Mixed Electrolyte Enabling Ultrafast Mass Transport and Compatibility with Polypropylene Separator for Stable and Low‐Cost Aluminum Ion Battery

The energy industry has taken notice of aluminum ion batteries (AIB) for their low cost, high safety, and high capacity. However, using the ionic liquid electrolyte results in the uneven Al electrodeposition and the reliance on expensive glass fiber separators, due to the sluggish mass transport and low wettability for the polypropylene separator. Herein, a mixed electrolyte is introduced by incorporating the co-solvent fluorobenzene into the traditional AlCl₃/1-ethyl-3-methylimidazolium chloride ionic liquid, in which the fluorobenzene (FB) mitigates electrostatic interactions between ions and facilitates the ion diffusion. The optimization principle for the mixed electrolyte is proposed based on maximizing the mass transportation, as indicated by the limiting current density. The optimized mixed electrolyte IL-FB (1:5) offers the highest limiting current density of 12 mA cm^-2, highly reversible plate/stripe of Al, and thus stable cycling for 7500 h with the high current density and capacity (8 mA cm^-2, 8 mAh cm^-2). Furthermore, IL-FB (1:5) also shows enhanced wettability for the polypropylene separator. The AIB with the polypropylene separator, exhibiting 60% decrease in cost, is achieved for the first time by using IL-FB (1:5), presenting a crucial step toward the initial practical application.