Reversible Electrochemical Intercalation of Aluminum in Mo6S8

A the rechargeable batteries beyond lithium chemistry, the ones based on aluminum (Al) are particularly promising: Al not only is the most abundant metal in the earth’s crust but also has attractive capacity due to its trivalency. To date, there were only scarce investigations on rechargeable Al batteries in literature. The initial investigations, as summarized in the review article by Li and Bjerrum, were focused on identifying Al-ion electrolytes from organic solvents and demonstrating potential cathode materials. However, these early attempts had little success due to the sluggish electrochemical Al deposition−dissolution in organic solvents. On the other hand, reversible electrochemical Al deposition− dissolution could be facilely achieved in ionic liquid (ILs) electrolytes composed of aluminum chloride (AlCl3) and organic salts such as 1-butylpyridinium chloride, 1-ethyl-3methylimidazolium chloride, and 1-butyl-3-methylimidazolium chloride ([BMIm]Cl). Utilizing IL electrolytes, aluminum− chlorine (Al−Cl2) rechargeable batteries were demonstrated by Gifford and Palmisano. Despite the high discharge voltage (>1.5 V), good capacity, and cycle stability, the gaseous Cl2 cathode was problematic. Furthermore, the Cl2 cathode had to be first generated from the electrolysis of electrolyte through charging, which was also undesirable. More recently, vanadium oxide, fluorinated graphite, chloroaluminate-doped conductive polymers, and graphitic carbons were also reported as cathode materials vs Al in the IL-based electrolytes. Unlike lithium, electrochemical Al intercalation into a host crystal structure can be very difficult due to the strong Coulombic effect induced by the three positive charges carried by the Al cation. Therefore, transition metal oxides, i.e., oxide anionic frameworks, may not be the ideal hosts for Al because of their strong electrostatic attraction with Al cations. It can hinder the redistribution of the charge of Al cations in the crystal, thus preventing the Al intercalation. On the other hand, sulfur has lower electronegativity than oxygen and is more polarizable due to its larger atom radius. Therefore, the charge redistribution in the sulfide anionic frameworks should be superior to oxides. Based on this concept, we demonstrate in this study the reversible electrochemical Al intercalation in Chevrel phase molybdenum sulfide (Mo6S8) for the first time. Mo6S8 has a unique crystal structure of stacked Mo6S8 blocks composed of an octahedral cluster of Mo atoms inside a sulfur anion cubic cell. It is known to have two types of sites between the sulfur cubes that are capable to accommodate small cations such as Li, Cu, and Mg2+. Aurbach and co-workers first demonstrated Mo6S8 as a cathode material for rechargeable magnesium-ion batteries. In this study, we synthesized Mo6S8 particles through a precipitation method modified from the reported works by Kumta et al. and Liu et al. As shown in the scanning electron microscopy (SEM) image in Figure 1a,