How Cyclic Aging at Different Temperatures Affects Safety of Li-Ion Pouch Cells

Abstract

Li-ion battery safety behavior is critical and should be examined carefully. However, safety testing is typically performed only on pristine cells. This neglects the significant effect of aging conditions, such as ambient temperature, on safety behavior. For example, aging at low temperatures results in Li plating and thus drastically reduces safety [1–3]. Cells aged at higher temperatures with a pronounced solid-electrolyte-interface (SEI) layer on the anode show an increased onset of self-heating leading to improved safety [3,4].The temperature dependent cyclic aging behavior of commercial Li-ion pouch cells is evaluated using an Arrhenius plot. Further Post-Mortem analyses of aged cells from both branches of the V-shaped Arrhenius plot confirm two dominant aging mechanisms: Li plating and SEI growth for the low and high temperature ranges, respectively.Heat-wait-seek (HWS) tests under quasi-adiabatic conditions in an accelerated rate calorimeter (ARC) are used to evaluate safety behavior and identify critical temperatures such as the onset of self-heating. Multiple sensors (voltage, resistance, temperature, strain, and ultrasonic transmission) are placed on the pouch cell to provide a better understanding of the processes taking place during HWS tests. These sensors show that some critical events, such as the onset of swelling, venting, or separator melting, are only slightly affected by aging. In contrast, the onset of self-heating depends on the aging mechanism and state of health (SOH) of the cell, as a more pronounced aging has a greater impact on the change in safety. This knowledge can be used, for example, by cell manufacturers to estimate the minimum safety of aged cells based on tests of new cells.