MOF-derived Co–Mo bimetallic heterostructures for the selective trapping and conversion of polysulfides in lithium–sulfur batteries

Abstract

Lithium–sulfur batteries (LSBs) are promising energy storage systems, but their practical application is hindered by the polysulfide shuttle effect and slow redox kinetics. To address these challenges, we constructed ZIF-67@CoS_x/MoO₃ with a core–shell structure and CoS_x/MoO₃ with a hollow structure as separator-modified materials for LSBs by varying the degree of sulfidation of ZIF-67. The high intrinsic conductivity of CoS_x facilitated ion transfer between the cathode and separator. Additionally, the introduction of MoO₃ formed a heterogeneous structure with CoS_x that enhanced the adsorption of LiPSs. Via in situ UV-vis and electrochemical impedance spectroscopy testing, we demonstrated the preferred selective trapping and conversion of LiPSs by CoS_x/MoO₃. As a result of the synergistic effect of the bimetallic heterogeneous structure, the modified LSB exhibited excellent cycling stability, with a capacity decay rate of only 0.041% after 500 cycles at 1C. Moreover, it achieved a high discharge capacity of 632 mA h g⁻¹ at 2C. This work provides a novel concept for MOF-derived heterogeneous structures to be applied in high-performance LSBs.

---

Introduction

The growing demand in the emerging electric vehicle and renewable energy storage markets has prompted the search for next-generation energy storage devices.^1–3 Rechargeable LSBs are considered a promising option due to their high theoretical energy density and low cost. However, the commercialization of these batteries faces challenges such as low sulfur utilization, short cycle life, and low charging and discharging efficiency. These challenges are primarily caused by the poor electrical conductivity of sulfur and its discharge products, as well as the dissolution of intermediate lithium polysulfides (LiPSs) and the shuttle effect.^4–6

To address these challenges, researchers have investigated various separator-modified materials to physically and chemically adsorb LiPSs. Transition metal compounds, particularly oxides, exhibit strong adsorption ability,^7,8 effectively preventing polysulfide shuttling. However, transition metal compounds typically present poor electronic conductivity and limited catalytically active sites, hindering the cycling process.

Recently, the development of metal–organic frameworks (MOFs) and their derivatives has garnered attention in the modification of lithium–sulfur battery separators^9–11 due to their unique porous structure and strong chemisorption properties for LiPSs. Additionally, MOFs have been utilized as templates to construct transition metal compounds, providing various reactive sites for polysulfide adsorption and facilitating their catalytic conversion.¹² Although MOF-derived sulfides exhibit high conductivity and catalytic ability, their polysulfide adsorption capability is relatively weak.¹³ Therefore, the construction of bimetallic transition metal sulfide/oxide heterostructures has been proposed as an effective approach for enhancing the cycling performance of LSBs.¹⁴ CoS_x can chemically bond with low-order LiPSs, reducing the energy barrier in the reaction. Co₉S₈ exhibits good catalytic activity and, in addition to its powerful adsorption ability, can accelerate the redox reactions of LiPSs.¹⁵ Based on these findings, Su et al. employed a simple electrospinning method to prepare a core–shell heterostructure anchored on nanocarbon fibers. This heterostructure combines the synergistic effects of chemical adsorption and electrochemical catalysis, resulting in improved sulfur utilization of high sulfur-loaded electrodes.¹⁶ At the same time, density functional theory calculations further indicate that the binding strength between MoO₃ and LiPSs is strong, making it an ideal host for anchoring LiPSs and improving cycling stability.¹⁷

Therefore, in this study, we utilized ZIF-67 as a template to fabricate two separator-modified materials, ZIF-67@CoS_x/MoO₃ and CoS_x/MoO₃, and investigated their performance in LSBs. The core–shell structured ZIF-67@CoS_x/MoO₃ was obtained by partially sulfurizing ZIF-67 and introducing the Mo element afterwards. Nevertheless, the hollow structured CoS_x/MoO₃ was achieved by complete sulfidation of ZIF-67, resulting in significantly improved performance as a separator in LSBs. In situ electrochemical impedance tests and in situ UV-vis confirmed that compared with ZIF-67@CoS_x/MoO₃, the hollow CoS_x/MoO₃ delivered much higher conductivity and can selectively capture LiPSs in the electrolyte and promote their conversion to insoluble LiPSs to a greater extent.

Results and discussion

Fig. 1a presents the preparation of ZIF-67@CoS_x/MoO₃ and CoS_x/MoO₃. ZIF-67 with a particle size of about 500 nm was synthesized in one step using a solvent method, and the scanning electron microscopy (SEM) image is shown in Fig. S1.† Then, by controlling the temperature and time of the hydrothermal reaction, precursor 1 (PR1) and precursor 2 (PR2) with different degrees of sulfidation were obtained upon mixing ZIF-67 with an ethanol solution of thiourea acetamide (TAA) (their SEM images are shown in Fig. S2a and b†). The X-ray diffraction (XRD) spectra of PR1 and PR2 are shown in Fig. S3,† with the characteristic peaks of ZIF-67 in PR1 weakened. No obvious diffraction peaks belonging to sulfides were observed, indicating that the obtained PR1 and PR2 may be amorphous.¹⁸ In the transmission electron microscopy (TEM) image (Fig. S4a†), the edges of PR1 appeared rough, and Fig. S4b† shows the hollow structure of PR2. Raman spectroscopy analysis further revealed that the peaks at 670 cm⁻¹ corresponded to the vibration of Co–S in Co₉S₈ (Fig. S5a†).¹⁹ Mixing PR1 and PR2 with NaMoO₄·2H₂O aqueous solution separately resulted in the formation of ZIF-67@CoS_x/MoO₃ and CoS_x/MoO₃, respectively; they all show a spherical shape and are coated with nanosheets on the outer layer (Fig. S2c and d†). Similar to PR1 and PR2, the vibration of Co–S was observed in Fig. S5b,† and the peaks at 817 cm⁻¹ and 941 cm⁻¹ were attributed to the asymmetric vibration of O–Mo–O originating from MoO₃.²⁰ Fourier transform infrared (FTIR) spectroscopy further confirmed the formation of heterostructures. The presence of Co–S (1084 cm⁻¹) and Mo–O bonds (564 cm⁻¹) can be attributed to CoS_x and MoO₃ (Fig. S5c†).^21,22 A high-resolution transmission electron microscopy (HRTEM) image (Fig. 1b) revealed the core–shell structure of ZIF-67@CoS_x/MoO₃. The energy-dispersive X-ray spectroscopy (EDX) elemental mapping (Fig. 1c and Fig. S6†) confirmed the successful introduction of Co, Mo, S, and O, while the lattice fringes in Fig. 1d indicated the (440) and (400) planes of Co₃S₄ corresponding to 0.162 nm and 0.241 nm, respectively, and 0.223 nm was attributed to the (150) plane of MoO₃.²³Fig. 1e and j show the selected area electron diffraction (SAED) patterns of ZIF-67@CoS_x/MoO₃ and CoSx/MoO₃, respectively. The Brunauer–Emmett–Teller (BET) surface area data (Fig. 1f) indicated the uniform microporous structure inside ZIF-67@CoS_x/MoO₃. In comparison, CoS_x/MoO₃ exhibited a hollow structure (Fig. 1g), and the element distribution in Fig. 1h showed the abundant distribution of Co, Mo, S, and O at the hexagonal edges. The lattice fringe analysis in Fig. 1i revealed the (422) plane of Co₉S₈ (0.208 nm) and the (400) plane of Co₃S₄ (0.236 nm). Furthermore, the XRD patterns of ZIF-67@CoS_x/MoO₃ and CoS_x/MoO₃ further confirmed the formation of the Co–Mo bimetallic heterostructure. As shown in Fig. S7a,† ZIF-67@CoS_x/MoO₃ retains the major ZIF-67 diffraction peaks, indicating a low degree of sulfidation. Nevertheless, diffraction peaks at around 26° and 37° were attributed to MoO₃ (PDF#75-0912), and diffraction peaks associated with Co₉S₈ (PDF#02-1459) and Co₃S₄ (PDF#47-1738) occurred mainly at around 30°, proving the presence of small amounts of CoS_x and MoO₃. Compared with ZIF-67@CoS_x/MoO₃, CoS_x/MoO₃ shows more pronounced diffraction peaks related to sulfide in Fig. S7b,† which is related to its deeper sulfidation. The BET data in Fig. 1k showed that CoS_x/MoO₃ possessed an uneven mesoporous structure, which is attributed to the hollow structure. The abundant mesopores in CoS_x/MoO₃ can not only facilitate the diffusion of electrolytes and the migration of lithium ions but also suppress the loss of soluble LiPSs through confinement.²⁴

Fig. 1 (a) Schematic of the preparation of ZIF-67@CoS_x/MoO₃ and CoS_x/MoO₃. Characterization of ZIF-67@CoS_x/MoO₃ and CoS_x/MoO₃: (b and g) TEM images, (c and h) elemental mapping images, (d and i) HRTEM images, (e and j) SAED patterns and (f and k) N₂ adsorption/desorption isotherms and pore diameter distributions.

In order to examine the uniformity and firmness of the sample loading on the separators, we conducted microscopic characterization and folding experiments. Fig. 2a–c and their insets show the SEM images of bare polypropylene (PP), ZIF-67@CoS_x/MoO₃/PP, and CoS_x/MoO₃/PP, respectively. The PP separator had a pore size of about 100 nm (Fig. S8†), which easily allowed the shuttle of LiPSs to the negative electrode, causing irreversible capacity loss. Coating ZIF-67@CoS_x/MoO₃ or CoS_x/MoO₃ on PP can facilitate the physical blocking of LiPSs. In addition, the highly conductive CoS_x can decrease the polarization inside the cell due to the increased thickness of the separator.²⁵ The cross-sectional SEM image (Fig. 2d) showed a uniform functional coating of CoS_x/MoO₃ on PP. Additionally, Fig. 2e and f show optical photographs of CoS_x/MoO₃/PP and folded–unfolded different separators. The unfolded CoS_x/MoO₃/PP did not exhibit obvious detachment, demonstrating good adhesion and flexibility of the coating. Therefore, thermal stability is significant for the safety of batteries. We conducted heating tests on different separators, and it is clear that the modified separators have better thermal stability and resistance to thermal shrinkage than PP (Fig. S9†). In the electrolyte wetting test, the modified separators also achieved better results, which facilitated the full wetting of the electrolyte (Fig. S10†).

Fig. 2 SEM images of (a) bare PP, (b) ZIF-67@CoS_x/MoO₃/PP, and (c) CoS_x/MoO₃/PP. (d) Cross-sectional SEM image of CoS_x/MoO₃/PP. (e) Photographs of CoS_x/MoO₃/PP. (f) Photographs of bare PP, ZIF-67@CoS_x/MoO₃/PP and CoS_x/MoO₃/PP under the same mechanical stress.

To investigate the adsorption capacity of the different samples for LiPSs, we performed static adsorption experiments of PR1, PR2, ZIF-67@CoS_x/MoO₃, and CoS_x/MoO₃ in a Li₂S₄ solution with equal mass (Fig. 3a). Benefiting from the synergistic effect of CoS_x and MoO₃, compared to the color of the liquid in the other bottles, the solution in bottle 5 became significantly lighter after 6 h of standing, and completed the adsorption of LiPSs in 24 h. The decrease in the absorption peak at 420 nm in the UV-Vis absorption spectra (Fig. 3b) further confirmed the efficient adsorption of LiPSs by CoS_x/MoO₃. However, the core of ZIF-67@CoS_x/MoO₃ mainly consisted of ZIF-67, which could limit the selective capture of LiPSs. Therefore, after 24 h, the Li₂S₄ peak in the corresponding solution was still evident. To demonstrate the inhibition of the shuttle effect more intuitively by the modified separator, we placed ZIF-67@CoS_x/MoO₃/PP and CoS_x/MoO₃/PP into H-type cells. As shown in Fig. 3c, LiPSs shuttle extensively into the transparent solution in the right compartment of the H-type cell with ZIF-67@CoS_x/MoO₃/PP in only 9 h. In contrast, after 18 h of standing, only a small amount of LiPSs entered the right compartment of the control group, further illustrating the effective suppression of the shuttle effect introduced by CoS_x and MoO₃. X-ray photoelectron spectroscopy (XPS) analysis was further conducted to study the chemical states of ZIF-67@CoS_x/MoO₃ and CoS_x/MoO₃ before and after LiPS adsorption and their full spectra are displayed in Fig. S11 and S12.† As shown in Fig. 3d and g, the SO_x species that appeared at high binding energy positions can effectively alleviate the shuttle effect, as reported in other articles.^26,27 The Co 2p_3/2 spectrum in Fig. 3e showed fitting peaks at 781.31 eV and 784.52 eV, and the spectrum in Fig. 3h showed fitting peaks at 781.72 eV and 784.01 eV, which can be attributed to Co²⁺ and Co³⁺, respectively.^28,29 The Mo 3d orbitals exhibited two strong peaks at 232.73 eV and 235.81 eV in Fig. 3f, as well as 232.69 eV and 235.75 eV in Fig. 3h, which can be assigned to Mo⁶⁺.¹⁷ As can be seen visibly, the binding energies of the metal ions in ZIF-67@CoS_x/MoO₃ and CoS_x/MoO₃ shifted after the adsorption of Li₂S₄. The binding energy of the Co³⁺ fitting peak in CoS_x/MoO₃ decreased by 2.36 eV that is much larger than that in ZIF-67@CoS_x/MoO₃, indicating its stronger chemical affinity with Li₂S₄ and higher adsorption capacity for LiPSs.³⁰ Additionally, the binding energy of the Mo⁶⁺ peak also shifted by 0.96 eV, indicating the involvement of Mo in the anchoring process of LiPSs. Therefore, the XPS analysis after adsorption confirmed that the hollow-structured CoS_x/MoO₃, with the assistance of bimetallic synergy and sulfur-rich sites, can selectively capture more soluble LiPSs and suppress the shuttle effect more efficiently. To investigate the advantages of the hollow bimetallic heterostructure in LSBs, we performed cyclic voltammetry (CV) tests on PR1/PP, PR2/PP, ZIF-67@CoS_x/MoO₃/PP, and CoS_x/MoO₃/PP (Fig. 4a and Fig. S13†). All samples exhibited typical redox peaks, but the peak currents of PR1 and PR2, which contained only single metal Co, were smaller than those of samples with the Co–Mo bimetallic heterostructure. Additionally, PR2 with a higher sulfidation degree showed better cycling stability than PR1. Meanwhile, CoS_x/MoO₃/PP demonstrated the best redox reversibility, showing almost no shift in the oxidation–reduction peaks after 3 cycles at a scan rate of 0.1 mV s⁻¹. Furthermore, compared to the reduction peak at around 2 V and the oxidation peak at around 2.4 V for ZIF-67@CoS_x/MoO₃/PP, the reduction peak of CoS_x/MoO₃/PP shifted positively, and the oxidation peak shifted negatively, indicating less internal polarization in the battery and faster reaction kinetics.¹⁴

Fig. 3 (a) Digital pictures of Li₂S₄ solutions before and after adding PR1, PR2, ZIF-67@CoS_x/MoO₃, and CoS_x/MoO₃ and (b) corresponding UV-Vis absorption spectra. (c) Li₂S₄ penetration in H-type cells based on different separators. High-resolution XPS spectra of (d) S (e) Co, and (f) Mo in ZIF-67@CoS_x/MoO₃ and ZIF-67@CoS_x/MoO₃/Li₂S₄. (g–i) High-resolution XPS spectra of (g) S (h) Co, and (i) Mo in CoS_x/MoO₃ and CoS_x/MoO₃/Li₂S₄.

Fig. 4 (a) 3-turn CV curves of LSBs with CoS_x/MoO₃/PP. (b) CV curves of LSBs with CoS_x/MoO₃/PP at different scan rates. (c) The linear relationship between I_p and ν^1/2 of LSBs with CoS_x/MoO₃/PP. (d and e) Extracted parts of the CV curves in (a) between 1.7 and 2.8 V and Tafel plots of Peak B and Peak C (inset). (f) Rate performance of LSBs with different separators. (g) Voltage gap between the charge/discharge platforms of the LSBs with different separators at 0.2C. (h) Cycling performance of different separators at 1C.

The higher polarization within ZIF-67@CoS_x/MoO₃/PP may be due to the low conductivity of ZIF-67 itself and the lower degree of sulfidation, which lead to limited catalytic sites and weak catalytic activity during the reaction. On the other hand, when comparing PR2 with CoS_x/MoO₃/PP, it can also be observed that the introduction of Mo enhances the conductivity and catalytic activity of CoS_x/MoO₃, resulting in the highest peak current and the smallest polarization for CoS_x/MoO₃/PP. CV tests were performed on the samples at different scan rates to further verify the improvement in battery stability achieved by the introduction of Mo (Fig. 4b and Fig. S14†). As the scanning rate increases, the CV curve of the LSB with CoS_x/MoO₃/PP still shows clear redox peaks and minimal polarization, indicating that CoS_x/MoO₃ can effectively promote the redox reaction in LSBs. Additionally, based on the CV results, we calculated the relationship between the peak current of different oxidation–reduction peaks (Peak A–C) and the corresponding scan rates for ZIF-67@CoS_x/MoO₃/PP and CoS_x/MoO₃/PP (Fig. 4c and Fig. S15†). The slope values of the fitted curves are related to the lithium-ion diffusion rate in the battery, and CoS_x/MoO₃/PP exhibited a steeper slope, indicating a faster lithium-ion diffusion rate.³¹ The flatter slope of ZIF-67@CoS_x/MoO₃/PP may be due to the lower degree of sulfidation and the insufficient ability to selectively adsorb and convert LiPSs, resulting in the dissolution of soluble LiPSs into the electrolyte during discharge, which increases the electrolyte concentration and increases resistance to lithium-ion transfer.³² The introduction of Mo and the deeper sulfidation can enhance the electron transfer rate of CoS_x/MoO₃ and greatly enhance the selective adsorption and conversion ability of CoS_x/MoO₃ for LiPSs.

To further examine the catalytic behavior of the CoS_x/MoO₃ modified separator on LiPSs, we extracted parts of the CV curves from Fig. 4a and Fig. S13† between 1.7 and 2.8 V, as demonstrated in Fig. 4d and e. The Tafel slope of the battery equipped with a separator modified by ZIF-67@CoS_x/MoO₃ is 50 mV dec⁻¹ for the reduction reaction, which is higher than that observed with a separator modified by CoS_x/MoO₃ (31 mV dec⁻¹). Meanwhile, the Tafel slope of the separator modified by ZIF-67@CoS_x/MoO₃ is observed to be 123 mV dec⁻¹ in the oxidation reaction, which is also higher than that of the CoS_x/MoO₃ modified separator (105 mV dec⁻¹). A smaller Tafel slope indicates a stronger charge transfer ability of the catalyst in the catalytic process. The above results suggest that the CoS_x/MoO₃ catalyst shows excellent charge transfer ability, which can accelerate the kinetics of polysulfide conversion. Therefore, it improves the discharge capacity and long cycle life of LSBs.

Therefore, according to the Randles–Sevcik equation,

In which I_p, D_Li⁺, and ν represent the peak current, lithium-ion diffusion coefficient, and scan rate, respectively, while n, A, and C_Li⁺ represent the number of electrons involved in the electrochemical reaction, the effective electrode area, and the lithium-ion concentration in the electrolyte. The lithium-ion diffusion rate can be observed through the linear correlation of the peak current and the square root of the sweep rate. The lithium-ion diffusion coefficients are obtained from the Randles–Sevcik equation (Fig. S16†), and D_Li⁺ on CoS_x/MoO₃ is always much higher than that on ZIF-67@CoS_x/MoO₃ and PP.

In addition, we performed galvanostatic charge–discharge (GCD) tests at different currents (0.1–2C) to examine the discharge performance of batteries with PR1/PP, PR2/PP, ZIF-67@CoS_x/MoO₃/PP, and CoS_x/MoO₃/PP (Fig. 4f). Due to the lack of selective capture of LiPSs and catalytic conversion ability, the capacity of PP decreases significantly at 1C current. After partial sulfidation of ZIF-67 and the introduction of Mo, the batteries with modified separators showed improved discharge performance at high currents. Fortunately, CoS_x/MoO₃/PP exhibited the best rate performance. LSBs with CoS_x/MoO₃/PP show a high discharge capacity of 632 mA h g⁻¹ at 2C rate. GCD testing is also a method used to measure the cycling stability of batteries. We tested the polarization ΔE of different samples during 100 cycles at 0.2C (Fig. 4g). At the beginning of the cycling, PP and CoS_x/MoO₃/PP had the highest and lowest polarization, respectively. With an increasing number of cycles, the polarization of PR1 and ZIF-67@CoS_x/MoO₃/PP increased significantly, which was related to the low conductivity of ZIF-67 itself and the lower degree of sulfidation, resulting in the coverage of catalytic sites. Thanks to the deeper sulfidation, CoS_x/MoO₃/PP maintained the smallest polarization after 100 cycles. Meanwhile, we disassembled the batteries after cycling to observe PP, ZIF-67@CoS_x/MoO₃/PP, and CoS_x/MoO₃/PP. Many pores of PP were blocked due to the lack of physical and chemical adsorption of LiPSs (Fig. S17a†). Partial detachment of the coating was observed in ZIF-67@CoS_x/MoO₃/PP after long cycles in Fig. S17b,† possibly due to the large volume expansion of the positive electrode during charge–discharge. In contrast, CoS_x/MoO₃/PP maintained its coating integrity after long cycles (Fig. S17c†), demonstrating the best cycling stability. After disassembling the battery after cycling, we found that the negative electrode surface of the battery equipped with the CoS_x/MoO₃ modified separator was smoother (Fig. S18†), which may be due to the ability of CoS_x/MoO₃ to quickly and selectively adsorb LiPSs, which greatly improves the “shuttle effect” of the battery.

As shown in Fig. 4h, all modified separators showed higher initial capacity than bare PP in the long cycling test at 1C. Furthermore, compared to other samples, CoS_x/MoO₃ showed the highest capacity retention (81.3%) and a capacity decay rate of 0.041% after 500 cycles. During cycling, the capacity of the battery with the CoS_x/MoO₃ modified separator did not monotonically decrease, further demonstrating the abundant active sites and improved electron transfer efficiency of CoS_x/MoO₃, which can accelerate the conversion of LiPSs and promote the reuse of dead sulfur, greatly improving battery life and cycling stability.³³ Compared with other transition metal compounds used as separators in LSBs (Table S1†), CoS_x/MoO₃ exhibited competitive cycling stability and rate capability.

To further investigate the internal resistance of batteries with ZIF-67@CoS_x/MoO₃/PP and CoS_x/MoO₃/PP during charge and discharge, galvanostatic intermittent titration (GITT) was performed at 0.1C. The polarization during cycling can be quantified by the relative magnitude of ΔiR in GITT, which is calculated as follows:

ΔiR(Ω) = |ΔV_QOCV–CCV|/I

where ΔV_QOCV–CCV is the voltage difference between the quiescent open circuit voltage (QOCV) and closed-circuit voltage (CCV) and I is the applied current. As shown in Fig. 5a and Fig. S19,† CoS_x/MoO₃/PP exhibited a smaller ΔiR than ZIF-67@CoS_x/MoO₃/PP at the Li₂S nucleation and activation points, indicating that CoS_x/MoO₃ can better reduce the hindrance of LiPS conversion.³⁴

Fig. 5 (a) GITT voltage profiles of CoS_x/MoO₃/PP. (b and c) In situ UV/Vis spectra of ZIF-67@CoS_x/MoO₃/Li₂S₄ and CoS_x/MoO₃/Li₂S₄. (d) In situ EIS spectra of the battery with a CoS_x/MoO₃ modified separator. (e) DRT calculated from EIS measurements of the battery with CoS_x/MoO₃/PP at different voltages and (f) corresponding contour plots. (g) Schematic diagram of the CoS_x/MoO₃ modified separator to improve the electrochemical performance of LSBs.

To investigate the dynamic process of Li₂S₄ adsorption by the samples, we performed in situ UV-Vis testing on the diluted Li₂S₄ solution containing ZIF-67@CoS_x/MoO₃ and CoS_x/MoO₃ samples. As can be seen in Fig. 5b and Fig. S20a,† the intensity of Li₂S₄ at 420 nm increased slightly at the beginning and decreased slowly afterwards. The optical image after Li₂S₄ adsorption showed a dark brown solution. This may be due to competition between the adsorption of Li₂S₄ by CoS_x/MoO₃ in the ZIF-67@CoS_x/MoO₃ sample and the adsorption of the solvent by parent ZIF-67. In contrast, the initial yellow solution of CoS_x/MoO₃ turned into a clear solution with almost no Li₂S₄ peak at 420 nm after 8 h of standing (Fig. 5c and Fig. S20b†). This is because the hollow bimetallic heterostructure obtained under deeper sulfidation enhances the affinity for sulfur, enables the selective capture of Li₂S₄ and achieves rapid adsorption.³⁵ To further characterize the impact of the samples on LiPS conversion, electrochemical impedance spectroscopy (EIS) testing was performed. Thanks to the introduction of Mo and the highly conductive sulfide, CoS_x/MoO₃ exhibited the lowest solution diffusion impedance and charge transfer resistance (Fig. S21†). Meanwhile, we performed in situ monitoring of the impedance of ZIF-67@CoS_x/MoO₃/PP and CoS_x/MoO₃/PP during discharge. The impedance of ZIF-67@CoS_x/MoO₃/PP suddenly increased during the LiPS generation stage as shown in Fig. S22,† indicating that the dissolution of LiPSs in the electrolyte hindered ion transfer. However, the impedance of CoS_x/MoO₃/PP showed no significant change during discharge as shown in Fig. 5d, indicating its effective adsorption and promotion of the conversion of LiPSs.³⁶ Additionally, semi-quantitative analysis of the EIS spectra was performed using the distribution of relaxation time (DRT) to further verify the stability and reversibility of the batteries with CoS_x/MoO₃/PP, as presented in Fig. 5e. The P1 peak corresponds to the contact resistance at the current collector and electrode interface, and the P2 and P3 peaks are related to ion transport at the anode and cathode, respectively. During discharge, the peaks of P2 and P3 both gradually shifted to lower frequencies and lower intensities, indicating a rapid mass transfer process at the cathode and anode. The P4 peak is associated with the charge transfer process at the positive electrode. As the voltage dropped below 2.3 V, the P4 peak increased slightly but decreased gradually as the discharge continued, which showed the rapid selective adsorption of LiPSs by the synergistic action of CoS_x and MoO₃ and prevented the mass transfer process from slowing down due to the dissolution of a large amount of LiPSs in the electrolyte.^37,38 The intensity color map of the DRT curves in Fig. 5f displayed more obvious changes in the P3 and P4 peaks. Overall, the improved sulfur cathode kinetics are illustrated in Fig. 5g. The hollow structure of the bimetallic heterostructure effectively suppresses the shuttle effect of LiPSs and accelerates their conversion into insoluble LiPSs.

Conclusions

In this work, MOF-derived heterostructure modified separators were designed to improve the electrochemical performance of LSBs. By varying the degree of sulfidation of ZIF-67, we successfully prepared core–shell structured ZIF-67@CoS_x/MoO₃ and hollow CoS_x/MoO₃, respectively. LSBs with CoS_x/MoO₃-modified separators show excellent electrochemical performance due to the complete sacrifice of ZIF-67 and the bimetallic synergistic system. The MoO₃ has strong chemical interactions with LiPSs; in addition, CoS_x has high conductivity that can speed up the transport of ions between the positive and negative sides and the conversion of LiPSs. The LSBs equipped with CoS_x/MoO₃/PP exhibit a high discharge capacity of 632 mA h g⁻¹ at 2C rate and a capacity decay rate of 0.041% after 500 cycles at 1C, demonstrating excellent cycling stability and rate performance. The excellent electrochemical performance of CoS_x/MoO₃ highlights the potential of MOF-derived heterostructure materials in the field of LSBs and provides new ideas for the development of high-performance LSBs.

Author contributions

Rongmei Zhu and Yuxuan Jiang conceived the ideas and wrote the paper; Yuxuan Jiang synthesized the materials and performed the experiments; Bingxin Sun performed the in situ experiments; Wang Zhang helped analyze the data; Rongmei Zhu and Huan Pang supervised the work and provided funding.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

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

Acknowledgements

This work was supported by the Natural Science Foundation of Jiangsu Province (Grant No. BK20230069, BX2023026 and BK20200044), the National Natural Science Foundation of China (U1904215), the Changjiang Scholars’ Program of the Ministry of Education (Q2018270), the Six Talent Peaks Project in Jiangsu Province, the Top Talent Project of Yangzhou University, and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2022R1A6A1A03063039). We also acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions and the technical support received from the Testing Center of Yangzhou University.

References

1. W. Zhang, Z. Zheng, L. Lin, X. Zhang, M. Bae, J. Lee, J. Xie, G. Diao, H. Im, Y. Piao and H. Pang, Ultrafast Synthesis of Graphene-Embedded Cyclodextrin-Metal-Organic Framework for Supramolecular Selective Absorbency and Supercapacitor Performance, Adv. Sci., 2023, 10, 2304062.
2. S. Zheng, Y. Sun, H. Xue, P. Braunstein, W. Huang and H. Pang, Dual-ligand and hard-soft-acid-base strategies to optimize metal-organic framework nanocrystals for stable electrochemical cycling performance, Natl. Sci. Rev., 2022, 9, nwab197.
3. C. Liu, Y. Bai, W. Li, F. Yang, G. Zhang and H. Pang, In Situ Growth of Three-Dimensional MXene/Metal–Organic Framework Composites for High-Performance Supercapacitors, Angew. Chem., 2022, 134, e202116282.
4. D. Luo, et al., Constructing Abnormal Step-Scheme Nano-Heterointerfaces as Sulfur Electrocatalysts with Desirable Electron Confinement for Practical Li-S Battery, Renewables, 2024, 2, 61–72.
5. X. C. Liu, et al., Design of Defect-Rich MgCo-Layered Double Hydroxide Microspheres as Highly Effective Sulfur Reduction Reaction Electrocatalyst for High-Performance Li-S Batteries, Renewables, 2024, 1–9.
6. Y. Jiang, M. Du, P. Geng, B. Sun, R. Zhu and H. Pang, CoO/MoO₃@Nitrogen-Doped carbon hollow heterostructures for efficient polysulfide immobilization and enhanced ion transport in Lithium-Sulfur batteries, J. Colloid Interface Sci., 2024, 664, 617–625.
7. L. Ni, G. Zhao, Y. Wang, Z. Wu, W. Wang, Y. Liao, G. Yang and G. Diao, Coaxial Carbon/MnO₂ Hollow Nanofibers as Sulfur Hosts for High-Performance Lithium-Sulfur Batteries, Chem. – Asian J., 2017, 12, 3128–3134.
8. Z. Wu, W. Wang, Y. Wang, C. Chen, K. Li, G. Zhao, C. Sun, W. Chen, L. Ni and G. Diao, Three-dimensional graphene hollow spheres with high sulfur loading for high-performance lithium-sulfur batteries, Electrochim. Acta, 2017, 224, 527–533.
9. X. Wang, X. Zhang, Y. Zhao, D. Luo, L. Shui, Y. Li and Z. Chen, Accelerated Multi-step Sulfur Redox Reactions in Lithium-Sulfur Batteries Enabled by Dual Defects in Metal-Organic Framework-based Catalysts, Angew. Chem., 2023, 135(42), e202306901.
10. L. Wu, J. Hu, X. Yang, Z. Liang, S. Chen, L. Liu, H. Hou and J. Yang, Synergistic effect of adsorption and electrocatalysis of CoO/NiO heterostructure nanosheet assembled nanocages for high-performance lithium–sulfur batteries, J. Mater. Chem. A, 2022, 10, 23811–23822.
11. S. Qiu, J. Zhang, X. Liang, Y. Li, J. Cui and M. Chen, Tunable MOFs derivatives for stable and fast sulfur electrodes in Li-S batteries, Chem. Eng. J., 2022, 450, 138287.
12. J. Wang and J. Li, Cobalt-based zeolitic imidazolate frameworks modified separator as efficient polysulfide adsorbent for high performance lithium-sulfur batteries, J. Colloid Interface Sci., 2021, 584, 354–359.
13. Y. Wen, Z. Shen, J. Hui, H. Zhang and Q. Zhu, Co/CoSe Junctions Enable Efficient and Durable Electrocatalytic Conversion of Polysulfides for High-Performance Li–S Batteries, Adv. Energy Mater., 2023, 13, 2204345.
14. Y. Luo, M. Wu, D. Zhang, J. Liu, Y. He, W. Zhang, S. Liu, Y. Dong, C. Xiang, L. Yang, H. Liu, H. Shu, X. Wang and M. Chen, Boosted Polysulfide Conversion by Co, Mn Bimetallic-Modulated Nitrogen–Carbon Material for Advanced Lithium–Sulfur Batteries, ACS Sustainable Chem. Eng., 2023, 11, 1087–1099.
15. X. Guan, M. Huang, L. Yang, G. Wang and X. Guan, Facial design and synthesis of CoS_x/Ni-Co LDH nanocages with rhombic dodecahedral structure for high-performance asymmetric supercapacitors, Chem. Eng. J., 2019, 372, 151–162.
16. B. Li, Q. Su, L. Yu, J. Zhang, G. Du, D. Wang, D. Han, M. Zhang, S. Ding and B. Xu, Tuning the Band Structure of MoS₂ via Co₉S₈@MoS₂ Core-Shell Structure to Boost Catalytic Activity for Lithium-Sulfur Batteries, ACS Nano, 2020, 14, 17285–17294.
17. K. Wen, L. Huang, L. Qu, T. Deng, X. Men, L. Chen and J. Wang, g-C₃N₄/MoO₃ composite with optimized crystal face: A synergistic adsorption-catalysis for boosting cathode performance of lithium-sulfur batteries, J. Colloid Interface Sci., 2023, 649, 890–899.
18. F. Wang, H. Fu, F.-X. Wang, X.-W. Zhang, P. Wang, C. Zhao and C.-C. Wang, Enhanced catalytic sulfamethoxazole degradation via peroxymonosulfate activation over amorphous CoS_x@SiO₂ nanocages derived from ZIF-67, J. Hazard. Mater., 2022, 423, 126998.
19. J. Bai, T. Meng, D. Guo, S. Wang, B. Mao and M. Cao, Co₉S₈@MoS₂ Core Shell Heterostructures as Trifunctional Electrocatalysts for Overall Water Splitting and Zn Air Batteries, ACS Appl. Mater. Interfaces, 2018, 10, 1678–1689.
20. J. Yang, Y. Qu, X. Lin, L. Wang, Z. Zheng, J. Zhuang and L. Duan, MoO₃/MoS₂ flexible paper as sulfur cathode with synergistic suppress shuttle effect for lithium-sulfur batteries, Electrochim. Acta, 2022, 418, 140378.
21. K. Ji, Q. Che, Y. Yue and P. Yang, Ni-Doped Co₃S₄ Hollow Nanobox for the Hydrogen Evolution Reaction, ACS Appl. Nano Mater., 2022, 5, 9901–9909.
22. J. Gong, W. Luo, Y. Zhao, M. Xie, J. Wang, J. Yang and Y. Dai, Co₉S₈/NiCo₂S₄ core-shell array structure cathode hybridized with PPy/MnO₂ core-shell structure anode for high-performance flexible quasi-solid-state alkaline aqueous batteries, Chem. Eng. J., 2022, 434, 134640.
23. D. Lei, W. Shang, X. Zhang, Y. Li, S. Qiao, Y. Zhong, X. Deng, X. Shi, Q. Zhang, C. Hao, X. Song and F. Zhang, Facile Synthesis of Heterostructured MoS₂-MoO₃ Nanosheets with Active Electrocatalytic Sites for High-Performance Lithium–Sulfur Batteries, ACS Nano, 2021, 15, 20478–20488.
24. Z. Wu, W. Wang, Y. Wang, C. Chen, K. Li, G. Zhao, C. Sun, W. Chen, L. Ni and G. Diao, Three-dimensional graphene hollow spheres with high sulfur loading for high-performance lithium-sulfur batteries, Electrochim. Acta, 2017, 224, 527–533.
25. J. Fu, C. Bie, B. Cheng, C. Jiang and J. Yu, Hollow CoS_x Polyhedrons Act as High-Efficiency Cocatalyst for Enhancing the Photocatalytic Hydrogen Generation of g-C₃N₄, ACS Sustainable Chem. Eng., 2018, 6, 2767–2779.
26. Y. Tang, F. Jing, Z. Xu, F. Zhang, Y. Mai and D. Wu, Highly Crumpled Hybrids of Nitrogen/Sulfur Dual-Doped Graphene and Co₉S₈ Nanoplates as Efficient Bifunctional Oxygen Electrocatalysts, ACS Appl. Mater. Interfaces, 2017, 9, 12340–12347.
27. J. Wang, J. Xu, X. Guo, T. Shen, C. Xuan, B. Tian, Z. Wen, Y. Zhu and D. Wang, Synergistic regulation of nickel doping/hierarchical structure in cobalt sulfide for high performance zinc-air battery, Appl. Catal., B, 2021, 298, 120539.
28. L. Yang, L. Zhang, G. Xu, X. Ma, W. Wang, H. Song and D. Jia, Metal–Organic-Framework-Derived Hollow CoS_x@MoS₂ Microcubes as Superior Bifunctional Electrocatalysts for Hydrogen Evolution and Oxygen Evolution Reactions, ACS Sustainable Chem. Eng., 2018, 6, 12961–12968.
29. J. Li, B. Gao, Z. Shi, J. Chen, H. Fu and Z. Liu, Graphene/Heterojunction Composite Prepared by Carbon Thermal Reduction as a Sulfur Host for Lithium-Sulfur Batteries, Materials, 2023, 16, 4956.
30. H. Liu, X. Yang, B. Jin, M. Cui, Y. Li, Q. Li, L. Li, Q. Sheng, X. Lang, E. Jin, S. Jeong and Q. Jiang, Coordinated Immobilization and Rapid Conversion of Polysulfide Enabled by a Hollow Metal Oxide/Sulfide/Nitrogen-Doped Carbon Heterostructure for Long-Cycle-Life Lithium-Sulfur Batteries, Small, 2023, 19, 2300950.
31. Z. Gu, C. Cheng, T. Yan, G. Liu, J. Jiang, J. Mao, K. Dai, J. Li, J. Wu and L. Zhang, Synergistic effect of Co₃Fe₇ alloy and N-doped hollow carbon spheres with high activity and stability for high-performance lithium-sulfur batteries, Nano Energy, 2021, 86, 106111.
32. Y. Yoshie, K. Hori, T. Mae and S. Noda, High-energy-density Li–S battery with positive electrode of lithium polysulfides held by carbon nanotube sponge, Carbon, 2021, 182, 32–41.
33. L. Zhang, T. Li, X. Zhang, Z. Ma, Q. Zhou, Y. Liu, X. Jiang, H. Zhang, L. Ni and G. Diao, Core–shell polyoxometalate-based zeolite imidazole framework-derived multi-interfacial MoSe₂/CoSe₂ @NC enabling multi-functional polysulfide anchoring and conversion in lithium–sulfur batteries, J. Mater. Chem. A, 2023, 11, 3105–3117.
34. Y. Wu, N. Wu, X. Jiang, S. Duan, T. Li, Q. Zhou, M. Chen, G. Diao, Z. Wu and L. Ni, Bifunctional K ₃PW₁₂O₄₀/Graphene Oxide-Modified Separator for Inhibiting Polysulfide Diffusion and Stabilizing Lithium Anode, Inorg. Chem., 2023, 62, 15440–15449.
35. Z. Ma, W. Liu, X. Jiang, Y. Liu, G. Yang, Z. Wu, Q. Zhou, M. Chen, J. Xie, L. Ni and G. Diao, Wide-Temperature-Range Li–S Batteries Enabled by Thiodimolybdate [Mo₂S₁₂]²⁻ as a Dual-Function Molecular Catalyst for Polysulfide Redox and Lithium Intercalation, ACS Nano, 2022, 16, 14569–14581.
36. L. Ni, J. Gu, X. Jiang, H. Xu, Z. Wu, Y. Wu, Y. Liu, J. Xie, Y. Wei and G. Diao, Polyoxometalate-Cyclodextrin-Based Cluster-Organic Supramolecular Framework for Polysulfide Conversion and Guest–Host Recognition in Lithium-sulfur Batteries, Angew. Chem., Int. Ed., 2023, 62, e202306528.
37. Y. Zhang, C. Kang, W. Zhao, Y. Song, J. Zhu, H. Huo, Y. Ma, C. Du, P. Zuo, S. Lou and G. Yin, d-p Hybridization-Induced “Trapping–Coupling–Conversion” Enables High-Efficiency Nb Single-Atom Catalysis for Li–S Batteries, J. Am. Chem. Soc., 2023, 145, 1728–1739.
38. Q. Liang, S. Wang, X. Lu, X. Jia, J. Yang, F. Liang, Q. Xie, C. Yang, J. Qian, H. Song and R. Chen, High-Entropy MXene as Bifunctional Mediator toward Advanced Li–S Full Batteries, ACS Nano, 2024, 18, 2395–2408.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi01249f

---