Alkali Etched Nanoporous Silicon Graphene Composite as Sodium Ion Battery Anode

2.  Experimental

2.1.  Active material synthesis

Two starting materials were used, a pure silicon powder (99%, Acros, Germany) and a mixture of silicon with graphite nanoplatelets (industrial quality graphite nanoplatelets, ACS material, USA) with a ratio of 2:1. The two samples are signed as (Si) and (Si-G) respectively. A total of  3 g of each sample was ball milled at 400 rpm for 5 min with 5 ml NaOH (min 99%, Carl Roth GmbH, Germany) solution (8 wt% in Milli-q water) and 5 ml n propanol (30 Vol%), the total solution volume was 10 ml. The ball milling machine is Fritsch Pulverisette 7, Germany, and the ball to powder ratio was 1:20. The produced slurry was washed many times using Milli-Q water, filtered, and dried at 50 ^OC overnight. The resulting powder was kept in a dry jar to prevent the oxidation of reactive Si powder.

2.2. Material characterization

Elemental composition of is analyzed using EDAX unit associated with JEOL JSM- 6360 LA, Scanning Electron Microscope (SEM).  Structural is analyzed using Shimadzu- XRD- 6000 Diffractometer, Japan, and Raman spectroscopy (SENTERRA, Bruker, Germany). Morphological studies are made by scanning electron microscope (JEOL JSM- 6360 LA SEM, Japan) and particle size analyzer (Beckman coulter N5 submicron PSA, USA). For PSA analysis, the powder samples are prepared by dispersing 2 mg of powder in 5 ml Dimethyl Sulfoxide (DMSO) and sonication in an ultrasonic bath for 30 minutes.

2.3.    Electrochemical Characterization

2.3.1. Electrode preparation

Electrodes are prepared by mixing active materials with conductive carbon (industrial quality graphite nanoplatelets, ACS material, USA) using ball milling at 400 rpm for 30 min. N-methyl pyrrolidone (NMP) (99.5%, Merck, Germany) containing 20 mg/ml PVDF (99%, Alfa Aesar, Germany) is added to the mixture. The final weight ratio of (active material: conductive carbon: PVDF) is (75:15:10). The slurry has been plated on copper foil while the unloaded parts of the current collectors are covered with isolating tape. The current collectors with an area of 1cm² are weighed before and after material loading. The weight of conductive additive and binder are subtracted to determine the active mass loading in the electrodes. 

2.3.2. Cell fabrication

Electrochemical cells are two-electrode systems. The working electrode acts as the anode, sodium metal as both the counter and reference electrodes with an approximate area of 1.13 cm², fiberglass filter paper (GFD Wattman, UK) as the separator, and 1M NaClO₄ (99% Across, Germany) in propylene carbonate (PC) (99%, Fluka, Germany) as the electrolyte. The components are assembled inside a homemade Teflon coin cell with an internal diameter of 11 mm. Cell assembly is conducted under a dry air-filled glove box.

2.3.3. Electrochemical Assessment

The electrochemical experiments are conducted using an Autolab workstation (Aut 87070). The capacity is calculated from galvanic charge/discharge by the formula (I x t/m), where I is the applied current (mA), t is the discharge time (hour), and m is the loading weight of active materials (grams). The first discharge curve is considered as the irreversible capacity. CV and impedance are also performed using the same cell. CV range is from 0 to 3 V and the scanning rate is 0.1 V/S. EIS is made at the open circuit potential of each electrode in a frequency range from 1MHz to 0.2 Hz.

3.   Results and discussion

Alkali etching process is a low-cost and environmentally friendly method that has been used for increasing the specific surface area of silicon powder [15]. The use of ball mail for a short time reduces the energy cost of the fabrication process. The mechanism of alkali etching is presented in equations 1- 4.

^*Si + 2NaOH + 2H₂O   →   SiO₂(OH)₂^-2 + 2 Na⁺ + H₂                                                                    (1)

SiO₂(OH)₂^-2 + 2Na⁺ + H₂   →  Si^* + 2Na⁺ +4 OH^-   →  Si(OH)₄^*+2Na⁺+ 2e^-                             (2)

Si + 4e^- + 4 H₂O + 2Na⁺   →   Si^* +2NaOH + 2(OH) ^- + 2 H₂                                                              (3)

*Si(OH)₄^*     →    SiO₂ + 2H₂O                                                                                                (4)

This treatment produces a surface etched silicon powder, the other soluble components (NaOH and isopropanol) are washed out by filtration. The color of pure silicon powder is converted from grey color to dark brown color after the etching process. This is because Si atoms of the different crystal planes have different activation energies for the etching reaction. The NaOH etching of Si is not diffusion-limited but etching-rate-limited, the etching process takes place anisotropically: The (100) and (110) planes are much more rapidly etched compared to the stable (111) plane [16]. The role of isopropyl alcohol in the alkali etching technique is to suppress the etching rate at (100) planes in alkaline solutions [17].

The EDAX graph for samples Si and Si-G is presented in Figure 1, the results of elemental analysis are summarized in Table 1. In addition to silicon and carbon, the formation of silica and the presence of sodium residues due to the etching process are also detected. The amount of oxidized silicon (silica) is about four times in graphene-containing sample indicating catalytic power of high surface area carbon. Also, the Si-G sample is characterized by the presence of sodium residues that might be strongly adsorbed to carbon.

XRD patterns of sodium iron phosphate Fe3 group samples are presented in Figure 2. The crystal structure matches (JCPDS #01-079-0613) with face-centered cubic (FCC) diamond structure (a = b = c = 5.431 Å), space group Fd-3m (no 227) [18]. There is no presence of peaks for crystalline silicon dioxide in both samples, graphite peak appears at 2Ɵ of 26.37° for the Si-G sample. The absence of peaks around 2Ɵ of 10.7°(specific to graphene oxide) indicates that no oxidation process had occurred for graphite and only silicon is oxidized during the etching process [19]. Crystallite sizes calculated for the prepared samples by applying Scherer’s equation for (111), (220), and (311) planes are summarized in Table 2. The crystallite sizes for sample Si and Si-G samples do not show large differences, indicating that the process of alkali etching has an insignificant effect on silicon’s crystal structure. The amorphous silica part of the material also may participate in the capacity as it was used as anode for lithium-ion batteries [20].

Raman spectra for anode samples are presented in Figure 3. The main one-phonon peak that appears at 510 cm^-1 is related to Si, beside the peaks at 295 and 934 cm^-1 which are also related to silicon structure [21]. No other peaks appear for both samples except the relatively weak D band and G band at 1340 and 1580 cm^-1 respectively which are related to graphite structure in Si-G sample. The absence of SiC peaks at around 790 cm^-1 indicates that there is no chemical binding between silicon and carbon, and only physical mixing had occurred [22]. Amorphous silica peak that is broad and very weak appeared at around 446 cm^-1 and hidden by the strength of Si peaks due to their low proportion on the silicon particle’s surface [23].

SEM images of Si and Si-G samples at different magnification powers are shown in Figure 4. Si samples show un-uniform particles with rough surface due to etching.  Si-G sample shows a uniform distribution of graphite particles with silicon, with apparent lower particle sizes and enhanced homogeneity. The mean particle size measured manually from SEM is compared with PSA results in Table 3, particle size distribution curve obtained from PSA is shown in Figure 5. SEM images show the presence of many particles with microscale in pure Si samples (left). This is due to the fact that ball milling with alkaline etching technique cannot be performed for long times, this is because hydrogen evolution during reaction may cause an explosion of milling vessel due to increasing pressure [24]. On the other side (right), SiG samples are more homogenous in particle size and shape. The presence of graphene during the ball milling alkaline etching process acts as a solid lubricant and prevents the formation of silica bridges between milled particles [25]. There is little or no presence of micro-scale particles in contrast to pure silicon samples. Charge-discharge curves for alkaline etched Si and Si-G samples Vs Na/Na⁺ are presented in Figure 6. For comparison, pure graphene (commercial graphite nanoplatelets) electrode was tested as an electrode with the same specific current density.

Table 1.  Elemental analysis of the anode samples.

Fig. 1  EDAX graph for treated silicon and silicon-graphene samples.