A Phenothiazine-Derived Organic Cathode for High-Capacity Aqueous Aluminum Batteries

Aqueous aluminum batteries (AABs) are considered as promising candidates due to their inherent safety and high theoretical volumetric capacity. However, the high charge density of Al³⁺ limits reversible intercalation in inorganic cathodes, often leading to severe structure collapse. Organic cathodes have recently emerged as ideal alternatives to traditional inorganic counterparts, given their renewable resources, molecular designability, and abundant reactive sites. Herein, we report a typical phenothiazine derivative, methylene blue (MB), as the organic cathode for advanced AABs. Benefiting from the –C═N– functional groups as the redox-active centers within the phenothiazine core, the MB cathode enables reversible co-insertion/extraction of one aluminum ion coordinated with two OTF⁻ anions to reduce desolvation penalty. Consequently, the MB cathode shows high reversible capacities of 138.8 and 57.0 mAh g⁻¹ at current densities of 50 and 200 mA g⁻¹, respectively, and retains 82.0 mAh g⁻¹ after 110 cycles at 50 mA g⁻¹. By introducing its structural analogue, neutral red (NR) molecule, it further demonstrates the universality of the phenothiazine framework for reversible aluminum storage. This work elucidates the fundamental processes governing charge storage of MB and highlights the great potential of phenothiazine-based organic cathode materials for next-generation aqueous batteries.