English

• 0.   Introduction

  Lithium-sulfur batteries (LSBs), with their exceptional theoretical energy density of 2 600 Wh·kg^-1 and naturally abundant sulfur cathode resources, are regarded as a leading candidate for next-generation energy storage systems^[1-2]. However, LSBs based on the solid-liquid-solid reaction mechanism face a fundamental and significant challenge: the detrimental "shuttle effect" of lithium polysulfides (LiPSs)^[3-4]. During charge/discharge cycles, soluble long-chain LiPSs (e.g., Li₂S₆, Li₂S₄) migrate towards the lithium anode under concentration gradients. This migration leads to parasitic reactions with metallic lithium, causing irreversible active material loss, abnormal electrolyte consumption, and anode corrosion^[5-7]. Concurrently, sluggish sulfur redox kinetics further exacerbate battery performance degradation^[8]. The insulating nature of the discharge end-product (Li₂S) and the formation of passivation layers on the electrode surface significantly impede charge transfer efficiency^[9-10]. Furthermore, the high decomposition energy barrier of Li₂S during charging creates reaction kinetic imbalances^[11-12]. Collectively, these factors result in rapid capacity fading and insufficient cycle life for LSBs, severely hindering their commercialization.

  To overcome these limitations, researchers have proposed a functional separator modification strategy. This involves constructing a functional coating on the polypropylene (PP) separator surface to achieve dual regulation of LiPSs: physical confinement to block LiPSs diffusion, and chemical adsorption coupled with electrocatalytic conversion to accelerate their redox kinetics^[13-15]. Initial research focused on single-component modifiers. For instance, metal oxides anchor LiPSs via Lewis acid-base interactions on their highly polar surfaces, but their poor intrinsic conductivity (< 10^-5 S·cm^-1) limits charge-transfer efficiency^[16-17]. Transition metal sulfides exhibit excellent catalytic activity and ion diffusion capability, yet generally possess low adsorption energies for long-chain LiPSs, failing to effectively suppress the shuttle effect^[18]. Carbon-based materials (e.g., graphene, carbon nanotubes, graphdiyne) offer good conductivity but suffer from weak adsorption due to their non-polar surfaces, rendering them ineffective as standalone solutions^[19-20]. Clearly, single-component materials struggle to simultaneously fulfill the synergistic requirements of strong adsorption, efficient catalysis, and rapid charge transport. Consequently, developing novel modifier layers with integrated multifunctionality has become a key research focus, with heterojunction catalytic materials demonstrating significant promise^[21-22].

  Recent studies reveal that precisely engineered heterointerfaces can induce electron structure reconstruction between components, generating synergistic catalytic effects surpassing those of individual constituents^[23-25]. MoS₂ and MoO₃ possess inherent lattice compatibility and complementary band structures. Theoretically, their interface can enhance charge transfer via a built-in electric field, optimizing the dual functionality of LiPSs adsorption and catalysis^[26]. It is worth noting that the MoS₂-MoO₃ heterointerface integrates the adsorption capability of MoO₃ with the catalysis of MoS₂, achieving a smooth transition from physical adsorption to electrochemical catalytic conversion. This synergistic effect not only effectively suppresses the shuttle effect in the first stage but also significantly enhances the reversibility of Li₂S oxidation-reduction in the second stage, thereby comprehensively improving the capacity, rate performance, and cycle life of LSBs. The design essence lies in the spatial proximity of functional components and the synergistic modulation of the interfacial electronic structure^[27-29]. However, heterostructures fabricated by conventional physical mixing or stepwise deposition often exhibit insufficient interfacial contact and limited exposure of active sites, significantly limiting their maximum catalytic efficacy.

  Addressing this heterointerface engineering bottleneck, we propose an innovative in-situ oxidation strategy for the one-step synthesis of MoS₂-MoO₃ heterojunctions. Compared to existing techniques, this method offers distinct advantages: (ⅰ) using MoS₂ as a precursor, surface Mo atoms are selectively oxidized to MoO₃ under controlled conditions, forming a coherent MoS₂-MoO₃ interface. This avoids phase separation issues inherent in traditional methods and maximizes interfacial electronic coupling. (ⅱ) The combination of MoO₃'s strong adsorption capability and MoS₂'s high catalytic activity simultaneously addresses the shuttle effect and sluggish kinetics. Performance validation confirms that LSBs employing this modified separator achieved a high capacity of 402.9 mAh·g^-1 after 1 000 cycles at 1C, with a low capacity decay rate of only 0.05% per cycle. This work not only provides a novel material system for battery separator design but also deepens understanding of the catalytic mechanisms governed by heterointerface electronic-structure modulation, paving the way for the development of efficient and long-lifespan energy-storage devices.

  1.   Experimental

  1.1   Materials

  All chemical reagents used in this work were commercially sourced and used as received without further purification, unless otherwise specified. Sodium molybdate dihydrate (Na₂MoO₄·2H₂O, AR) was purchased from China National Pharmaceutical Group Chemical Reagent Co., Ltd. Thiourea (CH₄N₂S, AR), sulfur powder (S, AR), and lithium sulfide (Li₂S, AR) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Multi-walled carbon nanotubes (CNTs, 20-30 nm) were sourced from Nanjing XFNANO Materials Tech Co., Ltd. Carbon paper (HCP020N) was purchased from Shanghai Jinchong Electronic Technology Development Co., Ltd. Deionized water was used in all experimental procedures.

  1.2   Synthesis of MoS₂

  MoS₂ was synthesized via a hydrothermal method. Specifically, 1.21 g of Na₂MoO₄·2H₂O and 1.9 g of CH₄N₂S were dissolved in 70 mL of deionized water under magnetic stirring for 1 h. The resulting homogeneous solution was transferred to a 100 mL Teflon-lined stainless-steel autoclave and maintained at 200 ℃ for 24 h. The obtained black precipitate was then collected, washed thoroughly with deionized water and absolute ethanol several times, and dried in an oven at 60 ℃ under vacuum.

  1.3   Synthesis of the MoS₂-MoO₃ heterojunction catalyst

  The as-prepared MoS₂ powder was finely ground and subsequently subjected to thermal oxidation in a muffle furnace at 300 ℃ under ambient air for 2 h, yielding the MoS₂-MoO₃ heterojunction catalyst. For comparative analysis, pure-phase MoO₃ was synthesized under identical conditions by increasing the oxidation temperature to 350 ℃.

  1.4   Material characterization

  X-ray diffractometer, X′Pert3Powder, PANalytical, Netherlands), with filtered Cu Kα radiation (λ=0.154 06 nm, 40 kV, 40 mA, as well as the scanning range of 5°-90°) was utilized to investigate the crystal structures of the obtained samples. Raman spectra were collected on a Raman Microscope (HORIBA Scientific LabRAM HR Evolution) equipped with a 532 nm laser. Scanning electron microscopy (SEM) images were obtained using the FEI Nova NanoSEM 450 field emission scanning electron microscope. Transmission electron microscopy (TEM) images were obtained using the JEM-2100F transmission electron microscope (accelerating voltage: 200 kV). Energy dispersive X-ray spectra (EDS) and elemental mappings were recorded by the JEM-2100F apparatus at acceleration voltages of 200 kV. X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, ESCALAB 250Xi) was used to determine the surface chemical composition and the valence states of the prepared samples. The binding energy was corrected using the C1s peak at 284.8 eV. Thermogravimetric (TG) analysis was carried out using STA499F5 thermogravimetric analyzer, and the temperature range was from room temperature to 800 ℃ with a heating rate of 10 ℃ min^-1.

  1.5   Fabrication of sulfur cathode and modified separators

  The sulfur cathode (S/CNTs) was prepared by thoroughly grinding a mixture of sublimed sulfur powder and CNTs at a weight ratio of 4∶1. The mixture was then heated in a horizontal tube furnace under high-purity nitrogen flow at 155 ℃ for 10 h. To remove excess surface sulfur, the temperature was further increased to 200 ℃ for 30 min. The sulfur content in the obtained S/CNTs composite was determined by TG analysis.

  For separator modification, functional coatings were applied to commercial PP separators via a slurry casting process. A slurry was prepared by dispersing MoS₂-MoO₃, Super P conductive carbon black, and polyvinylidene fluoride (PVDF) binder (mass ratio 8∶1∶1) in N-methyl-2-pyrrolidone (NMP). The slurry was uniformly coated onto the PP separator, vacuum-dried, and punched into disks (19 mm diameter). For control experiments, MoS₂-modified PP and MoO₃-modified PP separators were similarly prepared.

  1.6   Preparation of Li₂S₆ solution and adsorption tests

  5 mmol·L^-1 Li₂S₆ solution was prepared in an argon-filled glovebox (the contents of H₂O and O₂ were below 10^-7) by dissolving stoichiometric amounts of sublimed sulfur and Li₂S (the molar ratio of 5∶1) in a mixed solvent of 1, 2-dimethoxyethane (DME) and 1, 3-dioxolane (DOL) (1∶1, V/V). The mixture was magnetically stirred at 60 ℃ overnight until a homogeneous orange-yellow solution formed. For adsorption evaluation, 10 mg of MoS₂-MoO₃, MoS₂, or MoO₃ powder was separately added to 3 mL of the Li₂S₆ solution. After stirring for an identical duration and allowing sedimentation, the visual color change of the supernatant was recorded. The adsorption capacity was assessed using ultraviolet-visible (UV-Vis) absorption spectroscopy of the collected supernatant.

  1.7   Li-S battery assembly and electrochemical testing

  CR2025-type coin cells were assembled for electrochemical evaluation. The S/CNT composite cathode material was mixed with PVDF binder and Super P (8∶1∶1 by weight) in NMP to form a slurry, which was coated onto an aluminum foil current collector, vacuum-dried overnight at 45 ℃, and cut into 12 mm diameter electrodes. Li foil served as the counter electrode. The separators employed were: (ⅰ) pristine PP, (ⅱ) MoS₂-MoO₃-modified PP, (ⅲ) MoS₂-modified PP, and (ⅳ) MoO₃-modified PP. The electrolyte consisted of 1.0 mol·L^-1 lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in DOL/DME (1∶1, V/V) with the mass fraction of 1% LiNO₃ additive. Unless otherwise specified, the electrolyte/sulfur ratio was consistently 10 μL·mg^-1. All assembly steps were performed inside the argon glovebox.

  Galvanostatic charge/discharge cycling was conducted within a voltage window of 1.7-2.8 V using a LAND-CT2001A battery test system. Specific capacities were calculated based on the sulfur mass in the cathode. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed on a CHI660E electrochemical workstation. CV scans were recorded at 0.1 mV s^-1. EIS spectra were acquired over a frequency range of 10^-2 to 10⁵ Hz with an amplitude of 5 mV.

  1.8   Li₂S nucleation and dissolution experiments

  0.2 mol·L^-1 Li₂S₈ solution was prepared by dissolving sulfur and Li₂S (the molar ratio of 7∶1) in tetraglyme within the glovebox. Working electrodes were fabricated by ultrasonically dispersing 0.5 mg of the catalyst in 3 mL of ethanol, drop-casting the dispersion onto carbon paper, and drying. CR2025 coin cells were assembled using Li foil as the counter/reference electrode. 25 μL of the Li₂S₈ solution was added to the working electrode side, and 25 μL of standard LiTFSI-based electrolyte was added to the Li side.

  Li₂S deposition: the cell was first discharged galvanostatically at 0.112 mA to 2.06 V, followed by a potentiostatic hold at 2.05 V until the current decayed below 10^-5 A. The deposited Li₂S capacity was calculated by integrating the discharge curve using Faraday′s law.

  Li₂S dissolution: the cell was discharged at 0.112 mA to 1.70 V to convert soluble polysulfides to solid Li₂S. Subsequently, it was charged potentiostatically at 2.40 V until the current fell below 10^-5 A. The dissolved Li₂S capacity was similarly determined by curve integration.

  1.9   Assembly and testing of symmetric cells

  MoS₂-MoO₃, MoS₂, or MoO₃ was mixed with PVDF at a weight ratio of 9∶1 in NMP solvent. The resulting slurry was coated onto Al foil current collectors using a doctor blade, dried, and cut to serve as working electrodes for later use. In the glovebox, S and Li₂S were dissolved in a 1 mol·L^-1 LiTFSI solvent (DOL and DME with a volume ratio of 1∶1) at a molar ratio of 5∶1, and stirred overnight at 65 ℃ to prepare a 0.5 mol·L^-1 Li₂S₆ solution, which was used as the electrolyte for symmetric cells. Symmetric cells were assembled using two identical working electrodes with a mass loading of 1.0 mg·cm^-2, and 25 μL of electrolyte was added. CV measurements were performed on the symmetric cells at different scan rates, with a voltage range of -1.0 to 1.0 V.

  1.10   Theoretical calculation method

  Density functional theory (DFT) calculations were carried out using the Vienna ab initio simulation package (VASP). Ion-electron interactions were described using the projector augmented-wave (PAW) pseudopotential method. The generalized gradient approximation in the Perdew-Burke-Ernzerhof formalism revised for solids (GGA-PBEsol) was used to model the exchange-correlation effects. A plane-wave cutoff energy of 500 eV was applied. Van der Waals (vdW) interactions were corrected using Grimme′s DFT-D3 method. Convergence criteria were set to 0.5 eV·nm^-1 for ionic relaxation and 1×10^-5 eV for electronic optimization. The Brillouin zone was sampled using Monkhorst-Pack k-point meshes of 2×1×1 for geometry optimization, and denser meshes of 3×2×1 for electronic property calculations.

  2.   Results and discussion

  2.1   Morphology and structure analysis

  Fig. 1a illustrates the schematic fabrication process of the MoS₂-MoO₃ heterostructure catalyst and its application in LSBs. The synthesis procedure is remarkably straightforward: pure-phase MoS₂ is initially prepared through a hydrothermal reaction of sodium molybdate and thiourea, followed by controlled thermal oxidation to achieve partial oxidation of MoS₂, yielding a MoS₂-MoO₃ heterostructure. For comparison, complete oxidation of MoS₂ can also be performed to obtain pure-phase MoO₃. The as-prepared catalytic material was then coated onto commercial PP separators via a doctor-blade method for subsequent LSBs assembly and electrochemical evaluation. The coating thickness was about 7.8 μm, as measured by a micrometer and confirmed via cross-sectional SEM (Fig.S1a and S1b, Supporting information).

  Figure 1

  

  Figure 1.  (a) Schematic diagram of the preparation of MoS₂-MoO₃ and the separator modification; CA test of electrolyte on (b) MoS₂-MoO₃ modified PP separator and (c) PP separator; (d, e) TEM images of MoS₂-MoO₃; (f) HRTEM image of MoS₂-MoO₃ (left) and corresponding lattice stripe spacing (right); (g) EDS spectrum of MoS₂-MoO₃; (h) TEM image of MoS₂-MoO₃ and (i-l) corresponding elemental distribution mappings

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  Contact angle (CA) measurements of the electrolyte on modified and pristine PP separators are presented in Fig. 1b and 1c, respectively. The electrolyte exhibited a CA of 42.3° on the pristine PP separator (Fig. 1c), which significantly decreased to 9.5° after modification with the MoS₂-MoO₃ layer (Fig. 1b), demonstrating enhanced electrolyte wettability that facilitates lithium-ion transport. Combined characterization by SEM (Fig.S2) and TEM (Fig. 1d and 1e) reveals that the partially oxidized MoS₂-MoO₃ samples largely retain the morphological characteristics of MoS₂ (i.e., two-dimensional nanosheet assemblies, Fig.S3). In contrast, complete oxidation results in MoO₃ with distinctly different morphology (Fig.S4). This structural evolution arises because the layered architecture of MoS₂ relies on intralayer Mo—S covalent bonds and interlayer van der Waals forces, whereas oxidation disrupts this framework, transforming it into a three-dimensional network of Mo—O covalent bonds that ultimately forms aggregated bulk structures.

  Fig. 1f displays the high-resolution TEM (HRTEM) image of MoS₂-MoO₃, where measured lattice spacings of 0.20 and 0.27 nm correspond to the (200) plane of MoO₃ and (100) plane of MoS₂, respectively. EDS analysis (Fig. 1g) confirms the exclusive presence of Mo, S, O, and Cu (the latter originating from the TEM grid) without detectable impurities. Furthermore, elemental mapping (Fig. 1h-1l) clearly reveals that O distribution was predominantly localized at the material′s periphery, indicating that the oxidation process progresses gradually from the surface toward the interior of MoS₂.

  Fig. 2a shows the X- ray diffraction (XRD) patterns of MoO₃, MoS₂, and MoS₂-MoO₃. The diffraction peaks of MoS₂-MoO₃ obtained via partial oxidation correspond to orthorhombic MoO₃ (PDF No.05-0508) and hexagonal 2H-MoS₂ (PDF No.37-1492), with no detectable impurity phases. This confirms the successful construction of the heterojunction with high phase purity. The well-defined peak profiles suggested that the crystal structures remained largely undistorted upon heterojunction formation, providing a solid structural basis for subsequent investigation of interfacial interactions.

  Figure 2

  

  Figure 2.  (a) XRD patterns and (b) Raman spectra of MoO₃, MoS₂, and MoS₂-MoO₃; (c) XPS survey spectrum of MoS₂-MoO₃; High-resolution (d) Mo3d, (e) S2p, and (f) O1s XPS spectra of the samples

  Inset: corresponding enlarged pattern of MoS₂-MoO₃.

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  Fig. 2b displays the Raman spectra of the three samples. The characteristic vibrational modes of MoS₂ (E¹_2g and A_1g) exhibited shifts in wavenumber in the heterostructure—for instance, the E¹_2g peak shifts slightly to a higher wavenumber—indicating that the MoS₂ layers are under compressive stress, likely due to lattice constraint or electronic coupling from MoO₃^[30]. Additionally, the characteristic peak of MoO₃ (e.g., the stretching vibration at ca. 820 cm^-1) broadens in the heterostructure, suggesting interface-induced local disordering^[31]. These changes in Raman vibrational modes provide direct evidence of cross-phase mechano-electric coupling at the heterointerface, which may influence the conversion kinetics of LiPSs.

  To investigate the chemical states and elemental composition on the sample surfaces, XPS measurements were conducted on MoS₂-MoO₃, MoS₂, and MoO₃. The survey spectra confirm the presence of Mo, S, and O (along with signals from the carbon substrate), with no impurity-related peaks, affirming the chemical purity of the samples (Fig. 2c, S5). Fig. 2d shows the high-resolution Mo3d XPS spectra, which can be deconvoluted into spin-orbit doublets corresponding to Mo⁴⁺ (ca. 230.8/228.1 eV) and Mo⁶⁺ (ca. 234.9/231.7 eV) for MoS₂ and MoO₃^[32-33]. In the heterostructure, both doublets shift relative to those in the pure phases—for example, the Mo⁴⁺ peaks shift by ca. 0.5 eV toward higher binding energy—suggesting electron transfer between MoS₂ and MoO₃^[34].

  Furthermore, the S2p doublet (S2p_3/2 and S2p_1/2) in the heterostructure exhibited a chemical shift toward higher binding energy (ca. 0.2 eV), as shown in Fig. 2e, indicating a modified chemical environment for S due to the incorporation of MoO₃. This shift is attributed to interfacial electron redistribution, where S acts as an electron donor to MoO₃, resulting in reduced electron density on S and potentially enhancing the adsorption capability for LiPSs^[35]. The O1s spectrum (Fig. 2f) can be fitted to Mo—O bonds and surface hydroxyl groups (—OH). In the heterostructure, the Mo—O peak shift to higher binding energy, and the proportion of hydroxyl species increases, suggesting surface hydroxylation of MoS₂ induced by MoO₃. Such evolution of oxygen species may influence both electrolyte wettability and the availability of catalytically active sites on the material surface^[36].

  2.2   Electrochemical performance

  To evaluate the adsorption capability of MoS₂-MoO₃ toward LiPSs, a series of adsorption tests was conducted. Fig. 3a represents the UV-Vis adsorption spectra of the supernatant collected after 12 h of static adsorption in a Li₂S₆ solution, using identical masses of each catalyst (except the blank control). The insets in Fig. 3a and S6 show optical photographs taken at different adsorption time points. It can be observed that the absorbance of MoS₂-MoO₃ was significantly lower than that of pure-phase MoS₂ and MoO₃, approaching even the baseline level. The optical images further reveal that the solution containing the heterostructure exhibited the highest transparency (lightest color), indicating superior LiPSs adsorption capability. This enhanced adsorption provides a structural foundation for subsequent improvements in electrochemical performance.

  Figure 3

  

  Figure 3.  (a) UV-Vis adsorption spectra and the corresponding optical images (inset) of Li₂S₆ solutions adsorbed by different catalysts; (b) TG curve of the S/CNTs cathode; (c) CV curves of the first three cycles and (d) first three charge/discharge curves at 0.1C for the LSB with the MoS₂-MoO₃ modified PP separator

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  In addition, XPS analysis on MoS₂-MoO₃ after Li₂S₆ adsorption was also performed (Fig.S7). It was clearly observed that the characteristic peaks of Mo, S, and O exhibited significant shifts toward higher binding energies after the interaction between MoS₂-MoO₃ and Li₂S₆. It is evidenced that the Mo⁶⁺ ions on the MoO₃ surface engage in strong Lewis acid-base interactions (Mo⁶⁺-S_x^2-) with terminal S atoms in Li₂S₆ while undergoing partial reduction, which is in line with the following DFT calculations. This process effectively anchors soluble LiPSs onto the modified separator, thus suppressing the shuttle effect.

  To assess the electrochemical performance, MoS₂-MoO₃, along with reference pure-phase catalysts, was coated onto PP separators. Coin cells were then assembled as simulated LSBs, with S/CNTs composites serving as the cathode. The sulfur loading was determined to be 78% by TG analysis (Fig. 3b). Fig. 3c displays the first three CV curves of the LSB with the MoS₂-MoO₃ modified PP separator at a scan rate of 0.1 mV·s^-1. Compared to MoS₂ and MoO₃ (Fig.S8c and S8d), the LSB with the MoS₂-MoO₃ modified PP separator exhibited sharper reduction peaks at approximately 2.2 and 2.0 V, a more distinct oxidation peak around 2.4 V, and higher current densities. Moreover, the LSB with the MoS₂-MoO₃ modified PP separator demonstrated the smallest polarization voltage (0.12 V, Fig. 3d), whereas those of MoS₂ and MoO₃ reached 0.24 V (Fig.S8a) and 0.35 V (Fig.S8b), respectively. These CV results suggest that the heterointerface synergistically boosts electron transfer and reduces the energy barrier for LiPSs conversion.

  Fig. 4a presents the rate performance of LSBs with the three modified PP separators. At 0.1C, 0.3C, 0.5C, 0.7C, 1C, 2C, 3C, and 5C, the specific capacities of the LSB with the MoS₂-MoO₃ modified PP separator were 1245.3, 922.2, 791.2, 726.1, 632.1, 499.6, 414.6, and 299.5 mAh·g^-1, respectively. When the rate was reset to 0.1C, a reversible capacity of 940.9 mAh·g^-1 was recovered. The discharge/charge curves of the LSB with the MoS₂-MoO₃ modified PP separator at various rates are shown in Fig. 4b. Even at a high rate of 5C, well-defined plateaus were maintained, indicating that the synergistic effect of "interfatalytic centers" and "fast electron channels" in the heterostructure enables efficient LiPSs conversion under high rate, thereby preventing plateau collapse. Long-term cycling stability tests of LSBs with the three modified separators were performed at different rates (Fig. 4c and S9). The LSB with the MoS₂-MoO₃ modified PP separator delivered higher specific capacities than its pure-phase counterparts, both at 0.1C and 1C. After 1 000 cycles at 1C, it retained a specific capacity of 402.9 mAh·g^-1, outperforming that of MoS₂ (316.4 mAh·g^-1) and MoO₃ (290.7 mAh·g^-1). In addition, we characterized the anodes of the cells after 50 cycles at 0.1C, as shown in Fig.S10. The Li foil of the battery modified with MoS₂-MoO₃ exhibited the mildest corrosion, which indirectly demonstrates that MoS₂-MoO₃ can effectively suppress the shuttling of LiPSs intermediates in comparison with MoS₂ and MoO₃.

  Figure 4

  

  Figure 4.  (a) Rate capability at various rates and (c) long cycling stability of LSBs with MoS₂, MoO₃, and MoS₂-MoO₃ modified PP separators; (b) Charge/discharge curves of the LSB with the MoS₂-MoO₃ modified PP separator at various rates

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  Fig.S11 and 5a show the EIS of LSBs with different separators. Before cycling, charge transfer resistances (R_ct) corresponding to MoS₂-MoO₃, MoS₂, and MoO₃ separators were 151.2, 239.2, and 187.8 Ω. After 50 cycles, the values changed to 65.9, 166.7, and 377.6 Ω, respectively. The LSB with the MoS₂-MoO₃ separator exhibited the lowest overall impedance, indicating the most favorable electrochemical kinetics. The abundant heterointerfaces not only facilitate rapid ion/electron transport but also enhance interaction with LiPSs, thereby accelerating the redox kinetics of LiPSs^[37-39]. The catalytic capability of MoS₂-MoO₃ for LiPSs conversion was further examined using symmetric cells (Fig. 5b). Compared to MoS₂@Li₂S₆ and MoO₃@ Li₂S₆, the CV curve of MoS₂-MoO₃@Li₂S₆ exhibited higher peak currents and greater capacity, indicating more efficient catalytic conversion of LiPSs.

  Figure 5

  

  Figure 5.  (a) EIS of LSBs with MoS₂, MoO₃, and MoS₂-MoO₃ modified PP separators after 50 cycles; (b) CV curves of Li₂S₈ symmetric cells with different catalysts

  Inset: (a) the corresponding equivalent circuit diagram, where R_s stands for solution resistance, CPE stands for constant phase angle component, Z_W stands for Warburg impedance, and R_SEI stands for resistance of solid electrolyte interface (SEI) film; (b) Enlarged CV curves of Li₂S₈ symmetric cells with MoS₂ and MoO₃ catalysts.

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  Ion diffusion rate is a critical factor influencing the redox reaction kinetics of LiPSs. Fig. 6a, along with Fig.S12a and S12c, presents CV curves obtained at scan rates ranging from 0.1 to 0.5 mV s^-1, which were used to evaluate the lithium-ion diffusion capability and reaction kinetics during phase transformation. It can be observed that the redox currents of MoS₂-MoO₃, MoS₂, and MoO₃ electrodes increased with increasing scan rate, consistent with previous reports^[40-41]. Further linear analysis of the peak current (i_p) versus the square root of the scan rate (v^0.5) for each curve (Fig. 6b, S12b, and S12d) was conducted to determine the lithium-ion diffusion coefficients during the redox processes. The LSB with the MoS₂-MoO₃ modified PP separator exhibited steeper slopes in both the oxidation process (solid-liquid transition, Peak 1) and the reduction process (liquid-solid transition, Peak 3), indicating faster lithium-ion diffusion and improved electrochemical reversibility. Fig. 6c and 6d show Tafel plots and the corresponding fitted Tafel slopes derived from the CV curves (Fig.S13) of LSBs assembled with the three modified separators. Clearly, the LSB with MoS₂-MoO₃ modified PP separator demonstrates the smallest Tafel slope, further confirming the superior capability of the MoS₂-MoO₃-modified separator in facilitating the conversion of sulfur species.

  Figure 6

  

  Figure 6.  (a) CV curves at various scan rates and (b) i_p vs v^0.5 curves of the LSB with MoS₂-MoO₃ modified PP separator; Corresponding Tafel plots at (c) anodic peak 1 and (d) cathodic peak 2

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  Subsequently, the influence of the constructed heterointerface on catalytic activity was further investigated by analyzing the liquid-solid-liquid transitions associated with the deposition and dissolution of Li₂S on different catalytic materials, as shown in Fig. 7a-7d, S13, and S14. Potentiostatic discharge experiments were performed to record the nucleation and deposition processes of Li₂S. The results reveal that the Li₂S deposition capacity on MoS₂-MoO₃ reached 216.6 mAh·g^-1 (Fig. 7a), significantly exceeding those on MoO₃ (108.9 mAh·g^-1, Fig. 7b) and MoS₂ (55.7 mAh·g^-1, Fig.S14). These findings suggest faster nucleation and more efficient deposition of Li₂S on MoS₂-MoO₃, demonstrating its enhanced catalytic effect toward Li₂S nucleation and deposition. Moreover, in the corresponding potentiostatic charging experiments for Li₂S dissolution, the dissolution capacity on MoS₂-MoO₃ was 389.1 mAh·g^-1 (Fig. 7c), considerably higher than that on MoO₃ (261.6 mAh·g^-1, Fig. 7d) and MoS₂ (228.9 mAh·g^-1, Fig.S15). This indicates that Li₂S is more readily oxidized and decomposed under the electrocatalytic effect of MoS₂-MoO₃, thereby promoting the bidirectional catalytic conversion of sulfur.

  Figure 7

  

  Figure 7.  Potentiostatic nucleation of Li₂S with (a) MoS₂-MoO₃ and (b) MoO₃; Potentiostatic profiles of Li₂S dissolution with (c) MoS₂-MoO₃ and (d) MoO₃

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  2.3   DFT calculation

  In addition, DFT calculations were performed to systematically investigate the chemical synergistic adsorption effect of the MoS₂-MoO₃ heterostructure on LiPSs. Fig. 8a and 8b present the adsorption energies of the three catalytic materials toward various sulfur species and the reaction energy barriers for the stepwise conversion of S₈ to Li₂S (optimized structures of the catalysts are shown in Fig.S16). As illustrated in Fig. 8a, MoS₂-MoO₃ exhibited stronger binding affinity toward most intermediate species except S₈, particularly for short-chain Li₂S₂ and solid Li₂S, with adsorption energies (E_ads) significantly higher than those of pure-phase MoO₃ and MoS₂. In LSBs, the liquid-solid conversion (Li₂S₄→Li₂S₂→Li₂S) is typically the rate-limiting step^[42]. As shown in Fig. 8b, the energy barriers for these two steps on MoS₂-MoO₃ were only 0.52 and 0.51 eV, respectively, which are considerably lower than those on MoO₃ (0.84 and 0.17 eV) and MoS₂ (0.92 and 0.62 eV). These results indicate that constructing the MoS₂-MoO₃ heterostructure not only enhances the adsorption of LiPSs but also effectively reduces the energy barrier for liquid-solid conversion, thereby accelerating the reaction kinetics of sulfur species.

  Figure 8

  

  Figure 8.  (a) Calculated binding energies of LiPSs species adsorbed on MoS₂-MoO₃, MoO₃, and MoS₂ catalysts; (b) Gibbs free energy profiles for the conversion reactions of LiPSs involving MoS₂-MoO₃, MoO₃, and MoS₂; (c) Charge density difference map at the MoS₂-MoO₃ heterojunction interface; (d) Charge/discharge curves of MoS₂-MoO₃, MoS₂, and MoO₃ and (e) corresponding ΔE and Q_L/Q_H profiles; DOS curves of (f) MoO₃, (g) MoS₂, and (h) MoS₂-MoO₃

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  Combined with charge density difference (CDD) analysis (Fig. 8c), a pronounced electron depletion region is observed on the MoS₂ side, while an electron accumulation region is formed on the MoO₃ side. This interfacial charge redistribution establishes a built-in electric field within the composite, facilitating electron transfer from MoS₂ to MoO₃. As a result, the interfacial electron transport resistance is reduced, and the redox conversion of LiPSs is accelerated. The synergistic effects of electronic structure modulation and charge regulation provide strong support for enhancing the reaction kinetics and cycling stability of LSBs.

  Fig. 8d displays the galvanostatic charge/discharge curves of cells equipped with the three modified separators at 0.1C. The LSB with the MoS₂-MoO₃ modified PP separator clearly exhibited the smallest polarization potential (ΔE=0.12 V), which was lower than those of cells with MoS₂ (0.24 V) and MoO₃ (0.35 V) (Fig. 8e). Furthermore, Q_H and Q_L, defined as the capacities of the first and second discharge plateaus, respectively, serve as key indicators of sulfur species utilization^[43-44]. The Q_L/Q_H ratio reflects the catalytic activity of the modified separator toward LiPSs conversion—a higher ratio indicates stronger catalytic capability. Consistent with the above findings, the LSB with the MoS₂-MoO₃ modified PP separator demonstrated a Q_L/Q_H value of 1.87, surpassing those of the two pure-phase catalysts (Fig. 8e), confirming that MoS₂-MoO₃ effectively improves sulfur utilization.

  Fig. 8f-8h show the densities of states (DOS) of MoO₃, MoS₂, and MoS₂-MoO₃, as calculated using the Heyd-Scuseria-Ernzerhof 2006 (HSE06) functional. The surface band gap of MoO₃ was 2.64 eV, confirming its semiconductor nature^[45]. MoS₂ also exhibited semiconducting behavior with a band gap of 1.9 eV^[46]. As shown in Fig. 8h, the formed MoS₂-MoO₃ heterojunction exhibited a significantly reduced band gap with orbital hybridization near the Fermi level. Notably, the Mo d-orbitals contribute substantially at the Fermi level and display the most comprehensive total density of states (TDOS) distribution, providing efficient pathways for directional electron transfer. Compared to the individual components, the MoS₂-MoO₃ heterointerface reduces electron transport resistance, promotes rapid electron migration and accumulation at the interface, and facilitates the redox conversion of LiPSs, thereby enhancing the reaction kinetics and cycling stability of LSBs.

  3.   Conclusions

  In summary, a MoS₂-MoO₃ heterojunction was successfully synthesized via a controlled in-situ oxidation method and applied as a separator modifier in LSBs. The heterostructure synergistically combines the strong adsorption capability of MoO₃ and the high catalytic activity of MoS₂, effectively suppressing the polysulfide shuttle and accelerating sulfur redox kinetics. Electrochemical tests and DFT calculations confirm enhanced LiPSs adsorption, reduced energy barriers for liquid-solid conversion, and facilitated electron/ion transport. The resulting cells demonstrated exceptional cycling stability (the reversible specific capacity retained 402.9 mAh·g^-1 after 1 000 cycles at 1C, starting from an initial value of 828.8 mAh·g^-1, with a capacity decay rate of only 0.05% per cycle) and rate performance (delivering a reversible capacity of 300 mAh·g^-1 at 5C), underscoring the potential of interface-engineered heterojunctions as advanced separator coatings for high-energy-density LSBs.

  
  Supporting information is available at http://www.wjhxxb.cn
  Conflicts of interest: There are no conflicts to declare.
  
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• 

  Figure 1  (a) Schematic diagram of the preparation of MoS₂-MoO₃ and the separator modification; CA test of electrolyte on (b) MoS₂-MoO₃ modified PP separator and (c) PP separator; (d, e) TEM images of MoS₂-MoO₃; (f) HRTEM image of MoS₂-MoO₃ (left) and corresponding lattice stripe spacing (right); (g) EDS spectrum of MoS₂-MoO₃; (h) TEM image of MoS₂-MoO₃ and (i-l) corresponding elemental distribution mappings

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  Figure 2  (a) XRD patterns and (b) Raman spectra of MoO₃, MoS₂, and MoS₂-MoO₃; (c) XPS survey spectrum of MoS₂-MoO₃; High-resolution (d) Mo3d, (e) S2p, and (f) O1s XPS spectra of the samples

  Inset: corresponding enlarged pattern of MoS₂-MoO₃.

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  Figure 3  (a) UV-Vis adsorption spectra and the corresponding optical images (inset) of Li₂S₆ solutions adsorbed by different catalysts; (b) TG curve of the S/CNTs cathode; (c) CV curves of the first three cycles and (d) first three charge/discharge curves at 0.1C for the LSB with the MoS₂-MoO₃ modified PP separator

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  Figure 4  (a) Rate capability at various rates and (c) long cycling stability of LSBs with MoS₂, MoO₃, and MoS₂-MoO₃ modified PP separators; (b) Charge/discharge curves of the LSB with the MoS₂-MoO₃ modified PP separator at various rates

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  Figure 5  (a) EIS of LSBs with MoS₂, MoO₃, and MoS₂-MoO₃ modified PP separators after 50 cycles; (b) CV curves of Li₂S₈ symmetric cells with different catalysts

  Inset: (a) the corresponding equivalent circuit diagram, where R_s stands for solution resistance, CPE stands for constant phase angle component, Z_W stands for Warburg impedance, and R_SEI stands for resistance of solid electrolyte interface (SEI) film; (b) Enlarged CV curves of Li₂S₈ symmetric cells with MoS₂ and MoO₃ catalysts.

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  Figure 6  (a) CV curves at various scan rates and (b) i_p vs v^0.5 curves of the LSB with MoS₂-MoO₃ modified PP separator; Corresponding Tafel plots at (c) anodic peak 1 and (d) cathodic peak 2

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  Figure 7  Potentiostatic nucleation of Li₂S with (a) MoS₂-MoO₃ and (b) MoO₃; Potentiostatic profiles of Li₂S dissolution with (c) MoS₂-MoO₃ and (d) MoO₃

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  Figure 8  (a) Calculated binding energies of LiPSs species adsorbed on MoS₂-MoO₃, MoO₃, and MoS₂ catalysts; (b) Gibbs free energy profiles for the conversion reactions of LiPSs involving MoS₂-MoO₃, MoO₃, and MoS₂; (c) Charge density difference map at the MoS₂-MoO₃ heterojunction interface; (d) Charge/discharge curves of MoS₂-MoO₃, MoS₂, and MoO₃ and (e) corresponding ΔE and Q_L/Q_H profiles; DOS curves of (f) MoO₃, (g) MoS₂, and (h) MoS₂-MoO₃

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