Lithium–sulfur batteries are one of the most promising alternatives for advanced battery systems due to the merits of extraordinary theoretical specific energy density, abundant resources, environmental friendliness, and high safety. However, the sluggish sulfur reduction reaction (SRR) kinetics results in poor sulfur utilization, which seriously hampers the electrochemical performance of Li–S batteries. It is critical to reveal the underlying reaction mechanisms and accelerate the SRR kinetics. Herein, the critical issues of SRR in Li–S batteries are reviewed. The conversion mechanisms and reaction pathways of sulfur reduction are initially introduced to give an overview of the SRR. Subsequently, recent advances in catalyst materials that can accelerate the SRR kinetics are summarized in detail, including carbon, metal compounds, metals, and single atoms. Besides, various characterization approaches for SRR are discussed, which can be divided into three categories: electrochemical measurements, spectroscopic techniques, and theoretical calculations. Finally, the conclusion and outlook part gives a summary and proposes several key points for future investigations on the mechanisms of the SRR and catalyst activities. This review can provide cutting‐edge insights into the SRR in Li–S batteries.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/aenm.202202094

Sulfur Reduction Reaction in Lithium–Sulfur Batteries: Mechanisms, Catalysts, and Characterization

Although single-atom catalysts (SACs) have been largely explored in lithium-sulfur (Li-S) batteries, the commonly reported nonpolar transition metal-N4 coordinations only demonstrate inferior adsorption and catalytic activity toward shuttled lithium polysulfides (LiPSs). Herein, single Fe atoms with asymmetric coordination configurations of Fe-N3C2-C were precisely designed and synthesized as efficient immobilizer and catalyst for LiPSs. The experimental and theoretical results elucidate that the asymmetrically coordinated Fe-N3C2-C moieties not only enhance the LiPSs anchoring capability by the formation of extra π-bonds originating from S p orbital and Fe dx2-y2/dxy orbital hybridization but also boost the redox kinetics of LiPSs with reduced Li2S precipitation/decomposition barrier, leading to suppressed shuttle effect. Consequently, the Li-S batteries assembled with Fe-N3C2-C exhibit high areal capacity and cycling stability even under high sulfur loading and lean electrolyte conditions. This work highlights the important role of coordination symmetry of SACs for promoting the practical application of Li-S batteries.

Strengthened d-p Orbital Hybridization through Asymmetric Coordination Engineering of Single-Atom Catalysts for Durable Lithium-Sulfur Batteries.

Fundamental, Application and Opportunities of Single Atom Catalysts for Li-S Batteries

Implanting Single-Atom N2-Fe-B2 Catalytic Sites in Carbon Hosts to Stabilize High-Loading and Lean-Electrolyte Lithium-Sulfur Batteries

The complicated multielectron and multiphase electrocatalytic sulfur reduction reaction (SRR) occurring in the Li–S battery is demonstrated, which strongly influences the performances of this battery chemistry. Effective candidates for SRR are often based on heteroatom‐doped carbon‐based electrocatalysts. However, the electrocatalytic sulfur reduction activity of these catalysts is so far insufficiently explored. Herein, a series of graphene doped with nonmetal elements (nitrogen, phosphorus, and sulfur) are designed and synthesized. It is shown that nitrogen‐doped graphene has a superior SRR catalytic activity with highest electrochemical reversibility and best electrochemical kinetics for the liquid–solid two‐phase conversion from long‐chain soluble Li2Sx (4 ≤ x ≤ 8) and the solid‐state Li2S2 to Li2S conversion. The considerably improved kinetics of the liquid–solid and solid–solid phases conversion reduces the continued accumulation of lithium polysulfides in electrolyte and the passivation of the electrode, thus resulting in a significant improvement in electrochemical performance of Li–S cells. Density‐functional theory calculations demonstrates that the highest SRR performance of N/G is originated from the strongest adsorption of the sulfur species and lowest energy barriers for Li2S decomposition among three doped graphene samples. This study is believed to guide the design of efficient electrocatalysts to exceed the performance of the benchmark for Li–S battery.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/sstr.202200244

A Comparison Study of the Electrocatalytic Sulfur Reduction Activity on Heteroatom‐Doped Graphene for Li–S Battery

Carbon-based single metal atom catalysts (SACs) are being extensively investigated to improve the kinetics of the Li-S redox reaction, which is greatly important for batteries with cell-level energy densities >500 W h kg-1 . However, there are contradictory reports regarding the electrocatalytic activities of the different metal atoms and the role of the metal atom in LiS chemistry still remains unclear. This is due to the complex relationship between the catalytic behavior and the structure of carbon-based SACs. Here, the catalytic behavior and active-site geometry, oxidation state, and the electronic structure of different metal centers (Fe/Co/Ni) embedded in nitrogen-doped graphene, and having similar physicochemical characteristics, are studied. Combining X-ray absorption spectroscopy, density functional theory calculations, and electrochemical analysis, it is revealed that the coordination-geometry and oxidation state of the metal atoms are modified when interacting with sulfur species. This interaction is strongly dependent on the hybridization of metal 3d and S p-orbitals. A moderate hybridization with the Fermi level crossing the metal 3d band is more favorable for LiS redox reactions. This study thus provides a fundamental understanding of how metal atoms in SACs impact LiS redox behavior and offers new guidelines to develop highly active catalytic materials for high-performance LiS batteries.

Role of the Metal Atom in a Carbon-Based Single-Atom Electrocatalyst for LiS Redox Reactions.

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Role of the Metal Atom in a Carbon-Based Single-Atom Electrocatalyst for LiS Redox Reactions.