diffusivity and contact resistances determines the hot spot position – for example, whether critical zones tend to form in the electrode volume or in the vicinity of the separator [Gandert et al., 2025]. The surface emissivity of the separator and electrode surfaces also directly influences the sensitivity of imaging detection methods such as lock-in or IR thermography.
Hot spot detection: Operando metrology meets material characterization
For a reliable hot spot analysis, it is not enough to simply measure the external temperature of a cylinder or pouch cell. Spatially resolved temperature information and reliable material data are crucial. Operando IR thermography in combination with physics-based models makes it possible to derive internal temperature fields and quantify hot spots – provided the thermal diffusivity of the individual cell components is known [Wang et al., 2022]. New thermal wave sensors specifically use frequency-dependent thermal diffusion to draw conclusions about degradation states and local changes in thermal properties from the response to modulated thermal excitation.
A recent study on internal temperature evolution in Li-ion cells shows that the discrepancy between internal and external temperature measurement under operating conditions can be considerable and that hot spots and lithium plating on graphite anodes can only be fully quantified in this way [Alujjage et al., 2025]. Not only the absolute temperature level, but also the temporal development with known thermal diffusivity provides valuable information about local defects, inhomogeneities or aging zones. The coupling of operando measurement methods with experimentally determined diffusivities is therefore an effective tool for detecting weak points in the cell architecture as early as the material and cell concept phase.
Cell format and thermal diffusivity: round cell, pouch and prismatic in comparison
Thermal diffusivity has fundamentally different effects depending on the cell format – with direct consequences for the design of the thermal management system and the susceptibility to hot spots.
In round cells (18650, 21700), a pronounced anisotropy between axial and radial direction dominates. Anisotropic thermal conductivities of 0.20 W-m-¹-°C-¹ in the radial direction and up to 30.4 W-m-¹-°C-¹ in the axial direction have been measured for 18650 round cells. Heat generated in the cell core is therefore preferentially dissipated axially, while radial transport – in the direction of the cell surface and the cooling system – is strongly inhibited. At high C-rates, this results in considerable temperature gradients between the core and the cladding, which cannot be detected with pure external temperature measurement [Gandert et al., 2025].
Pouch cells have complementary characteristics: Pouch cells have inherently good in-plane heat dissipation due to their large surface area and flat design. However, as heat dissipation in the through-plane direction is less homogeneous, temperature gradients and hot spots can occur – particularly pronounced during fast charging. The thermal characterization of pouch cells therefore requires methods that capture both directions – laser flash analysis on representative layer stacks provides the most reliable input data for simulation models [Lin et al., 2022; Cloos et al., 2024].
Prismatic cells combine elements of both geometries. In prismatic and pouch cells, the thermal conductivity is decomposed along length, height and layer thickness, while in cylindrical geometries a decomposition in radial and axial direction is more appropriate. Here too, the through-plane diffusivity – perpendicular to the electrode layers – represents the dominant thermal bottleneck [Oehler et al., 2021].
This results in a clear requirement for measurement technology: a single scalar diffusivity measurement is not sufficient for any of these formats. Only the complete anisotropic characterization of realistic layer systems over the relevant temperature range provides the input parameters for reliable thermal simulations and hot-spot predictions [Gandert et al., 2025; Cloos et al., 2024].
Measurement technology: Flash analysis as the basis for realistic material parameters
A robust method for measuring the thermal diffusivity of graphite anodes, separators and composite structures is essential for use in R&D and quality assurance. An established approach is laser flash analysis (LFA): A short energy pulse heats a sample surface, and the temperature rise over time on the opposite side is recorded using an IR detector, from which the thermal diffusivity can be calculated [Balaji et al., 2024]. The combination with density and specific heat capacity then results in the thermal conductivity – the central input parameter for thermal simulation models.
For battery-relevant materials, it is important to examine not only bulk samples, but also realistic configurations: Graphite coatings on copper, separator foils or composite electrode stacks. Studies show that the effective thermal diffusivity of an electrode composite deviates significantly from the ideal value of pure graphite – in particular due to the interface with the copper foil and the distribution of polymeric and conductive additives [Cloos et al., 2024; Gandert et al., 2025].
Strategic consequences for battery development
For developers of cell architectures, there is a clear action plan: thermal diffusivity should be considered early in the material selection process – especially for graphite anode formulations and separator concepts. Anisotropies can be used in a targeted manner, for example through high in-plane diffusivity for lateral heat dissipation; at the same time, gradients through the layer thickness must be verified by measurement and modeling [Oehler et al., 2021]. Material and cell models should be systematically fed with experimentally determined diffusivity values in order to derive realistic temperature fields and thermal runaway scenarios. Operando methods – IR thermography, thermal waves, internal sensors – only develop their full potential in combination with exact thermophysical data: Hot spots thus become not only qualitatively visible, but also quantitatively assessable [Alujjage et al., 2025].
Thermal diffusivity is thus being transformed from an often neglected material parameter into a strategic development parameter that can be used to increase safety margins, extend fast-charging windows and mitigate degradation mechanisms in graphite anodes and separators at an early stage.
Bibliography
[Alujjage et al, 2025] Alujjage, N. et al: Internal Temperature Evolution Metrology and Analytics inLi-IonCells. Advanced Functional Materials, 2025. DOI: 10.1002/adfm.202417273 https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202417273
[Balaji et al, 2024] Balaji, C. et al: Thermal Transport and Thermal Diffusivity by Laser Flash Technique: A Review. International Journal of Thermophysics, 2024. DOI: 10.1007/s10765-024-03479-0 https://www.researchgate.net/publication/387526329_Thermal_Transport_and_Thermal_Diffusivity_by_Laser_Flash_Technique_A_Review
[Cloos et al., 2024] Cloos, L.; Herberger, S.; Queisser, O. et al.: Thermal Material Properties of Commercial NMC532 / Graphite Lithium-Ion Battery Cell. Karlsruhe Institute of Technology (KIT), 2024. DOI: 10.35097/kAlrZQzUaHBxWkIj https://publikationen.bibliothek.kit.edu/1000171382
[Gandert et al., 2025] Gandert, J. C.; Müller, M.; Paarmann, S.; Queisser, O.; Wetzel, T.: Challenges of the Measurement of the Effective Thermal Conductivity of Battery Electrodes with Laser Flash Analysis and Guarded Hot Plate Method. Energy Technology, 2025. DOI: 10.1002/ente.202501125 https://onlinelibrary.wiley.com/doi/10.1002/ente.202501125
[Li et al., 2025] Li, Y. et al.: Smart thermal-shutdown separators with fast response for safe Li-metal batteries. ScienceDirect / Journal of Power Sources, 2025. https://www.sciencedirect.com/science/article/pii/S3050914925000962
[Lin et al, 2022] Lin, J.; Chu, H. N.; Monroe, C. W.; Howey, D. A.: Anisotropic Thermal Characterization of Large-Format Lithium-Ion Pouch Cells. Batteries & Supercaps, 5, e202100401, 2022. DOI: 10.1002/batt.202100401 https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/batt.202100401
[Liu et al., 2021] Liu, W. et al: SaferLithium-IonBatteries from the Separator Aspect: Development and Future Perspectives. Energy & Environmental Materials, 2021. DOI: 10.1002/eem2.12129 https://onlinelibrary.wiley.com/doi/full/10.1002/eem2.12129
[Oehler et al., 2021] Oehler, D.; Seegert, P.; Wetzel, T.: Investigation of the Effective Thermal Conductivity of Cell Stacks ofLi-IonBatteries. Energy Technology, 2021. DOI: 10.1002/ente.202000722 https://onlinelibrary.wiley.com/doi/full/10.1002/ente.202000722
[Wang et al, 2022] Wang, W. et al: In-situ thermography revealing the evolution of internal short circuit of lithium-ion batteries. Journal of Power Sources, 2022. DOI: 10.1016/j.jpowsour.2022.231602 https://www.sciencedirect.com/science/article/abs/pii/S037877532200605X
