Table of Contents
Thermal behavior as the key to battery performance
The development of efficient and long-lasting batteries requires a detailed understanding of the thermophysical properties of cell components. Especially in the characterization of lithium iron phosphate (LFP), nickel manganese cobalt oxide (NMC) and solid electrolytes, the precise measurement of thermal parameters is crucial to understand and control aging mechanisms and efficiency losses during charge and discharge cycles. The Transient Hot Bridge (THB) method has proven to be the central method for the precise determination of the thermal conductivity, thermal diffusivity and specific heat capacity established.
The Transient Hot Bridge Method: Technical superiority
The THB-method improves the accuracy of measuring the thermal properties of cell components on several levels and offers decisive advantages over older methods such as the transient hot strip (THS) or classic heating wire methods. As an absolute measurement method, it requires no additional calibration or reference measurement, which eliminates systematic errors due to reference deviations (Linseis Messgeräte GmbH, 2024).
Technical structure and measuring principle
The sensor of the THB method is realized as a printed circuit foil made of nickel between two polyimide foils. The layout consists of four heating strips arranged in parallel and connected to form a Wheatstone bridge. At constant temperature, the bridge is inherently balanced, i.e. no calibration is required.
A particularly important advantage of the THB is the compensation of edge effects. While conventional heating wire methods are affected by heat losses via connections or edge areas, these edge effects are measured with the THB measurement method and can therefore be subtracted from the result.
The method covers a wide range of thermal conductivity measurements from 0.01 to 1000 W/(m*K) and complies with international standards such as ASTM D5930, ASTM D7896-19 and ISO 22007-2which ensures comparability and quality assurance. The short measuring times of often less than one minute are particularly advantageous.
Critical thermophysical parameters for battery cells
Several thermophysical parameters are of decisive importance for the ageing and efficiency of NMC and LFP cells during charging and discharging cycles:
The thermal conductivity determines how efficiently heat can be dissipated within the cell. High thermal conductivity ensures an even temperature distribution and prevents hotspots that can cause high local temperatures and thus accelerated ageing. Marconnet et al. (2024) show that the decrease in thermal conductivity due to ageing directly reduces the performance and safety of Li-ion batteries – in some cases by up to 75% after long high-temperature loads and several thousand cycles.
The specific heat capacity defines how much heat a cell component can absorb until the temperature rises. Materials with a high heat capacity buffer temperature fluctuations better and can therefore reduce cell damage during rapid charging/discharging processes. The specific heat capacity can change due to ageing and material fatigue and thus influences the temperature profiles during the cycle.
The thermal diffusivity indicates how quickly temperature changes spread through the material. Low thermal diffusivity leads to inertially changing temperature zones within the cell – particularly critical at high C rates, because in such cases dangerous temperature gradients can form, which promote locally accelerated ageing.
Practical application examples
Anode material characterization
A specific application example is the measurement of the thermal conductivity of anode material applied to a thin copper current collector. These measurements are important for the development, optimization and design of battery thermal management systems. The THB method makes it possible to characterize both the coating and the substrate material in their entirety.
Quality control in battery production
In industrial battery production, the THB method is used for continuous quality control of raw materials.
Development of new electrode materials
The method delivers results for solids and liquids as well as powders and pastes with high measurement accuracy, which makes it particularly valuable for the development of innovative electrode materials.
Material-specific considerations and ageing effects
LFP cells are known for their chemical stability and moderate temperature dependence, but microstructural damage to the electrode due to cyclic loading can significantly deteriorate the thermal conductivity and heat capacity. NMC cells often show a stronger temperature and aging dependence in their thermal properties, which places higher demands on thermal management and material characterization (Ali et al., 2023).
Solid electrolytes offer the potential for increased safety, but their sometimes low intrinsic thermal conductivity poses new challenges for temperature homogeneity and requires particularly precise and spatially resolved measurement methods such as THB. Steinhardt et al. (2022) experimentally confirm that strong temperature increases and gradients negatively influence both the ageing and the performance of the cells.
Comparison of methods: THB versus established methods
Comparison with Laser Flash Analysis (LFA)
The THB measurement method provides the thermal conductivity, thermal diffusivity and, if the density is known, the specific heat capacity. With the laser flash method only provides the thermal diffusivity. Furthermore, the measurements with the THB are very simple and can be carried out without prior knowledge. In addition, the measurements only take a few seconds to minutes.
The advantage of the LFA lies in the large temperature range from -150 to 2800°C that can be covered. The THB can be used in the temperature range from -150 to 700°C.
Advantages over conventional hot wire methods
Traditional hot wire methods suffer from edge effects and cable influences that can lead to systematic measurement errors. The THB eliminates these problems by:
- Measurement and compensation of boundary effects leads to significantly higher accuracies
- Bridge configuration greatly simplifies calibration and operation of the measuring device
Advantages over conventional hot wire methods
Traditional hot wire methods suffer from edge effects and cable influences that can lead to systematic measurement errors. The THB eliminates these problems by:
- Measurement and compensation of boundary effects leads to significantly higher accuracies
- Bridge configuration greatly simplifies calibration and operation of the measuring device
Significance for battery safety
Precise characterization of thermophysical properties is crucial for assessing battery safety. Regulatory authorities increasingly require detailed thermal models to predict behavior under abuse conditions. The standard-compliant THB method provides the necessary basic data for these safety assessments and contributes to the approval of battery products.
Conclusion for research and development
The Transient Hot Bridge method maximizes the accuracy of the measurement of thermal properties of cell components through calibration-free, boundary effect-compensated measurement, high material flexibility and short measurement times. Only through precise, reproducible measurements of all relevant thermophysical parameters can cell materials be efficiently evaluated today, new designs developed and quality standards guaranteed. For the characterization and optimization of modern battery materials – from electrodes and separators to solid electrolytes – it is therefore an indispensable tool in the laboratory and offers maximum precision and application flexibility specifically for the needs of modern battery research and development.
Bibliography
Ali, H. et al. (2023). “Assessment of the calendar aging of lithium-ion batteries for electric vehicle applications”. Frontiers in Energy Research.
Marconnet, A. et al. (2024). “Impact of aging on the thermophysical properties of lithium-ion battery electrodes”. Journal of Power Sources.
Hammerschmidt, U. “Transient Hot Bridge”. Physikalisch Technische Bundesanstalt Braunschweig.
Steinhardt, M. et al. (2022). “Experimental Investigation of the Thermal Conductivity of Lithium-Ion Battery Components”.
