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• Recently, lithium-ion batteries (LIBs) are widely applied as the essential energy storage with the development of science and technology. However, the safety challenges, high cost, and the relative low capacity of cathode are hardly to meet the urgent need for efficient and environmental friend energy storage [1,2]. Thus, it is essential to explore high-density and long-life secondary batteries. The development of novel secondary batteries such as Na⁺/K⁺ batteries and aqueous zinc-ion battery has been become an urgent problem to be solved [3–7]. Among of them, lithium-sulfur (Li-S) batteries are deemed to be one of the ideal candidates for storage systems with high energy density owing to their high theoretical energy density (2500 Wh/kg), theoretical specific capacity (1675 mAh/g), and abundant raw sulfur with inexpensive and environmentally friendly costs [8,9]. Moreover, the key challenges of Li-S batteries for practical applications focus on the low conductivity of sulfur, the loss of active material due to the lithium polysulfide shuttle effect, and the uncontrollable lithium dendrite growth [10,11]. In particular, the soluble lithium polysulfide generated by sulfur cathode can easily pass through the polypropylene (PP) separators and participate in irreversible byproduct reactions on the surface of lithium anode, which result in the collapse of electrode material and the degradation of cycle performance [12–15].

  Currently, various strategies were applied to improve the properties of Li-S batteries, involving the modification function in separators [16,17], cathode structure optimization [18,19], and electrolyte system upgrades [20,21]. The cathode modification method remarkably improves the conductivity of the cathode and the cycling stability of the batteries. However, the modification method leads to higher costs and increased process complexity [22,23]. Modifying the electrolytes reduces the polysulfide shuttle phenomenon and enhances the safety and durability of the batteries. However, the modification strategy in practical applications suffers from the compatibility of the modified electrolyte and the problem of cost [24]. Compared with the other two modification techniques, the separator modification technology exhibits significant advantages for enhancing the sulfide adsorption performance, catalytic conversion rate, and electrolyte affinity [25–27]. Hence, coating modified separators is regarded as an easy and effective method. In particular, transition metal-based complexes are widely employed in the practice of modifying separators, such as Co-Fe@NC [28], TiO@NC [29], Co₃Fe₇₋MXene [30], and others. In general, these modified separators exhibit superior performance advantages contributing to strong polysulfide adsorption, catalytic conversion efficacy, and lithium dendrite conversion.

  In addition, researchers discovered that metal-organic frameworks (MOFs) exhibit great potential as precursors for the preparation of promising electrode materials and separator modifier materials for Li-S batteries [31–35]. Specifically, MOFs are highly well-organized and porous crystal network structures based on the interactions of metal ions with organic ligands through covalent bonds. In Li-S battery systems, MOFs present several advantages, such as effective inhibition of polysulfide shuttling, improvement of electrochemical stability, and enhancement of reaction kinetics and cycling performance. But MOFs materials often can hardly fulfill the needs of powerful adsorption performance and high catalytic activity simultaneously [36,37]. Therefore, the bifunctional heterostructures synthesized with MOFs as precursors and transition metals can achieve the synergistic effect of adsorption and catalysis and significantly improve the electrochemical performance. Jin et al. worked out a ZnS@CoN—C hollow core-shell structure with ZIF-8/ZIF-67 and zinc sulfide, which were coated on PE separators for application in Li-S batteries. The results indicated that the batteries with the modified materials boosted redox kinetics and manifested chemisorption capacity for polysulfides [38]. Moreover, Hao et al. utilized ZIF-67/ZIF-8 bimetallic MOF as a precursor to the synthesis of Co₃O₄/ZnO dodecahedral heterojunctions with a heterogeneous porous framework. The Co₃O₄/ZnO modified separator batteries still maintained the specific capacity of 500.9 mAh/g over 400 cycles at 0.5 C, thus exhibiting superior cycling stability [39]. Hence, MOFs and their derivatives exhibit tremendous possibilities in Li-S battery applications.

  In this manuscript, dodecahedral cobalt-molybdenum bimetallic carbides (Co₆Mo₆C₂@Co@NC) hollow and porous heterostructure were successfully fabricated by etching of ZIF-67 precursor through ion exchange using sodium molybdate and the following pyrolysis/carbonization process. The obtained Co₆Mo₆C₂@Co@NC heterostructure exhibits hollow and mesoporous structure with large specific surface aera, which provide large amount of catalytic active sites with abundant adsorption space and efficient catalytic performance. When applied as the LSBs separator modifier layer through coating on PP separator surface, Co₆Mo₆C₂@Co@NC modified separator displays outstanding electrolyte wettable and excellent electrochemical and battery performance. Specifically, the battery of Co₆Mo₆C₂@Co@NC/PP separator maintains the specific capacity of approximately 782.6 mAh/g after 300 cycles at 0.5 C, accompanying by the coulombic efficiency (CE) near to 100%. Hence, the Co₆Mo₆C₂@Co@NC heterostructure presents powerful chemisorption and catalytic conversion ability for polysulfide, which mitigates the polysulfide shuttle effect and expedites polysulfide redox kinetics.

  A typical metal-organic framework (ZIF-67) were successfully produced and used as precursor by combining cobalt nitrate with 2-methylimidazole through a chemical coprecipitation method (Fig. S1 in Supporting information). Subsequently, ZIF-67 was etched by adding sodium molybdate. The precursors were pyrolytically carbonized at 800 ℃ under an argon atmosphere leading to the porous cobalt-molybdenum carbide composite Co₆Mo₆C₂@Co@NC. ZIF-67 provided structural templates and cobalt elements as a starting material, also converted by pyrolysis to carbon substrates and metallic Co. Hence, the formation of the Co₆Mo₆C₂@Co@NC material maintained the characteristic cubic dodecahedral architecture from ZIF-67 in their microscopic morphology. Meanwhile, the abundant active sites on the surface and inside of the Co₆Mo₆C₂@Co@NC material promote the catalytic performance in the batteries, improving the electrochemical performances.

  SEM experiments were conducted to observe the morphology and structure of Co-ZIF-67, Co@NC, Co/Mo-ZIF-67 and Co₆Mo₆C₂@Co@NC, respectively. The SEM image of Co-ZIF-67 reveals the formation from a smooth orthododecahedral structure with 100 nm (Figs. 1a and b). In Figs. S2a-c (Supporting information), Co@NC material was derived from the pyrolysis of Co-ZIF-67. The surface of the resulting orthododecahedral structure of Co@NC appears rough after pyrolysis, owing to the reduction of Co²⁺ to metallic Co by the carbon of Co-ZIF-67. After Co-ZIF-67 etching by sodium molybdate, multiple nanosheets are observed on the surface on the orthododecahedral structure of Co/Mo-ZIF-67 attributed to the binding of MoO₄^2- from Na₂MoO₄ to Co²⁺ in ZIF-67 (Figs. S2d-f in Supporting information). Eventually, the Co₆Mo₆C₂@Co@NC material features an orthododecahedral structure with multiple Co₆Mo₆C₂ and Co nanoparticles after pyrolysis, substantially elevating the specific surface area and augmenting the catalytically active sites for the conversion to polysulfides (Figs. 1c and d). Besides, TEM images of Co₆Mo₆C₂@Co@NC can also be observed in the rough and porous polyhedral structure (Figs. 1e and f). Moreover, The HRTEM image analysis of Co₆Mo₆C₂@Co@NC illustrates identified lattice spacings, with 0.193 and 0.112 nm correlating to the (201) and (220) planes of Co₆Mo₆C₂ and Co, respectively (Fig. 1g). In particular, the selected area electron diffraction (SAED) pattern from Co₆Mo₆C₂ material was analyzed to obtain the (201) facets of Co₆Mo₆C₂ with the (220) facets of Co, demonstrating the presence of Co₆Mo₆C₂@Co@NC (Fig. 1h). Elemental mapping in energy dispersive spectroscopy (EDS) evidences homogeneously distribution of Mo, Co, C, and N elements with Co₆Mo₆C₂@Co@NC (Figs. 1i-l).

  Figure 1

  

  Figure 1.  SEM images of (a, b) Co-ZIF-67, and (c, d) Co₆Mo₆C₂@Co@NC. (e, f) TEM images of Co₆Mo₆C₂@Co@NC. (g) HRTEM image of Co₆Mo₆C₂@Co@NC. (h) SAED pattern of Co₆Mo₆C₂@Co@NC. (i-l) Corresponding elemental mapping images of Co₆Mo₆C₂@Co@NC.

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  X-ray photoelectron spectroscopy (XPS) tests were characterized for the Co₆Mo₆C₂@Co@NC material, which studied the valence and electronic structure of the elements. In Fig. S3a (Supporting information), the Co₆Mo₆C₂@Co@NC spectrum has five different peaks for Co 2p, C 1s, Mo 3d, N 1s, and O 1s. And the high-resolution Co 2p spectrum reveals eight characteristic peaks corresponding to the previous adsorption (Fig. S3b in Supporting information). Meanwhile, the high-resolution Co 2p spectrum presents two characteristic peaks at 781 and 796.4 eV belonging to Co³⁺ before adsorption. The distinctive peaks of 784 and 798 eV, correspond to Co²⁺ [40]. Furthermore, two characteristic peaks of Co° correspond to 778.4 and 793.7 eV, suggesting that part of the Co⁰ is in the singlet state [41]. Similarly, In Fig. S3c (Supporting information), the Mo 3d spectrum before adsorption shows that the peaks at 228.5 eV and 233.1 eV respond to Mo⁶⁺ 3d_5/2 and Mo²⁺ 3d_5/2, respectively. Besides, the two peaks at 232.4 and 235.6 eV, correspond to Mo⁶⁺3d_3/2 and Mo²⁺ 3d_3/2 [42]. In Fig. S3d (Supporting information), the C 1s spectra display a C—C peak at 284.7, a C—N/C—O peak at 286.2 eV, and a C=O peak at 289 eV, respectively [43]. Finally, the N 1s peaks are observed at 394.6, 398.4, and 400.1 eV for Co₆Mo₆C₂@Co@NC, which represent pyridine N, pyrrolic N, and graphitic N, respectively (Fig. S4 in Supporting information) [44].

  Raman spectrum tests were performed to examine the defective properties of carbon atom crystals in the modified materials. Thus, the D peak is associated with the defects in the carbon atom lattice, while the G peak corresponds to the C—C bond stretching vibration in the plane formed with sp² hybridization orbitals of the carbon atoms. The ratio of I_D/I_G reflects the degree of defects in the carbon atom crystals. The Raman curves of Co₆Mo₆C₂@Co@NC and Co@NC show D and G peaks in approximately 1360 cm^-1 and 1590 cm^-1, respectively (Figs. S3e and f in Supporting information). Remarkably, the I_D/I_G ratio of the Co₆Mo₆C₂@Co@NC (0.86) material is lower than that of the Co@NC (1.07) material, which represents a superior degree in graphitization of Co₆Mo₆C₂@Co@NC. Therefore, the modified material features superior electrical conductivity. X-ray diffraction (XRD) patterns were studied and recognized for the crystal structures of materials (Figs. S3g and h in Supporting information). The XRD pattern of Co₆Mo₆C₂@Co@NC reveals the characteristic diffraction peaks corresponding to Co₆Mo₆C₂@Co@NC (PDF #80–0339) and Co (PDF #15–0806), showing the successful preparation of Co₆Mo₆C₂@Co@NC material, which is crucial for the adsorption of polysulfides. The specific surface area and pore size of Co₆Mo₆C₂@Co@NC were measured by Brunauer-Emmett-Teller (BET) at the N₂ adsorption-desorption isotherm (Fig. S3i in Supporting information). The BET surface area, total pore volume and adsorption average pore diameter are 33.0133 m²/g, 0.112561 cm³/g and 13.6383 nm for Co₆Mo₆C₂@Co@NC, respectively, confirming the excellent adsorption performance for polysulfides.

  Furthermore, the Co₆Mo₆C₂@Co@NC spectrum corresponds to six different peaks for Co 2p, Mo 3d, S 2p, C 1s, N 1s, and O 1s after adsorption (Fig. 2a). In Fig. 2b, the high-resolution Co 2p spectra after adsorption reduce the two characteristic peaks of Co° compared to that before adsorption. Moreover, the high-resolution Co 2p, Mo 3d, and C 1s spectra are transferred to the direction of low binding energy after Li₂S₆ adsorption, confirming the superior interaction and adsorption capacity between Co₆Mo₆C₂@Co@NC and polysulfides (Figs. 2b-d) [45]. In Fig. 2e, the high-resolution spectrum S 2p reveals the coexistence of polysulfides, thiosulfates, and LiPS. The thiosulfates are produced by the reaction with Co₆Mo₆C₂@Co@NC and polysulfides, confirming the existence of reversible transformations in LiPSs, and further proving the strong adsorption effect of Co₆Mo₆C₂@Co@NC on LiPSs [46,47].

  Figure 2

  

  Figure 2.  (a) The XPS of Co₆Mo₆C₂@Co@NC before and after the adsorption of Li₂S₆. (b-d) Co 2p, Mo 3d, and C 1s XPS spectra before and after Li₂S₆ adsorption. (e) S 2p XPS spectra after Li₂S₆ adsorption. (f) UV–vis spectra and corresponding image (inset) of pure Li₂S₆ solution before and after adsorption of Li₂S₆ by modified materials. (g) Li₂S₆ permeation measurements of PP, Co@NC/PP and Co₆Mo₆C₂@Co@NC/PP separators.

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  Furthermore, 75 mg of Co₆Mo₆C₂@Co@NC and Co@NC were added to 10 mL (DME:DOL = 1:1) of a solution containing 50 µL of Li₂S₆, respectively. After standing for 6 h, the supernatant was removed for ultraviolet-visible (UV–vis) absorbance test. In Fig. 2f, the characteristic peak intensity of Co₆Mo₆C₂@Co@NC material is smaller than that of Co@NC and Li₂S₆ solution without added adsorbent, confirming the superiority adsorption capacity of Co₆Mo₆C₂@Co@NC modified material for polysulfides [48].

  The permeability for LiPSs of modified materials and PP separators was tested by using an H-shaped glass tube (Fig. 2g). In particular, the electrolyte of Li₂S₆ (1.0 mol/L Li₂S₆, 1 mol/L LiTFSI in DME:DOL = 1:1, v/v) was incorporated on the left side of the H-shaped glass tube and the blank solution (DME:DOL = 1:1) was added to the right side of the glass tube. The inhibition of the shuttle effect for LiPSs was observed using three separators during the same period. In Fig. 2g, the glass device with the PP separators spread LiPSs on the right side after 6 h, and a dark yellow color appeared noticeable after 24 h With PP and Co@NC/PP separators, the glass devices with Co₆Mo₆C₂@Co@NC/PP modified separators exhibit pale yellow color on the right side even after 24 h, suggesting that the modified separator effectively suppressed the shuttle effect of LiPSs.

  SEM image of the PP separator features a porous structure of several hundred nanometers, which permits Li⁺ to shuttle (Fig. S5a in Supporting information). Fig. S5b (Supporting information) reveals that the Co₆Mo₆C₂@Co@NC material was coated with the PP separator. The thickness of the Co₆Mo₆C₂@Co@NC/PP modified layers is ~12.84 µm (Fig. S5c in Supporting information). Meanwhile, the diameter of the PP and Co₆Mo₆C₂@Co@NC/PP separators is 19 mm in the digital photograph (Fig. S5d in Supporting information). The modified separator retained the initial shape after continuous folding experiments, exhibiting that the Co₆Mo₆C₂@Co@NC/PP material and PP separator possess superior mechanical stability and flexibility in Fig. S5e (Supporting information). To detect the wettability of the electrolytes to the separators, the permeability test of the separators to the electrolyte were examined by dropping the electrolytes over the surface of the separators. The Co₆Mo₆C₂@Co@NC/PP separator possesses the fastest diffusion rate and diffusion area for the electrolyte (Fig. S5f in Supporting information). Fig. S5g (Supporting information) indicates that the contact angle of the Co₆Mo₆C₂@Co@NC/PP modified separator is 12.99° compared to that of the Co@NC/PP (13.57°) and PP separators (16.4°), attributing to the exceptional electrolyte affinity of the modified separator.

  The rate performance of Li-S batteries with Co@NC/PP, Co₆Mo₆C₂@Co@NC/PP, and PP separators were tested at various current densities in the range of 0.2–3 C (Figs. 3a-c). In Fig. 3b, the specific capacities of battery with Co₆Mo₆C₂@Co@NC/PP separator are 1090, 931, 842, and 738 mAh/g at different current densities of 0.2, 0.5, 1, and 2 C, respectively. The Co₆Mo₆C₂@Co@NC/PP battery demonstrates an excellent capacity of 617 mAh/g at a current density of 3 C compared to that of Co@NC/PP (363 mAh/g) and PP batteries (132 mAh/g), explaining that the Co₆Mo₆C₂@Co@NC/PP material accelerates the redox kinetics of polysulfides, thus confirming the high catalytic performance of the Co₆Mo₆C₂@Co@NC material (Figs. S7e and f in Supporting information). Besides, the capacity of the Co₆Mo₆C₂@Co@NC/PP battery achieves 933 mAh/g at 0.2 C, indicating the promising electrochemical reversibility of the Co₆Mo₆C₂@Co@NC material [49]. In Fig. 3d and Table S1 (Supporting information), the Co₆Mo₆C₂@Co@NC/PP modified material preserves a specific capacity of 782.6 mAh/g at 0.5C for 300 cycles with a low decay rate of 0. 13% compared with other separators. Besides, the Co₆Mo₆C₂@Co@NC/PP modified material maintains 842 mAh/g at 1 C, exhibiting the excellent performance of Co₆Mo₆C₂@Co@NC/PP modified material in Li-S batteries. Moreover, the long-term cycling performance of PP, modified material batteries was tested at 0.5 C, the Co₆Mo₆C₂@Co@NC /PP battery exhibits an initial discharge capacity of 1128 mAh/g (Figs. 3e and f). Additionally, the specific capacities of the Co₆Mo₆C₂@Co@NC/PP battery are 962, 820, 780, and 782.6 mAh/g after 50, 100, 150, 200, and 300 cycles, respectively. And the Coulombic efficiencies (CE) are all near to 100%. By contrast, Co@NC/PP and PP batteries only reach 524 and 365 mAh/g after 250 cycles at 0.5 C, respectively (Figs. S6a and b in Supporting information). The charges of the high discharge plateau (Q_H) and low discharge plateau (Q_L) of the Co₆Mo₆C₂@Co@NC, Co@NC and PP batteries represent the characteristics from the polysulfide conversion reaction (Fig. 3g). Specifically, the Co₆Mo₆C₂@Co@NC /PP batteries correspond to specific capacities of 293, 293, and 261 mAh/g after 500 cycles, respectively, at 2, 3, and 4 C. With the increasing of current density, the polarization voltage keeps growing and the specific capacity of discharge decreases slightly. The specific capacity of PP is nearly 3 mAh/g at 4 C, revealing significant catalytic performance at high current densities (Fig. 3h, Figs. S7a-d and S12 in Supporting information).

  Figure 3

  

  Figure 3.  (a) Rate capabilities of three samples. (b) Discharge/charge potential profiles of the Co₆Mo₆C₂@Co@NC/PP battery at various rates. (c) Electrochemical properties based on Co₆Mo₆C₂@Co@NC/PP, Co@NC/PP and PP batteries at 0.2 C. (d) Data comparison radar chart. (e) Cycling performances at 0.5 C. (f) Discharge/charge potential profiles of Co₆Mo₆C₂@Co@NC/PP modified separator at 0.5 C. (g) Q_H and Q_L values for three separators. (h) Cyclic performance at 1, 2, 3 and 4 C. (i) Cyclic performance under the sulfur loading of 4.096 and 2.432 mg/cm² at 0.5 C. (j) Cycling stability of Li||Li symmetric batteries of Co@NC/PP and Co₆Mo₆C₂@Co@NC/PP separators. (k-m) Cycling stability of Li||Li symmetric batteries of PP and Co₆Mo₆C₂@Co@NC/PP separators.

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  To accelerate Co₆Mo₆C₂@Co@NC/PP batteries for commercial applications, the electrochemical performance was estimated for Li-S batteries (Fig. 3i, Figs. S8a and b in Supporting information). The battery with Co₆Mo₆C₂@Co@NC/PP separator shows an initial discharge capacity of 903 mAh/g at 0.5 C in high sulfur loading (2.432 mg/cm²). Besides, the discharge capacity of the Co₆Mo₆C₂@Co@NC/PP battery delivers 585 mAh/g after 289 cycles at 0.5 C with a CE of over 96%. Notably, the battery with Co₆Mo₆C₂@Co@NC/PP separator obtains a reversible capacity of 431 mAh/g after 200 cycles at 0.5 C with 95% CE when the sulfur loading reaches as high as 4.096 mg/cm². In conclusion, the Co₆Mo₆C₂@Co@NC material with more catalytic sites contributes to the conversion of polysulfides dissolution and effectively reduce the formation of lithium dendrites. Besides, the effect of symmetric Li/Co@NC/Li and Li/Co₆Mo₆C₂@Co@NC/Li batteries on multiplicity performance was tested by enhancing the current density. In Fig. 3j, the Li/Co₆Mo₆C₂@Co@NC/Li battery presents a lower voltage difference in compared with that of Li/Co@NC/Li battery at the current densities of 0.5, 1, 2, 3, 4, and 5 mA/cm², respectively. Notably, The Li/Co₆Mo₆C₂@Co@NC/Li battery displays a voltage difference of merely 0.44 V in contrast to the Li/Co@NC/Li battery (0.6 V) at 5 mA/cm². Similarly, the voltage profiles of symmetric Li/PP/Li and Li/Co₆Mo₆C₂@Co@NC/Li batteries were examined at a current density of 2 mA/cm² (Figs. 3k-m). Among them, the battery of Li/PP/Li demonstrates a voltage difference of 0.052 V after 410 h cycling. In contrast, Li/Co₆Mo₆C₂@Co@NC/Li battery exhibits a voltage difference of 0.046 V after 1500 h cycling, which confirms that the modified cell mitigates the generation of lithium dendrites [50–54]. Furthermore, in comparison with Co@NC/PP and PP batteries, the Co₆Mo₆C₂@Co@NC/PP battery reveals a specific capacity of 601 mAh/g with a capacity decay rate of 0.0761% after 500 cycles at 1 C, inhibiting the shuttling effect of polysulfides and further enhancing cycling stability (Figs. 4a and b, Figs. S6c and d in Supporting information). To examine the performance of the modified separators for polysulfides barrier, SEM tests were conducted on Co₆Mo₆C₂@Co@NC/PP modified separators before and after cycling. In Fig. 4c, the PP separator coated by Co₆Mo₆C₂@Co@NC material displays obvious dodecahedral structure. After 500 cycles at 1 C, the surface SEM image of Co₆Mo₆C₂@Co@NC/PP separator still remains complete compared to that of Co@NC/PP and PP (Figs. S13a-c in Supporting information). Besides, the elemental content of S is high on the surface with Co₆Mo₆C₂@Co@NC separator, illustrating the ability of Co₆Mo₆C₂@Co@NC/PP modified material to effectively inhibit the sulfides (Figs. 4d, e and h). In summary, elemental mapping tests by EDS reveals a uniform distribution of the elements Mo, Co, C, S, and N, which indicates that the modified material facilitates the redox kinetics of the polysulfides and accelerates their transformations during the cycle process (Figs. 4f-k) [55].

  Figure 4

  

  Figure 4.  Electrochemical performance comparison of the batteries based on Co₆Mo₆C₂@Co@NC/PP, Co@NC/PP and PP separators. (a) Cyclic performance at 1 C. (b) Discharge/charge potential profiles of Co₆Mo₆C₂@Co@NC/PP modified separator at 1 C. (c) SEM image of the Co₆Mo₆C₂@Co@NC/PP separator furface. (d-k) SEM of images and the corresponding elemental mapping images of C, N, S, Co and Mo elements of Co₆Mo₆C₂@Co@NC/PP separator after 500 cycles at 1 C.

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  To detect the stabilizing performance of the modified separators and PP separator on the batteries, the modified material and PP batteries were operated for a self-discharge test after interruption of 72 h at 2.15 V. As illustrated in Figs. 5a-d, the capacity retention of Co₆Mo₆C₂@Co@NC/PP batteries is up to 83.2%. However, the capacity retention of Co@NC/PP and PP batteries are 76.45% and 51.12%, respectively. The Co₆Mo₆C₂ material enhances the LiPSs absorption and improves the self-discharge phenomenon. The batteries with modified material, and PP separators measured the internal resistances during charging and discharging at a current density of 0.1 C by GITT testing. The internal resistance of the battery with Co₆Mo₆C₂@Co@NC/PP separator (ΔIR = 0.02946 V) is smaller than the batteries of Co@NC/PP (ΔIR = 0.0312 V) and PP separators, illustrating that the Co₆Mo₆C₂@Co@NC/PP material promotes the transfer of Li⁺ (Figs. 5e-h). Besides, to further investigate the conversion of polysulfides by the modified materials of the separator, potentiostatic discharge plots were conducted at 2.05 V to measure the precipitation of Li₂S to the electrode surfaces of modified material batteries. From Figs. 5i and j, The Co₆Mo₆C₂@Co@NC/PP battery shows faster nucleation response time than that of Co@NC/PP battery. In addition, the Li₂S precipitation capacity of the Co₆Mo₆C₂@Co@NC/PP battery provides 373.16 mAh/g as compared to that of the Co@NC/PP battery (188.26 mAh/g). In summary, these results suggest that Co₆Mo₆C₂ material strengthens the redox reaction of polysulfides, which contributes to the conversion of polysulfides into Li₂S [56].

  Figure 5

  

  Figure 5.  (a-d) The discharge-charge voltage profiles of self-discharge behavior with Co₆Mo₆C₂@Co@NC/PP, Co@NC/PP and PP batteries. GITT voltage profiles of (e) Co₆Mo₆C₂@Co@NC/PP, (f) Co@NC/PP, and (g) PP batteries. (h) A columnar comparison of potential difference. (i) Capacities of Li₂S deposition on the Co@NC/PP separator in Li₂S₈ electrolyte solution. (j) Capacities of Li₂S deposition on the Co₆Mo₆C₂@Co@NC/PP separator in Li₂S₈ electrolyte solution. (k) CV curves of Li₂S₆ symmetric batteries using Co₆Mo₆C₂@Co@NC/PP, Co@NC/PP and PP as electrodes at the scan rate of 10 mV/s. (l) CV curves of Li₂S₆ symmetric batteries using Co₆Mo₆C₂@Co@NC/PP, as electrodes at the scan rate of 20, 30, 40 and 50 mV/s.

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  To further investigate the catalytic activity of the modified material, the CV curves of symmetric batteries were calculated at a range of voltages (−0.8 V to 0.8 V) with the homogeneous electrodes and Li₂S₆ (1.0 mol/L Li₂S₆, 1 mol/L LiTFSI in DME:DOL = 1:1, v/v) electrolyte (Fig. 5k). The symmetric battery with Co₆Mo₆C₂@Co@NC material reveals significantly elevated response currents and broader redox peaks compared to that with Co@NC material, which suggests that the Co₆Mo₆C₂@Co@NC-modified material expedites the catalytic conversion of LiPSs. Additionally, the cyclic voltammetry (CV) curves of the resulting Co₆Mo₆C₂@Co@NC symmetric batteries show a consistent trend in the voltage range from −0.5 V to 0.5 V with an increased scan rate from 20 mV/s to 50 mV/s, substantiating the stable electrochemical performance of the modified material (Fig. 5l). The symmetric battery of Co₆Mo₆C₂@Co@NC material presents smaller charge transfer resistance, which proves that modified material promotes charge transfer at LiPSs (Fig. S9 in Supporting information) [57].

  To investigate the effect of PP and modified materials on Li⁺ diffusion in batteries, cyclic voltammetry (CV) experiments were performed with various scanning rates in the voltage range of 1.7–2.8 V. In Figs. S10a-c (Supporting information), the response currents of modified materials separators in Li-S batteries exhibit a gradual increase with escalating scan rates. The CV curves of both modified material batteries show two various reduction peaks and one oxidation peak. As shown in Fig. S10a, two reduction peaks position at 2.071 V and 1.873 V, which correspond to the reduction of S₈ to long-chain soluble Li₂S_n (4 ≤ n ≤ 8) and reduction into short-chain insoluble Li₂S/Li₂S₂, respectively. Additionally, the oxidation peak responds to the oxidation reaction for the conversion of Li₂S/Li₂S_n (4 ≤ n ≤ 8) to S₈ at 2.456 V. Meanwhile, CV curves are often applied to investigate the influence of the diffusion rate of Li⁺ on the redox kinetics of Li-S batteries. Fig. S10b provides a linear relationship between the peak current (I_p) and the square root of the scan rate (v ^1/2), which is based on the Randles-Sevcik equation in Eq. 1, and the slope of the linear fit is proportional to the diffusivity of Li⁺[58].

  $ I_{\mathrm{p}}=2.69 \times 10^5 n^{1.5} A D_{\mathrm{Li}}^{0.5} C_{\mathrm{Li}} v^{0.5} $

  (1)

  In this formulation, I_p, A, n, v, C_Li and D_Li represent the peak current density, surface area, electron number, scan rate, concentration, and Li⁺ diffusion coefficient, respectively. In Figs. S10d and e (Supporting information), the Co₆Mo₆C₂@Co@NC/PP battery possesses the greatest slope, which explains the most rapid Li⁺ diffusion rate. Notably, the slopes of the CV curve fit for the Co₆Mo₆C₂@Co@NC/PP battery are 3.43, 4.41, and 7.49, while the respective slopes for Co@NC/PP are 2.44, 3.47, and 5.89 (Figs. S11a-c in Supporting information). These results indicate that the Co₆Mo₆C₂@Co@NC/PP separator promotes effectively Li⁺ transfer because of its improved electrolyte permeability (Fig. S10 h in Supporting information). In general, the synergistic effect of Co₆Mo₆C₂ and Co@NC materials contributes to the promotion of polysulfide redox kinetics and accelerates the conversion of polysulfides.

  Furthermore, the CV curves of the batteries were selected for the data to fit the Tafel slope (Figs. S10f and g in Supporting information). The Tafel slope was utilized to evaluate the rate of electron transfer, which indicated the kinetics of LiPSs conversion on the modified separator. Overall, Tafel slopes can be observed in the reduction reaction for the conversion of S₈ to Li₂S₈ for Co₆Mo₆C₂@Co@NC/PP (101.16 mV/dec) battery, which is weaker than slopes of Co@NC/PP (115.01 mV/dec) and PP (138.21 mV/dec) batteries. Similarly, the Tafel slope for the reduction conversion reaction of Li₂S_n (4 ≤ n ≤ 8) to Li₂S is concluded to be much slower for Co₆Mo₆C₂@Co@NC/PP (73.14 mV/dec) battery compared to that of Co@NC/PP (73.59 mV/dec) and PP (84.31 mV/dec) batteries, explaining that the synergistic catalytic effect of Co₆Mo₆C₂ metal carbide significantly speed up the polysulfide transformation. Moreover, the transfer rate of Li⁺ was also calculated. Co₆Mo₆C₂@Co@NC shows a greater Li⁺ transport rate than Co@NC in the polysulfide conversion process, thus implying that the modified material expedites the charging and discharging rate of the battery and enhances the efficiency of the battery. Additionally, modified materials batteries were analyzed by linear scanning voltammetry (LSV) (Fig. S10i in Supporting information). The Co₆Mo₆C₂@Co@NC/PP (3.37 V) battery provides a higher starting voltage than the Co@NC/PP (3.07 V) battery, which supports the better catalytic activity of Co₆Mo₆C₂ material to promote the conversion of LiPSs to redox.

  Furthermore, electrochemical impedance spectroscopy (EIS) of the batteries were analyzed to evaluate the effect of PP and modified separators on the electrochemical kinetics of batteries (Fig. S10j in Supporting information). The Co₆Mo₆C₂@Co@NC/PP battery provides much smaller resistance, which proves that the modified separator facilitates the diffusion rate of Li⁺, and further enhances the charge transfer to facilitate the redox conversion of LiPSs. Further, the impedance of the Co₆Mo₆C₂@Co@NC/PP and Co@NC/PP post-cycling were reduced compared to that before cycling after 500 cycles at 1 C (Fig. S10k in Supporting information). Comparatively, the impedance of Co₆Mo₆C₂@Co@NC/PP battery is lower than that of Co@NC/PP, confirming the effective inhibition of polysulfide shuttling effect by the Co₆Mo₆C₂@Co@NC/PP modified materials, thus slowing down the formation of passivation layer (Li₂S₂/Li₂S) on the Li surface.

  At last, the conductivity of the modified separators before cycling was performed to determine their capacity for Li⁺ transport (Fig. S10l in Supporting information). The formula for calculating the conductivity is in Eq. 2 [59]:

  $ \sigma=l / R_b \times A $

  (2)

  Specifically, σ is the conductivity of the separator (S/cm), R_b represents the impedance of the battery, l stands for the thickness of the separator, and A denotes the area of the separator. Among them, the conductivity of Co₆Mo₆C₂@Co@NC/PP is considerably superior to that of Co@NC/PP and PP batteries, suggesting that the modified material boosts Li⁺ transport and prolongs the lifetime of the batteries.

  In conclusion, hollow and mesoporous bimetallic carbides Co₆Mo₆C₂@Co@NC heterostructures have been successfully synthesized by etching dodecahedral ZIF-67 template using sodium molybdate as etching agent and the following pyrolysis and carbonation processes. The Co₆Mo₆C₂@Co@NC material was then coated on the conventional polypropylene (PP) separator for the improvement of LSBs properties. Specifically, the slopes fitted to the CV curves for the Co₆Mo₆C₂@Co@NC/PP battery displayed the largest current in compared with these of Co@NC/PP and PP, confirming superior electrochemical performance for the Co₆Mo₆C₂@Co@NC/PP battery. And the synergistic effect of Co₆Mo₆C₂ and Co@NC components contribute to the promotion of polysulfides redox kinetics. Subsequently, the battery with Co₆Mo₆C₂@Co@NC/PP separator maintained the high specific capacity of about 782.6 mAh/g after 300 cycles at 0.5 C, which demonstrates the excellent cycling stability of the Co₆Mo₆C₂@Co@NC/PP battery, revealing that the battery/PP separator significantly facilitates the polysulfide conversion reaction and effectively suppressed the polysulfide shuttle effect. Meanwhile, the Co₆Mo₆C₂@Co@NC/PP battery presented the capacity of 431 mAh/g after 200 cycles in the high sulfur loading (4.096 mg/cm²) at 0.5 C. This work provides an effective way to for the design of bimetallic carbides catalyst for synergistic regulation of polysulfides and lithium dendrites for the further development of high-performance Li-S batteries.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Xuanyang Jin: Investigation, Formal analysis. Xincheng Guo: Methodology, Formal analysis. Siyang Dong: Methodology, Conceptualization. Shilan Li: Methodology, Conceptualization. Shengdong Jin: Investigation, Data curation. Peng Xia: Formal analysis, Conceptualization. Shengjun Lu: Supervision, Resources. Yufei Zhang: Validation, Resources. Haosen Fan: Writing – review & editing, Project administration.

  Acknowledgments

  This work was supported by National Natural Science Foundation of China (Nos. 52472194, 52101243), Natural Science Foundation of Guangdong Province, China (No. 2023A1515012619) and the Science and Technology Planning Project of Guangzhou (No. 202201010565).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110604.

  
  
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• 

  Figure 1  SEM images of (a, b) Co-ZIF-67, and (c, d) Co₆Mo₆C₂@Co@NC. (e, f) TEM images of Co₆Mo₆C₂@Co@NC. (g) HRTEM image of Co₆Mo₆C₂@Co@NC. (h) SAED pattern of Co₆Mo₆C₂@Co@NC. (i-l) Corresponding elemental mapping images of Co₆Mo₆C₂@Co@NC.

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  Figure 2  (a) The XPS of Co₆Mo₆C₂@Co@NC before and after the adsorption of Li₂S₆. (b-d) Co 2p, Mo 3d, and C 1s XPS spectra before and after Li₂S₆ adsorption. (e) S 2p XPS spectra after Li₂S₆ adsorption. (f) UV–vis spectra and corresponding image (inset) of pure Li₂S₆ solution before and after adsorption of Li₂S₆ by modified materials. (g) Li₂S₆ permeation measurements of PP, Co@NC/PP and Co₆Mo₆C₂@Co@NC/PP separators.

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  Figure 3  (a) Rate capabilities of three samples. (b) Discharge/charge potential profiles of the Co₆Mo₆C₂@Co@NC/PP battery at various rates. (c) Electrochemical properties based on Co₆Mo₆C₂@Co@NC/PP, Co@NC/PP and PP batteries at 0.2 C. (d) Data comparison radar chart. (e) Cycling performances at 0.5 C. (f) Discharge/charge potential profiles of Co₆Mo₆C₂@Co@NC/PP modified separator at 0.5 C. (g) Q_H and Q_L values for three separators. (h) Cyclic performance at 1, 2, 3 and 4 C. (i) Cyclic performance under the sulfur loading of 4.096 and 2.432 mg/cm² at 0.5 C. (j) Cycling stability of Li||Li symmetric batteries of Co@NC/PP and Co₆Mo₆C₂@Co@NC/PP separators. (k-m) Cycling stability of Li||Li symmetric batteries of PP and Co₆Mo₆C₂@Co@NC/PP separators.

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  Figure 4  Electrochemical performance comparison of the batteries based on Co₆Mo₆C₂@Co@NC/PP, Co@NC/PP and PP separators. (a) Cyclic performance at 1 C. (b) Discharge/charge potential profiles of Co₆Mo₆C₂@Co@NC/PP modified separator at 1 C. (c) SEM image of the Co₆Mo₆C₂@Co@NC/PP separator furface. (d-k) SEM of images and the corresponding elemental mapping images of C, N, S, Co and Mo elements of Co₆Mo₆C₂@Co@NC/PP separator after 500 cycles at 1 C.

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  Figure 5  (a-d) The discharge-charge voltage profiles of self-discharge behavior with Co₆Mo₆C₂@Co@NC/PP, Co@NC/PP and PP batteries. GITT voltage profiles of (e) Co₆Mo₆C₂@Co@NC/PP, (f) Co@NC/PP, and (g) PP batteries. (h) A columnar comparison of potential difference. (i) Capacities of Li₂S deposition on the Co@NC/PP separator in Li₂S₈ electrolyte solution. (j) Capacities of Li₂S deposition on the Co₆Mo₆C₂@Co@NC/PP separator in Li₂S₈ electrolyte solution. (k) CV curves of Li₂S₆ symmetric batteries using Co₆Mo₆C₂@Co@NC/PP, Co@NC/PP and PP as electrodes at the scan rate of 10 mV/s. (l) CV curves of Li₂S₆ symmetric batteries using Co₆Mo₆C₂@Co@NC/PP, as electrodes at the scan rate of 20, 30, 40 and 50 mV/s.

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