Sustainable Aloe-Emodin as an Advanced Organic Cathode for High-Performance Aluminum-Ion Batteries

Anthraquinone derivatives, known for their redox properties, are widely recognized as promising cathode materials for aluminum-ion batteries. This study presents an environmentally benign extraction of aloe-emodin from aloe dry powder, a sustainable and economically viable resource, using ferric chloride as an oxidizing agent in an acidic medium. Emodin and its isomer, aloe-emodin, share a symmetrical polycyclic chemical architecture and carbonyl functionalities, but differ in the position of their hydroxyl groups. The shift of a hydroxyl group from the aromatic ring to the methyl moiety in emodin results in aloe-emodin, enhancing its redox potential. As a cathode material in aluminum-ion batteries, aloe-emodin demonstrates enhanced electrochemical performance compared to emodin, showcasing a high reversible specific capacity of 85.9 mAh/g at 50 mA/g, superior rate capability with 42.0 and 32.0 mAh/g at 1000 and 2000 mA/g, respectively, and remarkable long-term cyclability with a capacity retention of 50.3 mAh/g and a Coulombic efficiency of 99.05 % after 6000 cycles at 1000 mA/g. These findings contribute to a deeper understanding of the design principles for aluminum-ion batteries that leverage anthraquinone-based cathode materials.