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 as well as operation conditions like current load, temperature and pressure. 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 different aging mechanisms are depending not only on the temperature level but also on temperature distributions and changes during the aging process. Various aging induced changes of the microstructure at the electrode level lead to altered effective thermal material properties, resulting in reduced heat transfer among other changes. For an optimized thermal management throughout the whole lifetime of the electric vehicle, the information about these changes is necessary. Additionally, the alterations of the thermal cell behavior will also directly impact the electrical and electrochemical performance of the battery system. To assign the occurring aging mechanisms to the effects on the heat transfer, it is crucial to measure the electrodes individually because of the different aging mechanisms within anode and cathode.
Therefore, in this work, the influence of different thermal aging conditions on the heat transfer of cathode and anode stacks is investigated on electrode level to gain more insight on the individual underlying mechanisms. With an inhouse experimental methodology for the thermal characterization at electrode level (electrode stack, current collector and coating) the effective thermal conductivity can be determined by measuring the specific heat capacity with differential scanning calorimetry, the density with a gas pycnometer and the temperature diffusivity with laser flash analysis, taking the heterogenous microstructure into account.
The main focus of this contribution is the influence of homogenous and inhomogeneous temperature distributions during cyclic aging on the effective thermal conductivity and the comparison of the EoL to the BoL. The thermal aging conditions for the investigated cells were chosen over a wide temperature range to trigger different aging mechanisms within the cells. The investigated cells consist thereby of graphite anodes and blend metal oxide cathodes. Several samples were taken from aged cells of different aging conditions at EoL and the corresponding effective thermal material properties are determined using the described methodology. In addition, the impact of the porosity and its changes through aging on the thermal transport properties is investigated and presented.