Around the world, there is a critical need for sustainable energy storage systems for grid-level stationary energy storage. In the search for such systems, aluminum (Al)-ion batteries have recently attracted considerable attention due to their positive attributes including low cost, high abundance of raw materials, and long cycling life. Among all, chloroaluminate ionic liquid (IL) composed of aluminum chloride (AlCl 3 ) and a Lewis basic organic chloride (XCl) is the most widely employed electrolyte system in Al-ion batteries. In many existing Al-ion battery chemistries, chloroaluminate IL simultaneously functions as a medium for ion transportation and as an electroactive anode material. To provide fundamental insights into designing better performing Al-ion batteries, herein, we performed a systematic investigation of the electrochemical and transport properties of four AlCl 3 -XCl ILs, where X is EMIM (1-ethyl-3-methylimidazolium), BMIM (1-butyl-3-methylimidazolium), TMAH (trimethylammonium), or TEAH (triethylammonium), using a combination of density functional theory calculations and experimentation. Results show that the electrochemical stability window, ion transference number, conductivity, and ionicity of Lewis acidic chloroaluminate ILs are fundamentally governed by the (i) AlCl 3 /XCl molar ratio (r), which dictates the concentration of ionic species, and (ii) XCl type, which affects the overall degree of interactions among ionic species.

The achievable cell-level specific energy density of existing aluminum-ion batteries (AIBs) employing AlCl 4À intercalation type cathodes is intrinsically limited by the chloroaluminate anolyte. Towards achieving AIBs with higher specific energy, it is imperative to explore alternative cell chemistries that fundamentally tap the capacity of Al metal anode. Here, we report a benzo[1,2-b:4,5-b']dithiophene-4,8-dione (BDTD) organic electrode material with favorable AlCl 2 + intercalation mecha-nism. This BDTD cathode delivers a specific capacity of 143 mAh g À 1 , and the resulting battery exhibits a well-defined voltage plateau at ~1.2 V. This characteristic voltage plateau is mainly driven by the predominant diffusive charge storage in BDTD cathode, which accounts for up to 85 % of the total charge-storage contribution. As a result, the BDTD cathode demonstrates exceptional self-discharging resistance by recovering > 95 % of its capacity upon 24-h resting.