Synthesis of Si /rgo Nano-composites as Anode Electrode for Lithium-ion Battery by Ctab and Citrate Methods: Physical Properties and Voltage-capacity Cyclic Characterizations.


 In this paper, SiNPs / rGO nano-composites were prepared from porous silicon nanoparticles synthesized by silica mineral (SiO2). Reduced graphene oxide (rGO) was synthesized by thermal reduction method. The Si NPs/rGO nano-composite synthesized by three methods using the (a) CTAB as surfactant (b) CTAB as surfactant and citric acid as a functional group and (c) without CTAB and with citric acid under ultrasonic condition. The samples were analyzed by X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FE-SEM), VU-Vis, EDX and FTIR spectroscopy. The test of anode electrode sample for synthesis (a) carried out by potential (Li/Li+) vs. capacity measurements. The anode made of the resulting nano-composite showed an initial specific capacity of 1191 mAhg-1 and specific capacity of 845 mAhg-1 after 10 cycles at a current density of 100 mAg−1and a coulomb efficiency of approximately 99.4% after these cycles. Porous nanoparticles have improved the electrochemical properties of the nano-composite along with the conductivity and buffering properties of rGO by creating a suitable space for lithiation/delithiation process.


Introduction
Market push to acquisition high-capacity Li-ion battery (LIBs) technology for electronics and electrical vehicles, has driven the intense research and develop toward new anode materials. In addition, Silicon (Si) is considered as an attractive candidate because its high speci c capacity (4200 mAhg -1 ) and low discharging voltage (0.37mV vs Li/Li + ) that is very close to lithium [1][2][3][4].
Si-Graphene nano-composites have been widely considered in the fabrication of LIBS battery anodes since 2010 due to their potential to improve cycling performance [5]. The main factors that prevent the replacement of graphite-based anodes with Si-based one in these batteries are the enormous volume expansion during lithiation, slow diffusion rate of lithium and low electrical conductivity. Si nanostructures and carbon derivatives are used to overcome these problems [6]. Carbon compounds improve conductivity and the addition of Si stabilizes the solid electrolyte interface (SEI) layer [7].
Graphene has attracted much attention due to its unique electrical and optical properties [19,20]. The graphene band gap is zero [21,22]. This has limited its use in nanoelectronics. With the band gap engineering in graphene, many applications are obtained for it [23][24][25][26]. Graphene oxide (GO) and its relative reduction can be used to adjust the band gap in graphene [27,28]. It has a high potential for the synthesis of carbon-based nanostructures that can be obtained on a large scale from the oxidation of inexpensive graphite. The properties of the nal product can be adjusted by reducing GO in various ways and controlling the reduction conditions [29].
GO reduction increases the mobility of charge carriers and absorption and tune the band gap in which the photo responsivity can be adjusted by controlling defects and oxygen groups [30,31].
Go is generally electrically insulator because of the existence of substantial sp 3 hybridized carbon atoms bonded with oxygen. The alteration of sp 2 and sp 3 carbon segments present in graphene oxide are bene cial for the manipulation of its bandgap, therefore controlling the transformation of graphene oxide from an insulator to a semiconductor [32].
Various chemical, thermal, electrical, or a combination of methods have been reported to produce reduced graphene oxide (rGO) [27]. The rGO has a low reversible capacity as a host in the nano-composite, but as a conductor and buffer it can improve the electrochemical performance of Si nanoparticles.
Graphene reduces the surface contact of Si nanoparticles with the electrolyte (SEI) and improves the electrical conductivity of the electrode. The empty space between Si nanoparticles and graphene provides the space needed for volumetric changes in working cycles [33].
The physical methods for the synthesis of stable Si-graphene nanocomposites are di cult. Although, the CVD method is usually complex, time consuming and expensive [42], Zhu et al. synthesized nanocomposite of Si-graphene by using plasma-assisted milling route [43].
Thermal reduction method is a simple and mass-produced method for the production of Si-Graphene nano-composites [42]. This method has been used by different groups to prepare SiNPs-Graphene nanocomposites [44][45][46].
In this study, porous Si NP-rGO nano-composite was synthesized by thermal reduction method as an anode electrode for LiBs. Three different compounds were prepared by thermal reduction synthesis using (a) CTAB additive (b) CTAB and Citric acid and (c) Citric and only ultrasound assisted. The samples were analysis by X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FE-SEM), VU-Vis, EDS and FTIR spectroscopy. The structural properties of nano-composites including crystallinity, surface morphology, the optical behavior (optical band gap) and electrochemical performance for nanocomposite synthesized with CTAB additive were investigated. Also, test of anode electrode sample carried out by potential (Li/Li + ) vs. capacity measurements.

Synthesis of Si NP-rGO nano-composite
In this work, the synthesis of Si NPs-rGO nano-composite was performed in the following three synthesis: Synthesis 1: 0.05gr of reduced graphene oxide (rGO) was dissolved in 10 cc of deionized water. Then, 0.015 gr of prepared porous Si nanopowder, synthesized in the previous work [47], and 0.05gr of CTAB material were added to the rGO solution. CTAB as a surfactant consists of a hydrophilic cationic polar head and a hydrophobic sequence, it is able to bond Si nanoparticles to graphene.The resulting solution was stirred on a magnetic stirrer for 10 hours at T=100 °C and dried. Then, the obtained material was annealed in an oven under argon gas at T=500 ºC for 2 hours. It was then ltered using a paper lter and washed with HF, alcohol, deionized water and nally dried under vacuum at T=70 °C overnight.Synthesis 2: In another method, 0.05 g of reduced graphene oxide (rGO) was dissolved in 10 cc of deionized water and 0.015 g of Si nanopowder, 0.05 g of CTAB and 0.05 g of citric acid as a hydroxyl functional group were added.Citric acid is a hydroxy acid, a group of carboxylic acids to which a hydroxyl functional group has been added. Then, the re ux process was performed on the resulting solution at T=100 °C for 2 hours and then dried. After this step, the material was annealed like the previous method.Synthesis 3: 0.001g of Si nanopowder was mixed with 2 cc of deionized water and placed in ultrasonic for 10 minutes. Then, 1.25 cc of reduced graphene oxide (rGO) solution in water (1 mg / ml) was added to it and placed in ultrasonic for 2 hours.After drying, the resulting mixture was annealed in furnace under N 2 gas at T=750 ºC for 2 hours. After approximately 10 hours, the oven was cooled and the composition was taken out of the furnace and powdered in an agate mortar. In the last step, to remove impurities and residual oxide layers, it was washed with HF, alcohol, deionized water and nally dried under vacuum at T= 70 °C overnight. Figure 1  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 speci ed. The present patterns con rm 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 exibility 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 gure, 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 rst method, the number of graphene layers is limited, with a relatively at 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]. 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 −1 corresponding 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 −1 is 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 −1 is 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/ cm 2 . 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-lled glove box. 1.0 M LiPF 6 as electrolyte and polypropylene (Celgard 2300) as the separator (a layer of lithium-ion permeable lm 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 rst 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 lm formed by the decomposition of electrolyte on the anode surface. This lm protects the electrolyte from further decomposition and also affects safety, capacity, power, cycle life and battery performance [62]. mAhg -1 respectively, which are 3 times higher than the gravimetric capacity of graphite (372 mA h g -1 ). As a result, the coulombic e ciency (CE) is 98.2 %. As can be seen from the gure 10, the speci c 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 at tail with a plateau. It can be related to the delithiation from amorphous Li x Si phase. The CE stabilized at 95% to 98% for next 10 remaining cycles. In the rst lithiation cycle, a voltage drop slope can be seen approximately in the range 1.0 -0.2 V. It is due to the SEI lm development. In the discharge curves a sloping platform between 0.2 and 0.01 V is speci ed, that is consistent with the lithiation of Si to Li x Si.
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 e ciency and capacity retention of the Si NPs-rGO anode shows signi cant 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 nanocomposite 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.

Conclusion
In summary, the SiNPs-rGO nano-composite using the CTAB surfactant was successfully synthesized by thermal reduction. The FTIR, XRD and FESEM analysis con rm the nano-composite formation. The test of anode electrode sample carried out by potential (Li/Li + ) vs. capacity measurements. A suitable speci c capacity of 845 mAhg -1 after 10 cycles at current density of 100 mAg −1 and coulomb e ciency of approximately 99.4% was obtained. This high capacity of SiNPs-rGO to the role of porous Si nanoparticles which facilitate electron transfer and high surface area and conductivity of graphene sheets. Additional studies using nano-composites of silicon nanostructures and carbon derivatives to improve cyclic performance are ongoing in our laboratory.

Declarations Declaration of interests
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to in uence the work reported in this paper.
Con ict of Interest: The authors declare that they have no con ict of interest.
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