Activation of micropore-confined sulfur within hierarchical porous carbon for lithium-sulfur batteries

• Activation process of voltage plateau at 1.75 V during discharge is revealed.
• The reversible reaction appears upon cycling at 0.1C between 1 and 3 V.
• Micropore confined sulfur's role in the activation is elucidated.

Hierarchical porous carbon is often used in Li–S batteries due to the widely perceived benefits regarding the wide range of pore sizes. However, such notions are based solely on demonstrations of improved cyclic performances, and specific evidence to prove the utilization of the pores is yet to be found. Herein, we report, for the first time, the evidence for gradual activation of micropore-confined sulfur within porous carbon structures. By systematic comparison of microporous and hierarchical porous structures, we show that at sufficiently low current, sulfur infused hierarchical porous structures display a slowly activated and reversible reaction at 1.75 V vs Li/Li+ during discharge. This is in addition to the conventionally reported two voltage plateau at 2.3 and 2.1 V. Furthermore, the effects of LiNO3 decomposition on the system and the electrochemical mechanism behind the activation process is elucidated. Overall, the findings supplement the currently known electrochemical mechanisms occurring within porous structures and pave the way for more efficient utilization of hierarchical porous structures for applications in Li–S batteries.