Keggin-type Polyoxometalates as Bidirectional Redox Mediators for Rechargeable Batteries

Keggin-type polyoxometalates (POMs), which possess multiple redox centers, were investigated as bidirectional redox mediators in rechargeable batteries. A series of POMs have been synthesized and employed in sulfur electrodes where neither the active material nor the discharge product were electrically conductive. POMs were found to have multiple redox potentials covering the range of the equilibrium potentials of the redox reactions of sulfur, which consequently facilitated both charge and discharge reactions. In particular, [SiMo12O40]4− offered a large discharge capacity of 1270 mAh g−1 by accelerating the reduction of shorter, less soluble polysulfides, leading to a higher cycling performance. The mediator role was confirmed via an X-ray photoelectron spectroscopy study on 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.