SiO₂/C Composite as a High Capacity Anode Material of LiNi_0.8Co_0.15Al_0.05O₂ Battery Derived from Coal Combustion Fly Ash

Abstract

Abundantly available SiO₂ (silica) has great potential as an anode material for lithium-ion batteries because it is inexpensive and flexible. However, silicon oxide-based anode material preparation usually requires many complex steps. In this article, we report a facile method for preparing a SiO₂/C composite derived from coal combustion fly ash as an anode material for Li-ion batteries. SiO₂ was obtained by caustic extraction and HCl precipitation. Then, the SiO₂/C composite was successfully obtained by mechanical milling followed by heat treatment. The samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). Electrochemical properties were tested using an 18650 cylindrical cell utilizing LiNi_0.8Co_0.15Al_0.05O₂ (NCA) as the counter electrode. Based on the obtained results, the physiochemical characteristics and electrochemical performance, it was determined that SiO₂/C composites were greatly affected by the temperature of heat treatment. The best result was obtained with the SiO₂ content of 10% w/w, heating temperature of 500 °C, initial specific discharge capacity of 586 mAh g⁻¹ at 0.1 C (1 C = 378 mAh g⁻¹), and reversible capacity of 87% after 20 cycles. These results confirmed that the obtained materials had good initial discharge capacity, cyclability, high performance, and exhibited great potential as an anode material for LIBs.

1. Introduction

In recent decades, energy shortage and environmental pollution have become serious issues along with the world population growth. Extensive research on renewable energy sources has been rapidly performed. Current renewable energy cannot be separated from energy storage systems. Lithium-ion batteries (LIBs) have become the main candidates for a wide range of applications owing to their fast charging, long lifespan, low cost, high energy density, and ability to be used repeatedly with little degradation in performance [1,2,3,4,5]. Currently, graphite is widely used as a common anode material in the industry owing to its advantage of long lifespan and low cost [6,7,8]. In addition to graphite, Li₄Ti₅O₁₂ or LTO has also become a promising candidate because it has fast charging and long shelf life [9,10]. However, both of them exhibit low storage capacity (graphite = 372 mAh g⁻¹ and LTO = 160 mAh g⁻¹), which has become a huge obstacle to further application, especially for electric vehicles and electronic devices [1,11]. Various different anode materials with higher capacities have been studied as a candidate of the next generation of anode material. Among the various anode materials studied, Si is considered as the most promising anode material owing to its high theoretical capacity (4200 mAh g⁻¹), moderate operating voltage (0.1–0.5 V vs. Li/Li⁺), and low stable plateau potential [12,13]. Unfortunately, Si anode materials experience some challenges such as low electrical conductivity, large volume expansion (>300%) during lithiation and delithiation, and an unstable solid electrolyte interphase (SEI), which leads to poor cycling performance [1,9]. Therefore, silicon suboxide or SiO_x (x < 2) has been suggested as an alternative anode material. Even though SiO_x has a smaller theoretical specific capacity (1961 mAh g⁻¹) than Si, it exhibits better cycling stability, smaller volume expansion, lower cost, and eco-friendliness. SiO_x also has a drawback of poor conductivity and low Coulombic efficiency at the initial charge–discharge cycle. On the other hand, it is difficult to produce SiO_x owing to the partial oxidation of silicon or semi-reduction of SiO₂, which require complex and convoluted synthesis steps. It is advantageous to use SiO₂ as an anode material owing to its high availability and because it can be easily extracted from biomass-based waste. Similar to SiO_x, SiO₂ has low conductivity, which needs to be improved before being applied in full batteries [14,15].

Many approaches have been tried to overcome these problems such as inserting Li into SiO₂ and modifying the size of SiO₂ [14]. However, one of the most effective ways is to use composite SiO₂ with carbon. The composite improves electrical conductivity and also reduces volume expansion during the charge–discharge cycling to maintain a stable electrochemical performance [16]. For example, it has been reported that the SiO₂/C composite showed an impressive reversible capacity of 635.7 mAh g⁻¹ at the discharge current of 100 mA g⁻¹ [17]. The 3D SiO₂@graphene aerogel composite shows a reversible capacity of 300 mAh g⁻¹ at the discharge current of 500 mA g⁻¹ [18].However, the SiO₂-carbon (SiO₂/C) composite preparation for anode material usually requires many complex steps such as additional process to modify the morphology, expensive silicon precursor, e.g., Tetraethyl orthosilicate (TEOS), pretreatments [18] and multiple high temperature and long heating process [17], which greatly hinders further improvement. Thus, these additional processes will directly affect the overall production cost of SiO₂/C anode material. The use of cheap raw materials, especially from waste, and a simple processing method reduce the production cost in a significant amount. Hence, it is desirable to design a facile method to fabricate SiO₂/C composites to realize high-performance anode materials for LIBs. In this article, we reported a facile and inexpensive method for synthesizing the SiO₂/C composite from coal combustion-derived fly ash for active anode material by mechanically milling SiO₂/C and heat-treating it in an inert atmosphere. The use of silica from coal fly ash is a sustainable approach to recycle, reduce, and reuse industrial waste. SiO₂/C was directly used as the anode material for the LiNi_0.8Co_0.15Al_0.05O₂ battery. This approach has never been studied before.

2. Materials and Methods

2.1. Synthesize of the SiO₂/C Composite from Coal Fly Ash

Coal combustion-derived fly ash was provided by PT Semen Indonesia and was stored in an oven. The chemical composition of fly ash is shown in

Table 1. XRF analysis of coal combustion-derived fly ash from PT Semen Indonesia.

Component	O	Si	Fe	Ca	Al	Na	Mg	K	S	Ti	Cl	P	Mn	Trace Metals
wt%	36.6	20.4	18.1	9.4	4.0	3.5	3.1	1.2	1.1	0.7	0.5	0.4	0.3	0.7

. A total of 50 g of coal fly was extracted using 2 L of 2-M NaOH (Merck, Darmstadt, Germany). The mixture was heated at 90 °C for 4 h under stirring at 300 rpm. The separation of filtrate and solid particles was done by gravity filtration followed by the removal of the filter cake. SiO₂ particles were obtained by adding 10 N HCl (Merck, Darmstadt, Germany) to the filtrate until pH 6–7. The resulted precipitates were filtered, washed using hot demineralized water three times, and dried under oven at 100 °C for 12 h. Then, the SiO₂/C composite was successfully obtained through mechanical milling of SiO₂-C with various SiO₂ composition using a ball mill for 2 h and heating at 500 °C for 30 min under argon flow. Variations in heat treatment were applied to the optimized composite and performed at 400–600 °C under the same conditions. The overall process of SiO₂/C composite synthesis can be seen in Figure 1.

2.2. Material Characterizations

The crystallographic structure of materials was studied by X-ray diffraction (XRD, Rigaku Miniflex 600 Benchtop XRD). The compositional analysis of materials was performed with Fourier transform infrared spectroscopy (FTIR, Shimadzu IRPrestige-21) and X-ray fluorescence (Bruker XRF Spectrometer, Germany). The morphologies of the materials were investigated by scanning electron microscopy (SEM, JSM-6510LA).

2.3. Electrochemical Measurements

Electrochemical measurements were performed using a cylindrical cell (18650) assembled in an argon-filled glove box. LiNi_0.8Co_0.15Al_0.05O₂/NCA (MTI, CA, USA) was used as the counter electrode; monolayer polypropylene film (Celgard 2400) was used as the separator, and 1-mol/L LiPF₆ dissolved in a mixture of ethylene carbonate and diethyl carbonate (EC/DEC = 1:1, v/v) was used as the electrolyte. The working electrode was prepared with active materials, acetylene black, carboxymethyl cellulose (CMC), and styrene–butadiene rubber (SBR) obtained from MTI, CA, USA with the weight ratio of 93:1:1:5 in deionized water. The mixture was stirred for 5 h at room temperature; then, it was coated onto both side of copper foil using the doctor blade method. The copper foil was dried at 80 °C for 30 min in a vacuum oven. The load mass of active material was approximately 18 mg/cm². The overall assembly process of cylindrical Li-ion cells is described elsewhere [19,20]. The electrochemical performance of materials was studied by galvanostatic charge–discharge cycling tests using a battery testing system (Neware BTS 6000, Neware Electronic Co., Shenzen, China) at various current rates (1 C = 372 mAh g⁻¹) and room temperature (25 °C).

3. Results

3.1. SiO₂ Characterization

Figure 2 shows the as-prepared SiO₂ powder obtained from fly ash characterization by XRD (Figure 2a), FTIR (Figure 2b), SEM (Figure 2c), and XRF (Figure 2d). Based on the XRD pattern, the widening peak is detected at 15–35° of 2 theta; thus, it can be concluded that the samples exhibit amorphous properties. The impurity peaks (31° and 45°) of sodium chloride (NaCl) are detected in the sample. The presence of impurities can be caused by an unfinished cleaning process or salt entrapment in silica matrices. In addition to the impurity peak, all peaks from all samples are well indexed to ICSD Card no. 061662. Based on the FTIR spectra, the absorption peaks at 3400, 1630, and 950 cm⁻¹ are attributed to the stretch vibration of –OH from silicanol (Si–OH) or H₂O and Si–O from siloxane (Si–O–Si). The peak at 790 cm⁻¹ is attributed to symmetrical Si–O from Si–O–Si stretching vibration. Based on the XRD and FTIR result, SiO₂ is successfully obtained by caustic fusion and precipitation through following reactions [21].

SiO_2(s) + 2NaOH_(aq) → Na₂SiO_3(aq) + H₂O_(l)

(1)

Na₂SiO_3(aq) + 2HCl_(aq) → H₂SiO_3(aq) + 2NaCl_(aq)

(2)

H₂SiO_3(aq) + H₂O_(aq) → Si(OH)_4(s)

(3)

Si(OH)_4(s) → SiO_2(s) + 2H₂O_(g)

(4)

SEM images (Figure 2c) confirm that there are two types of particles, i.e., primary and secondary. The secondary particle has a diameter of ~7.5 µm, which consists of clustering sub-micron-sized primary particles and small adherents on the surface. These adherents or aggregates enhanced the overall surface area of silica, which resulted in good ionic transfer during Li insertion. The XRF result shows that mostly silicon was obtained, while there are also small sodium impurities (6%), which is confirmed by the XRD result. The NaCl impurities is still detected due to the entrapment of NaCl during the silica precipitation process and exist even after multiple washing steps.

The as-obtained SiO₂ powder was directly applied as anode in a NCA/SiO₂ battery. Figure 3 shows the electrochemical performance of the cell. The theoretical capacity of SiO₂ is adapted from a study by Liu et al. (1965 mAh/g), while the theoretical capacity of NCA is estimated at 200 mAh/g. In the final cell assembly, the SiO₂ anode was selected as the limiting reactant; therefore, the specific capacity was referred to as the specific capacity of SiO₂. Figure 2a shows that the specific charge and discharge capacity of SiO₂ is approximately 1396 mAh/g and 1188 mAh/g, respectively. The discharge capacity was maintained for 10 cycles. Even though SiO₂ has a considerably lower specific capacity than its theoretical capacity, the capacity was stable at 1/10 C. At increasing rate, as shown in Figure 2b, SiO₂ exhibits an inferior rate performance and high level of irreversibility. This can be caused by the formation of electrochemically inactive lithiated SiO₂ at high rate (>0.5 C). Previous studies claimed that the reaction mechanism between Li-ion and SiO₂ was not been completely understood yet; however, most studies showed a significant irreversible capacity loss during initial cycling [16]. This phenomenon directly indicates the importance of compositing SiO₂ with a more stable and highly conductive material such as artificial graphite or Mesocarbon microbeads (MCMB) [17,22].

3.2. Effect of SiO₂ Composition in the SiO₂/C Composite on Its Structure and Coulombic Capacity

As-prepared SiO₂ was mechanically milled with MCMB and heated at 500 °C for 30 min under flowing argon. Figure 4 shows the XRD analysis of the SiO₂/C composite with various SiO₂ wt%. Because as-obtained SiO₂ is in an amorphous form, the diffraction peaks of graphite are dominant in all samples, and the SiO₂ peak is not detected; however, there is an increase in the peak width, specifically at 2 theta of 27°, which may be caused by an increase in the amount of amorphous silica (SiO₂).

The initial charge/discharge curves of the samples at different SiO₂ content at 1/10 C are shown in Figure 5a. It is easily observed that the specific capacity of the SiO₂/C composite depends on the SiO₂ content in the composite. It is observed that the first capacity of the SiO₂/C composite considerably increased with an increase in the SiO₂ content. However, when the content of SiO₂ was greater than 10%, the capacity was insignificantly increased.

Table 2. Initial specific capacity of the SiO₂/C composite.

Content of SiO2 (wt%)	Initial Specific Capacity Based on the Anode (mAh g−1)	Columbic Efficiency (%)
0	340	85
1	356	83
3	365	80
5	423	78
10	541	76
30	552	72

shows that all samples have greater capacity than batteries that contain carbon materials (340 mAh g⁻¹) (0.1 V vs. Li/Li⁺) [1]. Increased capacity occurs because SiO₂ (1961 mAh g⁻¹) acts as an active material in composites and is responsible for increasing battery capacity. The increased capacity also indicates that SiO₂ used in the composite exhibits electrochemical activity [16,17,18]. Based on previous reports, the small amount of NaCl within silica matrices can potentially enhance the electrochemical performance of LIBs. Borong Wu et.al. claimed that 1% NaCl in electrolyte of Li-Graphite cell can improve the SEI formation on graphite surface which enhance the cycling performance. Study by Ikhsanudin et al. had stated that NaCl can improve the capacity of NCA cathode. Since the specific discharge capacity listed in Table 2 exceeds the capacity of pure graphite, we can conclude that the effect of a small amount of NaCl is still insignificant compared to the effect of silica composite on the graphite anode. However, the effect of NaCl additive on NCA/Graphite cell can be furtherly studied in future research [23,24].

However, an increase in the SiO₂ content in the anode material will also result in a decrease in battery stability (cyclability). Figure 5c shows that the reversible capacity of SiO₂/C 30% remains at 51% after 15 cycles. This is mostly due to the unstable solid electrolyte interphase (SEI) on the surface of Si [25], which causes lithium to be trapped in the active surface of Si and results in the rapid loss of irreversible capacity and low initial Coulombic efficiency (CE) [16,17,18]. These reactions that occur in the process can be summarized as follows:

2SiO₂ + 4Li⁺ + 4e⁻ → Li₄SiO₄ + Si

(5)

SiO₂ + 2Li⁺ + 2e⁻ → Li₂O + Si

(6)

Si + xLi + xe⁻ → Li₄Si

(7)

Reactions (2) and (3) result in the generation of irreversible capacity, which leads to capacity loss. In addition, Li₄SiO₄ compounds are reported to be electrochemically inactivated. However, Li₄SiO₄ and Li₂O act as buffer components and accommodate volume expansion that occurs in Li_xSi during lithiation and delithiation. This occurs because Li₄SiO₄ (2.39 g cm⁻³) and Li₂O (2.02 g cm⁻³) are denser materials than Li_xSi. The presence of oxide compounds shows a significant decrease in the volume expansion compared to when the material is homogeneous Li_xSi [16,26,27].

The rate performance of the SiO₂/C 10% composite was further examined, and the results are shown in Figure 5d; the samples tested at the discharge current of 0.1, 0.5, and 1 C provided the best performance. It is clearly observed that the sample still has a high specific capacity of approximately 395 mAh g⁻¹ at the current rate of 1 C. Figure 5c also shows that the reversible capacity slowly increased from 541 mAh g⁻¹ to 601 mAh g⁻¹ after 3 cycles and was stable after the first 15 cycles. The latest studies explain that an increase in reversible capacity is due to SiO₂, which is not fully active at the beginning of cycling and will be active in the next cycling [9,10,24]. Figure 5d shows that the sample has high stability. It was possible to obtain the reversible capacity at 87% after 31 cycles at 1 C and CE was always above 98%. These results confirm that the SiO₂/C composite with 10 wt% SiO₂ exhibited the best performance owing to its excellent specific capacity, good cyclability, and high performance, and showed considerable potential as an anode material for LIBs.

3.3. Effect of Heat Treatment on the SiO₂/C Composite

Figure 6a shows the FTIR spectra of SiO₂ in the composite after heat treatment at 400, 500, and 600 °C. The FTIR spectra of SiO₂/C show similar results at various heat treatment temperatures. The peaks at 3432, 3434, and 3433 cm⁻¹ are assigned to the –OH stretching in SiO₂/C heat-treated at 400, 500, and 600 °C, respectively. Furthermore, the peak at approximately 1600 cm⁻¹ indicates the presence of an –OH bending band. This hydroxide group indicates the presence of water molecules [15]. It is observed that an increase in the heat treatment temperature will decrease the water content in SiO₂/C (Figure 6b). The presence of SiO₂ is detected as the Si–O–Si stretching vibration of the sharp peak at approximately 1100 cm⁻¹ [28,29]. SiO₂/C heat-treated at 600 °C shows smaller transmittance than SiO₂/C heat-treated at 400 and 500 °C, specifically for the peak at approximately 1100 cm⁻¹. On the other hand, SiO₂/C heat-treated at 600 °C has better transmittance than SiO₂/C heat-treated at 400 and 500 °C for the peak at approximately 400 cm⁻¹. It can be concluded that heat treatment affects the SiO₂ surface chemistry. The FTIR spectra of graphite also show peaks at approximately 3400 and 1600 cm⁻¹, which indicates the presence of water molecules in graphite, as previously described.

Figure 7 shows the XRD patterns of SiO₂/C at various heat treatment temperatures. Based on the figure, all samples exhibit a graphite structure (JCPDS PDF 75-2078) owing to low content and crystallinity of silica compared to that of artificial graphite. The crystallite size of the samples, which is determined by the Scherer equation [19,30], is affected by the heat treatment temperature; it increased along with an increase in temperature, although heat treatment was performed for only 30 min.

The SEM characterization of artificial graphite and SiO₂/C composite heated at various temperatures is shown in Figure 8. Most graphite particles have a spherical shape with ~27-μm size. Flakes of carbon were observed between spherical particles. The abovementioned figure shows that SiO₂/C has rough surface morphology, owing to the presence of 10% of SiO₂ aggregates. SiO₂ secondary particles break into micron-sized primary particles during the ball milling process. The average primary particle size of SiO₂ on the graphite surface heated at 400, 500, and 600 °C is 344, 399, and 450 nm, respectively. Moreover, with an increase in temperature, the average particle size of SiO₂/C increases, owing to the agglomeration of particles [9,31].

Table 3. Initial specific capacity of the SiO₂/C composite depending on the heat treatment temperature.

Heat Treatment Temperature (°C)	Initial Specific Capacity (mAh g−1)	Columbic Efficiency (%)
400	506	74
500	586	76
600	705	82

shows that all samples have greater specific capacity than batteries using carbon materials. Compared to graphite, all samples have higher initial discharge capacity with an increase in temperature. Thus far, it can be concluded that with an addition of SiO₂ and increase in the heat treatment temperature, all of our samples exhibit high initial capacity; this improvement is achieved using an easy and simple method.

The initial charge/discharge curve of the samples at different heat treatment temperature of SiO₂/C is shown in Figure 9a. Based on the cyclability and rate ability of the cell, 500 °C is the optimum temperature to obtain the SiO₂/C composite. It is predicted that at higher temperature, the SiO₂/C composite is agglomerated into larger sizes, which lowers the diffusion kinetics of Li-ion during the charge–discharge process. At low temperature, the composite process did not completely occur.

The rate performance of the SiO₂/C composite depending on heat treatment variation is shown in Figure 9c. The samples were tested at the charge and discharge current of 0.1, 0.5, and 1 C. It is clearly observed that the samples treated at various temperatures have a high specific capacity of approximately 410 mAh g⁻¹ at the current rate of 1 C. Figure 9c also shows that the reversible capacity increases from 581 mAh g⁻¹ to 595 mAh g⁻¹ after two cycles at the heat treatment temperature of 500 °C. The latest research explains that an increase in reversible capacity owing to SiO₂ is not fully active at the beginning of cycling and will be active during next cycling [14,16,17]. The obtained results confirm that the SiO₂/C composite heat-treated at 500 °C produces the best results, owing to its excellent specific capacity, good cyclability, and high performance as anodes for LIBs [18,32,33]. Based on

Table 4. Comparative study of SiO₂ based anode electrochemical performance.

Precursors	Product	Methods	Voltage (V)	Initial Specific Capacity (mAh/g)	Retention Capacity	Rate Performances	Ref.
TEOS	SiO2/Graphene Aerogel	Hydrothermal	0–3 V	453 (half cell)	~100% (300 cycles)	103 mAh/g (5 A/g)	[18]
TEOS	SiO2/C Hollow sphere	Precipitation	0–3 V	400 (half cell)	421% (160 cycles)	-	[17]
TEOS	SiO2/C nanorods	Precipitation	0–3 V	498 (half cell)	95% (100 cycles)	345 mAh/g (1 A/g)	[33]
SiO2 nanoparticle	Lithiated SiO2	Heat treatment	0–3 V	1859 (half cell)	70% (50 cycles)	100 mAh/g (~2 A/g)	[14]
Coal derived Fly ash	SiO2/Graphite	Ball Milling-Heat Treatment	2.7–4.2 V	541 (full-cell)	87% (20 cycles)	410 mAh/g (372 mAh/g)	This Work

, the overall result is considered good since the starting material is far more inexpensive and the process is simple compared to previous reports.

4. Conclusions

In this study, a high-performance SiO₂/C composite was obtained by extracting SiO₂ from coal fly ash, mechanical milling of SiO₂-C at various SiO₂ content, and heating SiO₂-C composites at 400–600 °C. SiO₂/C composites exhibit specific gravity that is higher than that of carbon materials, which confirms that SiO₂ from fly ash acts as an active material and exhibits electrochemical activity. SiO₂/C composites with 10% of SiO₂ and heat treatment temperature of 500 °C shows the best result with the initial specific capacity of 586 mAh g⁻¹ at 0.1 C and the reversible capacity of 87% after 20 cycles. This method is advantageous for mass production owing to its low cost. Furthermore, the obtained SiO₂-C composites exhibit excellent reversible capacity, high initial specific capacity, good cyclability, and high performance of the anode. Based on these results, SiO₂ from coal fly ash is a great candidate for the anode material for LIBs.