Introduction

With the rapid development of power reserve systems in electric vehicles and portable electronic products, sodium-ion batteries (SIBs) have become a strong competitor to lithium-ion batteries (LIBs), because of the similar charge-discharge behavior to LIBs, low cost, and vast natural reserves [1,2,3]. And the electrochemical potential of Na (− 2.71 V vs the standard hydrogen electrode, SHE) is higher than that of Li (− 3.04 V) with 330 mV, which makes SIBs possible to meet large-scale energy storage demands [4,5,6]. However, the most important challenge in SIBs is the large volume expansion during the process of sodiation originated from the great strain derived from the larger radius of Na+ (1.02 Å) than Li+ (0.76 Å) [7, 8]. This will result in severe pulverization and exfoliation of active materials from copper foil and further lead to poor cycling performance. Therefore, rational design of anode materials is an impending concern.

Diverse anode materials have been reported for their high theoretical capacity, such as transition-metal sulfides (TMSs) [9,10,11,12,13,14], transition-metal oxides (TMOs) [15,16,17,18], phosphides [19,20,21,22], and carbon composites [23,24,25,26]. Among them, cobalt-based MSs (such as CoS, CoS2, Co3S4, and Co9S8) have attracted great attention for its near-metallic conductivity and easy accessibility [27,28,29]. Specifically, cubic Co9S8 attracts much attention for its great thermostability. Unfortunately, it is still impeded by the severe volume variation, slow Na+ diffusion rate, and poor conductivity [30,31,32]. Great efforts have been made to handle the shortcomings of Co9S8.

So far, most studies have also focused on designing novel carbon materials with heteroatom (N, P, S, B) doped, such as sandwich-like structures with N, S-doped RG O[33, 34], nanoflower-like N-C/CoS 2[35], Co9S8 coated with N-doped carbon nanospheres [36, 37], and N, S-doped nanofibers [38, 39]. Carbon coating can not only enhance the conductivity of TMSs, but also remit the stress stemming from the volume expansion. In particular, with the heteroatom doping, the electronic structure of carbon can be modified to improve the physical and chemical properties by generating extrinsic defects, expanding the interlayer distance and offering additional electron transfer route when heteroatoms are bonded with carbon atoms [40,41,42,43,44].

Herein, we synthesize double morphology of Co9S8, containing nanofibers and hollow nanospheres, both coated with N, S co-doped carbon (denoted as Co9S8@NSC), for highly stable SIBs. Nanofiber-like structure provides long-range continuous electron transport, while the hollow nanospheres enhance the infiltration of electrolyte. The N, S co-doped carbon layer can provide more free electrons, which benefit the adsorption of Na+ on the surface and enhance the integral conductivity. Due to the hardness of carbon coating and 3D network, volume variation during sodium ion insertion/extraction can be well alleviated from atomic and 3D level. And the high specific surface area can enhance the capacity of pseudo-capacitance contribution, leading to excellent rate performance. As a result, SIBs with Co9S8@NSC can deliver a stable capacity retention of 318 mA h g−1 after 800 cycles at 1 A g−1 with the coulombic efficiency of ~ 100%, making it a promising anode for large-scale SIBs.

Experimental Methods

Synthesis of Co9S8@NSC

In a typical process, the composites were prepared by coaxial electrospinning following alcohol-thermal method and carbonization.

Preparation of Electrospun Nanofibers

0.74 g PAN (Sigma-Aldrich, MW = 150,000) and 9 ml DMF was stirred for overnight to form the homogeneous outer solution, while 1.8 g Cobalt(II) acetylacetonate (Co(acac)2, Aladdin, purity ≥ 99%) and 0.74 g PAN (Macklin, MW = 150000) were mixed with 9 ml DMF and stirred at the same time as the dark red inner solution. Then, the two kinds of solutions were conducted by coaxial electrospinning (needle size: inner 17 G, outer:22 G). The distance between the needle and Al foil collector was 15 cm, and the condition temperature was maintained at 65 °C. Then, electrical potential was applied at 15 kV with flow rate of two syringes both at 1.5 ml h−1. The final precursor fibers were dried at 60 °C in vacuum for 24 h.

Sulfuration and Carbonization

The obtained nanofibers were firstly mixed with 50 ml ethanol containing thioacetamide (TAA, Aladdin, purity ≥ 99%) in 100 ml Teflon-lined stainless-steel autoclave at 120 °C for 6 h by solvothermal method. The final product was obtained by carbonizing at 700 °C for 1 h with a heating rate of 5 °C min−1 and cooling down naturally. For comparison, sample without cobalt (N, S co-doped carbon, denoted as NSC) was also prepared using the same method mentioned above without adding Co(acac)2.

Structural Characterization

The morphology and structure of the Co9S8@NSC were characterized by scanning electron microscopy (SEM, ZEISS Gemini 500) and transmission electron microscopy (TEM, JEM-2100HR). Thermal gravity analysis (TGA) test was performed to evaluate the content of Co9S8 by Netzsch STA449. The crystalline structures and surface valence state analyses were detected by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi), X-ray powder diffraction (XRD, Bruker D8 Advance), and Raman spectra. The specific surface area and pore size distribution were recorded from the Brunauer-Emmett-Teller (BET, Micromeritics ASAP-2020) analysis instrument.

Electrochemical Measurements

Slurries were obtained by mixing active materials, poly (vinylidene fluoride) (PVDF), and Super P (weight ratio of 8:1:1) with N-methylpyrrolidone (NMP). Then, the working electrode was prepared by coating the slurries uniformly on a precut copper foil (diameter 12 mm) and dried at 60 °C in air and vacuum overnight, respectively. The CR2032-type coin cells were assembled with sodium metal as the reference electrode, glass fiber membrane as the separator, and the as-prepared copper foil as the anode. The electrolyte was 1 M NaClO4 in EC/DMC (EC:DMC = 1:1, in volume) with 5.0% FEC. The assembly procedures were all carried out in an Ar-filled glove box (O2 < 0.1 ppm, H2O < 0.1 ppm). The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) results were obtained from an electrochemical workstation (CHI660E, Shanghai Chen Hua Instruments Ltd). And the galvanostatic discharge-charge tests were conducted in a NEWARE battery testing system.

Results and Discussion

The synthesis process of Co9S8@NSC is illustrated in Additional file 1: Scheme S1, including coaxial electrospinning, solvothermal sulfuration, and carbonization. The crystallinity of Co9S8@NSC and NSC after those procedures is shown in Fig. 1a. The XRD curve of Co9S8@NSC displays typical characteristic diffraction peaks in accord with the cubic Co9S8 phase (JCPDS no. 86-2273), while NSC only exhibits peaks of hard carbon. The broad peak at 24.8° is corresponding to (111) plane of amorphous carbon. Strikingly, it is lower than the standard value of 26.6°, indicating expanded interlayer distance and lower graphitization derived from N, S co-doped sites into the carbon [41]. The Raman spectra (Fig. 1b) also confirms the existence and composition of carbon in the composites. Co9S8@NSC and NSC both exhibit two obvious peaks of 1308 cm−1 and 1513 cm−1, representing the D band and G band of carbon, respectively. Besides, Co9S8@NSC owns a weak typical peak located at 671 cm−1, corresponding to the Co9S8. In detail, the D band is attributed to the structure defects of amorphous carbon, while the G band is due to the E2g vibration mode of Sp2 bond between graphitic carbon atoms [45]. The slightly larger ID/IG of Co9S8@NSC (1.31) than NSC (1.14) indicates more defects exist in the composites, resulted from N doping and S doping.

Fig. 1
figure 1

a X-ray diffraction (XRD) of Co9S8@NSC and NSC. b Raman spectra of Co9S8@NSC and NSC. c TG of Co9S8@NSC. d N2 adsorption and desorption isotherms of Co9S8@NSC and NSC

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The weight ratio of Co9S8 is measured by thermogravimetric analysis (TGA) as shown in Fig. 1c. After heating to 700 °C in air with 10 °C min−1, multistep reaction is involved with the final product as Co3O4, which can be ascribed to: (1) the evaporation of trace water (below 100 °C), (2) the oxidation of Co9S8 to CoSOx (from 200 to 400 °C), and (3) the decomposition of carbon and continuous oxidation of CoSOx (above 400 °C). According to these reactions, the content of Co9S8 in the composites is calculated as 40.1%. Co9S8@NSC exhibits typical type IV isotherm curves, indicating the mesoporous structure, while the NSC is microporous (Fig. 1d). The specific surface area of Co9S8@NSC (122.5 m2 g−1) is much larger than NSC (4.1 m2 g−1), which can benefit the infiltration of electrolyte for fast insertion/extraction of Na+ and remit the severe volume change. The pores of Co9S8@NSC with average size of 8.6 nm (shown in Additional file 1: Figure S1) are derived from the pyrolysis of chemical groups of polyacrylonitrile, dissolution of Co(acac)2 from the nanofibers, and the formation of hollow Co9S8 nanospheres.

X-ray photoelectron spectroscopy (XPS) is conducted to identify the chemical composition of Co9S8@NSC. The signals in the survey spectrum are accord with five elements of C, N, O, S, and Co in Fig. 2a. The presence of O should be due to the exposure of the sample in air with some oxygen adsorbed on the surface. The high-resolution spectrum of C 1s (Fig. 2b) displays four peaks, which are located at 284.6 eV (C-C/C=C), 285.0 eV (C-N), 285.8 eV (C-S), and 288.6 eV (C=N) [46]. The Co 2p spectrum was shown in Fig. 2c. Peaks located at 786.1 eV and 803 eV can be fitted to the satellite peaks of Co 2p3/2 and Co 2p1/2, respectively. In addition, peaks of 778.5 eV and 793.6 eV and another two peaks at 781.4 eV and 797.2 eV belong to Co2+ and Co3+, respectively [36]. Furthermore, the S 2p spectrum (Fig. 2d) is fitting into four peaks, consisting of 162.45 eV (S-Co), 163.7 eV (S-C), 165 eV (S-C), and 168.2 eV (sulfate) [47]. And N 1s spectra (Additional file 1: Figure S2) contain three peaks at 398.4 eV, 400.1 eV, and 401.0 eV, which correspond to pyridinic N, pyrrolic N, and graphitic N, respectively [48]. Compared to the alone carbon materials, introducing N,S co-doped carbon sites can provide more free electrons, which benefits the adsorption of Na+ on the surface and enhances the integral conductivity [41]. The results of all XPS spectrums confirm the presence of N, S co-doping in Co9S8@NSC.

Fig. 2
figure 2

a XPS survey spectrum. b C 1s. c Co 2p, and d S 2p of Co9S8@NSC

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The interesting morphologies and detailed internal structures of Co9S8@NSC and NSC are measured by SEM and TEM. Figure 3a–c depicts the Co9S8@NSC consist of two kinds of structure including hollow nanospheres and nanofibers. These nanofibers are composed by carbonization of PAN fibers. The formation of nanospheres adhered on the nanofibers may be attributed to the decomposition and sulfurization of Co(acac)2 which was dissolved from the inside of PAN electrospinning fibers. And the composites own the distinctly coarse surface, which can enhance the wettability of electrolyte. Additional file 1: Figure S3a–c displays the NSC morphology with only smooth nanofibers cemented to each other without nanospheres, which can be due to the absence of Co-based compound and melt of PAN nanofibers in the process of carbonization. The elemental composition is confirmed by EDS mapping (Additional file 1: Figure S4), with C, N, Co, and S elements uniformly distributing in the composites. And this further demonstrates the successful doping of N, S element.

Fig. 3
figure 3

a–c SEM and d–f TEM of Co9S8@NSC at different magnification. (The inset of (f) is the SAED of Co9S8@NSC.)

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As displayed in Fig. 3d–f, TEM images reveal the internal details of the morphologies of Co9S8@NSC. Figure 3d and e show the Co9S8 nanoparticles are embedded in carbon nanofibers and hollow nanospheres, which confirms the point proposed above about the formation of the hollow nanospheres. Figure 3f reveals the interplanar distance of 0.308 nm, matching well with the (311) planes of Co9S8, while the NSC in Additional file 1: Figure S3d–f manifests the traditional character of hard carbon. Co9S8 nanoparticles with size all bellowing 50 nm distribute uniformly in the composite, and the thickness of carbon coating layer is measured 3–5 nm (Fig. 3f). Owing to the rough surface, hollow structure, and carbon coating, severe pulverization and exfoliation of active materials resulting from volume variation might be alleviated effectively.

To investigate the electrochemical performance of Co9S8@NSC, 2032-type coin cells are assembled for electrochemical tests. As shown in Fig. 4a, the CV curves of Co9S8@NSC record the initial five cycles at a scan rate of 0.1 mV s−1. The first cycle is far different from the subsequent cycles with a broad peak at 0.476 V in cathodic sweep, which is assigned to the formation of solid electrolyte interface (SEI) film, irreversible intercalation of Na+, and the stepwise conversion of Co9S8 to Co and Na2S [49]. In the first anodic sweep, two oxidation peaks of 0.375 V and 1.682 V can be attributed to the multistep reaction of Co to CoSx [36]. The following CV curves of Co9S8@NSC are gradually overlapped, indicating the high electrochemical reversibility. For comparison, CV curves of NSC in Additional file 1: Figure S5a show the typical characteristic peaks of carbon, which represents the adsorption and insertion of Na+ in carbon nanofibers. Figure 4b and Additional file 1: Figure S5b display the charge/discharge curves for different cycles of Co9S8@NSC and NSC with the initial coulombic efficiency (CE) of 54.1% and 28.3%, respectively. The relatively low CE is caused by the irreversible formation of SEI film and electrolyte consumption [7]. The curves of these two samples manifest distinctive voltage platform of Co9S8 and carbon, which are in accord with the results of CV tests (Fig. 4a and Additional file 1: Figure S5a).

Fig. 4
figure 4

CV curves (a) and charge-discharge profiles (b) with different cycles of Co9S8@NSC. Rate capability (c) and cycling performance (d) of Co9S8@NSC compared with NSC

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Rate performance is shown in Fig. 4c. The capacity of Co9S8@NSC can reach 226 mAh g−1 at 5 A g−1, while NSC can only maintain 21 mAh g−1. And then, when the current density recovers to 100 mAh g−1, the capacity can be well returned to 440 mAh g−1. Figure 4d shows the cycling performance of Co9S8@NSC. The discharge capacities of Co9S8@NSC maintain at 423 mAh g−1 at 200 mA g−1 and 318 mAh g−1 at 1 A g−1 after 800 cycles with initial coulombic efficiency at 42.3% and 37.4%, respectively. This can be explained by the reaction mechanism transformation from intercalation/deintercalation at low current density to adsorption/desorption of Na+ at high current density [2]. Apparently, the capacities of Co9S8@NSC are all higher than those of NSC, which is due to the more active sites derived from Co9S8 and N, S co-doping. In addition, the capacity retention of 87.4% at 200 mA g−1 and 83.1% at 1 A g−1 after 800 cycles indicate the stable cycling performance and benign reaction reversibility of Co9S8@NSC.

To understand the interfacial properties and internal resistances of Co9S8@NSC and NSC, electrochemical impedance spectra (EIS) has been conducted. As presented in Additional file 1: Figure S6a, the Nyquist plots of Co9S8@NSC after different cycles show typical semicircles in high-frequency region (charge transfer resistance, Rct) and an oblique line in low-frequency region (Warburg resistance, W). Before the initial cycle, the largest Rct (about 1600 Ω) of Co9S8@NSC is caused by insufficient infiltration of electrolyte. After 5 cycles, the Rct becomes very small as 153 Ω for the formation of SEI film and well contact with electrolyte. Furthermore, the decrease of the Rct can be also attributed to the process of activation of the Co9S8@NSC electrode interfacial. After 10 cycles, it almost keeps the same value, which indicates the excellent stability. For comparison, EIS of NSC is also investigated in Additional file 1: Figure S6b–e. The initial Rct of NSC is smaller than Co9S8@NSC, indicating the higher electroconductivity of NSC. As the cycle is going on, the Rct of Co9S8@NSC gradually becomes smaller than that of NSC, owing to the larger specific surface area and enough infiltration of electrolyte. These results mentioned above support the benign cycling and rate performance of Co9S8@NSC.

To gain further insight into the electrochemistry of Co9S8@NSC electrode, a kinetic analysis is conducted. The CV curves at different sweep rates from 0.1 to 0.9 mV s−1 are collected and shown in Fig. 5a. Varying from common ionic diffusion, the peak current (I, mA) is not completely linearly dependent to v1/2 (v is the scan rate, mV s−1), indicating the coexistence of non-faradic and faradic behaviors [35, 50]. And the results can be verified by the relation between log(I) and log(v), according to equation of log(I) = b log(v) + log(a). When the value of b reaches 0.5 or 1, it indicates that the reaction mechanism is totally controlled by ionic diffusion or capacitive behavior, respectively [51, 52]. As shown in Fig. 5b, the calculated values of b are 0.7518 (cathodic peak) and 0.7792 (anodic peak), which means more capacitive behavior.

Fig. 5
figure 5

a CV curves of Co9S8@NSC at various scan rates. b The linear relationship between log (v) and log (I). c Capacitive contribution (red) in CV curve. d Contribution ratio of capacitive capacity at different scan rates

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Furthermore, the contribution of capacitive behavior can be obtained from the equation: i(V) = k1v + k2v1/2 [42], where i(V) is the current at a fixed voltage, v is the sweep rate, and k1v and k2v1/2 represent the capacitive capacity and ion diffusion capacity, respectively. When sweep rate is 0.5 mV s−1, the contribution ratio of the capacitive capacity reaches 69.2% (Fig. 5c). For different sweep rates, Fig. 5d illustrates an obvious tendency of the capacitive capacity ratio increasing with the scan rate from 0.1 to 0.9 mV s−1. The increasing capacitive contribution can be ascribed to high specific area and abundant active sites, which furthermore may be responsible for the excellent rate performance of the Co9S8@NSC electrode. All these results reveal a fast kinetic of Co9S8@NSC which resulted from the capacitive effect.

According to Fig. 5a, the Na+ diffusion coefficients (DNa+) can be estimated from the strongest peak current (Ip) and sweep rates (v) by Randles-Sevick equation [32]:

$$ {I}_p=2.69\times {10}^5{n}^{3/2}A{D}_{Na+}^{1/2}{v}^{1/2}C $$

where n, A, and C represent the number of transferred electrons in the process of Na+ intercalation/deintercalation, the surface area, and the molar concentration of Na+, respectively. The DNa+ of Co9S8@NSC is proportional to the slope of the linear relations between Ip and v1/2 (Additional file 1: Figure S7). As a result, the slope values of anodic peak and cathodic peak of Co9S8@NSC are much positive and negative than those of NSC, respectively, which means that the DNa+ of Co9S8@NSC is much higher than NSC in the process of Na+ intercalation/deintercalation. In detail, it can be ascribed to favorable infiltration of electrolyte and much exposed active sites derived from the larger specific surface area of Co9S8@NSC.

Conclusions

In summary, a novel double morphology of Co9S8, containing hollow nanospheres and nanofibers, with coating N, S co-doped carbon layer has been successfully synthesized using coaxial electrospinning following sulfurization by solvothermal method and carbonization. Owing to the larger specific surface area and carbon coating, Co9S8@NSC can accommodate the volume change during the charge/discharge process. More uniformly, active sites derived from Co9S8 and N, S co-doped position can not only contact with much more electrolyte, but also accelerate diffusion of Na+ and reversible reaction between Na+ and Co9S8@NSC. When applied as anode materials, Co9S8@NSC can deliver a high reversible specific capacity of 318 mAh g−1 after 800 cycles at 1 A g−1 with the coulombic efficiencies remaining almost 100%, while the large surface area and abundant N, S co-doped sites can lead to the excellent rate capability. The study offers more possibilities of cobalt sulfides in designing effective anode materials for SIBs.