Finding the Ideal Electrocatalyst Match for Sulfur Redox Reactions in Li-S Batteries

Lithium-sulfur (Li-S) batteries have emerged as a promising energy storage technology driven by their potential to reach very high theoretical specific energy densities of up to 2600 Wh<middle dot>kg(-1). This remarkably high energy density directly results from the reversible, multi-electron-transfer reactions between sulfur and lithium metal taking place during the charge and discharge cycles. However, the charge/discharge processes of Li-S batteries are invariably accompanied by changes in both the composition and the structure of the sulfur species in the cathode, all of which result in sluggish and incomplete sulfur transformation. It has been demonstrated that the application of electrocatalysts is an effective strategy to accelerate the sulfur reduction reaction (SRR). Recognizing this challenge, researchers worldwide have tried to develop efficient catalysts to accelerate the kinetics of the SRR and boost the overall performance of Li-S batteries. However, this goal necessitates an in-depth understanding of the intricate catalytic processes in the Li-S battery cathode. The effective characterization of the catalysts and a thorough investigation of the SRR process are essential steps to unraveling the underlying catalytic mechanism of sulfur reduction and to compare the performances of the different electrocatalysts. Nonetheless, this pursuit is hindered by the inherent complexity of the SRR process, which remains uniquely specific to the Li-S system under study. Throughout the SRR process, a multitude of intermediate products are created through catalytic conversions between liquid-solid and solid-solid phases. This complexity is markedly different from established heterogeneous catalytic processes, such as water-splitting reactions, where reactants, products, and reaction phases are relatively simple. Given these challenges, our response has been to design a series of catalysts with controlled structures to gain an in-depth understanding of the intricate reaction processes within Li-S catalysis. In this Account, we have undertaken a comprehensive analysis of the structure-function relationships governing the active sites of electrocatalysts in SRR. Our work thus encompasses three aspects-catalytic sites: their geometry and evolution during the reaction, catalytic mechanisms: a key factor that determines SRR kinetics, and catalytic materials: intelligent design toward optimized performance. Also presented in this Account is a brief discussion covering the broader domain of other electrocatalysts and sulfur-based electrochemical systems. Drawing upon the insights obtained from these works, we present future perspectives on potential opportunities and hurdles in the wider application of sulfur cathode electrocatalysts.