Lithium/Sulfur Cell Discharge Mechanism: An Original Approach for Intermediate Species Identification

Abstract

The lithium/sulfur battery is a promising electrochemical system that has a high theoretical capacity of 1675 mAh g(-1), but its discharge mechanism is well-known to be a complex multistep process. As the active material dissolves during cycling, this discharge mechanism was investigated through the electrolyte characterization. Using high-performance liquid chromatography, UV-visible absorption, and electron spin resonance spectroscopies, we investigated the electrolyte composition at different discharge potentials in a TEGDME-based electrolyte. In this study, we propose a possible mechanism for sulfur reduction consisting of three steps. Long polysulfide chains are produced during the first reduction step (2.4-2.2 V vs Li(+)/Li), such as S(8)(2-) and S(6)(2-), as evidenced by UV and HPLC data. The S(3)(•-) radical can also be found in solution because of a disproportionation reaction. S(4)(2-) is produced during the second reduction step (2.15-2.1 V vs Li(+)/Li), thus pointing out the gradual decrease of the polysulfide chain lengths. Finally, short polysulfide species, such as S(3)(2-), S(2)(2-), and S(2-), are produced at the end of the reduction process, i.e., between 2.1 and 1.9 V vs Li(+)/Li. The precipitation of the poorly soluble and insulating short polysulfide compounds was evidenced, thus leading to the positive electrode passivation and explaining the early end of discharge.

Capacity Fade Analysis of Sulfur Cathodes in Lithium–Sulfur Batteries

“…Some literature suggests that the transition from the high to the low discharge voltage plateau happens concurrently with the start of the formation of solid Li 2 S 2 and Li 2 S 6, 14. This stage contributes to the major portion of the capacity with a fixed voltage.…”

“…The ultimate discharge products are insoluble and nonconductive Li 2 S 2 /Li 2 S; however, there are a number of intermediate soluble products with different Li to S ratios (Li 2 S x , 3 ≤ x ≤ 8) 6. For one thing, the nonconductive sulfur (S 8 ) and Li 2 S 2 /Li 2 S greatly decrease the utilization of the active sulfur materials and pose major issues for power capability 7.…”

Elucidating the Electrochemical Activity of Electrolyte-Insoluble Polysulfide Species in Lithium-Sulfur Batteries

“…4,5 Multiple literature reports have reached different conclusions on the existence of the solid, electrolyte-insoluble species Li 2 S 2 and whether it forms as a solid intermediate during the discharge of Li-S batteries. [5][6][7][8][9][10][11][12] Recent studies have suggested there may be separate reaction pathways that are followed at different points during Li-S discharge: one where only Li 2 S is formed during reduction of soluble polysulfides (Li 2 S x , x > 2), and a second where both Li 2 S and Li 2 S 2 are formed simultaneously. 11 The question of whether any Li 2 S 2 that forms can then be reduced is debated in the literature.…”

The electrochemical properties of high-capacity sulfur/reduced graphene oxide with different electrolyte systems

“…3. As shown in Fig.3(a), three cathodic peaks are observed at 2.45, 2.05, and 1.9 V during the discharge process, showing typical sulfur cathode charge/discharge behavior [14,24]. Firstly, elemental sulfur in the solid phase is dissolved in the electrolyte, and then it is reduced to S 6 2-, S 4 2-, and S 2 2-(S 2-), respectively.…”