Metal catalysts synergistically enhance the sulfur reduction reaction (SRR) and sulfur oxidation reaction (SOR) in lithium-sulfur batteries through multiple mechanisms. However, different metals exhibit distinct catalytic mechanisms for sulfur conversion at the cathode, which are determined by their intrinsic properties. During the sulfur conversion process, transition metals with unique characteristics promote reactions via various catalytic mechanisms. These mechanisms include adsorption, accelerated electron transfer, reduced reaction activation energy, stabilization of intermediates, and surface reactions. Collectively, these mechanisms provide highly active catalytic sites, dynamic catalysis, suppression of the shuttle effect, enhanced electron transfer, and improved reaction reversibility, thereby significantly improving the performance and stability of lithium-sulfur batteries.

Adsorption effect: The adsorption effect mainly occurs on the surface of metal catalysts. Transition metal surfaces typically possess various active sites that can form chemical bonds with long-chain polysulfides, thereby stably adsorbing them onto the catalyst surface. Particularly sulfurophilic metals, such as vanadium (V)^[35], cobalt (Co)^[36], and others. This adsorption effect not only prevents free diffusion of polysulfides but also provides a favorable microenvironment for their subsequent catalytic conversion. At the microscopic level, the formation of these chemical bonds may involve electron cloud overlap between the d-orbitals of metal atoms and the p-orbitals of sulfur atoms in the polysulfides, forming stable coordination bonds.

Accelerated Electron Transfer: Some transition metal catalysts can promote chemical reactions by lowering the activation energy, a characteristic particularly prominent in noble metals with high electrical conductivity. Noble metals such as silver (Ag)^[9], platinum (Pt)^[37], and palladium (Pd)^[38] possess electronic structures and conductive properties that enable efficient electron transfer between the metal and polysulfides, significantly enhancing the rate of redox reactions involving polysulfides. At the microscopic level, the electron cloud distribution of these metals is relatively uniform, and they exhibit high electron mobility. When polysulfides come into contact with these metals, electrons can rapidly transfer from the metal to the polysulfide molecules or vice versa, enabling fast redox reactions. Additionally, the densely packed and orderly arranged atoms on the noble metal surfaces reduce electron scattering and loss during transmission, further improving the efficiency of electron transfer^[39].

Lowering the reaction activation energy :Some metal catalysts can promote chemical reactions by lowering the activation energy of the reaction. At the microscopic level, this is often related to the active sites on the metal surface. These active sites may have specific geometric configurations and electronic structures that can form unique interactions with reactant molecules, thereby reducing the energy barrier of the reaction. In addition, metal catalysts may also lower the activation energy by altering the reaction pathway or providing new reaction intermediates. For example, molybdenum (Mo)^[21]has been found to aggregate soluble LiPSs into droplet-like dense phases, thereby inducing instantaneous crystallization rather than the classical stepwise conversion.

Stable Intermediate Species :Metal catalysts can stabilize intermediate polysulfides during the sulfur conversion process. At the microscopic level, these interactions may include electrostatic interactions, van der Waals forces, and possible chemical bonding between metal atoms and polysulfide molecules. Due to the unstable and shuttle-prone nature of intermediate polysulfides (Li₂S₆^-/S₄^-), stabilizing these intermediates through methods such as adsorption or charge accumulation can lower the energy barrier of the entire reaction, thereby achieving a depolarization effect and enhancing capacity^[9].

Surface Reaction: At the surface of a metal catalyst, reactant molecules may undergo reconstruction or dissociation, generating species that are more reactive. This type of surface reaction is typically associated with catalytically active sites on the metal surface, surface defects, and the adsorption state of the reactant molecules. At the microscopic level, these processes may involve electron transfer and rearrangement between the d-orbitals of metal atoms and the atomic orbitals within the reactant molecules^[40]. Such surface reactions can accelerate the overall chemical reaction process^[41].

Synergistic effect: In some cases, metal catalysts may exhibit synergistic effects with other catalysts (such as other metals, transition metal compounds, or carbon-based catalysts), collectively promoting the reaction. This synergistic effect serves as the foundation for the design of bimetallic or multimetallic alloys, which can further enhance catalytic efficiency and selectivity^[42-43].

The improvement of catalytic materials on the battery performance of lithium-sulfur batteries mainly lies in enhancing their cycling stability, energy density and rate capability. As the most fundamental reaction in lithium-sulfur batteries, sulfur reduction reaction can cause several serious issues if it cannot proceed at an ideal reaction rate: (1) significant "shuttle effect"; (2) formation of large amounts of "dead sulfur" leading to rapid capacity decay and reduced battery lifespan; (3) poor rate performance. These issues hinder further applications of lithium-sulfur batteries. From this perspective, catalysts play an extremely important role. Transition metal-based catalysts, as commonly used catalysts, have already been extensively applied in lithium-sulfur batteries, where they improve battery performance in several key aspects.

Shuttle Effect Suppression: The "shuttle effect" of polysulfides is a major issue in lithium-sulfur batteries. Large amounts of soluble long-chain polysulfides migrate from the cathode side through the separator to the anode side, causing loss of active materials and self-discharge at the anode, which leads to localized short circuits. Sulfiphilic transition metal catalysts, such as vanadium (V) and cobalt (Co), effectively suppress the shuttle effect by anchoring polysulfides and accelerating their redox reactions, thereby enhancing the cycling stability and lifespan of the battery and reducing the probability of battery failure^[42]. In addition, suppressing the "shuttle effect" helps reduce the risk of internal short circuits and thermal runaway, thus improving the overall safety of the battery^[44-45].

Enhancing the Utilization of Active Materials: Enhancing the utilization of active materials refers to increasing the proportion of active sulfur effectively utilized during the electrochemical reaction. Through the synergistic effects of various catalytic mechanisms mentioned previously, metal catalysts can accelerate the redox reactions of sulfur, enabling more sulfur to participate in the reactions, thus improving the utilization efficiency of active materials and reducing the generation of "dead sulfur"^[46]. On one hand, this increases the capacity of lithium-sulfur batteries; on the other hand, the cycling life of lithium-sulfur batteries is significantly extended. By enhancing sulfur utilization, sulfur molecules within the limited battery space can be used more efficiently, thereby improving the overall reaction efficiency or the energy density of lithium-sulfur batteries. Transition metal catalysts help maintain battery stability, preserving high performance over multiple charge-discharge cycles^[47-48].

Accelerating reaction kinetics: Reaction kinetics refers to the rate at which active materials participate in electrochemical reactions. Metal catalysts significantly enhance the reaction kinetics of sulfur reduction reactions by accelerating electron transfer and inducing rapid lithium-ion transport, thereby improving the electrochemical reaction rate and high-current discharge capability of the battery^[29]. By increasing the reaction rate of polysulfides, the charge-discharge efficiency of lithium-sulfur batteries can be greatly improved, enabling the battery to deliver higher output power. This means the battery can operate at higher charge-discharge rates, enhancing its rate performance.

Ferrous metals mainly refer to iron, manganese, chromium and their alloys. These materials are characterized by high density and melting point, high strength and hardness, good plasticity and forgeability, as well as a tendency to form oxides. Ferrous metals, especially iron, manganese, chromium and others, have various applications in the field of electrochemical catalysis for batteries. For example, iron ions can provide active sites on the electrode surface, promoting oxygen molecule adsorption and reaction processes, thereby accelerating the electrocatalytic oxygen reduction reaction^[49-52]. Manganese can act as a catalyst to enhance energy conversion efficiency in batteries^[51,53-54].

Sun et al.^[52] designed a robust heterostructured material with nano-scale iron anchored on perovskite oxides for efficient catalysis of the oxidation and reduction reactions of lithium polysulfides (LiPSs), which was verified through density functional theory (DFT) calculations and experimental characterizations. As shown in Fig. 3a, the perovskite maintains high crystallinity after nano-scale transition metal exsolution, laying the foundation for its subsequent stable and favorable adsorption and catalytic performance. The elemental composition of the bifunctional heterostructured material STO@Fe was investigated using energy-dispersive spectroscopy elemental mapping. The results of elemental surface distribution indicated uniform distributions of Sr, Ti, and O elements. The Fe mapping image revealed enrichment of in situ-exsolved Fe species. Due to the strong chemical coupling interaction between nano-scale transition metals and polysulfides (LiPSs), STO@Fe exhibits excellent catalytic activity toward the redox reactions of LiPSs. Sun et al. employed in situ Raman spectroscopy to monitor real-time changes in sulfur and LiPSs during charge-discharge processes of lithium-sulfur batteries, based on which the catalytic role of iron nanoparticles was evaluated. As shown in Fig. 3b, under open-circuit voltage conditions of the STO@Fe electrode, peaks corresponding to S₈^2- were observed at 150, 220, and 475 cm^-1. During battery discharge, the peaks of S₆^2-/S₄^2- first increased and then decreased. The trend during charging was opposite. Comparison of STO@Fe with other cathodes revealed that STO@Fe exhibited more pronounced Raman peaks of S₆^2-/S₄^2-, indicating higher concentrations of polysulfide intermediates during catalytic conversion, meaning that the introduction of Fe nanoparticles significantly accelerated the redox process of sulfur. Fig. 3c illustrates this specific catalytic process, where green spheres represent nano-scale Fe anchored on the perovskite surface, blue spheres represent Li, and yellow spheres represent S. During discharge, S₈ dissolves, transforms into LiPSs, and eventually forms lithium sulfide. The charging process corresponds to the oxidation of lithium sulfide into LiPSs and ultimately back to S₈. STO@Fe as a cathode host demonstrates excellent rate capability and cycling stability, enabling high-performance Li-S batteries capable of stable cycling even under fast charge-discharge conditions. This work also indicates that metallic Fe participates simultaneously in both the sulfur reduction reaction (SRR) and sulfur oxidation reaction (SOR).

Ling et al.^[50] further demonstrated that iron metal can significantly promote the reduction of LiPSs to Li₂S. They employed a unique Fe-N-V pre-coordination strategy to regulate the content of "dissociated Fe³⁺" in solution, successfully constructing "urchin-like" hollow carbon nanospheres decorated with single-atom Fe-N₄ sites and VN nanoparticles (SA-Fe/VN@NMC). The Li-S battery based on SA-Fe/VN@NMC exhibited high cycling stability (a decay rate of 0.024% per cycle over 700 cycles at 1 C) and considerable rate performance (683.2 mAh·g^-1 at 4 C). Moreover, under a high sulfur loading of 5.6 mg·cm^-2, a high area capacity of 5.06 mAh·cm^-2 was maintained after 100 cycles. This indicates that nanoscale Fe metal already possesses strong catalytic ability for sulfur conversion, while atomic-scale Fe maximizes this sulfur catalytic capability.

Zhang et al.^[49] designed and synthesized iron single atoms with an Fe-N₃C₂-C asymmetric coordination configuration as efficient sulfur immobilizers and catalysts (Figure 3d). Figure 3e shows the X-ray absorption spectrum of Fe-N₃C₂-C, confirming the presence of Fe single atoms. Zhang et al. subsequently tested the material as a cathode for batteries, as shown in Figure 3f: The Fe-N₃C₂-C battery exhibited high discharge capacity and redox reaction reversibility (under 0.5 C conditions, the Fe-N₃C₂-C battery maintained a high discharge capacity of 979.4 mAh·g^-1 after 100 cycles; furthermore, it demonstrated excellent rate performance, maintaining a stable discharge capacity of 727.8 mAh·g^-1 even at a high current density of 6 C. When the current density recovered to 0.2 C, the battery achieved a discharge capacity of 1044.6 mAh·g^-1. Experimental and theoretical results indicated that the asymmetrically coordinated Fe-N₃C₂-C groups not only formed additional π bonds through S-p orbital and \mathrm{Fe} - {\mathrm{d}}_{x^2-y^2} / {\mathrm{d}}_{xy} orbital hybridization, enhancing the anchoring ability of LiPSs, but also improved the redox kinetics of LiPSs by reducing the Li₂S precipitation/decomposition barrier, thereby suppressing the shuttle effect.

Similar to ferrous metals, manganese (Mn) metal, as a common black iron-group metal, also exhibits excellent sulfur-fixing and catalytic activity. Figures 3g~i show the work of Zhang et al.^[55] on manganese single atoms. Based on an etching-evaporation defect engineering strategy, Zhang et al. fabricated Mn-N-doped porous carbon (Mn-N-C). Through Zn atom evaporation and NH₃ etching, abundant spatially confined sites and N dopants were formed in Mn-N-C, where the Mn loading could reach up to 2.31 wt% (Figure 3g). DFT calculations indicate that the Mn atoms in Mn-N-C play a crucial role in polysulfide adsorption and enhanced electrical conductivity (Figure 3h). Therefore, Mn-N-C-modified batteries exhibit high conductivity, strong sulfur-fixing characteristics, and excellent catalytic activity, which are beneficial for polysulfide conversion and Li₂S nucleation/dissolution, as shown in Figure 3i. Under a charge-discharge current density of 0.1 C, the initial discharge capacity of the Mn-N-C lithium-sulfur battery reaches as high as 1596 mAh·g^-1. In addition to the extremely high initial capacity, the battery's lifespan and stability are significantly improved (after 1000 cycles at 1 C, the decay per cycle is only 0.045%). This study demonstrates that Mn is a high-performance catalyst for lithium batteries and highlights the promising prospects of black metals in lithium-sulfur battery applications.

Non-ferrous metals refer to all metals except iron, manganese, and chromium. Compared with ferrous metals (mainly iron and iron alloys), non-ferrous metals are generally non-magnetic. Due to their unique physical and chemical properties, these metals are widely used in various fields. In addition, common non-ferrous metals can be further divided into heavy non-ferrous metals and light non-ferrous metals.

Among the transition metals, only titanium (Ti) metal is classified as a colored light metal^[56-60]. Due to its unique metallic properties, titanium is typically applied in the form of metal compounds. For example, Chen et al.^[58] investigated titanium-based MXene/carbon nanotube (CNT) porous microspheres as electrocatalysts for lithium polysulfide (LiPS) oxidation reactions. The internal surface stress endows MXene nanosheets with abundant active sites and shifts the d-band center of Ti atoms upward toward the Fermi level, thereby enhancing LiPS adsorption capacity and accelerating catalytic conversion. The macroporous framework provides uniform sulfur distribution, effective sulfur immobilization, and a large surface area. This tensile strain effect enables the lithium-sulfur battery to achieve an initial capacity of 1451 mAh·g^-1 at 0.2 C, significantly extending cycling life and achieving a maximum charge-discharge rate of up to 8 C.

As the most common types of metals in the fields of electrocatalysis and electrochemical reduction, non-ferrous heavy metals have been extensively studied due to their high stability, ductility, and relatively active chemical properties. Common non-ferrous heavy metals include copper (Cu)^[61-63], cobalt (Co)^[47,64-68], nickel (Ni)^[69-73], zinc (Zn)^[74], and others.

Copper (Cu): Copper exhibits excellent electrocatalytic reduction performance and has been widely applied in lithium-sulfur batteries. Currently, most studies attribute the superior catalytic properties of Cu metal to its sulfurophilicity and conductivity. For example, Wang et al.^[61] uniformly dispersed 10% highly conductive Cu nanoparticles into microporous carbon (MC) as a sulfur host, synthesizing MC confined sulfur composite with stabilized copper (MC-S-Cu). In the MC-Cu-S composite, the MC framework physically confines S/LiPSs, providing free space for volume expansion of sulfur during lithiation. The Cu nanoparticles dispersed within the MC further chemically interact with S/LiPSs through strong bonding interactions, enabling an increase in sulfur content within the C-S cathode material from 30% to 50% without compromising electrochemical performance in low-cost carbonate electrolytes. At a current density of 100 mA·g^-1, the MC-Cu-S cathode achieves nearly 100% Coulombic efficiency with capacity retention above 600 mAh·g^-1 over more than 500 cycles. Additionally, the Cu nanoparticles enhance the electronic conductivity of the MC-Cu-S composite, significantly improving the charge-discharge rate capability of lithium-sulfur batteries. This strategy of anchoring a small amount of metallic nanoparticles within MC to stabilize sulfur provides an effective approach for enhancing cycling stability, Coulombic efficiency, and sulfur loading in Li-S batteries. Shang et al.^[63] further validated the sulfur fixation effect of metallic Cu: by introducing metallic Cu to modify TiO₂ nanodots grown on N-doped carbon nanosheets (Cu/mTiO₂-NC), they effectively created a buffer layer that confines active sulfur species within the cathode region. Extensive experiments demonstrated that metallic Cu can react with polysulfides, further immobilizing them. Consequently, the Li-S battery exhibited a discharge capacity of 811 mA·h^-1 after 100 cycles at 0.2 C, which is 50% higher than that of batteries without Cu.

On the other hand, copper's electrical conductivity is also an important property that enhances its catalytic performance. Due to the insulating nature of S and Li₂S, during discharge, catalytic sites can become covered with these insulating materials, leading to reduced catalytic activity. To address this issue, Li et al.^[62] utilized single-atom copper, which exhibits excellent conductivity (Figure 4a), to lower the energy barrier for Li₂S deposition and dissociation. In addition to reducing the redox energy barrier for thermodynamic reduction, the metal Li₂S nuclei deposit onto the SA-Cu-modified nitrogen-doped carbon nanofiber foam (SA-Cu@NCNF), which facilitates superior electron transport. This results in three-dimensional nucleation and growth, forming three-dimensional spherical clusters instead of the traditional two-dimensional lateral morphology (Figure 4b), thereby significantly enhancing the Li₂S deposition capability and catalytic efficiency at sites covered by Li₂S. Figure 4c demonstrates a substantial increase in Li₂S nucleation. The results indicate that the SA-Cu@NCNF-based lithium-sulfur battery with a sulfur loading of 4 mg·cm^-2 delivers an area capacity of 1.60 mAh·cm^-2 after 500 cycles at 5 C (with a decay rate of 0.038% per cycle). Under conditions of 0.2 C, with a sulfur loading of 10 mg·cm^-2, the areal specific capacity reaches 8.44 mAh·cm^-2.

Cobalt (Co) and Nickel (Ni): Nickel and cobalt are widely applied in lithium-sulfur batteries, primarily leveraging their unique metallic properties, such as electrical conductivity and catalytic activity. On one hand, both nickel and cobalt exhibit good electrical conductivity, which is crucial for electron transport within the battery. On the other hand, both metals serve as catalysts at the cathode of lithium-sulfur batteries, promoting redox reactions of sulfur. However, from the perspective of catalytic mechanisms, these two metals may exhibit certain differences, with distinct catalytic pathways: cobalt metal mainly enhances interactions with LiPSs and improves adsorption capacity, while nickel-based catalysts tend to reduce reaction activation energy and suppress polarization effects. Zhao et al.^[66] synthesized porous CNTs/Co microspheres and used them as host materials for the sulfur cathode (Fig. 4d). Static adsorption experiments demonstrated that cobalt metal nanoparticles can achieve strong chemical adsorption of polysulfides (Fig. 4e) and rapid catalytic conversion. Fig. 4f shows the XPS spectra of Co/CNT and CNT after sulfur adsorption, indicating a strong chemical interaction between Co and LiPSs, which inhibits the dissolution and migration of LiPSs, thereby extending the cycling life of lithium-sulfur batteries. At 0.2 C, the S-CNTs/Co composite delivered a high initial discharge specific capacity of 1242.5 mAh·g^-1. Over 300 cycles, the capacity decay rate was only 0.066% per cycle.

Compared to cobalt metal, nickel metal utilizes its high electrical conductivity and lithiophilic characteristics to enhance the internal electron/ion transport rate of batteries, thereby promoting redox reactions of the sulfur cathode and inducing ordered growth of lithium dendrites on the anode. Yuan et al.^[72] prepared a composite material composed of Ni nanoparticles embedded in gradient porous carbon microspheres (Ni/PCMS), which functions as a microreactor and serves as a separator modification material. The embedded carbon substrate and nickel nanoparticles ensure sufficient electronic conductivity. The physical/chemical adsorption and high Li⁺ transference number of Ni/PCMS suppress both the shuttle effect and the growth of Li dendrites. Exposed Ni active sites provide high catalytic activity. As a result, the battery achieves a high initial discharge capacity of 1426.7 mAh·g^-1 at 0.1 C and a low capacity decay rate of 0.078% per cycle. The above research demonstrates that nickel metal is a promising catalyst for accelerating the conversion of active sulfur. Zheng et al.^[69] presented a method to further anchor Ni by utilizing MOF material coordination. After high-temperature pyrolysis in a nitrogen-doped nanocarbon matrix (Ni@NNC), a Ni single-atom catalyst with high loading (3.3 wt%) was synthesized. The strong adsorption capability of LiPSs provided by the MOF substrate combined with Ni's rapid catalytic conversion ability significantly enhances battery performance.

Zinc (Zn): As a metal with relatively mild chemical properties among non-ferrous metals, Zn metal itself possesses excellent electrical conductivity and sulfurophilicity. In the work by Chen et al.^[74], high-loading zinc single-atom catalysts (Zn SACs) were utilized, where nitrogen-anchored single-atom zinc on highly ordered nitrogen-doped carbon nanotube arrays acted as sulfur hosts to enable rapid conversion of LiPSs, thereby significantly enhancing the electrochemical performance of lithium-sulfur batteries. The state of single-atom Zn is shown in Figure 4g. The authors demonstrated the existence of Zn single atoms using extended X-ray absorption fine structure (EXAFS) wavelet transform spectroscopy (Figure 4h). To investigate the catalytic mechanism of Zn metal, in situ XRD was employed to examine the changes in Zn atoms during the battery's charging and discharging processes (Figure 4i). Based on the in situ XRD analysis, Chen et al. proposed the following catalytic mechanism for Zn single atoms: The Zn SACs achieve maximum atomic-level utilization and high catalytic activity through their highly dispersed metallic centers, effectively accelerating the redox reactions of polysulfides (LiPSs). Meanwhile, Zn SACs chemically adsorb LiPSs, suppressing their shuttle behavior during battery charging and discharging, thus reducing the loss of active materials and improving the cycling stability of the battery. Additionally, adjacent single atoms on Zn SACs synergistically enhance each other's effects, further promoting rapid multi-step sulfur reactions under high sulfur loading conditions, enabling the battery to maintain excellent electrochemical performance even at high energy densities.

Noble metals mainly include gold (Au), silver (Ag), and platinum group metals such as platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), and iridium (Ir). Most of these metals exhibit high ductility, electrical conductivity, and strong resistance to chemical corrosion. Due to their unique physical and chemical properties, these metals have been increasingly applied in lithium-sulfur batteries in recent years to improve the redox reaction kinetics of lithium-sulfur batteries, enhance conductivity, and accelerate the SRR and SOR reactions^{[34,37-38,46,75-77]}

Mei et al.^[46] introduced metallic iridium (Ir) into the cathode of the battery, utilizing the carbon framework structure derived from the calcination of Ir-organic frameworks to establish efficient conductive channels for rapid ion/electron transport (Fig. 5a). Additionally, the alloy of noble metal Ir with sulfurophilic metal Cu provides abundant active sites that can effectively capture LiPSs and accelerate the catalytic conversion process, as the introduction of Ir atoms can modulate the surface electronic structure of metal Cu, thereby influencing the 3d orbital distribution. Benefiting from the introduction of Ir, the redox kinetics of lithium-sulfur batteries are significantly improved (as evidenced by the CV curves in Fig. 5b). Moreover, Fig. 5c shows that the IrCu/NC battery delivers an initial specific capacity of 1288 mAh·g^-1 at 0.2 C, demonstrating enhanced sulfur conversion efficiency.

Chang et al.^[77] investigated the catalytic mechanism of noble metals on LiPSs. As shown in Figure 5d, they designed a sulfur host material based on ruthenium (Ru) nanocluster-modified TiO₂ nanotubes (TiO₂-Ru) through an interfacial engineering modification strategy. This sulfur host exhibited excellent rate performance in lithium-sulfur batteries (Figure 5e), enabling TiO₂, which is inherently semiconducting, to achieve conductivity comparable to that of conductive carbon materials. Figures 5f and 5g present the in situ Raman spectra of TiO₂-Ru during discharge and charge processes, respectively, showing the variation of Raman signals over time throughout the entire reaction. It can be observed that the concentration of soluble LiPSs remained consistently low within TiO₂-Ru, indicating that the TiO₂-Ru composite nanotubes possess appropriate catalytic activity, which facilitates the conversion of LiPSs and prevents the formation of "dead sulfur." The authors attributed this enhanced catalytic performance to the interface field effect of the TiO₂-Ru structure, combined with the hollow nanotube configuration and the strong chemical interaction of TiO₂, which together improve the trapping capability for LiPSs and suppress the "shuttle effect." Additionally, the high catalytic activity of Ru nanoclusters reduces the energy barriers of multistep reactions, thereby accelerating the redox kinetics in lithium-sulfur batteries.

A large body of research has demonstrated that noble metals exhibit the strongest sulfur catalytic effects among metals. Nevertheless, differences inherent to the noble metals themselves directly determine the reaction rates in lithium-sulfur batteries. Wan et al.^[34] investigated the catalytic performance of various noble metals in lithium-sulfur batteries and identified the noble metal with optimal catalytic properties. In this work, a surfactant self-assembly method was employed to synthesize a series of noble metals (Pd, Au, Pt, Rh, Ru) as catalysts on ordered mesoporous carbon (OMC). As shown in Fig. 5h, i, Wan et al. compared the first charge-discharge curves of lithium-sulfur batteries assembled with different noble metals and found that the Pd/OMC catalyst exhibited excellent catalytic performance, delivering the highest initial specific capacity. The catalytic mechanism of Pd metal was further explored using in situ Raman spectroscopy, as illustrated in Fig. 5j, k: OMC without Pd doping displayed obvious polysulfide Raman peaks (Fig. 5j). In contrast, during both discharge and charge processes, time-resolved Raman spectra collected from the cathode loaded with Pd/OMC showed almost no signals corresponding to soluble polysulfides (Fig. 5k). These results indicate that during the SRR at given voltages, polysulfides were fully reduced and precipitated on the cathode, significantly enhancing the reaction rate, which aligns well with the reduced apparent activation energy observed for the Pd/OMC catalyst. High sulfur utilization led to quasi-reversible redox reactions and high charge capacity.

Rare refractory metals are a class of rare metals with high melting points, including elements such as molybdenum (Mo), tungsten (W), and vanadium (V). Due to their excellent physical and chemical properties, particularly their high melting points and outstanding corrosion resistance, they can maintain stable performance even under extreme conditions. Because of these characteristics, metals represented by Mo and V exhibit various potential applications in lithium-sulfur batteries, such as serving as electrode support structures, catalysts, and cathode protective layers^[11,78-80].

Zhang et al.^[78] synthesized a Mo@N-G composite material by embedding monodisperse Mo nanoparticles onto nitrogen-doped graphene and utilized it as a cathode catalyst for lithium-sulfur batteries (see Figure 6a). This material can significantly enhance the battery capacity (initial specific capacity of 1359 mAh·g^-1 at 0.2 C), rate performance (specific capacities of 1365, 1195, 1025, and 842 mAh·g^-1 at 0.2, 1, 2, and 5 C, respectively), and cycling life (retaining a specific capacity of 615 mAh·g^-1 after 500 cycles at 1 C). To further verify the catalytic mechanism of Mo metal, Zhang et al. investigated the changes in Mo during the charging and discharging processes of the lithium-sulfur battery using ex-situ XPS, as shown in Figures 6b~d. For the XPS spectrum of Mo (Figure 6c), three distinct spin-orbit peaks correspond to Mo 3d_5/2 and Mo 3d_3/2 of Mo@N-G. Compared with the pristine sample, the peaks of Mo⁶⁺ and Mo⁴⁺ decrease during discharge, and a peak at 225 eV is observed, indicating the formation of Mo—S bonds. This occurs because the unfilled d orbitals of oxidized Mo can attract electrons from negatively charged LiPS intermediates and form Mo—S bonds, thus promoting the conversion of LiPSs. Importantly, compared to the pristine state, these spin-orbit peaks of Mo@N-G exhibit higher binding energies during charge/discharge, and the Mo—S bond disappears when charging reaches 2.7 V (point C2). These results demonstrate the chemical stability of the Mo@N-G catalyst during the LiPS conversion process. Therefore, the authors proposed that the unfilled d orbitals of Mo accept electrons from LiPS anions during the electrochemical process to form Mo—S bonds, thereby facilitating the rapid conversion of LiPSs. Inspired by molybdenum enzymes in biocatalysis featuring stable Mo—S bonds, Li et al. developed porous Mo-N-C nanosheets with atomically dispersed Mo-N₂/C sites as cathodes for lithium-sulfur batteries to promote Li-S batteries' adsorption and conversion of LiPSs. Due to the high intrinsic activity of S/Mo-N-C and the Mo-N₂/C coordination structure, which accelerates polysulfide reaction kinetics and suppresses the polysulfide shuttle effect, the rate performance and cycling stability of S/Mo-N-C are significantly improved compared to S/N-C. S/Mo-N-C exhibits a high reversible capacity of 743.9 mAh·g^-1 at 5 C and an extremely low capacity decay rate of 0.018% per cycle after 550 cycles at 2 C, outperforming most reported cathode materials. Density functional theory calculations reveal that the Mo-N₂/C site simultaneously reduces both the activation energy for the conversion of Li₂S₄ to Li₂S and the decomposition barrier of Li₂S, which accounts for its inherently high activity.

Rare earth metals are a collective term for 17 elements in group IIIB of the periodic table, including scandium, yttrium, and the lanthanide series. These elements possess unique physical and chemical properties, with luster ranging between silver and iron, and exhibit strong chemical reactivity. Rare earth elements are primarily applied in lithium batteries as cathode materials and electrolyte additives. By incorporating rare earth elements, the electrical conductivity and discharge capacity of lithium-ion batteries can be improved, thereby enhancing the battery's cycle performance and safety. In lithium-sulfur batteries, rare earth elements can similarly play a comparable role in enhancing the overall battery performance^[81-83].

Wang et al.^[81] utilized a metal-organic framework (MOF) loaded with cerium (Ce) combined with carbon nanotubes (CNTs) to form a Ce-MOF/CNT composite as a coating material for Li-S batteries. Due to the stability of the rare earth metal Ce, the conversion of LiPSs can be steadily catalyzed, and the battery exhibited excellent electrochemical stability and capacity retention. The Ce-MOF/CNT-coated Li-S battery delivered an initial specific capacity of 1021.8 mAh·g^-1 at 1 C and gradually decreased to 838.8 mAh·g^-1 after 800 cycles, with a fading rate of only 0.022% and a Coulombic efficiency close to 100%. This work confirmed that Ce exhibits excellent adsorption capability toward Li₂S₆ and effectively catalyzes sulfur conversion, thereby suppressing the polysulfide shuttle effect in Li-S batteries and enhancing their overall performance.

Rationally designing cathode/separators with high conductivity and strong interfacial adsorption towards lithium polysulfides is an urgent demand for lithium-sulfur (Li-S) batteries. Kang et al.^[82] reported a multifunctional pomegranate-like porous carbon nanofiber, in which lanthanum (La) was successfully doped into the carbon fibers (La@PCNFs) via electrospinning technology followed by a subsequent one-step carbonization process, applied in highly stable Li-S batteries. Interestingly, numerous hollow and mesoporous carbon particles with high graphitization degrees are uniformly and densely grown inside macroporous carbon skeletons, constructing fast hierarchical transfer channels and greatly enriching exposed active sites. More importantly, the polar interface modified by active ionic C-F bonds and La nanocrystals exhibits strong trapping effects on lithium polysulfides during cycling. Therefore, the La@PCNFs cathode delivers a high discharge capacity of 640 mAh·g^-1 with an average capacity decay of only 0.05%. This work further highlights the application potential of rare-earth metals in Li-S batteries. Currently, specific studies on the application of rare-earth metals in Li-S batteries are still ongoing. It is believed that more research will uncover additional applications of rare-earth metals in Li-S batteries, such as enhancing safety and regulating catalytic reaction rates.

In lithium-sulfur batteries, the catalytic activity of metal catalysts directly affects the battery performance. The catalytic activity of the catalysts is influenced by the metal's physicochemical properties, particle size, molecular structure, and doping modifications. In addition, the stability of metal catalysts also poses a significant challenge. To enhance both the activity and stability of catalysts, researchers have employed various strategies.

(1) Nanostructural design of catalysts (Fig. 7a)^[84], such as controlling the size of nanoparticles or roughening the metal surface to increase the effective catalytic area and enhance the specific surface area^[44,85]. Additionally, ordered rearrangement of atomic arrangements within the metal can achieve faster electron/mass transport rates^[86-87], thereby accelerating electrochemical reaction rates, which is also an effective modification strategy.

(2) The electronic structure and chemical properties of catalysts can be regulated through doping modification methods (Figure 7b), thereby enhancing their catalytic ability in sulfur-related reactions^[88-89]. For example, oxidizing pure metal catalysts into transition metal compounds (TMCs) can effectively improve the stability of transition metal catalysts^[21]. Currently, relevant studies have shown that the combination of transition metal compounds and pure metals can effectively utilize the catalytic performance of metals while leveraging the strong adsorption capability of TMCs, achieving lithium-sulfur batteries with both high capacity and high stability, thus verifying the feasibility of this optimization strategy.

(3) Alloying is also an effective strategy to enhance the stability of transition metal catalysts and functionalize single-metal catalysts.^[90-91] (Fig. 7c). Utilizing alloyed catalysts (e.g., bimetallic^[92], medium-entropy alloys^[90], and high-entropy alloys^[93-94]) can improve the catalytic activity and selectivity of metals, suppressing undesirable side reactions such as the shuttle effect of sulfur. This typically requires the catalyst to effectively guide the reaction pathway of the sulfur cathode, ensuring the efficient utilization of sulfur during the charging and discharging processes.

(4) Surface coating or adopting a core-shell structure for catalysts can also effectively prevent their degradation during charge-discharge processes^[95-96] (Fig. 7d). However, excessively thick coating layers tend to block catalytic sites, making the design of the coating structure critical to solving this issue. A common current approach is designing porous coating layers, where abundant pores ensure efficient transport of LiPSs and lithium ions, thereby enhancing catalyst lifespan while maintaining catalytic performance.

The application prospects of transition metal catalysts in lithium-sulfur batteries are evidently influenced by environmental friendliness and cost-effectiveness. However, the cost-benefit analysis of transition metal catalysts is a complex yet crucial process involving multiple considerations. This paper summarizes the costs and benefits of transition metals and provides a comprehensive evaluation to assess the feasibility of applying transition metal catalysts and to identify measures for reducing costs and enhancing efficiency.

Cost Analysis: Transition metal catalysts involve costs related to raw materials, preparation processes, and recycling. The preparation of transition metal catalysts requires metal oxides as raw materials, including costly base metals such as nickel, cobalt, and copper, and even precious metals like platinum and palladium. Fluctuations in the prices of these metals directly impact production costs. Additionally, the preparation process for transition metal catalysts is complex and requires specialized technology and equipment, which also contributes to the overall cost. Finally, recycling and reuse must be considered, as catalyst activity decreases with repeated use, necessitating regeneration or replacement. The recovery and reuse process demands specialized technology and equipment, further increasing costs.

Benefit Analysis: The benefits brought by transition metal catalysts are also considerable. From an economic perspective, transition metal catalysts exhibit high activity and high selectivity, enabling enhanced reaction rates of polysulfides and increased utilization efficiency of active materials under mild reaction conditions, thereby significantly improving battery capacity and service life. In addition, the recyclability of catalysts reduces production costs and further enhances economic benefits. From a social perspective, the use of catalysts can reduce energy consumption and environmental pollution, aligning with the development trend of green chemistry.

Cost-Benefit Comprehensive Evaluation: The cost-benefit analysis of transition metal catalysts requires a comprehensive consideration of cost factors such as raw materials, preparation processes, and recycling, as well as their economic and social benefits. By implementing cost control strategies such as improving preparation techniques, enhancing catalyst activity, and achieving catalyst reuse, the overall cost of catalysts can be effectively reduced. In addition, continuously optimizing catalyst performance and expanding application areas—for example, developing more efficient transition metal catalysts such as high-entropy alloys and heterojunction composite materials—can enhance their economic and societal benefits. Through strategies focused on cost control and benefit enhancement, sustainable development and widespread application of transition metal catalysts can be realized.

Exploration of Novel Catalysts: In recent years, researchers have continuously explored new types of transition metal catalysts aiming to find materials that can more efficiently promote sulfur conversion in the cathodes of lithium-sulfur batteries. From single-metal catalysts such as iron, cobalt, and nickel, to dual-metal, tri-metal, and even more complex medium-entropy and high-entropy alloys, or transition metal compounds, scientists are attempting to achieve superior catalytic performance by combining and doping multiple materials to maximize their advantages and mitigate disadvantages.

Innovation in synthesis methods: Innovative catalyst synthesis methods can effectively regulate the molecular structure and crystal phase of transition metal catalysts. In addition to traditional approaches such as chemical oxidation, sol-gel, and co-precipitation, many emerging synthesis techniques like electrodeposition, joule heating, magnetron sputtering, and plasma sputtering have emerged, enabling effective regulation of the properties of metal nanoparticles, thereby significantly enhancing the catalyst's activity, stability, and dispersion. These new methods not only improve the efficiency of catalyst preparation but also allow precise control over their physical and chemical properties.

Continuously improving battery performance: Metal catalysts significantly enhance the charging and discharging efficiency and cycling stability of lithium-sulfur batteries by reducing electrochemical polarization at the sulfur cathode and accelerating the transport of lithium ions and electrons. In the future, as catalyst research advances further, the performance of lithium-sulfur batteries will continue to improve.

Improving stability and environmental adaptability: The introduction of transition metal catalysts can also enhance the operational stability of lithium-sulfur batteries under various environmental temperatures, such as high temperature, low temperature, and high humidity conditions. This is particularly crucial for applications like electric vehicles and outdoor equipment, ensuring reliable performance across diverse environmental conditions.

Safety Assurance: Metal catalysts can not only improve battery performance but also enhance battery safety to some extent. By optimizing the structure and properties of the catalyst, lithium metal can be induced to deposit uniformly on the anode surface, preventing the disordered growth of lithium dendrites and reducing the risk of internal short circuits. At the same time, they reduce the shuttle effect of polysulfides, which can cause self-discharge and self-heating on the anode, thereby decreasing the occurrence of safety incidents such as thermal runaway.

Transition metal catalysts, as a common type of catalyst in lithium-sulfur batteries, play superior roles in the positive electrode sulfur conversion (SRR, SOR), suppression of polysulfide shuttling, and protection of the negative electrode. With further research, transition metal catalysts will continue to play a key role in the field of lithium-sulfur batteries, contributing to significant breakthroughs in battery performance, environmental adaptability, and safety, thereby promoting the continuous development of new energy battery technologies.