Reversible Electrochemical Intercalation of Aluminum in Mo₆S₈

Abstract

A mong 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, 1 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. 2−4 On the other hand, reversible electrochemical Al deposition− dissolution could be facilely achieved in ionic liquid (ILs) electrolytes composed of aluminum chloride (AlCl 3 ) and organic salts such as 1-butylpyridinium chloride, 1-ethyl-3-methylimidazolium chloride, and 1-butyl-3-methylimidazolium chloride ([BMIm]Cl). 5−7 Utilizing IL electrolytes, aluminum− chlorine (Al−Cl 2 ) rechargeable batteries were demonstrated by Gifford and Palmisano. 8 Despite the high discharge voltage (>1.5 V), good capacity, and cycle stability, the gaseous Cl 2 cathode was problematic. Furthermore, the Cl 2 cathode had to be first generated from the electrolysis of electrolyte through charging, which was also undesirable. More recently, vanadium oxide, 9,10 fluorinated graphite, 11 chloroaluminate-doped conductive polymers, 12 and graphitic carbons 13 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 (Mo 6 S 8 ) for the first time.Mo 6 S 8 has a unique crystal structure of stacked Mo 6 S 8 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 Mg 2+ . 14,15 Aurbach and co-workers first demonstrated Mo 6 S 8 as a cathode material for rechargeable magnesium-ion batteries. 16 In this study, we synthesized Mo 6 S 8 particles through a precipitation method modified from the reported works by Kumta et al. 17 and Liu et al. 18 As shown in the scanning electron microscopy (...

Trends in Aluminium‐Based Intercalation Batteries

“…As stated in [29] the highest discharge capacity obtained in the pre described battery configuration took place after the first discharge (148 mA h g −1 ) at 12 mA g −1 . A drawback here presents the current density that is very low.…”

“…The only explanation for this performance could be attributed to trapped Al atoms in the cathode structure. [29] Furthermore, by analysing the XRD patterns, Lee et al [81] proposed the final discharge product to be Al 4/3 Mo 6 S 8 instead of Al 2 Mo 6 S 8 as stated by Geng et al [29] In the same work, the discharge curve of the first cycle was discussed as well, explaining that the initial part of the curve is related to the electrolyte decomposition and not the intercalation mechanism. [81] However, after the first few cycles the discharge capacity remained to a moderate extent stable at the value of 70 mA h g −1 at the end of the 50 th cycle.…”

“…within its crystal structure. [11,79,80] Recently a paper [29] was published wherein reversible electrochemical Al intercalation (three positive charges) was reported.…”

“…In order to diminish this effect structures without oxygen (high electronegativity) can serve as an alternative. Guo and co-workers [29] proposed one alternative by synthesizing a Mo 6 S 8 cathode material (Figure 14). Figure 14 shows that the shape of the particles is cubic and approximately 1 µm in size.…”

“…In regards to intercalation, Al ions can be inserted into the larger Al 1 site easier than into the Al 2 site. [29] Importantly, the degree of volume change after Al 3+ intercalation is smaller when compared to Li + or Mg 2+ ion insertion, which is beneficial in regards to material stability. [81] The proposed redox reactions for the battery system with Mo 6 S 8 cathode and AlCl 3 imidazolium chloride are: [29] Discharge:…”