Emerging Aluminum-Ion Capacitors and Lessons from Battery Chemistry

Aluminum-ion capacitors (AICs) attract significant research interest as multivalent energy-storage devices that aim to increase energy density beyond that of conventional monovalent-ion supercapacitors. Owing to the trivalent charge of Al³⁺ and the abundance, low cost, and relative chemical stability of aluminum, AICs hold strong potential for sustainable and scalable energy storage. This review provides a comprehensive overview of their fundamental charge-storage mechanisms, recent advances in electrode materials, electrolyte engineering, and device architectures. Particular emphasis is placed on overcoming the intrinsic challenges associated with Al³⁺-based charge carriers, such as intricate solvation chemistry, sluggish desolvation kinetics, and limited ion transport within host frameworks, through structural design, defect engineering, and interfacial optimization. Progress in aqueous, ionic-liquid, deep-eutectic, and quasi-solid-state electrolytes is critically assessed alongside the key role of 2D materials, such as graphene and MXenes. Finally, insights from operando characterization and computational modeling are discussed to guide future directions toward high-rate, long-life, and flexible Al³⁺-based supercapacitors. By integrating these perspectives, this review provides a conceptual framework and roadmap for advancing AICs toward next-generation energy storage.