Single-dispersed polyoxometalate clusters embedded on multilayer graphene as a bifunctional electrocatalyst for efficient Li-S batteries.

The redox reactions occurring in the Li-S battery positive electrode conceal various and critical electrocatalytic processes, which strongly influence the performances of this electrochemical energy storage system. Here, we report the development of a single-dispersed molecular cluster catalyst composite comprising of a polyoxometalate framework ([Co 4 (PW 9 O 34 ) 2 ] 10- ) and multilayer reduced graphene oxide. Due to the interfacial charge transfer and exposure of unsaturated cobalt sites, the composite demonstrates efficient polysulfides adsorption and reduced activation energy for polysulfides conversion, thus serving as a bifunctional electrocatalyst. When tested in full Li-S coin cell configuration, the composite allows for a long-term Li-S battery cycling with a capacity fading of 0.015% per cycle after 1000 cycles at 2 C (i.e., 3.36 A g -1 ). An areal capacity of 4.55 mAh cm -2 is also achieved with a sulfur loading of 5.6 mg cm - 2 and E/S ratio of 4.5 μL mg -1 . Moreover, Li-S single-electrode pouch cells tested with the bifunctional electrocatalyst demonstrate a specific capacity of about 800 mAh g -1 at a sulfur loading of 3.6 mg cm -2 for 100 cycles at 0.2 C (i.e., 336 mA g -1 ) with E/S ratio of 5 μL mg -1 .