Lithium-Metal Stabilization through Tellurium Chemistry in Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are appealing owing to the high specific capacity (1672 mA h g^-1) and low cost of sulfur. Lithium-sulfur cells involve a solid-liquid-solid conversion of sulfur species, from S to intermediate lithium polysulfides to the end discharge product Li₂S, which results in a sluggish reaction kinetics due to the insulating nature of S and Li₂S. Furthermore, the dissolution of ploysulfides and their migration from the cathode to the anode severely hamper the reversible plating and stripping of lithium-metal anode. These problems impede the practical application of Li-S batteries. Numerous efforts have been devoted to overcoming these challenges, but the intrinsically low stripping and plating efficiency of Li and the corrosion from polysulfides necessitate excess Li and electrolyte.<br/><br/>This presentation will focus on the exploitation of the chemistry of tellurium, which lies in the same group as sulfur in the periodic table. The addition of a small amount of tellurium into sulfur cathodes helps enhance the cycle life of Li-S cells. However, due to the poor utilization of Te, a significant amount of Te is required to improve cell cycling performance, resulting in an increase in cost. To overcome this challenge, we have adopted two approaches: (i) use of tellurium nanowires (TeNW) synthesized <i>via</i> a hydrothermal method and (ii) incorporation of LiTe₃ synthesized by a one-step process as an additive into the electrolyte.<br/><br/>Coating TeNW onto the separator greatly enhances Te utilization and a significant improvement in cell cycle life. The versatility of TeNW is further demonstrated by utilizing it with carbon nanotubes as the anode substrate. The exceptional performance of TeNW is due to its high-surface area nanostructure and excellent conductive network, facilitating efficient electron transfer during cell cycling.<br/><br/>On the other hand, LiTe₃ reacts rapidly with polysulfides and functions as a redox mediator to significatly improve the cathode kinetics and the utilization of active material in the cathode. The formation of a Li₂TeS₃/Li₂Te-enriched interphase layer on the anode surface enhances ionic transport and stabilizes Li deposition. By regulating the chemistry on both the anode and cathode sides, the LiTe₃ additive enables a stable operation of anode-free lithium-sulfur cells with only 0.1 M concentration in conventional ether-based electrolytes. With a high utilization of Te, the additive enables significantly longer cycle life of anode-free pouch full-cells under lean electrolyte conditions.