The first-principles investigation on interaction mechanism of Co9S8 with polysulfides in alkali metal-sulfur batteries.

• Interaction mechanism of Co 9 S 8 was investigated via the first-principles. • Adsorption energies of Co 9 S 8 towards Na 2 S n are larger than those towards Li 2 S n. • Co 9 S 8 has a more effective suppression on shuttle effect in Na-S than Li-S system. • Co 9 S 8 is a promising cathode material for sulfur in alkali metal-sulfur batteries. Severe shuttle effect will deteriorate electrochemical performances of alkali metal-sulfur batteries involving lithium- or sodium-sulfur batteries. In this paper, the interaction mechanism between Co 9 S 8 and polysulfides (Li 2 S n /Na 2 S n) has been investigated thoroughly via the first-principles calculations. Both band structure and density of state show the metallicity of Co 9 S 8 , implying its positive role in conductivity improvement for cathodes. The adsorption energies of Co 9 S 8 towards long-chain Na 2 S n are larger than those towards Li 2 S n , indicating a more effective suppression of Co 9 S 8 on shuttle effect in Na-S than Li-S system. Co 9 S 8 also exhibits a catalytic property to beneficially decrease the Gibbs free energy barrier and thus speed up electrochemical reactions. Co 9 S 8 has been verified theoretically to be a promising cathode material for alkali metal-sulfur batteries. [ABSTRACT FROM AUTHOR]