The characterisation of the thermal behaviour of Li-ion battery cells is crucial for safe, reliable, and long-term operation. Temperature variations within the battery cells have been reported to potentially induce mechanical stresses. The currently used simulation technologies and material models are mostly based on phenomenological observations of the whole cell and can only reproduce the complex behavior of battery cells under multiaxial mechanical and thermal load effects to a certain degree. To be able to predict the behaviour of the battery cell more accurately, detailed layer-resolved battery models should incorporate anisotropic mechanical and thermal material properties to account for the differences in behavior along different axes. However, such detailed models need comprehensive material data of the individual battery cell components.
The present study focuses on the measurement and evaluation of the thermo-physical properties of the anode and cathode layers of a Li-ion battery cell. Specifically, the in-plane coefficient of thermal expansion (CTE) and the thermal diffusivity of each layer are measured using the Dantec Q400 system and a Laser Flash Analyzer (LFA), respectively. The materials tested are directly extracted from a commercial 21700 Li-ion battery cell.
For the CTE evaluation, square-shaped samples measuring 20 × 20 mm2 are cut from both, the extracted anode and cathode foils. The strains in the x- and y-directions are then measured using the digital image correlation technique with a heating rate of 1 °K/min over a temperature range of -40 °C to 250 °C. This technique involves tracking surface displacements using digital images captured at different temperatures. The results demonstrate that the behavior exhibited varies depending on the location of the cut specimen in the CTE measurements conducted for the anode and cathode materials. To this end, a grid of nine samples has been directly extracted from the material foils of a commercial Li-ion battery cell, and the CTE of the respective material has been determined by a least-squares fit to the measured data of all samples.
For the thermal diffusivity measurements, the thickness of the samples cut from both the extracted anode and cathode foils was measured to be 0.1 mm. Thermal diffusivity in the thickness direction was measured using the LFA measurement technique over a temperature range of -40 °C to 250 °C. The LFA technique involves subjecting a sample to a laser pulse and measuring the resulting temperature rise on the opposite side, which is then used to calculate the thermal diffusivity. This technique has been used to evaluate the thermal diffusivity of both the anode and cathode foils as a function of temperature. The evaluation of the thermal diffusivity combined with density and specific heat capacity measurements, facilitates the determination of thermal conductivity in materials.