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

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^-/I⁰/I⁺) in Al batteries, wherein AlCl₄ ^--deficient eutectic electrolytes are identified critical for stabilizing the conversion process. In contrast to ionic liquid electrolytes prone to parasitic Cl₂ evolution, eutectic systems facilitate the I^-/I⁰/I⁺ conversion process with high reversibility and significantly suppressed Cl₂ generation. Spectroscopic and theoretical investigations reveal AlCl₄ ^- 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 I₂ electrode to deliver a specific capacity of 358 mAh g^-1 and an energy density of 490 Wh kg^-1 (based on I₂ mass), along with excellent cycling stability (83.8% retention over 1000 cycles). High-loading I₂ 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.