Li-S batteries are promising candidates for advanced battery systems due to their high theoretical energy density (2600 Wh/kg) and specific capacity (1675 mAh/g), as well as the low cost of sulfur. However, the sluggish shuttling effect of LiPSs and volume expansion of the cathode during the cycle generally result in rapid capacity fading and low Coulombic efficiency, thus hindering the practical application of this battery system. Owing to their high conductivity and excellent sulfur-loading ability, carbon materials including carbon fibers, carbon spheres, and carbon nanosheets, have been demonstrated to be excellent sulfur host materials. Among them, carbon fibers have received great attention due to their adjustable structures and compositions (Sun et al. 2022). Electrospinning is an efficient technique for constructing high-surface-area carbon fibers with plentiful functional groups and active sites, which is desirable for sulfur host materials of Li-S batteries. Generally, precursors such as polyacrylonitrile (PAN) (Xing et al. 2021), polyvinylidene fluoride (He et al. 2021), polymethyl methacrylate (Jin et al. 2022), and polyvinyl pyrrolidone (PVP) (Xie et al. 2020) are employed to prepare porous carbon fibers, which are highly expensive and cannot meet the huge market demand.

Coal tar pitch, a high-yield by-product in the coal coking industry, is structurally rich in aromatic structures and heteroatoms (N, S, etc.), making it an economical and promising feedstock to synthesize heteroatom-doped porous carbon materials (Zhuang et al. 2021). Li et al. demonstrated that pitch extracted from coal residue liquefaction can be used to fabricate electrospun carbon nanofibers nonwovens, which show excellent rate capacity and high power density (Li et al. 2022). Using coal tar pitch as the raw material, Zhang et al. successfully prepared hierarchically porous carbon that exhibited fast and high-lux lightweight storage (Zhang Md, Qu et al. 2023). Wang et al. synthesized carbon materials with hierarchical porosity via a one-step carbonization activation method using coal tar pitch as the carbon source and KOH as the activator. The as-prepared multi-level porous carbon material with 90.5% micropores exhibited a specific capacitance of 475 F/g at a current density of 0.5 A/g (Wang Zh et al. 2023). Although coal tar pitch exhibits advantages in preparing porous carbon materials, it remains challenging to use coal pitch to manufacture carbon fibers because of the strong π-π conjugate in the structure of coal pitch. At present, numerous studies have chosen PAN as the main precursor of porous carbon fibers, while little literature focused on coal pitch (Zhang Ys et al. 2022; Zhujd, Cheng et al. 2022). To reduce the cost and cope with demand, it is attractive to employ modified coal pitch as the feedstock of porous carbon fibers for Li-S battery system.

Herein, porous N/O co-doped carbon fibers (CF_CP) were successfully constructed from coal pitch using an electrospinning method. As references, carbon fibers derived from PAN, and PVP (CF_PAN and CF_PVP) were also fabricated. The morphology, structure, and composition of these carbon fibers were investigated using various characterizations. The electrochemical performance of these carbon fibers as sulfur hosts in the Li–S battery system was compared and their electrochemical mechanisms were revealed. This study provides valuable insights into the engineering of coal pitch-base porous carbon fiber with high Li-S battery performance and a new path toward high value-added utilization of coal pitch.

2.1 Materials

The coal pitch used was obtained from Ma’anshan Iron and Steel Co., Ltd. Coal Coke Company It contains 93.34% C, 4.01% H, 1.03% N, 0.43% S, and 3.19% O. PVP, H₂SO₄, N, N-dimethylformamide, sulfur powder, HNO₃, HCl, and KOH were purchased from Sinopharm. Electrolytes, 1, 2-dimethoxyethane (DME), and 1, 3-dioxolane (DOL) were purchased from the Duoduo reagent network (battery grade). Lithium foil, Li₂S, acetylene black, and PVDF were purchased from the electrochemical material network. All reagents were analytical grade and used without further purification.

2.2 Preparation of the CFs

The coal pitch was first treated with HNO₃ and H₂SO₄ (HNO₃:H₂SO₄ = 3:7) for 12 h at 80 °C, then filtered and washed with deionized water until the filtrate pH approached neutral (pH = 7). To prepare porous carbon fibers, 1.2 g of the acid-treated pitch and 1.2 g of PVP were mixed with 10 mL of N, N-dimethylformamide to make a homogenous solution. Afterward, the solution was electrospun at a distance of 0.15 m and a voltage of 15 kV to produce carbon fiber precursors, which were subsequently oxidized in air at 280 °C for 2 h. The oxidized carbon precursor and KOH (mass ratio of 1:2) were carbonized for 2 h under Nitrogen in a tube furnace at 900 °C, followed by washing with deionized water and 2 M HCl, respectively. The cleaned samples are finally dried in a vacuum drying oven for 12 h to yield CF_CP. CF_PAN was synthesized similarly to CF_CP by replacing the acid-treated pitch with polyacrylonitrile. CF_PVP was obtained according to the same procedure using PVP alone without the acid-treated pitch or polyacrylonitrile.

2.3 Synthesis of S@CF_CP, S@CF_PAN and S@CF_PVP

The resultant CF_CP, CFP_AN, CF_PVP, and sulfur powder were combined at the mass ratio of 30:70 in N₂. Then the mixture was dried at 155 ^oC for 12 h to form a sulfur cathode. The sulfur content in S@CF_CP, S@CF_PAN, and S@CF_PVP were determined by thermogravimetry (STA449F3, Netzsch) analysis from ambient temperature to 600 °C at a ramping rate of 10 °C/min under Nitrogen₂ atmosphere.

2.4 Material characterizations

Morphological images were collected on field emission scanning electron microscopy (SEM, Germany, ZEISS sigma500) and field emission transmission electron microscopy (TEM, JEOL: JEM 2100 F and Tecnai G2 F20). X-Ray diffraction (XRD) patterns were acquired in a Rigaku D/Max-2400) diffractometer, operating at 30 kV and 30 mA with Cu-Ka radiation (λ = 0.154 nm). X-Ray photoelectron spectroscopy (XPS) data were obtained on a Thermo Fisher Scientific ESCALAB: 250Xi equipped with a monochromatic Al Ka (1486.6 eV) under vacuum at 15 kV and 10 mA. The N₂ physisorption isotherms were acquired using a physical adsorption apparatus (ASAP 2020, Micromeritics). Raman spectra were collected by a Renishaw inVia spectrometer equipped with a 514 nm Ar excitation source at a power of 5 mW.

2.5 Electrochemical measurements

The working electrodes contain S@CFs, acetylene black, and PVDF in a mass ratio of 60:30:10, with a mass of about 1.0 − 1.2 mg/cm² and a loading mass of approximately 1.1 − 1.3 mg·cm² of S. The CR-2032 coin cell battery is assembled using a lithium anode, Celgard-2500 separator, S@carbon fiber cathode, and electrolyte made of 1.0 M LiTFSI, DME/DOL (1:1, V/V) and 1.0% LiNO₃ under the protection of Ar. The constant current charge and discharge measurement (GCD) was acquired using a terrestrial battery measuring system (CT2001A, Lanhe) at the current density of 0.1–2 C at 1.7–2.8 V.

The cyclic voltammetry curves (CV) were measured on the electrochemical workstation (760E, Chenhua) between 1.7 and 2.8 V in a voltage window at a scanning rate of 0.1 mV/s, 0.2 mV/s, and 0.5 mV/s, respectively. The symmetrical electrode CV was performed at a scanning rate of 50 mV s^− 1 using an electrochemical workstation (Zahner Analysis) with an electrolyte of 0.5 M Li₂S₆ + 1 M LiTFSI + DOL/DME (1:1). Electrochemical impedance analysis (EIS) was carried out with on an electrochemical workstation (Zahner Analysis) at 100 kHz, 10 mHz and a voltage of 10 mV.

As shown in Fig. 1a-f, CF_CP prepared from coal tar pitch is a discontinuous and smooth fiber material with a diameter of approximately 1 μm, which is larger than CF_PAN prepared from PAN, with a diameter of 500 nm. The diameter of CF_PAN prepared from PVP is uneven, ranging from 1 to 2 μm. At the same time, the length of CF_CP is longer than that of CF_PAN and CF_PVP, which favors the enhancement of the conductivity of carbon fiber (Shimanoe et al. 2020). The diameter and length of CF_CP are larger than those of CF_PAN, probably originating from the rich oxygen-containing functional groups in the oxidized coal tar pitch, which could interact with PVP during the carbonization process. TEM images of CF_CP (Fig. 1g-i) reveal the presence of numerous worm-like micropores in CF_CP, as well as a disordered carbon structure on its surface. The EDS mapping graphs (Fig. 1j-l) suggest that C, N, and O are uniformly distributed in CF_CP.

As shown in Fig. 2a, the N₂ adsorption-desorption isotherms of CF_CP, CF_PAN, and CF_PVP belong to a combination of types I and IV isotherms (IUPAC), indicating the existence of microporous and mesoporous structures. According to Table 1, the specific surface areas of CF_PVP, CF_CP, and CF_PAN are 2822, 3311, and 3444 m²/g, respectively, suitable for sulfur storage (Yao et al. 2019). Figure 2b shows that the pore size distributions of CF_CP, CF_PAN, and CF_PVP are between 1 and 5 nm, which could offer numerous ion channels and facilitate the conversion of LiPSs in the anode material (Yao et al. 2020a, b). The thermogravimetric curves in Fig. 2c illustrate that the sulfur loads in S@CF_PVP, S@CF_CP, and S@CF_PAN are 67.0%, 69.1%, and 68.9%, respectively, due to the high surface area and large pore volume of the CFs.

SEM images of a-c CF_CP, d-f CF_PAN, and g-i CF_PVP; j-l TEM images of CF_CP; m-o EDS mapping images of CF_CP

a Nitrogen adsorption-desorption isotherms and b DFT pore width distributions of CF_PVP, CF_CP, and CFP_AN; c TG curves of S@CF_PVP, S@CF_CP, and S@CF_PAN

Two faint XRD diffraction peaks at roughly 24° and 43°, corresponding to the (002) and (101) lattice surfaces of the graphitic crystal, respectively, can be observed for the CF_PVP, CF_CP, and CF_PAN (Fig. 3a), reflecting the irregularity of the internal carbon structure of the three carbon fibers (Wang et al. 2020a, b). Raman spectra of CF_PVP, CF_CP, and CF_PAN (Fig. 3b) show a D band at 1342 cm^− 1, a G band at 1589 cm^− 1, and a 2D band at 2685–2915 cm^− 1, attributable to disordered graphite structure in porous carbon, sp² hybrid carbon in graphite microcrystals, and graphite layer thickness of carbon material, respectively. CF_CP has a lower I_D/I_G ratio and a stronger G band than those of CF_PVP and CF_PAN, suggesting that the graphitization degree of oxidized pitch-based carbon fiber is higher than that of CF_PVP and CF_PAN, which contributes to enhancing the electrical conductivity of carbon fiber.

a XRD patterns and b Raman spectra of CF_PVP, CF_CP, and CF_PAN

Figure 4a shows the XPS survey spectra of the three CFs, C 1s (285 eV), N 1s (400 eV), and O 1s (533 eV) peaks were observed, suggesting the presence of N and O on the surface of carbon fibers. Figure 4b shows that CF_CP has high carbon and N content. The high-resolution O 1s spectra of the CFs were deconvoluted into three peaks (Fig. 4c), corresponding to the configurations C = O (532 eV), C − OH/CO–C (532.8 eV) and O–C = O (533.9 eV), respectively (Wang et al. 2020a, b). The content of oxygen-containing functional groups on CF_CP and CF_PAN are almost identical, indicating that pitch could be an alternative of PAN to construct N, O-doped carbon fiber. As shown in Fig. 4d, the N 1s spectra of the CFs was deconvoluted into three peaks located at 398.2 eV, 400.1 eV, and 401.5 eV, corresponding to pyridinic-N, pyrrolic-N, and graphitic-N, respectively. Obviously, the content of pyrrolic-N in CF_CP is higher than that in others, which can play a role in boosting the catalytic conversion of LiPSs (Zhang et al. 2021). The N and O species in CF_CP can enhance the affinity of polar polysulfide/Li₂S for CFs, leading to improve the cycle performance and rate capacity of Li-S batteries (Kim and Lee 2021; Yi et al. 2017). From the viewpoints of carbon/sulfur cathodes, the large surface area, high N content, and the surface functional groups of CF_CP contribute to strong confinement of LiPSs and uniform deposition of sulfur and Li₂S, enabling CF_CP an ideal candidate for high-sulfur loading.

XPS spectra of CF_PVP, CF_CP, and CF_PAN a Survey; b Atomic ratio; c O 1s; d N 1s

The electrochemical performance of the Li-S batteries with S@CF_CP cathode was investigated and compared with that of S@CF_PAN and S@CF_PVP cathodes. As displayed in Fig. 5a, there are two reduction peaks at 2.2–2.4 V and 1.9–2.0 V, which corresponds to the stepwise conversion of long-chain soluble LiPSs to medium-long chain soluble LiPSs and then to insoluble lithium polysulfide (Li₂S₂/Li₂S) (Ouyang et al. 2021; Song et al. 2018). The oxidation peaks near 2.34 and 2.47 V represent the conversion of Li₂S₂/Li₂S to LiPSs and sulfur, respectively. The potential differences between the oxidation and reduction peaks for S@CF_PVP, S@CF_CP, and S@CF_PAN are 0.51 V, 0.4 V, and 0.49 V, respectively, indicating that S@CF_CP has lower polarization (Δ = 0.4 V) and stronger ability to promote the redox process of LiPSs compared to S@CF_PAN and S@CF_PVP. According to Figs. 5b-c, the ratios of specific discharge capacity Q_H/Q_L, corresponding to the reduction platform of S@CF_CP, S@CF_PAN, and S@CF_PVP electrodes, are 2.28, 2.31, and 2.21, respectively, indicating that S@CF_CP electrode can improve the utilization of LiPSss and accelerate their transformation (Tang et al. 2018; Su et al. 2017). Figure 5d shows the polarization curves of S@CF_PVP, S@CF_CP, and S@CF_PAN. Both the cathode and anode peak currents of the S@CF_CP electrode are much larger than those of the S@CF_PAN and S@CF_PVP electrodes, manifesting that more soluble polysulfides are transformed into insoluble polysulfides over the S@CF_CP electrodes which has a higher nucleation and deposition rate towards Li₂S.

a CV profiles, b-c Initial discharge curves at 0.1 C, and d Potentiostatic polarization curves of S@CF_PVP, S@ CF_CP, and S@CF_PAN

Figure 6a compares the rate performance of the S@CF_PVP, S@CF_CP, and S@CF_PAN cathodes. Notably, the S@CF_CP cathode shows superior rate performance with discharge specific capacities of 1309.5, 916.5, 779.5, 718.9, and 681.2 mAh/g at 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C, respectively. When the current density is restored to 0.1 C, the specific capacity approaches 883.0 mAh/g. Figure 6b depicts a typical galvanostatic charge − discharge (GDC) profile of the S@CFCP electrode. At 1 C, S@CF_CP had higher capacity and capacity retention than S@CF_PAN and S@CF_PVP cathodes. After 200 cycles, the discharge capacity of S@CF_CP remains at 701.1 mAh/g, with capacity retention and attenuation rates of 82.46% and 0.088%, respectively. As shown in Fig. 6d, at 2 C after 200 cycles, the discharge capacity of S@CF_CP was maintained at 611.4 mAh/g with the capacity retention capacity of 76.23%, capacity fade rate of 0.12%, and the average coulomb efficiency of 96.95%. Moreover, as shown in Fig. 6e, at the sulfur surface density of 3.6 mg/cm² and 1 C, the initial discharge capacity of the S@CF_CP electrode is 855.2 mAh/g and the discharge capacity kept at 610.0 mAh/g, with a capacity of 71.33% and a fading rate of 0.29% per cycle after 100 cycles. The performance of the S@CF_CP is superior to that of the S@carbon materials cathode reported in the literature (Yi et al. 2017).

a Rate performance of S@CF_PVP, S@CF_CP, and S@CF_PAN; b The representative GDC profiles of S@CF_CP; c-d Long-term cycling performance of S@CF_PVP, S@CF_CP, and S@CF_PAN at 1 C and 2 C; e Long-term cycling performance of S@CF_CP with high areal density at 1 C

To evaluate the adsorption capability and catalytic activity of porous carbon fiber towards LiPSs, as well as the charge transfer resistance of the electrode, Li₂S₆ adsorption test and EIS measurement were performed, and electrode currents at various sweep speeds were collected. As shown in Fig. 7a, the adsorption capacity of CF_CP for Li₂S₆ is slightly higher than that of CP_PAN and CP_PVP, as evidenced by the visualized adsorption tests (inset) and the UV-vis absorption spectra. EIS measurement results of S@CF_CP, S@CF_PAN, and S@CF_PVP electrodes (Fig. 7b) show that S@CF_CP has a low resistance, which is advantageous to improving the contact between insulating sulfur and conducting carbon, thereby accelerating the charge transfer electrode on the surface and the diffusion of lithium ion in the electrode (Su et al. 2017; Yao et al. 2020a, b). Based on the CV curves of S@CF_CP, S@CF_PAN, and S@CF_PVP at sweep speeds ranging from 1.5 to 2.8 V (Fig. 7c), the peak currents can be calculated using the Randles − Sevcik equation (Ren et al. 2018). As shown in Fig. 7d, the S@CF_CP electrode produces greater current than the S@CF_PAN and S@CF_PVP electrodes. Simultaneously, the slope of the S@CF_CP electrode is larger than that of the S@CF_PAN and S@CF_PVP electrodes, manifesting that CF_CP has a strong affinity towards LiPSs and brings about rich electrode/electrolyte active interfaces that are highly active for the catalytic conversion of LiPSs. Summarizing up, the S@CF_CP electrode shows a better performance in improving ionic diffusion and the kinetics of polysulfide species conversion than the S@CF_PAN and S@CF_PVP electrodes.

a UV − vis absorption spectra of the Li₂S₆ solution before and after adding S@CF_PVP, S@CF_CP, S@CF_PAN, and blank (inset: digital image of pure Li₂S₆ solution and Li₂S₆ solutions after 12 h); b EIS profiles of S@CF_PVP, S@CF_CP, and S@CF_PAN at an open-circuit voltage; c CV curves of S@CF_CP, S@CF_PAN, and S@CF_PVP at different scan rates; d CV peak current for cathodic reduction and anodic oxidation process versus the square root of the scan rates

N/O co-doped carbon fiber was produced by electrostatic spinning using H₂SO₄-HNO₃ pretreated coal pitch instead of polyacrylonitrile as the precursor. The as-prepared CF_CP exhibits a high specific surface area (3311 m²/g) and a large pore volume (2.0 cm³/g) with abundant N, O uniformly dispersed on the surface, which offers high conductivity, strong adsorption capacity, and fast catalytic conversion of lithium polysulfide as the sulfur host of Li-S batteries. The performance of the S@CF_CP electrode is superior to that of the S@CF_PAN electrode. At 1 C after 200 cycles, the S@CF_CP electrode shows a specific discharge capacity of 701.1 mAh/g, with a capacity retention capacity of 82.46% and a decay rate of 0.088%. At 1 C with a sulfur areal density of 3.6 mg/cm², the reversible capacity of the S@CF_CP electrode remains at 610 mAh/g after 100 cycles with a capacity retention capacity of 71.33% and a decay rate of 0.29%.