Facilitating Charge Reactions in Al‐S Batteries with Redox Mediators

Abstract

The Al−S battery is a promising next‐generation battery candidate due to high abundance of both aluminium and sulfur. However, the sluggish kinetics of the Al−S battery reactions produces very high overpotentials. Here, for the first time, it was demonstrated that the incorporation of redox mediators could dramatically improve the kinetics of Al−S batteries. On the example of iodide redox mediators, it was shown that the charging voltage of Al−S batteries could be decreased by about 0.23 V with as little as 2.3 wt% of redox mediator added as electrolyte additive. Control electrochemical measurements, without prior discharge of the battery, demonstrated that >97 % of the charge capacity was due to the desired oxidation of Al2S3 and polysulfides, and X‐ray diffraction experiments confirmed the formation of sulfur as the final charge product. The beneficial role of redox mediators was demonstrated with cheap and environmentally friendly electrolytes made of urea and AlCl3. This work showed that dramatic performance improvements could be achieved with low concentration of electrolyte additives, and therefore, much further performance improvements could be sought by combining multiple additives.

Advances and challenges of aluminum–sulfur batteries

“…Earth-abundant metals such as iron or aluminum are easily oxidized in aluminum electrolytes at the high voltages of >1.5 V vs. Al 3+ /Al used in positive electrode operation. Therefore, oxidatively stable conductive materials such as molybdenum 3,31,34,[37][38][39][40][41] , tantalum 33,35 , niobium 32 or nickel 25,[42][43][44] are usually used in Al-S batteries.…”

“…Considering the high kinetic barrier for the oxidation of Al 2 S 3 and AlPS, lowering their overpotentials upon charge and thus increasing the resulting capacity of the S cathode is an important aspect. From this perspective, redox mediators that have been actively studied in the field of Li–S batteries have also been tested for Al–S systems by Nuria’s group 40 . It has been shown that the addition of NaI or LiI as electrolyte additives, whose oxidation potentials are well matched to those of Al 2 S 3 oxidation, leads to a reduction of the charging voltage ( ca .…”

Polyoxometalates@Metal‐Organic Frameworks Derived Bimetallic Co/Mo2C Nanoparticles Embedded in Carbon Nanotube‐Interwoven Hierarchically Porous Carbon Polyhedron Composite as a High‐Efficiency Electrocatalyst for Al–S Batteries

“…Li et al successfully developed bromide-utilizing electrolytes that effectively reduce the energy barrier for Al 3+ decomposition. [32] In terms of cathode design, carbon-based materials have been used as sulfur hosts because their porous network and large specific surface area can somewhat restrict the shuttling of aluminum polysulfides (AlPSs). However, their effect on the shuttling process is not particularly significant.…”

Three-Dimensional Nitrogen-Doped Carbonaceous Networks Anchored with Cobalt as Separator Modification Layers for Low-Polarization and Long-Lifespan Aluminum–Sulfur Batteries

“…To date, extensive strategies have been developed to address these obstacles in Al–S batteries, including the design of sulfur hosts, electrolyte engineering, and separator modification. − Various catalysts as sulfur hosts are proven to reduce the polarization of sulfur batteries, e.g., transition metal/sulfides, , single-atom catalysts, − nitrogen-doped carbon . The cyclability of Al–S batteries is also enhanced with lithium salts employed as electrolyte additives, via promoting the chemical reactivation of aluminum polysulfides species .…”