Keggin-Type Polyoxometalates as Bidirectional Redox Mediators for Rechargeable Batteries

Abstract

Keggin-Type polyoxometalates (POMs), which possess multiple redox centers, were investigated as bidirectiona redox mediators in rechargeable batteries. A series of POMs have been synthesized and employed in sulfu electrodes where neither the active material nor the discharge product were electrically conductive. POMs wer found to have multiple redox potentials covering the range of the equilibrium potentials of the redox reactions o sulfur, which consequently facilitated both charge and discharge reactions. In particular, [SiMo12O40]4? offered large discharge capacity of 1270mAhg?1 by accelerating the reduction of shorter, less soluble polysulfides, leadin to a higher cycling performance. The mediator role was confirmed via an X-ray photoelectron spectroscopy study o the cycled cathodes. Density functional theory calculations showed that the redox potentials of POMs are tunable allowing selective design of suitable POM molecules for specific battery electrodes.