Electrochemical Reduction Mechanism of Sulfur Particles Electrically Isolated from Carbon Cathodes of Lithium-Sulfur Cells

Abstract

This work presents a new insight into the reduction mechanism of solid sulfur during the first step of cell discharge, that is, a 2.4 V plateau in Li-S batteries, by testing a specially designed cell with the solid sulfur electrically isolated from the carbon cathode and comparing it with a conventional cell. Importantly, the cell with the electrically isolated sulfur particles confined between two separators shows very normal operation even during the first cycle and provides the same result as a conventional cell after several cycles. Based on the controlled potentiostatic and galvanostatic experiments, we propose a reasonable reaction route: a portion of the electrically isolated solid sulfur (S 8 ) is dissolved in the electrolyte solution to form sulfur molecules, which can be electrochemically reduced to polysulfides on the carbon surface.

The Role of the Carbon Framework in Sulfur-Carbon Composite Cathodes in Li-S Batteries

“…Recently, Zhang presented that the initial mixing status of sulfur particles and conductive carbons in the sulfur cathode does not affect the electrochemical performance of Li/S batteries [34]. Furthermore, we found that sulfur particles electrically isolated from the cathode can participate in the electrochemical reaction via the reduction of dissolved sulfur at the carbon/electrolyte interface [30]. In practice, a cell composed of a carbon cathode and sulfur confined between separators showed normal operation, and its cycle performance was almost identical to that of a cell containing a C/S composite after 10 cycles [30].…”

“…Furthermore, we found that sulfur particles electrically isolated from the cathode can participate in the electrochemical reaction via the reduction of dissolved sulfur at the carbon/electrolyte interface [30]. In practice, a cell composed of a carbon cathode and sulfur confined between separators showed normal operation, and its cycle performance was almost identical to that of a cell containing a C/S composite after 10 cycles [30]. Based on these findings, we proposed that the highly enhanced performance of C/S composites is mainly due to the adsorption properties of mesoporous carbon and not the intimate electrical contact between carbon and sulfur.…”

“…The charge transfer does not occur at the carbon/solid sulfur interface but at the carbon/electrolyte interface [30]. This fact implies that the electrical contact of sulfur and carbon is not essential for initiating electrochemical redox reactions of sulfur in Li-S batteries.…”

“…However, in principle, the discharge process of sulfur proceeds via sulfur molecules dissolved in the electrolyte solution [30]. The charge transfer does not occur at the carbon/solid sulfur interface but at the carbon/electrolyte interface [30].…”

Boron-doped microporous nano carbon as cathode material for high-performance Li-S batteries

“…At the beginning of its cycle, the cell undergoes a reactivation process and the discharge capacity in the first 54 cycles increases. We assume that this increase results from the inability of the sulfur to solve into the electrolyte fast enough [32]. The highest discharge capacity (749.5 mAh/g) was obtained at the 54 th cycle.…”