It is well known that the composition of the materials used in lithium-ion (Li-ion) battery cells, combined with their historical use and state of charge (SOC), has a significant impact on the overall safety of these cells. This study investigates the safety characteristics of new or aged 21700 cells containing silicon-graphite blend anodes together with Ni-rich NMC cathodes, using accelerating rate calorimetry (ARC) over a range of SOC levels. Cells were cyclically aged at 0°C, ambient temperature or 50°C to induce different aging mechanisms including Li plating and solid-electrolyte interphase (SEI) growth. The quasi-adiabatic heat-wait-seek ARC tests show a decrease in temperatures of self-heating (SH), CID triggering, venting, and thermal runaway (TR) as SOC level increase, indicating reduced safety levels. Furthermore, both mass loss and TR intensity escalate as the SOC of the cell increases. Aged cells show a similar SOC dependence as new cells in terms 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 Li plated cells. Supplementary monitoring of cell voltage and on-line mass spectrometry analyses provide additional insights into the degradation mechanisms involved. The results of our study provide critical knowledge aimed at improving the safety and engineering of Li-ion batteries through the identification of hazardous cell conditions.