Boron-doping engineering of molybdenum carbide on nitrogen-rich carbon nanospheres: a synergistic adsorption–conversion modifier for high-performance lithium–sulfur batteries

Abstract

Lithium–sulfur batteries (LSBs) hold significant promise for next-generation energy storage due to the ultrahigh potential energy density. However, their commercialization is hindered by the shuttle effect and sluggish reaction kinetics of lithium polysulfides (LiPSs). Herein, a hierarchical catalyst composed of cubic Mo₂C nanoparticles anchored on N-doped carbon nanospheres (δ-B-Mo₂C@NC) is designed via facile boron-doping engineering, which simultaneously mitigates LiPS shuttling and facilitates sulfur conversion reactions. The incorporation of boron dopants into the δ-B-Mo₂C@NC framework significantly increases active sites and enhances electron/ion pathways, synergistically promoting strong adsorption and efficient catalytic conversion for LiPSs. Moreover, the electronic structure of δ-B-Mo₂C is optimized by upshifting the Mo d-band center. This enhancement promotes stronger Mo 4d/S 3p orbital hybridization between δ-B-Mo₂C@NC and LiPSs, thus accelerating sulfur redox kinetics. Consequently, the LSB equipped with the δ-B-Mo₂C@NC catalyst exhibits remarkable rate capability (459 mAh g⁻¹ at 3 A g⁻¹) and long-term cycling stability (a capacity decay of 0.045% per cycle over 500 cycles at 1 A g⁻¹). These findings highlight the potential of Mo₂C-based catalysts in suppressing the shuttle effect and pave the way for designing advanced electrocatalysts toward high-energy and long-life LSBs.

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New concepts

This work presents an innovative hierarchical catalyst, δ-B-Mo₂C@NC, in which cubic Mo₂C nanoparticles are anchored on N-doped carbon nanospheres through a facile boron-doping strategy. The introduction of boron into the δ-B-Mo₂C@NC framework plays a critical role by significantly increasing active site density and improving electron/ion transport pathways. This synergistic enhancement promotes strong chemisorption and efficient catalytic conversion of lithium polysulfides (LiPSs). Moreover, boron doping effectively modulates the electronic structure of Mo₂C by upshifting the Mo d-band center closer to the Fermi level, as confirmed by XPS analysis. This electronic optimization strengthens the hybridization between Mo 4d and S 3p orbitals, thereby accelerating sulfur redox kinetics. This boron-doping engineering offers a distinct advantage over most reported Mo₂C-based catalysts in lithium-sulfur batteries, which predominantly feature the hexagonal phase (β-Mo₂C). Conventional β-Mo₂C catalysts suffer from limited active sites and low conductivity, resulting in poor LiPS adsorption and sluggish kinetics. In contrast, the boron-engineered δ-B-Mo₂C@NC catalyst overcomes these limitations, exhibiting superior electrochemical functionality. This work thereby provides a novel pathway for designing high-performance hosts for advanced lithium-sulfur batteries.

Introduction

Lithium–sulfur batteries (LSBs) are highly desirable for portable electronics, electric vehicles, and grid-scale stationary storage due to their high theoretical energy density (2600 Wh kg⁻¹), environmental sustainability, and low cost. However, their commercial deployment is hindered by short cycle life and rapid capacity degradation.^1–6 Specifically, the LSB electrochemistry mainly involves multi-phase transformations among sulfur, lithium polysulfides (LiPSs), and the final discharge products Li₂S. During cycling, the soluble LiPSs tend to diffuse freely and shuttle between the electrodes, leading to low coulombic efficiency and poor sulfur utilization.⁶ Moreover, the inherently low electronic and ionic conductivity of both sulfur and Li₂S results in sluggish redox kinetics.^7,8 This further exacerbates the shuttle effect, thereby deteriorating the cycling stability and capacity performance of LSBs.

To address these obstacles, significant efforts have been directed toward the design of diverse porous carbon materials (graphene derivatives,⁹ porous carbon,¹⁰ carbon nanotubes,¹¹ etc.) to mitigate the shuttle effect in LSBs. Their high specific surface area and excellent electronic conductivity enable physical confinement and accelerate the redox kinetics of LiPSs. However, the inherent solvaphobicity of most carbons and their lack of sufficient active sites often result in limited ion diffusion and weak interactions of LiPSs. These limitations became particularly evident under high sulfur loadings, undermining the practical performance of LSBs. Consequently, research efforts have increasingly focused on integrating polar transition metal compounds (such as oxides,¹² sulfides,¹³ carbides,¹⁴ and nitrides¹⁵) into carbon supports. These materials not only exhibit strong anchoring ability toward LiPSs but also improve electrolyte wettability and lower the activation energy for sulfur redox reactions, thereby significantly enhancing the cycling stability and capacity of LSBs. Among various polar catalysts, those with metallic phases are especially advantageous due to their ability to provide a rapid and abundant electron supply, facilitating efficient reduction/oxidation of adsorbed LiPSs.^14–16 A prominent example is molybdenum disulfide (MoS₂), in which the metallic 1T-phase MoS₂ demonstrates significantly superior adsorption capability and electrocatalytic activity for LiPS conversion, compared to its semiconducting 2H-MoS₂ phase.^16,17

Molybdenum carbide (Mo₂C) has recently emerged as a highly promising electrocatalyst for LiPSs, owing to its favorable catalytic activity and intrinsic chemical stability properties derived from its platinum-like d-band electronic structure.^18,19 Extensive research has concentrated on the hexagonal phase (β-Mo₂C) due to its efficacy in suppressing the LiPS shuttle effect within sulfur cathodes. Nevertheless, the modest electrical conductivity and limited active sites of β-Mo₂C result in unsatisfactory LSB performance.^20,21 Heteroatom doping has arisen as an effective approach to induce phase transition into the metallic cubic phase (δ-Mo₂C), introduce defect sites, and precisely modulate the d-band center.^21–23 Such modifications concurrently increase catalytic site density and enhance conductivity, thereby offering a robust pathway for improving LiPS adsorption and facilitating electrocatalytic redox conversions.

Along this line of research, we present a metallic δ-Mo₂C-based catalyst synthesized via a boron-doping engineering strategy to suppress the shuttle effects and improve redox kinetics in LSBs. The catalyst exhibits a hierarchical porous structure consisting of boron doped δ-Mo₂C nanoparticles anchored on N-rich carbon nanospheres (denoted as δ-B-Mo₂C@NC). The introduction of boron dopants induces localized electron-deficient regions, providing a high density of active sites. These active sites function as strong “anchors” that effectively capture soluble LiPSs through chemical interactions, significantly suppressing the shuttle effect. Concurrently, boron-doping shifts the Mo d-band center positively, strengthening Mo 4d/S 3p orbital hybridization and accelerating the conversion kinetics of LiPSs. Owing to the boron-doping-induced formation of the cubic δ-Mo₂C phase and precise regulation of the electronic structure, the LSB incorporating the δ-B-Mo₂C@NC modifier demonstrates exceptional rate performance and durable cycling stability even at high sulfur loadings (>2 mg cm⁻²). Boron doping markedly enhances the electrical conductivity of the modifier and optimizes the anchoring and catalytic conversion kinetics of polysulfides at the interface, thereby effectively suppressing the shuttle effect and improving the utilization efficiency of active sulfur species (Fig. S1).

Results and discussion

Mo₂C-based catalysts with controlled crystalline phases were synthesized through a wet-chemical route followed by annealing, as illustrated in Fig. 1a. The process began with the fabrication of MoO₄²⁻-polydopamine (Mo-PDA) nanospheres via polymerization of a MoO₄²⁻-dopamine complex under controlled conditions (Fig. S2).²⁴ Subsequent pyrolysis of the Mo-PDA precursor under an argon atmosphere at 600 °C yielded MoO₂ nanoparticles embedded in N-doped carbon nanospheres (MoO₂@NC). Finally, boron-doping engineering was applied by reacting MoO₂@NC with boric acid at high temperature to produce δ-B-Mo₂C@NC. The boron-mediated phase transition in Mo₂C originates from the synergistic effects of the electronic structure modulation and introduced lattice strain (Fig. S3), attributable to boron's lower electronegativity (2.04) and smaller atomic radius compared with carbon. The resulting δ-Mo₂C-based modifier offers enhanced electrical conductivity and, more importantly, improved kinetics for LiPS conversion compared to the β-phase counterpart. In contrast, the undoped β-Mo₂C@NC was obtained under analogous conditions without boric acid. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images demonstrate that MoO₂@NC displays hollow spheres with MoO₂ nanoparticles decorated on the surface (Fig. S4). The β-Mo₂C@NC well retains a spherical flower-like morphology with porous features, closely resembling that of the precursor MoO₂@NC (Fig. S5). Following boron doping, SEM and TEM show that the δ-B-Mo₂C@NC material maintains its hollow spherical nanostructure (Fig. 1b–d), which is densely decorated with highly dispersed δ-B-Mo₂C nanoparticles with an average size of around 10 nm (Fig. 1e). This design is highly beneficial for adsorbing polysulfides and facilitating their subsequent redox reactions. The crystallinity of the δ-B-Mo₂C@NC composite was elucidated by the high-resolution TEM image (Fig. 1f). A distinct lattice fringe spacing of 0.24 nm was observed, which corresponds to the (111) plane of δ-Mo₂C, in agreement with the X-ray diffraction (XRD) results presented in Fig. 2a. The selected-area electron diffraction (SAED) pattern (Fig. 1g) further confirmed the crystalline nature of δ-B-Mo₂C@NC, exhibiting three diffraction rings with d-spacings of 0.24, 0.21, and 0.15 nm, which correspond to the (111), (200), and (220) planes of δ-Mo₂C, respectively. Additionally, the high-angle annular dark-field scanning TEM (HAADF-STEM) image (Fig. 1h) and the corresponding energy-dispersive X-ray spectroscopy (EDS) elemental mapping (Fig. 1i–l) clearly reveal the hollow nanostructure of δ-B-Mo₂C@NC, as well as the homogeneous spatial distribution of Mo, C, N, and B, with no detectable elemental segregation.

Fig. 1 (a) Schematic illustrating the synthesis procedure of δ-B-Mo₂C@NC. (b) and (c) SEM images of δ-B-Mo₂C@NC. (d) and (e) TEM images of δ-B-Mo₂C@NC showing nanoscale structures. (f) and (g) High-resolution TEM image and selected-area electron diffraction (SAED) pattern images of δ-B-Mo₂C@NC. (h–l) HAADF-STEM image and the corresponding EDS patterns of δ-B-Mo₂C@NC spheres.

Fig. 2 (a) and (b) XRD patterns and Mo 3d XPS spectra of δ-B-Mo₂C@NC and β-Mo₂C@NC. (c) B 1s high-resolution XPS spectra of δ-B-Mo₂C@NC. (d) UV-Vis spectra of Li₂S₆ solution after adsorption of δ-B-Mo₂C@NC and β-Mo₂C@NC (inset: the corresponding photo of visualized adsorption test). (e) and (f) CV curves and Nyquist plots of symmetric Li₂S₆ cells using different catalysts.

The crystal structures of the samples were analyzed by XRD. As shown in Fig. 2a, in the case of β-Mo₂C@NC, a set of characteristic peaks well matches hexagonal β-Mo₂C (PDF #35-0787). After b–oron-doping, the resulting pattern (red curve) corresponds to cubic δ-Mo₂C (PDF #15-0457). This metastable δ-Mo₂C phase has been demonstrated to possess higher catalytic activity towards LiPSs,^21,22 thereby enhancing the electrochemical performance of LSBs. X-ray photoelectron spectroscopy (XPS) was utilized to investigate the valence states and electronic structures of different elements in the two catalysts. In Fig. S6, the XPS wide-scan survey spectra reveal the presence of Mo, C, and N in β-Mo₂C@NC, while a distinct B signal is observed in δ-B-Mo₂C@NC, affirming the doping of B into the latter catalyst. High-resolution Mo 3d XPS spectra of β-Mo₂C@NC could be deconvolved into three doublets (Fig. 2b). The peaks located at 228.9/231.9 eV are assigned to the Mo²⁺ state of Mo₂C, while those at 229.6/232.5 eV and 233.1/235.9 eV correspond to Mo⁴⁺ and Mo⁶⁺ valence states, respectively.^23,25 Notably, compared to β-Mo₂C@NC, the Mo²⁺ peaks of δ-B-Mo₂C@NC exhibit a negative shift of 0.2 eV in binding energy. This shift originates from the lower electronegativity of B, which increases the electron density around Mo atoms in the hexagonal δ-phase.^21–23 Such electron transfer is likely a key factor in manipulating the d-band center of Mo to strengthen the Mo 4d/S 3p orbital hybridization between δ-B-Mo₂C@NC and LiPSs. These hybridized orbitals serve as active sites that not only anchor LiPSs but also facilitate electron transfer during electrocatalysis, significantly accelerating redox reactions. As shown in Fig. 2c, the B 1s XPS spectrum displays four components corresponding to B₂O₃ (193.2 eV), BCO₂ (192.3 eV), B–C₃ (191.2 eV), and Mo–B (189.6 eV) bonds,²³ further confirming the successful doping of B into the Mo₂C lattice.

To investigate the adsorption and catalytic capacity of different modifiers on LiPS intermediates, a static visual adsorption test was conducted by immersing equal mass of δ-B-Mo₂C@NC and β-Mo₂C@NC into the Li₂S₆ solution (2 mM). As shown in Fig. 2d and Fig. S7, after 6 h of static standing, the solution containing δ-B-Mo₂C@NC exhibits almost transparent color, whereas that containing β-Mo₂C@NC or NC shows incomplete discoloration. This observation is consistent with the intensity of the absorption band at 280 nm in the UV-Vis absorption spectra. These results confirm a stronger anchoring effect of Li₂S₆ species on the δ-B-Mo₂C@NC surface compared to the β-Mo₂C@NC surface. This phenomenon can be primarily due to the abundant adsorption sites on δ-B-Mo₂C@NC, along with an enhanced Mo 4d/S 3p orbital hybridization with LiPSs. The significant adsorption capability of δ-B-Mo₂C@NC effectively suppresses the shuttle effect and enhances sulfur utilization. To further evaluate the electrochemical catalytic performance of the two Mo₂C-based catalysts, symmetric cells were fabricated with identical catalyst loadings on both electrodes, using Li₂S₆ electrolyte as the electrolyte. As shown in Fig. 2e, the cyclic voltammetry (CV) curve of the δ-B-Mo₂C@NC-based cell exhibits higher peak currents and reduced voltage hysteresis compared to the β-Mo₂C@NC-based cell, indicating accelerated ion diffusion rate and electron transport efficiency for Li₂S₆ redox.^26,27 Furthermore, electrochemical impedance spectrum (EIS) results (Fig. 2f) reveal a significantly lower charge transfer impedance (R_CT) for the δ-B-Mo₂C@NC symmetric cell (137.7 Ω) than for its β-Mo₂C@NC counterpart (300 Ω). Therefore, adsorption tests and the symmetric cell results demonstrate that the δ-B-Mo₂C@NC electrocatalyst exhibits superior reaction kinetics during the Li₂S₆ conversion process.

We systematically evaluated the impact of different crystalline phases of Mo₂C-based catalysts on the electrochemical performance of LSBs using a carbon/sulfur composite as the cathode, a modified polypropylene (PP) membrane as the separator, and Li metal as the anode. CV was initially performed on LSBs equipped with a pristine PP separator, as well as those modified with δ-B-Mo₂C@NC and β-Mo₂C@NC, within a voltage window of 1.7–2.7 V. As depicted in Fig. 3a, all CV curves display two obvious reduction peaks (R_I: ∼2.3 V and R_II: ∼2.0 V) and one oxidation peak (O: ∼2.4 V), corresponding to the conversion of Li₂S₈ to Li₂S₄ and Li₂S₄ to Li₂S, and the reverse oxidation of Li₂S to S₈,^28–30 respectively. In contrast to PP and β-Mo₂C@NC, the LSB with the δ-B-Mo₂C@NC configuration shows the lowest polarization potential (ΔE = 443 mV) and the highest current response, suggesting more favorable reaction kinetics of sulfur species. Notably, during the reduction process, the R_I current peak is significantly enhanced and positively shifted upon the introduction of the δ-B-Mo₂C@NC modifier, indicating efficient conversion of long-chain LiPSs to Li₂S₂/Li₂S rather than promoting the shuttle effect.^31,32 EIS measurements of LSBs at different temperatures under the discharge state of 2.1 V confirm the reduced reaction energy barrier (E_a) associated with the conversion of Li₂S₄ to Li₂S, consistent with the aforementioned CV results. As shown in Fig. S8a–c, the fitting results demonstrate that the δ-B-Mo₂C@NC-based LSB exhibits a relatively lower R_CT across a temperature range of 298–328 K. Based on the Arrhenius equation (R_CT⁻¹ = Ae^−E_a/RT), the calculated E_a values are 0.27 eV for the δ-B-Mo₂C@NC-based battery and 0.37 eV for the β-Mo₂C@NC-based battery (Fig. S8d). The lower E_a value indicates enhanced catalytic activity of the δ-B-Mo₂C@NC towards the conversion from Li₂S₄ to Li₂S. Furthermore, the cycled Li anodes and separators were harvested from the LSBs to visually inspect the dissolution of LiPSs. As displayed in Fig. S9, the Li anode in the δ-B-Mo₂C@NC-based cell exhibited a markedly cleaner surface, which appeared free of visible deposits. This observation indicates the superior catalytic activity of the δ-B-Mo₂C@NC composite in facilitating efficient conversion of LiPSs, which effectively suppresses the shuttle effect and thus enhances the cycling stability of the LSB.

Fig. 3 (a) Cyclic voltammetry (CV) profiles of LSBs with various separators at 0.2 mV s⁻¹. (b) Cycling stability of LSBs at a current density of 0.2 A g⁻¹. (c)–(f) Rate performance and corresponding charge/discharge curves of the three LSBs with charging rates from 0.2 to 3 A g⁻¹. (g) Long-term cycling performance of the LSB with the δ-B-Mo₂C@NC modified separator at 1 A g⁻¹. (h) Cycling performance of δ-B-Mo₂C@NC-based LSB under high sulfur loading and lean electrolyte.

Fig. 3b shows the cycling performance of the LSBs with different separators at a current density of 0.2 A g⁻¹. The δ-B-Mo₂C@NC-based LSB demonstrates a superior reversible capacity of 735.6 mAh g⁻¹ after 100 cycles, with an average capacity decay rate of 0.54% per cycle. Additionally, the cell achieves and maintains a high Coulombic efficiency exceeding 99% after the initial activation cycles. In comparison, LSBs constructed with β-Mo₂C@NC and plain PP exhibit lower reversible capacities of 581.5 and 174.6 mAh g⁻¹ after 100 cycles, corresponding to higher decay rates of 0.65% and 0.77% per cycle, respectively. This performance degradation is primarily attributed to the shuttle effect of soluble LiPSs. Additionally, the rate capability and typical charge/discharge profiles of the LSBs were evaluated at various current densities (Fig. 3c–f). Compared to LSBs equipped with PP or β-Mo₂C@NC separators, the battery with a δ-B-Mo₂C@NC-based separator exhibited the best electrochemical performance, delivering specific capacities of 1207, 935, 736, 548, and 459 mAh g⁻¹ at rate densities of 0.2, 0.5, 1, 2, and 3 A g⁻¹, respectively. Notably, the δ-B-Mo₂C@NC battery achieved a significantly higher capacity and a smaller ΔE than its β-Mo₂C@NC counterpart, particularly at a high current density of 3 A g⁻¹. This superior performance is attributed to the enhanced adsorption of LiPSs and improved ion transport properties resulting from boron doping into Mo₂C. It is noteworthy that although the β-Mo₂C particles within the β-Mo₂C@NC composite are smaller, their catalytic performance is inferior to that of δ-B-Mo₂C@NC, which is attributed to the superior intrinsic electronic structure and higher density of active sites towards LiPSs in the δ-phase Mo₂C

Long-term cycling stability is crucial for the practical application of LSBs. The δ-B-Mo₂C@NC-based electrode demonstrated remarkable stability under demanding conditions (a sulfur loading of 2.5 mg cm⁻² and a current density of 1 A g⁻¹). As shown in Fig. 3g and Fig. S10, it sustained a long cycle life of 500 cycles while maintaining a stable reversible capacity of 366 mAh g⁻¹ (0.92 mAh cm⁻²), corresponding to a high capacity retention of 78% and a low capacity decay rate of only 0.045% per cycle (referencing to the 10th cycle). To achieve high energy density, the areal sulfur mass loading in the δ-B-Mo₂C@NC-based LSB was further increased to 4.0 mg cm⁻² while the electrolyte-to-sulfur (E/S) ratio was reduced to 10 µL mg⁻¹. As depicted in Fig. 3h, the battery delivered an initial discharge capacity of 421 mAh g⁻¹ (1.68 mAh cm⁻²) at a current density of 0.5 A g⁻¹ (2 mA cm⁻²). After 100 cycles, a stable capacity of 408 mAh g⁻¹ (1.63 mAh cm⁻²) was retained, corresponding to a high capacity retention of 97%. These improvements stem from the high density of active sites and the favorable structure of cubic δ-B-Mo₂C, tailored through boron doping. As a result, the δ-B-Mo₂C@NC modifier exhibits significantly improved electrical conductivity, optimized adsorption/desorption behavior, and accelerated charge transfer kinetics for LSBs.

To gain deeper insights into the alterations in sulfur reaction kinetics facilitated by the δ-B-Mo₂C@NC modifier, we performed CV measurements at multiple scan rates and applied Randles–Sevcik analysis to assess the Li⁺ diffusion capability.^33–36 The relationship is described by the following equation:

I_p = (2.69 × 10⁵)n^1.5AD^0.5Cv^0.5

where I_p is the peak current density, n is the number of electrons transferred, A is the area of the electrode, D is the Li⁺ diffusion coefficient, C is the concentration of Li⁺ in the electrolyte, and v is the scan rate.

As depicted in Fig. 4a–c, the δ-B-Mo₂C@NC cell demonstrates a higher current response across all scan rates compared to the control samples, particularly at the A_II and O peaks corresponding to the conversion of Li₂S₄ to Li₂S and Li₂S₄ to S₈, respectively.^37,38 These findings suggest that the majority of Li₂S₄ participates in the electrochemical reactions rather than migrating and shuttling to the Li anode.^39,40 Based on the Randles–Sevcik equation, the D values in the δ-B-Mo₂C@NC cell were found to be 1.02 × 10⁻⁸, 3.20 × 10⁻⁸, and 5.98 × 10⁻⁸ cm² s⁻¹ for A_I, A_II, and O peaks (Fig. 4d–f and Fig. S11), which is much higher than that in the β-Mo₂C@NC cell. This is attributed to the unique cubic crystal structure of δ-B-Mo₂C, which features three-dimensionally interconnected ion diffusion channels with a significantly lower energy barrier compared with β-Mo₂C.^21,22 Furthermore, the boron-doped δ-B-Mo₂C exhibits significant metallic conductivity. As a result, during the conversion between Li₂S₄ and Li₂S or between Li₂S₄ and S₈, electrons tend to be transferred from the conduction band of δ-B-Mo₂C to Li ions. This process not only accelerates redox reactions but also effectively screens the ionic charges, thereby facilitating ion diffusion and reducing the energy barriers associated with localized charge accumulation.^14–16,36 On the other hand, doped boron atoms are shown to form lower-energy B-Li bonds, thereby improving the kinetics of lithiation/delithiation processes.^21,22,41 The synergistic combination of these advantages in the δ-B-Mo₂C@NC modifier contributes to overall improvement in LSB performance (Table S1).

Fig. 4 (a)–(c) CV curves of δ-B-Mo₂C@NC, β-Mo₂C@NC, and PP-separator based LSBs measured at different scan rates. (d)–(f) Corresponding linear fitting results of the peak current versus the square root of the scan rate.

Conclusions

In summary, a hierarchical δ-B-Mo₂C@NC modifier was constructed through phase-transition engineering for synergistic adsorption–conversion regulation of LiPSs. The δ-B-Mo₂C@NC offers abundant adsorption sites and efficient electron/ion pathways, which effectively improve LiPS adsorption and promote sulfur conversion kinetics. Furthermore, the electronic structure of δ-B-Mo₂C was manipulated by upshifting the Mo d-band center, leading to strengthened Mo 4d/S 3p orbital hybridization at the δ-B-Mo₂C@NC interface during interaction with LiPSs, thereby accelerating sulfur redox reactions. As a result, LSBs equipped with the δ-B-Mo₂C@NC catalyst exhibit enhanced rate capability (459 mAh g⁻¹ at 3 A g⁻¹) and long-term stability (366 mAh g⁻¹ after 500 cycles at 1 A g⁻¹ with a low capacity decay of 0.045% per cycle). These results demonstrate the exceptional efficacy of δ-B-Mo₂C@NC in electrocatalytic sulfur redox reactions, highlighting its potential for applications in energy storage catalysis and beyond.

Experimental

Synthesis of δ-B-Mo₂C@NC nanostructures

Firstly, the Mo-polydopamine (Mo-PDA) hybrid was prepared via a self-polymerization method under alkaline conditions. Briefly, ammonium molybdate tetrahydrate (250 mg) was dissolved in 80 mL of deionized water. Dopamine hydrochloride (300 mg) was then added to this solution under stirring. Subsequently, 150 mL of ethanol and 400 µL of aqueous ammonia (NH₃·H₂O) were added to the above solution. After continuous stirring for 2 h, the resulting orange Mo-PDA was washed several times with ethanol and vacuum-dried at 60 °C. After that, the as-obtained Mo-PDA was carbonized at 600 °C for 2 h under an argon atmosphere, affording MoO₂@NC. Finally, to synthesize δ-B-Mo₂C@NC, a homogeneous mixture of MoO₂@NC and boric acid (mass ratio 1 : 2) was thoroughly ground and then annealed under an argon atmosphere. The annealing process involved heating to 900 °C at 5 °C min⁻¹, maintaining this temperature for 2 h. For comparison, β-Mo₂C@NC was synthesized following the same procedure without adding the boric acid source.

Characterization methods

The morphological analysis of the catalysts was conducted with a scanning electron microscope (SEM, ZEISS GeminiSEM 360). High-resolution transmission electron microscopy (TEM) imaging and selected area electron diffraction (SAED) were performed using a JEOL JEM-2100Plus TEM system operating at 200 kV. High-angle annular dark-field scanning TEM (HAADF-STEM) images and corresponding elemental mappings were performed using a FEI Tecnai G2 F30 electron microscope equipped with an energy-dispersive X-ray spectroscope (EDS) system. The phase and crystalline properties of δ-B-Mo₂C@NC and β-Mo₂C@NC were examined on a X-ray diffraction (XRD, Rigaku SmartLab SE) diffractometer equipped with Cu-Kα radiation (λ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha) with a monochromatic Al Kα X-ray source (hν = 1486.6 eV) was used to analyze the electronic binding energies of samples. All binding energies were referenced to the C 1s peak at 284.8 eV. The UV-vis absorption curves of Li₂S₆ solutions were measured using a UV-vis spectrophotometer (HITACHI UH4150) within the wavelength range of 250-400 nm.

Fabrication of the δ-B-Mo₂C@NC-modified separator

The synthesized δ-B-Mo₂C@NC powder and poly(vinylidene difluoride) (PVDF) in a mass ratio of 9 : 1 were dispersed in N-methyl-2-pyrrolidone (NMP) under continuous grinding. Then, the above slurry was uniformly coated on one side of the polypropylene (PP) membrane (Celgard 2400), followed by vacuum drying at 50 °C for 12 h. After that, the obtained δ-B-Mo₂C@NC-modified film was cut into circular discs with a diameter of 19 mm for further use. The mass loading of the modifier on PP is controlled at approximately 0.5 mg cm⁻². Similarly, a β-Mo₂C@NC-modified separator with the same mass loading was prepared according to the same procedure as the reference samples.

Fabrication of the sulfur cathode

Sulfur powder, acetylene black, and PVDF were sufficiently ground at a weight ratio of 6 : 3 : 1, dispersed in NMP, and spread onto aluminum foil to obtain the sulfur cathode. Sulfur loading mass was around 0.8 mg cm⁻². For the fabrication of high-loading sulfur cathode (m_sulfur > 2 mg cm⁻²), commercial carbon cloth (CC) was used as a current collector. The sulfur slurry was cast onto CC sheets by doctor blading and vacuum dried at 60 °C for 12 h.

Visualized adsorption of Li₂S₆ and measurement of symmetric cells

The Li₂S₆ solution was prepared by dissolving sulfur and Li₂S at a molar ratio of 5 : 1 in a mixed solution of 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) (volume ratio of 1 : 1). To compare the adsorption capability between different catalysts and LiPSs, 50 mg of δ-B-Mo₂C@NC and β-Mo₂C@NC powders were added into 5 mL of the as-obtained Li₂S₆ solution (2 mM), respectively. After standing for 6 h, the supernatants were further characterized by UV-vis spectroscopy. The mixed slurry, composed of 90% δ-B-Mo₂C@NC (or β-Mo₂C@NC) and 10% PVDF, was cast onto an aluminum foil and vacuum-dried at 60 °C to fabricate the electrode with a mass loading of 1.0 mg cm⁻². Symmetric cells were then assembled in CR2032 coin-type configurations, using the prepared electrodes as both working and counter electrodes, a PP membrane as the separator, and 40 µL of 0.2 M Li₂S₆ solution as the electrolyte. Cyclic voltammetry (CV) and EIS measurements were performed on a CHI 660E electrochemical workstation. The CV tests were conducted at a scan rate of 1 mV s⁻¹ over a voltage window of -1 to 1 V. The EIS measurements were carried out in the frequency range from 0.1 to 10⁵ Hz with a 5 mV perturbation.

Electrochemical measurements

CR2032 coin-type LSBs were assembled in an argon-filled glovebox (H₂O < 0.1 ppm, O₂ < 0.1 ppm, Mikrouna) using the prepared sulfur cathode, modified PP separator, and Li metal anode. The electrolyte was 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved in a mixed solvent of DME and DOL (v/v = 1/1) with 2.0 wt% LiNO₃ addition. Galvanostatic discharge/charge curves and rate performance tests of LSBs were carried out on a Land battery cycler (LAND CT-2001A Instrument). The specific capacities of all LSBs were calculated based on the mass of sulfur in the cathode. The CV curves were obtained at different scanning rates using an electrochemical workstation (CHI 660E) within a voltage range of 1.7–2.7 V versus Li⁺/Li. The cycled LSB was disassembled in an Ar-filled glovebox.

Author contributions

Pengqian Guo designed the project and wrote the manuscript. Jing Lin and Wenxuan Hu performed the synthesis and structural characterization of the materials. Jinchi Huang and Pangquan Huang carried out the electrochemical experiments. Weixin Chen contributed to material analysis and data interpretation. Xiuwan Li, Xia Lu, and Xinhua Guo conceived the concept and supervised the research. All authors discussed the results and approved the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

All data supporting the findings of this study within the article and its supplementary information (SI) are available from the corresponding author, upon reasonable request. Supplementary information: a schematic illustration of B-doping engineering, SEM/TEM images of MoO₂@NC and β-Mo₂C@NC, wide-scan XPS spectra of δ-B-Mo₂C@NC/β-Mo₂C@NC, adsorption test result of NC, temperature-dependent EIS data for LSBs, optical images of the cycled Li anodes, and Li⁺ diffusion coefficients in LSBs with different separators. See DOI: https://doi.org/10.1039/d5nh00712g.

Acknowledgements

This work was supported by the General Project of the National Natural Science Foundation of China (52277049); the National Key Research and Development Program of China (2022YFB2502802); the Xiamen City's Universities and Research Institutes Industry-University-Research Projects (2024CXY0231), and the Scientific Research Funds of Huaqiao University (605-50Y24011).

Notes and references

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