Single-atom composites (SACs), which consist of isolated metal atoms dispersed or anchored on matrices, have been regarded as rising stars in the field of catalysis recently [46]. With the merits of maximum atomic utilization, uniformly distributed active sites and splendid catalytic activity and selectivity, SACs have been widely employed in numerous electrochemical reactions, for instance, oxygen reduction reactions [47], hydrogen evolution reactions [48] and advanced energy systems [49-51].

Among these, the latest researches revealed that the non-precious iron-based SACs, with stable and explicit active centers, can demonstrate the same or even better electrochemical performances than other traditional precious metal catalysts. In particular, iron-based SACs coordinated with nitrogen-doped carbon matrix have achieved much attention and great progresses as efficacious anchors and catalysts [52-54], especially in Li-S batteries [55-57]. For example, one of the early studies using iron-based SACs as sulfur hosts was reported by Liu and co-workers in 2018 [58]. The researchers fabricated the sulfur host material of single iron active sites in porous nitrogen-doped carbon (Fe-PNC, as shown in Fig. 5a(i)) through polymerizing and carbonizing diphenylamine in the presence of iron phthalocyanine and a hard template. With a sulfur loading of 1.3 mg/cm², the as-prepared Fe-PNC/S cathode displayed an initial specific capacity of 1138.6 mAh/g at 0.1 C and retained 427.1 mAh/g after 300 cycles (Fig. 5a(ii)). The morphology of Li₂S in both Fe-PNC/S and PNC/S after the first and second cycles at 0.1 C were also observed to clarify the comparison of phase nucleation overpotential for the formation of nanoscale Li₂S between Fe-PNC and PNC. It was notable that Li₂S nanospheres deposited on Fe-PNC/S cathode surface were smaller and held the morphology better in Fig. 5a(iii). Encouraged by the attractive surface chemistry of single iron atoms, Jiang et al. [59] developed novel self-supporting carbon nanofibers with hierarchical porous structure and Fe/N adsorption/nucleation centers (Fe/N-HPCNF) as high-performance sulfur hosts via a facile co-spinning method. Benefiting from the porous carbon fiber structure, electrolyte infiltration, conductivity, Li ion transportation rate and sulfur redox kinetics were all improved even with high sulfur loading. The evenly distributed Fe/N heteroatoms served as restraints on polysulfide diffusion through strong chemisorption and as regulators for homogeneous sulfur nucleation. Consequently, S@Fe/N-HPCNF cathodes presented a high initial specific capacity of 1273 mAh/g for over 500 cycles at 0.5 C (the sulfur areal density equals 3.5 mg/cm²). There has been an increasing interest on utilizing iron-based SACs for Li-S battery systems as sulfur cathodes [60-63], separators [38, 64, 65] or cathode interlayers [66] to improve the polysulfide immobilization or sulfur species conversion. For instance, Wang et al. [60] fabricated a holey Fe, N co-doped graphene (HFeNG) via a one-step scalable calcination process. The corresponding sulfur cathodes delivered a high-rate capacity of 1154 mAh/g at 0.5 C initially and cycled stably with a slight capacity decay of 0.083% per cycle as displayed in Fig. 5b(ii). The electrochemical performance advancement can be attributed to the unique holey structures and the strong adsorption forces of Fe-N₂ at the edges which are represented in Fig. 5b(i). Considering the utilization efficiency of SACs, Zhang et al. [64] proposed a multifunctional commercial polypropylene separator coated by graphene foam impregnated with Fe SACs catalysts. As a result, the cycling stability improved hugely, with an initial specific capacity of 891.6 mAh/g and 83.7% retention after 750 cycles at 0.5 C.

Given the performance advancement arising from iron-based materials, many researches have been focused on the mechanist insights about the chemisorption capability enhancement and catalytic effect by theoretical analysis. Furthermore, Zhang et al. [67] investigated the anchoring mechanism of Fe-N₄/graphene to LiPSs species by means of comprehensive DFT computations. The calculation results of binding energy suggested that the long-chain polysulfides can bind with Fe-N₄/graphene stably, with two interaction configurations of Fe-S bond and Li-N bond. The computed binding energies of S-containing clusters on various metal-N₄/graphene as shown in Fig. 5c revealed that Cr-, Mn-, Fe-, Co- and Cu-nitrogen/graphene exhibited optimal interaction with soluble lithium polysulfides due to the synergistic effects between the metal and N atoms, which not only effectively trapped the soluble lithium polysulfides to suppress the shuttle effect, but also well kept their cyclic structures. Notably, the Fe-N₄/graphene displayed an appropriate binding energy among them, which facilitates a smoother trapping-diffusion-conversion process of LiPSs [68, 69]. The researchers also compared the electronic band structures of Co-N/graphene before and after polysulfides adsorption and found that there was a small band gap of less than 0.50 eV, which indicated the obvious intrinsic semiconducting nature of Co-N₄/graphene even after polysulfides adsorption. This theory also provides some experiences for Fe-N₄/graphene. However, the relationship between these two binding mechanisms still remained mysterious. Liang et al. [70] explored that there is a competitive relationship between Li-bonding and S-bonding. Moreover, the S-bonding configuration usually has better performance for both chemisorption and catalysis than Li-bonding. Similar results were obtained by Lin et al. [71] and Zhang et al. [40].

Owing to the low cost, easy availability and strong functional groups on the surface, iron oxides, including FeO, Fe₂O₃, Fe₃O₄, etc. have been extensively applied in lithium-ion battery and sodium-ion battery since early stages [72-74]. However, the poor conductivity and uncontrollable size seriously retard the electrochemical performance of iron oxides. Hence, to overcome these limitations, hybridizing iron oxides with various carbon-based materials has been an effective way.

Iron oxides were used in lithium sulfur battery until 2017 by Yang et al. [32] as we mentioned above. Considering the large surface area and abundant pore structures which benefit sulfur utilization, iron-based metal-organic frameworks (MOFs) have been widely applied as precursors to synthesize iron oxides/carbon composite materials [34, 75, 76]. For example, Fan et al. [34] prepared a jujube pit like Fe₃O₄/C composite (Fe₃O₄/C) calcinated from Fe-MOFs (MIL-53), of which the schematic diagram is presented in Fig. 6a(i). After a facile melt diffusion technique, sulfur was loaded into the internal hollow space of Fe₃O₄/C. The static polysulfides adsorption tests and relative UV–vis adsorption spectrum proved the promotional adsorption capability of Fe₃O₄/C as shown in Fig. 6a(ii). Consequently, such Fe₃O₄/C/S cathodes displayed a high specific capacity of 819 mAh/g at 1 C, with 642 mAh/g retained after 300 cycles (Fig. 6a(iii)). Then Ding et al. [75] proposed an ordered microchannel graphene scaffold with incorporated catalytic MOF-derived Fe₃O₄ nanocrystals and porous carbon (denoted as Fe₃O₄/NC/G) as a multifunctional sulfur host, which exhibited a reversible specific capacity of 1007 mAh/g at 0.1 C, with a small capacity decay rate of less than 0.01% after 100 cycles as shown in Fig. 6b(i). Cyclic voltammetry tests of symmetric cells were performed to verify the catalytic effects of Fe₃O₄/NC/G towards polysulfide reaction kinetics and stability in Fig. 6b(ii). The higher current density and stronger redox peaks demonstrated that polar Fe₃O₄ nanocrystals accelerated the electrochemical reactions and expeditious conversion with catalytic effects. Furthermore, the conversion mechanism of Fe₃O₄/NC/G for Li-S battery was studied through in-situ X-Ray Diffraction (XRD) during cycling for the first 2 cycles at 0.1 C as elucidated in Fig. 6b(iii). During the discharge process, the signals of polysulfides intermediates almost disappeared, suggesting that the diffusion of long-chain LiPSs to the Li anode was greatly reduced and then the undesired anode corrosion can be effectively suppressed. DFT calculations were also carried out to analyze the chemical interaction between Fe₃O₄ and polysulfides. The inferior bonding energy of Fe₃O₄ towards LiPSs (0.44–3.35 eV) was much higher than that of pristine carbon (0.10–0.52 eV), which can be credited to the strong polar-polar interaction between Fe₃O₄ and polysulfides. The accurate calculation results of binding energies of polysulfides on the surface of Fe₃O₄ were displayed in Fig. 6b(iv). Besides these Fe-MOFs derivatives, various iron oxide/carbon nanomaterials emerged through diverse synthesis methods. Lu et al. [77] reported a new approach to fabricate strongly coupled Fe₃O₄ and N-doped carbon which were grown on self-standing activated carbon fiber cloth, namely Fe₃O₄-NC@ACC. This unique nanostructure was obtained via pyrolysis of molecular mixtures of Fe(CN)₆⁴⁻ and pyrrole which were coated on ACC. During the oxidation polymerization of pyrrole prompted by Fe(CN)₆⁴⁻, the reduced Fe(CN)₆⁴⁻ compounds would be incorporated in the PPy (polypyrrole) framework due to the chemical interaction between the C≡N ligands and pyrrole, which means, the Fe₃O₄ nanocrystals would be embedded in the carbon framework with rigid carbon layers covering the surface. In the LiPSs adsorption evaluation, the Li₂S₄ solution immersed with Fe₃O₄-NC@ACC exhibited nearly complete LiPSs adsorption within only 30 min as displayed in Fig. 6c(i), which can be ascribed to the chemical binding of strongly coupled Fe₃O₄ for LiPSs. The following UV–vis adsorption spectra and XPS analysis of Fe₃O₄-NC@ACC composite before and after LiPSs adsorption further proved that in Figs. 6c(ii) and (iii). Beyond that, the discharge capacity of S@Fe₃O₄-NC@ACC cathodes with varied crystallinity and Fe/N loading levels were analyzed in correlation with the reaction stages. By controlling the pyrolyzing temperatures and the amount of pyrrole, the discharge capacity occurred in stage Ⅲ (from Li₂S₄ to Li₂S₂ and Li₂S) differentiated, suggesting the strongly coupled Fe₃O₄ and N-carbon played a key role in the improvement of short-chain polysulfide conversion. With these merits, as shown in Fig. 6c(iv) the S@Fe₃O₄-NC@ACC cathodes presented a high specific capacity of 1316 mAh/g at 0.1 C, and a retention of around 70% after 1000 cycles at 0.2 C under a high sulfur areal density of 4.7 mg/cm². Similar synthesis mechanism was utilized by Zhang et al. [78], but with different morphologies. Herein, novel hierarchical mesoporous nitrogen-rich carbon nanospheres comprising 1D carbon nanotubes encapsulating Fe₃O₄ nanoparticles (Fe₃O₄@CNTs nanospheres) were manufactured as sulfur hosts. Supported by the theoretical simulations, the soluble polysulfides interacts with iron oxides stronger than with carbonaceous material, thus alleviating the shuttling effect of LiPSs effectively. Benefiting from the synergistic effect of chemical adsorption and redox reaction catalyzing, the S/Fe₃O₄@CNTs cathodes displayed the specific discharge capacity change from 937.6 mAh/g initially to 538.5 mAh/g after 1800 cycles at 1 C. Iron oxides can also be obtained through the pyrolysis of iron oleate according to He et al.s' report [79]. The researchers assembled core-shell structured Fe₃O₄@C nanodots with 5 nm diameter as a valid sulfur host, of which the specific surface area reached 216.2 m²/g owing to the nanosized structures of Fe₃O₄ particles. Due to the porous structures and loose surface of Fe₃O₄@C, the sulfur content of S/Fe₃O₄@C host material was calculated to be 66%. And the resultant S/Fe₃O₄@C cathode showed a high initial capacity of 1089 mAh/g at 0.2 C and 816 mAh/g at 1 C.

Iron carbides have attracted intensive attention because of their potential utilization in the field of catalysis and energy storage systems, which can be obtained through versatile synthesis methods, such as sol-gel method, high temperature organic phase method, physical vapor deposition [80]. Notably, the unique electronic, catalytic and magnetic properties of iron carbides originated from their intrinsic structures favor their usage in Li-S batteries [81-83].

For instance, inspired by the work of introducing ferroelectric BaTiO₃ (BTO) into the sulfur cathode to control the "shuttle effect" of polysulfides by the "spontaneous polarization" of BTO [84], Gao et al. [85] introduced ferromagnetic iron/iron carbide (Fe/Fe₃C) nanoparticles with a graphene shell (Fe/Fe₃C/graphene) onto a flexible activated cotton textile (ACT) fiber to prepare the ACT@Fe/Fe₃C/graphene sulfur host. The innovation spot was that the path of the dissolved negative polysulfide ions can be altered via Lorenz force since the built-in magnetic field brought by ferromagnetic iron carbides, namely the "shielding effect" via ferromagnetic nanoparticles. In order to study the polysulfide trapping mechanism of magnetic Fe/Fe₃C nanoparticles, custom-design liquid cells of ACT@Fe/Fe₃C/S were performed in comparison with that of ACT/S. The strikingly different polysulfide diffusion behavior shown in Figs. 7a(i) and (ii) unveiled that the magnetic field can alter the diffusion path of LiPSs effectively through Lorenz force. In the following schematic illustration of Fig. 7a(iii), it can be found that owing to the instinctive magnetization of Fe/Fe₃C NPs, the generated magnetic field manipulated the motion of polysulfide ions through attracting them to the cathode side, thus resulting in a darker brown region of electrolyte around. Wei et al. [86] fabricated ordered mesoporous graphitic carbon/iron carbide nanocomposites (GC/Fe₃C) as sulfur host materials via an evaporation-induced self-assembly (EISA) procedure combined with Teflon-assisted solid-state decomposition. Herein, the Fe₃C nanocrystals contributed to catalyzing the graphitization of amorphous carbon, increasing its pore size. The large quantity of micropores and small-size mesopores enabled the hybrid framework a large specific surface area of 3105 m²/g and pore volume of 3.32 cm³/g, which guaranteed a high sulfur loading (85%) and alleviated the volume expansion to some extent. The as-prepared cathode displayed excellent capacity and cycling stability, with an initial discharge capacity of 1203 mAh/g at 0.2 C and a retention of 91.4% after 500 cycles. Wang et al. [87] also designed and synthesized a unique porous and conductive nanocomposite for which Fe₃C nanoparticles are embedded in nitrogen-doped porous carbon sheets (Fe₃C@NPCS). The porous carbonaceous nanostructures can provide more accommodation space for sulfur species and Fe₃C nanoparticles can enhance the electronic conductivity of the composite (Fig. 7b(i)). According to DFT simulations presented in Fig. 7b(ii), Fe₃C exhibited strong affinity towards polysulfides through Fe-S bonds. Benefiting from the outstanding conductivity of Fe₃C nanocrystals, the Fe₃C@NPCS-S cathode showed a reduced polarization and enhanced reversibility compared with NPCS-S in cyclic voltammogram (CV) measurements as demonstrated in Fig. 7b(iii). More importantly, as exhibited in Fig. 7b(iv), the Fe₃C@NPCS-S electrodes delivered greatly improved specific discharge capacities of 1251, 1127, 1020, 907, 802, 731 and 647 mAh/g at 0.2 C, 0.3 C, 0.5 C, 1 C, 2 C, 3 C and 5 C, respectively. The well-performed rate capability further proved that the introduction of Fe₃C nanocrystals facilitated the electrochemical kinetics, internal resistance and charge transfer as previous research reported. Apart from the increasing utilizations in sulfur hosts [88-91], iron carbides have also represented their advantages of superior conductivity in the application of interlayers for separators [92, 93] or electrodes [94-96]. For example, as Song et al. [92] proposed, the MOF-derived Fe₃C nanocrystals compounded with the nitrogen-doped graphene-like carbon nanosheet (Fe₃C/NG) were coated onto separators given the catalytic and conductive properties of Fe₃C. The Li-S cell with a lightweight Fe₃C/NG-coated separator demonstrated splendid rate performances and cycling stability. Specifically, the as-prepared cell delivered an initial discharge specific capacity of 954.5 mAh/g even at a high current density of 6 C and maintained 439.9 mAh/g after 500 stable cycles with a sulfur loading of 1 mg/cm². Zhang et al. [94] fabricated freestanding graphitized carbon interlayers decorated with Fe/Fe₃C nano-catalysts acquired by thermal treatment of cellulose paper with adsorbed ferric nitrate at 1000 degrees. The in-situ grown Fe nanoparticles can promote the graphitization of carbon, enhancing the conductivity of interlayers. Thanks to the rationally designed nanoarchitecture and the efficacious acceleration of Fe/Fe₃C nanocrystals towards polysulfide conversion, the Li-S cell with an optimal carbon/Fe/Fe₃C interlayer displayed an initial specific capacity of 1556 mAh/g at 0.1 C and stabled at around 772 mAh/g at 1 C after 200 cycles.

Originated from the multi-electron orbitals of phosphorus, the superior chemical properties of iron phosphides have prompted their thriving development in catalytic applications in hydrogen evolution reaction (HER), and oxygen reduction reaction (ORR), lithium-ion batteries (LIBs), sodium-ion batteries (SIBs) and supercapacitors [97, 98]. As reported in ORR catalysis, the lone pair electrons in the 3p orbital of phosphorus can result in a correlated local concentrated charge density, and the empty 3d orbital can supply with accommodation for the lone pair electrons from the p orbital of oxygen molecule, thus promoting the activity of phosphide catalysts [99], besides, the low cost, high conductivity and special electronic structures boost the application of iron phosphides in catalysis and energy storage systems.

Until recently, iron phosphides have demonstrated their superiorities in Li-S batteries [100-104]. Chen et al. [105] manufactured a phosphorized rGO/CNTs hybrid aerogel with embedded FeP nanocubes (p-HA/FeP) through the instant freezing and phosphorization of Fe-MOF precursors as shown in Fig. 8a(i). With the strong affinity of FeP NPs towards polysulfide immobilization, and the sufficient interfaces for charge transformation offered by the conductive and microporous carbon scaffold, as well as the potential catalytic effects of FeP NPs for accelerating polysulfide redox reaction, the corresponding p-HA/FeP/S cathode delivered a high specific capacity of 1312.3 mAh/g at 0.2 C and maintained a lifespan of over 500 cycle at 1 C with only 0.037% capacity decay rate per cycle (Figs. 8a(ii) and (iii)). Li et al. [106] reported a facile approach to synthesize hierarchically structured composite of Fe₂P@nitrogen, phosphorus co-doped carbon (Fe₂P@NPC). Herein, Fe₂P nanoparticles were produced via the direct biological recycling of iron metal from electroplating sludge using bacteria, resulting in the uniform distribution of nanosized Fe₂P. According to DFT calculations, Fe₂P can not only suppress the polysulfide shuttling effect through strong chemical adhesion ability, but also facilitate the decomposition of insoluble Li₂S during the delithiation process. Meanwhile, with the merits of the multiple channels for ions transportation and adequate space for volume expansion, the Fe₂P@NPC/S cathode exhibited an obviously improved initial specific capacity of 1555.7 mAh/g at 0.1 C and prolonged cycling for 500 cycles with a reversible specific capacity of 761.9 mAh/g at 1 C.

Considering that the d orbital electron layer of the metal cation in transitional metal sulfides is more easily to obtain or lose electrons, which facilitates their utilizations in electrochemical catalysis, iron sulfides have been expected as cost-efficient and efficient alternatives for noble Pt-based catalysts. Additionally, iron sulfides usually possess high conductivity and appropriate chemical bond intensity for catalyzing conversion reactions [68, 107]. However, the irresistible volume change during cycling and difficult synthesis methods hinders the practical applications of iron sulfides. Hence, in the field of Li-S battery, an increasing number of efforts have been devoted to explore and develop iron sulfides with different morphologies and hybridized matrices to overcome these obstacles [108-110].

After systematical investigation of the immobilization effects and mechanisms of transitional metal sulfides (Fe, Co, Ni, Cu, Zn, etc.) by Chen et al. [36], diverse iron sulfides have been dedicated to sulfur cathodes. For instance, Zeng et al. [111] fabricated sulfur cathode composites with FeS₂ nanoparticles encapsulated in hollow carbon shells through a facile way (Fig. 8b(i)). In addition to the enhanced adsorption capability towards polysulfides arising from FeS₂ which was confirmed by FTIR, Raman, and XPS analysis, more importantly, FeS₂ also demonstrated remarkable electrocatalytic effects in Li-S battery. In the CV measurements of FeS₂@C-S and MC/S cathodes, the cathodic and anodic peaks of FeS₂@C-S cathode were sharper than those of MC/S cathodes (mesoporous carbon/S), as well as the higher current densities, suggesting the improved redox reaction kinetics attributed to FeS₂. The FeS₂@C-S electrode exhibited a reversible specific discharge capacity of 800 mAh/g after cycling for 200 rounds at 0.2 C, while for the MC/S electrode, the specific discharge capacity faded to 200 mAh/g after only 50 cycles at 0.2 C. Even at a current density of 5 C, the FeS₂@C-S electrode still delivered a reversible discharge specific capacity of ~400 mAh/g after cycling for 400 times as shown in Fig. 8b(ii). Similarly, Li et al. [112] reported a rationally designed nanoarchitecture of N-doped carbon-coated iron sulfide (abbreviated as FeS/N-C) as sulfur hosting material fabricated through a simple pyrolysis method. With the virtues of abundant sulfiphilic sites for immobilizing polysulfides and catalyzing polysulfide conversion, the FeS/N-C based sulfur cathode showed a significant cycling stability promotion, with a stable discharge specific capacity of ~729 mAh/g retained after 500 cycles at 0.5 C.

In addition to the simple inorganic compounds, there have been increasing attention in other binary inorganic or organic iron-based compounds or heterostructures for ameliorating the electrochemical performance of Li-S battery, such as iron-based MOFs [113-115], multi-metallic alloy [116-119] and other iron-based compounds or heterostructures [120].

Taken the marvelous porosity and crystallinity of pristine MOFs in account, most studies in the past decades focused on the modification of MOFs derivatives due to the poor conductivity of MOFs, however, the application of pristine MOFs as cathode materials in Li-S battery attracts researchers' attention until currently. For example, Benítez group [113] synthesized MIL-88A based on iron fumarate, which possessed a central prismatic portion and pyramidal terminal portions, resulting in a dual micro-mesoporous MOF system. The relative MIL-88A@S composite cathode displayed an initial discharge specific capacity of around 400 mAh/g at 0.1 C. Meanwhile, iron-based multi-metallic alloys exhibited great potential as catalysts for Li-S battery since their simpler and more suitable synthesis methods for practical utilizations. According to Chen et al.s' report [116], during the thermal treatment at a high temperature, Fe³⁺ and Ni²⁺ can be transformed into FeNi₃ as well as catalyze the graphitization of carbon materials, thus upgrading the electronic conductivity of carbon-based sulfur hosting materials. Confirmed by DFT calculations and experimental results, the bimetallic alloy FeNi₃ played a vital role in impeding polysulfide shutting effect through its chemical adsorption ability and catalyzing polysulfide conversion reaction.

Recently, in the case of metal catalysts, transition-metal-based alloys or intermetals have gained promising applications in catalyzing gas involved electrochemistry due to their high activity and stability arising from their good electronic and chemical properties. Considering the similar chemical property of oxygen and sulfur, researchers explored the functional mechanism of intermetallic catalysts in Li-S electrochemistry. For example, Manthiram group [117] reported a cost effective hexagonal close-packed (hcp)-phase Fe-Ni alloy serve as an efficient electrocatalyst to promote the LiPS conversion reaction in Li-S batteries. The electrocatalysis mechanisms of Fe-Ni toward LiPS conversion is considered that both the pristine nanosized Fe-Ni phase and the thin layer coated on the Fe-Ni alloy consists of various sulfurized phases provide high catalytic activity, thus the Fe-Ni alloy delivers a long lifespan over 800 cycles and high areal capacity of 6.1 mAh/cm² under lean electrolyte conditions with high sulfur loading of 6.4 mg/cm². Yang et al. [118] reported Ni₃Fe as electrocatalyst to enhance efficient polysulfide-involving surface reactions. They claimed the incorporation of iron into nickel phase induce strong electronic interaction and lattice distortion and promotes the redox kinetics of the multiphase conversion in Li-S electrochemistry. The 70 wt% sulfur cathodes with Ni₃Fe-modified separator delivers initial capacities of 1310.3 mAh/g at 0.1 C and 598 mAh/g at 4 C as well as stable long cycle life of 1000 cycles with low-capacity fading rate of ~0.034% per cycle.

Considering the benefits of iron-based compounds, our group has explored the promotion effects of iron nitrides [121] towards Li-S battery system. The yolk-shelled Fe₂N@C nanoboxes were designed and fabricated through hard-templating method using Fe₂O₃ as precursor and etching core for the conversion into Fe₂N (Fig. 9a). The as-prepared yolk-shelled Fe₂N@C nanoboxes can provide strong chemical affinities, efficacious catalytic effects and electronic conductivity arising from polar Fe₂N cores, and sufficient loading space for sulfur due to the large internal void. With the support of Fe₂N cores, the binding energy of LiPSs on the surface of the as-proposed cathode composites were further confirmed to be greatly advanced through DFT calculations as shown in Fig. 9b. And the advancement can be attributed to the chemical Fe-S and Li-N binding configurations. In the relative rate performance as shown in Fig. 9c, the S/Fe₂N@C NBs and S/Fe₃O₄@C NBs electrodes delivered similar electrochemical performance from 0.1 C to 2 C, however, when the current density raised to 3 C, the S/Fe₂N@C NBs electrode still exhibited a specific capacity of 778 mAh/g while the S/Fe₃O₄@C NBs electrode reached only 161 mAh/g, demonstrating the superior rate capability of S/Fe₂N@C NBs electrode under higher current densities, which can be ascribed to the faster charge transfer dynamics and accelerated redox kinetics prompted by Fe₂N dominantly.

Benefiting from the considerable electrical conductivity, vigorous catalytic effect and cost effectiveness, iron-based chalcogenide compounds including iron-sulfides, iron-selenides have been widely employed in Li-S battery and displayed extraordinary electrochemical performance. Notably, iron-based selenides have displayed their advantages in ORR [122], which enlightens their potential utilizations for high-performance Li-S batteries since the similar redox mechanism between oxygen and sulfur electrochemistry. In this work, our group encapsulated FeSe₂ nanoparticles inside the hollow carbon nanocubes, promoting the electrochemical performance of Li-S cell through strong polysulfide immobilization and catalytic effect [123]. The binding energy of Li₂S_n (n = 4, 6, 8) on the FeSe₂(111) surface were simulated through DFT calculations as shown in Fig. 9d, which ranged from 4.27 eV to 9.03 eV, much higher than those on Fe₃O₄(311) surface. Furthermore, in the long-term cycling stability test, the S/FeSe₂@C electrode still retained 684 mAh/g discharge specific capacity at 1 C after 700 cycles, with an ultralow decay rate of 0.04% per cycle. In order to investigate the nature of the remarkable enhancement of electrochemical performance triggered by FeSe₂ NBs, DOS analysis of pristine FeSe₂ and Fe₃O₄ were performed in comparison as shown in Fig. 9e. Combined with the corresponding calculated energy gap of FeSe₂ and Fe₃O₄ after absorbed by S₈, Li₂S₄, Li₂S₆, and Li₂S₈, respectively (Fig. 9f), the patterns revealed that the energy gap values of original FeSe₂ and FeSe₂ after adsorbed polysulfides were all smaller than those of Fe₃O₄, which enable Se ions more easily to interact with polysulfides.

To achieve multiple functions of individual component, heterostructured materials attracted increasing attention due to their extraordinary strengths towards effective adsorption and catalytic conversion of LiPSs. The rational design and construction of multiple solid-state materials into heterostructures which take advantageous synergistic effects provide a common strategy to address the longstanding intrinsic issues of Li-S batteries. Lee et al. [124] fabricated conductive Co_5.47N/Fe₃N heterostructures wrapped with 3D nitrogen-doped CNTs and graphene framework (3D Co_5.47N/Fe₃N@N-CNT-G) to improve the electron and lithium-ion transport. The synergistic effect of high electrocatalytic activity of polar Co_5.47N/Fe₃N heterostructure can provide strong LiPS adsorption, accelerate the LiPS conversion reaction, control the kinetic behaviors of dissolved LiPS, and promote the Li₂S nucleation. The sulfur loaded 3D Co_5.47N/Fe₃N@N-CNT-G/S cathode delivered a capacity of ~1293 mAh/g at 0.1 C and a low degradation capacity of 0.019% per cycle for 500 cycles at 1 C rate.