Space-confined synthesis of CoSe2-NC nanoclusters anchored on honeycomb-like carbon framework towards high-performance lithium sulfur battery

Lithium–sulfur (Li-S) battery has been considered to be one of the next-generation high-energy-density rechargeable battery systems due to the high theoretical energy density, low cost, and environmental friendliness. However, the commercial application of Li-S battery still faces problems such as sluggish redox kinetics and infamous shuttle effect of sulfur cathode, which result in low sulfur utilization, poor cycle life, and unsatisfied rate performance. Herein, we proposed a CoSe2-NC nanocluster anchored honeycomb-like carbon framework (CoSe2-NC@HCF) as sulfur host aiming to accelerate sulfur conversion and inhibit polysulfide shuttle in Li-S electrochemistry via space-confined growth and in situ selenization. The obtained CoSe2-NC@HCF provides strong chemical adsorption capability and massive polar cobalt active sites as well as abundant and continuous hierarchical pores supplying adequate sulfur storage space and physical confinement. The S/CoSe2-NC@HCF cathode with sulfur content of 83.24 wt% delivers high sulfur utilization with initial discharge capacity of 1212.9 mAhg−1 at 0.1 C, excellent rate performance with 1094.7 mAh·g–1 at 1C rate, and good cyclability with low-capacity decay rate of 0.12% up to 600 cycles.


Introduction
The lithium-ion battery is currently utilized extensively in electric and communication devices, vehicles, airplanes, and other equipment due to its portability, light weight, high specific energy, long lifespan, and other advantages [1,2].Lithium-sulfur (Li-S) battery is widely acknowledged as a frontier in energy storage devices, endowed with immense potential for development.The theoretical energy density of Li-S battery is about 2510 Wh kg −1 , nearly 10 times that of conventional lithium-ion batteries [3][4][5].Despite the high energy density of Li-S battery, its commercial application is still problematic due to the low coulombic efficiency, short cycle life, and poor rate performance [6][7][8][9].The researchers have developed various strategies to overcome the challenges faced by Li-S battery, including electronic/ion conductive network design, creating buffer space for volume change, enhancing the adsorption effect of polysulfides, accelerating polysulfide conversion and catalysis, and developing anode carriers with excellent lithium sulfide properties.Through optimization and improvement of these approaches, the problems associated with Li-S batteries can be effectively addressed [10][11][12][13][14].
The electrochemical reaction process of the sulfur cathode in Li-S battery involves a complex, multi-phase conversion reaction that has a crucial impact on the battery's electrochemical performance.As a result, the investigation of the sulfur cathode has become a topic of widespread discussion in recent years [15][16][17][18].Conductive carbon-based materials are extensively researched as sulfur hosts, which provide hierarchical pores to accommodate active sulfur and buffer volume expansion during cycling as well as creating diverse micro/nano networks for the transfer of electrons/ ions within the electrode [19][20][21][22][23].However, carbon-based sulfur hosts often struggle to effectively inhibit the shuttle effect of polysulfides, as their non-polar surface properties lead to a low binding energy between the carbon host and polar polysulfides [24,25].To address this issue, researchers have introduced strong polar transition metals, such as Co and their compounds like CoSe 2 , into the carbon host to enhance chemical interactions with polysulfides and effectively prevent the irreversible loss of active sulfur species [26][27][28][29].
The use of MOF-derived carbon materials with polar metal-based nanoparticles has gained significant attention as sulfur host.This is due to their exceptional properties such as high specific surface area, rich catalytic active sites, well-defined pore structure, excellent structural controllability, and outstanding chemical stability.These features make them ideal candidates for efficient sulfur host materials [30,31].However, MOFs such as ZIF-8 and ZIF-67, synthesized through conventional solution method, typically have a large size exceeding 200 nm.Consequently, after the large MOF crystals subjected to high-temperature carbonization, the MOF-derived carbon materials tend to form agglomerates of large particles, which hinders the provision of abundant adsorption and catalytic active sites [32][33][34][35].The carbon host's restricted adsorption or catalytic sites weaken the adsorption or catalytic effect of polar transition metals or metal compounds, particularly for sulfur cathodes with high sulfur loading [36][37][38][39].
In this study, we propose a space-confined strategy to synthesize CoSe 2 -NC nanoclusters anchored on a honeycomblike carbon framework as a sulfur host.This approach provides efficient adsorption and catalytic active sites, as well as a continuous 3D porous conductive network [24,[40][41][42].The honeycomb porous carbon, derived from sodium citrate carbonization, was utilized as a semi-closed nanoreactor to provide limited space for the growth of ZIF-67 crystals, resulting in the formation of the ZIF-67@HCF composite.This composite was then selenided with selenium powder to obtain the CoSe 2 -NC@HCF composite, in which sub-10 nm CoSe 2 nanocrystals dispersed among the nitrogen-doped carbon host to provide strong polysulfides adsorption and high catalytic effects [43,44].The resulting S/CoSe 2 -NC@HCF cathode, with a sulfur content of 83.24 wt%, exhibits high sulfur utilization, with an initial discharge capacity of 1212.9 mAhg −1 at 0.1 C. It also demonstrates exceptional rate performance, with a capacity of 1094.7 mAh•g -1 at a 1C rate, and good cyclability, with a low-capacity decay rate of 0.12% over 600 cycles.Compared to S/HCF materials and S/CoSe 2 -NC, the S/CoSe 2 -NC@HCF honeycomb composite structure exhibits a lower polarization potential, smaller interfacial impedance, stronger adsorption of lithium polysulfide, and better electrochemical properties [45].Our findings provide insight into the design of space-confined synthesis of polar nanoclusters anchored on an open carbon framework, which possesses efficient electrocatalysts and a conductive network for high-performance sulfur cathodes [46][47][48][49].

Synthesis of HCF
To prepare the HCF, 60 g of sodium citrate was first placed in a 150°C oven for 6 h to remove crystalline water.The dry sodium citrate was then ball-milled at 400 r•min −1 for 30 min and placed in a corundum crucible.It was heated to 700°C for 1 h under an argon atmosphere.The resulting product was washed with deionized water and ethanol 2-3 times, followed by vacuum drying at 55°C for 6 h.This resulted in the HCF material.

Synthesis of ZIF-67@HCF and ZIF-67
To prepare the ZIF-67@HCF precursor, 20 g of Co(NO 3 ) 2 was first dissolved in a 10 mg•mL −1 methanol solution.Ten grams of HCF was then added to the solution in a mass ratio of Co(NO 3 ) 2 :HCF=2:1.After stirring for 10 h, an equal volume of 2-methylimidazole/methanol solution was quickly added in a molar ratio of Co(NO 3 ) 2 :2-MI=1:4.The solution was stirred at 15°C for 24 h, and the resulting product was washed with deionized water and ethanol 2-3 times, followed by vacuum drying at 55°C for 24 h.As a control experiment, ZIF-67 was prepared using the same method without HCF being added to the Co(NO 3 ) 2 /methanol solution.

Synthesis of CoSe 2 -NC@HCF and CoSe 2 -NC
The 2 g of ZIF-67@HCF and 1 g of selenium powder were mixed and ground evenly in a crucible, maintaining a mass ratio of Se:ZIF-67@HCF of 2:1.The mixture was annealed at 350°C for 6 h with a heating rate of 2°C per minute under an argon-hydrogen mixture atmosphere (Ar:H 2 =9:1 (vol%)).The resulting product, after cooling to room temperature, was the CoSe 2 -NC@HCF.As a control experiment, pure ZIF-67 was produced using similar procedures without the addition of HCF, and the resulting product was referred to as CoSe 2 -NC.

Synthesis of S/CoSe 2 -NC@HCF, S/HCF, and S/ CoSe 2 -NC
The 1 g of CoSe 2 -NC@HCF and 4 g of sublimed sulfur were ground evenly in a mortar, maintaining a mass ratio of CoSe 2 -NC@HCF: sublimed sulfur of 1:4.The 2 g of carbon disulfide was added to the mixture and stirred continuously in a fume hood until the carbon disulfide was completely volatilized.The resulting black powder was placed in a glass bottle and annealed at 155°C for 10 h with a heating rate of 2°C per minute under an Ar atmosphere.It was then annealed at 210°C for 10 min with a heating rate of 2°C per minute under the same atmosphere to obtain the S/ CoSe 2 -NC@HCF composite material.In the control group, the preparation process of S/HCF was the same as that of S/ CoSe 2 -NC@HCF without the addition of CoSe 2 -NC.The preparation process of S/CoSe 2 -NC in the control group was the same as that of S/CoSe 2 -NC@HCF, but with pure CoSe 2 -NC instead of CoSe 2 -NC@HCF.

Characterization
Scanning electron microscopy (SEM) images of all samples in this study were acquired at 10.0 kV using a field emission scanning electron microscope (FESEM, Hitachi S-4800, Japanese scanning electron microscope).Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were recorded on the FEI Tecnai 2100 device operating at 200 kV, equipped with a GatanGIF quantum 963 energy dispersion spectroscope system.Crystal structure information from 10 to 80°was analyzed using X-ray diffraction (XRD) measurements.X-ray photoelectron spectroscopy (XPS) measurements were performed on the Thermo Fisher K-Alpha 1063 device.It is worth noting that all binding energies refer to the C 1s peak of surface adventitious carbon at 284.8 eV to correct for displacements caused by charge effects.Thermogravimetric analysis (TGA) was used to confirm the content of cobalt and sulfur with the Netzsch STA 449F3 TGA instrument.The specific surface area and pore size distribution of all samples were determined using the V sorb 2008 nitrogen adsorption/desorption assay device.

Electrochemical measurements
To prepare the electrodes for the corresponding sulfur composites, the meticulously prepared active material (S/CoSe 2 -NC@HCF, S/HCF, and S/CoSe 2 -NC) is first mixed with ultra-P and water-soluble polymer-acrylate (LA133) binder in a weight ratio of 80:12:8.The mixture is stirred magnetically in a water solvent for 10 h.The resulting slurry is then evenly coated onto aluminum foil.After drying overnight at 55°C in a vacuum oven, an electrode with a controlled sulfur density of about 1.23~1.76mg•cm −2 is obtained.To analyze the electrochemical properties, a CR-2016 coin-type battery with an S/CoSe 2 -NC@HCF or S/HCF electrode and lithium metal foil with thickness of 400 um is assembled with an electrolyte/sulfur ratio (E/S ratio) of approximately 12 ml g −1 in a glove box filled with an argon atmosphere.The electrolyte consists of 1M lithium bis(trifluorometha nesulfonimide) (LiTFSI) dissolved in a binary solvent of DOL and DME (volume 1:1), with 2% LiNO 3 added.Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests are performed on the Princeton Versa STAT electrochemistry workstation.Charge and discharge measurements are made over a voltage range of 1.7-2.8V.The Li/Li + uses a LAND CT 2001A instrument with different current densities.The Li 2 S 6 solution for static polysulfides adsorption tests was diluted from the as-prepared Li 2 S 6 solution which is comprised of 5 mM Li 2 S 6 and DME/DOL (1:1 of volume ratio) solvent.

Results and discussion
Figure 1 shows the schematic of the preparation of a honeycomb carbon structure CoSe 2 -NC@HCF through a nanoconfined growth process.First, sodium citrate was carbonized at high temperature, etched with hydrochloric acid, and vacuum dried to obtain honeycomb carbon HCF.The obtained HCF and Co(NO 3 ) 2 were immersed in a methanol solution and stirred for 10 h, followed by the addition of 2-MI solution, inducing the growth of ZIF-67s in the HCF cavity to obtain ZIF-67s@HCF.The resulting material was then mixed with selenium powders, and ZIF-67s@HCF was heat-treated under a high-temperature inert atmosphere to obtain a honeycomb CoSe 2 -NC@HCF.
Scanning electron microscopy was used to analyze the topography of HCF, ZIF67@HCF, CoSe 2 -NC@HCF, and S/CoSe 2 -NC@HCF.The microscopic morphology of honeycomb carbon in Fig. 2a shows that the pore structure is relatively regular, with mostly large pores ranging from 100 to 500 nm, and the pore walls are extremely thin, resembling a honeycomb and forming a semi-closed continuous pore environment.During the carbonization and decomposition of sodium citrate, the hexagonal crystalline carbonate converted from Na + plays a role in providing hard formwork support, thereby forming a three-dimensional network porous carbon material that resembles a honeycomb.As shown in Fig. 2B, the morphological structure of ZIF-67@ HCF reveals that after 24 h of stirring, the honeycomb pore structure is still maintained, and a large number of ZIF-67 nanoparticles with sizes of 20-50 nm can be observed in the macroporous structure.This indicates that the semi-closed continuous pore structure of honeycomb carbon can be used as a nanoreactor to provide a confined growth environment for small-size MOFs.According to Fig. 2c, the overall morphological structure of CoSe 2 -NC@HCF obtained after selenization remains intact.Notably, the ZIF-67 particles have been converted to porous carbon embedded with CoSe 2 nanoparticles, with a slightly reduced size.As shown in Fig. 2d, the composite material after sulfur melting still maintains a relatively complete honeycomb structure, and no large particles of elemental sulfur are found.This indicates that the sulfur element is relatively uniformly filled into the pore structure of the honeycomb composite material, forming a uniform composite structure with CoSe 2 -NC@HCF.Since ZIF-67 was uniformly loaded on HCF with particle sizes ranging from 20 to 50 nm and CoSe 2 nanocrystalline crystals with sizes around 10 nm were uniformly loaded in a nitrogen-doped carbon frame, HCF was used to prevent CoSe 2 nanocrystalline agglomeration and growth.According to Fig. 2c, the overall morphological structure of CoSe 2 -NC@HCF obtained after selenization remains intact.Notably, the ZIF-67 particles have been converted to porous carbon embedded with CoSe 2 nanoparticles, with a slightly reduced size.As shown in Fig. 2d, the composite material after sulfur melting still maintains a relatively complete honeycomb structure, and no large particles of elemental sulfur are found.This indicates that the sulfur element is relatively uniformly filled into the pore structure of the honeycomb composite material, forming a uniform composite structure with CoSe 2 -NC@HCF.Since ZIF-67 was uniformly loaded on HCF with particle sizes ranging from 20 to 50 nm and CoSe 2 nanocrystalline crystals with sizes around 10 nm were uniformly loaded in a nitrogen-doped carbon frame, HCF was used to prevent CoSe 2 nanocrystalline agglomeration and growth.
Scanning electron microscopy and transmission electron microscopy were used to characterize ZIF67 and ZIF-67@ HCF.It can be seen in Fig. 3a that the size of ZIF-67 nanoparticles is the same as in Fig. 2b, with a size of 20-50 nm in the macroporous structure.As shown in Fig. 3b, the size of ZIF-67 nanoparticles was 120-180 nm without HCF poured into the Co(NO 3 ) 2 /methanol solution, which was larger than the size of ZIF-67 nanoparticles in the macroporous structure in Fig. 3a.This indicates that the semi-closed continuous pore structure of honeycomb carbon can provide a smaller size for ZIF-67.As shown in Fig. 3c, there are more ZIF-67 particles in the honeycomb large pores, with a size of about 20-50 nm, which is smaller than the size of pure ZIF67 (120-180 nm) in Fig. 3d.This is consistent with the SEM observations, and the dispersion is relatively uniform, further demonstrating that the continuous pore structure of honeycomb carbon can be used as a nanoreactor for the limited growth of small-size MOFs.
Transmission electron microscopy was used to characterize ZIF-67@HCF, CoSe 2 -NC@HCF, and S/CoSe 2 -NC@HCF.Figure 4a-f shows the elemental mappings of ZIF-67@HCF, revealing that C, O, N, and Co elements are mainly present and evenly distributed.The N element and Co element are mainly derived from the organic and inorganic components in ZIF-67, respectively.The distribution positions of N and Co are consistent with the morphology of ZIF-67.Figure 4g shows the TEM diagram of CoSe 2 -NC@HCF, where the honeycomb structure is still well-maintained.The CoSe 2 -NC particles generated by in situ selenization of ZIF-67 are evenly dispersed in the honeycomb-shaped large pores, with a size of about 10-30 nm, and the black dots embedded in the porous carbon are CoSe 2 nanocrystals.The high-resolution transmission electron microscope image of CoSe 2 -NC@HCF shown in Fig. 4h reveals that the size of the CoSe 2 -NC grain is about 15 nm, and the measured crystal plane spacing is 0.249 nm, corresponding to the (120) plane of the CoSe 2 crystal.Moreover, a nitrogen-doped amorphous carbon coating layer is formed on the surface of the CoSe 2 grain, which can effectively enhance the conductivity of the CoSe 2 particles.Figure 4i shows that C, N, Co, Se, S, and other elements are present in S/CoSe 2 -NC@HCF, Fig. 3 ZIF-67@HCF: a scanning electron microscopy images and c transmission electron microscopy picture; ZIF-67: b scanning electron microscopy images and d transmission electron microscopy picture further illustrating the successful compounding of elemental sulfur and CoSe 2 -NC@HCF, which is consistent with previous SEM observations.From Fig. 4j, the morphological structure of the S/CoSe 2 -NC@HCF composite after sulfur loading has not changed significantly.The elemental surface scan diagram (Fig. 4k-p) reveals that the distribution of Co, Se, and other elements is basically the same as the position distribution of CoSe 2 nanoparticles, while the distribution of N elements is intertwined with the distribution of C elements due to the formation of nitrogen-doped porous carbon after carbonization of organic ligands in ZIF-67.It is worth noting that the distribution of S element near the CoSe 2 nanoparticles is more concentrated, indicating that the CoSe 2 nanoparticles have a certain adsorption capacity for the active substance sulfur.
To further clarify the phase composition and crystallization status of the relevant products, X-ray diffraction characterization of HCF, ZIF-67@HCF, CoSe 2 -NC@HCF, and S/ CoSe 2 -NC@HCF was carried out, and the results are shown in Fig. 5.The crystal diffraction spectra of honeycomb carbon shown in Fig. 5a reveal that the sample has a diffraction peak of amorphous carbon at 23.1° and 42.5°, corresponding to (002) and (101) crystal planes, respectively.The diffraction peak of the (101) crystal plane in Fig. 5a indicates that a sp2 hybrid structure has appeared in the carbon material.The XRD diffraction spectra of the ZIF-67@HCF composite material obtained by the nano-limiting method are shown in Fig. 5b, and the samples have sharp diffraction peaks at 12.7° and 18.0°, which are consistent with the ZIF-67 diffraction signals reported in the literature.The XRD diffraction pattern of the CoSe2-NC@HCF obtained after selenization is shown in Fig. 5c, and the position of the diffraction peak corresponds to the standard diffraction peak of the CoSe2 card (PDF #53-0449), indicating that ZIF-67 has been converted to CoSe2 after selenization.In addition, diffraction peaks of CoSe2-NC@HCF appear around 23.1°, corresponding to amorphous carbon, which is mainly derived from honeycomb carbon and ZIF-67-derived carbon materials.The diffraction spectrogram of the sulfur-laden composite is shown in Fig. 5d, and it can be observed that the diffraction peak corresponds to the crystalline elemental sulfur (PDF #79-1517), indicating that the sulfur can be successfully recombined with CoSe 2 -NC@HCF by the melting method, which is consistent with the observations of TEM and EDS.
To determine the composition and the chemical state of S/CoSe 2 -NC@HCF, X-ray photoelectron spectroscopy was used for characterization, and the results are shown in Fig. 6.The characteristic peaks in the full spectrum of S/CoSe 2 -NC@HCF shown in Fig. 6a reveal the presence of C, N, Co, Se, and S elements, which are consistent with the EDX results.As shown in the fitted spectra of C1s in Fig. 6b, the characteristic diffraction peaks appear at 284.6, 285.6, and 286.5 eV and are corresponding to the C-C bond, C-N bond, and C-O bond of sp 2 hybridization, respectively.The signal peak of the C-C bond is very sharp, indicating the presence of graphitized carbon in the S/CoSe 2 -NC@HCF, while the C-N bond indicates the doping of nitrogen in CoSe 2 -NC@ HCF, corresponding to the carbonization of organic ligands in ZIF-67.As shown in the fitted spectra of N1s in Fig. 6c, characteristic peaks appear at 399.0, 400.1, 400.9, and 402.5 eV, corresponding to the pyridine nitrogen, pyrrole nitrogen, graphite nitrogen, and pyridine oxide nitrogen, respectively.Among them, graphite nitrogen accounts for the highest proportion, indicating the good conductivity of the sample [50].As shown in Fig. 6d, four characteristic signal peaks can be observed in the Co2p fitting spectrum.The signal peaks at 779.3 and 794.2 eV correspond to Co 3+ , while the signal peaks at 780.6 and 797.1 eV correspond to Co 2+ .Additionally, satellite peaks at 785.8 and 803.3 eV are also present [51].As shown in the fitted spectra of Se 3d in Fig. 6e, the signal peak at 54.8 eV corresponds to Se3d 5/2 from Se-Se bond, while the signal peak at 55.6 eV corresponds to Se3d 3/2 from the Se-Co bond, which further confirms the existence of CoSe 2 in the composite.Additionally, the signal peak at 59.5 eV corresponds to the Se-O-Se bond, which is mainly due to partial oxidation of the surface of CoSe 2 [52].
The surface area and pore size characteristics of CoSe 2 -NC@HCF and S/CoSe 2 -NC@HCF were analyzed using nitrogen adsorption and desorption, and the results are shown in Fig. 7.The nitrogen adsorption-desorption curve of CoSe 2 -NC@HCF presents an IV isotherm, accompanied by an H1-type hysteresis ring, indicating that the composite material has smaller mesopores.After sulfur loading, the  adsorption of the composite material decreases significantly, indicating that the pore structure is essentially filled with elemental sulfur at this point.Figure 7b further confirms the mesoporous structure of CoSe 2 -NC@HCF composites, with a concentration of mesopores in the range of 2-5 nm.The smaller pores are mainly derived from the porous carbon structure of ZIF-67.The pore size distribution of S/CoSe 2 -NC@HCF is similar to that of the CoSe 2 -NC@ HCF except with a significant reduction in distribution.This mesoporous hybrid material with a high specific surface area has a favorable effect of accelerating the adsorption and redox reaction of sulfur substances and provides regulatory conditions for volume expansion during lithiation.
TGA thermogravimetry was used to characterize the ZIF-67 content in ZIF-67@HCF, the CoSe 2 -NC@HCF, and the sulfur content in S/CoSe 2 -NC@HCF and S/HCF.The content of ZIF-67 in ZIF-67@HCF was obtained by indirect calculation.As shown in Fig. 8a, ZIF-67 is completely converted to Co 3 O 4 when heated to about 400°C under an oxygen atmosphere, and the remaining C and N components are oxidized and removed.Finally, the mass of the remaining Co 3 O 4 accounts for 27.18% of the total mass of ZIF-67.Similarly, ZIF-67 in ZIF-67@HCF is converted to Co 3 O 4 by high-temperature treatment under an oxygen atmosphere, and the remaining C and N components are oxidized and removed.Finally, the mass of the remaining Co 3 O 4 accounts for 13.86% of the total mass of ZIF-67@HCF, and the ZIF-67 content in ZIF-67@HCF can be calculated to be 50.99%.Figure 8b shows the TGA curve of CoSe 2 -NC@HCF.Under an oxygen atmosphere, CoSe 2 -NC@HCF starts to lose weight at 300°C and continues until complete weightlessness at about 550°C.This corresponds to the two reactions The corresponding substance content in ZIF-67@HCF, CoSe 2 -NC@HCF, S/HCF, S/ CoSe 2 -NC@HCF of 3CoSe 2 +2O 2 →Co 3 O 4 +6Se↑ and C+O 2 →CO 2 ↑, respectively.The Co content is calculated from the final remaining Co 3 O 4 content, which corresponds to the CoSe 2 content in CoSe 2 -NC@HCF.It is calculated that the CoSe 2 content in CoSe 2 -NC@HCF is 48.59%.Figure 8c shows the TGA weight loss curve of S/CoSe 2 -NC@HCF and S/HCF, which is used to determine the sulfur load.The weight loss part of the curve corresponds to the volatilization of elemental sulfur.It can be seen that the sulfur load in S/HCF is 76.34%, while the sulfur load in S/CoSe 2 -NC@HCF is 83.24%.This indicates that the sulfur-carrying performance of CoSe 2 -NC@HCF is better than that of honeycomb carbon [53].
To further analyze the role of the honeycomb-like composite structure in the lithium-sulfur battery system, the electrochemical properties of S/HCF and S/CoSe 2 -NC@ HCF composite sulfur cathodes were compared.The aim of this comparison is to clarify the synergistic mechanism of the honeycomb-like composite structure prepared by the nano-domain method for improving the electrochemical performance of lithium-sulfur batteries.To exclude interference from other factors, all process parameters were kept constant except for the type of cathode plates used in the two sets of experimental samples.
Firstly, coin cell batteries were assembled using S/HC and S/CoSe 2 -NC@HC composite, and cyclic voltammetry was performed to analyze their specific electrochemical behavior by observing the peak changes in CV curves.The electrochemical window was set to 1.7-2.8V in the CV tests, with a scanning rate of 0.1 mV•s −1 .The initial 6 cycles of CV curves are shown in Fig. 9.
Both groups of samples showed two oxidation and two reduction peaks.O1 and O2 correspond to the oxidation conversion from Li 2 S 2 /Li 2 S to Li 2 S n (4≦n≦8) and from Li 2 S n to S 8 , respectively.R1 and R2 correspond to the reduction process from S 8 to Li 2 S n and from Li 2 S n to Li 2 S 2 /Li 2 S, respectively.These results indicate that the good electrochemical activity of sulfur in the honeycomb carbon-based composite shows ideal multi-electron conversion reactions of lithium polysulfides.The CV curve of the S/CoSe 2 -NC@ HCF composite shown in Fig. 9a indicates that the oxidation peaks appeared at 2.389 V and 2.438 V, while the reduction peaks appeared at 2.326 V and 2.011 V during the first cycle.As the cycles increased, both oxidation peaks significantly shift to the left, while there is not much change in the reduction peaks.This indicates that the polarization in the battery has weakened at this time.In the CV curve of the S/HCF composite shown in Fig. 9b, oxidation peaks appeared at 2.411 V and 2.450 V in the first loop, while two reduction peaks appeared at 2.334 V and 2.020 V, respectively.As the cycles increased, two oxidation peaks shifted slightly to the left, indicating that Li 2 S n could complete the oxidation process at a lower potential.Additionally, the second reduction peak also shifted to the left, indicating the reduction process was more difficult at this time.In comparison, although the polarization of the S/HCF composite in the cycle process gradually weakened to a certain extent, the polarization of the S/CoSe 2 -NC@HCF composite was more obvious.This not only shows the applicability of honeycomb carbon host in Li-S batteries but also demonstrates that the S/CoSe 2 -NC@HCF composite has excellent electrochemical reversibility and cyclic stability.
Figure 10 shows the cyclic and rate performance of S/ HCF, CoSe 2 -NC@HCF, and S/CoSe 2 -NC@HCF cathodes.It is evident that the capacity of the S/CoSe 2 -NC@HCF composite at different rate is superior to that of the S/HCF and CoSe 2 -NC@HCF composite.At 0.1 C, 0.2 C, 0.5 C, and 1 C, the S/CoSe 2 -NC@HCF composite sulfur cathode exhibits a discharge specific capacity of 1264.1, 1109.8,959.6, and 867.4 mAh•g −1 , respectively.Even at a higher rate of 2 C, it still maintains a specific capacity of 740.2 mAh•g −1 .Upon returning to 0.1 C, the S/CoSe 2 -NC@HCF composite electrode retains a capacity of 964.9 mAh•g −1 .This demonstrates that the S/CoSe 2 -NC@HCF cathode can maintain good electrochemical reversibility under different discharge rates, indicating the outstanding structural stability of S/CoSe 2 -NC@HCF.In comparison, the initial discharge specific capacity of CoSe 2 -NC@HCF is only 946.9 mAh•g −1 at 0.1 C, and when the discharge rate increased to 2 C, the capacity falls to 559.4 mAh•g −1 .The initial discharge specific capacity of S/HCF is only 814.3 mAh•g −1 at 0.1 C, and when the discharge rate increased to 2 C, the capacity decays to 483.3 mAh•g −1 .It is evident that the S/CoSe 2 -NC@HCF composite exhibits superior rate performance Figure 10b shows the impressive long-term performance of S/HCF, CoSe 2 -NC@HCF, and S/CoSe 2 -NC@HCF cathode at 0.2 C. It is evident that the initial discharge capacity of the S/HCF cathode is only 926.5 mAh•g −1 , and the capacity decays to 607.3 mAh•g −1 after 120 cycles, with a minimal capacity degradation rate of 0.37% per cycle and Coulomb efficiency of 97.76%.The initial discharge capacity of the CoSe 2 -NC@HCF cathode is only 1106.2 mAh•g −1 , and the capacity decays to 686.1 mAh•g −1 after 120 cycles, with a negligible decline in capacity of only 0.37% per cycle and Coulomb efficiency of 98.24%.In contrast, the S/CoSe 2 -NC@HCF cathode exhibits a significantly improved initial discharge capacity of 1196.8 mAh•g −1 , which only attenuates to 942.9 mAh•g −1 after 120 cycles, corresponding to an insignificant decrease in capacity by a mere 0.37% per cycle and Coulomb efficiency of 99.05%.It is evident that the S/ CoSe 2 -NC@HCF cathode exhibits improved cycle performance, indicating that CoSe 2 -NC particles play a crucial role in lithium-sulfur battery.On one hand, honeycomb carbon embedded with CoSe 2 nanoparticles exhibits superior ion/electron transport performance, effectively accelerating the conversion reaction of lithium polysulfide.On the other hand, the CoSe 2 nanoparticles and ZIF-67-derived nitrogendoped porous carbon exhibit a stronger chemical adsorption effect on lithium polysulfides, effectively inhibiting the shuttle effect of lithium polysulfides, and strongly supporting the comprehensive improvement of the electrochemical performance of Li-S batteries.
To further validate the advantages of the S/CoSe 2 -NC@ HCF composite with high sulfur loading, the cycling performance of S/CoSe 2 -NC@HCF composite with sulfur areal densities of 1, 2, and 3 mg•cm -2 was tested at 0.2 C. As shown in Figure S6, the three electrodes exhibited initial discharge specific capacities of 1092.7,1108.7, and 1126.8 mAh•g -1 , respectively.After 100 cycles, the discharge capacities declined to 758.7, 813.8, and 859.4 mAh•g -1 , corresponding to capacity retention of 69.4%, 73.4%, and 76.2%, respectively.Overall, the cycling performance of the S/CoSe 2 -NC@HCF composite remained relatively good at high sulfur areal densities.
To explore the long-term cycle stability of the S/CoSe 2 -NC@HCF cathode, an extensive cycle testing was performed at a 1C rate.As shown in Fig. 10c, the initial discharge specific capacity of the S/CoSe 2 -NC@HCF cathode is 1107.2mAh•g −1 , and the discharge specific capacity at cycles 200, 400, and 600 is maintained at 706.1, 518.7, and 393.1 mAh•g −1 , respectively, with Coulomb efficiencies of 98.5%, 98.4%, and 98.2% (as is shown in Figure S7).According to the cycle performance, the gradual deterioration of capacity the S/CoSe 2 -NC@HCF cathode mainly occurs in the pre-cycle period, and the capacity decay rate during1-200, 201-400, and 401-600 cycles is 0.20%, 0.17%, and 0.17%, respectively.It is evident that as the electrochemical reaction deepens, the capacity decay rate of the S/CoSe 2 -NC@HCF gradually decreases and eventually stabilizes, mainly due to the relatively open porespace of the honeycomb carbon.In the early stage of the cycle, when the adsorption components and catalytic components have not been fully activated, the active substances dispersed on the outside of the host are easily dissolved and suffer shuttle effect, resulting in irreversible losses.As the composite structure gradually exerts its advantages, the cycle stability also gradually improves.
To further analyze the synergy mechanism of the CoSe 2 -NC@HCF composite structure in the performance improvement of Li-S batteries, the efficiency enhancement mechanism of CoSe 2 -NC@HCF in Li-S batteries was comprehensively analyzed from the adsorption and catalytic effects of this composite structure on Li-S batteries, respectively.
To investigate the adsorption effect of the CoSe 2 -NC@ HCF composite structure on lithium polysulfides, a static adsorption experiment of lithium polysulfide was conducted, and the results are presented in Fig. 11.The HCF, CoSe 2 -NC@HCF, and SP were added to the Li 2 S 6 solution and left to stand for 48 h.It can be observed that the solution after SP treatment hardly changed, indicating that it has almost no adsorption capacity for lithium polysulfides.The solution after honeycomb carbon adsorption treatment becomes pale yellow, indicating that honeycomb carbon has a certain adsorption effect on Li 2 S 6 .This effect may be attributed to the pore structure of the honeycomb carbon material and the joint action of van der Waals force between the carbon-based material and Li 2 S 6 .In contrast, the supernatant after CoSe 2 -NC@HCF adsorption treatment is almost colorless, indicating that CoSe 2 -NC@HCF has a stronger adsorption effect on Li 2 S 6 .The ultraviolet spectroscopic test results shown in Fig. 11b further confirm the adsorption of honeycomb carbon and CoSe 2 -NC@HCF on Li 2 S 6 .It can be observed that the peak position located between 400 and 450 nm corresponds to the absorption peak of Li 2 S 6 .After honeycomb carbon adsorption, the peak strength is significantly weakened, indicating that honeycomb carbon has a certain adsorption effect on Li 2 S 6 .Moreover, the Li 2 S 6 supernatant after CoSe 2 -NC@HCF adsorption is almost invisible to the absorption peak, which is consistent with the optical picture observation.
To analyze the catalytic effect of the composite structure during battery charge and discharge, cyclic voltammetry was conducted as an indirect method.Figure 12a shows that in the first cycle, both the S/HCF and S/CoSe 2 -NC@HCF composite sulfur cathodes exhibited double oxidation peaks and double reduction peaks.Typically, the potential difference between the first oxidation peak O1 and the second reduction peak R2 in the cyclic voltammetry curve of lithium-sulfur batteries can be used to indirectly reflect the degree of polarization in the battery system.The greater the potential difference, the more severe the polarization phenomenon, and the oxidation and reduction processes become more difficult.Comparing the oxidation peak positions of the two sample sets, it can be observed that the polarization potential of the S/HCF composite sulfur cathode is 0.391 V, while the polarization potential of the S/CoSe 2 -NC@HCF composite sulfur cathode is 0.378 V, which is significantly lower than that of S/HCF.In particular, the peak position of the two during the reduction process is not significantly different, indicating that the conversion process from elemental sulfur to lithium polysulfide is similar for both.During the oxidation process, the two oxidation peaks of the S/CoSe 2 -NC@ HCF composite sulfur cathode are significantly lower than those of the S/HCF composite sulfur cathode.This indicates that the S/CoSe 2 -NC@HCF composite sulfur cathode can achieve the conversion reaction of lithium polysulfide to elemental sulfur at a lower potential.These results demonstrate that the redox reaction kinetics of lithium polysulfide in the S/CoSe 2 -NC@HCF composite sulfur cathode are more rapid, primarily due to the catalytic action of CoSe 2 -NC.The surface of CoSe 2 derived from small ZIF-67 particles exposes more catalytically active sites, and the doped nitrogen derived from ZIF-67 can also play a catalytic role in the conversion reaction of lithium polysulfide to a certain extent.
To further verify the catalytic effect of the CoSe 2 -NC@ HCF composite structure in the battery system, the charge and discharge curves of S/HCF and S/CoSe 2 -NC@HCF composite sulfur cathodes were analyzed at different  12b and c, the observations reveal that the two platforms appear in the charging and discharging process at different discharge rates, which is consistent with the results observed in the cyclic voltammetry curve.This indicates that the conversion reaction of the active substance in the honeycomb carbon-based sulfur cathode carrier is more thorough, further confirming the catalytic effect of the CoSe 2 -NC@ HCF composite structure in the battery system.
Upon comparing the two curves, it is evident that the S/CoSe 2 -NC@HCF composite sulfur cathode maintains better charging and discharging platforms at different discharge rates.Furthermore, the discharge platform voltage at 0.1 C, 0.2 C, and 0.5 C remains stable, indicating superior capacity performance.To analyze the polarization phenomenon of the two battery systems at different discharge rates, a comparison diagram of polarization potentials is shown in Fig. 12d.The comparison diagram in Fig. 12d reveals the polarization potential of the S/ CoSe 2 -NC@HCF composite sulfur positive electrode at 0.1 C magnification is 0.256 V, which is lower than the polarization potential (0.263 V) of S/HCF.Moreover, the polarization potential of the S/CoSe 2 -NC@HCF composite sulfur positive electrode remains consistently lower than that of S/HCF at the remaining magnifications.As the discharge rate increases, the gap between the polarization potentials of the two becomes wider, indicating that the effect of S/CoSe 2 -NC@HCF composite sulfur cathode in improving the polarization phenomenon is more significant at higher discharge magnifications.
In the study of electrochemical impedance spectroscopy (EIS), the results are shown in Fig. 13a.The impedance spectra of S/HCF and S/CoSe 2 -NC@HCF are composed of two parts: a semicircle in the high-frequency region and a Rs of the two batteries is similar.The charge transfer resistance R ct of S/CoSe 2 -NC@HCF (15.68 Ω) is significantly lower than that of S/HCF (79.7 Ω).This indicates that the CoSe 2 -NC@HCF composite structure can effectively reduce the charge transfer resistance of the cathode carrier material, thereby greatly improving the charge transfer rate.This improvement is beneficial for boosting the rate of conversion reaction of lithium polysulfide.Figure 13 b and c illustrate the mechanism of action of S/HCF and S/CoSe 2 -NC@HCF composite sulfur cathodes during the cycle process.These schematics provide a simulation of the process.Compared to the conventional hollow structure, the honeycomb CoSe 2 -NC@HCF provides a larger surface area for the ultra-fine CoSe 2 nanoparticles to be exposed to more active sites.This feature effectively inhibits the shuttle effect of lithium polysulfide and promotes the redox reaction kinetics of lithium polysulfide.Additionally, the nitrogen-doped carbon derived from the organic ligand in ZIF-67 also has a positive impact on the lithium-sulfur battery system.It enhances the performance of the battery system.

Conclusions
In summary, we have proposed a novel synthesis strategy to construct honeycomb composite structures.This approach is based on the concept of space-confined growth, which utilizes honeycomb porous carbon as a semi-closed nanoreactor to induce uniform growth of nano ZIF-67 particles within the honeycomb pores.The resulting ZIF-67@HCF composites have particle sizes primarily between 20-50 nm.The rational design of the three-dimensional network structure, which is constructed by the nano-CoSe 2 -NC particles in coordination with honeycomb carbon, exhibits significant structural characteristics.These include the physical constraint of accommodating high sulfur loading in internal void spaces and the physical constraint and chemical adsorption synergistic effect of effectively inhibiting the shuttle effect of polysulfide intermediates.Additionally, the strong adsorption and catalytic effect of CoSe 2 -NC on lithium polysulfide promotes the redox kinetics of lithium polysulfide.As a result, the S/Co-NC@HCF electrode demonstrates high reversible stability, good circulation ability, and exceptional rate performance.

Fig. 4
Fig.4ZIF-67@HCF: a-f elemental mapping of CoSe 2 -NC@HCF.g Transmission electron microscopy image and h high-resolution transmission electron microscopy pictures of S/CoSe 2 -NC@HCF.i EDS

Fig. 6 a
Fig. 6 a X-ray photoelectron full spectrogram of S/CoSe 2 -NC@HCF; b C 1s fitted spectra; c N 1s fitting spectra; d Co2p fitting spectra; e Se 3d fitted spectra

Fig. 9 a
Fig. 9 a CV curves of the S/ CoSe 2 -NC@HCF electrode under 0.1 mV S −1 scanning rate; b CV curves of the S/HCF electrode under 0.1 mV S −1 scanning rate

Fig. 11 a
Fig. 11 a Optical picture of HCF, CoSe 2 -NC@HCF, SP lithium polysulfide adsorption experiment.b Li 2 S 6 and UV spectroscopy test after adsorption by HCF and CoSe 2 -NC@ HCF, respectively

Fig. 12 a
Fig. 12 a Comparison of the initial CV curves of the S/HCF, S/ CoSe 2 -NC@HCF composite at 0.1 mV•s −1 .Charge and discharge curves of b S/CoSe 2 -NC@HCF composite and c S/HCF composite at