Coexistence of extended flat band and Kekul\'e order in Li-intercalated graphene

Doping graphene near the 1/4 filling to shift the extended flat band and van Hove singularity below E$_F$ has been highly desirable. Here we report the experimental observation of an extended flat band below E$_F$ in Li-intercalated graphene. Strong electron-phonon interaction is clearly identified by notable kinks in the band dispersion. Moreover, the evolution of the band structure upon Li intercalation shows that the extended flat band and the Kekul\'e order emerge simultaneously. Our work provides opportunities for investigating flat band related instabilities and its interplay with the Kekul\'e order