The crystal structures and morphologies of the as-prepared Fe3O4/rGO and Fe3O4 precursors are characterized by XRD, SEM, and TEM. From Fig. 1a, it can be seen that the main peaks in the XRD patterns are indexed to magnetite Fe3O4 (JCPDS card No. 75-0449). For the Fe3O4/rGO composite, a weak peak at about 22.5° is detected, which can be indexed to rGO. From Fig. 1b, c, one can see that both the precursors are mainly composed of uniform nanoparticles about 80 nm in diameter. Figure 1c shows that each nanoparticle is surrounded by thin graphene nanosheets. The TEM image shown in Fig. 1d further confirms the enwrapped structure of the composite. Moreover, it is obvious that each nanoparticle is connected by rGO to form an integral 3D network.
Figure 2 shows the crystal structure and morphologies of the as-prepared FeS2 and FeS2/rGO samples. As shown in Fig. 2a, both the samples show high-intensity XRD peaks, all of which can be indexed to pyrite FeS2 (JCPDS card No. 06-0710), demonstrating the high purity and good crystallinity of the two samples. Figure 2b, c shows that the FeS2 sample is composed of irregular particles like the Fe3O4 precursor. However, it can be observed that the particles tend to aggregate and become larger than the precursor, which is caused by the sulfuration process. Figure 2d shows that the general morphology of the FeS2/rGO composite is similar to that of the Fe3O4/rGO precursor. The TEM image shown in Fig. 2e further reveals that the FeS2 nanoparticles continue to be evenly dispersed in the graphene networks and that the particle size remains largely unchanged, compared to its precursor. Figure 2f shows that each nanoparticle is surrounded by graphene, which effectively prevents the aggregation of the FeS2 nanoparticles. The thickness of the graphene layer is 2–3 nm (Fig. 2g). High-resolution TEM images (HRTEM, Fig. 2g, h) display clear lattice fringes with an interplane distance of 0.16 nm, corresponding to the (311) plane of pyrite FeS2. The selected-area electron diffraction (SAED) pattern of FeS2/rGO (Fig. 2i) shows well-defined rings, indicating that the as-prepared FeS2 is polycrystalline.
According to the N2 adsorption–desorption measurements (Fig. 3b), the specific surface areas of FeS2 and FeS2/rGO are 25.6 and 58.1 m2 g−1, respectively, indicating that the introduction of rGO significantly increases the surface areas. To determine the rGO content in the composite, thermogravimetric analysis is carried out in an air atmosphere (Fig. 3a). Both the samples display a minor weight loss (∼ 6–8%) under 200 °C, which is due to the vapor of the residual water in the materials. Then a large weight loss of about 35% is observed in the range 400–500 °C for pure FeS2, which corresponds to the conversion of FeS2 to Fe2O3. (The theoretical weight loss is ∼ 33.3%.) For FeS2/rGO, a more significant weight loss of about 42% is observed between 400 and 600 °C, which may be caused by the phase change of FeS2 to Fe2O3 and rGO to carbon dioxide. Based on the thermogravimetric analysis, the weight content of FeS2 in the FeS2/rGO composite can be calculated to be about 79.1%. According to the above analysis, the FeS2/rGO composite contains integral nanostructures, with the FeS2 nanoparticles enwrapped in the 3D rGO networks. This unique structure endows the composite with high structural stability and super electron conductivity, which may be beneficial for the cycling stability and rate performance of the FeS2 electrode material for sodium storage.
Figure 4a, b shows the cyclic voltammetry (CV) curves of pure FeS2 and the FeS2/rGO composite at a scan rate of 0.1 mV s−1 between 0 and 2.5 V (vs. Na/Na+). During the initial cathodic scan, a large peak appears at 1.0 V and a broad peak appears at 0.25 V for the FeS2 electrode, which corresponds to Na+ intercalation and the formation of the NaxFeS2 (x < 2) phase, Fe and Na2S, and the formation of a solid-electrolyte interface (SEI) layer [13, 16, 32]. For the FeS2/rGO electrode, a large peak at ~ 0.65 V and a small peak at ~ 0.1 V are detected, which may be due to a similar electrochemical process with the FeS2 electrode. The differences in the peaks of the two samples may be caused by the nanostructure and the introduction of rGO. During the subsequent anodic scan, the peaks observed at ~ 1.4 and ~ 1.8 V can be attributed to be the desodiation process, with the formation of Na2FeS2 and Na2−xFeS2 . During the subsequent cycles, the CV curves are quite different from those in the initial cycle, which may be due to the irreversible formation of the SEI layer and the decomposition of the electrolyte [19, 33,34,35]. It can be observed that the FeS2/rGO electrode shows much better repeatability and a larger closed curve area than those of the pure FeS2 electrode, demonstrating its much better cycling stability and higher specific capacities.
Figure 5a presents the charge–discharge curves of FeS2/rGO electrode at a current density of 100 mA g−1. An initial discharge plateau at ~ 1.0 V (vs. Na/Na+) and charge plateau at ~ 1.3 V are observed, which are in good agreement with the CV curves. In the subsequent cycles, the charge–discharge curves do not change much, showing good electrochemical reversibility. The cycling performances of the two samples are further evaluated at 100 mA g−1. As shown in Fig. 5b, both the electrodes have quite good cycling stability. However, the FeS2/rGO electrode has obviously higher specific capacities than does the pure FeS2 electrode, which may be due to the higher utilization of the active materials after the introduction of rGO. The FeS2/rGO composite displays a high initial discharge capacity of 1263.2 mAh g−1 and charge capacity of 759.4 mAh g−1, showing a low coulombic efficiency of 60.1%, which is mainly caused by the irreversible formation of the SEI layer and electrolyte decomposition in the initial cycle. Moreover, the dissolution of sodium polysulfides into organic liquid electrolytes causes a parasitic redox shuttle, leading to unfavorable side reactions with sodium, reducing the charging efficiency and resulting in serious capacity decay [36,37,38]. In the following cycles, the coulombic efficiency increases over 95%. From the second cycle, the discharge and charge capacities are stable and remain at 609.5 and 581.7 mAh g−1, respectively, after 100 cycles.
The rate capability of the two FeS2 electrodes is evaluated using varying current densities from 0.1 to 10 A g−1 and back to 0.1 A g−1. As shown in Fig. 5c, the average specific capacities for FeS2/rGO electrodes are 705, 672, 613, 555, 496, 426, and 344 mAh g−1 at 0.1, 0.2, 0.5, 1, 2, 5, and 10 A g−1, respectively, which are remarkably higher than those for pure FeS2 electrode, demonstrating its superior rate performance. When the current density is altered back to 0.1 A g−1, the reversible capacity remains at ~ 655 mAh g−1 after 90 cycles, further confirming the excellent cycling stability of the FeS2/rGO composite. We further investigate the electrode process kinetics of the two materials through EIS. As shown in Fig. 5d, both the Nyquist spectra are composed of a semicircle in the high-frequency region and an inclined line in the low-frequency region. The bigger semicircle for the FeS2 electrode illustrates the poor electrical conductivity of the active materials. According to the Z-view program in the Sai software set, R
ct for FeS2 and FeS2/rGO electrodes is 1055.1 and 291.9 Ω, respectively, illustrating the better charge transfer kinetics of the FeS2/rGO electrode.
The FeS2/rGO composite displays much higher specific capacity and better rate capability than does the pure FeS2 electrode. It is inferred that several features may contribute to the excellent electrochemical properties. First, the intimate contact of the FeS2 nanoparticles with rGO and the integral conductive rGO networks provide a facile electron transport pathway, ensuring good rate performance [27, 30]. Second, the unique enwrapping structure can effectively improve the structural stability and buffer the volume change of FeS2 during the charge–discharge process [26, 28]. To investigate the structural stability, the nanostructures of the freshly prepared FeS2/rGO electrode and the FeS2/rGO electrode after 100 cycles are investigated by SEM and TEM. From Fig. 6a, it can be seen that the morphology of the FeS2/rGO composite does not change. After 100 sodiation–desodiation cycles, the nanoparticles are not very regular but are still enwrapped in the graphene networks (Fig. 6b, c). The high-resolution TEM test shows that the nanoparticles transform into smaller nanocrystals (Fig. 6d), which are still surrounded by rGO. It is obvious that the graphene network can effectively prevent the collapse of the structure and the aggregation of FeS2 nanoparticles, thus improving the cycling stability of the FeS2/rGO composite. Moreover, the improvement of the BET surface area increases the contact area between the active material and the electrolyte, which helps improve the utilization of active materials, endowing the FeS2/rGO composite with high specific capacitance.