Experimental Investigation of the Influence of Different Thermal Aging Conditions on the Thermal Conductivity of Battery Electrodes
Authors: Pfeifer Leonie , Herberger Sabrina , Wetzel Thomas , Seegert Philipp 
Published: December 2024

A thermal management system is required to ensure the safe and optimized operation of a lithium-ion battery systems in electric vehicles. The design of the thermal management systems is thereby commonly based on the respective cell behavior only at the Begin of Life (BoL). During operation, various aging mechanisms occur on both the anode and cathode side, depending on the cell chemistry. These mechanisms change the properties of the battery cells. For a more profound understanding and design of thermal management systems the dependency of the thermal cell behavior on the State of Health (SoH) until the End of Life (EoL) is crucial.

The aging mechanisms themselves depend on the temperature during aging. The changes in the microstructure at the electrode level lead to changes in the effective thermal material properties, resulting in reduced heat transfer among other changes. Alterations of the thermal cell behavior will influence the electrical and electrochemical behavior. In literature, few studies have been carried out investigating the changes in the integral thermal conductivity of pouch cells on cell level, which have shown an influence of the SoH on the thermal conductivity (for example Tendera et al. 2023). However, from these studies it is not directly possible to assign the individual aging effects and the observed changes in thermal conductivity to the individual processes at the anode and cathode.

Therefore, in this work, the influence of different thermal aging conditions on the thermal conductivity of cathode and anode stacks is investigated on electrode level to gain more insight on the individual underlying mechanisms. An experimental method for the thermal characterization at electrode level (electrode stack, current collector and coating) was developed. In combination with measurements of the specific heat capacity with differential scanning calorimetry, the density with gas pycnometer and the temperature diffusivity with laser flash analysis, regarding the microstructure, the effective thermal conductivity can be determined.

The thermal aging conditions for the investigated cell were chosen over a wide temperature range (from 0 °C to 50 °C) to trigger different aging mechanisms within the cells. The investigated cell consists of a graphite anode and a NCA/LCO blend cathode. For lower temperatures, for example, lithium plating occurs increasingly on the anode side. At higher temperatures, particle cracks can occur in the cathode particles and side reactions resulting in cover layers are enhanced. In addition, various inhomogeneous temperature distributions were considered, such as the change in temperature during charging / discharging and inhomogeneous temperature fields over the geometric dimensions of the cell.

The main focus of this contribution is the influence of the inhomogeneous temperature distribution during aging and the comparison to homogenous aging conditions. The aging mechanisms are investigated through various post-mortem analysis methods and compared to the effective thermal conductivity for the different samples. Several samples are taken from aged cells of each aging condition and the corresponding effective thermal material properties are determined using the described methodology. Through comparison with the results of the post-mortem analysis and the homogenously aged samples the aging effects responsible for the changes of the thermal properties are investigated.