3.1: Structural studies of Si NPs- rGO nano-composites by XRD and EDX analysis
In order to study the structural properties of Si NPs / rGO nano-nano-composites, XRD patterns was taken from three synthesis processes. The XRD spectra of the Si NPs-rGO nano-composite are shown in Figure 2. The peaks observed at the angles 2θ = 28.52 º, 47.23 º, 56.31 º, 69.30 º and 2θ = 76.41 º correspond to the crystal plates (111), (220), (311), (400) and (331) respectively, are related to Si nanoparticles.The graphite peak (C) in the samples at 2θ = 26.28 º, the peak for rGO at 2θ = 44º and the broad peak in the range 2θ = 20 - 27º (maximum at 2θ = 23º) has been specified. The present patterns confirm the formation of Si NPs- rGO nano-composite [48, 49].Also, the EDAX spectrum from the nano-composite and the results of elemental analysis using its data are shown in Fig 7 and Table 1 respectively.
3-2: Morphology of Si NPs-rGO Composition
The FESEM images of the synthesis1 for nano-composite using CTAB as a surfactant, are shown in Figure 4 (a) to (c).
These images show the layered rGO and folded structure (with a relative thickness of 50 nm according to the scale of the images). The presence of Si nanoparticles as a mass on graphene plates indicates the formation of Si NPs-rGO nano-composite.
In the synthesis 2, CTAB was used as surfactant and citric acid as a functional group. The FESEM images of the nano-composite synthesized by this method are shown in Figure 5 (a) to (d).
The use of organic molecules such as citric acid as a binder has a unique effect on Si anode stabilization due to its flexibility and non-toxic properties.
This acid reacts with GO functional groups due to carboxyl and hydroxyl functional groups and therefore causes Si nanoparticles to adhere to graphene plates [50]. Also, the presence of this acid and the CTAB prevented the nanoparticles from bonding to each other and caused the nanoparticles to spread on all rGO surfaces.As previously mentioned, in the synthesis 3, ultrasonic was used for the initial combination of Si and GO powder. The FESEM images of the nano-composite synthesized by this method are shown in Figures 6 (a) and (b). The dimensions of the nanoparticles are shown in the figure, which are in the range of 36-57 nm.As can be seen from the microscopic images in Fig 6, the layered graphene structure exists in all three methods. In the first method, the number of graphene layers is limited, with a relatively flat surface with Si nanoparticles attached to the graphene plates (binding state). In the second method, the number of graphene layers is increased, the degree of smoothness of graphene surfaces is reduced and Si nanoparticles are placed on the plates and between the graphene plates (layering state). In the third method, graphene plates are Tangled in shape with more layers that contain Si nanoparticles (folding state).It should be noted that the ratio of Si nanoparticles to graphene have a particular importance in the performance of the nano-composite fabricated as the anode of lithium batteries. The higher the Si value, the higher the initial battery capacity and the faster the capacity fading. The ratio of 40% -60% carbon for this nano-composite has been reported by researchers [51].The operating conditions of Si-rGO nano-composites depend on their morphology. The best-case scenario is when Si NPs are homogeneously distributed among the graphene plates.The uniform distribution of graphene sheets also increases the electrical conductivity of this nano-composite [52]. These conditions are observed in the second and third synthesis methods so that the nanoparticles are placed with a more uniform distribution between the graphene plates. The use of ultrasonic in the third method, is effective in the uniformity and homogeneous distribution of Si nanoparticles.3-3: Optical band gap for Si NPs-rGO nano-compositeIn order to determine optical band gap of samples, the curves (αhν)1/2 and (αhυ)2 in terms of hʋ were plotted in Figures 7(a) and (b). The intersection of the red lines with the horizontal axis indicates the amount of the gap. As can be seen from the graph, the value of the indirect band gap is equal to 4.15 eV, that indicates a slight increase over the pure sample of Si nanoparticles. The value of the direct band gap showed two values of 3.45 eV and 5.36 eV.In synthesized nano-composite samples, the addition of Si nanoparticles opens the optical gap and due to the hybridization between the s and p orbitals of C and Si atoms, the σ bonds are slightly altered while π bonds are substantially modified.3-4: Molecular structure of synthesized nano-composite samples by FTIR
Figure 8 (a) and (b) shows the FTIR spectra of SiNP-rGO-CTAB and SiNP-rGO-CTAB-Acid Citric nano-composites respectively.
The peaks at 3445 cm-1 and 1147 cm-1 are the anti-symmetric stretching vibration of -OH and the typical asymmetric stretching vibration of Si-O-Si of SiNPs. The next peaks at 1735, 1624, 1226, and 1049 cm-1 of GO correspond to the C=O stretching, C=C stretching, C-OH stretching, and C-O stretching vibrations, respectively [53-55]. O-Si-O bending vibration, symmetric elastic vibration of Si-O-Si and Si-O-Si asymmetric elastic vibration are ascribed to peaks at about 468 cm−1,816 cm−1, and 1087 cm−1 respectively [56].For GO, the presence of the two peaks at 1715 cm−1 and 1050 cm−1 corresponding to the stretching vibration of C=O for −COOH and C−O for C−OH, respectively. new peak forming at 1628 cm−1 is attributed to the C=O stretching vibration of amide [57].GO exhibits many absorption peaks due to it functional groups,with the peak at 1730 cm−1, 1608 cm−1, 1220 cm−1, and 850cm−1corresponding to C=O stretching vibrations, C=C stretching vibrations, C−O symmetric stretching and deformation vibrations of the epoxy groups, respectively [58].
The IR peaks corresponding to 2927 cm−1 and 2849 cm−1 are due the asymmetric and symmetric CH2 stretching of GO respectively while the peak around 1619 cm−1 is attributed to C=C stretches from unoxidized graphitic domain. The peak at around 1720 cm−1is attributed to C=O stretch of carboxyl group, 1224 cm−1 corresponds to C-OH stretch of alcohol group [59]. The peak at around 2923 cm−1is attributed to C-H [60].
3-5: Electrochemical measurements of NP-Si/ r-GO nano- composite for anode electrode
In order to perform the battery test, CR2035 half-cell battery was prepared. The working electrode was prepared by doctor-blade method, combining the active substance with a mass ratio of 90% SiNP-rGO powder, 5% CMC (Carboxy Methyl Cellulose-Sodium) as binder and 5% black carbon. The slurry was then coated on a 100 µm thick copper foil, pressed and dried under vacuum at T=100°C for approximately 24 hours. Mass loading of electrode was 3.3 mg/ cm2. Since the electrolyte used in the battery and the subsequent assembly process is very sensitive to moisture, the coin cell was assembled in an argon-filled glove box. 1.0 M LiPF6 as electrolyte and polypropylene (Celgard 2300) as the separator (a layer of lithium-ion permeable film for preventing direct contact between the anode and the cathode) were used. The cut-off voltage used for charging and discharging was 0.001V and 3V (versus Li/Li+), respectively. The electrochemical performances were tested on a NEWARE battery test system at room temperature. Fig 9 (a-c) shows the anode and coin cell battery made and a view of the battery tester used for analyze. Electrochemical measurements were used to investigate the kinetics of lithium-ion transfer during the processes of lithiation/delithiation for the anode made of SiNP-rGO nano-composite.
During the formation (the first charge / discharge steps after cell assembly) the part of lithium that is available by the electrolyte and the positive electrode, used to formation of SEI layer on The surface of graphene [61].
The SEI layer is a surface film formed by the decomposition of electrolyte on the anode surface. This film protects the electrolyte from further decomposition and also affects safety, capacity, power, cycle life and battery performance [62].
Figure10 shows the charge-discharge curve for the first 4 cycles and the 10th cycle of Si-rGO nano-composite with a current density of 100 mAg-1 (0.1C) and voltage range of 0.001–3.0 V.
The first discharge and charge capacities of the SiNP-rGO anode are approximately 1191 and 1212 mAhg-1 respectively, which are ̴ 3 times higher than the gravimetric capacity of graphite (372 mA h g-1). As a result, the coulombic efficiency (CE) is 98.2 %. As can be seen from the figure 10, the specific capacity of nano-composite has been maintained at about 850 mAh·g−1 after 10 cycles. As the number of cycles is added, the capacity decreases further.
The initial discharge curve shows lengthy flat tail with a plateau. It can be related to the delithiation from amorphous LixSi phase. The CE stabilized at 95% to 98% for next 10 remaining cycles. In the first lithiation cycle, a voltage drop slope can be seen approximately in the range 1.0 -0.2 V. It is due to the SEI film development. In the discharge curves a sloping platform between 0.2 and 0.01 V is specified, that is consistent with the lithiation of Si to LixSi.
In Figure 11 the cycling performance of Si NPs-rGO nanonano-composite, prinstine Si NPs [63] and rGO [64] has been compared at a current density of 100 mAg-1. As can be seen the Si NPs show a rapid capacity fading from ̴ 1990 to ̴ 164 mAh g-1 after 11 cycles, rGO a capacity fading with gentle and gradual slope from ̴ 451 to ̴ 175 mAh g-1, while the SiNPs- rGO nanonano-composites show capacity fading from 1212 to 843 mA h g-1 After this number of cycles.
The rGO represent better reversibility than the Si NPs while the reversible capacity and initial CE is relatively low.
The initial coulombic efficiency and capacity retention of the Si NPs-rGO anode shows significant improvement over Si and rGO anode. This can be attributed to the role of graphene layers in improving the cyclic performance due to the increase in electrical conductivity and stabilization of the nano-composite structure.
The decay of reversible capacity of the Si NPs-rGO over 10 cycles can result from the pulverization of Si nanoparticles during lithiation/delithiation, leads to the gradual damage of the intimate attachment between the Si nanoparticles and graphene and the missing the electrical connectivity between them.