Utilization of Coal Fly-Ash derived Silicon (Si) as Capacity Enhancer of Li-Ion Batteries Anode Material

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Introduction
Fly ash is a by-product of the coal combustion process with a crystalline phase often referred to as ferroaluminisilicate. Fly ash contains material components including SiO2 = 59.11%,Al2O3 = 25.82%,Fe2O3 = 5.30%, and CaO = 4.66% [1].As a waste, fly ash is considered unsafe if improperly discarded.Thus, it must still be processed with the precautionary principle to reduce waste or minimization.One of the efforts to reduce the accumulation of fly ash waste is to utilize the silicon content in SiO2 as an active material composite material for lithium-ion battery anode [2].

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Lithium-ion batteries' main components are the cathode, anode, separator, and electrolyte.This research focuses on the lithium-ion battery anode material, which is generally graphite.However, graphite anodes have a low specific discharge capacity of 372 mAh/g [3], so the addition of silicon material is needed as an additional material to increase battery capacity because silicon has a high theoretical capacity of 4,200 mAh/g [4].Silicon material has the advantages of having a large energy capacity, high chemical stability, and environmentally friendly [5].Recent studies showed that nano-structured silicon with unique morphology improved the structural stability of Si.However, silicon precursors, such as tetraethyl orthosilicate (TEOS) or silicon tetrachloride (SiCl4), are highly expensive [6,7].The utilization of cheap silicon sources, like combustion ash, will improve sustainability and reduce the overall production cost of Si [8,9].Using water-based reagents, the hydrometallurgical method extracts and purifies SiO2 from fly ash at low temperatures.Then, the SiO2 material is reduced to Si through the magnesiothermal process under N2 flow, followed by purification.Using N2 instead of Ar or H2 will reduce Si's production and operating costs.The as-prepared Si was used as a capacity enhancer of graphite, a commercially established lithium-ion battery anode.The result of this study will significantly affect the development of energy storage technology while implementing the waste-to-product scheme.

SiO2 Extraction Process from Fly ash
Making silicon (Si) material begins with extracting silica (SiO2) material in fly ash.The extraction process uses an alkaline reaction because SiO2 can react with strong bases such as NaOH.This extraction process aims to isolate the SiO2 content in the form of NaSiO3 compounds and separate SiO2 from other materials contained in fly ash, such as Al2O3, Fe2O3, CaO, and Sulfur.The extraction process used technical grade NaOH with a concentration of 20% at a temperature of 80 ℃ for 3 hours.The reaction can be seen in Equation 1. SiO2 (s) + 2NaOH (aq)  Na2SiO3 (aq) + H2O (l) (1)

SiO2 Reduction Process to Silicon
The reduction process is a reaction to reduce the oxidation state due to the release of oxygen.SiO2 is reduced to Si by adding Mg (Magnesium) in this process.Magnesium added to SiO2 has a mass ratio of 1 1.The material is heated in a muffle furnace at a temperature of 750 ℃ for 2 hours with additional N2 gas flow.The reaction can be seen in Equation 4.

Purification of Si
MgO compounds are impurity compounds that can be separated from silicon using HCl compounds with a mole ratio of HCl to silicon of 1 2. The process is carried out by gradually adding silicon material containing MgO in an HCl solution.Furthermore, heating and stirring are carried out by maintaining the temperature at 60 ℃ for 2 hours.During the reaction, MgCl2 and H2O compounds are produced, which can be separated from silicon by washing.The washing process removed MgCl2 to obtain pure Si.

Si/C composite material and cell fabrication
The anode sheet was made with 5% Si/C composite Active Material (AM).A 5% Si/C composite with a base material of 1 gram requires 0.05 grams of silicon and 0.95 grams of graphite.The anode fabrication process requires additional materials: Acetylene Black (AB), Carboxymethyl Cellulose (CMC), and Styrene-Butadiene Rubber (SBR).Composition of AM: AB: CMC: SBR = 90: 3 : 3: 4. All materials were mixed to form an anode slurry and coated on Cu foil with a thickness of 0.01 mm.

Characterization and Electrochemical Performance Evaluation
TG/DTA (Differential Thermal Analysis/Thermal Gravimetry) analysis was performed to investigate the reduction process.FTIR (Fourier Transform Infra-Red) spectroscopy and Xray diffraction analyses were conducted to evaluate the characteristics of Si.SEM (Scanning Electron Microscope) was used to examine the morphological features of the sample.The as-prepared electrode was assembled into the cylindrical cell.LiNi0.8Co0.1Mn0.1O2electrode and 1 M LiPF6 solution were used as the counter cathode and the electrolyte, respectively.The cell was charged and discharged at the 2.5-4.3V voltage window at 0.05 mA/g current density using a BST8 SERIES battery analyzer.The Differential Thermal Analysis (DTA) analysis in Fig. 1. shows three peaks, two of which are upward and one downward.The first two peaks at "a" region indicate the occurrence of endothermic reactions.Specifically, at 53.93 ℃, endothermic dehydration with an enthalpy value of +463.69J/g occurred due to the presence of entrapped water molecules in the silica matrices.At the same time, the upward-facing peak is an exothermic reaction.The peak at 383.69 ℃ with an enthalpy value of +799.68 J/g confirmed the endothermic decomposition of residual acetic acid.The sharp peak at 651.46 ℃ has an enthalpy value of -452.38 J/g, interpreted as an exothermic reduction reaction of SiO2 to Si.

Structural Analysis of Si and SiO2
The diffraction pattern of silicon has sharp peaks, indicating that silicon has a good crystal structure.The peaks are at angles 2θ = 28.48°;47.3°; 56.08°, and 69,22 ° corresponding to standard silicon [10].The XRD test results of SiO2 derived from fly ash show that the sample has an amorphous characteristic with a main diffraction peak at 2θ of around 22° [11].No obvious impurities were detected in the X-ray diffractograms; thus, both SiO2 and Si samples have high purity.The presence of the silicon phase also confirms that silica reduction via the magnesiothermic process successfully occurred, which is supported by the previous TG/DTA analysis in Fig. 1.

Fig. 3. Infrared Spectra of SiO2 of Silicon Materials
Based on Fig. 3., the FTIR spectra of the Si sample show several compound groups.Si-O groups appear at bending vibrations 861.48 /cm and 457.83 /cm, and Si-O-Si groups appear at 1.100 /cm.OH groups appear at 3,394.5 /cm.These peaks are by the previous study, where the Si-O groups appear at a bending vibration of 820 /cm, Si-O groups rocking vibration of 450 /cm, and OH groups appear in the range of 3,000 -3,500 /cm [12].
Compared with the as-prepared SiO2 sample, the FTIR spectra also confirm the presence of several compound groups.The Si-O group on Si-O-Si appears at symmetric stretching vibrations 800 /cm and 1,100 /cm, the OH group on Si-OH appears at bending vibrations 1,600 /cm and 3,400 /cm, the Si-O-Si group appears at turning vibrations 2,400 /cm [13].
Overall, it can be concluded that the sample has high silicon content.However, the findings It is expected that the SiO2 is mainly present on the surface of the silicon material.Many studies have suggested a post-treatment of the sample using HF etching [14].However, to maintain the Eco-friendliness of the process, the etching using HF is avoided in this study.Based on the SEM analysis in Fig. 4., The surface morphology of SiO2 particles has an irregular shape and a rough surface.At a larger magnification of 2,500x, the SiO2 structure has fibrous grains resembling clouds due to the gelling process [15].Small fiber particles indicate a large surface area of the sample, which is beneficial for electronic applications.Table 1 confirms the composition of the as-prepared SiO2 sample.The sample has high purity and is free of anionic impurities due to the use of acetic acid as the gelling agent.Based on Fig. 5., the surface morphology of silicon is a dense particle with an irregular shape and a rough surface.Still, when viewed with greater magnification (1,000x), the silicon structure has denser grains than SiO2 and has a rock-like shape.The sample is densified, which results from high-temperature processing and the sintering effect during the reduction process [16].Meanwhile, silicon peaks at angles 28.48°, 47.3°, 56.08°, and 69.22°.This shows that the Si/C composite is successfully formed [15].

Charge-Discharge Analysis of Si
Electrochemical performance tests were conducted on a cylindrical cell battery.The anode was a composite material of silicon and graphite with a 5% silicon content.The incorporation of silicon in graphite increases the capacity of the anode, which by adding 5% silicon, the theoretical capacity of the anode, initially 372 mAh/g, is estimated to increase to 563.4 mAh/g.Battery characteristics were evaluated using the charge-discharge method to determine its energy storage capability.In the charging process, the sample is tested up to a voltage of 4.3 V followed by a discharging process until the voltage dropped to 2.5 V.The electrochemical performance of the anode can be indicated by the specific capacity (mAh/g), reflecting the amount of current flowing in a unit of time discharge curve can be seen in Fig. 7, displaying initial specific charge capacity of 600 mAh/g, while the specific discharge capacity is 473.56 mAh/g.The difference in specific capacity between charge and discharge proves a lost capacity of 126.44 mAh/g with an efficiency of 78.92% in the battery sample.The capacity loss is due to the formation of electrode-electrolyte interphase layer on the surface of the active material.This means that the Si can effectively enhance the capacity of graphite by a simple composite formation during the electrode fabrication [18].there is a high decrease in specific capacity, and the capacity lost between charge and discharge is irregular.Cycles 8-19 were conducted at an elevated current rate of 0.2 mA/g showing a constant small decline in specific capacity, and the lost capacity between charge and discharge is not too large, proofing a higher coulombic efficiency of ~100% [19].

Conclusion
Extensive characterization techniques on the SiO2 derived from fly ash confirmed that SiO2 material has an amorphous structure and high purity.In contrast, silicon material has a crystalline structure and has a slight impurity in the form of MgO.The battery capacity with 5% Si / C composite anode has a charge capacity of 600 mAh / g, while the discharge

Table 1 .
Quantitative analysis of SiO2 using EDX

Table 2 .
Table 2 confirms the quantitative and qualitative analysis of the Si sample.Mg indicates the unfinished leaching process during the Si•MgO purification.Since the MgO phase does not appear in the XRD result, the presence of MgO can be considered doping, improving silicon material's electrical properties.XRD test of Silicon Material Composition