Operando Investigation of Al Plating Regimes on HOPG in [EMImCl]:AlCl3 by Electrochemical Reflection Anisotropy Spectroscopy

Rechargeable aluminium batteries show promise as next-generation systems with a more abundant material base than lithium technology. However, the stable native oxide on top of aluminium metal electrodes leads to poor cell performance. Graphite, on the other hand, is a so far rarely investigated alternative that can be used as both the anode and cathode. Here, metallic aluminium is deposited at the anode, while AlCl₄⁻ is intercalated at the cathode. For both cases, understanding the electrode–electrolyte interface is crucial for improving the performance of the battery. In this work, we use reflection anisotropy spectroscopy to study the evolution of the interface under applied potentials. We find that the cathode exhibits an irreversible swelling of the topmost graphite layer due to AlCl₄⁻ intercalation as well as the formation of an SEI during the first voltammetry cycle. On the anode, the electrodeposition of aluminium is initially well-ordered. However, the evolution of the surface morphology depends on the applied potential, with island-like growth at less cathodic potentials, and layer-by-layer growth at more anodic potentials. With the optical operando spectroscopy, we can follow these qualitatively different plating and stripping regimes in a time-resolved manner.