<i>In Situ</i> Monitoring of the Al(110)‐[EMImCl] : AlCl₃ Interface by Reflection Anisotropy Spectroscopy

Abstract Recently, Al‐batteries (AlBs) have become promising candidates for post‐lithium batteries, with [EMImCl] : AlCl 3 (1 : 1.5) as the most commonly used electrolyte. However, progress in the development of AlBs is currently hindered by the lack of understanding of its solid‐electrolyte interface. Monitoring the structure of this interface under operational conditions by complementary spectroscopy could help to identify and overcome bottlenecks of the system. Reflection anisotropy spectroscopy (RAS), an optical in situ technique, provides access to physical and chemical properties of electrochemical interfaces on an atomistic level. Herein, we report the first example of RAS as an in situ characterization technique for non‐aqueous battery systems, investigating an Al(110)‐based model system. During chemical pre‐treatment in [EMImCl] : AlCl 3 , the Al(110) surface passivation film is modified. The oxide film is partially etched while an inhomogeneous passivation layer forms, increasing the surface roughness. Upon electrochemical cycling, applied potential‐dependent oscillations of the anisotropy are observed and demonstrate the applicability of RAS to monitor phenomena such as plating/stripping and surface passivation in real‐time.