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

Abstract

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.

The Catalyst Design for Lithium‐Sulfur Batteries: Roles and Routes

“…Metals and semiconductors with high conductivity have been used as a catalyst to promote the conversion [108–110] . Liu et al .…”

“…Metals and semiconductors with high conductivity have been used as a catalyst to promote the conversion. [108][109][110] Liu et al synthesized a Pt@Ni core-shell bimetallic catalyst with a patch-like Ni shell which can form the conductive intermediates with greatly enhanced conductivity. [87] The closely contacted metal induced the electronic migration from Ni to Pt and largely promoted the transformation of LiÀ SÀ Li to NiÀ SÀ Li.…”

Theoretical and Experimental Understanding of Metal Single-Atom Electrocatalysts for Accelerating the Electrochemical Reaction of Lithium–Sulfur Batteries

“…Up to now, numerous efforts have been devoted to designing effective catalytic materials with inherent polarity and superior conductivity, including metal nanoparticles (Pt, Co, and Ni) and transition metal oxides, sulfides, nitrides, and phosphides. − These catalysts can promote adsorption, transportation, and conversion for polysulfides and lower the energy barrier for the nucleation/decomposition of Li 2 S. With these catalysts, the reaction kinetics of Li–S batteries have been effectively enhanced, leading to improved cycle stability and rate performance. , Among various catalysts, metal single-atom catalysts recently have aroused tremendous attraction. With almost 100% atom utilization efficiency, metal single-atom catalysts can offer high efficiency and distinctive selectivity in promoting the mutual conversion between high-order polysulfides and Li 2 S 2 /Li 2 S. − In addition, the unique features including the quantum size effect, unsaturated coordination environment, and specific electronic structures endow the single-atom catalysts with a strong competitive edge . Transition metal single-atom catalysts with conventional metal–nitrogen–carbon (M–N x –C) moieties have been reported to achieve an encouraging performance in Li–S batteries due to the high-efficiency catalysis. − For example, Xiong et al prepared a tungsten (W) single-atom catalyst immobilized on nitrogen-doped graphene (W/NG) as a multifunctional separator modifier for Li–S batteries .…”

“…With almost 100% atom utilization efficiency, metal singleatom catalysts can offer high efficiency and distinctive selectivity in promoting the mutual conversion between highorder polysulfides and Li 2 S 2 /Li 2 S. 17−21 In addition, the unique features including the quantum size effect, unsaturated coordination environment, and specific electronic structures endow the single-atom catalysts with a strong competitive edge. 22 Transition metal single-atom catalysts with conventional metal−nitrogen−carbon (M−N x −C) moieties have been reported to achieve an encouraging performance in Li− S batteries due to the high-efficiency catalysis. 23−27 For example, Xiong et al prepared a tungsten (W) single-atom catalyst immobilized on nitrogen-doped graphene (W/NG) as a multifunctional separator modifier for Li−S batteries.…”

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

“…Single-atom catalysts (SACs) with monodispersed metal atoms supported on different matrices have attracted considerable attention for energy storage and conversion. − When applied to Li–S batteries, SACs could effectively confine and catalytically promote the conversion of soluble LiPSs. , Generally speaking, the d–p orbital hybridization between the central metal atoms of SACs and sulfur species dominates the adsorption strength and thus the catalytic ability toward LiPSs . Therefore, various approaches have been proposed to regulate the electronic structure of d orbitals of SACs in order to enhance the adsorption strength with LiPSs, mainly including the modulation of central metal atoms and coordination atoms. − For example, Cheng et al clearly elucidated that the d–p orbital hybridization is closely related to the d orbital electron filling of the central metal atoms, resulting in distinct catalytic performance for different SACs with the same coordination configuration . However, it should be noted that, for the commonly reported SACs with nonpolar TM-N 4 coordinations (TM means transition metal), the d–p orbital hybridization mainly originates from the hybridization between d z 2 orbital of TM and p z orbital of sulfur as well as d xz/yz orbital of TM and p x / y orbital of sulfur, leading to the formation of a σ- and π-bond, respectively.…”