Interfacial ion depletion at high concentration guides electrolyte design for aqueous aluminum-iodine batteries

Concentrated aqueous electrolytes have emerged as a central strategy for stabilizing high-energy multivalent batteries, yet their design has largely relied on the presumption that increased additive content ensures stronger interfacial enrichment. Here, we show that this assumption does not hold universally: in AlCl 3 -ChCl hybrid electrolytes, choline cations become sequestered in the bulk phase through selective coordination with polychloroaluminate species ([Al(H 2 O) x Cl y ] 3−y ), where the hydroxyl group of choline acts as an anchoring site. Consequently, excessive additive levels diminish polyiodide confinement and accelerate capacity fading, whereas an intermediate regime (3Al + 1Ch) achieves optimal interfacial accessibility, enabling 260 mAh g −1 at 0.5 A g −1 and stable cycling over 1,100 cycles at 2 A g −1 . These findings introduce a concentration-guided interfacial design framework that identifies a functional concentration window to balance bulk coordination and interfacial enrichment, informing the design of durable, high-rate multivalent battery electrolytes.