Synergetic insights into Nb single atoms and lithiophilic support for high-efficiency sulfur catalysis in Li–S batteries

Yiyang Mao† ^ab, Yan Zhang†^b, Mingyu Su^a, Yanbin Ning^b, Jirui Shao^b, Kai Zhu*^a and Shuaifeng Lou*^b
aKey Laboratory of Superlight Materials and Surface Technology (Ministry of Education), College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China. E-mail: kzhu@hrbeu.edu.cn
^bState Key Laboratory of Space Power-Sources, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China. E-mail: shuaifeng.lou@hit.edu.cn

First published on 22nd August 2025

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Broader context With the popularization of electric vehicles and consumer electronics, the contradiction between the growing demand for higher energy density energy storage devices and the limited energy density of Li-ion batteries is gradually emerging. As a representative of high specific energy battery systems, lithium–sulfur (Li–S) batteries have received widespread attention because of their ultra-high theoretical capacity (1675 mAh g−1) and theoretical energy density (2600 Wh kg−1). In particular, the sulfur reduction reaction (SRR) is the core of Li–S batteries, and a comprehensive understanding of the SRR contributes to catalyst design for high-performance Li–S batteries. Here, with reference to the classical theory of other electrocatalytic reactions, we propose a Li+-assisted sulfur reduction mechanism and synthesize the Nb single-atom catalyst. Experiments and DFT calculations demonstrate that *Li+-rich catalysts can accelerate LiPS conversion through changing the Li+ attack pathway. The Nb single atom not only exhibits strong Nb–S interactions, but also enhances the electronic delocalization on the support to increase the amount of *Li+, which synergistically accelerates the LiPS conversion. Our study contributes to a comprehensive understanding of the SRR and provides a new direction for the rational design of potential catalytic materials for Li–S batteries.

Introduction

Lithium–sulfur (Li–S) batteries are considered to be promising next-generation batteries because of their exceptional theoretical capacity (1672 mAh g⁻¹) and ultrahigh theoretical energy density (2600 Wh kg⁻¹).^1–4 Despite considerable efforts being devoted to enhancing the practical performance of Li–S batteries, there is still a lack of systematic understanding of the sulfur reduction reaction (SRR). As an intermediate in the reaction, lithium polysulfides (LiPSs) have received great attention for their interaction with catalysts.^5–9 The electronic structure and d-orbital levels of the catalysts are the most widely studied due to their determined roles in the chemical adsorption of LiPSs.^10–14 The d–p orbital hybridization produces a bonding orbital and an antibonding orbital, and the adsorption of LiPSs has a negative correlation with the electron filling of the antibonding orbital.^15,16 It has been demonstrated as a feasible way to analyze the SRR in terms of the interaction between catalysts and LiPSs. However, as another important reactant, the adsorption and migration of Li⁺ on the catalyst have received little attention.¹⁷ In recent years, several works have begun to focus on Li⁺ regulation to enhance the electrochemical performance of Li–S batteries, but the mechanism of Li⁺-assisted catalysis has not been clarified. A comprehensive elucidation of the SRR especially the role of Li⁺ is essential for the rational design of catalysts that can target specific steps to fundamentally solve the slow sulfur reduction kinetics.

The classical theory of other electrocatalytic reactions (like the electrochemical nitrate reduction reaction, NO₃RR) can be referred to, where the strong adsorption of NO₃⁻ and their reduction to NH₃ is analogous to the reduction of LiPSs.¹⁸ These multistep hydrogenation reactions have a crucial intermediate surface-active hydrogen¹⁹ and we conjecture that the SRR also has a crucial intermediate. Many attempts have been made to establish a connection between surface-active hydrogen and NO₃RR activity, including the Eley–Rideal (ER) mechanism and the Langmuir–Hinshelwood (LH) mechanism.²⁰ The LH mechanism proposes the adsorbed-hydrogen-mediated pathway that explains the origin of fast nitrate reduction kinetics. To further regulate the surface-active hydrogen and quantify its function in the reaction, efforts have focused on the molecular orbital, which correlates the surface-active hydrogen on the catalysts with the occupancy of antibonding orbitals.²¹ Inspired by the success of the surface-active hydrogen theory, we propose surface-active lithium (*Li⁺) as another crucial intermediate for the SRR and relate the Li⁺ diffusion barriers and the reduction actively. Concretely, *Li⁺ can directly react with the adsorbed LiPSs to avoid the slow migration of Li⁺ in the bulk electrolyte and enhance sulfur reduction kinetics. It is noteworthy that a rich *Li⁺ interface can also effectively inhibit Li dendrite formation owing to fast Li⁺ migration and replenishment.^22,23 This means that *Li⁺ not only can accelerate LiPS conversion but also can regulate Li deposition, and it is significant to study *Li⁺ for Li–S battery dual-function catalysts.

Given the presence of cation–π interactions between the cation and the electron-rich conjugated system, we considered whether promoting the cation–π interactions through fabricating an electron-rich π system as a plausible strategy to regulate *Li⁺. Herein, Nb–C₃N_4−x single-atom catalysts were synthesized which enhance the electronic delocalization through the synergy of metal–support interactions and N-defects.^24,25 Concretely, the transfer of electrons occurs from the Nb site to the support, while the construction of N-defects allows more electrons to participate in the formation of π bonds, and the five-membered ring structure enables the lone-pair electron of N to enter the π bond to form an electron-rich π system.²⁶ Experimental and density functional theory (DFT) calculations prove that π-electron delocalization in the support enhances Li⁺ adsorption. Notably, the Nb single atom also weakens the S–S bond through strong Nb–S interactions, inhibiting the shuttle effect and further enhancing the SRR kinetics (Scheme 1). Meanwhile, because of the advantages of abundant *Li⁺ and strong Nb–S interactions with LiPSs, Nb–C₃N_4−x can simultaneously optimize the anode and cathode of Li–S batteries, and the battery based on the Nb–C₃N_4−x catalyst has good cycling stability even with a low negative-to-positive capacity ratio and lean electrolyte addition. The LiPS reduction reaction involves multiple complex processes, and the discovery of *Li⁺ provides a new perspective for catalyst design, and greatly expands the catalyst selection for practical Li–S batteries.

Results

Theoretical investigation of the Li⁺-assisted sulfur reduction reaction

The SRR is a 16-electron complex reaction in which sulfur undergoes a solid–liquid–solid reaction and is eventually converted to Li₂S.¹¹ The typical discharge curve of a Li–S battery is shown in Fig. 1a. The high plateau region (Q_H) corresponds to the solid–liquid conversion of S₈ to Li₂S₄, which accounts for 1/4 of the total capacity. The low plateau region (Q_L) corresponds to the conversion of Li₂S₄ to the solid Li₂S, accounting for 3/4 of the total capacity.^1,27–30 To analyze the performance of the model catalyst at each stage of the SRR, the cell was fully discharged to 1.7 V at 0.1C (Fig. 1b). Compared to C₃N₄, C₃N_4−x and Nb–C₃N_4−x cells have higher capacities in both high and low plateau regions. It is noteworthy that C₃N_4−x exhibits analogous high plateau capacities like Nb–C₃N_4−x, but the capacity of the low plateau region is significantly reduced related to the “shuttle effect” of LiPSs.

Aiming to dissect the reasons for the remarkable performance of C₃N_4−x and Nb–C₃N_4−x, a thorough investigation was conducted on the intrinsic relationship between the catalysts and LiPSs utilizing DFT. Fig. S1 shows the optimized model of three catalysts, where the lower energy “corrugated” structure was chosen for C₃N₄.³¹ C₃N₄ is a typical polymer semiconductor with Fermi energy levels between the valence band and the conduction band. The introduction of defects and the Nb single atom improves the conductivity of the catalyst by impurity energy levels (Fig. S2).²⁴ In the previous study, it was found that the metal sites in catalysts are LiPS adsorption centers by the d–p hybridizing with the S in LiPSs.¹⁵ Take Li₂S₆ as an example, the electrons move from the Nb site to Li₂S₆, and the Nb–S bond is 2.17 Å (Fig. 1c). Meanwhile, the Bader charge results illustrate the transfer of 0.41 |e| to Li₂S₆ (Fig. 1d). The electron transfer proves the strong interaction between the Nb single atom and LiPSs, which favors further conversion. Furthermore, C₃N₄ and C₃N_4−x were proposed, which exhibit a highly conjugated system that tends to hybridize with the s-orbitals of Li and as Li⁺ adsorption sites. The formation of defects upgrades the p-band center energy level of C₃N_4−x (−4 eV, −5.35 eV for C₃N₄) leading to a decrease in antibonding orbital occupancy and an increase in *Li⁺ yield (Fig. 1e). Therefore, by comparing the three model catalysts, we can decouple the roles of the Nb single atom and lithiophilic support in LiPS catalytic conversion.

The bond energy of S–S bonds is commonly utilized to predict the conversion ability of long-chain LiPSs; the weaker S–S bond means that LiPSs are more easily converted. The projected crystal orbital Hamilton population (pCOHP) based on the overlap-population-weighted densities of states was introduced to unravel the strength of the S–S bond in LiPSs.^32,33 As shown in Fig. 1f, the integral of the part of the −COHP below the Fermi level (−ICOHP) was calculated. Therein, Li₂S₆ adsorbed on Nb–C₃N_4−x exhibits the smallest −ICOHP value and the weakest S–S bond which is more prone to be reduced to short-chain LiPSs. Notably, the −ICOHP value is largest when Li₂S₆ is adsorbed on the C₃N_4−x catalyst and it is larger than pristine Li₂S₆ when adsorbed on either C₃N_4−x or C₃N₄. To analyze the reasons for this occurrence, the electronic structure of Li₂S₆ was studied. Fig. S4 shows the projected density of states (PDOS) of the S p orbitals after adsorption. We integrate below the Fermi energy level and found that the IPDOS is inversely proportional to the −ICOHP, which indicates that S–S bonds are more likely to break when the catalyst gives electrons to LiPSs (Fig. 1g).³⁴ Bader charge analysis leads to the same conclusion; for Nb–C₃N_4−x, electrons move from the catalyst to Li₂S₆; however, for C₃N_4−x and C₃N₄, electrons flow from Li₂S₆ to the catalyst (Fig. 1d and Fig. S5). By studying the bonding and electronic structure of Li₂S₆, we found that the metal site mainly provides electrons through the M–S bond to reduce the S–S bond energy to accelerate LiPS conversion, while *Li⁺ improves the reaction kinetics by changing the attack pathway.

To analyze the non-covalent interactions between *Li⁺ and the catalyst, we focus on the conjugated effect, which correlates *Li⁺ on the catalysts with π-electron delocalization. C₃N₄ has a rich conjugate structure; however, owing to the stronger electronegativity of the N-site, electrons accumulate at the N-site. Fig. S6 shows the electron localization function (ELF), which is similar to the previous analysis in that the electrons are mainly concentrated in the N-site of C₃N₄. The N-defect and Nb single atom allows more electrons to delocalize at the C site, which improves the adsorption of Li⁺ and increases the *Li⁺ on the catalyst,³⁵ and is consistent with the previous analysis of the p-band center. Meanwhile, the electrostatic potential of three model catalysts provided support for this conclusion (Fig. 1h). N defects increase the electron density of the system and enhance the cation–π interactions. Notably, the electrons transfer from the Nb site to the support and the electron cloud density of the support is enhanced because of the metal–support interactions, and further increases the amount of *Li⁺ on the catalyst.

As the most direct parameter for determining the lithiophilicity of materials, the Li nucleation overpotential test gave the same conclusion.³⁶ As shown in Fig. S7a, the nucleation overpotential of C₃N_4−x was lower than that of C₃N₄ (45 mV vs. 59 mV) at 1 mA cm⁻², indicating that the N-defects improved the lithiophilicity of the catalyst and increasing *Li⁺. Meanwhile, the mass transfer control potentials (μ_mtc) of C₃N_4−x and Nb–C₃N_4−x are also much higher than C₃N₄, which suggests that N-defects significantly improve Li⁺ migration on the catalyst surface (Fig. S7b). Based on the above theoretical and experimental analyses, we conclude that the N-defect and Nb single atom synergistically improve the adsorption capacity of Li⁺ by enabling the electron into the π bond to form an electron-rich π system (Fig. 1i). More *Li⁺ on the catalyst surface is probably the key to the excellent performance of C₃N_4−x,³⁷ while the synergy of the Nb single atom and lithiophilic support leads to the best performance of Nb–C₃N_4−x. We tentatively propose *Li⁺ as a crucial intermediate on the SRR that can influence the LiPS conversion kinetics.

Synthesis and characterization of catalysts

For a proof of concept, three catalysts were synthesized, and the effect of *Li⁺ on the SRR was analyzed through experiments. Nb–C₃N_4−x was synthesized with a two-step annealing method, whereby the initial step involved pre-sintering under an air atmosphere to form C₃N₄. Subsequently, the sample underwent sintering under an Ar atmosphere following the addition of an Nb source (Fig. 2a).³⁸ Moreover, the N-defect C₃N₄ was prepared by tuning the sintering atmosphere to Ar (Fig. S8). Fig. 2b shows the X-ray diffraction (XRD) patterns of catalysts, where the (100) and (002) characteristic peaks of C₃N₄ are shown around 12.5° and 27°, respectively. For Nb–C₃N_4−x, the peak at 12.5° disappeared while the peak at 27° with reduced intensity and broadened width was obtained, which suggests that the Nb single atom introduced produced defects in the C₃N₄ structural unit.³⁹ The inset shows the optical photographs of three catalysts. It can be seen that the color of the catalysts becomes darker with the addition of defects, which is consistent with the results of the DOS in Fig. S2. Similarly, X-ray photoelectron spectroscopy (XPS) can prove this, with the ratio of N–CN and CN decreasing, which proves that the number of N defects is increasing (Fig. S10a).^24,39,40 Meanwhile, the ratio of pyrrolic-N is increasing and pyridinic-N is decreasing (Fig. S10b). Pyrrolic-N with abundant lone-pair electrons facilitates the adsorption of Li⁺ and enables to build a rich *Li⁺ interface. The unique Nb 3d_3/2 and 3d_5/2 characteristic peaks of Nb–C₃N_4−x in the Nb 3d spectra prove the loading of the Nb element (Fig. S10c).

Through transmission electron microscopy (TEM), the structural characteristics of Nb–C₃N_4−x were further obtained and Nb–C₃N_4−x is a 2D lamellar structure (Fig. S11). No information on Nb nanoclusters was found in the high-resolution TEM images (HRTEM), and no diffraction spots were found after the Fourier transform, proving that Nb is atomic-level dispersed (Fig. 2c). Moreover, Nb elements were observed in the corresponding energy-dispersive spectroscopy (EDS) and uniformly distributed on the C₃N_4−x support (Fig. 2d). Then, the spherical aberration-corrected scanning TEM (AC-STEM) was chosen, to visually demonstrate the successful preparation of single-atom Nb, and a large number of bright spots were directly monitored in the high-angle annular dark-field (HAADF) images, which indicated the successful loading and uniform dispersion of Nb (Fig. 2e). The atom-overlapping Gaussian-function-fitting mapping provides more spatial information with a typical spacing with two Nb atoms of about 0.81 nm, demonstrating the successful synthesis of single-atom Nb (Fig. 2f and g). Spectroscopic and electron microscopic characterization studies proved the successful preparation of three model catalysts.

To clarify the electronic structure and coordination states of the Nb site, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) were performed, respectively. Fig. 2h shows the XANES spectra, and Nb–C₃N_4−x is between Nb foil and Nb₂O₅, showing that the oxidation state of Nb is between Nb⁰ and Nb⁵⁺. Furthermore, the coordination information of the Nb site was obtained by the Fourier transform of the extended edges in the k-space. Nb–C₃N_4−x exhibited a strong peak for the Nb–N bond at 1.29 Å, and no characteristic peaks for the Nb–Nb (2.60 Å) and Nb–O bonds (1.69 Å) were observed (Fig. 2i).^15,39,41 This suggested that Nb coordinates are mainly located with the N site on the C₃N_4−x support and that Nb sites are atomic-level dispersed. The wavelet transform also showed the same situation, with a maximum peak for Nb–C₃N_4−x at k = 2.78 Å for the Nb–N bond, which is completely different from the Nb foils and Nb₂O₅ (Fig. 2k–m). The data in R-space were fitted with a coordination number of 3.16 ± 0.43 for Nb in the first coordination layer, and there were two different Nb–N bonds with bond lengths of 1.87 Å and 2.08 Å, respectively (Fig. 2j and Table S3). The two Nb–N bonds were categorized as Nb–N_pyridinic and Nb–N_pyrrolic, which corresponds to Fig. S10b.⁴² Next, we evaluated them in terms of their adsorption and conversion of LiPSs, respectively, and analyzed the roles of the lithiophilic support and Nb single atom.

Evaluation of adsorption and conversion kinetics for LiPSs

Adsorption of LiPSs by catalysts is an important parameter in suppressing the “shuttle effect” and evaluating the performance of Li–S batteries. Through visualization experiments, the adsorption of LiPSs by the three catalysts was compared, and the orange-colored Li₂S₆ solution in Nb–C₃N_4−x was almost colorless at 0.5 h of adsorption. Notably, Li₂S₆ still showed no significant color change after 5 h of adsorption with C₃N_4−x and C₃N₄ (Fig. 3a inset). For a more visual comparison of the adsorption capacity of LiPSs, UV-vis absorption spectroscopy was used. As shown in Fig. 3a, the intensities of the remaining S₆²⁻ after 5 h adsorption followed the order of C₃N₄ ≈ C₃N_4−x > Nb–C₃N_4−x. The similar S₆²⁻ intensities of C₃N_4−x and C₃N₄ indicated that the lithiophilic sites are hardly favorable for the adsorption of LiPSs. Chemical interactions between LiPSs and Nb–C₃N_4−x were studied by XPS.⁵ The typical characteristic peaks of Nb–S bonds on the adsorbed Nb–C₃N_4−x surface indicated the strong interaction of the Nb sites with S, which shows that the Nb–S interaction is crucial for the adsorption of LiPSs (Fig. 3b). Conversely, there were only very weak peaks for terminal sulfur (S_T⁻¹) and bridged sulfur (S⁰_B) on the C₃N_4−x and C₃N₄, illustrating the weak adsorption of Li₂S₆ (Fig. S12). The peak shift of Nb before and after Li₂S₆ adsorption indicated the direction of charge transfer. The Nb peak shifted to high binding energy, which proves that the electron density of the Nb atom is significantly reduced, which is consistent with the DFT (Fig. 3c).¹²

Furthermore, the adsorption of all the intermediates of S cathodes on the three catalysts was systematically studied by DFT, and their adsorption configurations on the catalyst are shown in Fig. S13. Consistent with the previous analysis, C₃N_4−x and C₃N₄ are typical lithiophilic supports, whereas the Nb site in Nb–C₃N_4−x behaves as an S adsorption site through the strong Nb–S interaction. Benefiting from the strong interaction with LiPSs, Nb–C₃N_4−x has the largest adsorption energy for all intermediates, and we believe that the larger adsorption energy of C₃N_4−x compared to C₃N₄ is owing to its stronger interaction with Li (Fig. 3d). The DFT results show that the Nb single atom effectively enhances the LiPS conversion kinetics and inhibits the “shuttle effect” through strong LiPS adsorption.

To comprehensively evaluate the adsorption of LiPSs on the catalyst, the catalyst was coated on the surface of the PP separator, which was coated on one side and the catalyst was facing the cathode (Fig. S14). The catalyst was uniformly coated on the surface of the separator with a thickness of about 15 μm (Fig. S15). The coating did not affect the mechanical properties of the separator, and the coating layer remained intact after two folds. In Li–S batteries, the main intermediates involved in the “shuttle effect” are liquid long-chain LiPSs (i.e., Li₂S₈, Li₂S₆, and Li₂S₄), and the conversion between them occurs in the high plateau region. First, the cell is discharged to a constant voltage, and then undergoes a constant voltage discharge; the stabilized current is the shuttle current of LiPSs (Fig. S16).^43–45 Here, five voltages were selected to compare the ability of the catalyst to suppress the “shuttling effect” of LiPSs. In particular, Nb–C₃N_4−x showed the lowest shuttling current, indicating that the catalyst can prevent LiPS shuttling through strong chemical adsorption (Fig. 3e). Anomalously, C₃N_4−x exhibits higher shuttle currents, it can be attributed to the fact that the catalytic effect of C₃N_4−x on the SRR produces more long-chain LiPSs, and the higher LiPS concentration exacerbates the “shuttle effect”. Furthermore, the cell with a LiNO₃-free electrolyte is assembled to amplify the “shuttle effect”. The lack of LiNO₃ leads to the intensification of the spontaneous reduction reaction between LiPSs and the Li metal anode, generating a nonuniform Li₂S SEI, which is manifested in the charge/discharge curves as greatly higher charging capacity.⁴⁶ As shown in Fig. 3f, Nb–C₃N_4−x shows the highest coulombic efficiency of 71.35%, which means that Nb–C₃N_4−x inhibits the LiPS shuttling effectively. Based on the above analysis, we found that C₃N_4−x has no obvious inhibition of the “shuttle effect” which is attributed to the weak adsorption of soluble long-chain LiPSs. In contrast, the introduction of the Nb single atom greatly enhances the adsorption of LiPSs on the catalyst and significantly inhibits the “shuttle effect”. Therefore, we believe that the lithiophilic catalysts cannot exhibit inhibition of the “shuttle effect”, therefore attention should be paid how *Li⁺ affects the Li–S battery in terms of the conversion kinetics of LiPSs.

In cyclic voltammetry (CV) curves, Nb–C₃N_4−x exhibits the highest peak currents and significant cathodic/anodic peak shifts, proving its excellent reaction kinetics (Fig. 3g). Counterintuitively, C₃N_4−x shows a weaker peak current compared to C₃N₄. We tend to ascribe it to the “shuttle effect” of LiPSs, and therefore, the Tafel curves of the three catalysts at different reaction stages were plotted (Fig. 3h, i and Fig. S17). The fitted slope values of peak I are 42.9, 43.5 and 46.2 mV dec⁻¹ with Nb–C₃N_4−x, C₃N_4−x and C₃N₄, respectively. Peaks II and III show the same trend, indicating that increasing material lithiophilicity can effectively improve the reaction kinetics of the Li–S battery, and the Nb–C₃N_4−x catalyst, which takes into account the dual advantages of abundant *Li⁺ and strong Nb–S interaction, exhibits the most excellent performance. Fig. 3j–l shows the contour plots of the CV patterns for three catalysts at scan rates of 0.1–0.5 mV s⁻¹. Nb–C₃N_4−x exhibits the largest peak current and lowest polarization, which is consistent with the previous analysis. We observe that all peak currents have a linear relationship with the square root of the scan rate, indicating a diffusion-limited process (Fig. S18a–c).¹² Here, the classical Randles–Sevcik equation was used to describe the Li⁺ diffusion:

Ip = 2.69 × 105n1.5ADLi+0.5CLi+ν0.5	(1)

where I_p is the corresponding peak current, n is the transferred electrons for the reaction (for the Li–S battery, n = 2), A is the electrode area (1.54 cm²), D_Li⁺ is the lithium-ion diffusion coefficient, C_Li⁺ is the concentration of Li⁺ in the electrolyte (1 mol L⁻¹) and ν is the scan rate. We can find that C₃N₄ has the lowest Li⁺ diffusion coefficient, which is attributed to the slow LiPS conversion and weak adsorption, leading to an electrolyte viscosity increase (Fig. S18d). In contrast, Nb–C₃N_4−x exhibits faster Li⁺ diffusion thanks to the fast LiPS conversion kinetics.

Furthermore, electrochemical impedance spectroscopy (EIS) was used to evaluate the charge transfer kinetics of the Li–S battery (Fig. S19). Nb–C₃N_4−x exhibits the lowest charge transfer impedance (R_ct = 24.51 Ω), while C₃N₄ has the highest charge transfer impedance of 26.91 Ω. Like the previous tests, the CV and EIS of the symmetric cells show similar trends (Fig. S20). Nb–C₃N_4−x and C₃N_4−x show higher oxidation/reduction peak current, indicating their excellent catalytic activity which is stronger than C₃N₄ (Fig. S20a). Theory and experiments confirm the excellent performance of Nb–C₃N_4−x in the adsorption and conversion of LiPSs. Furthermore, quantifying the role of *Li⁺ facilitates a comprehensive understanding of the SRR.

Li⁺-rich and Li⁺-lean conversion models for the SRR

In situ Raman spectroscopy was applied in characterizing the variation of the Li⁺ concentration on the catalyst.⁴⁷ The assembly of the in situ Raman electrochemical cell is shown in Fig. S21, and the laser was focused on the junction of the cathode and the separator to observe the surface environment of the catalyst during the sulfur reduction.¹¹ Through monitoring the Raman signal of TFSI⁻, the concentration changes of Li⁺ on the surface of catalysts during discharge were observed (Fig. 4a and b, left). The stronger TFSI⁻ signal was observed on the surface of Nb–C₃N_4−x at the beginning of the reaction, which confirms the rich *Li⁺ environment on Nb–C₃N_4−x. It is noteworthy that the signal intensity of TFSI⁻ keeps changing with the reaction, indicating that the Li⁺ on the catalyst undergoes repeated loss and replenish (Fig. 4a, left). In contrast, the TFSI⁻ signal in PP cells was weak and stable, demonstrating that LiPSs at the lean *Li⁺ system tend to react with Li⁺ in the electrolyte rather than *Li⁺ (Fig. 4b, left). Furthermore, the evolution of soluble LiPSs during discharge was observed by in situ Raman spectroscopy to study the primary influence steps of *Li⁺ in the SRR (Fig. 4a and b, right). In the early stage of discharge, a large number of Li₂S₈ and Li₂S₄ signals were observed in the Nb–C₃N_4−x cell and disappeared in the late stage of discharge, demonstrating that *Li⁺ can significantly enhance the conversion between soluble LiPSs. In contrast, the stable Li₂S₆ signal was observed throughout the discharge in PP cells, indicating that significant amounts of soluble LiPSs were not involved in the reaction.

The SRR is a complex multi-step reaction, and it is necessary to analyze the reaction kinetics of each step for studying the conversion of LiPSs. Galvanostatic intermittent titration technique (GITT) measurements were conducted at 0.1C to analyze the dynamic conversion process of LiPSs (Fig. S22). Nb–C₃N_4−x exhibits smaller voltage differences between the quasi-open circuit voltage (QOCV) point and the closed-circuit voltage (CCV) point compared to C₃N_4−x and C₃N₄, indicating the accelerated SRR kinetics facilitated by the Nb–C₃N_4−x. Furthermore, the corresponding internal reaction impedance (ΔiR) was calculated; in the Li₂S nucleation stage, the ΔiR of the three cells appeared to be significantly different, suggesting that *Li⁺ has a certain promotion effect on Li₂S nucleation (Fig. 4c). Meanwhile, the lowest ΔiR of Nb–C₃N_4−x also demonstrates that the *Li⁺-rich interface and strong M–S interactions can enhance the sulfur reduction kinetics simultaneously. To quantify the SRR kinetics, the activation barrier under different voltages for the SRR was determined by fitting the charge transfer resistance at different temperatures (Fig. S23). The reaction barriers at different voltages were calculated based on the Arrhenius formula (Fig. 4d and Fig. S24). Compared to C₃N₄, both Nb–C₃N_4−x and C₃N_4−x exhibit smaller reaction barriers before 1.9 V (Li₂S₂/Li₂S deposition), which proves that *Li⁺ acts mainly in the liquid–liquid conversion and initial stages of liquid–solid conversion (Fig. 4e).

As the rate-determining step of the SRR, Li₂S₄ to Li₂S directly affects the LiPS conversion kinetics, and the reaction occupies 3/4 electron transfer which is the most important step. Through Li₂S deposition experiments, the Li₂S deposition kinetics on the surface of different catalysts were compared. Fig. 4f shows the constant–voltage discharge curve of Li₂S nucleation, which was performed at 2.05 V.⁴⁸ Nb–C₃N_4−x showed the fastest current response and the highest deposition capacity (T_peak = 3543 s, 523.95 mAh g_s⁻¹), and C₃N_4−x was slightly lower (T_peak = 3639 s, 392.74 mAh g_s⁻¹). It is noteworthy that the Li₂S deposition capacity on C₃N₄ is significantly reduced (also clearly noticed in SEM images in the inset), which indicates the importance of the *Li⁺ on the catalyst surface for LiPS conversion.

To systematically study how the lithiophilic support and Nb single atom act in the various steps of the SRR, the Gibbs free energy for the sulfur reduction pathway was calculated (Fig. 4g). Normally, the liquid–solid conversion reaction is the rate-determined step of the SRR. After calculations, we found that the reaction of Li₂S₂ to Li₂S has the largest Gibbs free energy, and here we analyze this step in focus. Here, C₃N₄ exhibits the highest Gibbs free energy change (1.16 eV) which is in agreement with the experimental results. The Gibbs free energy of the solid–solid conversion step is markedly lowered by Nb–C₃N_4−x and C₃N_4−x, and the critical role of *Li⁺ in the SRR has been thermodynamically demonstrated. Here, relying on the classical theory of the NO₃RR (ER and LH mechanisms),^19–21 Li⁺-rich and Li⁺-lean models of the SRR were proposed (Fig. 4h). It means that when Li⁺ is enriched on the surface of the catalyst, the adsorbed LiPSs and Li⁺ react directly. In contrast, when Li⁺ is scarce on the catalyst surface, the adsorbed LiPSs react with Li⁺ in the electrolyte, which greatly slows down the reaction rate. The lithiophilic support further enhances the performance of the Nb single atom catalysts through *Li⁺, and synergy between the Nb single atom and the lithiophilic support of the Nb–C₃N_4−x catalysts exhibit optimal performance. Through the Li⁺-rich model, we explained the effect of *Li⁺ on the LiPS conversion successfully, and we suggest that *Li⁺ can be used as a crucial intermediate to guide the cathode/anode dual-function catalyst design for Li–S batteries.

Effect of Li⁺-rich catalysts on the Li metal anode

Based on Sand's time theory, we suggest that fast Li⁺ migration and prompt Li⁺ replenishment are important parameters to achieve Li dendrite-free deposition.⁴⁹ Therefore, the catalyst was coated on the surface of Cu foil to study the ability of Li deposition. First, based on DFT calculations, the effects of N-defects both on Li⁺ adsorption and migration were investigated from a theoretical viewpoint. In the above study of electron delocalization on the catalyst, N-defects were found to facilitate electron delocalization in the C₃N₄ backbone, which promotes the s–p hybridization between C and Li.⁵⁰ For this reason, the adsorption energy of C₃N_4−x for Li is −4.39 eV and much higher than the −3.54 eV of C₃N₄ (Fig. 5a). The chemical interaction between Li⁺ and catalysts has been revealed at the atomic scale by the electron density difference. Larger charge transfer in the C-site was observed in C₃N_4−x–Li⁺ systems, indicating that N-defects enhance electronic coupling between C and Li, which is the reason for the larger adsorption energy (Fig. S25). Meanwhile, the more delocalized electrons favor the migration of Li⁺ on the catalyst, and the lower migration energy barrier of Li⁺ on C₃N_4−x (only 2.11 eV) (Fig. 5b). Attributed to the strong adsorption and lower migration energy barriers, Li was deposited more uniformly on the C₃N_4−x with a lower face average roughness (Fig. 5c and Fig. S26).

Experimental and theoretical analyses have shown that N-defects on C₃N₄ facilitate Li deposition, and Nb–C₃N_4−x@Li, C₃N_4−x@Li and C₃N₄@Li anodes have been prepared by electrodeposition. According to Sand's time theory,²³ Li dendrites arise from the rapid depletion of Li⁺ at the electrode/electrolyte interface, resulting in a transient Li⁺ “vacuum zone”. Therefore, good Li⁺ conductivity at the electrode/electrolyte interface is necessary to enhance Li anode performance. Here, the Bruce–Vincent method is used to calculate the Li⁺ migration number for three anodes (Fig. 5d–f). The Li⁺ migration numbers are 0.44, 0.40 and 0.37 with Nb–C₃N_4−x, C₃N_4−x and C₃N₄, respectively. Meanwhile, we analyzed and compared the Li deposition reaction kinetics of three anodes through Tafel curves. As shown in Fig. 5g, the exchange current density of the Li deposition reaction on the Nb–C₃N_4−x@Li anode is 0.59 mA cm⁻², which is much higher than those on C₃N_4−x@Li and C₃N₄@Li anodes (0.46 mA cm⁻² and 0.24 mA cm⁻²).⁵¹ Faster interface Li⁺ conductivity and Li deposition reaction kinetics are conducive to achieving dendrite-free Li deposition, and we believe that Nb–C₃N_4−x could inhibit dendrite effectively.

To further study the regulation of Li deposition by catalysts on the anode side, a Li||Cu asymmetric cell was assembled to analyze the Li deposition behavior on the electrode surface. The Nb–C₃N_4−x cell showed the highest coulombic efficiency and the longest cycle life at 1 mA cm⁻² and 1 mAh cm⁻² (Fig. 5h and Fig. S27), in that Nb–C₃N_4−x effectively inhibits Li dendrite growth and reduces the “dead Li”. Through Aurbach CE profiles, the coulombic efficiency of Li plating/stripping was studied and evaluated anode reversibility.⁵² The Nb–C₃N_4−x anode has the highest plating/stripping Coulomb efficiency (up to 99.79%), and ultra-high reversibility means less Li loss during the cycle (Fig. 5i). A symmetric cell allows for a direct comparison of the cycling stability in Li anodes. Next, the electrodes were prepared by electrodeposition using an assembled Li||Li symmetric cell. The Nb–C₃N_4−x anode exhibits the strongest plating/stripping stability, and the battery can be cycled stably for more than 2000 h, which is 10 times longer than C₃N_4−x and 20 times longer than C₃N₄ (Fig. 5j). Meanwhile, the Nb–C₃N_4−x cell shows a lower overpotential of only 17 mV, and smaller overpotential means faster Li plating/stripping kinetics. The deposition behavior of Li on Nb–C₃N_4−x was observed in situ with the aid of an optical microscope (Fig. 5k). The Li deposition on Nb–C₃N_4−x was uniform and dense, in contrast to the obvious Li dendrites observed on the surface of Cu foil after 20 min deposition. Through defective engineering, Nb–C₃N_4−x increases the s–p hybridization between C and Li, which improves the *Li⁺ on the catalyst surface and effectively suppresses Li dendrites. Therefore, we suggest that the *Li⁺ can be a crucial intermediate for screening the high-performance dual-function catalysts which are capable of improving both the cathode and the anode.

Performance and application of Nb–C₃N_4−x-based Li–S batteries

Battery performance is an indicator for comprehensive evaluation of catalysts. The galvanostatic charge–discharge profile of Nb–C₃N_4−x at 0.5C delivers the highest initial capacity (1098 mAh g⁻¹) and the smallest polarization potential (179 mV), indicating enhanced reaction kinetics (Fig. 6a). The value of Q_L/Q_H can reflect the ability of the catalyst to convert long-chain LiPSs to short-chain LiPSs, and the values of Nb–C₃N_4−x, C₃N_4−x, and C₃N₄ are 2.31, 2.27 and 2.19, indicating that the conversion ability becomes weaker sequentially which is in line with the previous analysis (Fig. S28). Fig. 6b shows the cycling performance of the three catalysts at 0.5C. The cell with Nb–C₃N_4−x@PP exhibits the best performance with a capacity retention of up to 85.6% after 200 cycles. We attribute the rapid capacity decay of the C₃N_4−x cell to the severe “shuttle effect”. C₃N₄ exhibited the worst cell performance at both high and low rates, which is related to its worst catalytic ability. However, C₃N_4−x is only slightly inferior to Nb–C₃N_4−x at a low rate, which is because there is sufficient time to complete the conversion although the adsorption of LiPSs on C₃N_4−x is weak, explaining the sharp decrease in capacity at the large rate (Fig. 6c and Fig. S29). Nb–C₃N_4−x catalysts with both abundant *Li⁺ and strong Nb–S interactions exhibit excellent performance, showing superior rate performance compared to recent literature (Fig. 6d).^33,53–65

To fully examine Nb–C₃N_4−x, the long-term cycling stability of the cell with a sulfur loading of 1.3 mg cm⁻² was tested. The reversible capacity after 400 cycles at 2C was 584.01 mAh g⁻¹ with a high-capacity retention of 77.99% and an average capacity decay of about 0.055% per cycle (Fig. 6e). To ensure reproducibility, six independent cells were tested in parallel, and the cells showed excellent repeatability at 2C for 100 cycles (Fig. S30). The TEM image of Nb–C₃N_4−x after 100 cycles is shown in Fig. S31a, and the two-dimensional structure of C₃N₄ is well maintained during the cycling. No obvious lattice fringes were observed in the HRTEM images, confirming that Nb remained atomically dispersed and the catalyst was able to remain stable during cycling (Fig. S31b and c). Meanwhile, a uniform distribution of Nb is also observed by EDS (Fig. S31d). To simulate real-world application scenarios, the full cell with Nb–C₃N_4−x@PP as the separator and Nb–C₃N_4−x@Li as the anode was assembled. The cell is tested under extreme conditions with a S loading of 5.17 mg cm⁻², a negative-to-positive capacity ratio (N/P) of 3.33, and an electrolyte-to-sulfur ratio (E/S) of 4.62 μL mg_s⁻¹ (Fig. 6f). Even under these conditions, the cell continued to operate stably for more than 38 days with 87.01% reversible capacity, demonstrating the potential of the Nb–C₃N_4−x catalyst for practical applications. As a proof of concept, the Nb–C₃N_4−x pouch cell was assembled. The pouch cell works stably for 100 cycles at 0.1C and still powers the LED lamp when 90° bend (Fig. S32). Furthermore, an Ah-level pouch cell was assembled and test under 50 mA g⁻¹. The pouch cell has an initial discharge capacity of 1.32 Ah and delivers a high energy density (Fig. 6g and Fig. S33). The application of the cell in a variety of scenarios is shown in Fig. 6h. The Nb–C₃N_4−x cell can drive the electric fan, and even more surprisingly the cell can power a smartphone, which further proves the potential of the application of the Nb–C₃N_4−x cell. Benefiting from the strong Nb–S interaction on the Nb single atom and the abundance *Li⁺ on the lithiophilic support, Nb–C₃N_4−x-based cell exhibits excellent performance. The results on Li⁺-assisted catalysis contribute to our comprehensive insights into the high-efficiency SRR for Li–S batteries.

Conclusions

In summary, we elaborately synthesized Nb–C₃N_4−x catalysts, and the Nb single atom not only exhibited strong Nb–S interactions, but also enhanced the electronic delocalization on the support to increase *Li⁺, which synergistically accelerates the LiPS conversion. Experiments and DFT demonstrated that *Li⁺ shows a direct relationship with sulfur reduction kinetics and *Li⁺-rich catalysts can accelerate LiPS conversion through changing the Li⁺ attack pathway. Based on the above view, Nb–C₃N_4−x catalysts were designed with excellent performance which significantly enhance the SRR kinetics and inhibit the “shuttle effect”. Li–S batteries with the Nb–C₃N_4−x catalyst delivered an average capacity attenuation rate of 0.055% during 400 cycles at 2C. Meanwhile, the regulation of Li⁺ by the catalyst also inhibits Li dendrites, and the symmetric cell can cycle stably for more than 2000 h at 1 mA cm⁻² and 1 mAh cm⁻². The Nb–C₃N_4−x based full cell can stably work for more than 100 cycles under harsh conditions (S loading = 5.17 mg cm⁻², N/P = 3.33 and E/S = 4.62 μL mg_s⁻¹), with a capacity retention rate of 87.01%. The *Li⁺-mediated reaction pathway contributes to a more comprehensive insight into the SRR and provides a new direction for the rational design of potential catalytic materials for Li–S batteries.

Author contributions

Y. M., Y. Z., K. Z. and S. L. conceived the idea and supervised the project. Y. M., Y. Z. and S. L. designed the experiments and wrote the manuscript. Y. M., Y. Z. and Y. N. carried out the synthesis and characterization of samples. Y. M. and J. S. conducted the electrochemical measurements. Y. M. and M. S. performed the DFT calculations. Experimental results were collected and analyzed through contributions from all authors.

Conflicts of interest

The authors declare no competing financial interests.

Data availability

The data of this study are available from the corresponding authors upon reasonable request.

The supplementary information includes details of the experiment and electrochemical testing. It also contains some electrochemical testing data that is presented in the main text. See DOI: https://doi.org/10.1039/d5ee02048d

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22279026 and 2247090373), the Natural Science Foundation of Chongqing (CSTB2022NSCQ-MSX1401), and the China Postdoctoral Science Foundation (2024M764198).

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Footnote

† Yiyang Mao and Yan Zhang contributed equally to this work.

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