Because of their high energy density, low cost, and environmental friendliness, lithium-sulfur (Li-S) batteries are one of the potential candidates for the next-generation energy-storage devices. However, they have been troubled by sluggish reaction kinetics for the insoluble Li2S product and capacity degradation because of the severe shuttle effect of polysulfides. These problems have been overcome by introducing transition metal compounds (TMCs) as catalysts into the interlayer of modified separator or sulfur host. This review first introduces the mechanism of sulfur redox reactions. The methods for studying TMC catalysts in Li-S batteries are provided. Then, the recent advances of TMCs (such as metal oxides, metal sulfides, metal selenides, metal nitrides, metal phosphides, metal carbides, metal borides, and heterostructures) as catalysts and some helpful design and modulation strategies in Li-S batteries are highlighted and summarized. At last, future opportunities toward TMC catalysts in Li-S batteries are presented.

Understanding the Catalytic Kinetics of Polysulfide Redox Reactions on Transition Metal Compounds in Li-S Batteries.

Lithium-sulfur (Li-S) batteries have great potential as high-energy-density energy storage devices. Electrocatalysts are widely adopted to accelerate the cathodic sulfur redox kinetics. The interactions among the electrocatalysts, solvents, and lithium salts significantly determine the actual performance of working Li-S batteries. Herein, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), a commonly used lithium salt, is identified to aggravate surface gelation on the MoS2 electrocatalyst. In detail, the trifluoromethanesulfonyl group in LiTFSI interacts with the Lewis acidic sites on the MoS2 electrocatalyst to generate an electron-deficient center. The electron-deficient center with high Lewis acidity triggers cationic polymerization of the 1,3-dioxolane solvent and generates a surface gel layer that reduces the electrocatalytic activity. To address the above issue, Lewis basic salt lithium iodide (LiI) is introduced to block the interaction between LiTFSI and MoS2 and inhibit the surface gelation. Consequently, the Li-S batteries with the MoS2 electrocatalyst and the LiI additive realize an ultrahigh actual energy density of 416 W h kg-1 at the pouch cell level. This work affords an effective lithium salt to boost the electrocatalytic activity in practical working Li-S batteries and deepens the fundamental understanding of the interactions among electrocatalysts, solvents, and salts in energy storage systems.

Regulating Lithium Salt to Inhibit Surface Gelation on an Electrocatalyst for High-Energy-Density Lithium-Sulfur Batteries.

The industrialization of lithium–sulfur (Li–S) batteries is simultaneously impeded by the shuttle effect of lithium polysulfides and dendrites growth on lithium anode. To address both issues, a novel sulfiphilic and lithiophilic interlayer of Mo2N quantum dots decorated N‐doped graphene‐nanosheet (Mo2N@NG) are presented on polypropylene separator via a facile scalable method. Benefiting from the strong chemisorption ability, eminent electrocatalysis for LiPSs, and high chemical affinity with lithium‐ion (Li+), Mo2N@NG can efficiently catalyze the rapid transformation of LiPSs and induce uniform deposition of Li+. Theoretical calculation and in situ Raman synergistically elucidate the inhibition of shuttle effect and alleviation of dendrite growth. As a result, the assembled Li–S cell with Mo2N@NG/PP separator exhibits remarkable rate performance (860.2 mA h g–1 at 4 C), good cycling stability (0.039% capacity decay per cycle after 800 cycles at 2 C), a high areal capacity of 3.89 mA h cm–2 of Li–S pouch cell (4.5 mg cm–2 and 6 µL mg–1 at 0.2 C), and steady performance in protecting the lithium anode (at 5 mA cm–2 for 1500 h). This present strategy of quantum dots in a hybrid framework has great potential to be generalized to other transition metal‐based catalysts for advanced Li–S batteries.

Mo2N Quantum Dots Decorated N‐Doped Graphene Nanosheets as Dual‐Functional Interlayer for Dendrite‐Free and Shuttle‐Free Lithium‐Sulfur Batteries

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

Lithiated metallic molybdenum disulfide nanosheets for high-performance lithium–sulfur batteries

Rechargeable zinc-air batteries call for high-performance bifunctional oxygen electrocatalysts. Transition metal single-atom catalysts constitute a promising candidate considering their maximum atom efficiency and high intrinsic activity. However, the fabrication of atomically dispersed transition metal sites is highly challenging, creating a need for for new design strategies and synthesis methods. Here, a clicking confinement strategy is proposed to efficiently predisperse transitional metal atoms in a precursor directed by click chemistry and ensure successful construction of abundant single-atom sites. Concretely, cobalt-coordinated porphyrin units are covalently clicked on the substrate for the confinement of the cobalt atoms and affording a Co-N-C electrocatalyst. The Co-N-C electrocatalyst exhibits impressive bifunctional oxygen electrocatalytic performances with an activity indicator ΔE of 0.79 V. This work extends the approach to prepare transition metal single-atom sites for efficient bifunctional oxygen electrocatalysis and inspires the methodology on precise synthesis of catalytic materials.

A clicking confinement strategy to fabricate transition metal single-atom sites for bifunctional oxygen electrocatalysis

Lithium–sulfur batteries (LSBs) are regarded as promising next‐generation energy storage systems owing to their remarkable theoretical energy density (2600 Wh kg‐1) and low cost. However, sluggish electrochemical kinetics, lithium polysulfides (LiPS) shuttling, and uncontrollable Li dendrite growth seriously hamper the commercial application of LSBs. Herein, dual‐functional 3D interconnected free‐standing fibers embedded with TiO2‐TiN heterostructures as an advanced skeleton are designed for concurrently regulating both the sulfur cathode (S/hollow TiO2‐TiN) and Li anode (Li/solid TiO2‐TiN). As a cathode skeleton, the hollow TiO2‐TiN fibers afford synergistic functions of chemical anchoring, physical confinement, and excellent electrocatalysis for LiPS. Meanwhile, the multifunctional skeleton with remarkable lithiophilicity and high conductivity can accomplish uniform Li deposition and homogeneous Li ion flux for inhibiting the growth of dendrites. Benefiting from these advantages, the full battery (S/hollow TiO2‐TiN || Li/solid TiO2‐TiN) exhibits excellent electrochemical performance, including high cycling stability (988.8 mAh g−1 after 200 cycles at 0.5 C) and impressive rate properties (639.3 mAh g−1 at 2 C). This work inaugurates a novel strategy from experimental and theoretical aspects for fabricating LSBs with robust electrochemical performance.

“One Stone Two Birds” Design for Dual‐Functional TiO2‐TiN Heterostructures Enabled Dendrite‐Free and Kinetics‐Enhanced Lithium–Sulfur Batteries

The performance of rechargeable lithium (Li) batteries is highly correlated with the structure of solid electrolyte interphase (SEI), which governs the ion transport at the anode/electrolyte interface and directly impacts the cycle lifespan of the anode. The properties of a working anode are vital factors in determining the structure of SEI; however, the correspondingly poor understanding hinders the rational regulation of SEI. Herein, the role of electrode potential and anode material, two critical properties of a working anode, in dictating the structural evolution of SEI was investigated theoretically and experimentally. The anode potential is identified as the crucial role in dictating the SEI structure. The anode potential determines the reduction products of electrolyte thermodynamically, ultimately giving rise to the mosaic and bilayer SEI structure at high and low potential, respectively. In contrast, the anode material does not cause a significant change of the SEI structure but merely changes the ratio of organic and inorganic compounds in SEI. This work discloses the crucial role of electrode potential in dictating SEI structure and provides rational guidance to regulate SEI structure.

Xi‐Yao Li

Papers

A clicking confinement strategy to fabricate transition metal single-atom sites for bifunctional oxygen electrocatalysis

“One Stone Two Birds” Design for Dual‐Functional TiO2‐TiN Heterostructures Enabled Dendrite‐Free and Kinetics‐Enhanced Lithium–Sulfur Batteries

Regulating Lithium Salt to Inhibit Surface Gelation on an Electrocatalyst for High-Energy-Density Lithium-Sulfur Batteries.

The Crucial Role of Electrode Potential of a Working Anode in Dictating the Structural Evolution of Solid Electrolyte Interphase.

Boosting sulfur redox kinetics by a pentacenetetrone redox mediator for high-energy-density lithium-sulfur batteries