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ISSN : 1225-0112(Print)
ISSN : 2288-4505(Online)
Applied Chemistry for Engineering Vol.35 No.6 pp.572-580
DOI : https://doi.org/10.14478/ace.2024.1066

Cobalt Catalyst Supported on Nitrogen-Doped Metal Organic Frameworks (MOF)-Derived Carbon Electrodes for Lithium-Sulfur Cells

Yongwoo Lee*, Yongju Jung**, Seok Kim*,***†
*School of Chemical Engineering, Pusan National University, Busan 46241, Republic of Korea
**Department of Chemical & Biological Engineering, Korea University of Technology and Education, Cheonan 31253, Republic of Korea
***Institute of Environment and Energy, Busan 46241, Republic of Korea
† Corresponding Author: Pusan National University School of Chemical Engineering, Busan 46241, Republic of Korea Tel: +82-51-510-3874 e-mail: seokkim@pusan.ac.kr
September 25, 2024 ; October 22, 2024 ; October 25, 2024

Abstract


Lithium-sulfur batteries are recognized as potential next-generation energy storage devices due to their superior energy density and affordability. However, the dissolution of lithium polysulfides (LiPS) in the electrolyte induces a shuttle phenomenon, depleting active materials and shortening battery lifespan. This study examines the use of Co single-atom (CoSA) catalysts supported on an N-doped carbon matrix to mitigate the polysulfide shuttle effect. By varying the concentrations of Zn2+ and Co2+, the synthesis of ZnCo-Zeolitic imidazolate framework (ZIF) was optimized, enabling the uniform dispersion of Co single atoms within an N-doped carbon matrix. The Co2SA-CN@S electrode, derived from ZnCo-ZIF with an optimal Zn2+ to Co2+ concentration ratio of 8:2, achieved a specific capacity of 1113 mAh g–1 at 0.1 C and demonstrated excellent rate performance of 647 mAh g–1 at 1.0 C. This study confirms that the concentrations of Zn2+ and Co2+ during the synthesis of ZnCo-ZIF significantly influence Co particle aggregation and the formation of Co single atoms after heat treatment. The N-doped metal- organic framework-derived carbon, supported by single cobalt atoms, referred to as Co single-atom carbon nanomaterials (CoSA-CN), synthesized with optimal reactant concentrations, effectively enhances polysulfide conversion during redox reactions, minimizes LiPS migration, and suppresses the shuttle effect. This research reveals that controlling metal ion concentrations (Co2+ and Zn2+) is an effective strategy to limit an aggregation of metal catalysts, thereby producing single atoms more efficiently and ensuring their uniform distribution within the carbon matrix.


Li-S cells , Single atom catalysts , N-doped carbon , Zeolitic imidazolate frameworks , Metal-organic frameworks

초록

    1. Introduction

    Portable electronic devices and electric vehicles are becoming increasingly popular, driving an urgent need for advanced rechargeable batteries that offer high energy and power densities, long cycle lives, and affordability. Currently, Li-ion batteries (LIBs) dominate the secondary battery market. These batteries store chemical energy, which is converted into electrical energy through the movement of lithium ions into and out of the electrodes. Despite their widespread use, significant challenges remain in improving LIB performance. The low energy density of LIBs (200 Wh kg–1) hinders efficient electrical energy storage [1]. Furthermore, the reliance on certain materials, such as cobalt, makes battery production expensive[2]. Due to the inability of LIBs to fully meet these growing demands[3,4], researchers are exploring new battery chemistries.

    Li-sulfur batteries (LSBs) have garnered significant attention due to their impressive theoretical specific capacity (1675 mAh g–1), exceptional energy density (2600 Wh kg–1), and low cost[5-7]. Sulfur, an abundant and environmentally friendly element, is primarily produced through the hydrodesulfurization of petroleum, making it a byproduct of pollutant removal from fossil fuels[8]. Consequently, sulfur is both inexpensive and free from resource scarcity issues. To commercialize LSBs, researchers are addressing limitations such as the low conductivity of sulfur, the formation of insoluble lithium sulfide (Li2S), slow reaction kinetics, and the shuttle effect during charge-discharge cycles[9].

    A major obstacle hindering the development of LSBs is the dissolution of the lithium polysulfides (LiPSs) generated at the cathode into the electrolyte. If not immediately converted or adequately contained, LiPSs can migrate between the cathode and anode, a process known as the “shuttle effect.” This continuous movement persists until the LiPSs are converted into insulating Li2S, which deposits on the anode, reducing active material utilization and causing metal anode corrosion[7]. Additionally, the significant volume expansion (ca. 80%) during LiPS formation further contributes to performance degradation[10,11].

    A well-known approach to addressing the challenges of LSBs involves designing sulfur hosts capable of immobilizing LiPSs. A common strategy is to incorporate sulfur into microporous or mesoporous carbon hosts[12-18]. While numerous sulfur/carbon composites with different nanostructures have shown potential for trapping LiPS, the interactions between carbon and sulfur are primarily physical, making shuttle effect suppression inadequate[19,20]. Consequently, recent studies have focused on developing chemisorption mechanisms to immobilize LiPS[11]. In particular, metal-organic framework (MOF)-based materials have gained attention as sulfur hosts owing to their excellent porous structures and high heteroatom content[21,22]. These MOF-derived carbonaceous cage-like hosts attract and trap a significant number of soluble LiPSs through polar interactions with heteroatoms. However, once the active binding sites are saturated, LiPS begins to leak from the cathode, continuing to degrade LSB performance. This is largely due to the dissolution and slow conversion kinetics of LiPS during charge/discharge cycles. Recent advancements in LSBs have focused on improving cycle life and maintaining capacity by improving sulfur electrocatalysis. By promoting faster conversion of soluble polysulfides (Li2Sx, 4 ≤ x ≤ 8) into insoluble Li2S2/Li2S during discharge, LiPS does not have sufficient time to shuttle. The success of this electrocatalytic strategy relies on utilizing the smallest electrocatalytic particles to accelerate conversion kinetics[23-27]. In Fact, previous research demonstrated sulfur hosts for Na–S batteries using atomic cobalt- decorated hollow carbon nanospheres[28]. Similarly, another research designed sulfur hosts for LSBs with monodispersed cobalt atoms embedded in N-doped graphene[29], thereby improving battery performance. These studies clearly demonstrate that isolated cobalt atoms act as catalysts, facilitating conversion reactions during charge and discharge. Therefore, single-atom catalysts (SACs) represent the most effective approach where individual metal atoms are dispersed on a substrate, maximizing atomic utilization and improving catalytic efficiency[30,31]. Moreover, SACs have shown great potential because they can effectively load more sulfur while using a minimal amount of metal[30]. MOFs serve as excellent precursors for SACs due to their tunable porous structures and high heteroatom content[31]. These porous structures allow MOFs to stably anchor metal atoms, making them an ideal material for SAC fabrication. MOF-derived SACs not only provide abundant active sites for immobilizing LiPS but also enhance electron transfer, significantly improving the cycling stability and performance of LSBs[31]. As a result, the incorporation of SACs into sulfur hosts has emerged as a key strategy to rapidly convert LiPS into Li2S, mitigate the shuttle effect. However, metal aggregation remains a significant challenge for successfully doping single atom catalysts into MOF precursors.

    In this paper, a cobalt single-atom catalyst-based sulfur host for LSBs, derived from a distinctive zeolitic imidazolate framework (ZIFs) precursor, is reported. As a precursor for SACs and a subgroup of MOFs, ZIFs consist of tetrahedral metal ions (such as Zn and Co) connected by imidazolate linkers. ZIF-8 and ZIF-67, two representative ZIFs, consist of zinc and cobalt ions, respectively, as their metal centers, with 2-methylimidazole serving as the nitrogen-containing ligand [32,33]. Given their identical topologies, a ZnCo-ZIF precursor can be synthesized in which Zn and Co were uniformly distributed because of their similar coordination with 2-methylimidazole[32,33]. Upon heat treatment, Zn²⁺ is removed, creating open sites for nitrogen atoms to occupy, which is crucial for preventing the coalescence of cobalt single atoms (CoSAs) and enhancing their catalytic activity in converting soluble LiPS into insoluble Li2S. During ZnCo-ZIF synthesis, variations in electrochemical performance were observed, depending on the concentrations of Zn2+ and Co2+, indicating that reactant concentrations significantly affect the ZnCo-ZIF synthesis mechanism. In this study, we optimized Zn²⁺ and Co²⁺ concentrations to synthesize ZnCo-ZIF precursors at the ideal ratio. The ZnCo-ZIF synthesized with optimal reactant concentrations minimized Co aggregation and maximized the formation of CoSAs. These CoSAs enhance redox reaction kinetics and suppress the lithium polysulfide shuttle effect[34,35]. CoSAs are highly dispersed on the carbon matrix, allowing every atom to participate in the catalytic reaction. Both theoretical and experimental results demonstrated that ZnCo-ZIF synthesized at optimal reactant concentrations not only accelerated reversible conversion reactions but also provided stable cycling performance over extended cycles.

    2. Experimental

    2.1. Materials

    Zinc nitrate hexahydrate (Zn(NO3)2⋅6H2O ≥ 98.0%, Sigma-Aldrich), cobalt nitrate hexahydrate (Co(NO3)2⋅6H2O ≥ 98.0%, Sigma-Aldrich), 2-methylimidazole (2-mIM, 99%, Sigma-Aldrich), methanol (99.5%, Daejung), and sulfur powder (99.98% trace metals basis, Sigma-Aldrich) were utilized without prior treatment.

    2.2. Synthesis of ZnCo-ZIF

    Co(NO3)2⋅6H2O (0, 2, 5, and 8 mM) and Zn(NO3)2⋅6H2O (10, 8, 5, and 2 mM) were dissolved in 80 mL of methanol to achieve Co ratios of 0:10, 2:8, 5:5, and 8:2, respectively, for the preparation of each sample(Table 1). Separately, 2-Methylimidazole (40 mM) was dissolved in 80 mL of methanol. The two solutions were then combined and ultrasonicated for 15 min. Afterward, the mixture was stirred vigorously for 2 h and aged at room temperature for 24 h. Following centrifugation at 6000 RPM, the mixture was washed several times with methanol and freeze-dried for 48 h to obtain powders of ZnCo- ZIF crystals with varying concentrations.

    2.3. Synthesis of CoSA-CN and CN

    The synthesized ZnCo-ZIF was placed in a tube furnace under an Ar atmosphere, heated to 900 °C at a rate of 5 °C/min, and held at this temperature for 3 h. After cooling to room temperature, the resulting black product was collected and labeled as CoSA-CN, specifically named Co2SA-CN, Co5SA-CN, and Co8SA-CN according to the Co ratio). A similar method was used to synthesize CN.

    2.4. Synthesis of CoSA-CN@S and CN@S

    Sulfur and CoSA-CN were vigorously mixed in a 7:3 mass ratio and heated to 155 °C for 12 h to allow the molten sulfur to impregnate the CoSA-CN. After cooling to room temperature, the final product, CoSA-CN@S, was obtained. A similar method was used to produce the CN@S composite.

    2.5. Characterization

    The morphologies and structures of the samples were analyzed using field-emission scanning electron microscopy (FE-SEM, SUPRA25, SUPRA40VP), field-emission transmission electron microscopy (FE-TEM, TALOS F200X), energy-dispersive X-ray spectroscopy (EDS, X-max 80 mm2), X-ray diffraction (XRD, XRD-Xpert), and Fourier-transform infrared spectroscopy (FT-IR, Perkin Elmer Spectrum GX).

    2.6. Electrochemical characterizations

    For the electrochemical measurements, a slurry consisting of 70 wt% CoSA-CN@S composite, 20 wt% Super-P, and 10 wt% polyvinylidene fluoride (PVDF) was prepared using N-methyl-2-pyrrolidone (NMP) as the solvent[36]. This slurry was evenly spread onto aluminum foil and dried for 12 h in a vacuum oven at 60 °C. A CR2016 coin cell was assembled in an Ar-filled glove box using the fabricated cathode, a Celgard 2400 separator, a lithium metal anode, and an electrolyte. The electrolyte comprised of 1 M LiTFSI in a 1:1 volume ratio of 1,2-dimethoxyethane to 1,3-dioxolane with the addition of 2.0 wt% LiNO3. Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and rate capability measurements for the Li-S coin cells were conducted using a Won-A Tech WBCS-3000S battery tester (Korea). CV tests were performed within a voltage range of 1.6~2.8 V at a scan rate of 0.2 mV s–1. GCD tests were performed at a C-rate of 0.1 C. Rate capability tests were conducted over the same voltage range with C-rates of 0.1, 0.2, 0.5, 0.8, 1.0, and 0.1 C, with ten cycles at each C-rate (1 C = 1675 mAh g–1)[37,38].

    3. Results and discussion

    3.1. Material characterization

    Scheme 1 illustrates the preparation process for obtaining CoSACN@ S composites. ZnCo-ZIFs with varying Co2+ to Zn2+ molar ratios were first prepared. A porous carbon host modified with Co single atoms was synthesized through a one-pot solution method at room temperature. The samples were then carbonized at 900 °C to volatilize the Zn. The synthesis of ZIF-based crystals, such as ZIF-8, ZIF-67, and ZnCo-ZIF, involves three stages: nucleation, crystal growth, and the stationary phase. Crystals initially formed via nucleation upon mixing and stirring the two solutions. The crystal structure then developed and continued to grow until precipitation from the solution occurred[39].

    The morphology and chemical composition of the precursor ZnCo- ZIFs and their pyrolyzed products, CoSA-CNs, as determined using SEM, are presented in Figure 1a-h. The ZnCo-ZIF precursors with Zn contents of 100%, 80%, 50%, and 20%, are shown in Figure 1a, c, e, and g, respectively. ZIF-8, Zn8Co2-ZIF, Zn5Co5-ZIF, and Zn2Co8-ZIF all exhibit uniform dodecahedral structures, indicating that stable precursor synthesis is possible irrespective of the Co concentration because Zn and Co have the same phase structure and similar coordination with the 2-methylimidazole ligand. However, the morphology of each material after heat treatments shows clear differences. Figure 1b, d, f, and h show the samples obtained by pyrolyzing each precursor (CN, Co2SA-CN, Co5SA-CN, and Co8SA-CN, respectively), demonstrating that the Co concentration significantly affects the ZIF-based porous carbon structure. Notably, Co clusters are observed in Figure 1f and 1h, with more clusters seen in Co8SA-CN, which had the highest Co concentration. Interestingly, no Co clusters were detected in Co2SA-CN, indicating that Co was dispersed in a single-atom state within the carbon matrix without aggregation. This suggests that the loading of Co single atoms does not depend on the porous carbon structure. Additionally, while the CN sample showed some structural collapse after heat treatment, Co2SA-CN exhibited a more stable porous carbon structure owing to the incorporation of Co single atoms, demonstrating superior thermal stability. This indicates that Co single atoms enhance the stability of the carbon matrix and prevent structural collapse, potentially leading to better electrochemical performance.

    Figure 2 shows the presence of Co single atoms confirmed through the TEM image, and the elemental distribution within the Co2SACN@ S composite obtained as determined by EDS elemental mapping. Figure 2a demonstrates the successful formation of the Co single-atom catalyst, showing bright dots that represent Co atoms uniformly dispersed across the polyhedral matrix in the TEM image. The reason why the Co single atoms appear bright is their higher atomic number relative to the surrounding elements[40,41]. Figure (c)–(f) illustrate the uniform distribution of C, Co, N, and S, respectively. Figure (b) displays the micrograph of Co2SA-CN@S, where no distinct sulfur structures are visible apart from the ZIF-derived carbon nanostructure, indicating uniform impregnation of sulfur into the Co2SA-CN matrix. The absence of a Zn signal confirms that most of the Zn was volatilized and removed during thermal treatment. The uniform distribution of C, Co, N, and S demonstrates that nitrogen was doped into the carbon matrix during the thermal process and that Co single atoms are evenly distributed within the nitrogen-doped carbon matrix.

    Figure 3 presents the XRD patterns of Co2SA-CN, Co5SA-CN, and Co8SA-CN, with different Zn2+ and Co2+ concentrations. Characteristic diffraction peaks at 2θ values of 44.2°, 51.5°, and 75.8° corresponding to the (111), (200), and (220) crystal planes of face-centered cubic Co[ICDD file no. 15-0806, space group Fm3m (225)][42]. The pyrolyzed Zn8Co2-ZIF (red line) exhibits almost no diffraction peaks corresponding to these crystal planes of FCC-structured Co, indicating the amorphous nature of the pyrolyzed Zn8Co2-ZIF and suggesting that the Co atoms are randomly dispersed in a single atomic form within the carbon matrix, thus preventing the formation of an X-ray diffraction pattern. In contrast, the pyrolyzed Zn5Co5-ZIF (green line) and Zn2Co8-ZIF (blue line) clearly show diffraction peaks from the (111), (200), and (220) crystal planes, indicating Co atom aggregation into a crystalline FCC structure. Notably, the pyrolyzed Zn2Co8-CN (blue line) exhibits stronger diffraction peak intensities, implying a greater degree of Co aggregation and the formation of a more crystalline Co FCC structure. As a result, Co2SA-CN based on Zn8Co2-ZIF has a higher active surface area and the less particle aggregation compared to Co5SA-CN based on Zn5Co5-ZIF and Co8SA-CN based on Zn2Co8-ZIF. This suggests that the single atomic dispersion of Co in Co2SA-CN provides higher catalytic efficiency in electrochemical reactions.

    Figure 4 shows the XRD patterns of elemental sulfur, Co2SA-CN, and sulfur-impregnated Co2SA-CN@S. Major sulfur diffraction peaks are identified, with distinct and strong peaks of elemental sulfur confirmed by the reference pattern (JCPDS No. 08-0247)[43]. A comparison of the XRD patterns of the three samples reveals that Co2SA-CN exhibits almost no diffraction peaks due to its amorphous nature, whereas the Co2SA-CN@S composite shows diffraction peaks similar to those of elemental sulfur. This indicates that the XRD pattern of Co2SA-CN@S reflects both the amorphous nature of Co2SA-CN and the characteristics of sulfur, resulting in a unique pattern. These findings confirm successful impregnation of sulfur into the Co2SA-CN composite. However, the intensity of the sulfur diffraction peak in Co2SA-CN@S appears slightly reduced compared to that of elemental sulfur. This reduction is attributed to either a decrease in sulfur size or a reduction in intensity due to encapsulation within the porous structure of Co2SA-CN.

    Figure 5 shows the results of FT-IR spectroscopy analysis for the chemical functional groups and structural characteristics of the Co2SA-CN@S and CN@S samples. The distinct peak observed around 850-1000 cm⁻¹ indicates the presence of nitrogen-containing ring structures[44], representing the CN heterocycle vibrational modes formed within the carbon matrix. Specifically, these peaks result from the substitution of some carbon atoms with nitrogen atoms in the carbon matrix, suggesting that nitrogen was successfully doped into the carbon framework through structural changes during the heat treatment process. This nitrogen doping alters the electronic properties of the ring structures, leading to a shift in the peaks observed in the FTIR spectrum. Specifically, the asymmetric bending vibration of C-N bonds and the vibrational modes of N-C-N bonds can be observed within this range, reflecting the characteristics of the nitrogen-containing ring structures.

    The peaks observed in the 1025~1250 cm⁻¹ region correspond to the stretching vibrations of C–N bonds, while peaks observed around 1480~1660 cm–1 are attributed to vibrational modes due to the resonance effects of C=C and C=N bonds within the pyridine rings[45]. These peaks represent the structural characteristics of the N-doped carbon matrix, indicating the presence of pyridinic and other N-containing ring structures. The N-doping causes electron delocalization within the carbon matrix, contributing to the distinct characteristics of the C–N and C=N bonds in the pyridine structure. Additionally, C–N bond vibrations in the aniline radical structures may also contribute to the peaks observed around 1250 cm–1[46]. While the C–N bond peaks appear strongly in the CN@S sample, they are barely visible in Co2SA-CN@S. This indicates that the Co single atoms form Co–N bonds with the N-doped C, leading to the disappearance or significant weakening of the existing C–N bond vibrations. These results provide crucial evidence of the successful formation of Co–N bonds in the Co2SA-CN@S sample.

    3.2. Electrochemical characterization

    The electrochemical performance of the CoSA-CN@S composites, CN@S, and carbon black as sulfur hosts was analyzed through CV, GCD, and rate capability tests. The carbon-black@S composite was prepared using a previously reported method[47]. The cathode was prepared using 70 wt% carbon-black@S composite, 20% Super-P, and 10% PVDF.

    Figure 6 shows the CV curves of the electrodes using Co2SA-CN@S composites, CN@S, and carbon black as sulfur hosts. The CV curves were used to evaluate the electrocatalytic capability of CoSA in promoting the conversion of LiPS. CV measurements were performed at a scanning rate of 0.2 mV s–1. The Co2SA-CN@S electrode, which exhibited the best performance, showed two distinct reduction peaks at 2.30 V and 1.98 V, corresponding to the reactions of S8 → Li2Sx (4 ≤ x ≤ 8) and Li2Sx → Li2S2/Li2S, respectively. The reduction peaks of the Co2SA-CN@S electrode were positioned further to the right than those of the other samples, indicating that the conversion of LiPS proceeded most rapidly and reversibly at the Co2SA-CN@S electrode. In addition, at the oxidation peak, the reduced Li2S/Li2S2 was converted back into Li2Sx, with a strong oxidation peak observed at 2.43 V. This oxidation peak indicates that Li2Sx was completely oxidized back to elemental sulfur (S8)[48]. The shift in both the cathodic and anodic peak positions clearly demonstrates the strong catalytic effect of CoSA, further promoting the conversion reactions of LiPS. Moreover, a higher current density indicates more active electrochemical reactions. In other words, samples with larger current density peaks imply that more electrons are being transferred, leading to more efficient redox reactions than in samples with smaller peaks[49-51]. As a result of the CV analysis, Co2SA-CN@S recorded the highest peak values, confirming the higher charge capacity by electrochemical reactions.

    Figure 7 shows the GCD curves of the electrodes using Co2SA-CN @S, CN@S, Co5SA-CN@S, Co8SA-CN@S, and carbon black as sulfur hosts. The first plateau in the discharge curve corresponds to the first reduction peak in the CV curve, representing the reaction S8 → Li2Sx (4 ≤ x ≤ 8). The second plateau reflects the second reduction peak in the CV curve, indicating the Li2Sx → Li2S2/Li2S conversion reaction. The Co2SA-CN@S electrodes exhibited a discharge voltage peak shifted to a higher value and a charge voltage peak shifted to a lower value compared to the other samples. These results suggest enhanced reaction kinetics and reduced polarization in the charge/discharge plateaus owing to the strong affinity and catalytic activity of Co2SA-CN towards LiPS. The specific discharge capacities of each sample at 0.1 C were: Co2SA-CN@S (1135 mAh g–1), CN@S (973 mAh g–1), Co5SA-CN@S (944 mAh g–1), Co8SA-CN@S (795 mAh g–1), and carbon-black@S (743 mAh g–1). The Co2SA-CN@S electrode exhibited the highest specific discharge capacity of 1135 mAh g–1 with a long charge–discharge plateau, indicating its effective promotion of LiPS conversion and maximization of sulfur activation and reactivity during the charge–discharge process. Furthermore, Co2SA-CN exhibited the smallest overpotential (0.17 V), demonstrating reduced polarization and suggesting that the electrochemical conversion reactions occurred more easily. Table 2 demonstrates that the electrochemical performance of the Co2SA-CN@S electrode surpasses that of previously reported LSBs, emphasizing its strong potential as an effective sulfur host for high-performance LSBs.

    Figure 8 shows the rate capabilities of electrodes using Co2SACN@ S, CN@S, Co5SA-CN@S, Co8SA-CN@S, and carbon-black@S as sulfur hosts. The experiment was conducted at varying current densities of 0.1, 0.2, 0.5, 0.8, and 1.0 C, before returning to 0.1 C to evaluate the electrochemical performance of each electrode. Table 3 presents the average discharge capacities of the samples at each C-rate. The Co2SA-CN@S electrode exhibited high reversible capacities of 1113, 1016, 904, 779, and 647 mAh g–1 at 0.1, 0.2, 0.5, 0.8, and 1.0 C, respectively, demonstrating the best rate capability among the samples. These results suggest that the CoSA catalyst effectively minimizes performance degradation during high-rate charge–discharge cycles because of its high activation and promotion of LiPS conversion reactions. Notably, when the current density was returned to 0.1 C, the capacity recovery was very high, further confirming the electrochemical stability of Co2SA-CN@S. Thus, the Co2SA-CN@S electrode shows significant promise as a high-performance sulfur host material for LSBs. These kinds of composite electrode study could be applicable to the other Li based batteries[55,56].

    4. Conclusion

    In summary, the Co2SA-CN@S composite electrode exhibited the highest discharge capacity and outstanding reversibility. The Co2SACN@ S electrode achieved a high specific capacity of 1113 mAh g–1 at 0.1 C and exhibited excellent rate performance of 647 mAh g–1 at 1.0 C. Additionally, after cycling at 0.1 C, 0.2 C, 0.5 C, 0.8 C, 1.0 C and returning to 0.1 C for a total of 60 cycles, the electrode maintained a capacity of 1053 mAh g–1, with a reduction of approximately 5.4% from the initial capacity, thereby demonstrating excellent cycling stability. This performance was attributed to the formation of Co single atoms, rather than Co clusters, during the thermal treatment of the ZIF precursor synthesized at the optimal Zn2+ to Co2+ ratio of 8:2, which maximized the catalytic effect. This study demonstrated that the Co–N structure in the CoSA-CN host can strongly chemisorb LiPS, promoting the conversion reaction of LiPS and effectively suppressing the shuttle effect. Conversely, in the Co5SA-CN@S and Co8SA-CN@S samples, the increased number of Co clusters suppressed LiPS adsorption, reduced the efficiency of the LiPS conversion reaction. Co2SA-CN@S exhibited the highest electrochemical performance as a cathode host for LSBs, due to the synergy between its robust porous structure and the extensive catalytic sites provided by CoSA. This demonstrates that Co2SA-CN@S is the optimal material for minimizing the size of Co clusters and maximizing Co single atoms. Overall, this study presents a novel approach to synthesizing a promising single- atom catalyst-based sulfur host by optimizing Co concentration in LSBs, suggesting significant potential for the development of catalysts and other electrochemical cells.

    Acknowledgement

    This work was supported by a two-year research grant from Pusan National University.

    Figures

    ACE-35-6-572_S1.gif
    Schematic diagram of the preparation process for obtaining CoSA-CN@S composites.
    ACE-35-6-572_F1.gif
    FE-SEM images of (a) ZIF-8, (b) CN, (c) Zn8Co2-ZIF, (d) Co2SA-CN, (e) Zn5Co5-ZIF, (f) Co5SA-CN, (g) Zn2Co8-ZIF, and (h) Co8SA-CN.
    ACE-35-6-572_F2.gif
    (a) TEM image of Co2SA-CN@S. (b) Micrograph of Co2SA-CN@S and (c)–(f) Corresponding EDS elemental mapping images of C, Co, N, and S, respectively.
    ACE-35-6-572_F3.gif
    XRD patterns of the prepared (a) Co2SA-CN, (b) Co5SA-CN, and (c) Co8SA-CN.
    ACE-35-6-572_F4.gif
    XRD patterns of pure sulfur, Co2SA-CN, and Co2SA-CN@S.
    ACE-35-6-572_F5.gif
    FT-IR spectra of Co2SA-CN@S and CN@S composites.
    ACE-35-6-572_F6.gif
    CV curves of CoSA-CN@S, CN@S, and Carbon Black@S electrodes at a scan rate of 0.2 mV s–1 within a voltage range of 1.6~ 2.8 V.
    ACE-35-6-572_F7.gif
    Initial galvanostatic charge–discharge profiles of CoSA-CN @S, CN@S, and carbon-black@S electrodes at 0.1 C.
    ACE-35-6-572_F8.gif
    Rate capability of CoSA-CN@S, CN@S, and carbon- black@S electrodes from 0.1 C to 1.0 C.

    Tables

    Composition of the ZnCo-ZIF Samples
    Electrochemical Performance of Sulfur Hosts
    Average Specific Capacity of CoSA-CN@S, CN@S, and Carbon-black@S Electrodes at Each C-Rate

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