Sulfur Cathode Electrocatalysis in Lithium-Sulfur Batteries: A Comprehensive Understanding

Abstract

Lithium-sulfur (Li-S) batteries have emerged as promising candidates for next-generation secondary power batteries given that they exhibit extremely high discharge specific capacity (1672 mAh·g⁻¹) when sulfur is used as the positive electrode. Despite the potential of Li-S batteries for commercial applications, two significant issues need to be addressed: the shuttle effect of dissolved high-order lithium polysulfides (Li₂S_n, 4 ≤ n ≤ 8) during charge/discharge processes and the slow redox kinetics of sulfur species. Fortunately, the introduction of electrochemical catalysis is an effective strategy to mitigate the above problems. In the context of electrochemical catalysis, in this paper we discuss the existence forms of polysulfides and draw clear conclusions. Specifically, in ether electrolyte systems, the dominant form of polysulfide is the neutral molecule, while a smaller proportion exists as anions and cations. In addition, we also propose the corresponding solutions for different forms of polysulfides. Unlike previous reports, we analyze the conversion mechanism of polysulfides from two perspectives: adsorption-catalysis and reactive intermediates. In terms of the strength of the interaction force between the substrate materials and polysulfides, adsorption-catalysis can be classified into physisorption-catalysis and chemisorption-catalysis. The differences between both types are analyzed and discussed in-depth. Additionally, the reactive intermediates are further classified into sulfur free radicals, thiosulfates, and organosulfur molecules based on different electrochemical reaction pathways. The mechanisms involved in the reactions of these intermediates are subsequently analyzed in detail. We also evaluate different strategies and list the types of catalysts that may correspond to each mechanism. Finally, the quantitative evaluation method of catalytic performance is also summarized, which paves a new way for the design of high-efficiency electrocatalysts in Li-S batteries. The nucleation transformation ratio (NTR) is a quantitative measure we developed to assess the catalytic properties of materials. When the reaction is ideal, the NTR should be equal to 3. A calculated NTR close to 3 indicates that the reaction from Li₂S₆ to Li₂S₄ occurs rapidly, suggesting that the material is highly catalytic to polysulfide nucleation. This quantitative approach enables researchers to determine the adsorption and catalytic effects of cathode materials on polysulfides, allowing the study of lithium-sulfur battery cathode materials to move from qualitative description to quantitative evaluation with specific factors. As a result, we can move from a qualitative description of lithium-sulfur battery cathode materials to their quantitative evaluation.

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