3.1 Preparation and structural characterization of PSi@PEDOT/GO
Figure 2a shows the XRD patterns of PSi@PEDOT/GO-2, PSi@PEDOT, PSi, and Al–Si. The presence of aluminum and silicon diffraction peaks in the Al–Si alloy patterns indicates that the silicon in the matrix material exists in the form of crystalline silicon. However, after the hydrochloric acid treatment, the PSi, PEDOT, and GO coated composites only exhibited diffraction peaks corresponding to crystal silicon, with no other heterogeneous peaks observed, which proved that aluminum was etched and the crystal state of silicon did not change after being coated.
FT-IR spectroscopy was used to further elucidate the chemical structure of the composites. Figure 2b shows the infrared spectra of PSi@PEDOT/GO-2, GO, PEDOT and PSi. The peak near 3450 cm− 1 corresponds to the –OH group, the peak at 2930 cm− 1 is the stretching vibration of -CH, the peak at 1381 cm− 1 is the stretching vibration of the characteristic peak C = C of PEDOT on the thiophene ring, and the peak between 16383 cm− 1~1750 cm− 1 is the characteristic peak of C = O group of GO and PSi@PEDOT/GO-2 composites [26, 27]. Therefore, the above FT-IR results show that there are PEDOT and GO in PSi materials, and PSi@PEDOT/GO-2 composites have been successfully prepared.
The elements on the surface of PSi@PEDOT/GO-2 composites were characterized by XPS.From the full spectrum of Fig. 3a, it can be seen that there are O, Si, S, C and other elements on the surface of the composites, and there is still a very small amount of Al element (the proportion of Al atom content in XPS semi-quantitative calculation is 1.75%), indicating that there is still a small amount of Al in PSi, a small amount of Al is beneficial to improve the ionic conductivity of the material [28]. Figures 3b, 3c correspond to the XPS spectra of Si 2p, S 2p, respectively. The characteristic peaks at the binding energy of 98.6 eV correspond to the Si-Si bond. The peaks at the binding energy of 102.8 eV and 103.3 eV are the characteristic peaks of SiOx [6], corresponding to the Si-O-Si bond. The reason for the appearance of Si-O-Si bond may be that the silicon surface of PSi@PEDOT/GO-2 composite is slightly oxidized to form SiOx [29] during the fabrication process. The main peaks with binding energies of 165.0 eV and 166.2 eV correspond to S 2p3/2 and S 2p1/2 of the C-S-C covalent bond, respectively, while the two small peaks with binding energies of 168.5 and 169.5 eV are attributed to the C-S (O)2-C bond of sulfur oxide [30].
The silicon content of PSi @ PEDOT and PSi@PEDOT/GO-2 composites can be calculated according to the mass loss rate of the thermogravimetric curve. As shown in Fig. 4, the composites have two obvious mass losses between 300°C and 600°C, which is due to the chemical changes such as partial dehydrogenation, oxidation and decomposition of organic matter when heated. The mass of PSi begins to rise slightly near 300°C, which is due to the oxidation of some Si into SiOX compounds [31], resulting in an increase in mass. Therefore, the content of Si in PSi @ PEDOT and PSi@PEDOT/GO-2 composites can be calculated by mass loss difference to be 77.5% and 65%, respectively. The content of PEDOT is 22.5%, and the content of GO is 12.5%.
3.2 Morphological characterization of PSi@PEDOT/GO
Figure 5 shows the SEM images of the Al–Si alloy, PSi, PSi@PEDOT, and PSi@PEDOT/GO-2 composites and the EDS distribution of PSi@PEDOT/GO-2. Figure 5a shows the original Al–Si alloy with uniform spherical particles and a smooth surface. After selective etching with dilute hydrochloric acid, the spherical structure retained its shape with numerous holes. These porous structures provide sufficient internal space for the huge volume expansion of silicon during lithium embedding. Figure 5c shows an SEM image of PSi coated with PEDOT. It is evident that the number of holes in the sphere decreases, indicating that PEDOT was successfully coated with a thin –layer of GO, as shown in Fig. 5d. Some PSi@PEDOT particles were embedded in GO, whereas others were exposed. Figures 5e–j show the distribution of Si, C, O, S, and Al in the PSi@PEDOT/GO-2 composites. Evidently, these five elements are evenly distributed, indicating a relatively uniform coating. The external PEDOT and GO layers not only improved the electrical conductivity of the PSi@PEDOT/GO composites but also prevented excess silicon from reacting with the electrolyte to form irreversible SEI films. They can also buffer the volume change in silicon during lithium de-embedding, thereby maintaining the stability of the spherical structure [32, 33]. These structural properties laid the foundation for the excellent electrochemical performance of the PSi@PEDOT/GO composites.
3.3 Electrochemical properties of PSi@PEDOT/GO
The electrochemical properties of PSi@PEDOT/GO-2 composites were studied by cyclic voltammetry (CV). As shown in Fig. 6a, two upward oxidation peaks appeared near 0.35 V and 0.53 V, which corresponded to the process of dealloying of silicon from amorphous to crystalline. A downward reduction peak potential appears near 0.18 V, which corresponds to the process of silicon lithium insertion from crystalline to amorphous alloying [31, 34]. In the first cycle, a downward reduction peak potential appears at 0.43 V, which is a process of forming a SEI film [35]. With the increase of the number of scans, the oxidation peak and the reduction peak become more and more obvious, which may be due to the full exposure of the active site of the material. Due to the full utilization, and the coincidence degree of the following several cycle curves is high, indicating that the composite material has high redox stability and good cycle stability.
Figure 6b shows the charge–discharge curves of the PSi@PEDOT/GO-2 composite materials. The first three laps are the charge–discharge curves of the formation (current density 0.1 A/g), whereas the remaining three laps are the charge–discharge curves of different cycles (0.3 A/g). The slope of the curve began to increase at approximately 0.75 V, followed by a long and flat plateau, indicating that the active material began to intercalate a large amount of lithium and a certain amount of Li+ was consumed to form the SEI film [36–38]. As the reaction progressed, the capacity retention rate of PSi@PEDOT/GO-2 was relatively stable, indicating that appropriate PEDOT and GO coatings can provide a continuous conductive path between the porous silicon particles and maintain the structural integrity of the material. Simultaneously, it prevents excess active substances from reacting with the electrolyte to generate an SEI film, thereby preventing an irreversible capacity loss.
Figure 7 The electrochemical performance of PSi@PEDOT with different proportions of GO was tested.
To explore the relationship between the electrochemical performance and impedance of the Si-based anode active materials, the AC impedances of the PSi, PSi@PEDOT, and PSi@PEDOT/GO-2 electrodes were investigated, and the results are shown in Fig. 6d. By comparing the semicircle diameters in the high-frequency region, the high-frequency semicircle diameter of the PSi@PEDOT/GO-2 electrode after cycling is much smaller than those of the electrodes of the other two materials, indicating that the combination of PSi with PEDOT and GO can effectively reduce the impedance value of the battery. In addition, the slope of the oblique line in the low-frequency region is closely related to the diffusion resistance of the electrode material and the lithium ions in the electrolyte [39]. The larger the slope, the smaller the diffusion resistance. The slope of the PSi@PEDOT/GO-2 electrode is larger than those of the PSi and PSi@PEDOT electrodes, indicating that the PEDOT and GO coatings are beneficial for improving the ion and electron transport rates of the electrode material. Therefore, the PSi@PEDOT/GO-2 composite, with the highest electrical conductivity, exhibited the best electrochemical performance.
Figure 7a shows the cycling performance of the PSi@PEDOT/GO composites with different GO contents at a current density of 0.3 A/g. The composites are named according to the different mass ratios of GO as follows: PSi@PEDOT/GO-1 (mass ratio = 6:4:3), PSi@PEDOT/GO-2 (mass ratio = 6:4:4), and PSi@PEDOT/GO-3 (mass ratio = 6:4:5). From the cyclic data, it was found that the cyclic stability and rate performance of the composites improved with increasing GO content. However, an excessive GO content results in the deterioration of the cycle stability and rate performance, indicating that an appropriate amount of GO could maintain the structural stability of the material and improve the conductivity of the electrode; however, the coating layer was too thick when the GO content was excessive. This affects the intercalation and deintercalation of lithium ions, which in turn affects the cycling and rate performance of PSi@PEDOT/GO. The impedance spectrum in Fig. 7c shows that when the mass ratio was 6:4:4, both the charge transfer impedance and diffusion impedance were optimal; therefore, an appropriate amount of GO coating can improve the lithium-ion battery cycle and rate performance.