AlCl ₄ ⁻ ‐Deficient Eutectic Electrolytes Enable Reversible Iodine Redox‐Amphoteric Conversion for Aluminum Battery Cathodes

Abstract

Abstract Aluminum (Al) batteries are promising for sustainable and large‐scale energy storage due to the inherent safety, low cost, and attractive metrics of the Al anode. However, the development of high‐voltage and high‐capacity cathodes remains a key challenge. Herein, we achieve the reversible iodine redox‐amphoteric conversion (i.e., I − /I 0 /I + ) in Al batteries, wherein AlCl 4 − ‐deficient eutectic electrolytes are identified critical for stabilizing the conversion process. In contrast to ionic liquid electrolytes prone to parasitic Cl 2 evolution, eutectic systems facilitate the I − /I 0 /I + conversion process with high reversibility and significantly suppressed Cl 2 generation. Spectroscopic and theoretical investigations reveal AlCl 4 − as the dominant species limiting anodic stability of the electrolyte, and its reduced presence in eutectic electrolytes directly enhances iodine conversion reversibility. The optimized electrolyte allows the I 2 electrode to deliver a specific capacity of 358 mAh g −1 and an energy density of 490 Wh kg −1 (based on I 2 mass), along with excellent cycling stability (83.8% retention over 1000 cycles). High‐loading I 2 electrodes (8.52 mg cm −2 ) achieve a high areal capacity of 2.25 mAh cm −2 and demonstrate practical feasibility in a single‐layer pouch cell. This work establishes a new design framework for high‐energy‐density Al batteries and opens avenues for advancing conversion chemistries in multivalent systems.