Thermal management is a critical component of battery safety. Extensive amount of research has been done on the thermal aspects of battery safety,electrochemical stability of the cell components, initiation of internal electrical failure due to short-circuits, inadvertent inclusion of impurities during the fabrication process, and swelling of cells due to chemical reactions. However, limited results have been reported on how the safety of lithium ion cells changes with aging of the cells. The lowering of capacity as the cell ages may contribute to marginal improvements to the abuse performance of the cells; whereas the increase in resistance and changes to the mechanical and thermal stability window of the components result in narrower windows for safe operation.

In this talk, we present some results on how the thermal, electrical and mechanical properties of lithium ion cells change under different cycling conditions. The testing on the components was performed after careful disassembly of large format (40 Ah) lithium ion cells. Thermal conductivities were measured using the xenon nanoflash technique. Compression and tensile tests were performed on cell components harvested from the cells before and after cycling. Electrochemical resistance build up was characterized using impedance spectroscopy. Cycling conditions included different depth of discharge windows, cycling temperatures as well as charge/discharge rates. Initial results (Fig. 1) show that for the cells tested, under the cycling conditions we used, the anode undergoes a significant amount of degradation in terms of thermal and mechanical properties. The subsequent build up of electrochemical resistance and over heating of the cells lead to lowering of the safety threshold.

We then proceed to integrate the properties of cell components that were experimentally measured, into mathematical models that are used to assess cell-level performance under abuse. The models are used to perform several what-if analyses to identify safety thresholds for the individual cell components for our cell design. We conclude with case studies illustrating how safety thresholds for each cell component is different, how the different factors interact with each other to determine the outcome of cell-level safety, and how to best incorporate these findings to design safer lithium ion cells.