A series of lithiated graphite anodes (LiₓC₆, 0 < x < 1) from 18650-type Li-ion batteries were analyzed using ex situ high-resolution X-ray and neutron diffraction to study their thermal structural behavior across a wide temperature range. The study revealed that at high temperatures, lithiated graphite undergoes irreversible decomposition, accompanied by lithium ion loss. The structural behavior was analyzed for different states of charge (SOC) from 0% to 100%. At low temperatures, no significant degradation was observed, except for minor changes in reflection intensities and the appearance of additional reflections from frozen electrolyte components. At high temperatures, degradation of lithiated graphite was observed starting around 350 K, which is close to the operational temperature range of state-of-the-art Li-ion batteries. The degradation process was kinetically controlled by temperature and involved the formation of Li₂O and LiF phases. SEM and EDX analyses showed that the surface of the graphite electrode was passivated by reduction products, and silicon was identified in the graphite anode. The study highlights the importance of the solid electrolyte interface (SEI) layer during battery cycling and its thermal degradation. The results indicate that the thermal stability of graphite anodes is crucial for the safety and performance of Li-ion batteries, especially in fast charging applications. The findings emphasize the need for improved high-temperature stability of battery components to mitigate thermal runaway risks.A series of lithiated graphite anodes (LiₓC₆, 0 < x < 1) from 18650-type Li-ion batteries were analyzed using ex situ high-resolution X-ray and neutron diffraction to study their thermal structural behavior across a wide temperature range. The study revealed that at high temperatures, lithiated graphite undergoes irreversible decomposition, accompanied by lithium ion loss. The structural behavior was analyzed for different states of charge (SOC) from 0% to 100%. At low temperatures, no significant degradation was observed, except for minor changes in reflection intensities and the appearance of additional reflections from frozen electrolyte components. At high temperatures, degradation of lithiated graphite was observed starting around 350 K, which is close to the operational temperature range of state-of-the-art Li-ion batteries. The degradation process was kinetically controlled by temperature and involved the formation of Li₂O and LiF phases. SEM and EDX analyses showed that the surface of the graphite electrode was passivated by reduction products, and silicon was identified in the graphite anode. The study highlights the importance of the solid electrolyte interface (SEI) layer during battery cycling and its thermal degradation. The results indicate that the thermal stability of graphite anodes is crucial for the safety and performance of Li-ion batteries, especially in fast charging applications. The findings emphasize the need for improved high-temperature stability of battery components to mitigate thermal runaway risks.