The displacement of a commercial pouch cell during discharge is modeled and validated using a thermally coupled physico-chemical model. To achieve high accuracy, we derive five different profiles for a concentration-dependent volume change in the graphite particles from literature. Based on the particle volume changes in the positive and the negative electrodes, the pouch cell displacement is calculated and compared to experimental data for all derived graphite characteristics at a low current discharge scenario. One graphite profile is chosen for investigating the pouch cell displacement at higher discharge rates. The thermal expansion of the pouch cell is determined by pulse excitation measurements to distinguish between thermal displacement and intercalation displacement. Our model is capable of describing the cell potential, the cell temperature and the cell displacement with high accuracy. Even the characteristic displacement plateau, which vanishes at higher discharge rates, can be represented. The plateau corresponds to the graphite staging behavior. The effect of the vanishing plateau with higher discharge rates is ascribed to concentration gradients within the graphite electrode. This observation is verified by investigating the displacement relaxation after a constant current discharge at 2C for 1200 s.

No abstract

Inhomogeneity within lithium-ion battery modules can occur due to variations in capacity and impedance of the connected cells as well as due to thermal gradients or cell connector design. We present a model for describing xSyP battery modules during operation, which is able to study these effects. The multidimensional multiphysics model includes a physicochemical model describing the electrochemical behavior of each cell. The electrical model accounts for the conservation of electric charge and energy between the cells to reach electrical consistency according to the respective module topology and cell interconnections. The model is capable of investigating the influence of defective and asymmetric cell connectors on the inhomogeneity of module operation. To evaluate this electrical influence, the observed inhomogeneities are compared to the influence of thermal gradients between the cells. The resulting inhomogeneous current distribution is presented for a module of two parallel connected lithium iron phosphate-graphite cells under constant current discharge operation for variations in cell capacity, cell impedance and ambient temperature at different module contact scenarios. From the observed impact of both, electrical and thermal variations between parallel connected cells, a matching strategy is derived and discussed which can enhance a module's performance during e.g. second life applications.

Thermal impedance spectroscopy (TIS) is a non-destructive method for characterizing thermal properties of entire battery cells. Heat capacity, thermal conductivity and heat exchange with environment are determined by an evaluation of the heat transfer behavior of the battery. TIS measurements are usually conducted with contact-based temperature sensors, such as thermocouples or thermistors, which show drawbacks at higher convection rates and higher temperature differences between battery and environment.To elude drawbacks in these kinds of sensors, an infrared-based temperature sensor system for battery surface temperature measurements is implemented. TIS measurements are conducted with this sensor system and with conventional, contact-based temperature sensors. Accuracy and reliability of thermal parameter identification is analyzed for the different sensor systems. Moreover, thermal parameters are identified for different cylindrical 18650 Li-ion cells with capacities between 1.1 Ah and 2.7 Ah.The comparison of different types of temperature sensors shows that contact-based sensors underestimate surface temperatures even at low temperature differences to environment. This causes an error in thermal parameter identification. The TIS measurements performed with contact-based sensors show divergence of 20 -60 % for heat capacity, 30 -70 % for thermal conductivity and 20 -60 % for convective heat exchange with environment.With our IR temperature sensor system, parameter identification is performed for different batteries. Resulting values for specific heat capacity are in a range between 900 and 1020 J/kgK and thermal conductivities in radial direction lies between 3.1 and 3.6 W/mK.Our investigations show that IR-based temperature sensors are an effective progression for TIS measurements and improve quality of parameter identification at low cost. Moreover, discrepancies mentioned in TIS literature can be explained by our findings.

Thermal impedance spectroscopy (TIS) is a non-destructive method for characterizing thermal properties of entire battery cells. Heat capacity, thermal conductivity and heat exchange with environment are determined by an evaluation of the heat transfer behavior of the battery. TIS measurements are usually conducted with contact-based temperature sensors, such as thermocouples or thermistors, which show drawbacks at higher convection rates and higher temperature differences between battery and environment.To elude drawbacks in these kinds of sensors, an infrared-based temperature sensor system for battery surface temperature measurements is implemented. TIS measurements are conducted with this sensor system and with conventional, contact-based temperature sensors. Accuracy and reliability of thermal parameter identification is analyzed for the different sensor systems. Moreover, thermal parameters are identified for different cylindrical 18650 Li-ion cells with capacities between 1.1 Ah and 2.7 Ah.The comparison of different types of temperature sensors shows that contact-based sensors underestimate surface temperatures even at low temperature differences to environment. This causes an error in thermal parameter identification. The TIS measurements performed with contact-based sensors show divergence of 20 -60 % for heat capacity, 30 -70 % for thermal conductivity and 20 -60 % for convective heat exchange with environment.With our IR temperature sensor system, parameter identification is performed for different batteries. Resulting values for specific heat capacity are in a range between 900 and 1020 J/kgK and thermal conductivities in radial direction lies between 3.1 and 3.6 W/mK.Our investigations show that IR-based temperature sensors are an effective progression for TIS measurements and improve quality of parameter identification at low cost. Moreover, discrepancies mentioned in TIS literature can be explained by our findings.