The development of an in situ high-temperature X-ray diffraction technique for lithium-ion battery materials is crucial for understanding the detailed mechanism of thermal runaway. We realized such a technique and employed it on a C6Lix electrode with an LiPF6-based electrolyte, thereby revealing multiple transformations through several intermediate stages, i.e., C6Li → C12Li → C18Li/C24Li → C36Li → C6, which could be helpful to improve the thermal stability.