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. 2023 Nov 18;8(48):45232–45244. doi: 10.1021/acsomega.3c01851

Controlled Construction of Cobalt-Doped Carbon Nanofiber–Carbon Nanotubes as a Freestanding Interlayer for Advanced Lithium–Sulfur Batteries

Jia Liu †, Huijie Zhang †, Cheng Ma ‡,*, Jitong Wang †,§,*, Wenming Qiao †,§, Licheng Ling †,§
PMCID: PMC10702321  PMID: 38075826

Abstract

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The major challenges for the realistic application of lithium–sulfur batteries (LSBs) lie in the great difficulties in breaking through the obstacles of the sluggish kinetics and polysulfides shuttle of the sulfur cathode at high sulfur loading for continuous high sulfur utilization during prolonged charge–discharge cycles. Herein, cobalt-doped carbon nanofibers containing carbon nanotubes (Co@CNF-CNT) were prepared via electrospinning and chemical vapor deposition (CVD) methods while using polyacrylonitrile (PAN) as the carbon source and cobalt nanoparticles as the catalyst. The obtained uniform thickness film with high mechanical strength can be cut and used directly as a functional freestanding interlayer for LSBs. The appearance of one-dimensional “dendritic” carbon nanotubes on the surface of carbon nanofibers not only enhanced the capture ability of lithium polysulfide (LPSs) but also further improved the conductivity of the materials and increased the electron transport path for Li2S deposition. The results show that under the synergistic effect of porous structure, nitrogen doping, cobalt nanoparticles, and high-conductivity carbon nanotubes, the Co@CNF-CNT interlayer can effectively raise the polysulfide adsorption and conversion efficiency, and provide remarkable rate performance and excellent cycling stability even at high sulfur mass loading. The LSBs with Co@CNF-CNT interlayer have a discharge capacity of 656 mAh g–1 at a high rate of 3C, and the capacity decay rate at 1C after 1000 cycles was only 0.045% per cycle. When fitted with a high sulfur loading cathode of 5.3 mg cm–2, the battery could still maintain a discharge capacity as high as 0.045% mAh g–1 after 70 cycles at 0.2C.

1. Introduction

As one of the new-generation battery systems, lithium–sulfur batteries (LSBs) have been regarded as one of the most promising choices to satisfy the increasing demand for high-energy storage systems due to their high theoretical specific capacity (1675 mAh g–1) and high energy density (2600 Wh kg–1).1−3 Meantime, because sulfur, the active substance of LSBs, possesses the characteristics of abundant reserves, low cost, and environmental friendliness, LSBs have a huge potential commercial value. Although LSBs have many advantages, there are still many problems that restrict their commercial applications, such as poor conductivity of S/Li2S,4 huge volume expansion as high as 80% during cycling,5 and the “shuttle effect” caused by the dissolution of lithium polysulfides (LPSs).6 These problems are ultimately manifesting in the rapid decay of capacity, inferior cycling performance, and poor rate capability.7

To solve the above problems, the design and preparation of materials on the cathodic side including sulfur hosts, interlayer, and modified separator, which is the main existing space of active materials, has been widely studied. Porous carbon materials, such as porous carbon nanofibers (CNFs),8 carbon nanotubes (CNTs),9 reduced graphene oxide (rGO),10 and carbon spheres,11 have already demonstrated excellent potential as host materials. Their porous nature offers ample space for the encapsulation of active sulfur, which helps prevent dissolution into the electrolyte and improves the conductivity of the cathode side. Ji8 demonstrated an electrode prepared from a porous CNF–S nanocomposite with 42 wt % S, which delivered stable capacites of about 1400 and 900 mAh g–1 at 0.05C and 0.2C, respectively. It has been reported that doping nitrogen atoms in porous CNFs can also contribute considerable adsorption ability to polysulfides and catalytic activity to convert polysulfides.12−14 Recently, a great number of studies have shown that it has a positive effect on controlling the migration of polysulfide and improving the utilization rate of sulfur by inserting a functional interlayer between the sulfur cathode and separator.15,16 The functional interlayer can serve as a secondary current collector, facilitating the conversion reaction of polysulfides by providing additional reaction sites. This promotes the efficient utilization and recycling of sulfur, resulting in improved cycling stability and rate capacity of LSBs.17,18 Among numerous functional interlayer materials, porous carbon nanofiber interlayer could play the role of an absorbent to retain polysulfides and reduce their dissolution into the electrolyte due to its high specific surface area and porous structure.16,19 Notwithstanding, weak intermolecular interactions still limit the effectiveness of nonpolar carbon materials in mitigating the shuttle effect. Current research indicates that the introduction of transition-metal-based electrocatalysts, such as Pt,20 Co,21 Ti,22 and Ni,23 into materials could remarkably enhance the chemical adsorption of LPSs and accelerative effects on polysulfides conversion that fundamentally reduce the shuttle effect of polysulfides.24 For example, Zhou22 reported a TiO2/TiN heterostructure as coating on the separator, which achieved a rapid process of trapping, diffusion, and catalytic conversion of LPSs across the interface and offered capacity retentions of 73 and 67% at 1C for sulfur loadings of 3.1 and 4.3 mg cm2, respectively, over 2000 cycles. However, the transition metal and its composites still face the disadvantages of intrinsically poor electrical conductivity25 and low specific surface area.26 Therefore, a promising strategy is to couple these metal composites with porous CNF materials to optimize the overall conductivity, porosity, and catalytic kinetics.27 For example, Chen28 designed a Co-PCNF sandwich structure electrode, fastening the active materials between two layers of porous carbon fiber membrane embedded with Co nanoparticles (Co-PCNF) by electrostatic spinning, which delivered an initial discharge specific capacity of 1013.3 mAh g–1, and the decay rate was only 0.04%/cycle over 500 cycles at 0.5C. Wang12 also prepared a hierarchical nitrogen–phosphorus co-doped carbon nanofibers with TiC nanoparticles by electrostatic spinning and incorporated the film between the separator and cathode of the lithium–sulfur battery. The battery exhibited a high capacity of 737.1 mAh g–1 at 5C over 500 cycles with a slow capacity fading rate of 0.06%/cycle.

However, the electrical conductivity and catalytic activity of metal composites with porous CNFs is still limited. The introduction of carbon nanotubes (CNTs) has become a favorable alternative for the interlayer material in LSBs on account of its significant improvement in electrical conductivity, as well as its limitations on the diffusion of LPSs by physical resistance and its excellent mechanical flexibility. Chemical vapor deposition (CVD) methods have been proven to be effective in improving the electrical conductivity and catalytic activity by depositing CNTs and constructing active components on materials with high defective sites.29 As one-dimensional (1D) carbon materials, CNFs and CNTs could be assembled into multidimensional internetworked architectures in order to achieve the characteristic advantages of three-dimensional (3D) hierarchical structures.10 The abundant mesoporous and microporous structures of those CNFs with “dendritic” CNTs also allow space for the deposition of Li2S/S, facilitating the utilization of dissolved polysulfide to achieve increased utilization of sulfur cathode. Besides, those porous CNFs provide an effective electronic conduction path and their network-like structure forms a stable structure of the S electrode.30 In addition, freestanding interlayer has such unique advantages as high mechanical strength, good flexibility, and high structural integrity without peeling, which could hardly be achieved by the coated interlayer.

Based on the above considerations, we adopted a strategy via electrospinning and CVD methods to prepare cobalt-doped carbon nanofibers combined with carbon nanotubes (Co@CNF-CNT) using polyacrylonitrile (PAN) as the carbon source and metallic cobalt as the catalyst, which could serve as the freestanding functional interlayer in LSBs. The CVD method enables the uniform growth of CNTs on CNFs and a uniform loading of cobalt in the form of nanoparticles on carbon-based materials. The regulated CNF-CNT has an abundance of pore structure and a high specific surface area, which could furnish sufficient space for the trapping and conversion of LPSs. The uniformly dispersed cobalt nanoparticles supply adequate catalytic active sites for the efficient conversion of LPSs. Under the synergistic effect of porous structure, carbon intrinsic defects, well-dispersed and ultrafine cobalt nanoparticle doping, and high-conductivity carbon nanotubes, the Co@CNF-CNT could function as a freestanding interlayer to physically capture polysulfides and restrain their shuttling between the cathode and anode. Moreover, it can effectively improve the polysulfide adsorption and conversion efficiency and provide remarkable rate performance (656 mAh g–1 at 3C) and excellent cycling stability (0.045% capacity decay per cycle at 1C after 1000 cycles), together with large areal capacity at a very high sulfur mass loading of 5.3 mg cm–2 (744 mAh g–1 after 70 cycles at 0.2C). This unique reticular structure may provide an advance in the structural design of interlayers for LSBs.

2. Materials and Methods

2.1. Preparation of Cobalt Salt-Doped Polyacrylonitrile Nanofibers (Co@PAN)

All of the chemicals were purchased from Shanghai Titan Technology Co., Ltd. without further purification. In the synthesis of Co@PAN, 2.489 g of Zn(Ac)2·2H2O and 0.141 g of Co(Ac)2·4H2O were dispersed in 14.00 g of N,N-dimethylformamide (DMF) solution and stirred until completely dissolved. Then, 1.659 g of polyacrylonitrile (PAN, Mw = 15 W) was dissolved in the obtained pink-purple solution and stirred at 60 °C for 6 h. This precursory solution was then transferred into a 20 mL syringe for the electrospinning process by using a 21 G stainless syringe needle under 15–16 kV voltage and a distance of 16 cm between the collector and steel syringe needle. The flow velocity was 0.75 mL h–1, and the electrospinning process was carried out with ambient humidity under 45% relative humidity (RH). After spinning, the cobalt salt-decorated polyacrylonitrile nanofibers were dried at 80 °C for 12 h and were named Co@PAN.

2.2. Preparation of Cobalt-Doped Carbon Nanofiber–Carbon Nanotube (Co@CNF-CNT) and Cobalt-Doped Carbon Nanofiber (Co@CNF)

After electrospinning, the fiber precursor Co@PAN was heated to 280 °C for 1 h with a heating rate of 2 °C min–1 to stabilize in air. The growth of CNT was performed on CNF substrates by the CVD method. The process was started by heating to 650 °C at a heating rate of 4 °C min–1 under an argon atmosphere. Then, a mixture flow of argon and H2 (Ar/H2 = 90/10, v/v) was introduced for constant-temperature reduction for 1 h, followed by introducing C2H4 for 10 min with a flow rate of 80 mL min–1 for CNT growth. After the CVD process, the sample was further heated to 950 °C for 4 h at the rate of 4 °C min–1 and then cooled down naturally under argon. The obtained product was named Co@CNF-CNT. For comparison, the preoxidized fiber was carbonized at 950 °C for 4 h at a rate of 4 °C min–1 under an argon atmosphere and was named Co@CNF.

2.3. Material Characterizations

The morphologies and structures of Co@CNF and Co@CNF-CNT interlayers were investigated by scanning electron microscopy (SEM, Nova Nano SEM 450) and high-resolution transmission electron microscopy (HRTEM, JEOL, JEM-2100F). The specific surface areas and poremsize distribution of all samples were calculated by the Brunauer–Emmett–Teller (BET) equation and the density functional theory (DFT) method based on the N2 adsorption–desorption measurement (Quadrasorb SI). XRD patterns were measured on an X-ray diffractometer (Rigaku D/mA X 2550) using Cu Kα X-ray (λ = 1.5406 Å) over the 2θ range of 10–80° with a scan speed of 5° min–1. Raman spectra were characterized on a Renishaw inVia Reflex instrument with a 514.5 nm excitation laser. X-ray photoelectron spectroscopy (XPS) was performed on Thermo Scientific Kα using a monochromatic Al Kα (1486.8 eV) X-ray source at 12 KV. The electrical conductivity of the interlayer was measured by pinching the thin interlayer with the 4-probe tester (RTS-8), and the thickness of the Co@CNF and Co@CNF-CNT interlayers used for the measurement was ca. 30–50 μm.

2.4. Cell Assembly and Electrochemical Characterization

The sulfur and Ketjen Black (KB) were ground in a 9:1 weight ratio and heated under vacuum at 155 °C for 12 h. The compound was mixed with Super C and poly(vinylidene difluoride) (PVDF) with a weight ratio of 8:1:1 in NMP solvent with stirring for 12 h, followed by coating the slurry on an Al foil. After drying in an oven at 50 °C, the electrode was cut into 12 mm diameter electrodes (KB/S) with a mass loading of 1.0–1.5 mg cm–2 (sulfur content of 72 wt %). The obtained Co@CNF and Co@CNF-CNT interlayer materials were directly cut into φ16 mm disks as interlayers. CR2025 coin cells using the KB/S as the cathode, Co@CNF or Co@CNF-CNT as interlayers, PP separators (19 mm), and 0.6 mm thick lithium foil (φ16 mm) as the counter electrode were assembled in a glovebox filled with argon. The electrolyte dosage was 45 μL and was composed of 1.0 M LiTFSI with 1 wt % LiNO3 in DOL and DME (1:1, v/v).

The performances of the cells were determined by a LAND CT3001A analyzer at a voltage interval from 1.7 to 2.8 V. The cyclic voltammogram (CV) curves were measured on CHI-600E electrochemical workstation (CHI Instruments, Shanghai) at a scan rate from 0.1 to 0.5 mV s–1 and at 10 mV s–1 with a voltage range between 1.7 and 2.8 V. Electrochemical impedance spectra (EIS) analysis was performed on the GAMRY Reference 600+ electrochemical workstation in the range of 100 kHz to 0.01 Hz (Gamry Instruments).

3. Results and Discussion

Co@CNF-CNTs were synthesized via electrospinning from PAN-nanofiber precursor containing Zn(Ac)2·2H2O and Co(Ac)2·4H2O as an in situ template and metal precursor, and the growth of carbon nanotubes was determined by the CVD method, as schemed in Figure 1a. During the CVD process, C2H4 was used as the carbon source and cobalt in the carbon nanofiber was used the catalyst for the growth of carbon nanotubes. The digital picture of the fiber membrane at each stage during the heat treatment process is shown in Figure 1b. It could be found that the color of the fiber membrane undergoes a series of changes with the heat treatment process: the precursor is first white, then turns to brown after preoxidation, and finally becomes black because of carbonization and CVD process. Although the area of the fiber membrane gradually decreases during the whole process, its shape remains intact, and the surface is free of wrinkles and damage. As shown in Figure 1c, the folding/recovery test shows that the fiber is able to be restored to its original shape after folding and does not produce any cracks or creases on the surface, suggesting that the fiber fabrics have good mechanical strength and flexibility.31

Figure 1.

Figure 1

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(a) Schematic illustration of the synthesis of Co@CNF-CNT; (b) digital photos of the Co@CNF-CNT at each heat treatment stage; and (c) folding/recovery tests of the Co@CNF-CNT.

Figure 2 shows the morphology and microstructure features of the materials by SEM and TEM characterizations. It is seen that continuous fibrous webs with almost even diameters of 300 nm form by electrospinning. A comparison of Figure 2a,b and 2c–f shows that the surface of Co@CNF-CNT and Co@CNF becomes rough while the precursor Co@PAN has a smooth surface. After carbonization, a large number of “bubble holes” appear on Co@CNF-CNT and Co@CNF, which may be caused by the collapse of hollow carbon bubbles from the large local stress change in the fiber during the deoxidization and evaporation of Zn/CoOx. Those “bubble holes” not only provide a certain specific surface area and flexibility for materials, which alleviates the mechanical stress generated during bending, but also enhance the trapping ability of the material to lithium polysulfide and might be profitable to uniform deposition of Li2S/S.

Figure 2.

Figure 2

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SEM images of (a, b) Co@PAN, (c, d) Co@CNF-CNT, and (e, f) Co@CNF. (g) TEM images and element mapping images of Co@CNF, and (h–m) TEM and HR-TEM images of Co@CNF-CNT.

As seen in Figure 2c,d, under the catalysis of cobalt, dendritic CNTs are generated on the outside of the carbon nanofibers. Those nanobranches entangle with each other at the fiber junction, forming a more compact conductive network based on the original network. This structure further improves the ability of a material to capture polysulfides in the electrolyte while providing a continuous and rapid electron transfer channel for Li2S/S deposition and reuse of the dissolution cathode. It is clear from the TEM images of Co@CNF-CNT (Figure 2h–m) and Co@CNF (Figure S1) that some cobalt nanoparticles are uniformly dispersed on the surface of the carbon nanofiber with the size of 10–20 nm, which could greatly promote chemical adsorption and reduction of LPSs. The clear lattice stripes (Figure 2m) of the high-resolution TEM image further confirm the metallic cobalt nanoparticles.32 The elemental mapping analysis of Co@CNF for the response signal of C, N, O, and Co in Figure 2g further demonstrates the uniform distribution of cobalt nanoparticles. As shown in the results of air thermogravimetry in Figure S2, the ash content of Co@CNF-CNT decreased after hydrogen reduction. ICP analysis also shows that the mass percentages of Co in the Co@CNF and Co@CNF-CNT are 7.2 and 3.9 wt %, respectively. Compared with that on the Co@CNF, the cobalt nanoparticles are distributed more uniformly on Co@CNF-CNT due to the addition of hydrogen during the CVD process, since the addition of hydrogen contributes to the reduction of metal precursors. These uniformly distributed cobalt nanoparticles can provide more catalytic active sites and improve the efficiency of adsorption catalysis.

During the CVD process, when the temperature reaches a certain value, C2H4 is adsorbed on the surface of cobalt as the carbon source; the reaction of carburization happens and metal carbide forms; and when carbon solubility exceeds a threshold value, carbon atoms lose their mobility in the solid solution forming metal carbides, which gives rise to precipitation of carbon nanotubes at the metal carbide surface.33 The growth of CNTs improves the electrical conductivity of the material. At the same time, the metal catalyzes the transformation of amorphous carbon into graphitized carbon during carbonization, which also increases the conductivity of nanofibers to a certain extent.34 It could be found in Figure 2h–m that some carbon fibers are also generated in addition to carbon nanotubes, which is mainly caused by the rapid deposition of carbon atoms on the catalyst surface. There are also a considerable number of pores generated after etching, which would also have some delightful advantages. They would be able to immobilize sulfur and polysulfides while contributing to the combination of sulfur with the host. Furthermore, they act as channels to accelerate the electron transport. The one-dimensional structure of the carbon nanofiber can not only raise the electron transport rate but also inhibit the agglomeration of metal nanoparticles.

The porosity also plays an important role in the electrochemical performance of cobalt-doped carbon nanofiber fabrics, as well as the morphology. The porosity of the Co@CNF-CNT and Co@CNF was characterized by nitrogen adsorption–desorption isotherms (Figure 3a) to understand the effect of CNTs growth on the specific surface area and pore structure of materials. The two materials have very similar type IV isotherm with a hysteresis loop (Figure 3a),35 which suggests the presence of abundant mesoporosity in the samples.36 As mentioned above, Zn(Ac)2 was added in the precursor as an in situ template in the preparation of fiber. During the oxidation process, Zn(Ac)2 decomposes to form ZnO. In the further high-temperature charring process, ZnO reacts with C to form Zn and CO. When the temperature further increases to 950 °C, zinc and part of cobalt volatilize to produce a well-developed mesoporous structure.37 As shown in Table 1, the calculated BET surface area of Co@CNF is 704.9 m2 g–1, which is somewhat higher than that of Co@CNF-CNT (302.8 m2 g–1), but the pore size distribution of both remains consistent (Figure 3b), and the pore size is concentrated at 1.5 and 3.4 nm. It is clear that after the growth of CNTs, the pore volume of the material also decreases (0.37 cm3 g–1), which would be considered that some carbon generated by C2H4 dissociation is directly deposited on the fibers during the CVD process, leading to the blockage of pores and the small pore volume of carbon nanotubes. After the CVD process, the electrical conductivity of the material (Table 1) reveals that the conductivity of Co@CNF-CNT (257 S m–1) is significantly higher than that of Co@CNF (183 S m–1). The improvement of conductivity is due to the good conductivity of CNTs and the 3D network of dendritic CNTs on the CNFs, which could enhance LPS conversion kinetics and facilitate uniform deposition of Li2S/S.

Figure 3.

Figure 3

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(a) N2 isotherm adsorption–desorption curves and (b) the corresponding pore size distributions. (c) XRD patterns and (d) Raman spectrum of Co@CNF and Co@CNF-CNT.

Table 1. Porosity Parameters and Electrical Conductivity of Co@CNF and Co@CNF-CNT.

samples SBET (m2 g–1) SMicro (m2 g–1) VTotal (cm3 g–1) VMicro (cm3 g–1) conductivity (S m–1)
Co@CNF 704.9 489.8 0.69 0.20 183
Co@CNF-CNT 302.8 75.9 0.37 0.03 257
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X-ray powder diffraction (XRD) patterns of the Co@CNF-CNT and Co@CNF exhibit two clear diffraction peaks coincident with the characteristic peaks of cobalt at 44.2 and 51.5°. These diffraction peaks are derived from the crystal plane diffraction of the crystalline lattice facets (111) and (200) from cobalt (PDF: 15-0806), respectively, which indicates that cobalt mainly exists in the form of metallic cobalt nanoparticles in the materials.32 The Raman spectra of Co@CNF and Co@CNF-CNT display two strong peaks (Figure 3d), corresponding to D-band at 1350 cm–1 and G-band at 1600 cm–1. Co@CNF-CNT presents a higher degree of disorder as ID/IG = 1.26 than Co@CNF (ID/IG = 0.91), which means much more defect sites on the surface of Co@CNF-CNT. It seems that multiwalled carbon nanotubes with large defects and high amorphous carbon have been formed, which is also beneficial to high electrical conductivity.38,39

The surface chemistry of the material also has an important influence on the electrochemical properties. The X-ray photoelectron spectra (XPS) (Figures 4 and S3) reveal the chemical state of C, N, O, and Co of Co@CNF and Co@CNF-CNT. Both materials have obvious response peaks of C, and the response peaks of Co are relatively weak due to the low cobalt content in the samples. The C 1s, N 1s, and O 1s spectra of the two materials are similar, indicating that the growth of CNTs has little influence on the chemical states of C, N, and O elements in the materials. As shown in Figure 4c, the high-resolution N 1s spectra visualize the presence of four forms of N species for all of the samples, namely, pyridinic N (398.6 eV, N-6), pyrrolic/pyridine N (399.9 eV, N-5), graphitic N (401.5 eV, N-Q), and pyridine N-oxide (402.4 eV, N-X),40 which are mainly obtained by carbonization of nitrogen-containing precursor polyacrylonitrile.41 Among them, N-6 and N-5 locate at the edges of graphene layers, and these nitrogen species are favorable chemisorption sites for LPSs, which effectively inhibits the “shuttle effects”.42 It could be found in Figure 4d that the proportion of N-6 and N-5 in the Co@CNF-CNT is significantly increased compared with that in the Co@CNF, while the proportion of N-Q is reduced. Therefore, the Co@CNF-CNT might have a stronger adsorption capacity for LPSs. The high-resolution Co 2p spectra (Figure 4e,f) have two characteristic peaks at 778.3 and 793.3 eV assigned to metallic cobalt,43,44 which is consistent with the TEM and XRD analysis. The peaks at 779.1 and 794.1 eV assigned to Co(III) and the peaks at 781.7 and 796.7 eV assigned to Co(II) also exist in Co@CNF and Co@CNF-CNT, indicating that there are many valence states of cobalt. These results show that the proportion of Co(0) and Co(II) in Co@CNF-CNT is higher than that in Co@CNF, mainly because the introduction of hydrogen during the CVD process contributes to the reduction of cobalt oxide. Combined with high nitrogen heteroatoms, high electrical conductivity, and flourishing pore structure, Co@CNF-CNT is favorable for hosting elemental sulfur, polysulfide trapping, and conversion, which endow the LSBs with efficient sulfur utilization even at high areal sulfur loading.

Figure 4.

Figure 4

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(a) XPS spectrum survey of Co@CNF and Co@CNF-CNT, (b) high-resolution C 1s spectra of Co@CNF-CNT, (c) high-resolution N 1s spectra of Co@CNF and Co@CNF-CNT, (d) nitrogen content distribution of Co@CNF and Co@CNF-CNT, and high-resolution Co 2p spectra of (e) Co@CNF and (f) Co@CNF-CNT.

The “shuttle effect” caused by the dissolution of LPSs is one of the key problems to obtain high-performance lithium–sulfur batteries. Therefore, the interlayer which possesses high interaction with polysulfides, such as the strong chemical or physical adsorption ability for polysulfides, is preferred.45 In order to evaluate the adsorption of polysulfides by the Co@CNF-CNT and the Co@CNF, the visualized adsorption of Li2S6 is further evaluated, as shown in Figure 5a. After immersion of the Co@CNF-CNT and Co@CNF for 12 h, the color fading comparison for the Li2S6 solution indicates the stronger affinity between Li2S6 and both materials. Moreover, the solution that basically faded to colorless proves that the adsorption capacity of the Co@CNF-CNT is significantly better. As shown in the UV–vis spectra of the solution after adsorption (Figure 5b), it is obvious that the peak intensity of S62– at 250–300 nm with the Co@CNF-CNT declines more than that with the Co@CNF, which also reveals that more Li2S6 molecules are adsorbed to the Co@CNF-CNT surface than the Co@CNF surface.46 To further demonstrate the mechanism of the Co@CNF-CNT and the Co@CNF for polysulfide trapping and conversion, the samples after Li2S6 adsorption were characterized by XPS tests. As displayed in Figure 5c, the S 2p spectra of the Co@CNF-Li2S6 and the Co@CNF-CNT-Li2S6 can be divided into four small peaks. The increased binding energy of bridging sulfur (SB0) and terminal sulfur (ST-1) in the S 2p spectrum after adsorption suggests lower electron cloud density for sulfur atoms, which proves the strong interactions between the materials and Li2S647 The interactions between the Co@CNF-CNT and Li2S6 are more intense since the peak offsets of bridging sulfur (SB0) and terminal sulfur (ST-1) are greater, transferring to 162.2 and 164.1 eV, respectively.48 Moreover, the appearance of peaks corresponding to thiosulfates (166.8 eV) and polythionates (168.8 eV) demonstrates the sulfur-chain catenation effects on improving sulfur conversion. It could be seen that there are more thiosulfates and polythionates in the Co@CNF-CNT-Li2S6 than those in the Co@CNF-Li2S6, which indicates that in addition to the physical adsorption of pore structure, more LPSs are partially oxidized to insoluble substances and are adsorbed on the Co@CNF-CNT surface.49 The interactions not only effectively anchor LPSs but also reduce the concentration of lithium polysulfide in the system, which can be further proved in the high-resolution Co 2p XPS spectra of samples after adsorption in Figure 5d. After the adsorption of Li2S6, the peaks corresponding to each valence state of cobalt in Co@CNF-CNT are transferred to a higher binding energy. Compared with that before the adsorption, the proportion of Co0 and Co2+ is significantly reduced while the proportion of Co3+ is significantly raised, which reveals that part of the low-valence cobalt is oxidized to the high-valence state during the adsorption process.50 This may also be the reason why the Co@CNF-CNT has better adsorption capacity although it possesses a lower specific surface area than that of Co@CNF. Besides, the conversion of LPSs is further investigated by CV curves using symmetric cells with two Co@CNF-CNT or Co@CNF electrodes and Li2S6 electrolyte. As shown in Figure S4, an obvious current response peak appears only when Li2S6 is added to the electrolyte, implying that the material itself could not provide additional response current. Through comparison, it can be found that the response current of Co@CNF-CNT is stronger, and the voltage difference between each pair of redox peaks is smaller, indicating that Li2S6 has better reaction kinetics on Co@CNF-CNT. Furthermore, the symmetric cell with Co@CNF-CNT has a clearer reaction path and higher reaction activity in each step of the conversion reaction of LPSs. When the scanning rate increased to 10 mV s–1, the polarization of the electrode becomes greater, but the oxidation and reduction peaks corresponding to Co@CNF-CNT still display lower reaction overpotential than those of Co@CNF, suggesting good reactivity (Figure 5e).51

Figure 5.

Figure 5

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(a) Visualized adsorption and (b) UV–vis spectra of Li2S6 adsorption experiment. XPS spectra of samples after adsorption: (c) S 2p, (d) Co 2p; CV curves of (e) Co@CNF-CNT and Co@CNF symmetric cells at 10 mV s–1 and the current curves of Li2S precipitation on the surface of (f) Co@CNF-CNT and (g) Co@CNF.

In order to further demonstrate the electrocatalytic effect of the material in the LPS conversion, potentiostatic discharge Li2S precipitation experiments were conducted with Li2S8 as the positive electrolyte (Figure 5f,g), and the material surface was observed by SEM before and after Li2S precipitation (Figure S5). The potentiostatic discharge curve of the cell with the Co@CNF-CNT interlayer shows a sharper and higher peak shape and earlier responsivity peak time (5051 s) than that with the Co@CNF interlayer (5801 s). After fitting analysis, the cell with the Co@CNF-CNT interlayer has a higher Li2S nucleation capacity (349.8 mAh g–1) than Co@CNF (218.3 mAh g–1),52 suggesting a higher LPSs precipitation efficiency on the Co@CNF-CNT surface, which is more conducive to the conversion of LPSs and the nucleation and growth of Li2S. From the SEM images before and after Li2S precipitation, it is clear that Li2S is completely coated on the fiber surface after precipitation, and the fiber remains a network structure. The coating layer on the surface of Co@CNF is relatively tight and smooth, while on the surface of Co@CNF-CNT, due to its denser network, besides the fiber surface, there is also Li2S precipitation on the CNT. More Li2S is deposited on the surface of Co@CNF-CNT, indicating its good promoting effect on the precipitation of lithium sulfides.

To further confirm the superiority of Co@CNF-CNT and Co@CNF as the interlayers between the sulfur cathode and separator for the LSBs, CR2025 button cells were respectively assembled using lithium plate as the anode and carbon/sulfur (sulfur mass loading of 1–1.5 mg cm–2) as the cathode. Then, the corresponding electrochemical performance was systematically tested and analyzed. Figure 6a shows CV curves of Co@CNF-CNT and Co@CNF at a scanning speed of 0.2 mV s–1 within the voltage range of 1.7–2.8 V. CV curves of Co@CNF-CNT and Co@CNF both contain a pair of oxidation peaks and a pair of reduction peaks. However, the reduction peak of the LSB with the Co@CNF-CNT interlayer is higher at about 2.10 V, which means that the insertion of the Co@CNF-CNT interlayer can promote the reaction corresponding to the second discharge voltage platform of the LSB. Although the insertion of the interlayer on the cathode side can effectively alleviate the shuttle effect, it may also hinder the migration of lithium ions between the anode and cathode, affecting the conversion process of LPSs.53 Therefore, CV measurements of Co@CNF-CNT and Co@CNF at the scan rates of 0.1–0.5 mV s–1 are presented. With the increase in scanning rate, the current peak gradually shifts to the direction of a high driving force. As can be seen from Figures 6b and S6, at a high scanning rate, the polarization of Co@CNF-CNT is relatively small, and the corresponding current peak is larger, suggesting that the LSB with the Co@CNF-CNT interlayer has a faster electrochemical reaction rate. Figure 6c displays a curve fitted according to the Randles–Sevcik equation, and the slope of the curve is proportional to the lithium-ion diffusion coefficient (DLi+). It can be seen from Figure 6c that the DLi+ of Co@CNF-CNT in reaction I (Peak I: S8 → Li2S4/Li2S6) and reaction II (Peak II: Li2S4/Li2S6 → Li2S2/Li2S) is greater than that of Co@CNF, and that reaction III (Peak III: Li2S2/Li2S → Li2S8/S8) is similar, indicating that although Co@CNF-CNT has a denser conductive network and smaller specific surface area, it has little effect on lithium-ion diffusion rate and has better promotion effect on reaction I and reaction II. EIS tests results are shown in Figure 6d, where the cell with Co@CNF-CNT shows a smaller semicircle than the Co@CNF, which reveals that the existence of Co@CNF-CNT is helpful to reduce the impedance of the battery reaction process and accelerate the reaction rate,50 which also corresponds to the rate performance of the LSBs.

Figure 6.

Figure 6

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(a) CV curves of Co@CNF-CNT and Co@CNF at 0.2 mV s–1, (b) CV curves of Co@CNF-CNT at various scan rates, (c) linear fits of peak currents of Co@CNF-CNT and Co@CNF, (d) EIS curves, (e) cycle performance at 0.2 C, (f) EIS curves. rate capability and Galvanostatic discharge/charge profiles of (g) Co@CNF and (h) Co@CNF-CNT, (i) comparison of capacity contribution on different discharge plateaus, (j) long-cycle performance at 1C of the Li-S batteries with interlayer, and (k) cycle performance of the high-loading sulfur cathode (4.2 and 5.3 mg cm–1) with Co@CNF-CNT interlayer.

As shown in the cycle performance at 0.2C (Figure 6e), the cell with Co@CNF-CNT displays a superior initial discharge capacity of 1158 mAh g–1 and remains 784 mAh g–1 after 100 cycles, which is significantly higher than that of Co@CNF as the interlayer. The rate discharge capability (Figure 6f) of the cell with Co@CNF-CNT reaches 1059, 904, 826, 724, and 656 mAh g–1 at 0.2, 0.5, 1, 2, and 3C, respectively, showing outstanding rate capability, especially at a high rate. Moreover, when the current returns to 0.2C, the specific capacity is close to the initial value, indicating that the cell with Co@CNF-CNT still has good structural stability after multiple cycles.54 The excellent rate capability of cells with Co@CNF-CNT should give credit to its good conductivity and effective capture capability for LPSs. There are two typical voltage platforms in the discharge curve of LSB, from which platform II at 2.10 V corresponds to the conversion of soluble lithium polysulfide to solid lithium polysulfide.55 The capacity contributed by this step is three-quarters of the theoretical capacity. Since the reaction kinetics of the corresponding step of platform II is relatively slow, this step is also the rate control stage of the entire electrochemical reaction.56 To further reveal the electrochemical performance at different rates, Figure 6g,h shows the galvanostatic discharge/charge voltage profiles of the cell with Co@CNF and Co@CNF-CNT. When it comes to Co@CNF-CNT, two discharge plateaus could be still maintained even at 3C, proving the large rate capability and rapid redox reactions.57 However, the cell with Co@CNF could hardly hold platform II when the current exceeds 1C, resulting in a significant reduction in cell capacity. The capacity contribution of platform I and platform II are shown in Figure 6i. The difference in total capacity mainly comes from the capacity QL generated by platform II. As the rate increases, the gap between the QL corresponding to different interlayers gradually raises, which further reflects the advantages of Co@CNF-CNT as a functional interlayer of LSB. This might be due to the presence of metal nanocobalt, which can catalyze the conversion of soluble Li2S6/Li2S4 to insoluble Li2S2/Li2S.58 Co@CNF-CNT shows a higher proportion of Co, which could be justified in Figure 4f.

To evaluate the potential for realistic applications, the long-term cycling performance was further tested at a higher rate of 1C with a S mass loading of 1.0 mg cm–2 (Figure 6j), and the capacity of 508 mAh g–1 can be retained after 1000 cycles, with a capacity decay of 0.045% per cycle by the cell with Co@CNF-CNT. It was reported that the energy density of LSBs would be promoted only when the mass loading of sulfur was higher than 4 mg cm–2.14 So, for future practical applications, the electrochemical performance of Co@CNF-CNT as the interlayer under larger sulfur loading was also tested at 0.2C. The electrochemical tests at high sulfur loading were carried out at S area loadings of 4.3 mg cm–2 and 5.3 cm–2 and a sulfur content of 72 wt %, with an electrolyte dosage of 45 μL. As shown in Figure 6k, the specific capacity of 4.3 and 5.3 mg cm–2 after 70 cycles are up to 905.7 and 744.3 mAh g–1, respectively. By introducing a Co@CNF-CNT functional interlayer to the battery, the material’s abounding pore structure and sulfophilic surface could serve as a second sulfur host, providing plenty of space for the volume expansion of sulfur that intensifies under high sulfur loading, thus relieving the volume expansion of the cathode and preventing the exfoliation and structural collapse of the cathode material. The excellent electrical conductivity and sulfur storage potential of the material can reduce the amount of conductive agent and polymer binder on the cathode side at high sulfur loading, hence increasing the gravimetric/volumetric energy density of lithium–sulfur batteries.59

The cycled cells with different interlayers were disassembled and ex situ postmortem characterizations were performed to investigate the stability of the lithium metal and interlayers after cycling. As shown in Figure S7, the Co@CNF-CNT interlayer effectively inhibits the shuttle of yellow lithium polysulfide. SEM images of the Li metal anode and interlayers on the surface of the cathode side were also obtained after cycling in Figure S8. The lithium film of the cell with Co@CNF interlayer shows a very rough surface after cycling. This phenomenon can be attributed to the shuttling of LiPSs and heterogeneous lithium deposition during the electrochemical reaction. In contrast, the lithium metal in cells with Co@CNF-CNT interlayer is smoother after cycling, showing a more effective suppression of LiPSs shuttling. After sufficient discharge (discharged to 1.7 V), a layer of Li2S was uniformly deposited on the surface of the Co@CNF-CNT interlayer on the cathode side (Figures S8 and S9), demonstrating the strong interaction between the material and the polysulfide. As shown in Figure S10, the signal of the Li2S peak in XRD also indicates that the interlayer has a facilitated adsorption effect on the deposition of Li2S. XPS analysis was performed to investigate the surface features of the interlayers in the discharged states (Figure S11). The shift of the S-peak toward the high binding energy in the XPS results further indicates the presence of interaction between the material and the polysulfide. The appearance of peaks in the material with thiosulfate and polysulfates also reveals the occurrence of redox reactions with polysulfides.60 In the EIS measurements after cycling, the Nyquist plot of Co@CNF-CNT shows a much smaller charge transfer resistance (Rct) than before cycling. In contrast, the Nyquist plot of Co@CNF shows two semicircles corresponding to the passivation film (in the high-frequency region) and Rct (in the low- and medium-frequency region) (Figure S12). The formation of the passivation film comes from the deposition of dissolved polysulfides on the electrode.11 This result indicates that the electrode’s charge transport properties are improved and is also a side evidence of the effective limitation of dissolved polysulfides by Co@CNF-CNT.

4. Conclusions

In summary, cobalt-containing carbon nanofibers and carbon nanotubes (Co@CNF-CNT) interlayer was prepared by electrospinning and CVD process, and was introduced into the cathode side of LSBs. The obtained freestanding functional interlayer with good conductivity has been proven to improve the overall performance of LSBs, mainly owing to the serial benefits of Co@CNF-CNT: (1) the “dendritic” structure of CNTs interweaves with CNFs to form a denser conductive network and a denser physical barrier layer, and the high conductivity of carbon nanotubes further raises the conductivity of the network as well, which significantly enhances the rate performance; (2) the high specific surface area and pore volume of CNF allow space for the capture and the immobilization of LPSs; and (3) the introduction of CNTs adds to the inherent defects of porous CNFs. Combined with the uniformly loaded cobalt nanoparticles in the fibers, the polysulfide adsorption and conversion efficiency are significantly boosted, and the cycle stability of the battery is also greatly improved; (4) the high mechanical strength, good flexibility, and superior structural integrity of the freestanding interlayer could considerably upgrade the stability of the battery as well. Therefore, this functional interlayer exhibits a long life span with excellent capacity, especially at a high rate, and even delivers a stable cycle performance with fair sulfur loading, which has certain reference significance for the commercial application of LSBs.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. U21A2060, 22178116, 21978097), Shanghai Pujiang Program (No. 21PJD019), Natural Science Foundation of Shanghai (No. 22ZR1417400), and Fundamental Research Funds for the Central Universities (JKD01231701).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c01851.

  • Additional experiment section, TEM image of thermogravimetric analysis (TGA) curves, S 2p spectrum of the as-synthesized samples, CV curves of the symmetric cells, SEM images of the samples after Li2S precipitation, CV curves of Co@CNF, digital photographs of separators in the cycled cells, and SEM-EDS images of the samples in the cycled cells (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao3c01851_si_001.pdf (1.1MB, pdf)

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