SEM images of Fig. 1a shows the microstructure of as-obtained CNF nanofibers sample. The nanofibers of CNF exhibit clean surfaces with the diameters of 200–300 nm. It can be observed that CNF is composed of large amounts of fibers with well-distributed diameter and thickness, which ensures the uniform growth and distribution of NiCo2S4 particles. The NiCo2S4 nanorods arrays are generated around the carbon fibers after hydrothermal reaction, and the morphological changes of the carbon fibers before and after hydrothermal treatment are shown in Fig. S1. As shown in Fig. 1b and c, NiCo2S4 nanorods arrays are dispersed on top of CNF and tightly intertwined with each other to form NiCo2S4@CNF electrodes. The dispersed nanorods arrays of NiCo2S4 can exposed more active sites for sodium and the conductive network of CNF ensures faster electrolyte transport. Furthermore, the energy dispersive spectroscope (EDS) spectrum and corresponding mapping images reveal the uniform distribution of C, S, Ni and Co elements in the NiCo2S4@CNF sample (Fig. 1d).
The crystalline feature of as-obtained samples was elucidated by XRD in Fig. 2a. It is noticed that both the NiCo2S4@CNF and pure NiCo2S4 display same diffraction peaks located at 2θ = 23.8°, 27.1°, 31.9°, 38.6°, 47.7°, 50.8°, 55.6°, 65.4°, 69.6° and 78.5°, respectively. All the characteristic peaks can be indexed to the standard PDF card of NiCo2S4 (JCPDS 01-073-1704)[9]. The peak intensity of NiCo2S4@CNF is slightly weaker than that of NiCo2S4, mainly due to the composite effect of carbon fiber membrane. As shown in Fig. 2b, the chemical state of NiCo2S4@CNF electrode was investigated by X-ray photoelectron spectroscopy (XPS) measurements. The survey spectrum manifests that NiCo2S4@CNF contains Ni, Co, S, O and C elements. The oxygen content in NiCo2S4@CNF is attributed to the natural properties of carbon fibers and the exposure of sample to air. High-resolution C 1 s signal of NiCo2S4@CNF in Fig. 2c can be fitted into a series of peaks located at 284.8, 285.5, 286.2 and 288.9 eV, which correspond to C-C, C-O, C = O and O-C = O bonds, respectively. In the S 2p spectrum of Fig. 2d, the peaks located at 161.4 eV and 162.5 eV are assigned to S 2p3/2 and S 2p1/2 orbitals of S2−. Ni-S and Co-S can be also observed, confirming the formation of multi-metal sulfides. The Ni 2p spectrum in Fig. 2e reveals two strong peaks located at 856.2 eV and 873.9 eV, corresponding to Ni 2p3/2 and Ni 2p1/2, respectively[10]. From the Co 2p spectrum of Fig. 2f, strong peaks at 781.5 eV and 796.8 eV correspond to Co 2p3/2 and Co 2p1/2, respectively. Furthermore, the corresponding elemental analysis is summarized in Table S1. These results are in accordance with the reported characteristics of NiCo2S4.
The sodium ion storage characteristics of NiCo2S4@CNF electrodes were investigated in CR2032 coin type half-cell, as shown in Fig. 3. Figure 3a exhibits the rate performance of NiCo2S4 and NiCo2S4@CNF samples in the range of current densities from 0.1 A g− 1 to 5.0 A g− 1. It is obvious that the NiCo2S4@CNF electrode delivers a higher discharge specific capacity than NiCo2S4, which are 683.6, 551.9, 470.2, 403.6, 345.1 and 256.7 mAh g− 1 at the current densities of 0.1, 0.2, 0.5, 1.0, 2.0, and 5.0 A g− 1, respectively. As for the NiCo2S4 electrode, only a specific capacity of 478.8 mAh g− 1 released at a current of 0.1 A g− 1, and it decayed rapidly. When the current density increases to 1.0 A g− 1, there is almost no capacity. Figure 3b shows the galvanostatic charge and discharge (GCD) curves of the NiCo2S4@CNF electrode at a current density of 0.1 A g− 1. The NiCo2S4@CNF electrode represents an initial capacity of 756.2 mAh g− 1 and secondary capacity of 683.6 mAh g− 1, with a high initial coulombic efficiency (ICE) of 90.4%.
As shown in Fig. 3c, the EIS measurement was conducted to analyze the kinetic feature of NiCo2S4@CNF electrode toward sodium ions. The semicircle district represents the resistance of charge transfer (Rct), which is related to the electrochemical kinetics of the electrodes. Meanwhile, the oblique line represents the Warburg impedance (Zw), which is determined by the diffusion of sodium ions in the electrode. Based on the equivalent circuit (the inset in Fig. 3c), the Rct of NiCo2S4@CNF and NiCo2S4 are calculated to 187.6 Ω and 392.7 Ω respectively. The smaller Rct value of NiCo2S4@CNF indicates lower charge transfer resistance and higher conductivity. Figure 3d reveals the first three CV curves of NiCo2S4@CNF electrode in the potential range of 0.01-3 V at a scan rate of 0.1 mV s− 1. In the first cathodic scan, two peaks located at ∼0.5 V should be assigned to the reversible formation of solid electrolyte interphase (SEI) film and activation process of Na+ insertion into NiCo2S4@CNF. The subsequential anodic peaks at 1.88 V correspond to the extraction of Na+ and the formation of NiS and CoS. In the following scan, two pair of conspicuous peaks reflected the reverse process between NiS/CoS and Na. The profile of successive cycles fits well, confirming the excellent stability of the electrode. The long-term cycling stability of the NiCo2S4@CNF electrode was also evaluated under high current density and the result is shown in Fig. 3e. At a high current density of 2.0 A g− 1, the reversible capacity still maintained at 283.2 mAh g− 1 after 400 cycles with the CE over 99%, indicating the superior stability of NiCo2S4@CNF. In addition, the comparison between the reported literatures on NiCo2S4 materials in sodium ion batteries in Table S2 also confirms the superiority of NiCo2S4@CNF[11–16].