  MXenes are a rapidly growing family of 2D materials that exhibit a highly
versatile structure and composition, allowing for significant tuning of the
material properties. These properties are, however, ultimately limited by the
surface terminations, which are typically a mixture of species, including F and
O that are inherent to the MXene processing. Other and robust terminations are
lacking. Here, we apply high-resolution scanning transmission electron
microscopy (STEM), corresponding image simulations and first-principles
calculations to investigate the surface terminations on MXenes synthesized from
MAX phases through Lewis acidic melts. The results show that atomic Cl
terminates the synthesized MXenes, with mere residual presence of other
termination species. Furthermore, in situ STEM-electron energy loss
spectroscopy (EELS) heating experiments show that the Cl terminations are
stable up to 750 {\deg}C. Thus, we present an attractive new termination that
widely expands the MXenes functionalization space and enable new applications.
