Revealing the Catalysis Modes for Sulfur Conversion Kinetics in Molten Salt Aluminum-Sulfur Batteries.

Abstract

Molten salt aluminum-sulfur (MSAS) batteries operated at a sub-water-boiling temperature are attractive for large-scale energy storage because of the low costs along with moderate energy density. However, the sulfur positive electrode is plagued by the sluggish reaction kinetics of polysulfides, which hampers the rate and cycling performance. Herein, carbon materials integrated with single-atom catalysts (SACs) are utilized as the model sulfur hosts for MSAS batteries and comprehensively investigate the active modes of these SACs on cell performance. Results show that the cell utilizing cobalt-SACs shows an initial specific capacity of 1064 mA h g^-1 at 0.5C, retaining 88.5% capacity after 100 cycles. Even at 3C, the cell achieves a specific capacity of 533 mA h g^-1 and demonstrates record-long cycle performance with an ultralow capacity decay rate of only 0.0014% per cycle over 6000 cycles. By combining theoretical calculations and experimental characterizations, a new mechanism is undercovered wherein - owed to differentiated binding toward varied Al-polysulfides - the cobalt-SAC tunes down the conversion of long-chain to short-chain polysulfides while accelerating that from short-chain polysulfides to solid Al₂S₃. This mechanism alleviates polysulfide accumulation in the molten salt electrolyte, mitigates the shuttle effect, and enhances the overall sulfur reaction kinetics.