Revisiting the Roles of Carbon in the Catalysis of Lithium–Sulfur Batteries

Carbon materials are the key hosts for the sulfur cathode to improve the conductivity and confine the lithium polysulfides (LiPSs) in lithium–sulfur batteries (LSBs), owing to their high electronic conductivity and strong confinement effect. However, physical or chemical trapping methods have limitations in preventing the dissolution and accumulation of LiPSs in the electrolyte. Catalysis has emerged as a fundamental solution to accelerate the sluggish redox kinetics, and carbon materials acting as catalyst supports or direct catalysts significantly impact the reaction efficiency. Herein, the roles of carbon in the catalysis of LSBs are systematically discussed, focusing on the influence of surface area, pore structure, and surface chemistry on sulfur conversion. Then, two modification strategies, vacancy defects and heteroatom doping, that endow carbon with catalytic activity are summarized. Finally, the remaining challenges and solutions are outlined in terms of the preparation and characterization of the functional carbon in LSBs. This perspective provides essential insights and guidance for the rational design of carbon‐based catalysts in LSBs.


Introduction
[3] Nevertheless, several critical issues hinder their practical application.The first one is the insulative nature of sulfur and the discharged products lithium sulfide (Li 2 S), [4] thus making conductive carbon an indispensable material in the cathode.Various carbon materials have been investigated as the sulfur host, such as porous carbon, [5,6] carbon black, [7,8] graphene, [9][10][11] carbon nanotubes, [12,13] and so on.[16][17] Thus, many early researches focus on designing complicated carbon structures to confine or physically adsorb the LiPSs to limit their diffusion and uncontrollable deposition.However, only limited improvements can be achieved in suppressing LiPS shuttling and enhancing the electrochemical performance of LSBs, even under low sulfur loading and high electrolyte usage. [18,19]Under the practical requirements such as high sulfur loading (>5 mg cm À2 ) and lean electrolyte (<5 μL mg s À1 ), the accumulation of LiPSs in the electrolyte, which induces more severe shuttling and higher polarization, troubles the nominal operating of LSBs due to the sluggish redox kinetics from LiPSs to Li 2 S. [20][21][22] Introducing catalysis to promote sulfur conversion is a promising solution in which carbon materials play essential roles as catalyst supports or catalysts due to their adjustable surface area, pore structure, and surface chemistry. [18,19,23]Various carbon-supported catalysts, such as metal nanoparticles, [24] metal compounds, [25,26] and single atoms , [27,28] have been developed for LSBs.For example, our group [29] designed α-Fe 2 O 3 loaded on reduced graphene oxide (rGO) as the host to sulfur cathodes inspired by the industrial desulfurization activity of Fe 2 O 3 .The rGO has a high surface area and can uniformly disperse α-Fe 2 O 3 nanoparticles (NPs).In addition, it also has good conductivity.Thus, the conversion of LiPSs to insoluble discharge products is enhanced, resulting in better battery performance than without the catalyst.Notably, carbon also shows activity in the redox reactions of LiPSs by incorporating vacancies [30] and heteroatoms . [31]owever, a systematic summary and discussion on the functions of carbon in the catalysis for LSBs is still missing, hindering the rational design of the carbon structure.
In this perspective, we first give an overview of recent research progress on carbon as catalyst support and catalyst in LSBs.The influences of surface area, pore structure, and surface chemistry on the catalysis conversion process are thoroughly discussed.The large surface area for uniform dispersion of active sites, the pore structure as nanoreactors for confined catalytic conversion, and the enhanced polarity of carbon surface to modulate the adsorption and catalytic conversion of LiPSs are highlighted.Finally, future directions for carbon materials in the catalysis for LSBs are prospected.

Carbon as Catalyst Support for LSBs
Carbon materials have found extensive applications as catalyst supports in various catalysis processes.They have high electronic conductivity, excellent structural stability, tuneable pore structure with tailored pore size and surface area, adjustable surface chemistry, and diverse morphologies (particles, fibers, nanosheets, etc.), greatly affecting the distribution and catalytic efficiency of catalysts. [32,33]It has been demonstrated that the reaction of sulfur species in LSBs undergoes trapping, diffusion, and catalytic conversion at the three-phase interface formed by carbon support, catalyst, and electrolyte. [34,35]Therefore, the rational design of the interfacial structure between carbon materials and catalysts helps improve the catalytic performance of carbon-supported catalysts.
The most commonly synthetic process of carbon-supported heterogeneous catalysts involves impregnating the carbon supports in the precursor solution.A rich porous structure and large surface area facilitate the monodispersion of catalysts. [36]esides, oxygen-containing groups can reduce the hydrophobicity of the carbon surface, making it easier to interact with precursors and enhancing the anchoring effect to suppress the aggregation of the catalyst NPs. [34]In addition, carbon surfaces with vacancy defects or heteroatom doping (nitrogen, sulfur, etc.) exhibit enhanced effectiveness in reducing the size of catalysts through coordination to form clusters or even single atoms. [37]he carbon support also plays a crucial role in influencing the electronic structure of the catalyst through their interaction. [38]or example, tuning the coordination atoms in carbon-supported single-atom catalysts can significantly alter the electronic symmetry of the central atom, which substantially affects catalytic performance. [27]It is evident that manipulating the physical and chemical properties of the carbon support can lead to alterations in this interaction.In LSBs, preventing sulfur species' sulfidation or strong adsorption on the catalyst is crucial to ensure long cycling stability.The encapsulation of catalysts by carbon layers represents a typical protection method.The ultrathin carbon shell is significant to balance stability and activity, but this consideration is often overlooked in many reports. [39]Therefore, we will emphasize the effect of different characteristics of carbon supports on the interaction between carbon and catalyst, as well as their influence on the catalytic activity of LSBs (Figure 1).

Effect of Carbon Surface Area and Pore Structure
The surface area and pore structure of carbon materials play a crucial role in the distribution of catalytically active phases and sulfur, further affecting the catalytic effect and battery performance.Generally, the conductive carbon with a large surface area is desired as a sulfur host, which can realize uniform dispersion of active sulfur, rapid charge transfer, and electrolyte infiltration. [40,41]Nazar et al. [42] prepared ordered mesoporous carbon spheres with ultrahigh surface area (2445 m 2 g À1 ), achieving a reversible charge capacity of 1200 mAh g À1 at 1 C and excellent cycle stability.Similarly, a large surface area is beneficial to uniformly disperse the catalyst NPs and expose more catalytic sites for the sulfur conversion, which is important to enhance the "solid-solid" contact between the sulfur/Li 2 S and the catalyst, and the "liquid-solid" contact between LiPSs and catalysts.To comprehend the effect of surface area on the catalytic process in LSBs, it is necessary to distinguish between outer and inner surfaces (Figure 2a,b).The former is mainly provided by the open surface of various nanosized carbon materials, such as 0D carbon nanoparticles, 1D carbon nanotubes (CNTs) and carbon nanofibers (CNFs), and 2D graphene (G) nanosheets.In contrast, the latter is mainly provided by carbon materials with 3D structures rich in micropores, mesopores, or macropores. [43]able 1 summarizes the information on the pore structure and surface area of different carbon supports in the catalytic research of LSBs.
As a representative material with an open surface, 2D G is entirely composed of monoatomic layer sp 2 hybridized carbon, and its huge open surface can provide a theoretical surface area Figure 2. Sulfur storage in a) the external surface of 0D, 1D, and 2D carbon materials and b) the internal surface of porous carbon and hollow carbon sphere.Reproduced with permission. [43]Copyright 2016, Wiley-VCH.c) Synthetic illustration of Ni, Pt electrocatalysts supported on G. d) Field emission scanning electron spectroscopy image of Pt nanoparticles anchored G layers.Reproduced with permission. [45]Copyright 2015, American Chemical Society.TEM images of e) NG/G and f ) NG. Reproduced with permission. [47]Copyright 2019, Wiley-VCH.Hollow carbon nanorods separated by empty channel voids [61]   Activated carbon 800-3000, mainly contributed by inner microporous and mesoporous surfaces 0.8-35 0.1-0.9(micropore) Amorphous, characteristic twisted geometry of planar layered structures [108,109]  of up to 2600 m 2 g À1 , further increasing the catalytic sites. [44]herefore, it is more suitable as a template for achieving high dispersion of catalytic materials than 0D and 1D carbon materials with limited open surfaces.Based on this advantage, Al Salem et al. loaded Pt and Ni nanocatalysts onto G (Figure 2c) and compared with the direct use of Ni and Pt-based metal 3D current collectors; [45,46] the large open surface of G ensured the uniform dispersion of catalysts and improved their utilization (Figure 2d).Lin and co-workers decorated G with nitrogen-rich (17%) carbon (NC/G) by one-step synthesis (Figure 2e,f ). [47]Abundant N active sites promoted the fast conversion of LiPSs into insoluble products by reducing the energy barrier.Simultaneously, the G skeleton effectively resisted the stacking of NC/G and enhanced the surface area, providing ample space for the interaction of catalytic active sites with LiPSs.The dual effect improved the electrochemical performance of LSBs.However, although 2D G can offer more space for catalytic reactions, its open surface cannot restrict the diffusion of soluble intermediate LiPSs.This problem can be overcome by utilizing the self-assembly behavior of graphene oxide (GO) to construct a 3D rGO network.The porous structure composed of interconnected G sheets plays a physical confinement effect on the diffusion of LiPSs.At the same time, the 3D rGO network also retains the advantages associated with the large surface area offered by monomeric G.
Moreover, the uniformly loaded catalyst on rGO effectively manipulates the sulfur conversion.Along this line, our group has developed a series of catalytic systems, such as α-Fe 2 O 3 , [29] In 2 O 3 , [48] VN, [34] and Bi 2 S 3 [49] in situ supported on a 3D rGO network.
In addition to the open surface, the internal pore structure determines the surface area of most porous carbon materials.According to the classification standard of the International Union of Pure and Applied Chemistry (IUPAC), pores are divided into micropores (<2 nm), mesopores (2-50 nm), and macropores (>50 nm) by their size.The surface area is mainly contributed by micropores and small-sized mesopore structures.Therefore, varieties of 3D porous carbon with rich and fine small-sized pore distribution are expected to be applicable templates for synthesizing catalytic materials.[52][53] On the other hand, the contact area between the catalytic material and the carbon matrix can be enlarged to broaden the transformation path of electrons.In this regard, Chen and co-workers [54] achieved in situ growth of ultrafine Ta 2 O 5-x nanoclusters (1.2 nm) in the microporous carbon nanosphere using a wet-impregnation strategy (Figure 3a).With the merits of micropores, the unique structure, like a "ship  [54] Copyright 2020, Elsevier.c) The distribution of catalysts is restricted by small micropores.d) Charge and discharge process of S 2-4 confined in ultrasmall microporous carbon.Reproduced with permission. [55]opyright 2012, American Chemical Society.e) Schematic illustration of NiS@C-HS as sulfur hosts in LSBs.Reproduced with permission. [56]Copyright 2017, Wiley-VCH.f ) Schematic illustration of built-in catalysis and the structure of confined nanoreactor.Reproduced with permission. [57]Copyright 2020, American Chemical Society. in a bottle", was elaborately constructed as an efficient 3D conductive nanoreactor, in which the Ta 2 O 5-x nanoclusters were uniformly filled in the microporous carbon (Figure 3b).As a result, the confined catalysis of Ta 2 O 5Àx in the microporous structure enabled a rapid sulfur conversion.Along similar lines, they also developed a micropore-confined Nb 2 O 5-x catalytic system. [36]lthough microporous materials have a high surface area, the excessively small pore size can give rise to certain challenges.It is mainly manifested in three aspects: 1) The spatial confinement effect brought by small micropores usually leads to the restricted distribution of catalytic active phase and sulfur species in the pores, which will inhibit the catalytic effect (Figure 3c).2) The spatial limitation in ultrasmall-sized micropores (<0.5 nm) forces sulfur to form noncyclic allotropes S 2-4 instead of S 8 (Figure 3d). [55]At the same time, due to the isolation of the solvent, the small molecular sulfur exhibits a "solid-solid" conversion mode with slow kinetics.There is still no suitable method for effective catalyst loading inside such ultrasmall-sized micropores.
3) The small pore volume of the microporous structure greatly limits the sulfur loading and electrolyte penetration, making it difficult to meet the practical requirements of high energy density for LSBs.
Therefore, few works have used carbon-supported catalytic materials containing only micropores as sulfur hosts, which would greatly limit the sulfur content.Most designs choose microporous carbon as the outer shell of the nanoreactor to achieve confined catalysis by inhibiting the diffusion of internal sulfur species.For example, Qiao et al. [56] proposed a hollow carbon sphere with a large-volume cavity inside to store sulfur species, NiS catalysts loaded on the inner wall to accelerate the conversion of LiPSs, and the outer microporous carbon shell provided physical confinement to LiPSs diffusion (Figure 3e).Wu et al. [57] developed a 3D-framework nanoreactor with ordered mesopores, microporous carbon shell, and catalyst layout based on aluminum metal-organic frameworks (Figure 3f ).Specifically, the Al 2 O 3 generated during pyrolysis acted as a sacrificial template, leading to loose carbon layers and mesoporous voids as the main storage sites for sulfur species.The abundant microporous structure in the external carbon framework effectively prevented the outward diffusion of LiPSs.Afterward, the MoS 2 sheets were loaded in the mesopores, and a nanoreactor with the functions of spatial confinement and catalytic conversion was constructed.Benefiting from the built-in catalysis confined by the microporous carbon shell, the S-C@MoS 2 cathode enabled excellent rate performance and cycle stability.
Generally, the high pore volume of mesoporous carbon materials allows for high sulfur loading and guarantees enough space for Li migration.At the same time, its considerable surface area also contributes to the uniform loading of catalysts.Therefore, mesoporous carbon materials are suitable supports for catalyst loading and sulfur storage in LSBs.Based on this line, Li et al. [58] developed a mesoporous N-doped carbon nanosheet loaded with Co catalytic sites (MC-NS) for LSBs (Figure 4a).The volatilization of Zn during preparation brought about a large amount of mesoporous structure and considerable pore volume (Figure 4b), which provided ample space for storing sulfur and more adsorptive and catalytic sites for LiPSs conversion.Such mesoporous-rich catalytic material achieved a high sulfur content of 86 wt% and excellent rate performance.Therefore, carbon materials with abundant ordered mesopores are an excellent choice for accomplishing a high-loading sulfur cathode.These ordered pores facilitate smooth internal mass transfer, while mesoporous structures offer effective confinement and sufficient space for sulfur, electrolyte, and catalyst storage.
Although mesoporous carbon can achieve uniform dispersion of catalysts and high sulfur loading, some studies have also shown that the battery performance will be greatly reduced, if sulfur completely fills the mesopores.Thus, the maximum sulfur loading of mesoporous materials is restricted. [59]In this regard, hierarchical pore structures are beneficial, as the micropores can provide a uniformly dispersed surface for catalysts to facilitate efficient adsorption and rapid conversion of LiPSs, and mesopores can act as electrolyte reservoirs to ensure the smooth transport of ions.Especially large mesopores are necessary to achieve a high pore volume and sulfur loading.Following this idea, Wu et al. [60] developed a carbon-based catalytic material with a hierarchical porous structure and uniformly dispersed Co active sites, which was constructed by a ZIF-67-derived carbon framework dispersed and connected by CNTs (Figure 4c).The introduction of CNTs broadened the pore size range from the original 1.22-10 nm to 1.66-100 nm, and the corresponding pore volume also increased from 0.73 to 1.55 cm 3 g À1 (Figure 4d).Such hierarchical nanostructure and high pore volume could provide the smooth mass transfer and uniform storage of components with different sizes in electrodes, including infiltrated electrolyte, adsorbed LiPSs, and high-loading sulfur.This ingenious design finally improved the catalytic effect, allowing LSBs to cycle stably at a high sulfur content of 92.4 wt% and a high sulfur loading of 13.1 mg cm À2 .
Although the hierarchical pore structure can help to physically confine LiPSs and increase the sulfur loading, the inhomogeneous mass transfer caused by the complex pore tortuosity severely reduces the uniformity of catalyst size and distribution while concurrently elongating the Li þ transport pathway.Therefore, the relationship between pore structure and catalyst loading and Li þ transport is an important factor that must be considered in designing carbon supports.The ordered pore structure allows faster and more uniform mass transfer, and this feature is very important in catalytic reactions for LSBs.On the one hand, the size of catalysts and their spatial distribution on the carbon support will be more uniform.On the other hand, the ordered pores also provide fast transport channels for Li þ .For example, CMK-3 has well-ordered mesoporous pores and is widely used as the catalyst support in many catalysis reactions and the sulfur host in LSBs. [61]Wang et al. [62] proposed CMK-3 loaded with MoP clusters (cMoP) as a catalytic host for LSBs (Figure 4e).Transmission electron microscopy (TEM) image in Figure 4f revealed that the cMoP with an average size of 0.8 nm were uniformly anchored onto the inner wall of pores of CMK-3, which was attributed to the uniform diffusion and confined phosphating growth of the precursor solution in the highly ordered channels of CMK-3.In addition, the strong interaction between cMoP and CMK-3 enhanced the local electric field to concentrate Li þ inside the channels (Figure 4g), thus significantly accelerating the electrochemical reaction and improving sulfur utilization to up to 90%.

Effect of Carbon Surface Chemistry
Carbon surface chemistry is also an essential factor affecting the catalytic performance of carbon-supported catalysts in LSBs.The surface chemistry determines the contact mode between the carbon surface and the catalytically active phase during the catalyst preparation, thus directly affecting the parameters such as the loading, size, and distribution of catalysts.On the other hand, the physical interaction between nonpolar carbon and polar LiPSs is too weak, hindering the migration and catalytic conversion of LiPSs.In contrast, high specific surface carbon materials treated with surface chemical regulation such as heteroatom doping [63][64][65] or functional group modification [11,66,67] provide more accessible polar sites to chemically trap LiPSs based on strong polar-polar interaction or Lewis acid-base interaction, resulting in a smoother trapping-diffusion-catalytic conversion path.
The surface chemical modification of carbon supports is usually achieved through the treatment of chemical reagents, and the adjusted surface chemistry is related to the type of carbon material and the modification methods.Several modification technologies, such as oxidation, nitriding, etc., are commonly used to introduce O and N into the carbon network in atomic doping or functional groups.For the oxidation process, chemical  [58] Copyright 2019, Wiley-VCH.c) Schematics illustration of LiPSs conversion on CoN-C, CoN-DC, and IRA-DC.d) N 2 adsorption-desorption isotherms and pore size distribution of IRA-DC.Reproduced with permission. [60]Copyright 2022, Elsevier.e) Illustration of the preparation process for cMoP-CMK-3 and the confined catalytic effect inside the channels.f ) Top-view scanning transmission electron microscopy image of cMoP-CMK-3, and the inset illustrates the distribution of cMoP.g) Theoretical simulation for the concentration of Li þ within the cMoP-CMK-3.Reproduced with permission. [62]Copyright 2022, Elsevier.
reagents, including HNO 3 , H 2 SO 4 , H 2 O 2 , K 2 MnO 4 , KOH, etc., are usually applied.Treatment with different acids/bases leads to various oxygen-containing functional groups on the carbon surface.Concentrated acids usually introduce acidic oxygen groups, such as lactone (OCO), carboxylic acid (COOH), phenolic (COH), carbonyl (CO), and quinone (OCCO), on the carbon surface, while alkaline treatment usually introduces hydroxyl (OH) groups. [68]For nitriding, urea, melamine, aniline, polypyrrole, ammonia, dicyandiamide, and phenanthroline are mainly used as nitrogen (N) sources.N atoms are introduced into the carbon surface by reacting N precursors with carbon materials or directly in situ synthesis. [69]Figure 5a shows several representative N-and O-based surface chemical modification methods on the carbon network. [70]Other elements, such as P, S, halogen, etc., can be introduced similarly.
The influence of carbon surface chemistry on the catalyst preparation is reflected in its interaction mode with precursors.Generally, the carbon support needs to attract ions in the precursor solution by electrostatic interaction to complete the loading of catalysts, which is closely related to the charged state of the carbon surface.The widely used GO sheets are negatively charged due to the abundant oxygen-containing functional groups, which help them attract metal cations to achieve uniform loading of metals or metal oxides.This is also a commonly used catalyst loading method in the catalytic research for LSBs.In this regard, our group [29] successfully synthesized α-Fe 2 O 3 nanoparticles  [70] Copyright 2012, American Chemical Society.b) TEM image of α-Fe 2 O 3 dispersed on the rGO.Reproduced with permission. [29]Copyright 2017, Elsevier.TEM images of VN c) without and d) with melamine.Reproduced with permission. [34]Copyright 2021, Wiley-VCH.Schematic illustration of e) 4-NPA, TnPcCo, TaPcCo, and F-MWCNT, f ) TaPcCo-MWCNT, (g) TaPcCo and MWCNT linked by C-NH-C covalent, and h) the LiPSs catalytic conversion on TaPcCo-MWCNT.Reproduced with permission. [71]Copyright 2020, Wiley-VCH.
with a size of about 20 nm uniformly supported on rGO by utilizing the electrostatic interaction between the GO surface and Fe 3þ (Figure 5b).The α-Fe 2 O 3 exhibited strong chemical interaction toward LiPSs and catalytic activity to promote its conversion.
However, the above preparation method is only suitable for the positively charged precursor ions.When the precursor is an anion, it is necessary to change the charged state of GO by reasonable surface chemical modification means.The model of rGO-loaded VN nanoparticles proposed in the follow-up study of our group can serve as a typical example. [34]The precursor VO 3 2À (NH 4 VO 3 ) was negatively charged and could not establish a good interconnection with the similarly negatively charged GO surface.Therefore, the synthesized VN particles would agglomerate without any modification (Figure 5c).To address this issue, we introduced melamine (MA) to modify the surface chemistry of GO.The N atoms on the three amino groups (-NH 2 ) of MA could combine with ionized hydrogen ions in water to form positively charged -NH 3 þ .Therefore, the surface of GO treated with MA exposed many positively charged groups, which could produce a good connection with VO 3 2À .This surface chemical modification strategy finally led to ultrasmall and highly dispersed VN nanoparticles with an average particle size of 2.7 nm (Figure 5d), greatly improving the catalytic performance and long-term cycle stability of LSBs.
Covalent interaction is another combination mode between catalyst and carbon support besides electrostatic interaction during preparation.It is likewise affected by carbon surface chemistry.Yang et al. [71] proposed to support the catalytically active phase cobalt (II) tetraaminophthalocyanines (TaPcCo) on the surface of multiwalled carbon nanotubes (MWCNTs) to improve the sluggish sulfur conversion kinetics for LSBs.They adopted a fluorination strategy to change the surface chemistry of MWCNTs during the preparation to enhance the binding of TaPcCo to the support surface.As shown in Figure 5e-h, using a simple pyridine-catalyzed reaction, the amino groups substituted the fluoro groups to form ÀNHÀ covalent bonds between the phthalocyanine complexes and the MWCNT skeletons.Such strong covalent linkage enabled smooth electron transport to the Co catalytic center, ensuring high electrocatalytic activity.
In addition to the influence on the preparation and dispersion of the catalyst, the carbon surface chemistry can also affect the interaction with LiPSs.In some cases, surface chemically modified carbon support and the catalyst show a synergistic effect, jointly affecting the adsorption, diffusion, and catalytic conversion of sulfur species.A typical example is the single Co atom catalyst loaded on N-doped G support by forming a CoÀN-C coordination center through chemical bonding interactions.The Co atom in the Co-N 4 center preferred to bond with the S atom, while the Lewis acid N atoms on the carbon support could strongly interact with the Li atom in the sulfur species (Figure 6a).Thus, the synergistic effect between the support and the Co catalytic sites significantly enhanced the anchoring of LiPSs. [72]he surface chemistry of carbon supports can also modulate the electronic structure of catalysts, thereby affecting the catalytic activity.Ding et al. [73] investigated the effects of carbon supports with different surface chemistries (CNT-OH, CNT-NH 2 , CNT-COOH) on the catalytic activity of molecular catalysts hemin.They found that the carboxyl functional group-modified CNTs formed a unique Fe-O coordination with hemin, which enhanced the ability of the Fe catalytic center to trap and catalyze Figure 6.a) Schematic illustration of the interaction between cobalt phthalocyanine and Li 2 S 2 .Reproduced with permission. [72]Copyright 2019, Elsevier.b) Schematic illustration of LSBs with CNTs-FG@hemin cathodes (FG: -OH, -NH 2 , -COOH) and the mechanism of LiPSs chemically adsorbed on CNTs-COOH@hemin.Reproduced with permission. [73]Copyright 2020, Wiley-VCH.c) Structural illustration of Fe-N 4 sites on wG.Reproduced with permission. [74]Copyright 2022, Wiley-VCH.
the LiPS conversion (Figure 6b).In contrast, due to the absence of Fe-O coordination between hemin and CNT-NH 2 /CNT-OH supports, negligible or only weak electronic interactions were observed between Fe and LiPSs in these systems.In addition, J. Kim et al. explored the catalytic activity of a single Fe atom catalyst on GO support with different oxygen group content.Due to the decrease of oxygenated carbon, they observed the formation of wrinkles (wG, Figure 6c), which caused nonplanar distortions of the Fe-N 4 center and greatly affected the local symmetry of its electronic structure.Compared to FeNC/G, Fe atoms in FeNC/wG exhibited more electron-poor and lower d-band center, thus accelerating the sluggish sulfur redox reaction. [74]rom the above examples, it can be seen that surface chemistry modification can change the polarity of carbon, affect the loading state of the catalyst, enhance the adsorption of sulfur species, and more importantly, it can regulate the interaction between carbon support and the active phase to enhance the catalytic ability, thereby improving the electrochemical performance of LSBs.

Carbon as Catalyst for LSBs
In addition to serving as a catalyst support, numerous studies have explored the direct utilization of carbon as a catalyst.This dual role of carbon reduces the energy barrier for sulfur conversion and enhances the electrochemical performance of LSBs (Table 2).Moreover, carbon catalysts can decrease the addition of inactive materials, thus improving the energy density of the batteries.However, two disadvantages of carbon, weak polarity and chemical inertness, prevent it from interacting with LiPSs.For these constraints, modulating the electronic structure of carbon by inducing vacancy [75][76][77] and heteroatom [78,79] has been considered as effective means to endow carbon surfaces with catalytic activity.

Effect of Vacancy Defect
Vacancy defects, including topological and edge defects, are essential to stimulate potential active sites by tailoring the charge distribution of carbon. [80,81]Topological defects will convert lattice units into unstable structures with high energy states, such as pentagons, heptagons, etc., to change the charge or spin density distribution of adjacent carbon atoms.[84] For example, the previous study of Zhang et al. [85] revealed that the binding energies of Li 2 S 6 on porous carbon nanotube microspheres with abundant topological defects enhanced significantly to the maximum value of À1.62 eV through density functional theory calculations in Figure 7a.According to the Lewis acid-base interaction principle, such improvement was due to partially positive carbon atoms forming in the asymmetrical lattice, which tended to accept electrons from LPSs.In addition, Cui et al. [86] used spray drying followed by a chemical activation method to synthesize an etched cotton@petroleum asphalt carbon (eCPAC) with abundant edge defects (Figure 7b).They found that the sawtooth-edged carbon atoms with many unpaired π-electrons had higher charge density, which could effectively reduce the energy barrier of the nucleation and dissociation of Li 2 S. Thus, eCPAC achieved the bidirectional catalysis of Li 2 S, experimentally demonstrated by a potential intermittent titration technique with relatively early nucleation and dissociation peaks and higher corresponding capacity.Jiang et al. [87] further investigated the catalytic mechanism.They explained that the vacancy defects led to the upshift of the p-band center and increased the electron density of carbon atoms near Fermi energy to promote more charge transfer between carbon and LiPSs, providing stronger interaction.Specifically, it is found that defective carbon could bind with both Li and S atoms of Li 2 S 4 to enlarge the S─S bond length and catalyze the decomposition reaction of Li 2 S 4 (Figure 7c,d).

Effect of Heteroatom Doping
Compared to vacancy defects, introducing nonmetallic heteroatoms (such as N, B, S, etc.) with different electronegativities in the carbon framework provides a more comprehensive means to control the periodic structure of carbon and rational distribution of local electron cloud density. [88,89]N, with an atomic radius similar to carbon, is considered an ideal dopant due to the alleviation of lattice distortion.Besides, high-electronegativity N dopant will attract electrons of adjacent carbon atoms, resulting in them with a positive charge, which is beneficial for the adsorption and dissociation of LiPSs. [88,90,91]Various N-doped carbons have been studied to trap and catalyze sulfur species in LSBs.For example, Du et al. [79] synthesized an N-doped carbon nanocage with a layered structure.They compared the free energy of sulfur species between the isolated states and the adsorbed states on different sites, including pyridinic N@zigzag edge, pyridinic N@armchair edge, graphitic N, zigzag edge, armchair edge, and graphite plane (Figure 8a).It was found that pyridinic N-doped carbon could chemically trap LiPSs and catalyze sulfur conversion.Further, to explore the dependence of catalytic reactivity on the N-dopants concentration, Xu et al. [47] proposed G with different pyridine N structures, including G-2N (two pyridine N), G-3N (three pyridine N), and G-4N (four pyridine N).
From the energy barrier of each step displayed in Figure 8c, it was found that G-4 N exhibited the lowest energy barrier, followed by G-3N (0.41 eV), G-2N (0.44 eV), and G (0.67 eV), indicating that the sequence of catalytic activity was G-4N > G-3N > G-2N > G.They confirmed for the first time that high N content on G with large surface area could provide abundant active sites to enhance catalytic performance (Figure 8b).In addition to the amount of dopant, the synergistic effect between codoped atoms also significantly affects the catalytic properties of carbon. [92]Along this line, Pang et al. [93] first proposed N, S-doped cellulose-derived carbon (NSC) as a sulfur host.On the one hand, strong Li-N interaction was confirmed because of the electron-donating properties of pyridine N. On the other hand, thiophene-like S dopants with positive charge tended to form highly specific bonds with terminal sulfur.Thus, both N and S dopants synergistically achieved the substantial chemisorption of LiPSs (Figure 8d).In addition, Peng et al. [94] investigated the catalytic process of sulfur species on N, S-doped holey graphene framework (N, S-HGF).They found that edge carbon atoms adjacent to N and S served as catalytic centers.Especially, N, S-HGF exhibited the minimized reaction overpotential (Figure 8e) because the moderate p-band center provided by N, S dual doping led to a mild bonding strength with the catalytic intermediate LiS radical.As a result, the N, S-HGF electrode obtained the smallest polarization voltage gap at various current densities, considerably lower than those of N-HGF, S-HGF, and undoped HGF.
Another common modification strategy is incorporating metal elements into the carbon matrix as single-atom catalysts (SACs).Owing to their maximal atom utilization efficiency and welldefined catalytic active sites, SACs hold significant potential in facilitating the conversion of LiPSs.Therefore, utilizing SACs with abundant adsorption sites and strong electrochemical activity is suitable for capturing LiPSs and catalyzing sulfur chemistry, effectively mitigating the "shuttle effect" in LSBs.A more comprehensive description has been discussed in Section 2.2.
These examples indicate that vacancy defect and heteroatom doping efficiently improve carbon catalytic activity and address sluggish kinetics.However, excessive heteroatom doping will severely damage the lattice structure of carbon, resulting in a significantly decreased electronic conductivity. [95]Thus, it is required for the precise regulation of dopant content. [96]In addition, the stability of defects needs to be further examined during long-term operation.Future efforts can be invested in rationally designing carbon catalysts and exploring catalytic mechanisms under the guidance of simulation calculation.

Conclusions and Perspectives
In this perspective, we have outlined the application of carbon materials as catalyst supports and catalysts in sulfur conversion.While significant progress has been made in carbon-based catalysis for LSBs, there are still challenges in elucidating the working mechanism and realizing their uses in practical batteries.Future research directions mainly focus on the following aspects: 1) As catalyst supports, carbon materials have demonstrated remarkable versatility in tuning conductivity, porosity, surface chemistry, and catalyst-support interactions, leading to advancements in catalytic applications in LSBs.However, a more comprehensive understanding of the chemical and physical characteristics is lacking.Theoretical investigations should be undertaken to elucidate the influence of carbon surface, electronic, mechanical, and morphological characteristics on catalyst loading and sulfur conversion.These studies can provide valuable insights and predictive models to facilitate the rational design of carbon materials.2) As catalysts, carbon materials with vacancies and heteroatoms have shown promise in boosting the sluggish sulfur chemistry in LSBs.However, the direct detection of sulfur conversion at the defected carbon skeleton is currently limited.To this end, developing high-resolution electron microscopy and in situ testing setups holds promise for atomic-scale observations.In addition, excess defects may reduce the electronic conductivity, and thus, future efforts can be made to precisely regulate the content and ratio of heteroatom and vacancy to improve catalytic performance.3) For further commercialization, energy density is an important evaluation index of LSBs.However, whether as catalysts or supports, most carbon materials have low density and limited sulfur loading, reducing the gravimetric and volumetric energy density of devices.From this perspective, high-density and high-porosity carbon materials are required for LSBs.8. a) Six typical configurations in hNCNC and hCNC, including pyridine N@zigzag edge, pyridinic N@armchair edge, graphitic N, zigzag edge, armchair edge, and graphite plane.Reproduced with permission. [79]Copyright 2018, Elsevier.b) Schematic illustration of the LiPSs transformation on N-doped G (left) and N-rich G (right).c) Theoretical simulations of the reaction pathways from S 8 to Li 2 S on the G, G-2N, G-3N, G-4N.Reproduced with permission. [47]Copyright 2019, Wiley-VCH.d).Schematic configurations of Li 2 S 2 binding with NSC.Reproduced with permission. [93]Copyright 2015, Wiley-VCH.e) A volcano plot associating the adsorption energy of LiS radical intermediate on different active sites with the overpotential of Li 2 S 2 conversion to Li 2 S, and active sites include triangle, square, and circle represent the active sites of armchair edge, zigzag edge, and inner defect edge.Reproduced with permission. [94]Copyright 2019, Springer Nature.
The compact porous carbon prepared by the self-assembly and dense shrinkage of GO can meet the above conditions.Meanwhile, the abundant surface functional groups of GO are helpful for the surface modification and uniform loading of catalysts.4) For the fabrication of carbon materials, developing environmentally friendly and cost-effective techniques will ensure the feasibility of large-scale production and contribute to the further commercialization of LSBs.Although numerous carbon electrode materials in published research exhibit remarkable electrochemical performance, they are synthesized using multistep methods, costly precursors, and complex equipment, making it challenging to scale up and resulting in significant amounts of waste.Therefore, exploring sustainable and scalable synthesis methods for these optimized carbon materials remains critically important.
Overall, clarifying the roles of carbon in sulfur catalytic conversion and achieving the rational design of functional carbon materials are crucial to realizing the commercialization of LSBs through catalytic strategies.

Figure 1 .
Figure 1.The schematic diagram of carbon-based catalysis for LSBs and factors affecting catalytic activity.

Figure 3 .
Figure 3. a) Scheme illustration of Ta 2 O 5-x confined in microporous carbon nanosphere and LPSs catalytic conversion in nanoreactor.b) The high-angle annular dark-field scanning TEM image of A-Ta 2 O 5-x /MCN.Reproduced with permission.[54]Copyright 2020, Elsevier.c) The distribution of catalysts is restricted by small micropores.d) Charge and discharge process of S 2-4 confined in ultrasmall microporous carbon.Reproduced with permission.[55]Copyright 2012, American Chemical Society.e) Schematic illustration of NiS@C-HS as sulfur hosts in LSBs.Reproduced with permission.[56]Copyright 2017, Wiley-VCH.f ) Schematic illustration of built-in catalysis and the structure of confined nanoreactor.Reproduced with permission.[57]Copyright 2020, American Chemical Society.

Figure 4 .
Figure 4. a) The dissolution and diffusion of LPSs happen on conventional carbon materials and are suppressed by ordered 2D carbon with honeycomb-like nanostructures.b) The pore volume distribution curves of MC-NS, C-NS, and C-NP, respectively.Reproduced with permission.[58]Copyright 2019, Wiley-VCH.c) Schematics illustration of LiPSs conversion on CoN-C, CoN-DC, and IRA-DC.d) N 2 adsorption-desorption isotherms and pore size distribution of IRA-DC.Reproduced with permission.[60]Copyright 2022, Elsevier.e) Illustration of the preparation process for cMoP-CMK-3 and the confined catalytic effect inside the channels.f ) Top-view scanning transmission electron microscopy image of cMoP-CMK-3, and the inset illustrates the distribution of cMoP.g) Theoretical simulation for the concentration of Li þ within the cMoP-CMK-3.Reproduced with permission.[62]Copyright 2022, Elsevier.

Figure
Figure8.a) Six typical configurations in hNCNC and hCNC, including pyridine N@zigzag edge, pyridinic N@armchair edge, graphitic N, zigzag edge, armchair edge, and graphite plane.Reproduced with permission.[79]Copyright 2018, Elsevier.b) Schematic illustration of the LiPSs transformation on N-doped G (left) and N-rich G (right).c) Theoretical simulations of the reaction pathways from S 8 to Li 2 S on the G, G-2N, G-3N, G-4N.Reproduced with permission.[47]Copyright 2019, Wiley-VCH.d).Schematic configurations of Li 2 S 2 binding with NSC.Reproduced with permission.[93]Copyright 2015, Wiley-VCH.e) A volcano plot associating the adsorption energy of LiS radical intermediate on different active sites with the overpotential of Li 2 S 2 conversion to Li 2 S, and active sites include triangle, square, and circle represent the active sites of armchair edge, zigzag edge, and inner defect edge.Reproduced with permission.[94]Copyright 2019, Springer Nature.

Table 2 .
Performance of LSBs with various carbon-based catalysts.