Effects of Main Aging Mechanism and State of Charge on the Safety of 21700 Li-Ion Battery Cells with Ni-Rich NMC Cathode

It is known that both the material used in Li-ion battery cells, as well as their aging history and state of charge (SOC), strongly impact the safety of such cells. This study investigates the safety characteristics of new or aged 21700 cells containing silicon-graphite blend anodes together with Ni-rich NMC cathodes by accelerating rate calorimetry (ARC) at different SOC. Cells underwent cyclic aging at 0 °C, room temperature, or 50 °C to induce different aging mechanisms including Li plating and solid electrolyte interphase growth. The quasi-adiabatic heat-wait-seek ARC tests show lower temperatures for self-heating (SH), CID triggering, venting, and thermal runaway (TR) with increasing SOC, indicating reduced safety levels. Furthermore, the mass loss and TR intensity increase as the SOC of the cell increases. Aged cells show a similar SOC dependence as new cells in view of venting and TR, although both temperatures are reduced. The onset of SH at around 35 °C, independent of SOC, reveals a significant safety issue in cells with Li plating. Additional cell voltage monitoring and on-line mass spectrometry provide further insights into the decomposition processes. Our findings provide essential knowledge to improve the safety and design of Li-ion battery cells by identifying unsafe states.