Aging Effect of Temperature Gradients in Li-ion Cells Experimental and Simulative Investigations and the Consequences on Thermal Battery Management

The occurrence and simulative reconstruction of local aging inhomogeneities in Li-ion cells caused by temperature gradients are described, which typically result from active battery cooling in automotive applications. The interactions of thermal, electrical and degradation behaviour are investigated on a prismatic Li-ion cell of 60Ah capacity comprising a graphite / NCM/LMO-blend system. Therefore a transient 3D thermal FVM model combined with a 3D impedance based FNM-model is implemented in order to consider thermal-electrical inhomogeneities. The model is validated by thermal measurements on operated Li-ion cells that are prepared with 8 internal thermocouples inside the jelly roll of the battery. For degradation investigations a weighted charge throughput cycle life model is discretized analogue to the thermal-electrical model. Temperature, current rate and SoC swing are each considered by an aging stress pre-factor. The aging model is exemplary parameterized by five isothermal cycle-life tests. The simulative results show the inhomogeneous capacity loss and impedance rise inside the battery cells with thermal gradient for two aging scenarios. As a result, also temperature and current density distributions inside the cell changes over lifetime. According to both aging indicators, a Li-ion battery cell with temperature gradient comprises an aging relevant temperature that equates the volumetric averaged jelly roll temperature plus 10% of the maximum temperature difference in the jelly roll.