The study on the reason why ZnS catalyst cannot suppress the shuttle effect of polysulfides in lithium-sulfur (Li–S) battery

The catalytic effect of zinc sulfide (ZnS) on the transformation of polysulfides (Li 2 S 8 ) to lithium sulfide (Li 2 S) has been proven by the enhancement in the rate performance of lithium-sulfur (Li–S) batteries incorporating ZnS particles into the cathode. Accordingly, the shuttling effect of polysulfide should also be suppressed. However, the expected phenomenon is not found in the Li–S batteries with ZnS particles being added in the cathodes. In order to find the intrinsic mechanism of this phenomenon, detailed catalytic effects of ZnS particles on different polysulfide molecules are investigated. It is shown that ZnS can only enhance the reaction from Li 2 S 4 to Li 2 S and it has almost no catalytic effect on the reaction from Li 2 S 6 to Li 2 S 4 . This is why the shuttle effect of Li 2 S 6 cannot be effectively prohibited. This mechanism is further verified via combining ZnS particles with S–C@PVA yolk-shell particles containing sulfur (S) and carbon black (C) as the core and PVA as the shell. Although the reaction from Li 2 S 6 to Li 2 S 4 cannot be directly catalysed by ZnS particles, it can still be enhanced when ZnS particles are distributed within the S–C@PVA yolk-shell structure (S–C–ZnS@PVA) because the reaction speed from the Li 2 S 4 to Li 2 S is accelerated. The fast consumption of Li 2 S 4 will result in the enhancement of the reaction from Li 2 S 6 to Li 2 S 4 , which will largely reduce the diffusion of Li 2 S 6 out of the PVA shell. While if ZnS particles are located outside the S–C@PVA yolk-shell structure (S–C@PVA-ZnS), large amount of Li 2 S 6 can still diffuse out of the PVA shell and enter into the electrolyte since the transformation from is Li 2 S 6 to Li 2 S 4 relatively low. Both the rate performance and cycle stability of the batteries with S–C–ZnS@PVA cathodes are higher than those with S–C@PVA-ZnS ones. These results will give a guidance for the preparation of Li–S battery with high performance.