Al-based batteries are considered an attractive alternative to Li metal batteries due to the use of Al metal as an anode with lower cost, lower reactivity and larger abundance compared to Li. Despite the aforementioned attributes, few electrolytes permit reversible electrodeposition of Al metal at room temperature. Thus, for the last 30 years, expensive imidazolium-based ionic liquids have dominated the field. Even though extensive research has been conducted on imidazolium-based ionic liquids, a systematic study that sheds light on the structural requirements, physicochemical and transport properties required to enable high reversibility has not been conducted. Here, we investigated two ammonium-based systems that show comparable reversibility to imidazolium-based ionic liquids with Coulombic efficiencies up to 99.1% for over 1000 cycles (1 mAh/cm2, 4 mA/cm2). We reveal that high reversibility in ammonium- and imidazolium- based electrolytes is achieved when a critical ratio of the solvation species (AlCl4- and Al2Cl7-) is obtained, where Lewis’s acidity and beneficial interfacial reactions continuously etch the Al2O3 resistive interfacial layer and uniform current distribution is attained on the anode. Insights were obtained via construction of phase diagrams, 27Al quantitative NMR, structural considerations, chemical analyses, transport properties and electrochemical measurements. The findings yield quantitative molecular-level understanding of Al electrodeposition in room-temperature ammonium-based molten salts that are up to five times cheaper than the commonly used imidazolium-based electrolytes. The fundamental understanding gained on the structural, physicochemical and transport properties that enable high reversibility of Al electrodeposition in electrolytes can guide engineering approaches for other electrolyte systems for Al-based batteries. Furthermore, the development of low-cost, high-rate capability and long-term stability electrolytes can accelerate the development of high-capacity cathodes leading to higher energy densities relevant for electric vehicles.

Understanding the reversible electrodeposition of Al in low-cost room temperature molten salts

ACS Spring 2022

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