Axial fluorine coordination boosts the activity and durability of Fe single-atom catalysts in room-temperature Na–S batteries

Abstract

Single-atom catalysts (SACs) hold promise for addressing challenges of polysulfide shuttle and sluggish sulfur reduction reaction (SRR) in room-temperature (RT) Na–S batteries. However, their structural durability under harsh electrochemical conditions remains a critical concern. Herein, we propose an effective strategy to optimize and stabilize the active sites of Fe single atoms (Fe_SACS) by modulating the local geometries through axial fluorine (F) coordination, thus significantly alleviating the stability problems faced by conventional high-performance but deactivation-prone Fe–N–C catalysts. Density functional theory (DFT) calculations and experimental results confirm that the enhanced Fe–F interactions in the second shell layer play a key role in maintaining the structural integrity of the single atoms during synthesis and operation and effectively inhibit the agglomeration behavior of Fe atoms. The F axial coordination with the optimized electronic structure enhanced the d–p hybridization between the Fe 3d orbitals and the sulfur intermediates, which significantly promoted the SRR kinetics and catalytic durability. Through comprehensive spectroscopic investigations, we further elucidate that the sulfur species undergo quasi-solid–solid conversion pathways on Fe_SACS-FCNT@S electrodes, effectively suppressing polysulfide dissolution. This work establishes a universal paradigm for designing durable SAC systems through rational coordination engineering while providing fundamental insights into structure–stability relationships for advanced metal–sulfur batteries.

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Introduction

Lithium-ion batteries (LIBs) have long been a core energy storage technology underpinning modern sustainable energy systems, but further large-scale application of LIBs is increasingly limited due to the high cost of lithium resources and limited scope for performance improvement.^1–3 In this technological context, the RT Na–S battery system based on multi-electron transfer chemistry has become one of the most competitive candidates for large-scale grid energy storage due to its high theoretical specific capacity (1675 mA h g⁻¹), abundant resources and low costs of Na and S.^4,5 However, the high solubility of sodium polysulfides (NaPSs) during operation and the resulting shuttle effect, as well as severe slow reaction kinetics of the sulfur cathode, can trigger severe capacity degradation and limit cycle life. Addressing these challenges requires innovative strategies in catalyst design to regulate the complex sulfur phase transition and achieve the efficient kinetics required.^6–9

d-Block single-atom catalysts, characterized by atomically dispersed metal sites that maximize active interface exposure, have emerged as a transformative paradigm for regulating NaPS conversion kinetics.^10–12 The well-defined site structures and the coordination environments of isolated metal sites are easier to regulate than those of metal nanoparticles and nanoclusters, which provide an atomic-scale operating space for precise modulation of their catalytic activities.^13–15 Among various coordination architectures, planar tetracoordinated M–N₄ moieties have been identified as effective catalytic centers for NaPS transformation.^16,17 Nevertheless, the inherent symmetry of these configurations induces anisotropic adsorption behavior toward intermediates, fundamentally constraining the optimization of the geometric and electronic structures of active sites.^18–21 In stark contrast, second-shell axial engineering—achieved through heteroatom incorporation (O/S/P/Cl)—leverages long-range d–p orbital coupling to induce directional charge transfer.^22–25 The resultant electron density gradient not only amplifies localized polarized electric fields but also strategically modulates d-band center positions, thereby synergistically enhancing sulfur species adsorption and subsequent transformation dynamics.^16,26–28

While substantial progress has been made in deciphering the structure–activity relationships of SACS for RT Na–S batteries, the critical challenge of sustaining atomic dispersion under operational conditions remains underexplored—a prerequisite for practical implementation.^29–31 It is well-established that the irreversible leaching of SACs into clusters/nanoparticles during prolonged cycling fundamentally compromises their durability.^32,33 Current research predominantly focuses on optimizing intrinsic activity through coordination engineering, yet crucially overlooks the dynamic stability of metal centers during continuous sulfur redox events. Extended reaction durations inevitably exacerbate atomic aggregation, accelerating catalyst deactivation. This underscores the imperative to engineer^34,35 both the thermodynamic stability of isolated sites and kinetic resistance to reconstruction—a dual stabilization paradigm intrinsically linked to the macroscopic performance of SACs.

Herein, we present a fluorine-mediated axial coordination strategy to precisely regulate the spatial configuration and local electronic structure of Fe single-atom sites (FeN₄) anchored on a carbon matrix. Systematic DFT calculations coupled with experimental validation reveal that fluorine coordination induces the 3d orbital reconfiguration of Fe centers and strengthens d–p hybridization with sulfur species, thereby significantly enhancing the intrinsic SRR kinetics. Remarkably, the optimized Fe_SACS-FCNT@S demonstrates exceptional structural robustness, retaining atomic dispersion even after 150 cycles at 0.2 A g⁻¹ while delivering a high capacity retention of 589.5 mA h g⁻¹. Combining various characterization studies such as operando sulfur K-edge XAS, TOF-SIMS and ex situ XPS, we reveal that Fe_SACS-FCNT@S is capable of inducing a quasi-solid sulfur conversion pathway, effectively inhibiting the generation and diffusion of soluble NaPSs. This work provides mechanistic insights into the design of atomically dispersed catalysts for RT Na–S battery systems and emphasizes the critical role of axial ligand engineering in achieving efficient sulfur conversion and long-term stability.

Results and discussion

In order to elucidate the nature of the kinetic behavior of the FeN₄–F active site-catalyzed SRR and its interaction mechanism with the reaction intermediates, DFT simulations were carried out from the intrinsic properties of the electronic structure of the 3d orbitals of Fe atoms. First, the structural models of FeN₄–F and FeN₄ were constructed and optimized, and the adsorption behaviors of the sulfur intermediates were investigated on the basis of the optimized substrate configurations. In RT Na–S batteries, d–p orbital hybridization is widely recognized as a critical descriptor of catalytic activity.^36,37 Projected density of states (PDOS) analysis revealed distinct orbital features before and after F coordination (Fig. 1a).³⁸

Fig. 1 Electronic structural analysis and orbital interactions from DFT calculations. (a) The PDOS of FeN₄ and FeN₄–F models. (b) Schematic diagram of the modulation of the d–p orbital hybridization. (c) –pCOHP analysis of the Fe–S bond after Na₂S₂ adsorption. (d) Electronic structure of Na₂S₂ after adsorption of FeN₄(up) and FeN₄–F(down). (e) Electron density differences of Na₂S₂ on FeN₄ and FeN₄–F. (f) Free energy diagrams of the sulfur reduction reaction on the surface of FeN₄ and FeN₄–F (the insets show the corresponding adsorption configurations).

For the FeN₄ site, the d_x²−y² orbitals hybridized with four N ligands formed deeply occupied states and high-energy empty states, while the d_z² orbital exhibited two localized bands. Upon F coordination, the d_z² bands became delocalized due to strong Fe–F electronic coupling, with empty states near the Fermi level shifting to higher energies. Additionally, the degenerate d_yz/d_xz orbitals showed elevated energy levels under axial F perturbation. Collectively, axial F coordination induced an upward shift of the Fe 3d orbitals, resulting in a higher d-band center. Such electronic restructuring enhances the electron density within the d-band of the FeN₄–F model, promoting orbital overlap with sulfur intermediates and optimizing the adsorption–desorption equilibrium of reactive species, thereby accelerating SRR kinetics (Fig. 1b). To further elucidate the orbital hybridization mechanism between FeN₄ sites and sulfur species, we performed crystal orbital Hamiltonian population (–pCOHP) and PDOS analyses (Fig. 1c and S1) to quantify the modulation of Fe 3d–S 3p orbital coupling by F coordination.³⁹ The results demonstrate that F doping effectively modulates the coordination environment of FeN₄. This electronic restructuring is critical for catalyzing S–S bond cleavage. The elevated Fe 3d orbital energy levels enhance the coupling with the antibonding orbitals of polysulfides, thereby populating and weakening the S–S bonds.^40,41 Simultaneously, the strengthened d–p hybridization facilitates electron transfer from the Fe site to the adsorbed intermediates, dynamically promoting S–S bond rupture during reduction.⁴² These synergistic effects significantly enhance the material's capacity to adsorb and convert polysulfide intermediates. Furthermore, by comparing the –pCOHP curves of FeN₄ with those of FeN₄–F systems, it was found that the σ* and σ bonding/anti-bonding orbitals exhibited a more pronounced energy level splitting between them upon the introduction of the F axial ligand. Notably, the σ* orbitals below the Fermi energy level are almost unoccupied in the FeN₄–F system, resulting in a more negatively skewed overall –pCOHP value, suggesting that stronger and more stable metal–sulfur valence bonds are formed in this configuration. Fig. 1d depicts the geometrical configurations of Na₂S₂ molecules adsorbed on the FeN₄ and FeN₄–F models, respectively. It is noteworthy that the Fe–S bond lengths of the two models are only slightly different. In such cases, the nature of the orbital interaction becomes a more decisive factor than the absolute bond length in determining binding strength. –pCOHP analysis reveals that the FeN₄–F model displays significantly stronger metal–ligand orbital interactions, which contribute to enhanced structural stability. This finding highlights the pivotal role of axial F coordination in modulating the electronic coupling between the Fe center and sulfur species. In addition, charge density difference analysis further revealed the charge transfer behavior during the adsorption process (Fig. 1e): in the FeN₄–F system, the electron density of the Fe site was significantly reduced, while a significant electron accumulation was observed around the neighboring S atoms, suggesting that an effective charge transfer from the Na₂S₂ to the Fe site occurred. This charge rearrangement reinforces the stability of the adsorbed configuration. We further analyzed the Gibbs free energy changes of each intermediate state in the reduction pathway of S₈ to Na₂S₂. As shown in Fig. 1f, the conversion of Na₂S₄ to Na₂S₂ is accompanied by a significant energy barrier, suggesting that this step is the rate-determining step for the complete SRR. Notably, the energy barrier for this step in the FeN₄–F system is 1.39 eV, which is significantly lower than that of 1.72 eV in the FeN₄ model, suggesting that the F-ligand modulation effectively reduces the reaction energy barrier and provides a more favorable thermodynamic pathway to facilitate the final reduction of sulfur species. The above results demonstrate the positive modulation effect of F axial coordination engineering on catalyzing the conversion of sulfur species.

Guided by DFT computational insights, the axial F-coordinated FeN₄ active sites demonstrate enhanced catalytic potential for accelerating SRR kinetics. To experimentally validate this design, we developed a fluorine-doped carbon tube (FCNT) architecture through a templated synthesis strategy. As illustrated in Fig. 2a, MnO₂ nanorods served as sacrificial templates for the co-condensation of phenolic resin with fluorine-doped carbon dots (FCDs),⁴³ followed by pyrolysis at 700 °C under an argon atmosphere to yield FCNTs. A control sample (CNT) was synthesized under identical conditions without FCD addition to elucidate the role of fluorine modulation. Transmission electron microscopy (TEM) images show that both are tube-like structures (Fig. S2 and S3) and energy dispersive X-ray (EDS) elemental mapping analyses show the corresponding F signals (Fig. S3), which prove the successful doping of F. Subsequently, the FeCl₃–phenanthroline complex was anchored onto the CNT or FCNT substrate via a facile wet impregnation method, followed by thermal treatment to remove surface ligands, yielding the Fe_SACS-CNT and Fe_SACS-FCNT catalysts. TEM imaging combined with EDS elemental mapping confirmed the structural integrity of the carbon tubes after metal incorporation, with detectable Fe signals but no observable metallic nanoparticles (Fig. S4 and S5).

Fig. 2 (a) Schematic illustration of the synthetic procedure of Fe_SACS-FCNT@S. (b) XRD patterns of Fe_SACS-CNT and Fe_SACS-FCNT. HAADF-STEM images of (c and d) Fe_SACS-FCNT and (e) Fe_SACS-CNT. STEM image (f) and (g–k) elemental mapping images of Fe_SACS-FCNT@S.

X-ray diffraction (XRD) patterns further revealed the absence of crystalline Fe phases in both Fe_SACS-FCNT and Fe_SACS-CNT catalysts, suggesting the possible presence of atomic dispersion of Fe species (Fig. 2b). However, in comparison to the two blank carbon-based controls without Fe incorporation (CNT and FCNT), a slight difference was observed, which manifested specifically as a narrowing of the diffraction peak around 20° (Fig. S6) indicative of enhanced graphitization degree of the carbon material following the secondary calcination process.⁴⁴ Atomic-resolution aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) imaging provided direct evidence of the highly dispersed Fe species in Fe_SACS-FCNT, where numerous isolated bright spots—assigned to individual Fe atoms—are uniformly distributed across the carbon matrix (Fig. 2c and d) (encircled by white dashed circles). In contrast, the reference Fe_SACS-CNT sample not only exhibited discrete single-atom-like features but also showed Fe clusters (encircled by blue dashed circles), indicating partial aggregation (Fig. 2e). These observations suggest that F axial modification strengthens the covalent interaction between the metal site and the support, thereby stabilizing the isolated Fe centers and suppressing cluster formation. This highlights the critical role of F doping in enhancing the structural stability of Fe_SACS during synthesis. Inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis further confirmed nearly identical Fe loadings in both Fe_SACS-FCNT and Fe_SACS-CNT (1.15 wt% and 1.13 wt%, Table S1), ruling out any loading-dependent differences in catalytic performance. The as-synthesized catalysts and the two control carbon tube samples were further loaded with sulfur to fabricate a cathode for RT Na–S batteries. TEM images reveal that all four sulfur-loaded samples still retained the carbon tube shape (Fig. S7–S10). Sulfur is uniformly distributed both on the surface and within the interior of the tubes, which can be attributed to the hollow structure that facilitates efficient S₈ encapsulation. This structural feature also helps accommodate volumetric expansion during cycling, thereby mitigating structural degradation.

The corresponding EDS (Fig. 2f–k and S7–S10) results confirm the presence of sulfur signals, and XRD patterns (Fig. S11) exhibit characteristic diffraction peaks of orthorhombic sulfur,^11,45 further confirming successful sulfur incorporation. BET surface area and pore size distribution analyses show that Fe_SACS-FCNT and Fe_SACS-CNT possess high surface areas and well-defined mesoporous structures before sulfur loading. However, after sulfur infiltration, a significant decrease in surface area and near disappearance of mesopores are observed (Fig. S12), indicating effective penetration of sulfur into the porous framework—further evidence that S₈ has been successfully confined within the carbon tubes.^46,47 Moreover, Raman spectroscopy reveals comparable D-band and G-band intensities among all four sulfur-loaded samples (Fig. S13),^48,49 suggesting similar graphitization degrees and indicating that the introduction of single-atom Fe has negligible impact on the structural integrity of the carbon matrix. The surface chemical composition and elemental states of the as-prepared samples were investigated by X-ray photoelectron spectroscopy (XPS). The corresponding N 1s spectra (Fig. S14) exhibit three prominent peaks at 398.38, 400.08, and 401.68 eV, which are attributed to pyridinic N, pyrrolic N, and graphitic N, respectively.^50,51 In the F 1s spectra of both FCNT and Fe_SACS-FCNT samples, a peak corresponding to C–F bonds is clearly observed. Notably, an additional peak assigned to Fe–F bonds is identified in the Fe_SACS-FCNT sample,^43,52 providing direct evidence for the effective coordination between F atoms and the central Fe active sites (Fig. S15). The electronic structure evolution of Fe centers in Fe_SACS-FCNT and Fe_SACS-CNT was systematically investigated through X-ray absorption near-edge structure (XANES) spectroscopy. As illustrated in Fig. 3a, the normalized absorption edges of Fe_SACS-FCNT and Fe_SACS-CNT are positioned between metallic Fe foil and Fe₂O₃, indicative of partially Fe species with intermediate valence states (0 to +3). Notably, a distinct positive shift in the absorption edge energy is observed for Fe_SACS-FCNT relative to Fe_SACS-CNT, evidencing the elevated oxidation state of Fe induced by the axial Fe–F coordination.^53,54 Fig. 3b illustrates the results of the Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) analysis of Fe_SACS-FCNT, Fe_SACS-CNT and their selected standard samples. In both materials, the main peak appears at ∼1.5 Å, which is attributed to the Fe–N scattering path. No Fe–Fe bonding peak is observed at ∼2.2 Å,⁵⁵ demonstrating that the Fe species exist in an atomically dispersed form (Fig. 3c).

To further verify the presence of Fe–F axial coordination, FT-EXAFS curves were fitted for both sets of samples. The best fit for Fe_SACS-FCNT showed the coexistence of Fe–N and Fe–F bonds, with the calculated actual N and F coordination numbers of 4.48 and 0.68 (Fig. 3d, e and Table S2), respectively, suggesting that the coordination configuration consists of ^56,57 four in-plane nitrogen atoms and one F atom in the axial direction. In contrast, the control sample Fe_SACS-CNT contains only isolated FeN₄ sites (Fig. 3f and Table S3).

Fig. 3 (a) XANES spectra and (b) corresponding FT-EXAFS results in R space of Fe_SACS-FCNT, Fe_SACS-CNT, Fe foil, and Fe₂O₃. (c) Two forms of alien atom-doped single-atom structures. (d) EXAFS oscillations of Fe_SACS-FCNT and Fe_SACS-CNT. Fitting results of (e) Fe_SACS-FCNT and (f) Fe_SACS-CNT in R space. (g) WT analysis of Fe_SACS-CNT, Fe_SACS-FCNT, Fe foil, and Fe₂O₃.

To resolve the subtle differences in bonding configurations, wavelet transform (WT) analysis was employed for its superior resolution in both k-space and R-space (Fig. 3g).^58,59 The WT profiles of both Fe_SACS-FCNT and Fe_SACS-CNT at about 5.0 Å⁻¹ (k-space) and 1.5 Å (R-space) show one of the strongest peaks, which is attributed to the Fe–N and Fe–N/F coordination. Meanwhile, the peak of Fe_SACS-FCNT shifts to higher k-space and R-space, which is attributed to the stronger backscattering effect of the F ligand. This phenomenon provides direct spectral evidence for the successful construction of Fe–F axial bonds, which synergistically optimize the charge transfer kinetics and electronic polarization of Fe centers. Such precisely engineered coordination environments establish a fundamental structure–activity relationship for sulfur redox electrocatalysis in RT Na–S batteries. To systematically evaluate the electrochemical performance of Fe_SACS-FCNT-based and Fe_SACS-CNT-based sulfur cathodes, coin cells were assembled and tested within a voltage window of 0.8–2.8 V. The cyclic voltammetry (CV) test at 0.1 mV s⁻¹ was employed to investigate the sulfur redox behavior. As shown in Fig. 4a, two distinct cathodic peaks during the discharge process correspond to the stepwise reduction pathway of S₈ → soluble polysulfides (Na₂S_x, 4 ≤ x ≤ 8) → solid Na₂S₂/Na₂S, while the single anodic peak during charge arises from the reverse oxidation of Na₂S to polysulfides.^60,61 Remarkably, the Fe_SACS-FCNT@S electrode exhibits enhanced cathodic peak current and reduced polarization voltage, suggesting faster redox reaction kinetics.

Fig. 4 (a) CV curves of the Fe_SACS-FCNT@S and Fe_SACS-CNT@S cathodes measured at 0.1 mV s⁻¹. (b) Rate capability at 0.2, 0.5, 1.0, 2.0, and 5.0 A g⁻¹, respectively. (c) Cycling performance at 0.2 A g⁻¹. (d and e) XANES spectra and corresponding FT-EXAFS results in R space of Fe_SACS-FCNT@S, Fe_SACS-CNT@S, Fe foil, and Fe₂O₃ after 150 cycles at 0.2 A g⁻¹. (f and g) HAADF-STEM images of Fe_SACS-FCNT@S and (h and i) Fe_SACS-CNT@S after 150 cycles at 0.2 A g⁻¹. (j) Separation energies of FeN₄ sites in Fe_SACS-FCNT and Fe_SACS-CNT.

Fig. 4b shows the rate performances of the electrodes at different current densities from 0.2 A g⁻¹ to 5.0 A g⁻¹. Compared to the control group without Fe_SACS, both Fe_SACS-FCNT@S and Fe_SACS-CNT@S exhibit significantly enhanced initial capacity, verifying the superior performance of Fe_SACS in catalyzing the conversion of NaPSs. More notably, under the same test conditions, the cells based on the Fe_SACS-FCNT@S structure exhibited higher specific discharge capacity compared to other control groups. Even when reverting back to the current density of 0.2 A g⁻¹ after high-rate cycling, a reversible capacity of up to 832.60 mA h g⁻¹ is still maintained, showing excellent structural stability and cycling durability. The long-term cycling stability of Fe_SACS-FCNT@S and Fe_SACS-CNT@S was systematically evaluated (Fig. 4c and S16–S18). Remarkably, Fe_SACS-FCNT@S exhibited superior performance at both 0.2 A g⁻¹ and 1 A g⁻¹, at a current density of 0.2 A g⁻¹, and the capacity of Fe_SACS-FCNT@S is still maintained at 589.5 mA h g⁻¹ after 150 cycles, while that of Fe_SACS-CNT@S is only 409.4 mA h g⁻¹. In addition, at a high current density of 1.0 A g⁻¹, Fe_SACS-FCNT@S retained a capacity of 319.3 mA h g⁻¹ after 1600 cycles, which is much higher than that of the control group (115.3 mA h g⁻¹). These results demonstrate that the FeN₄ sites with axial F-coordination can enhance the kinetics of the SRR. Subsequently, the GITT-derived Na⁺ diffusion coefficient (D_Na⁺) tests were carried out to investigate the reaction kinetics of Fe_SACS-FCNT@S and Fe_SACS-CNT@S. It is noteworthy that D_Na⁺ confirmed (Fig. S19) that Fe_SACS-FCNT@S achieved a D_Na⁺ value higher than that of Fe_SACS-CNT@S, confirming faster Na⁺ diffusion kinetics, which contributes to the improvement of the overall electrochemical performance. Furthermore, in situ electrochemical impedance spectroscopy (EIS) was employed to reveal the kinetic nature of Fe_SACS-FCNT@S and Fe_SACS-CNT@S during the SRR, and the in situ characterization was performed in the first discharge/charge cycle. Typically, the EIS curve consists of three components: the electrolyte internal resistance (R_o), the interface resistance (R_s) and the Warburg impedance (R_o).⁶² As shown in Fig. S20, at the beginning of the discharge process, the R_o of both groups of samples slightly increased, which may be related to the change of electrolyte viscosity due to the dissolution of a small amount of NaPSs. With the increase of discharge depth, R_o decreased due to the gradual conversion of soluble NaPSs into insoluble Na₂S. During the charging process, the above changes are basically reversible. Notably, Fe_SACS-FCNT@S consistently exhibited smaller R_o and R_s throughout the charging and discharging cycles, suggesting that its electrodes have faster charge transfer rate and more efficient electron transfer capability. Combined with the aforementioned characterization results, it further confirms that Fe_SACS-FCNT@S possesses excellent kinetic performance in the sulfur conversion process. In addition, the ex situ EIS was conducted on the four sample groups (CNT@S, FCNT@S, Fe_SACS-CNT@S, and Fe_SACS-FCNT@S). As shown in Fig. S21, the results clearly demonstrate that Fe_SACS-CNT@S and Fe_SACS-FCNT@S electrodes exhibit superior electronic conductivity and enhanced charge transfer kinetics, attributable to the presence of Fe-based single-atom catalysts. The effect of F-axial coordination on the structural properties of the material itself has been confirmed earlier by a series of characterization studies. However, under operating conditions, metal atoms may detach from the carbon support due to chemical reactions during the catalysis process and form severe aggregates, leading to a loss of catalytic activity. Therefore, we further analyze the effect of F-axial coordination on the stabilization of single-atom systems. First, we performed the XANES test on two sets of samples after 150 cycles to check the morphological evolution of Fe_SACS. As shown in Fig. 4d, the valence of Fe in Fe_SACS-FCNT@S remained between 0 and +3, indicating the existence of mainly atomic Fe species with good structural stability. In contrast, the valence of Fe in Fe_SACS-CNT@S tends to approach the 0-valence of Fe metal particles. Furthermore, FT-EXAFS analyses and WT transform (Fig. 4e and S22) analysis reveal differences in the coordination environments of the two samples after cycling: the first shell coordination of Fe in Fe_SACS-CNT@S is cleaved into two peaks and a peak belonging to the Fe–Fe bond is observed at ∼2.2 Å,⁴⁷ indicating degradation of the Fe central structure and irreversible metal leaching. In contrast, Fe in Fe_SACS-FCNT@S maintained similar coordination interactions to the initial state throughout the cycling process, with only minor changes observed in the oxidation state and coordination environment of the FeN₄ center and thus was able to significantly retain its catalytic activity throughout the cycling process, which is also consistent with its excellent electrochemical performance. Post-cycle TEM and HAADF-STEM characterization of the two electrodes further corroborated the above findings. As shown in the TEM images (Fig. S23), no Fe nanoparticles were discerned in Fe_SACS-FCNT@S, whereas they were clearly observable in Fe_SACS-CNT@S. HAADF-STEM analysis provided further visual evidence as displayed in Fig. 4f and g, and the majority of Fe single atoms in Fe_SACS-FCNT@S remained atomically dispersed without significant agglomeration. In contrast, pronounced aggregation of Fe species occurred in Fe_SACS-CNT@S, leading to the formation of large metal clusters and particles (Fig. 4h and i).

In order to further understand the potential demetallization trend of FeN₄ active sites in RT Na–S cells, we performed DFT calculations. We evaluated the energy required to detach the metal atoms from the carbon matrix, as shown in Fig. 4j. In the Fe_SACS-FCNT structure, Fe atoms require a higher detachment energy to escape from the carbon matrix, suggesting that F doping significantly enhances the anchoring stability of the Fe single atom centers. In addition, we calculated the –pCOHP to evaluate the bonding strength between the metal atoms and their coordination ligands. The –pCOHP values for the Fe–N and Fe–F bonds are −2.54 eV and −2.80 eV, respectively (Fig. S24), which suggests that the additional axial coordination of Fe–F in the Fe_SACS-FCNT system has stronger bonding interactions than pure FeN₄ planar coordination bonding, and thus Fe single-atom leaching during synthesis and cycling is less likely. These calculations are in good agreement with our experimental observations and provide strong support for the role of F axial coordination in enhancing the stability of FeN₄ active sites. The dissolution of NaPSs is widely recognized as a critical factor leading to the capacity degradation of RT Na–S batteries.^7,63 In order to elucidate the mechanism by which FeN₄-catalysts promote the multistep conversion of NaPSs, we assessed the dissolution behavior of S species by qualitative measurements. We conducted spectroscopic studies to assess the reactivity between Na₂S₆ and various substrates (CNT, FCNT, Fe_SACS-CNT and Fe_SACS-FCNT). As shown in the inset of Fig. 5a, after immersing four different substrates into Na₂S₆ solution for 4 h, the two groups of Fe_SACS-loaded samples faded away from the yellow color to a much greater extent compared to the two groups of pure carbon tube samples, and the solution was almost completely transparent, which indicated their strong adsorption capacity for Na₂S₆, which was further confirmed from UV-Vis absorption spectra. As shown in Fig. 5b and S25, we calculated the adsorption energies of the two Fe-based catalysts for different intermediates to assess their ability to stabilize sulphur species.¹⁷ Although Fe_SACS-CNT exhibits stronger adsorption towards NaPSs, this strong binding hinders S–S bond cleavage and product desorption, thereby slowing down the subsequent reduction steps^64,65—particularly during the Na₂S₄ → Na₂S₂ transformation, which is the rate-determining step of the overall reaction. In contrast, Fe_SACS-FCNT shows a moderate interaction with intermediates, enabling faster sulfur conversion and highlighting the crucial balance between atomic utilization and sulfur redox efficiency.

Fig. 5 (a) UV-vis absorption spectra (inset: the optical image of visualized adsorption of Na₂S₆ on CNT, FCNT, Fe_SACS-CNT and Fe_SACS-FCNT). (b) Optimized configuration of adsorbed NaPSs on the surface of Fe_SACS-CNT and Fe_SACS-FCNT. Ex situ sulfur K-edge analysis of the Fe_SACS-CNT@S (c) and Fe_SACS-FCNT@S (d) electrodes at different discharged states (original, 1.75, 1.05, and 0.8 V) and charged states (1.6, and 2.8 V). (e) TOF-SIMS 3D rendering NaS⁻ and NaS₄⁻ depth profile image of Fe_SACS-FCNT@S (up) and Fe_SACS-CNT@S (down). (f) Schematic of Fe_SACS-FCNT and Fe_SACS-CNT in a RT Na–S battery.

In order to gain a deeper understanding of the evolutionary behavior of sulfur species during the reaction process on the surface of the two catalysts, we also carried out operational sulfur K-edge XAS characterization.^66,67 As shown in Fig. 5c, for the Fe_SACS-CNT@S electrodes, only the characteristic peak of element S₈ (2472.7 eV) was observed in the initial state, and this signal can still be detected at different reaction stages, suggesting that part of the sulfur exists in the form of irreversible deposition, which fails to participate in the reaction effectively. The typical characteristic peak (2468.2 eV) corresponding to NaPSs appeared at the beginning of the discharge, which gradually weakened as the discharge continued, and the characteristic absorption of Na₂S appeared at 0.8 V, indicating that the soluble polysulfides were gradually converted to insoluble Na₂S. In the two charging states (1.6 V and 2.8 V) the conversion of Na₂S → NaPSs → S₈ through a reversible transformation process was also verified. Similarly, identical tests were conducted on Fe_SACS-FCNT@S (Fig. 5d) samples at the same voltage, revealing only the characteristic peaks of S8 and the final product Na₂S.^17,47 Upon charging back to 2.8 V, the photon energy of S₈ intensified, indicating its recovery. This observation confirms the reduction of sulfur and the gradual breaking of S–S bonds, demonstrating a highly reversible process. This reversibility was further validated by S 2p XPS test analysis (Fig. S26). These findings suggest that both electrodes undergo reversible transformations from S8 to various sulfides, reaffirming the feasibility of enhancing the SRR with Fe single atoms. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) further verified the spatial domain-limiting effect of the FeN₄-based sulfur anode on sulfur species during cycling. The depth analysis by TOF-SIMS elucidated the spatial distribution of the NaPS components. As shown in Fig. 5e, the signal intensity of NaS₄⁻ at the Fe_SACS-CNT@S electrode was significantly stronger than that of Fe_SACS-FCNT@S samples after 150 cycles at a small current density of 0.2 A g⁻¹, and furthermore, the content of NaS⁻ was more distributed in the Fe_SACS-FCNT@S electrode. This verified that the Fe_SACS-CNT@S samples presented higher NaPS aggregation concentration, both indicating that the Fe_SACS-FCNT@S system had a stronger ability to catalyze the transformation of polysulfides. In conjunction with the above tests, it was found that no long-chain intermediate sulfur species were detected in the Fe_SACS-FCNT@S system due to the relatively moderate adsorption strength of these intermediates (Fig. 5c), which are rapidly converted to the next intermediate up to Na₂S₂, and thus are not readily detected in the ex situ tests. This further highlights the increased conversion efficiency of NaPSs through F-axis coordination. This interaction transformed the redox pathway from the traditional liquid–solid–solid conversion to a more efficient quasi-solid–solid conversion, thus effectively suppressing the shuttle effect within the electrolyte.^68–70 Fig. 5f schematically illustrates the different reaction pathways for Fe_SACS-CNT@S and Fe_SACS-FCNT@S. The conventional sulfur transformation pathway can be represented as S₈ ↔ Na₂S_8/6 ↔ Na₂S₄ ↔ Na₂S₂/Na₂S. For the Fe_SACS-CNT@S cathode, the introduction of FeN₄ catalytic sites promotes the rapid conversion of polysulfides to Na₂S, forming a fast liquid–solid–solid reaction pathway. Further optimization of the sulfur formation pathway is achieved through axial F coordination, which refines the local environment around Fe single atoms, enabling a quasi-solid–solid reaction route. This leads to significantly improved suppression of polysulfide shuttling and markedly enhanced sulfur utilization efficiency.

Conclusions

In summary, a new strategy based on fluorine ligands to modulate the coordination environment of the second shell layer is proposed using FeN₄-type single-atom catalysts as a model system, aiming at solving key problems such as the slow kinetics of the SRR and the serious shuttle effect of NaPSs in RT Na–S batteries. The local electronic structure of the Fe active centre is precisely modulated by F doping, and DFT calculations show that the strategy effectively modulates the 3d orbital distribution of Fe and enhances its d–p orbital interactions with NaPSs, which significantly improves the adsorption and conversion kinetics of NaPSs. The experimental results further validate the dual advantages of this modification strategy in enhancing the intrinsic activity and structural stability in the FeN₄ single-atom system. In short, Fe_SACS-FCNT@S exhibits superior performance and cycling stability compared with the conventional Fe_SACS-CNT@S battery system. In addition, Fe_SACS-FCNT@S shows that the iron species remain atomically dispersed after 150 cycles, whereas Fe_SACS-CNT@S shows obvious metal agglomeration. In addition, we have also combined various characterization studies to systematically reveal the dynamic evolution of sulfur species during the charging and discharging process, identified two different reaction pathways, and established a conformational relationship between ‘coordination environment–electronic structure–catalytic activity’.

Author contributions

All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Supplementary information: Experimental procedures and additional characterization details, DFT calculations, SEM, TEM, XRD, BET, Raman analysis, XPS, discharge/charge profiles, GITT, EIS, WT analysis and tables. See DOI: https://doi.org/10.1039/d5sc05972k.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22379165, 52074359, U21A20284, and U22B2069) supported by the Fundamental Research Funds for the Central Universities of Central South University (2024ZZTS0389).

Notes and references

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