The active reuse of automotive lithium-ion batteries (LIBs) no longer utilized in electric vehicles for onboard applications is an effective way to achieve carbon neutrality. However, the safety of used LIBs, whose chemical composition of components has changed from that of new ones due to degradation, is still unknown, and the relationship between degradation and safety needs to be understood. The purpose of this study is to clarify the effect of degradation on the thermal stability of the battery by numerical simulation for LiNi0.8Co0.1Mn0.1O2 (NCM811)/graphite battery using high-Ni layered oxide NCM811, which has recently attracted attention as a high-capacity positive electrode. The numerical simulation was employed to clarify the effect of degradation on the thermal stability of the battery. After degradation, the temperature at which the thermal runaway starts increased by 15 °C compared to the initial batteries, indicating that the thermal stability as a battery has improved. It is because the heat generated by the positive electrode near 200 °C, which triggers thermal runaway of batteries, has decreased due to battery degradation.