Synergistic Regulation of Bidirectional Conversion of LiPSs and Li2S Using Anthraquinone as a Redox Mediator

Lithium-sulfur (Li-S) batteries are strong contenders as energy storage options in the next-generation, primarily because of their potential for delivering high energy densities. Nonetheless, their widespread commercialization faces several obstacles, including sluggish sulfur redox kinetics, the insulating properties of the Li2S discharge product, and significant reaction energy barriers. In this work, anthraquinone (AQ) was introduced as a redox mediator and incorporated onto Co-doped carbon materials through pi-pi interactions. The results showed that synergistic effect between AQ and Co atoms facilitated the bidirectional conversion of lithium polysulfides (LiPSs) and Li2S. During charging, AQ lowered the reaction energy barrier for Li2S oxidation and thereby enhanced the reversibility of sulfur redox reactions. Density functional theory (DFT) calculations showed that AQ-Li2S x exhibits a lower energy for the lowest unoccupied molecular orbital (LUMO) and a higher energy for the highest occupied molecular orbital (HOMO). Experimental results demonstrated that an impressive initial discharge specific capacity of 1290 mAh g-1 was achieved by the fabricated S@AQ/Co-N-C electrode at 0.1 C. After 600 cycles at 1 C, it retained 64% of this capacity and exhibited a minimal 0.06% capacity decay rate per cycle.