High-efficiency metal selenide as an electrocatalyst in a separator for lithium–sulfur batteries†

Yujuan Hu, Bo Jin* and Hui Liu
Key Laboratory of Automobile Materials, Ministry of Education, and School of Materials Science and Engineering, Jilin University, Changchun 130022, China. E-mail: jinbo@jlu.edu.cn

First published on 23rd April 2025

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Introduction

The rapidly developing field of batteries for electric cars and electronic devices requires improvements in cycle longevity and fast-charge performance which lithium-ion batteries seem to be unable to provide.^1–3 LSBs possess extremely high theoretical specific capacities (1675 mAh g⁻¹), are environmentally friendly, and are also rich in sulfur.^4,5 Thus LSBs have a bright future for development for use in electric vehicles.^6–8 However, the capacity loss caused by their shortcomings has slowed down the pace of commercialization. Most of these problems are caused by the shuttle effect, which arises from the dissolution of multiple intermediate sulfides. In addition, the insulating properties of S₈, with an electronic conductivity of 5 × 10⁻³⁰ S cm⁻¹, and Li₂S_x (x = 1, 2) are also specific reasons for the delay in commercialization of LSBs. Therefore, strategies are urgently needed to accelerate the application of LSBs.

In recent years, researchers have tried different methods to improve the problems of LSBs. On the one hand, for the cathode, various structural engineering approaches and different catalytic particles are used to effectively anchor LiPSs and accelerate the multistage reaction dynamics to inhibit the shuttle effect. Polar substances, such as phosphates, transition metal oxides, nitrides, and sulfides, have been rapidly developed to chemically anchor LiPSs at polar sites.^9–15 On the other hand, for the protection of the lithium anode, coating compounds such as Li₃N/LiF are available, and an in situ solid electrolyte interface (SEI) for coating onto the Li metal surface, which induces uniform deposition of Li, can be synthesized by artificial means.^16,17 From the perspective of these two strategies alone, designing a second adsorption layer (a functional separator) is understood to be another more comprehensive and effective strategy. Functional separators are favored by researchers because they are simple to manufacture and they provide an obvious improvement in electrochemical performance after modification, and protect both the cathode and the anode at the same time. The functional separator has made its own contribution in three aspects: by inhibiting LiPSs shuttle, by accelerating reaction kinetics, and by reducing lithium dendrites.

Transition metal sulfides/selenides (TMS/TMSe) have attracted the attention of researchers due to their semi-metallic properties, excellent electronic conductivity, sulfur affinity, and robust adsorption of LiPSs. TMS like MoS₂ has been proved to be the efficient catalyst in the field of photoelectric conversion.^18,19 On account of the electronegativity between the TM and S, the changing valence of S, as well as the potential formation of S–S and M–M bonds, TMS possess the high catalytic activity due to the abundant defects grown on its surface.²⁰ TMS have the stable catalytic effect on LiPSs, and also have the strong adsorption capacity for LiPSs because of its polar peculiarity.²¹ TMSe, formed from Se which is in the same chemical family as S, have similar crystal structure characteristics as TMS with the upper electronic conductivity of 1 × 10⁻³ S cm⁻¹ (>5 × 10⁻³⁰ S cm⁻¹, S₈). Sulphophilic TMSe can adsorb LiPSs due to their polarity and catalyze LiPSs due to their abundant active sites.^22,23 The unique d-electronic structures of TMSe also result in good electrocatalysis of LiPSs.²⁴ Lin et al. synthesized Fe₃Se₄@PNCNFs, via an electrospinning technology, which shows strong adsorption and catalysis of LiPSs, whilst the low internal impedance of the battery accelerated the lithium-ion migration rate.²⁵ An intermediate of Li_xFeS_y (facilitating 3D deposition of Li₂S) was generated to improve the invertibility and rate performance, thus providing a potential and improved method to improve the LSBs’ performance. Zhang et al. designed an amide-modified carbon nanofiber combined with a CoFe₂O₄ composite (CFOANF) as a membrane electrode, which had high specific capacity and good cyclic stability under high sulfur loading.²⁶ Wang's team prepared a modified functionalized CNTs separator coated with K₃PW₁₂O₄₀ nanospheres (KPW@FCNTs) for a lithium–sulfur battery, with an initial discharge capacity of 630 mAh g⁻¹ at a low temperature (−10 °C).²⁷ By coating the separator with polar nanoparticles, the smooth separator with few holes prevents the LiPSs shuttle effect whilst the chemical adsorption of polar substances on LiPSs effectively blocks the LiPSs at the cathode side.

In this article, an efficient porous polyhedron nanostructure (CoSe/NCC) is synthesized via a simple solvothermal and in situ calcination (selenization) method. The introduction of polar CoSe effectively adsorbs LiPSs through Lewis acid–base interactions. The synergistic effect of the highly conductive CNTs and NC, as well as the polar CoSe enhances the transplantation rate of Li⁺ and the anchoring ability of LiPSs, thus accelerating the kinetics of the transformation of multistage intermediates. As a consequence, a lithium–sulfur battery with a MWCNTs/S cathode and a CoSe/NCC/PP separator has a high discharge capacity of 932 mAh g⁻¹ after 100 cycles at a low current density of 0.5 C. There is still a discharge capacity of 300 mAh g⁻¹ after 1000 cycles at a high current of 5 C, thus providing insights into the excellent rate performance (quick-charge) and long run of LSBs. At an electrolyte/sulfur (E/S) ratio of 3.65 μL mg⁻¹ and a sulfur loading of 4.1 mg cm⁻², a battery with good electrochemical performance is achieved.

Experimental section

Synthesis of ZIF-67

Co(NO₃)₂·6H₂O (2.71 g) was added into a beaker filled with 100 mL of carbinol to form solution A, and dimethylimidazole (2.4 g) was put in a beaker filled with 100 mL of carbinol to make solution B. Solution A was quickly added into solution B. Then the mixture was agitated for 240 min to mix the solution evenly, and it was left to stand for 3 h. A purple powder was obtained by centrifugation, and was vacuum-dried in an oven at 60 °C after cleaning.

Synthesis of Co/NC and Co/NCC

A mixed solution of 0.5 g of ZIF-67, 1 g of C₃H₆N₆, and 50 mL of ethanol was stirred overnight at 80 °C. After drying, the as-obtained product was placed into a porcelain boat and carbonized for 2 h at 800 °C (ramp rate, 2 °C min⁻¹) in an Ar atmosphere to obtain the Co/NCC. When C₃H₆N₆ was not added, Co/NC was also obtained by the same method.

Synthesis of CoSe/NCC

The prepared Co/NCC and selenium powder were mixed well at a weight ratio of 1:2. Under an Ar atmosphere, the temperature was controlled to 800 °C (ramp rate, 2 °C min⁻¹), and the heat treatment lasted for 3 h. The sediment was collected at 25 °C to obtain the CoSe/NCC.

Preparation of MWCNTs/S

MWCNTs/S was made via a classical method. The MWCNTs (0.2 g) were uniformly mixed with sulfur powder (0.6 g). The mixture was annealed for 12 h at 155 °C in a tubular furnace under an Ar atmosphere. Finally, the sample was ground into fine powder for further use.

Preparation of the CoSe/NCC/PP separator

0.21 g of CoSe/NCC, 0.06 g of acetylene black (AB), and 0.375 g of polyvinylidene fluoride (PVDF) were mixed evenly, and the appropriate amount of N-methylpyrrolidone (NMP) was added. The mixture was agitated at room temperature for 6 h. The obtained slurry was evenly coated onto a commercial PP separator as a way of modifying the separator. Then, the coated separator was put in a vacuum oven at 60 °C for more than 12 h. Finally, the coated separator was cut into 16 mm diameter disks using a microtome, and the resulting material was named as the CoSe/NCC/PP separator. Following the same steps, Co/NCC/PP and Co/NC/PP separators were also made for comparison. The mass loading on the separator was ≈0.8 mg cm⁻².

Assembly of symmetric cells

Li₂S and S (at a molar ratio of 1:5) were put into a mixed solvent of 1,2-dimethoxyethane and 1,3-dioxolane (1:1, v/v), followed by successive stirring for 24 h at 60 °C to prepare a Li₂S₆ solution. 70 wt% active material (CoSe/NCC or Co/NCC or Co/NC), 20 wt% acetylene black (AB), and 10 wt% polyvinylidene fluoride (PVDF) were blended evenly, and the appropriate mount of 1-methyl-2-pyrrolidinone (NMP) was added to achieve a uniform slurry. The obtained slurry was coated onto Al foil, and then vacuum-dried for 12 h at 60 °C. Finally, the Al foil was cut into 12 mm round plates in a slicer. The mass loading of the active material was ≈0.8 mg cm⁻². The same electrode was used as both the positive and negative electrodes for the symmetric cell. 0.5 M Li₂S₆ (15 μL) was utilized as an electrolyte.

Adsorption test

S and Li₂S were mixed (a molar ratio of 5:1), and then the mixture was loaded into the component solvent of 1,3-dioxolane (DOL) and 1,2-dimethoxymethane (DME) (volume ratio of DOL to DME = 1:1). In a glove box filled with an argon atmosphere, the as-obtained mixture was stirred at 65 °C for 24 h to obtain a Li₂S₆ solution. 50 mg of CoSe/NCC, Co/NCC, or Co/NC were poured into the 5 mM as-synthesized Li₂S₆ solution. The observation time was set to 6 h, and the UV-Vis absorption spectra of the supernatants of the different solutions were recorded.

Materials characterization

The crystal structures of the materials were verified using X-ray diffraction (XRD). Raman spectra were collected using a Raman spectrometer (Renishaw micro-Raman). Nitrogen adsorption/desorption isotherms were isothermically tested at −196 °C to assess the Brunauer–Emmett–Teller (BET) specific surface area and porosity using an analyzer (Micromeritics ASAP 2460). X-ray photoelectron spectroscopy (XPS) tests were performed for studying the elements and valence states of the samples using an ESCALAB-250Xi. The microstructures and morphologies of the samples were investigated using scanning electron microscopy (SEM, JSM-7900F), transmission electron microscopy (TEM), high-resolution TEM (HRTEM, JEOL JEM-2100F, 200 kV), and energy dispersive spectroscopy (EDS). The weight of the sulfur in the MWCNTs/S was surveyed using thermal gravimetric analysis (TGA) under a temperature gradient of 10 °C min⁻¹ from 25 to 600 °C. UV-Vis spectra of the adsorption experiments were tested using an Evolution 300 UV-Vis spectrophotometer.

Electrochemical measurements

0.21 g of MWCNTs/S, 0.06 g of AB, and 0.375 g of PVDF were mixed uniformly with the appropriate amount of NMP solvent and agitated for 6 h. The mixture was evenly daubed onto Al foil. Furthermore, the coated aluminum foil was placed in an oven overnight before being cut into round plates. A lithium–sulfur battery was assembled with the MWCNTs/S as the cathode, CoSe/NCC/PP, Co/NCC/PP, or Co/NC/PP as the separator, and lithium metal plate as the anode. The diameter of the cathode was 12 mm, the diameter of the lithium anode was 15.6 mm, and the diameter of the separator was 16 mm. The remaining information on electrolyte dosage and active material loadings is given in Table S1.† 1 M trifluoromesulfonimide lithium salt containing 2 wt% LiNO₃ in DOL and DME (volume ratio of DOL to DME = 1:1) was used as an electrolyte. LIR2025 button-type batteries were fabricated in an argon atmosphere-filled glove box (water and oxygen indicators were below 0.1 ppm) to explore the correlative electrochemical performance. An electrolyte/sulfur (E/S) ratio of 15 μL mg⁻¹ was usually utilized in the lithium–sulfur battery, and different E/S ratios were chosen for comparison. A LAND CT2001A battery system (Wuhan, China) was selected to measure the rate and cycling performance of the LSBs, and the voltage range was 1.7–2.8 V. Cyclic voltammetry (CV) curves of the LSBs were tested using an electrochemical workstation (CHI650D) at a scan rate of 0.1 mV s⁻¹. The electrochemical impedance spectroscopy (EIS) spectra were derived from the same workstation, the frequency range was 0.01 to 100 kHz, and the alternating current potential was 5 mV. For symmetric cells, the CV curves were measured at a sweep rate of 10 mV s⁻¹. Tafel curves were tested at a scan rate of 1 mV s⁻¹ in the voltage range of −0.1 to 0.1 V.

Results and discussion

Fig. 1 displays the synthesis of CoSe/NCC and how the as-synthesized CoSe/NCC is coated onto the PP separator as a modified layer, which is used as a functional separator in the lithium–sulfur battery. The separator in the battery plays a role in separating the positive electrode from the negative electrode to avoid a short circuit, conducting ions, and insulating electrons. The CoSe/NCC-modified separator has a dense surface which can physically block the shuttle of LiPSs. Polar CoSe uses its Lewis acid–base interactions to adsorb LiPSs, limiting LiPSs to the positive side as much as possible, alleviating the shuttle effect, and thus improving the electrochemical performance of lithium–sulfur batteries. ZIF-67 is synthesized through a simple solution method, resulting in uniform dodecahedral particles with average particle sizes of 500 nm (Fig. 2a and Fig. S1†). ZIF-67 is directly carbonized at a high temperature of 800 °C to form a nitrogen-doped carbon shell with a crumpled surface covered with cobalt nanoparticles (Co/NC), as shown in Fig. S2.† In Fig. S3,† it is shown that the Co, C, and N are uniformly distributed in Co/NC. The mixture of ZIF-67 and melamine is carbonized at the same temperature (800 °C) to form NC covered with cobalt nanoparticles that are surrounded by carbon nanotubes (CNTs) with a diameter of approximately 50 nm (Fig. S4†). Fig. S5† confirms the even distribution of Co, C, and N in Co/NCC as well as the presence of Co nanoparticles on the carbon shells. For the formation of CoSe/NCC, Co/NCC is directly mixed with selenium powder and then selenized in situ at 800 °C to form CoSe/NCC. By comparing the SEM images of Co/NCC (Fig. S4†) and CoSe/NCC (Fig. 2b), it is found that the quantity of CNTs in CoSe/NCC decreases significantly after two high temperatures treatments at 800 °C (carbonization and selenization). In Fig. S6,† it is obvious that there are CNTs in CoSe/NCC. The dodecahedral structure of CoSe/NCC remains intact after the two high-temperature treatments, which proves that the mechanical strength of the grains is sufficient. The complete dodecahedral structure and the CNTs distributed in the outer layer can be seen in the TEM images in Fig. 2c and Fig. S7.† The significant (101), (002), and (102) crystal faces of CoSe are observed in Fig. 2d and e, and their crystal spacings are 0.269, 0.264, and 0.202 nm, respectively. CoSe is surrounded by graphitic carbon with a (002) crystal face (crystal spacing: 0.34 nm).^28–30 The SAED pattern of CoSe/NCC in Fig. 2f shows the (101) and (110) crystal faces of CoSe. In Fig. 2g–k, it is evident that the Co, Se, C, and N are uniformly distributed in CoSe/NCC.

Through the qualitative characterization of the material using XRD, it is concluded from Fig. 3a that the crystal structures of Co/NCC and Co/NC are the same, and their diffraction peaks at 44.2°, 51.5°, and 75.8° correspond to the (111), (200), and (220) crystal faces in the standard PDF card of Co (JCPDS No. 89-4307), respectively. The above results verify the successful synthesis of Co/NCC and Co/NC. Compared to Co/NCC and Co/NC, the diffraction peaks of CoSe/NCC are significantly different due to two high-temperature treatments required for the synthesis of CoSe/NCC. The three most obvious peaks at 33.2°, 44.8°, and 50.4° correspond to the (101), (102), and (110) crystal faces in the CoSe standard PDF card (JCPDS No. 70-2870), respectively, indicating strong crystallinity and the successful preparation of CoSe/NCC. The XRD patterns of the precursor (ZIF-67) and the cathode material (MWCNTs/S) also match with their corresponding PDF cards, which are presented in Fig. S8.† Combining carbon materials with sulfur is the current mainstream modification method, and the ratio of MWCNTs to S is 1:3. Fig. 3b exhibits the TGA curve of MWCNTs/S. With an increase in temperature, sulfur volatilizes in the material, accounting for 72.7% of the total mass. Moreover, the CNTs are loaded with sulfur inside and outside, and the sulfur and carbon are evenly distributed in the MWCNTs/S (Fig. S9†). Furthermore, for the sake of investigating the degree of defects in the carbon-based material, Raman characterization was carried out. Fig. 3c shows that the two peaks at around 1358 and 1593 cm⁻¹ belong to disordered sp³ carbon and graphitized sp² carbon, respectively, which represent the D and G bands of carbon, respectively.³¹ The intensity ratio of the D and G bands (I_D/I_G) can demonstrate the degree of defects in carbon-doped materials. The I_D/I_G values of Co/NCC (1.02) and CoSe/NCC (1.00) are higher than that of Co/NC (0.99), indicating higher defect degrees.³² In addition, as shown in Fig. 3d, for CoSe/NCC, the isothermal hysteresis loop of the nitrogen adsorption–desorption curve is a type IV isotherm. Both Co/NCC and Co/NC also have the same isotherm type (Fig. S10 and S11†). In addition, the Brunauer–Emmett–Teller (BET) specific surface area (S_BET) of Co/NCC (221.3 m² g⁻¹) is higher than that of Co/NC (125.4 m² g⁻¹) because the addition of the CNTs increases the S_BET. Compared to Co/NCC, the decrease in the S_BET of CoSe/NCC (131.8 m² g⁻¹) is due to the selenization of the material, which reduces the S_BET by covering some pores with CoSe. Notably, the average pore size distribution of the three samples is approximately 3.8 nm. The microporous structure is not only conducive to ion transport, which can allow lithium ions to pass through quickly but also limits the passage of long chain polysulfides. Furthermore, the high S_BET promotes the penetration of the electrolyte, and exposes a good number of active sites.³³

To get a more detailed understanding of the valence states of the material, we conducted an XPS test and analysis of the material. As shown in Fig. 4a, the full XPS spectrum shows that C, N, Co, Se, and O exist in CoSe/NCC. The C 1s XPS fine spectrum (Fig. 4b) has two characteristic peaks at 285.54 and 284.8 eV, belonging to the C–C/CC and C–N/C–O bonds, respectively.³⁴ The N 1s spectrum is fitted to four main peaks, namely, oxidized N (405.4 eV), graphitized N (402.8 eV), pyrrolic N (400.9 eV), and pyridinic N (398.8 eV),³⁵ as presented in Fig. 4c. It is found that in CoSe/NCC, C and N exist simultaneously and are bonded. CoSe/NCC richly N-doped, which improves the transition reaction rate of LiPSs and the electron donation ability, thus reducing the shuttling effect.^36,37 Furthermore, to explore the LiPSs adsorption ability of Co/NC, Co/NCC, and CoSe/NCC, we conducted static adsorption experiments.³⁸ After 24 h, the control sample Li₂S₆ is yellow-black, and the Li₂S₆ solution containing CoSe/NCC becomes the clearest, followed by the solutions containing Co/NCC and Co/NC. The supernatant of each solution was extracted and its ultraviolet absorption spectrum was recorded (Fig. 4d). It is found that two characteristic peaks of Li₂S₆ appear near 270 and 281 nm.³⁹ All indications indicate that the characteristic peaks of CoSe/NCC are the weakest and that CoSe/NCC adsorbs LiPSs the best, and thus the shuttle effect is slowed down to improve the electrochemical performance of the LSBs. Before adsorption, there are peaks for Co³⁺ at 778.12 and 793.68 eV, for Co²⁺ at 780.73 and 797.22 eV, and two satellite (Sat.) peaks at 785.25 and 802.57 eV.^36,37 After adsorption of Li₂S₆, the peak positions for Co³⁺ are at 778.15, 793.81 eV, for Co²⁺ at 780.82, 797.29 eV, and the satellite peaks appear at 785.44, 802.80 eV (Fig. 4e). All the peaks in the Co 2p spectrum of the Li₂S₆-adsorbed material shift in the direction of increased binding energy, demonstrating the strong interaction of the exposed Co sites with the surrounding robust electronegative sulfur ligand.^40–42 In the Se 3d spectrum of CoSe/NCC (Fig. 4f), the peaks at 53.99 and 54.73 eV are attributed to Se 3d_5/2 and Se 3d_2/3, respectively. The other two peaks belong to Se–O (58.86, 59.96 eV).⁴³ After adsorption of Li₂S₆, the peaks all move in the direction of higher binding energy (to the left) because of Lewis acid–base interactions,^44,45 and the corresponding peak positions of Se 3d_5/2, Se 3d_2/3, and Se-O are at 54.27, 55.24, and 58.97 as well as 60.17 eV, respectively. Fig. S12† shows the five peaks for CoSe/NCC after Li₂S₆ adsorption. The S 2p_3/2 and S 2p_1/2 peaks are located at 160.8 and 163.8 eV, respectively. In addition, high binding energy peaks are detected at 166.4 and 168.7/169.9 eV, which are caused by thiosulfate and polythionate, respectively.

In this study, a lithium–sulfur battery uses modified PP as a separator, MWCNTs/S as the positive electrode, and lithium metal as the negative electrode. Based on the results of the adsorption tests, CoSe/NCC adsorbs LiPSs the best, and the electrochemical performance was verified by CV tests of three different batteries at conversion sweep speeds over a voltage range of 1.7–2.8 V. Two successive reduction peaks appear when moving from high potential to low potential. The reduction peak at 2.3 V (high potential) represents the conversion of S₈ into soluble Li₂S₈, Li₂S₆, and Li₂S₄. The reduction peak near 2.0 V (low potential) is the main output of the specific capacity of the discharge reaction (meaning the reduction of Li₂S₄ to Li₂S₂/Li₂S).⁴⁶ In addition, by comparing the first cycles of the three batteries together, it is clearly shown in Fig. 5a that CoSe/NCC shows the biggest peak current, and its peak area is the largest. It is followed by Co/NCC and Co/NC, which verifies that CoSe/NCC has the greatest ability to accelerate the redox kinetics. As displayed in Fig. S13,† CoSe/NCC still has the smallest polarization voltage and maximum peak current, indicating that it has the best reversibility and consistency. The CV curve of the lithium–sulfur battery made from CoSe/NCC/PP at 0.1–0.5 mV s⁻¹ is shown in Fig. 5b. The obvious redox peak gradually increases with the increase of the sweep speed, and the area of the redox peak also increases. The voltage of the low-voltage reduction peak also shifts in the low-voltage direction when the sweep speed is increased, while the oxidation peak in the high-voltage location shifts toward the high-voltage direction. When the sweep speed is faster, the redox peaks of the three batteries become more obvious. As can be seen from Fig. S14† and Fig. 5b, the coincidence degree and the peak current of CoSe/NCC are the best. A Li//Li symmetrical cell equipped with the modified separator was assembled to study the reaction kinetics of the lithium metal electrode and evaluate the effect of the modified separator on the performance of the lithium metal electrode. The Tafel equation is as follows.

η = a + blogi	(1)

Here, a is the Tafel intercept, b is the Tafel slope, i is the current density, and η is the overpotential. The exchange current density (ECD) is derived from i when η is zero. The ECD characterized by the Tafel curve (Fig. 5c) is a means to quantitatively measure the catalytic activity of a material.^47,48 CoSe/NCC has the fastest ion transport capacity due to having a higher exchange current density (ECD = 1.16) than Co/NCC (ECD = 0.95) and Co/NC (ECD = 0.54). Faster ion reaction kinetics means that a steadier SEI is generated on the surface of CoSe/NCC, which is more beneficial to the lithium–sulfur battery's electrochemical performance.⁴⁹

Fig. S15† displays the results of the toughness tests conducted on the PP separator coated with CoSe/NCC after drying. After being folded in half twice and then expanded, there is no loss or damage at all, indicating that the separator has good flexibility. An excellent battery separator must not only have good toughness and mechanical strength, but also play a key role in the degree of infiltration for battery performance when it is in contact with the electrolyte. Therefore, the contact angles of the electrolyte on the four separators were tested (Fig. S16†). The results show that when the electrolytic liquid is dropped onto the CoSe/NCC/PP separator it is immediately wetted, and the contact angle is almost 0°. The permeability of the electrolyte in the Co/NCC/PP, Co/NC/PP, and empty PP separators is not as good as that of the CoSe/NCC/PP separator, and produces contact angles of 14°, 20°, and 32°, respectively. This also verifies that CoSe/NCC/PP is better than the other separators at improving the infiltration rate of the electrolyte. The good affinity between the electrolyte and the separator may be related to the polar groups of CoSe/NCC. According to the SEM image in Fig. S17a,† it is generally observed that there are many obvious holes in the pure PP separator to enable ions to pass through smoothly, but this is also the drawback that leads to the shuttle effect of polysulfide. The larger holes make soluble polysulfide pass through the separator into the negative side and react directly with lithium, contributing to the loss and damage of Li, thus causing an irreversible shuttle effect. As presented in Fig. S17b and c,† the modified Co/NCC/PP and Co/NC/PP separators have reduced pores, but the cracks on the surfaces are clearly visible, and the shuttle effect is not that well alleviated. Fig. S17d† shows that the PP separator modified with CoSe/NCC has a flat surface and a further reduction in the number of pores, which aid the blocking of the LiPSs on the positive side. CoSe/NCC is not only a physical barrier, but can also adsorb LiPSs effectively because of the chemical action of the polar CoSe species (Lewis acid–base interactions). As shown in Fig. S18,† the thickness of the modified CoSe/NCC/PP separator is 32 μm, and the thickness of the modified layer is 6 μm. The lithium-ion diffusion coefficient (D_Li⁺) is evaluated by linear fitting of the CV curves at different sweep speeds. The D_Li⁺ values for the LSBs with the different separators are obtained by taking advantage of the following formula (Randles–Sevcik equation).⁵⁰

Ip = 269000n1.5ACLiDLi+0.5v0.5	(2)

Here, I_p symbolizes the peak current, n expresses the number of electrons transferring through the redox process, A is the electrode area, C_Li is the lithium-ion concentration, D_Li⁺ signifies the diffusion coefficient of the lithium ion, and v is the scan rate.

The Li⁺ diffusion coefficients for the LSBs with CoSe/NCC/PP, Co/NCC/PP, and Co/NC/PP separators are obtained from the liner relationship of I_p and v^1/2 in Fig. 5d–f. Taking oxidation peak A as an example, the D_Li⁺ values of the LSBs with the CoSe/NCC/PP, Co/NCC/PP, and Co/NC/PP separators are 11.6 × 10⁻⁸, 8.3 × 10⁻⁸, and 3.2 × 10⁻⁸ cm² s⁻¹, respectively. It is apparent that the lithium–sulfur battery with the CoSe/NCC/PP separator has the highest Li⁺ diffusion coefficient, the Li⁺ diffusion coefficient of the lithium–sulfur battery with the Co/NCC/PP separator is the second largest, and the D_Li⁺ value of the lithium–sulfur battery with the Co/NC/PP separator is the smallest. The trend observed for peak A is not an isolated case and the trend in the D_Li⁺ values for each peak is the same, which is clearly observed in Fig. 5g. Thus, it is proved that CoSe/NCC obviously contributes to promoting the redox kinetics. A detailed understanding of the electrocatalytic performance of the material was carried out, and symmetrical cells containing Co/NC, Co/NCC, and CoSe/NCC were assembled with electrolyte containing Li₂S₆ added into each symmetrical cell. The voltage test range was from −1.0 to 1.0 V. In Fig. 5h, there are no obvious redox peaks in the symmetrical cells constructed from Co/NC and Co/NCC. The peak current decreases on the order of CoSe/NCC > Co/NCC > and Co/NC, showing that CoSe/NCC possesses the highest current response, the greatest catalytic activity, and the fastest redox reaction kinetics.⁵¹ Fig. 5i shows the results of the EIS tests carried out on the symmetrical cells that were assembled and left to stand for 10 h. It is clear that the CoSe/NCC cell has the smallest semicircle (charge transfer resistance, R_ct). When the polarization is small, the Butler–Volmer equation can be linearized to the following formula.

	(3)

Here, i is the current density, i₀ is the exchange current density, η is the overpotential, n is the electron transfer number, F is the Faraday constant, R is the gas constant, and T is the absolute temperature.

According to Ohm's law:

	(4)

and the relational expression of R_ct and i₀ is obtained as follows:

	(5)

where R_ct is inversely proportional to i₀. The smaller the value of R_ct, the larger the value of i₀, and the easier it is for the reaction to proceed, i.e., the kinetics is enhanced. The separators modified with the three different materials were assembled in stainless steel symmetric cells for EIS testing, as shown in Fig. S19.† The lithium-ion conductivity is calculated using the following formula:²⁸

	(6)

where S, d, and R_b are the contact area, thickness, and resistance of the separator, respectively. According to the calculations, the lithium-ion conductivities of the CoSe/NCC/PP, Co/NCC/PP, and Co/NC/PP separators are 1.16 × 10⁻³, 1.03 × 10⁻³, and 7.41 × 10⁻⁴ S cm⁻¹, respectively. Overall, CoSe/NCC/PP has the highest ionic conductivity which certainly contributes to the cells with this separator having the best electrochemical performance.

The feasibility of using the studied material is judged by assembling a lithium–sulfur battery containing a modified separator to test the cycle performance at a small current density of 0.5 C. As displayed in Fig. 6a, the initial discharge capacity of the lithium–sulfur battery with a CoSe/NCC/PP separator reaches 1270 mAh g⁻¹, and after 100 cycles, the battery has still a discharge capacity of 932 mAh g⁻¹ with a capacity retention rate of 73%. The initial discharge capacities of the LSBs with Co/NCC/PP or Co/NC/PP separators are 1114 and 1008 mAh g⁻¹, respectively. The good cycle performance is due to the anchoring of the polar CoSe onto the LiPSs to slow down the shuttle effect. Fig. 6b shows the EIS spectra of the LSBs with different separators before cycling, and each Nyquist plot consists of a semi-circle and a vertical line. In addition, the first intersection point with the X axis represents the electrolyte resistance (R_e). It is clearly seen that the lithium–sulfur battery with the CoSe/NCC separator has the smallest R_ct, demonstrating the best charge transfer kinetics.^52,53 In addition to the R_e and R_ct in the high frequency region, the line segment in the low frequency region indicates the Warburg impedance (Z_w), which is ascribed to the Li⁺ transmission in the electrode. The constant phase element (CPE) indicates the interfacial capacitance, as presented in the equivalent circuits (Fig. S20†). The lithium–sulfur battery with the CoSe/NCC separator has the highest slope and the fastest transfer rate of Li⁺, which is validated by the results obtained from the Z_w value of the equivalent circuit fitting.⁵⁴ The Z_w values for the LSBs with the CoSe/NCC/PP, Co/NCC/PP, and Co/NC/PP separators are 45, 162, and 538 Ω, respectively. It is intuitively visible that the resistance before cycling is significantly higher than that after cycling, indicating that the internal resistance of the battery decreases after cycling. After fitting to the equivalent circuit diagram (Fig. S20†), the R_e value is 2 Ω, and the R_ct values of the LSBs with the CoSe/NCC/PP, Co/NCC/PP, and Co/NC/PP separators before cycling are 38, 50, and 73 Ω, respectively. After 100 cycles, two semicircles appear (Fig. 6c), and the high frequency and low frequency semicircles represent the solid electrolyte interface resistance (R_SEI: the solid exterior of the SEI film is shaped) and the R_ct, respectively.^55,56 The lithium–sulfur battery containing the CoSe/NCC/PP separator has the smallest R_SEI and R_ct values, both values are 10 Ω, while the battery containing the Co/NCC/PP separator possesses a R_SEI of 18 Ω and a R_ct of 20 Ω. In addition, the lithium–sulfur battery assembled from Co/NC/PP has a maximum R_SEI of 30 Ω and a R_ct of 27 Ω. To test the influence of the quantity of electrolyte on the capacity of the battery, the cycling capability of the lithium–sulfur battery with CoSe/NCC/PP separator at 0.5 C was tested by adding less electrolyte, as shown in Fig. S21.† The results show that the initial discharge capacity of the battery with a E/S ratio of 10 μL mg⁻¹ is lower than that of the battery with a E/S ratio of 15 μL mg⁻¹ (Fig. 6a). To enhance the practical value of the material, we assembled LSBs containing the CoSe/NCC/PP separator with different sulfur loadings and E/S ratios for testing at 0.2 C (Fig. 6d). When the sulfur loading is 1.1 mg cm⁻² and the E/S ratio is 13.6 μL mg⁻¹, the initial discharge capacity of the battery is 1214 mAh g⁻¹. When the sulfur loading is 2 mg cm⁻², the first discharge capacity of the battery is still maintained at 452 mAh g⁻¹ when E/S = 7.5 μL mg⁻¹, and the capacity retention rate is 77.9% after 100 cycles. When the sulfur loading is increased to 4.1 mg cm⁻², and the E/S ratio is decreased to 3.65 μL mg⁻¹, the lithium–sulfur battery still maintains good cycling stability.

Fig. 6e reveals the rate performance of the LSBs with the CoSe/NCC/PP, Co/NCC/PP, and Co/NC/PP separators at various current densities, with a minimum current density of 0.2 C and a highest current density of 8 C. The lithium–sulfur battery composed of CoSe/NCC/PP possesses discharge capacities of 1664 mAh g⁻¹ at 0.2 C, 1041 mAh g⁻¹ at 0.5 C, 846 mAh g⁻¹ at 1 C, 637 mAh g⁻¹ at 3 C, 528 mAh g⁻¹ at 5 C, and 441 mAh g⁻¹ at 8 C, respectively. However, the rate performance of the LSBs composed of Co/NCC/PP and Co/NC/PP is inferior to that of the lithium–sulfur battery with CoSe/NCC/PP, and their discharge capacities at 0.2, 0.5, 1, 3, 5, and 8 C are 1615 and 1067, 951 and 830, 833 and 724, 634 and 592, 472 and 460, 313 and 216 mAh g⁻¹, respectively. The charge and discharge platforms corresponding to the rate performance of the lithium–sulfur battery with the CoSe/NCC/PP separator are shown in Fig. 6f. The charge and discharge platforms at 0.2 C are very long, making clear that the LSB with CoSe/NCC/PP separator has good reversible charge and discharge capacity. During the first discharge, a high initial discharge capacity (1664 mAh g⁻¹) at 0.2 C can be realized, but the dissolution and shuttle of polysulfides results in the loss of active material, and SEI film formation consumes some of the lithium ions and electrolyte, and thus the active substances cannot be fully recovered during charging. The first charge capacity is small, which is significantly lower than the initial discharge capacity.^1,47,57,58 Upon increase of the current density, the platforms gradually become shorter. However, even at a high current density of 3 C, there are still certain charging/discharging plateaus. As shown in Fig. S22,† at high current densities, the charge and discharge platforms of the LSBs with the Co/NCC/PP and Co/NC/PP separators seem to be absent. A careful study of the constant current charge/discharge test platforms derived from the cycling performance of the LSBs with the CoSe/NCC/PP, Co/NCC/PP, and Co/NC/PP separators at 0.5 C is carried out in Fig. 6g. It is found that the lithium–sulfur battery with the CoSe/NCC/PP separator has the smallest polarization voltage difference of 0.19 V, while the polarization voltage differences of the LSBs with the Co/NCC/PP and Co/NC/PP separators are 0.23 and 0.25 V, respectively. For the purpose of verifying the quick-charge ability of the battery, a long cycle performance test of the lithium–sulfur battery with the CoSe/NCC/PP separator was performed under different large current densities. A charge–discharge reaction with a higher current density means a higher commercial value. As shown in Fig. 6h, at a high current density of 5 C, the lithium–sulfur battery with the CoSe/NCC/PP separator was cycled 1000 times and has a discharge capacity of 300 mAh g⁻¹ after 1000 cycles. Fig. 6i shows that at a current density of 1 C, the initial discharge capacity of this battery is 874 mAh g⁻¹, and there is still a discharge capacity of 648 mAh g⁻¹ after 500 cycles with a capacity retention rate of 74% and capacity decay rate per turn of 0.051%. As presented in Fig. S23,† when the current density is 2 C, the first discharge capacity of this battery is 869 mAh g⁻¹, and 800 continuous cycles are still obtained. In summary, the reasons for the good electrochemical properties of the battery and its ability to capture LiPSs are listed below: (1) the mechanical strength of the MOF-derived NC skeleton lays the foundation for the charge and discharge cycles; (2) the adsorption of LiPSs via Lewis acid–base interactions with the polar CoSe species is combined with the conductive CNTs to alleviate the shuttle effect of LiPSs; and (3) the modified separator material prevents LiPSs migration by acting as a physical barrier and via chemisorption.

In this work, we demonstrate a lithium–sulfur battery assembled from MWCNTs/S as the positive electrode and CoSe/NCC/PP as the separator. To show its use in a practical application, a string of tulip flowers is lit up using two LSBs with MWCNTs/S cathodes and CoSe/NCC/PP separators connected in series, as shown in Fig. S24.† Through the comparison with other literature (Table S2†), it is confirmed that the lithium–sulfur battery assembled from a MWCNTs/S cathode and CoSe/NCC/PP separator offers fresh ideas and insight for high-performance LSBs.

Conclusion

We have successfully synthesized a dodecahedral nitrogen-doped carbon shell with a MOF substrate as a precursor upon which carbon nanotubes and polar metal selenide (CoSe/NCC) were grown in situ to create a modified PP separator. The comprehensive CV and Tafel tests demonstrate that the CoSe/NCC separator effectively promotes the kinetics of the redox reaction and reduces polarization. Ultraviolet adsorption testing verifies that CoSe/NCC can effectively adsorb properties LiPSs. Excellent ionic conductivity and lithium-ion diffusion coefficients are important for the promotion of lithium-ion transport. These surprising qualities give the lithium–sulfur battery with the CoSe/NCC separator and MWCNTs/S cathode an initial discharge capacity of 1270 mAh g⁻¹ at 0.5 C with a capacity retention rate of 73% after 100 cycles, as well as a capacity decay rate of 0.051% per turn over 500 cycles at 1 C. The battery can also undergo 1000 cycles at 5 C and 800 cycles at 2 C. This work offers insights into the good rate performance (quick charge) and longevity of LSBs with modified metal selenide separator materials, materials which could be applied to the other secondary batteries.

Data availability

The data that support the findings of this study are available from the corresponding author, Bo Jin, upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 52130101) and the Project of Science and Technology Development Plan of Jilin Province in China (no. 20210402058GH and 20220201114GX).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5qi00452g

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