Single-atom electrocatalysts for lithium–sulfur chemistry: Design principle, mechanism, and outlook

Abstract

Lithium–sulfur batteries (LSBs) have been regarded as one of the promising candidates for the next-generation “lithium-ion battery beyond” owing to their high energy density and due to the low cost of sulfur. However, the main obstacles encountered in the commercial implementation of LSBs are the notorious shuttle effect, retarded sulfur redox kinetics, and uncontrolled dendrite growth. Accordingly, single-atom catalysts (SACs), which have ultrahigh catalytic efficiency, tunable coordination configuration, and light weight, have shown huge potential in the field of LSBs to date. This review summarizes the recent research progress of SACs applied as multifunctional components in LSBs. The design principles and typical synthetic strategies of SACs toward effective Li–S chemistry as well as the working mechanism promoting sulfur conversion reactions, inhibiting the lithium polysulfide shuttle effect, and regulating Li⁺ nucleation are comprehensively illustrated. Potential future directions in terms of research on SACs for the realization of commercially viable LSBs are also outlined.

INTRODUCTION

Commercialized lithium-ion batteries (LIBs) have been the main choice as state-of-the-art rechargeable energy storage devices over the past decades. Nevertheless, the practical capacity and energy density of LIBs are approaching the theoretical limits, hardly fulfilling the growing demand for renewable storage, portable electronics, and electric vehicles. Therefore, advanced battery systems “beyond LIBs” with superior energy density and robust lifespans are urgently required. Among them, lithium–sulfur batteries (LSBs), as an emerging alternative to LIBs, have advantageous characteristics of an environmentally friendly nature, relatively low cost, and remarkable energy density (2600 Wh kg^–1 and 2800 Wh L^–1). Sulfur, an Earth-abundant material, can endow the battery with a reversible multi-electron conversion reaction, coupled with lithium metal with low negative potential (–3.04 V vs. standard hydrogen electrode), giving rise to a high theoretical capacity (1672 mAh g^–1). However, the actual application of LSBs is plagued by unsatisfactory electrochemical performances, mainly due to the low discharge capacity and inferior cyclic stability.^1–4 These issues mainly stem from the notorious lithium polysulfide (LiPS) shuttle effect, retarded sulfur reaction kinetics, and dendrite growth of lithium.^5–7

To date, considerable efforts have been made toward optimization of sulfur and lithium evolution behaviors from various perspectives.^8,9 Accordingly, some favorable progress has been achieved by utilizing various hosts with tailored morphologies, controllable structures, and rich surface chemistries to alleviate the LiPS shuttle effect and propel sulfur redox kinetics.^10,11 Previous studies have demonstrated that the study of sulfur reaction kinetics is the key to breaking through the gap between academic investigations and actual implementation of LSBs. Along this line, recent years have witnessed ever-growing interest in the design of various electrocatalysts with diverse catalytic mechanisms for Li–S chemistry.^12–15 These include the following aspects: (i) heteroatom-doped carbonaceous catalysts. Inspired by the hydrogen bond, the concept of Li–bond chemistry reflects the interaction between LiPSs and doped heteroatoms, and plays a vital role in promoting sulfur reaction kinetics and thus enabling satisfactory battery performances. Zhang's group revealed that the active Li–bond could promote LiPS binding and electron transfer.¹⁶ Dong's group further demonstrated that the doped N could promote the oxidation of Li₂S₆ to Li₂S₈ and then to solid S₈.¹⁷ (ii) Metal-based catalysts. In the context of Li–S chemistry, metal-based catalysts encompassing oxides,¹⁸ sulfides,¹⁹ nitrides,²⁰ carbides,²¹ and phosphides²² can efficiently promote the evolution of LiPS and thus enhance the LSB performance. Wang and co-workers²³ designed defect-rich MoSe₂ as a superior catalyst for enhancing sulfur electrochemistry, where Se vacancies effectively boosted the electrochemical conversion reactions between Li₂S₂ and Li₂S species. However, challenges remain in the comprehensive regulation of surface reactivity and LiPS adsorption ability and ion and electron diffusion properties to achieve excellent catalytic activity for Li–S chemistry. (iii) Heterostructural catalysts. Heterostructures combine the merits of different components, enabling the balanced management of adsorption, diffusion, and conversion of LiPSs. In this regard, a recent study carried out by us confirmed the higher catalytic activity of VO₂–VN,²⁴ TiO₂–graphene,²⁵ and MgO–VTe₂ heterostructures²⁶ in comparison to single-component catalysts (Figure 1). Although excellent strategies have been proposed and practiced, the electrocatalytic efficiencies are relatively low due to the unsatisfactory activity of electrocatalysts, leading to the low energy density of LSBs. Moreover, the notorious LiPS shuttle effect and slow reaction kinetics pose a major threat to battery performances, and essentially, effective electrocatalysts are still needed to overcome these issues.

Figure 1. Electrocatalyst evolution diagram of LSBs. Heteroatom-doped carbon: B-, N-doped graphene. Reproduced with permission: Copyright 2022, Elsevier.27 N-, O-, S-doped carbon. Reproduced with permission: Copyright 2022, Elsevier.28 Metal-based catalysts: TiO2/MXene. Reproduced with permission: Copyright 2020, Royal Society of Chemistry.29 MoSe2 with Se vacancy. Reproduced with permission: Copyright 2021, Wiley-VCH.23 Heterostructural catalysts: VTe2–MgO. Reproduced with permission: Copyright 2019, American Chemical Society.26 CoSe2–MoS2. Reproduced with permission: Copyright 2021, Wiley-VCH.30 Single-atom catalysts: Fe–N4/hollow carbon. Reproduced with permission: Copyright 2020, Royal Society of Chemistry.31 Fe–N4 and Fe–N2/graphene. Reproduced with permission: Copyright 2019, Wiley-VCH.32 LSB, lithium–sulfur battery.

In light of this, shrinking the catalyst unit from nanoparticles to single atoms (SAs) indeed represents a potentially useful way to address the key problems and understanding the underlying mechanism of LiPS evolution. Single-atom catalysts (SACs) have well-defined atomic structures, maximum metal atom dispersion, and multiple actives sites, implying a new frontier in the field of energy-related applications such as electrocatalysis, some of which have been verified to have extraordinary electrocatalytic activity for reaction kinetics and long-term cycling stability for lifespan. It is particularly noteworthy that the concept of SACs also attracts considerable research interest in the field of LSBs. The electronic and geometric structures of metal-based catalysts can be determined by their sizes (Figure 2), affecting their catalytic activity. For SAs, the electronic structure is strongly linked to the coordination environment. As for the metal cluster, nanoparticles, and bulk, the electronic structures are more complicated on account of the orbital overlapping between metal atoms.³³ Moreover, SAs have electronic states that change from positively to negatively charged and do not have adjacent SAs, which is markedly different from metal catalysts with larger sizes such as nanoparticles, and so forth.³⁴ In terms of geometric structures, SAs may show minimal geometric changes during the reactions due to chemical anchoring by supports. For clusters, the geometric structures are affected by the reaction conditions. However, nanoparticles tend to show quite stable geometric structures, which is quite different from the SAs and clusters. The pioneering research by Dong's group confirmed that Co–N–C showed the remarkable functionality of promoting the kinetic conversion of sulfur intermediates under the real operation conditions of LSBs.¹⁷ Moreover, recent findings at the atom level indicated that the remarkable electron structure of SACs with ample active metal centers and different energy levels could efficiently accelerate the sulfur redox reaction kinetics.³⁵ Besides the unique behaviors in sulfur cathode of LSBs, SAC materials are also found to be surprisingly effective in realizing the uniform deposition of Li on the anode side.³⁶ Despite many impactful advances, there are very few detailed and in-depth reviews that discuss the design strategies and action mechanism of SACs for accelerating sulfur redox reactions and suppressing dendritic Li growth, so as to enhance the overall electrochemical performances of LSBs.

Figure 2. Electronic and geometric structures of SAs, clusters, and nanoparticles. Reproduced with permission: Copyright 2018, American Chemical Society.33 SA, single atom.

In this tutorial review, current progress in the field of SAC-enabled Li–S chemistry is comprehensively described. We first focus on discussing the key issues of sulfur reaction kinetics, followed by a detailed presentation of the design principles of objective-oriented active SACs and the corresponding typical synthesis routes. Then, this review predominantly focuses on the role of SACs in Li–S chemistry. Finally, in the final section, we propose the practical challenges of and future perspectives on SAC-enabled Li–S chemistry. By a systematic discussion, this review offers comprehensive and detailed insights into the action mechanism of SACs in sulfur reaction processes and Li deposition, which can potentially enable realization of practically viable LSBs.

ELECTROCATALYSIS AND CHALLENGES

The electrochemical reaction in the LSB system can be described using the reaction, S₈ + 16Li ⇌ 8Li₂S, which contributes to an excellent theoretical specific capacity of 1672 mA h g^–1. The first stage of the discharge procedure, involving the lithiation of S₈ into soluble high-order LiPSs (S₈ → Li₂S₈ → Li₂S₆ → Li₂S₄), endows the battery with a quarter (418 mA h g^–1) of the whole theoretical specific capacity. The second stage of the discharge procedure, involving further conversion of soluble high-order LiPSs into insoluble low-order LiPSs (Li₂S₄ → Li₂S₂ → Li₂S), provides the rest of the three quarters (1254 mA h g^–1) of the capacity (Figure 3A,B). The reversible reaction pathways can be completely adapted to the charge procedure. Apparently, such an actual electrochemical reaction involves multi-electron chemistry and multi-step solid–liquid–solid phase transitions, which lead to the intractable nature of Li–S chemistry, thereby resulting in insuperable issues and posing fatal threats to the real implementation of LSBs. These include the following aspects: (i) Due to the concentration gradients of soluble LiPSs between two electrode sides and the electric field, the back-and-forth migration of LiPS intermediates in between is the so-called “LiPS shuttle” effect. The movement of LiPSs into the electrolyte inevitably causes low sulfur utilization and anode corrosion.^37–39 (ii) The bottleneck in Li–S systems lies in the stepwise conversion of S₈ into Li₂S. Such phase-transition reactions have sluggish kinetics and excessively high polarization.^40,41 (iii) The vast volume change (S: 2.03 g cm^–3 vs. Li₂S: 1.66 g cm^–3) of the cathode can lead to the deformation of the electrode structure and consequently short lifespan of LSBs.⁴² (iv) The growth of dendritic Li and continuous cracking of the solid electrolyte interphase during the repetitive discharge/charge can lead to low Coulombic efficiency (CE), a short lifespan, and even the risk of battery explosion (Figure 3C).^43,44

Figure 3. (A) Scheme of a basic LSB configuration. (B) Typical charge and discharge curves of LSBs with two plateaus. Reproduced with permission: Copyright 2021, Wiley-VCH.14 (C) Challenges existing in the cathode and anode of LSBs. Reproduced with permission: Copyright 2019, Wiley-VCH.43 LSB, lithium–sulfur battery.

Besides, another technical challenge when applying SACs to LSB systems lies in how to perform atomic-level structural observations and thus substantiate the intrinsic reaction mechanism and pathways. Conventional techniques like X-ray diffraction are insufficient for distinguishing SACs from nanoparticle clusters. For this, advanced ex situ/in situ characterization tools, like high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray absorption near-edge structure (XANES), element-selective X-ray absorption fine structure (EXAFS), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM), are useful.^45,46

DESIGN PRINCIPLES OF SACS FOR LI–S CHEMISTRY

Recent investigations have confirmed the advantages of SACs in contrast to the traditional particle electrocatalysts in the field of Li–S chemistry. Along this line, particular attention has been paid to the exploration of the structure–performance relation. In this section, the design principles of SACs targeting high-efficiency and long-life battery systems, based on an optimized coordination environment and defect configuration, as well as high SAC loadings, are summarized and discussed.

Optimized local coordination environments

SAC-based catalysts usually present different electronic structures due to their tunable atomic–local coordination environments, thus providing distinct electrocatalytic ability. By optimizing the local coordination environment, the intrinsic activity of SACs for Li–S chemistry can be effectively manipulated. Several key points should be listed accordingly. First, various supports represent different types of coordination configurations. Second, the coordination stability of SACs is an essential precondition for studying their adsorptive and electrocatalytic activity. Heteroatom dopants in carbonaceous supports are frequently applied, as they can strongly coordinate with metal atoms during the production process, giving rise to the uniform distribution of SAs exclusive of aggregation. Third, the selection of the coordination environment depends on the lithiophilicity chemistry, as the Li binding contributes to the strong interaction between LiPSs and the polar substrate in principle. Specifically, an electron-rich donor X (Lewis base) can interact with Li−Y, where Li is a Lewis acid. Obviously, higher electronegativity of the bonding atom is preferred in Li–S chemistry.^16,47

Nitrogen (N) dopants can provide N coordination atoms for SAs. Zhou et al. explored the coordination configurations of SAs (Fe, Mn, Ru, Zn, Co, V, Cu, and Ag) on N-doped graphene (NG) for LiPS evolution (Figure 4A). SAs, including Fe, Mn, Ru, Zn, Co, and V, with NG substrates, retained pristine atomic configurations after Li₂S adsorption. However, the SAs of Cu or Ag on NG presented deformed atomic lattices, due to the breaking of coordinated bonds or the replacement of Li atoms in LiS clusters by a metal atom. This indicated that the structural stability of SAs during the long-term cycling process depends on the distinct coordination environments. The binding of SAs with Li₂S is closely correlated with the nucleation process of Li₂S, which can be used to evaluate the activity of SACs for sulfur reduction procedures. In this regard, SA-Cu@NG and SA-Ag@NG were incapable of promoting sulfur reaction kinetics. This study further compared the Li₂S decomposition barrier on various SACs, reflecting the catalytic ability of SACs for sulfur reaction procedures. As shown in Figure 4B, SA-V on NG was screened as the most promising catalyst for alleviation of the LiPS shuttle effect.⁴⁸ Clearly, the M–N₄ configuration was mostly explored in the initial stage of the emergence of SACs in the field of LSBs.^49–52 For instance, Chen's group demonstrated that mono-dispersed Fe atoms in N-doped hollow spheres acted as multifunctional nano-reactors for long-term Li–S pouch cells.³¹ The Fe–N₄ square–planar configuration was confirmed by XANES spectra of the Fe edge, as shown in Figure 4C, where the fingerprint of the pre-edge peak at 7114 eV was observed. The assembled pouch cell with this Fe-SAC could maintain a capacity retention rate of 77.1% after 200 cycles.³¹ The coordination form can be affected by the synthesis procedures, resulting in different LiPS affinities and electrocatalytic activities. In addition, besides the intrinsic activity of M–N_x, the electrocatalytic property of M–N_x-coordinated SACs also relies on the design of supports including ion/electron conductivity, pore structure, and so forth, indicating that the electrochemical activity for SACs can be manipulated, which represents an effective strategy for facilitating Li–S chemistry.

Figure 4. (A) Scheme of SA–N4 for Li–S catalysis toward fast redox kinetics. (B) Dissociation barriers of Li2S on different catalysts (the inset shows the decomposition path of Li2S on SA-V@NG. Reproduced with permission: Copyright 2020, American Chemical Society).48 (C) XANES profile at the Fe K edge of Fe–N/MHCS, FePc, and an Fe foil. Reproduced with permission: Copyright 2020, Royal Society of Chemistry.31 (D) Schematic illustration of Li–S batteries with bi-Janus Celgard separators: Co–O4 arrays on 2D MOF-Co nanosheets enable the anchoring of polysulfides upon Lewis acid–base interaction and homogenization of Li+ flux. Reproduced with permission: Copyright 2020, Wiley-VCH.53 (E) Upper: binding energies of NSC, FeNC, and FeNSC with Li2S6 from DFT calculations, with their respective atomic configuration on the right; down: schematic illustration of how FeN3S − C promotes LiPS conversion. Reproduced with permission: Copyright 2021, American Chemical Society.54 (F) Scheme of the synthesis process of FeN4 − NC and FeN2 − NC catalysts. Reproduced with permission: Copyright 2022, Elsevier.55 (G) EXAFS fitting curves in K and R spaces, respectively, and the proposed Ni−N5 structure (inset). (H) Schematic illustration of Ni–N5 moieties promoting sulfur redox kinetics. Reproduced with permission: Copyright 2021, American Chemical Society.56 DFT, density function theory; EXAFS, element-selective X-ray absorption fine structure; MOF, metal–organic framework; XANES, X-ray absorption near-edge structure.

Besides N dopants, heteroatoms including sulfur (S), oxygen (O), and phosphorus (P) can potentially coordinate with metal atoms. Among them, heteroatoms such as N and O can solely act as coordinated atoms for SACs.⁵⁷ For instance, M–O₄ sites were investigated as active catalyst species for LSBs, due to the strong interaction between Li and O atoms.^58,59 Guo and co-workers applied metal–organic framework (MOF)-derived Co-SACs to modify the polypropylene (PP) separator, endowing the LSB with mitigated shuttle effect of LiPSs and high utilization of sulfur (Figure 4D). Density function theory (DFT) calculations further demonstrated the assembly of S−Co and Li−O bonding between Co-MOF and LiPSs based on the classic adsorption model. The binding energies of Li₂S_x (1 ≤ x ≤ 8) with the Co−O₄ moiety increased gradually during the redox reactions, implying the effective mitigation of the shuttle effect in LSBs.⁵³ Considering that MOF-derived SACs usually can be attained by combining pyrolysis and acid etching procedures, an interesting phenomenon can also be noticed wherein the reservation and removal of particles also result in distinct electrochemical activity for SACs. On the one hand, removal of the particles can enable the full use of the isolated metal atoms to optimize the sulfur redox reaction, which can be useful for the fundamental and practical investigations of SACs in LSB systems. On the other hand, the particles represent the second active centers, which implies additional catalytic activity, which can promote high-efficiency redox reactions in Li–S chemistry. In a subsequent study by the same group, it was reported that Ru–O₄@CNF-coated Janus separators could tackle the challenges of both the sulfur and lithium sides in LSBs.⁵⁸ Moreover, N and O could also collaboratively coordinate with SAs as an efficient Li₂S conversion catalyst and a LiPS immobilizer, respectively, which was demonstrated by Xiong and co-workers, where a unique W–O₂N₂ coordinated catalyst with a high W loading of 8.6 wt.% was prepared using a facile self-template and self-reduction approach.⁶⁰ By contrast, dopants, such as S, and so forth, normally act as supportive coordinated atoms together with N for SACs. It was reported that Fe, Co, and Ni-SAs could indirectly bond with S via nitrogen coordination sites, generating FeN₂S₂, CoN₃S₁, and NiN₃S₁ active centers, respectively.⁶¹ Zhao et al.⁵⁴ reported that FeN₃S–C had a higher binding energy, as compared with FeN₄–C and N, S–C, thereby effectively mitigating the LiPS shuttle effect (Figure 4E). To date, the bare metal–P coordination for LSBs has still not been investigated. However, recently, P atom-altering local chemistry has been explored for further increasing the activity of SACs in the LSB field. The substitution of an N atom in Fe–N₄ by a P atom led to the formation of Fe–N₃P configuration, resulting in increased chemical affinity with LiPSs and decreased energy barrier for Li₂S dissociation.⁶² This strategy of delicately manipulating the local metal–N coordination composition by a P atom also represents a new method for the rational design of active promotor-targeted optimized sulfur species evolution throughout the whole discharge–charge process. As such, the indirect coordination of metal atoms represents a new method to increase the activity of SACs to mitigate the LiPS shuttle effect and catalyze the generation and decomposition of Li₂S. Use of properly coordinated heteroatoms can lead to favorable electron allocation in coordinate centers, improving the interaction strength for SACs with LiPSs and thus the ultimate battery performance.

The currently reported SACs in LSBs mainly include SA–X₄ (X: N, O, S, etc.) configurations; however, it is known that the symmetric planar configuration of SA–X₄ with a symmetric electronic structure cannot always be useful for optimizing the catalytic capability. Other coordinated structures SA–X_n (n = 2, 3, 5) have also been proven to be effective.^{55,56,63–66} Wang and co-workers proposed that the unsaturated Fe–N₂ centers acted as multifunctional sites for anchoring LiPSs and boosting the redox conversion of LiPSs, mainly attributed to the powerful hybridization of Fe 3dzz and S 3py orbitals in the Fe–N₂ configuration (Figure 4F).⁵⁵ The corresponding LSB could achieve a high areal capacity of 4.3 mAh cm^‒2 with increased sulfur loading (5 mg cm^‒2) and a lean electrolyte condition (E/S = 5.3 mL g^‒1) at 1 C. By contrast, using a self-templating strategy, a supersaturated SAC structure of Ni–N₅ located in nitrogen-doped porous carbon was reported by Zhang et al. (Figure 4G,H). The oversaturated Ni–N₅ moiety led to moderate adsorption ability and reduced energy barrier toward Li₂S_x (1 ≤ x ≤ 8), enabling the ultimate LSB to achieve excellent areal capacity at high sulfur loading.⁵⁶ The coordinated elements and their corresponding content can be rationally modulated to tune the electronic structure and further optimize the catalytic activity of SACs.

Rich surface defects

Defect engineering can increase the anchoring sites and bonding strength for SAs and thereby prevent the agglomeration of SAs by selecting appropriate supports. Defect engineering has become an effective strategy to restrict the migration of SAs and increase their loading, accordingly resulting in highly efficient electrocatalysis for Li–S chemistry. The defects, which are generally categorized into edge and vacancy defects in the supports, can tune the electronic and coordination environments of the surrounding atoms to form unsaturated coordination centers, thus improving the catalytic ability of SACs.

Due to the existence of unsaturated coordination around nanosheet edges, the atoms located at edges are more likely to absorb SAs and stabilize the corresponding hybrid configuration. Transition-metal dichalcogenide nanosheets are one of the ideal supports for SAs. For example, it is difficult to substitute atoms in molybdenum sulfide (MoS₂) nanosheets, due to the strong Mo–S bonds. Nevertheless, the S atoms at the edge sites can be ideal coordination centers because of the lone pair electrons. Ji et al.⁶⁷ developed MoS₂ material, by sulfurization and reduction (MoO₃ → MoS₃ → MoS₂), as a support to adsorb Cu²⁺ on the edge S^2– atoms to form Cu@MoS₂ SAC (Figure 5A). Moreover, the intrinsic step edges existing in metal oxide supports can also capture metal atoms and alter the structure and electronic states of SACs. Dvořák and co-workers reported that the (111) planes of CeO₂ contained step edges and could result in tunable density. The step edges with excessive oxygen could promote the stabilization of Pt²⁺ and give rise to monodispersed Pt SAs on CeO₂ (Figure 5B).⁶⁸ Different from the carbonaceous supports, the active metal compound substrates can serve as functional centers, suggesting that loading SAs on the active can increase the electrochemical activity of SACs. However, there is lack of in-depth research in this respect.

Figure 5. (A) Scheme of the preparation of Cu@MoS2 SAC. Reproduced with permission: Copyright 2019, Elsevier.67 (B) Left: STM images of Pt anchored on the CeO2 surface, where the bright dots represent the Pt atoms. Right: Pt adsorption sites on the CeO2 (111) surface obtained from DFT calculations. Reproduced with permission: Copyright 2016, Springer Nature.68 (C) Left: FE-SEM images of Au–WO3/TiO2. Right: HADDF-TEM images of Au–WO3/TiO2. Single Au sites are highlighted by green circles. Reproduced with permission: Copyright 2022, Royal Society of Chemistry.69 (D) Scheme of the electrochemical exfoliation process of MXene with immobilized single Pt atoms. Reproduced with permission: Copyright 2018, Springer Nature.70 DFT, density function theory; FE-SEM, field emission scanning electron microscopy; HADDF-TEM, high-angle annular dark-field transmission electron microscopy; SAC, single-atom catalyst; STM: scanning tunneling microscopy.

Anchoring of metal atoms on unsaturated edges can be easily achieved; nevertheless, it is difficult to obtain SACs with high SA mass loading, because of the limited density of edge vacancies. The vacancies are considered to be optimal defect sites to immobilize metal atoms, owing to the low adsorption energy and favorable restriction capability of the migration of metal atoms. The most commonly existing surface vacancies are S, O, and N vacancies. Reduction agents and electro-reduction treatment can break the bond between the metal and O/S atoms, giving rise to vacancies in the substrate. Wang and collaborators demonstrated that the electro-reduced Na₂SO₄ could remove the O atoms from WO₃/TiO₂ nanotubes to generate O vacancies, where Au SAs were then readily anchored to achieve a superior SA-Au@WO₃/TiO₂ catalyst (Figure 5C).⁶⁹ Also, Zhang's group attempted to develop Ru SAs anchored on S vacancies in MoS₂ using a chemical etching method.⁷¹ By altering the strain, the synergistic effect between Ru SAs and S vacancies could be modulated, thus altering the catalytic behavior and improving the reaction efficiency. Furthermore, metal vacancies were also found to be effective for anchoring SAs. Gogotsi and his collaborators trapped Pt atoms in MXene (Mo₂TiC₂T_x) with sufficient exposed basal planes and Mo vacancies via the interaction between the protons and surface groups on MXene (Figure 5D).⁷⁰ Such metal and nonmetal vacancies indeed provide effective sites for SA loading. Along this line, tuning of the vacancy level is important to increasing the SA loading and thus optimizing LSB systems.

To sum up, defect sites including edges and vacancies can be ideal sites for SAs; the synergistic effect between SAs and defects can regulate the electronic environment and tune the catalytic behaviors of SACs. Defect engineering can further increase the number of anchoring sites and bonding strength for SAs and efficiently prevents the agglomeration of metal atoms. By selecting appropriate substrates and optimizing heteroatom coordination, defect engineering can serve as an effective strategy to increase the loading of SAs to improve catalytic capability.

Increased metal atom loading of SACs

The utilization efficiency of SAs in SACs is nearly 100%. Based on this premise, the long-standing aim in SAC synthesis is to increase the loading of metal atoms on the supports, which is particularly important for highly effective electrocatalysis.^72,73 However, achievement of high SA loading (>5 wt.%) can be challenging due to the high surface energy of SAs, which results in spontaneous agglomeration and thus the formation of clusters or particles, decreasing the utilization efficiency of SACs for Li–S chemistry.

The key to high-loading SAs lies in the stable anchoring of SAs in the optimum configuration. In principle, whether the metal atom would evolve into a metal nanoparticle or an isolated atom depends on the competition between the strength of the metal–substrate interaction and metal–metal bonds.^74,75 Along this line, the focus has been on increasing the anchoring sites, bonding strength, and surface area of the supports to achieve high loading of SAs. The above-mentioned heteroatoms such as N, O, and S in carbonaceous supports with lone pairs of e^– can provide strong affinity with SAs.^76–78 For example, Sun and his group adopted a salt-template method to obtain a high SA loading of 15.3 wt.% in a N-doped carbon nanosheet, achieving LSB with excellent cycling stability and areal capacity (Figure 6A).⁷⁶ The HAADF-STEM image and FT-EXAFS spectra in Figure 6B,C indicate that Co atoms were uniformly distributed in the carbon support without a Co–Co bond. Such a template route can represent a rational substrate design by enlarging the surface area and optimizing the morphologies for increased SA loading. Such a strategy would inspire interest in research on tailored SACs for LSBs. In addition, different types of heteroatoms usually show distinct SA anchoring ability. For instance, pyridinic N species show stronger binding strength with SAs, in comparison to pyrrolic and graphitic N species, indicating their potential for the design of high-loaded SAs.^32,79 Therefore, in-depth explorations of active N-enabled SACs are also quite necessary for the theoretical and experimental investigations of active electrocatalysts.

Figure 6. (A) Scheme of the preparation and application of a CoSA–N–C catalyst in LSBs. (B) HAADF-STEM image of CoSA–N–C, where the red circled regions represent the Co atoms. (C) FT-EXAFS spectra of Co3O4, a Co foil, and CoSA–N–C. Reproduced with permission: Copyright 2020, Elsevier.76 (D) Scheme of the synthesis and model structure of atomically dispersed noble metal catalysts. Reproduced with permission: Copyright 2019, Science.77 (E) Schematic illustration of as-prepared Ru/mono–NiFe. Reproduced with permission: Copyright 2019, Royal Society of Chemistry.80 (F) Schematic illustration of the structural evolution of h-Pt1-CuSx. Blue, purple, and white balls, respectively, indicate Cu, Pt, and S atoms. Reproduced with permission: Copyright 2019, Elsevier.81 HAADF-STEM, high-angle annular dark-field scanning transmission electron microscopy; LSB, lithium–sulfur battery.

Other coordinated S atoms showed stronger capability on bonding with metal atoms, as compared with nitrogen and oxygen.⁷⁴ Wang et al. reported a single monovalent Pt center on mesoporous S-doped carbon. The ultrahigh content of sulfur and the large surface area of the carbon substrate led to high loading of SA Pt via strong Pt–S interaction. This universal strategy was successfully also used for the preparation of other SAs like Ru, Rh, and Ir with high-density loading (>10 wt.%) (Figure 6D).⁷⁷ Similarly, Uzun's group utilized rGO with abundant surface oxygen groups and a large surface area (1000 m² g^‒1) as the support to effectively anchor Ir SAs with a high metal loading of 14.8 wt.%.⁸² On the basis of these reports, the N-rich derivatives of C₃N₄, melamine formaldehyde, dopamine, and so forth, can act as preferred substrates for the preparation of SACs with remarkable SA loadings.^35,83

Other substrate materials and strategies have also been exploited to prepare SACs with high SA loading. Wang et al. proposed a one-step coprecipitation strategy to homogeneously distribute Ru SAs on top of Fe atoms in a monolayer NiFe double hydroxide, with a high metal loading of 7.0 wt.% (Figure 6E).⁸⁰ The interaction between SA Ru and O in NiFe LDH could prevent the spontaneous agglomeration of SAs. Analogously, Li and co-workers prepared SA Pt with an ultrahigh density of 24.8 atom% loading on hollow CuS_x via the strong interaction strength of Cu–S bonds present in the Pt atom agglomerations (Figure 6F).⁸¹ Other emerging supports such as MOFs and covalent organic frameworks (COFs) with spatially confined structures can also be applied as molecular-scale cages to restrain SAs in the limited space.^84,85 Defect engineering on supports has also been used to prevent the agglomeration of atoms by confining SAs in defective centers including vacancies and unsaturated sites of supports to realize atomically dispersed SACs.^{46,70,86–88} In addition, the strategies of sacrificial templates, frozen solutions, atom trapping, and electrochemical corrosion have also been applied to prepare SACs with high SA loadings.⁴⁶ The above-mentioned three fundamental design principles, targeting highly efficient electrocatalysis for Li–S chemistry, are shown in Figure 7.

Figure 7. Schematically illustrated combination of a coordination environment, surface defects, and metal atom loading to increase the electrocatalytic activity of SACs. SAC, single-atom catalyst.

TYPICAL FABRICATION OF SACs

As one of the most important electrocatalyst systems currently, the electrocatalytic activity of SACs can be determined by the selection of fabrication strategies. Thus, to achieve SACs with high SA loadings and rational coordination environments, the investigation and development of fabrication methodologies for SACs have been in the spotlight. The bottom-up and top-down routes as the two typical strategies for SAC fabrication have been proposed thus far. This section will compare the merits and demerits of the two strategies to guide the rational design of highly active SACs for Li–S chemistry.

Bottom-up routes for SACs

Bottom-up routes are performed by precipitating the metal atoms onto the selected supports. The metal precursors are adsorbed, reduced, and then confined in a limited space, such as various defect sites of supports. Carbonaceous materials including graphene,^89,90 CNTs,⁵⁹ C₃N₄,⁸³ MOFs,⁵⁰ and COFs⁹¹ and metal sulfide including VS₂ and⁹² MoS₂⁹³ usually act as substrates for metal atoms. The defects on the carbonaceous supports can be realized using various strategies such as heteroatom doping and vacancy construction. However, these defect sites are generally inhomogeneous, implying that it is difficult to achieve precise adjustment of SA structures. In this respect, the rational construction of defects on supports to obtain well-designed SACs using advanced techniques is highly desired.

Considerable attempts have been focused on bottom-up strategies for the fabrication of SAs. Wang et al.⁹⁴ summarized the currently applied bottom-up routes, for example, the mass-selected soft-landing technique and atomic layer deposition (ALD), as well as the wet chemical approach (Figure 8A). The ALD technique is similar to chemical vapor deposition, and it depends on a sequence of molecular-level, self-limiting surface reactions of precursors with substrates, emerging as a novel technique for SAC constructions. Both types of bottom-up routes have considerable advantages in maintaining atomic dispersion of metal atoms in supports under realistic fabrication and reaction conditions. Nevertheless, because of the high cost and the low yield, it is difficult to use these two routes for large-scale production.

Figure 8. (A) Scheme of the process of atomic layered deposition, the mass selective soft-landing method, and the wet chemistry method. Reproduced with permission: Copyright 2018, Chinese Chemical Society.94 (B) Scheme of the synthesis of Co2+@COF-300. Reproduced with permission: Copyright 2019, Wiley-VCH.91 (C) Scheme of the preparation of Co@C3N4. Reproduced with permission: Copyright 2019, Elsevier.83 (D) Preparation procedures of Fe–N2 on C3N4. Reproduced with permission: Copyright 2022, Wiley-VCH.35 (E) Scheme of the preparation of M1/CN (M = Pt, Ir, Pd, Ru, Mo, Ga, Cu, Ni, or Mn). Reproduced with permission: Copyright 2020, Springer Nature.95 (F) Scheme of the ionic exchange method to achieve Ni–SAC. Reproduced with permission: Copyright 2021, Elsevier.96 COF, covalent organic framework; SAC, single-atom catalyst.

Many wet chemistry routes such as the typical bottom-up strategy, usually implemented by applying mononuclear metal precursors, have become more desirable from the practical viewpoint, owing to the low cost and facile operation. Song et al. synthesized a N–HP–Co SAC by dissolving a Co(acac)₂ precursor into aqueous dopamine-derived carbon dispersions, and then annealing at 900°C under a N₂ atmosphere,⁹⁷ and it was found that the local N atoms stemming from the dopamine promoted the monodispersed distribution of Co atoms. For the non-N supports, foreign N doping into carbon can bridge the support and metal atoms to generate a “M–N–C” coordination structure, which is favorable for preventing SA aggregations. In addition, SA spatial confinement and coordination rationalization in COFs have also become the main research focus for the wet chemistry synthesis of SACs.⁵⁰ For example, Sun reported COF-derived Co–NC with isolated Co SAs via wet chemistry synthesis and pyrolysis of Co@COF-300 (Figure 8B).⁹¹ Co SAs were stabilized at the edge of standing carbon layers. Similarly, COF-derived Fe–NC was fabricated for excellent ORR activity using a “COF–adsorption–pyrolysis” method.⁹⁸ Although the wet chemistry route has been widely applied in fabricating various SACs, it is still inadequate to realize precise and atomic-level control in terms of composition and loadings. in addition, due to the complex liquid phase environment, it may be difficult to avoid oxidation and increase the purity of SACs. In this regard, special protection measures or treatment for wet chemistry reactions should be taken into consideration.

Top-down routes for SACs

To a certain extent, the development of top-down routes for SACs, including one-step pyrolysis, use of ligands, and host guests, can address the shortcomings of the bottom-up routes. MOFs have been considered as one of the potential candidates for the synthesis of SAs because of their porous structure, outstanding specific surface areas, and chemical bonding ability. Remarkably, the inherent atomic-scale distribution of metal centers creates a natural coordination environment, making it promising to synthesize highly active SACs for Li–S chemistry. Typically, the high surface areas of MOFs can enable the adsorption of SAs and further form metal–heteroatom–carbon or metal–carbon coordination structures throughout the routine thermal annealing procedure. Wu et al. designed MOF-derived Co@C₃N₄ as an active electrocatalyst for LSBs. Such Co SAs were obtained through the reaction of a series of precursors involving melamine, cyanuric acid, and cobalt acetate in dimethyl sulfoxide (DMSO) and subsequent annealing in an Ar atmosphere (Figure 8C).⁸³ Using a diatomite-template strategy, Ding and his collaborators designed undercoordinated Fe–N₂ on a C₃N₄ support with a high metal loading of 6.32 wt.% (Figure 8D).³⁵ It is also noteworthy that the derivatives of MOFs comprise of low-content SAs as well as high-loading metal compounds. Zhang et al.⁹⁹ successfully prepared V₂O₃@C, V₈C₇@C, and V₂O₃/V₈C₇@C as hosts for sulfur cathodes by pyrolysis of MIL-47 at different temperatures. However, the existence of metal compounds gives rise to a huge challenge in improving the purity of SACs. In this regard, the strategy of hydrochloric acid etching has been corroborated to be effective to remove the particles and thus reserve the SAs. Impressively, both the structural adjustability of MOFs and the operational controllability of pyrolysis can be favorable for tailoring well-designed SACs for Li–S chemistry.

Recently, other new top-down routes have also been developed to prepare active SACs. For instance, Li et al. proposed a universal host–guest strategy to construct SAs on N-doped carbon supports (M₁/CN, M = Pt, Ir, Pd, Ru, Mo, Ga, Cu, Ni, or Mn).⁹⁵ In this host–guest strategy, the metal precursor guests were first in situ trapped in cages of porous MOF hosts during the crystallization process, followed by pyrolysis at 800/900°C for 3.0 h under an Ar atmosphere to fabricate the SAC products (Figure 8E). Wang et al.⁵⁰ encapsulated Fe atoms into nanocages of ZIF-8 to prepare Fe-SA, coordinated with organic frameworks, as a cathode catalyst for LSBs. It was found that it was relatively difficult for the reduced Fe atoms to migrate under extremely high temperatures. The ionic exchange strategy has also been introduced to aid the fabrication of SACs. Hao et al. developed ionic exchange of ZIF-8 to achieve Ni–SA (Figure 8F).⁹⁶ In the recent work of Ma et al.,¹⁰⁰ Zn nodes in ZIF-8 were successfully exchanged by Cu ions at a high temperature of 900°C, leading to the generation of Cu-SAC. The ionic exchange route can result in the accurate modulation of SA structures at the atomic level, and hence serves as a new method for the fabrication of active SAs targeting promoted sulfur redox reaction kinetics. In spite of some progress in other fields, reports on the preparation of SACs by host–guest and ionic exchange routes are rare in the LSB field.

All these desirable strategies have been developed with the aim of synthesizing SACs with favorable SA loading and a well-designed coordination environment. However, there are obvious differences in the formation mechanism of bottom-up and top-down strategies. The former strategy usually involves the deposition of only low-loading SAs onto substrates and needs to overcome the aggregation issue resulting from the large surface energies. The latter involves the top-down pathway for SAC preparation and is intended to endow them with SA loading and coordination optimization. The one-step pyrolysis of MOFs as a representative of top-down strategies can retain the metal nodes by carbonizing the organic linkers, leading to the final synthesis of SACs. A comparison of the features of bottom-up and top-down strategies is also presented in Table 1.

Table 1 Comparison of the features of bottom-up and top-down strategies for single-atom catalyst synthesis

Preparation strategy	Type involving	Structural features	Application features
Bottom-up strategy	i. Wet chemistry ii. Mass-selected (MS) route iii. Atomic layer deposition (ALD)	i. Low SA loading ii. Easy operation for the wet chemistry route ii. Hard coordination control iii. Low yields for MS and ALD routes	i. Low electrocatalytic activity ii. Low industrial application potential for MS and ALD routes iii. Wide use range iv. Expensive or complicated equipment for MS and ALD routes
Top-down strategy	i. One-step pyrolysis ii. Ionic exchange iii. Host–guest route iv. Ligand-assisted route	i. High SA loading ii. Delicate structure control iii. Limited purity for one-step pyrolysis iv. Large yield	i. High electrocatalytic activity ii. Favorable industrial application potential iii. Wide use range

SAC STRATEGY FOR LI–S CHEMISTRY

To overcome the limitations of LSBs, reducing the size of a catalyst into a single atomic level has been described as an ideal strategy. SACs are molecularly monodispersed on a solid substrate, resulting in low weight density, high electronic conductivity, and abundant active sites. Therefore, they can achieve almost 100% atom utilization efficiency, thereby giving rise to higher electrocatalytic ability than a metal-based nanoparticle catalyst.^37,101 The density and location of coordination sites, as well as neighboring atoms of SACs, can be rigidly modulated to be selective and efficient for optimizing Li–S systems. Such independent and well-defined sites can be used to simulate the essential path and detect the in-depth mechanism of electrochemical reactions.^46,73

Recent years have witnessed exciting achievements resulting from exploration of the use of SACs with local atomic configurations such as M–X₄ and M–X₂ (X: N, O) in the cathode, anode, and interlayer as well as the electrolyte of Li–S batteries. Along this line, these advances have confirmed the effectiveness of SACs in the aspects of anchoring LiPSs, accelerating Li₂S_x (1 ≤ x ≤ 8) conversion, and mitigating the growth of Li dendrites.^37,102 In this section, we will comprehensively present the updated advances in the development of SACs for LSBs. The relation between the local environment of SACs and the “LiPS adsorption–diffusion–conversion” process, as well as the working lithium interface in LSBs, is elucidated in detail, along with advanced characterization for monitoring the role of SACs. The first and second parts will, respectively, focus on the adsorption principle of soluble LiPSs and the catalysis path for Li₂S_x (1 ≤ x ≤ 8) conversions. In the end, current breakthroughs and opportunities will be presented upon the application of SACs in the field of LSBs.

Alleviation of the LiPS shuttle effect by SACs

The shuttle effect caused by the soluble LiPSs can result in the irreversible loss of active sulfur and thus rapid capacity degradation. Based on the principle of LiPS adsorption, the support of SACs is usually expected to have a large specific surface area and porous structures. The strong chemical adsorption between SACs and sulfur species is mainly responsible for the alleviation of the shuttle effect. In light of this, the mass loading and the local coordination environment play a critical role in suppressing the LiPS shuttle effect. Extensive efforts have been made to attain highly adsorptive SACs, and insights into the mechanism are further provided. In principle, SACs have metal centers with unsaturated coordination and are capable of serving as anchoring sites for sulfur species. Along this line, the Lewis acid–based theory has been used to unravel the underlying adsorption mechanism. For example, Liu et al.¹⁰³ confirmed that the strong adsorption interaction between electrophilic Fe (II) in Fe (II)–N₄ coordination centers and nucleophilic S₄^2– species endowed the Fe SA (Fe–PNC) with remarkable LiPS anchoring ability (Figure 9A). In 2019, Niu et al. further reported the synthesis of catalysts with Ni on N-doped graphene using a pyrolysis approach, to modify the separator for LSB, as shown in Figure 9B. According to the Lewis model, the half-filled d orbital of Ni could accommodate the electron from LiPS anions, with the formation of Ni–S bonds. Together with the simultaneous interaction between the Li cation and the N atom with a lone pair electron, LiPSs could be trapped properly on the surface of Ni–N₄. DFT simulations also confirmed that the embedding Ni atoms endowed NG with enhanced binding energy, thereby efficiently immobilizing LiPSs.¹⁰⁴ The atom size of SACs, the metastable states of LiPS species, the limited detection range, and the resolution of current analytical techniques including Raman, Ultraviolet–visible (UV–vis) spectra, and so forth, led to lack of clarity on the working mechanism of SACs in terms of alleviation of the LiPS shuttle effect. This problem still remains to be resolved by development of advanced probe tools with higher resolution and wider range.

Figure 9. (A) Schematic demonstration of the LiPS evolutions on the surface of Fe–PNC. Reproduced with permission Copyright 2018, American Chemical Society.103 (B) Left: Molecular structure of Ni@NG with the Ni–N4 sites; middle: binding energy profiles of Ni@NG and NG on sulfur species; right: reproduced with permission Copyright 2019, Wiley-VCH.104 (C) Schematic illustration of how CoSA–N–C contributes toward improving the conversion kinetics between the solid (S, Li2S) and LiPSs and mediating the deposition of Li2S. (D) XPS spectra of (left) S 2p and (right) Li 1s of CoSA–N–C/Li2S6. Reproduced with permission Copyright 2020, Elsevier.76 (E) Schematic illustration of a M1/NG modified separator applied in LSB (the inset shows the digital photo of bare and coated separators). (F) Left: Binding energy of Li2S6 adsorption on Fe/NG and NG; right: adsorption capacity of Li2S6 on NG and M1/NG. Inset: Digital photo of Li2S6 with samples over 12 h. Reproduced with permission Copyright 2019, American Chemical Society.105 (G) Left: Side view of charge density difference of Li2S adsorption on graphene, NG, SACo, and V on NG; right: calculated binding energies of Li2S6 on graphene, NG, and various SAs on NG. The inset shows the side view for the Li2S6 adsorption configurations on SAV@NG. Reproduced with permission Copyright 2020, American Chemical Society.48 (H) Left: Schematic of the design strategy of a porous host with a Co–N–C catalyst. Right: Time-of-flight secondary ion mass spectrometry (ToF-SIMS) profiles on the S ion distribution of different cathodes after 100 cycles at 1.0 C. Reproduced with permission Copyright 2021, Springer Nature.106 LiPS, lithium polysulfide; LSB, lithium–sulfur battery; XPS, X-ray photoelectron spectroscopy.

Apart from Fe and Ni atoms, Co−X₄ (X: N, O) active moieties have also emerged as effective catalysts for LiPS conversions. Sun and co-workers⁷⁶ applied Co SAs as excellent host materials for superior LSBs (Figure 9C). The XPS spectra and the adsorption test revealed the strong adsorption capability of Co–NC toward Li₂S₆. The shift of terminal S and the existence of Li−N bonds indicated the electron transfer from nucleophilic LiPSs to electrophilic Co–N₄, with the formation of Co−S₂^6– and N−Li bonds (Figure 9D). The calculated adsorption energies of Li₂S₆ on Co−NG and NG were –0.50 and –0.35 eV, respectively, further confirming the dual lithiophilic–sulfiphilic feature of Co−NG contributing toward alleviating the shuttle effect. On the one hand, the Co–N₄ center was Lewis acid in nature, and due to the empty 3d orbitals, it tended to bond with S atoms in LiPSs (Lewis base). On the other hand, the Lewis-based N atom in the Co–N₄ moiety could adsorb the Lewis-acidic Li atom in LiPSs. Co and N atoms in the matrix could collaboratively act as the anchor for LiPSs. Xiao et al. also developed SA-Cu@NCNF as an electrocatalyst for sulfur cathodes.¹⁰⁷ Cu–NC centers showed excellent affinity and catalysis capability toward LiPSs. Meanwhile, CNF foam provided continuous 3D electronic transport networks.¹⁰⁸ Co atoms cannot always be useful in adsorbing LiPSs. One study carried out by Wan's group showed that the Co−NC moiety only showed average adsorption capability, as compared with NC.

In those studies, considering that SACs are randomly chosen and prepared, one may speculate on the most applicable SACs in terms of the practical requirements of LSBs. The optimization of atomic–local coordination environments by selecting various single-metal atoms can be beneficial for tailoring the LiPS anchoring ability targeting practically viable LSBs. Along with this, Xie and co-workers¹⁰⁵ applied three SAs (Fe, Co, and Ni) on N-doped graphene as separator modifications for LSBs with prolonged cyclic life (Figure 9E). The authors adopted quantitative adsorption and a permeation test to verify the adsorption ability of the three SAs, where Fe atoms outperformed the other two SAs. Furthermore, the shortened distance of Fe···S (2.248 Å) and N···Li (2.135 Å), as well as rotation of Li₂S₆ toward the catalyst, based on DFT calculations, implied the stronger adsorption of Li₂S₆ and Li₂S₄ on Fe−NC, in contrast to NC, due to the strong attraction between the Fe atom and LiPSs. The adsorbed quantities of Li₂S₆ by Fe1/NG, Co1/NG, Ni1/NG, and NG were 4.10, 3.05, 2.97, and 1.53 μmol m⁻², respectively, according to UV–vis measurements (Figure 9F). Analogously, Cui's group carried out a theoretical simulation, attempting to identify effective SACs for advanced LSBs.⁴⁸ The chemical interactions between the Li₂S₆ and a variety of metal atoms (including Fe, Mn, Ru, Zn, Co, Cu, V, and Ag) were systematically determined (Figure 9G). The binding energies of Li₂S₆ on SAV@NG, SARu@NG, SACo@NG, SAZn@NG, SAFe@NG, and SAMn@NG were separately calculated to be 3.38, 1.69, 1.67, 1.02, 0.95, and 0.84 eV, respectively. As such, the SAV@NG presented the highest binding energy value of 3.38 eV. As expected, the adsorption experiments confirmed that V SAs possessed better absorption capability for S₆²⁻, as compared with other counterparts, further indicating the distinct advantage of V SAs in immobilizing the dissolved LiPS and thus retarding the notorious shuttle effect. The structure stability of SACs in chemically anchoring sulfur species needs to be focused on. The bond of a metal atom and a neighboring coordinated nonmetal atom may be broken or the key atom may be replaced by a Li atom of LiPSs. Therefore, calculation simulations and experimental confirmation should be conducted. Additionally, the delicate design of supporting materials can help to exploit the full potential of SACs in Li–S chemistry. For instance, Zhao's group proposed an ordered macroporous configuration with evenly embedded ZnS nanoparticles and Co SAs. These oriented macropores could significantly increase the sulfur content, improve ion transport, and eliminate the LiPS shuttle effect, especially under high sulfur loading (Figure 9H). Accordingly, this electrocatalyst demonstrated great potential in practical Li–S pouch cells with only 100% excess Li and sulfur loading of 1.2 g, capable of a smooth operation over 80 cycles.¹⁰⁶ The substrates can help to synchronously immobilize the LiPS shuttle effect by introducing polar centers and constructing porous structures.

To conclude, the superior adsorption capability of SACs on LiPSs is mostly elucidated according to the Lewis acid-based theory, where the electron transfers from the LiPS anions (base) to unoccupied d orbitals of SA (acid) in the M–X₄ center, generating a M–S bond. Besides, the interaction of the Li cation and lone-pair X can induce a strong Li–X bond. The theoretical simulations can provide evidence that the binding energies of SACs for LiPSs are usually stronger than that of the countering sample without SAs.

Promoted reaction kinetics by SACs

The reaction at the cathode is a complex multi-step electrochemical procedure involving the conversion process of S → Li₂S₈ → Li₂S₆ → Li₂S₄ → Li₂S₂ → Li₂S. Long-chain soluble LiPSs cause the shuttle effect, while the poor conductivity of S, insoluble Li₂S₂, and Li₂S leads to sluggish conversion with a high energy barrier.⁴⁰ SACs possess unique properties of an unsaturated coordination environment, high surface energy, and sufficient active sites, showing great potential in achieving unprecedented electrolysis toward sluggish LiPS conversions. SAs should act as effective catalytic sites, reducing the activation energy and Gibbs free energy of the rate-limiting step, to promote faster conversion kinetics of LiPSs. The possible explanations behind the superior catalysis of SACs upon LSBs can be elucidated as follows: (i) From the thermodynamics aspect, the surface free energy of the metal can markedly increase if tracking down into the scale of SACs, which can promote sulfur redox reactions. (ii) SACs can realize the rational regulation of an electronic structure to improve catalytic efficiency by adjusting the position of d-bands, filling d orbitals to different degrees, and coupling the structure between the adsorbate and d-band electron states of metal sites. The redistributed electron around the metal centers can reduce the band gap near the Fermi level, consequent to the faster electron transfer. (iii) The atoms adjacent to support SAs can also act as cocatalysts to synergistically accelerate the electrochemical process.^46,101 SACs with unfilled orbitals have been proven to be effective in entrapping and catalyzing LiPS evolution as described in the previous section. This section will mainly discuss the synergy and division catalysis mechanism of metal centers and heteroatoms in Li–S chemistry.

Considering the rarity of noble metals such as Pt and Au, transition metals like Fe, Co, and Ni are ideal SAs for applications in SACs. Zhang's group utilized Fe–NC moieties as sulfur hosts and pioneeringly proposed the catalytic conversion pathway of SAC and supported heteroatoms for LSBs.¹⁰⁹ As the decomposition of Li₂S was considered as the rate-determining step during the redox process, the authors first calculated the reaction barrier Li₂S delithiation. Based on the DFT result, the highly active Fe–SAs markedly lowered the barrier energy during the breakage of the Li−S bond (Figure 10A,B). During the in situ XAS observation, the K-edge of Fe shifted to a lower-energy position during charge, implying the reduction of Fe, which was caused by the interaction between nucleophilic sulfur species and electrophilic Fe atoms. Encouragingly, this motion of the Fe K-edge could be restored to the original energy value, indicative of highly reversible electrocatalysis ability (Figure 10C). Inspired by the experimental and theoretical results, the authors proposed a catalytic pathway for Fe SAs in Li−S chemistry. During charge, Fe atoms first interacted with Li₂S, giving rise to a prolonged and weakened Li−S bond. Then, Li could easily be deintercalated, coupled with electron withdrawing. The partially oxidized intermediates could coordinate with adjacent Li₂S species to repeatedly form longer polysulfide chains. This delithiation process continuously proceeded until LiPSs were released from Fe atoms at a certain charge state. When the produced LiPSs were detached from the reaction spot, consequently, more Li₂S was exposed to Fe atoms to repeat the same conversion process. Also, the highly capable Fe–SA could interact with over one Li₂S molecule, hence initiating multiple redox processes simultaneously. The synergistic effect of lowered activation energy in multi-reactions yielded LSBs with excellent rate performance and ultra-long lifespan. Similarly, Fe–NG was used as a separator modifier in LSBs, and in situ Raman spectroscopy was used to investigate the conversion chemistry.¹⁰⁵ Upon discharge, the earlier appearance of S₆²⁻ and S₄²⁻ at 2.42 V, complete disappearance of S₈ signal at 2.24 V, and a dominant Li₂S peak at 2.10 V indicated the promoted discharge depth and sulfur utilization. This process was also reversible in the charge process. The superior catalytic ability of Fe atoms toward LiPSs was beneficial for improving the ultimate capacity and cyclic performance. The electrochemical reversible evolution based on the K-edge energy of the optimized LSBs by a Ni SA-modified separator was observed in 2019.¹⁰⁴ Based on further XPS spectra, it was believed that the continuous electron delocalization between the unfilled d orbitals of Ni atom and negative LiPSs (positive Li⁺ and N atoms with lone-pair electrons) constantly suppressed the shuttle effect and enhanced the conversion kinetics. The DFT calculation on the Gibbs free energy of phase transitions also confirmed the effect of coordinated Ni SAs in reducing the energy barrier at each step and accelerating the inter-conversions in Li–S chemistry.

Figure 10. (A) Proposed mechanism scheme of SAFe catalyzing Li2S decomposition. (B) Comparison of decomposition energy barriers of pristine Li2S and Li2S@NC with/without Fe SA. (C) In situ XANES spectra of the Li2S@NC:SAFe cathode at different oxidizing states under 0.1 C. Reproduced with permission Copyright 2019, Elsevier.109 (D) Variations of (left) S K-edge XANES and (right) intensities of peaks B (LiPSs: 2469.0 eV) and D (Li2S: 2474.7 eV) during cycling. Reproduced with permission Copyright 2019, American Chemical Society.107 (E) Current evolutions during the PITT test. Reproduced with permission Copyright 2019, Wiley-VCH.110 (F) Scheme demonstrating the conversion process on SAV-NC and NC for Li–S batteries. (G) Images of X-ray three-dimensional nano-computed tomography of Li2S deposition on V–N–C and N–C with various rotation angles. (H) Potentiostatic discharge profile of a Li2S8/tetraglyme solution with V–N–C. Inset: SEM images of precipitated Li2S. Reproduced with permission Copyright 2021, Elsevier.111 (I) Schematic illustration of the redox process/LiPS adsorption on C3N4 and Fe-N2@ C3N4 for LSBs. (J) Free energies of LiPS reduction to Li2S on 3DFeSA-CN and CN. Reproduced with permission Copyright 2022, Wiley-VCH.35 LSB, lithium–sulfur battery; PITT, potentiostatic intermittent titration; SEM, scanning electron microscopy; XANES, X-ray absorption near-edge structure.

Wan's group developed Co–N–C coordination centers surrounded by NC that served as bifunctional electrocatalysts to facilitate the redox process of Li₂S during discharge and charge.¹⁰⁷ The operando XANES measurement revealed that Li₂S nucleated at a favorable reaction time and overpotential during cycling, indicative of the accelerated reaction dynamics for the phase changes (Figure 10D). DFT calculations further demonstrated that the energy barrier for the formation and decomposition of Li₂S with Co–SA was lower, as compared with NG. The extensive exploration of Co–SACs can lead to an understanding of the working mechanism of SACs in electrocatalyzing Li–S chemistry redox reactions. Huang and his collaborators also experimentally verified the superior electrocatalytic effects of Co–NC moieties on the nucleation and growth of Li₂S using the potentiostatic intermittent titration technique, as shown in Figure 10E.¹¹⁰ The precipitation and decomposition of Li₂S highly depended on the applied current, affinity capability, and nucleation sites of the host. Due to the higher response current and earlier response time, together with the larger nucleation capacity of cells, Co–NC was indeed an effective electrocatalyst to expedite the ultimate step of the reduction. Besides, it has been claimed that Co SA is also a mediator in inducing the homogeneous propagation of Li₂S on the electrode surface. Co-based SACs show satisfactory electrocatalytic ability for sulfur redox reactions. More importantly, the currently reported Co–SACs can maintain structural stability throughout the whole cycling procedure. These conspicuous merits make the Co–SACs competitive catalysts in LSB systems.

Various SAs including Fe, Mn, Ru, Zn, Co, and V were collectively investigated in terms of the decomposition barriers of Li₂S by Cui et al.⁴⁸ Theoretical and experimental results revealed that SACo@NG (0.72 eV) and SAV@NG (0.84 eV) have a relatively lower Gibbs energy barrier toward the rate-determining step from Li₂S₂ to Li₂S. In addition, in our recent work also it was found that the dual active site of nitrogen-coordinated V–SAC with VN nanoparticles anchored on a carbonaceous framework with superior Li⁺/e⁻ migration capability toward Li–S chemistry, leading to uniform propagation and decomposition of Li₂S upon charge/discharge (Figure 10F).¹¹¹ In particular, this study utilized the synchrotron three-dimensional nano-computed tomography (X-ray 3D nano-CT) technique to demonstrate Li₂S precipitation on the electrocatalyst with different angles (Figure 10G). The V–N–C catalyst induced homogeneous growth of small Li₂S particles, indicative of the lower dissociation energy barrier of Li₂S on SAV–N–C, which was in good agreement with the corresponding Li₂S deposition test result (Figure 10H). In view of the limited loading of SAs, the additional integration of special nanoparticles into SACs can help to elevate the electrocatalytic activity for the conversion reactions of LiPSs to a certain extent. This new concept of SAs and nanoparticles may represent a new frontier in the field of catalyst design. The coordinative Fe–N₂ interaction on C₃N₄ was also demonstrated by Sun's group as a promising carbon-based host material for LSBs (Figure 10I).³⁵ The Li₂S₆ symmetric cell and Tafel curves of solid–liquid Li₂S oxidation proved that Fe atoms could create lower overpotentials toward LiPS evolutions and faster reaction kinetics. From the experimental and theoretical results, it was evident that the energy barrier of the rate-determinant step (liquid–solid conversion) was markedly suppressed with 3DFeSA–C₃N₄ (Figure 10J). The use of SACs in cathodes or separators can optimize the electrochemical reactions in the cathode side to a huge extent, which is favorable for effectively dealing with the critical issues resulting from the use of a sulfur cathode.

Besides the application as host materials and interlayer/separator modifiers, soluble SACs can be used as special redox reaction mediators in an electrolyte. The soluble atoms in the electrolyte may be beneficial for completely uncovering solid discharge and charge products and to promote close contact with LiPSs in the electrolyte, as compared with an electrocatalyst in bulk host materials. Generally, the use of an electrocatalyst in cathodes or separators represents the heterogeneous catalysis mode. Soluble SACs as additives in an electrolyte may lead to the creation of a new homogeneous catalysis concept in Li–S chemistry, which provides an avenue for investigation of new catalysts in the LSB field. A comparison of the electrochemical performance of SAC-enabled LSBs is shown in Table 2.

Table 2 Summary of the electrochemical performance of various single-atom catalyst-promoted lithium–sulfur batteries

SA type	Coordinations	Substrate	SA loading	Function	Initial capacity (mAh g‒1, C)	Cycling performance (capacity, cycles, rate [C], S loading [mg cm‒2])	Ref.
Fe	Fe–N2	C3N4	6.32 wt.%	Host	1363.5, 0.2	400 mAh g‒1, 2000, 1, 1.5; 5.16 mAh cm‒2, 55, 0.2, 5.75	[35]
Zn	Zn–N4	Co-in PNCSh	1.4 wt.%	Host	1185, 0.2	680 mAh g‒1, 800, 1.0 C, 1.0; 4.5 mAh cm‒2, 100, 0.2, 5.1	[49]
Fe	Fe–N2	NC	-	Host	1397, 0.2	800 mAh g‒1, 500, 1.0, 1.5; 4.5 mAh cm‒2, 80, 0.1, 5.0	[55]
Ru	Ru–O4	rGO	6.3 wt.%	Separator modifier	1080, 0.2	500 mAh g‒1, 800, 1.0, 1.2; 4.4 mAh cm‒2, 50, 0.1, 5.8	[58]
Ni	Ni–N5	Hollow NC	0.6 wt.%	Host	1123, 0.2	798 mAh g‒1, 500, 0.5, 1.3~1.6; 4.0 mAh cm‒2, 80, 0.1, 5.1	[56]
Co	Co–N2	NC	1.23 wt.%	Host	1280, 0.2	950 mAh g‒1, 100, 1.0, 1.2; 6.5 mAh cm‒2, 100, 0.6, 9.0	[106]
V	V–N4	NC	0.7 atom%	Host	1132.5, 0.2	450 mAh g‒1, 1000, 2.0, 1.6; 6.0 mAh cm‒2, 40, 0.05, 8.1	[111]
W	W–O2N2	NG	8.6 wt.%	Separator modifier	1389, 0.2	605 mAh g‒1, 1000, 2.0, 1.1; 6.24 mAh cm‒2, 100, 0.2, 8.3	[60]
Fe	Fe–N5	NC	0.018 wt.%	Host	1170, 0.2	662 mAh g‒1, 500, 1.0, 1.0; 6.0 mAh cm‒2, 50, 1.0, 8.2	[65]
Fe	Fe–N4	NG	1.33 wt.%	Separator modifier & host	1242, 0.2	604.7 mAh g‒1, 500, 4.0 (801.3 mAh g‒1, 500, 1.0), 1.5; 5.02 mAh cm‒2, 100, 0.1, 5.0	[52]
Fe	Fe–N3S	N,S-CSh	3.2 wt.%	Host	1093, 0.2	477.1 mAh g‒1, 1000, 1.0, 1.0	[54]
Co	Co–N4	NCSh	4.1 wt.%	Separator modifier	1022, 0.2	460 mAh g‒1, 700, 2.0, 1.0; 4.23 mAh cm‒2, 100, 0.5, 7.2	[59]
Co	Co–N4	NCSh	15.3 wt.%	Host	1088, 0.2	675 mAh g‒1, 1000, 1.0, 1.2; 4.24 mAh cm‒2, 120, 0.2, 4.9	[76]
V	V–N4	NG	4.3 wt.%	Host	1230, 0.2	551 mAh g‒1, 400, 0.5, 2.0; 2.42 mAh cm‒2, 200, 0.5, 5.0	[48]
Fe	Fe–N4	NC	1.14 wt.%	Host	1123, 0.2	403 mAh g‒1, 500, 4.0, 1.4	[50]
Fe	Fe–N4	NCS	1.03 wt.%	Host	1110, 0.2	834 mAh g‒1, 1000, 1.0, 2.0; 5.24 mAh cm‒2, 100, 0.1, 5.4	[31]
Fe	Fe–N2	NC	11.4 wt.%	Host	1301, 0.2	620 mAh g‒1, 2000, 2.0, 1.4; 5.4 mAh cm‒2, 100, 0.2, 5.6	[66]
Co	Co–N4	NG	0.77 atom%	Host	1120, 0.2	681 mAh g‒1, 500, 1.0, 2.0; 5.1 mAh cm‒2, 100, 0.2, 6.0	[107]
Ni	Ni–N4	NG	-	Separator modifier	1400, 0.2	826.2 mAh g‒1, 500, 1.0, 1.5; 3.02 mAh cm‒2, 100, 1.0, 6.0	[104]

In brief, the key to an effective “adsorption-conversion process” depends on the delocalization of unpaired electrons in unfilled d orbitals of SAC with negative LiPS anions. Atomically dispersed lithiophilic and sulfiphilic sites of SACs on doped or defective substrates can completely promote atomic-efficient electrocatalysis toward the reversible conversion of sulfur species. On the one hand, the uniformly distributed SAs can efficiently catalyze LiPSs before migrating into the electrolyte, due to the ultrahigh reactivity. The weakened LiPS transportation can also protect the lithium anode from the corrosion of LiPS depositions. On the other hand, the SA–NC moieties on a substrate with a large surface and sufficient pores can strongly suppress LiPS shuttling via physical and chemical confinement. Besides, such substrates can accommodate the volume change of the active sulfur and intermediate material during cycling. These synergistic effects can lead to the creation of advanced LSBs with high capacity, remarkable efficiency, and long cycling, even at elevated sulfur loading (Figure 11).

Figure 11. Schematic diagram of the promoting effect of SACs on Li–S chemistry. SAC, single-atom catalyst.

Mitigated lithium dendrite by SACs

Lithium metal anode is an ideal choice for high-energy-density rechargeable batteries because of the extremely high theoretical capacity (3860 mAh g⁻¹) and low electrochemical reduction potential (−3.04 V vs. a standard hydrogen electrode). However, the huge volume expansion and uncontrolled Li dendrite growth over stripping/plating can lead to low CE, rapid capacity decay, and short lifespan, which hinders the practical use of lithium metal anodes.^16,112 In particular, the irregular dendrite formation and continuous SEI disruption can result in major safety concerns. It is noteworthy that the hostless nature of Li deposition and uneven Li⁺ distribution are the main causes of dendrite growth over cycling. Thus, increasing the lithiophilicity and homogenizing Li⁺ reflux can be effective strategies to achieve stabilized cycling performance. Along with this, SACs have recently attracted attention due to their surprising capability to function as nucleation seeds of Li, accelerating the diffusion rate of Li⁺, and thereby inducing the uniform deposition of Li and restricting Li dendrite formation. Moreover, the substrate of SACs generally has a porous structure with a high specific area to accommodate huge volume changes.

The superior affinity of SA toward Li was reported by Zhang and his co-workers, with an example of Co SA on N-doped graphene.¹¹³ The strong Co–N configurations could induce electron redistribution in the material matrix, contributing to the remarkable lithiophilicity  of Co–N_x molecular. The atomically disseminative Co–N_x species promoted stronger Li⁺ adsorption and ensured smooth nucleation behavior, without dendrite formation (Figure 12A). Also, Fe atoms anchored on NC were demonstrated by Sun et al. to act as lithiophilic seeds to lower the overpotential of Li nucleation from 18.6 to 0.6 mV (Figure 12B).¹¹⁴ Molecular dynamic simulations and the radial distribution function were used to reveal that Li⁺ could be easily attracted around the Fe–NC coordination, suggestive of favorable affinity between Li⁺ and Fe–NC species, resulting in homogeneous plating behavior (Figure 12C,D). During the CE test, the FeSA-NC cell showed higher cycling stability over 120 cycles with a high CE of 98%, as compared with the counterpart, at 3 mA cm^‒2/1 mAh cm^‒2. Gong's¹¹⁵ groups successfully loaded three kinds of SA (Ni, Pt, and Cu) on a graphene scaffold as a stable host for a Li anode. The populated SA, coordinated with a N atom on graphene, could improve the adsorption energy upon the local space around SA sites, forming a moderately grading profile of Li adsorption (Figure 12E). Additionally, the authors claimed that the incorporation of SAs into the N-doped graphene improved the stability of the atomic structure of the whole material and eliminated the potential damage caused by Li–N stretching. This synergistic effect could suppress the loss of active Li, defend fragile solid electrolyte interphase (SEI), and induce uniform deposition, resulting in long-term cycling with high CE. Yang and his collaborators¹¹⁶ demonstrated that Zn SAs immobilized on MXene easily led to governable plating/stripping behavior of Li. During the initial plating, Li preferred to nucleate on the surface of Zn-MXene and deposit vertically along the edge of Zn-MXene, due to the strong lightning rod effect, generating bowl-like Li (Figure 12F). Accordingly, this design led to an ultralow overpotential of 11.3 ± 0.1 mV, long cycle life of 1200 h, and deep stripping/plating of 40 mAh cm⁻² with a smooth surface (Figure 12G). Another study of Zn SA sites again revealed that Zn SAs served as nucleation seeds to induce homogeneous Li deposition.¹¹⁷ It can thus be concluded that the unique nucleation agent role of SACs, based on various substrates, can lead to lowered nucleation barrier and faster Li reaction kinetics.

Figure 12. (A) Schematic representation of the SACo–NC material and the preferred Li nucleation spot. The gray, blue, and deep pink balls represent carbon, nitrogen, and cobalt atoms, respectively. Reproduced with permission Copyright 2019, Wiley-VCH.113 (B) Schematic diagram of the Li nucleation path and barrier on C@Cu and FeSA–N–C@Cu. (C) Molecular dynamics simulations of FeSA–N–C in DOL/DME electrolytes, with Li+ marked in green. (D) Radial distribution function of Li–C and Li-FeSA-N-C. Reproduced with permission Copyright 2020, American Chemical Society.114 (E) Distribution maps of Li adsorption energy and schematic of the nucleation and plating process of metallic Li on a SA metal (Ni, Pt and Cu)–NG electrode. Reproduced with permission Copyright 2019, Wiley-VCH.115 (F) Schematic diagram of SAZn on MXene layers (Zn-MXene) for the nucleation and growth of Li. (G) SEM images of Zn-MXene Li anodes at a different Li plating capacity of 40 mAh cm−2. Reproduced with permission Copyright 2020, American Chemical Society.116 (H) Schematic illustration of the design strategy of a multifunctional fibrous skeleton with single-atomic Co–Nx species for Li dendrite inhibition and sulfur redox enhancement. (I) Density functional theory calculation of the binding energy of a Li atom on a carbon nanofiber (CNF), Li, and SACo-PCNF. Reproduced with permission Copyright 2021, American Chemical Society.118 SA, single atom; SEM, scanning electron microscopy.

As mentioned earlier, besides excellent lithiophilicity, the favorable surface energy and migration path can also be determinants for uniform Li deposition. Qian and co-workers found that Zn SA had a higher surface energy and lower Li migration barrier, driving the two/three-dimensional deposition, rather than forming a one-dimensional framework (dendrite).¹¹⁹ The enhanced nuclei density, reduced nucleation relaxation time, and decreased nucleation polarized potential could lead to improved deposition kinetics, consequently inhibiting the loss of active Li and inducing uniform deposition. Besides, Co–N_x moieties on light carbon fiber, serving as an artificial interlayer for a Li anode, also increased the absorption energy toward Li and promoted the distribution of Li⁺ flux, thus preventing the growth of Li dendrites. SACo-NCNF was bifunctional in effectively suppressing LiPS shuttling and promoting sulfur utilization, achieving long-life LSBs (Figure 12H,I).¹¹⁸ The second proposed view focused on promoting favorable Li migration toward a nearby area with the introduction of SAC, and could aid uniform deposition rather than aggregate locally, thus greatly inhibiting the formation of Li dendrites.

In summary, there are two essential requirements in the design of SA for a durable Li anode: (i) the SAs must evenly distribute within the active space of the entire matrix to induce boosted lithiophilicity and localized current density and (ii) a suitable coordination heteroatom is of utmost importance to create moderate affinity between M–X_n and Li⁺. The recent reports of the use of SACs in the anode side of LSB are shown in Table 3.

Table 3 Electrochemical performance of various single-atom catalyst-optimized Li anodes for lithium–sulfur batteries

SA	Coordinations	Substrate	Function	Coulombic efficiency (current [mA cm‒2], capacity [mAh cm‒2], cycles)	Cycling performance (current [mA cm‒2], capacity [mAh cm‒2], period [h], overpotential [mV])	Li loading (mAh cm‒2)	Ref.
Fe	Fe–Nx	NC	Host	1, 1, 200; 3, 1, 120; 5, 1, 110	-	5, ED	[114]
Ni	Ni–Nx	NG	Host	1, 2, 150; 1,4, 100; 2, 1, 140	-	5, ED	[115]
Co	Co–Nx	NC	Host	2, 2, 400; 5, 2, 350; 10, 2, 200	-	Roller press (Li foil)	[113]
Zn	Zn–N4	NC	Host	-	1, 1, 800, 29	3.5, ED	[119]
Zn	-	MXene	Host	-	1, 1, 1200, 15.2; 1, 5, 500, 32; 1, 40, 500, 40	5~42, ED	[116]
Zn	Zn–N4	NCS	Host	1, 1, 100	3, 3, 1200, 16	15, ED	[117]
Co	Co–O4	MOF	Separator modifier	2, 1 120	2, 1, 400, 26; 5, 5, 300, 76;	Li foil	[53]
Co	Co–Nx	NCF	Interlayer	1, 1, 200; 3, 3, 50	3,3, 1500, -; 5,5, 1200, -	Li foil	[118]
Co	-	Fe1-xS	Interlayer	-	0.5,1, 1600, 12.5; 2, 1, 450, -; 3, 1, 135, -	Li foil	[120]
Pt	-	In2S3/Ti3C2	Separator modifier	-	1, 1100, 49.6	Li foil	[121]

It can be concluded that SACs can effectively enable the comprehensive management of suppression of the LiPS shuttle effect, promotion of sulfur reactions, and lithium metal anode optimization toward long-life LSBs. An LSB system involves complex phase conversion reactions that result in unclear correlations between sulfur conversion and lithium evolution.^122–126 Therefore, identification of the working mechanism of SACs in lithium dissolution, and transfer and deposition behaviors is still an intractable challenge.

CONCLUSION AND PERSPECTIVES

This review highlights the updated progress of SAC applications as versatile components for advanced LSBs. We focus on the design principle, typical synthesis strategy, and working mechanisms of SAC in LSBs. The proper coordination environment, sufficient defective sites, and high SA loading can ensure the full use of SACs in Li–S chemistry for the alleviation of the shuttle effect, enhancement of sulfur redox, and inhibition of dendritic growth. Using advanced characterization tools and theoretical calculations, direct identification of SACs and in situ observation of the relevant catalytic process can be achieved. The catalytic mechanisms related to the impact of the local atomic configuration of SACs on the immobilization and conversion of LiPSs in the sulfur redox reactions are illustrated in detail. Although favorable progress has been made in SAC development for LSBs, grant challenges for researchers in this field are yet to be addressed (Figure 13):

• (i)

  Relatively low loading of SACs and their small-scale production. The most intricate step in SAC preparation involves the difficulty in achieving good dispersion owing to their high surface energy; namely, it is challenging to prevent aggregation into nanoparticles on increasing their concentration. Although some progress has been achieved in preparing SACs with relatively high loading, the universality of the synthesis route, substrate selection, and cost remain unsatisfactory. Hence, it is desirable to develop a large-scale, low-cost, and facile preparation technique for stable and high-loading SACs for LSBs. In this respect, the use of conductive metal-based substrates including MXene, and so forth, and heteroatom-rich C₃N₄ derivatives can offer abundant sites for stabilization of foreign SAs to obtain high-loading SACs. In situ pyrolysis of MOFs can also be used to achieve this target because of their inherent high-content metal nodes embedded in organic linkers. In addition, tunable defect engineering including H₂O₂ etching, plasma irradiation, and confined engineering focused on the design of carbon substrates with high defect density and large surface area can serve as another useful strategy. Additionally, chemical functionalization of substrates can also help with the high-load design of SACs.

• (ii)

  Limited coordination configurations and support materials. Currently, the typical coordination structure and substrate are mainly SA–N₄ and N-doped carbons, respectively. Exploration of other coordinative atoms (P, S, or O) and numbers (1, 2, 3, 5, etc.) that may also act as active centers should be performed for efficient LiPS adsorption and conversions. Moreover, carbonaceous supports are normally light and have poor thermal stability, and are unfavorable for the construction of high volumetric energy density and inflammable LSBs. Therefore, other non-carbon substrates like MXenes, metal oxides/sulfides/selenides, and layered double hydroxides (LDHs) should be researched more to develop practical SACs for LSBs. These active substrates can act as additional centers for adjusting the evolution behaviors of sulfur and lithium species, which is conducive to further achieving efficient Li–S redox reactions.

• (iii)

  Unclear catalytic mechanism of SAC in Li–S chemistry. Although several advanced characterization tools have been utilized to identify the existence of SACs, the specific electronic interaction between SAC species and LiPSs, as well as the evolution of reaction intermediates of SACs, can be difficult to monitor on an atomic scale. The metastable states of sulfur species in a working LSB and the atom size of metal centers increase the practical difficulty in disclosing the working mechanism of SACs. In response, operando characterization techniques with high resolution and high sensibility should be synergistically applied to explore the underlying reaction mechanisms and the rate-determining step for guiding the rational design of SACs with optimized intrinsic activity for Li–S chemistry.

• (iv)

  The ambiguous stability level of SAC during repeated charge/discharge. The stability of a catalyst is an essential factor when assessing the utility of LSBs in practical conditions. The high-surface-energy feature enables the easy agglomeration of SA during repeated charge/discharge, possibly causing decay of density and activity of SACs over cycling. Thus, stability tests of SACs as different functional components require further investigation to promote the commercialization of LSBs. Machine learning can enable building of calculation models based on the database and then use the experience to forecast and obtain stable theoretical coordination configurations without practical experimental programming. Such an emerging subset of artificial intelligence can be useful to obtain highly efficient theoretical coordination configurations with high stability and guide the practical synthesis of stable SACs.

• (v)

  Restricted kinds of SAs and their combinations. The current reports of SACs for LSBs are centered on several common atoms including Fe, Ni, Co, and Zn. However, other SAs can also act as potential electrocatalysts for LSBs. Furthermore, reversible conversions of LiPSs during cycling and different steps of multi-electron reactions in Li–S chemistry may require rational selection of SAs in specific stages. For this reason, the combination of double and triple SACs might be promising for accelerated redox kinetics and improved efficiency of advanced LSBs. For instance, the synergy of Zn–Ni,¹²⁷ Ru–Co,¹²⁸ Ni–Cu,¹²⁹ Ru–Ni,¹³⁰ and Co–Te diatomic sites ¹³¹ has led to tunable electron structures in other electrocatalytic systems, which may also be extended to the LSB field for performance enhancement. In this regard, the emerging d-p orbital hybridization principle can guide the rational construction of SACs. The p orbitals of sulfur species can hybridize with the d orbitals of transition-metal atoms in SACs, leading to the manipulation of their electronic states, which are strongly related to the electrocatalytic ability in Li–S redox reactions. Also, effective d-p orbital hybridization by the transition-metal atoms in SACs and sulfur species results in strengthened LiPS adsorption and weakened S–S bonds in the sulfur chain and Li–S bonds in the Li₂S cluster, which can lead to optimization of Li–S chemistry reactions. For instance, Ti-SAC shows higher electrocatalytic ability for sulfur evolution compared with other metal SACs (M = Cr, Cu, Mn) owing to the more effective d-p orbital hybridization.¹³²

Figure 13. Schematic illustration of the future directions of SACs targeting practically viable LSBs. LSB, lithium–sulfur battery; SAC, single-atom catalyst.

ACKNOWLEDGMENTS

This study was supported by the National Natural Science Foundation of China (Grant Nos. 52172239 and 51902346), the Project of State Key Laboratory of Environment-Friendly Energy Materials (Grant Nos. 21fksy24 and 18ZD320304), the Natural Science Foundation of Hunan Province (Grant No. 2021JJ40780), the Science and Technology Innovation Program of Hunan Province (HuXiang Young Talents, Grant No. 2021RC3021), the Start-up Funding of Yangtze Region Institute (Huzhou), and the University of Electronic Science and Technology (U03220102).

CONFLICTS OF INTEREST

The authors declare no conflicts of interest.