Interfacial Oxygen Migration Underlies Performance Limitations in High‐Loading Aluminum‐Ion Batteries

Rechargeable aluminum batteries (RABs) are considered promising for long‐duration, cost‐effective energy storage applications due to their intrinsic safety and potential cost advantages. RABs are nonetheless fundamentally hindered by thermodynamics, which favor formation of a high bandgap, thin aluminum oxide (Al 2 O 3 ) layer at the aluminum (Al) anode–electrolyte interface—passivating the anode. The electrode‐level capacities achieved in the literature reports are also typically lower than expected from the redox reactions at the electrodes. Here, we investigate root causes and report a previously unrecognized, yet dominant degradation mechanism associated with interfacial chemistry at the Al anode. Specifically, we find that strong Lewis acid–base interactions between the passivating oxide layer and chloroaluminate species in electrolytes promote formation and migration of oxochloroaluminate species to the cathode. We report further that these species infiltrate the graphite cathode and impede ion transport, rendering a large fraction of the cathode electrochemically inaccessible at high mass loadings. We propose a single‐step chemical etching strategy that eliminates the native oxide layer from conventional Al anodes without compromising electrochemistry and transport in the cathode. The effectiveness of the approach is illustrated in full‐cell RABs able to achieve high reversibility (Coulombic efficiency (CE) > 98%) at higher cathode mass loadings (e.g., 13 mg cm −2 ).