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.