Advanced aqueous electrolytes toward better aluminum-ion batteries: A comprehensive review

Rechargeable aluminium-ion batteries are emerging as promising next-generation energy storage systems, benefiting from the high mass/volumetric energy density, low cost, and intrinsic safety of Al anodes. However, their practical deployment is hindered by the limitations of conventional non-aqueous electrolytes, including moisture/oxygen sensitivity, cost, and toxicity. Aqueous aluminium-ion batteries (AAIBs) offer a compelling alternative, with advantages in cost, safety, and environmental impact. Yet, the scarcity of suitable electrolytes, particularly those capable of supporting reversible three-electron Al 3+ /Al redox chemistry, poses a major obstacle to commercialization. Meanwhile, developing low-cost, chemically/electrochemically stable electrolytes capable of functioning across diverse temperatures remains a critical challenge and a central research direction for advancing AAIB technology. This review comprehensively surveys advances in AAIB electrolytes, focusing on high-concentration electrolytes systems for electrochemical stability, hydrogel electrolytes that mitigate degradation through solid-phase confinement, and hydrated eutectic electrolytes for wide-temperature operation. In particular, we delve into the latest innovations in AAIB electrolyte design, provide a comprehensive perspective on current opportunities and unresolved hurdles, discuss the strengths, limitations, and potential solutions for each electrolyte type, while outline future pathways for the development of AAIB electrolytes. Schematic depiction of the frame construction of this review, mainly focusing on "water-in-salt" electrolytes (WISEs), Hydrogel electrolytes (HEs) and hydrated eutectic electrolytes (HEEs) of aqueous aluminium-ion batteries (AAIBs). • We provide a dedicated review of aluminium-ion batteries (AIBs) and aqueous AIBs (AAIBs), benchmarking their origins, structural configurations, working principles, and overarching challenges. • Currently, a systematic review that comprehensively synthesizes the current landscape and emerging trajectories of major electrolyte systems in aqueous aluminum-ion batteries is lacking. • We address the scientific challenges in optimizing AAIB performance, analyzing existing electrolyte strategies and proposing feasible developmental trajectories. • We conclude by outlining targeted recommendations and providing expert-level outlooks on the future of this potential energy storage technology.