The Effects of the Addition of Silica Mol Fraction (x = 1.5; 2; 2.5) as a Solid Electrolyte on Ion Conductivity of NASICON (Na1-xZr2SixP3-xO12) Using Solid-State Method

Energy is a very important in modern life and need innovations to develop it. One innovation is the application of energyfor storage devices, such as batteries, capacitors, fuel cells, etc. For 30 years, the application of the NASICON (Na1+xZr2SixP3-xO12) into the NASICON gas sensor material was successfully prepared by using solid-state method. The raw materials such as SiO2, Na2CO3, ZrO2, and NaH2PO4 with a little methanol were mixed in Ballmill equipment. The silica powder was made by the extraction of bagasse ash by using sol-gel method. The x-ray diffraction patternshowedthat the result of silica extraction was amorphous and the NASICON structure wassynthesizedto bemonoclinic. The scanning electron microscopy results indicated that silica had non-uniform surface morphology and the NASICON had good surface morphology only on the form of Na3Zr2Si2PO12. The ionic conductivty of NASICON wasshown on LCR Nyquist plot of the three compositions. The highest NASICON conductivity was found inthe composition of x = 2.0, i.e. 1.142x10-8 S/m.


Introduction
NASICON is an acronym for sodium (Na) Super ionic Conductor, which usually refers to a family of solids with the chemical formula of Na 1+x Zr 2 Si x P 3-x O 12 , 0 < x < 3. In a broader sense, it is also used for similar compounds where Na, Zr, and/or Si are replaced by isovalent elements. NASICON compounds have high ionic conductivities, on the order of 10 −3 S/cm, which is a rival of those liquid electrolytes. They are caused by hopping of Na ions among interstitial sites of the NASICON crystal lattice [1].
The main application that envisaged for NASICON materials is as a solid electrolyte in a sodiumion battery. Some NASICON exhibits a low thermal expansion coefficient (< 10 −6 K −1 ), which is useful for precision instruments and household ovenware. NASICONs can be doped with rare-earth elements, such as Eu, and used as phosphors. Their electrical conductivity is sensitive to molecules in the ambient atmosphere, a phenomenon that can be used to detect CO 2 , SO 2 , NO, NO 2 , NH 3 , and H 2 S gases. Other NASICON applications include as catalysis, immobilization of radioactive waste, and sodium removal from water [ Besides, the production of baggase ash from the waste of sugar cane industries was run over. On the other hand, the resulting ash was rich in silica of around 59%. It was as a raw material ofsilica gel and silica powder production. Generally, the silica gel was produced from the acidification of sodium silicate. Fabrication of sodium silicate was used commercially by reacting materialswhich contain silica, for example, quartz sand, and soda ash in furnaces at temperatures of over 1300 о C [2]. It was clear that this technique consumes large amounts of energy, which can prevent the sugar industry from changing the ash into silica gel. Therefore, it would be advantageous to develop a simple, inexpensive, and low energy consuming method for the production of silica gel from the produced ash so that it can support the use of waste for the economic value-added products. Reported alkali extraction method can be used to increase the concentration of silica from rice husk ash using a low-temperature reaction [3].
This research concerned to analyze the effect of silica (SiO 2 ) addition from an extraction of bagasse ash by sol-gel/alkali method on the formation of NASICON.

Synthesis of Silica Powder.
In this paper, two-step synthesis method was applied to synthesize the silica powder. The first step of the method was extraction. 10 gram of bagasse ash was added into 60 mL NaOH 2 M and then stirred and heated under boiling temperature for 1 hour. Then the extraction result was filtered to separate the extract and ash residue. Secondly, the extract was titrated by HCL for gelation. Next, the silica gel was aged for 18 hours and washed by demineralized water until the salt resulted from the titration procces was not present. The pure silica gel was subsequently heated to degradate the water in an oven for 24 hours [4].

Preparation of NASICON (Na 1+x Zr 2 Si x P 3-x O 12 ).
The NASICON material was produced by preparing SiO 2 , ZrO 2 , NaH 2 PO 4 , and Na 2 CO 3 with proper compositions which were then mixed on a Ballmill with methanol sufficienly. The mol silica was varied to x=1.5, 2, and 2.5 to make the NASICON (Na 1+x Zr 2 Si x P 3-x O 12 ). The ball mill was rotated at a speed of 800 rpm for 4 hours. Then the result of pre-NASICON was calcined on horizontal furnace for 4 hours at 850 о C and was sintered by the same tool for 5 hours at 1000 о C [5].

General Characterization.
The samples were characterized by X-ray Diffraction (X'Pert PRO PANalytical) to know the phase and purity of the composing materials and NASICON. Scanning Electron Microscophy (FEI Inspect S50) was employed to know the morphology of the particle surface of silica and surface pellet of NASICON, and LCR meter was used to know the ionic conductivity of the super ionic conductor of NASICON. 3

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The  The results of x-ray diffraction analysis were shown in Figure 1. The phase of silica powder was amorf and 100% pure without impurities (Figure 1a) and the zirconia powder, sodium carbonat, and sodium dihydrogen phosphate phases were monoclic with suitable properties of conducting materials.

X-Ray Diffraction of NASICON
The results of x-ray diffraction analysis for NASICON which has been formed after the milling process for 4 hours in a ball mill with a rotation rate of 800 rpm are shown in Figure 2.a. The black line shows the diffraction pattern of the sample with the variable of silica mole fraction of x = 1.5. the formed NASICON has a chemical formula of Na 3.12 Zr 2 Si 2.12 P 0.88 O 12 and its crystal structure is monoclinic according to JCPDS 01-084-1317. While the red line shows the diffraction pattern of the sample with the variable of silica mole fraction of x = 2. The formed NASICON has a chemical formula of Na 3.1 Zr 1.78 Si 1.24 P 1.76 O 12 and its crystal structure is rhombohedral in accorandce with JCPDS 01-077-1266 but this NASICON was not fully formed because an unreacting zirconia was still found. The blue line shows the diffraction pattern of the sample with the variable of silica mole fraction of x = 2.5. The formed NASICON has a chemical formula of Na 5.27 Zr 0.5 Si 0.5 P 2.5 O 12 and its crystal structure is rhombohedral according to JCPDF 01-087-0617.
The results of x-ray diffraction test for NASICON has done calcination to renew or change the shape of the crystal structure of the system that has been in the milling NASICON shown in Figure 2. The black line shows the diffraction pattern of the sample with variable silica mole fraction x = 1.5. NASICON formed has a chemical formula Na 3.12 Zr 2 Si 2.12 P 0.88 O 12 and its crystal structure is monoclinic according to JCPDS 01-084-1317. These results are similar to the diffraction test results before calcination. While the red line shows the diffraction pattern of the sample with variable silica mole fraction x = 2, NASICON formed has a chemical formula Na 3 change. Such changes occur at concentrations of zircon and silica that had not yet reacted becomes react back when done calcination. On the blue line shows the diffraction pattern of the sample with variable silica mole fraction x = 2.5, NASICON formed has a chemical formula Na 3.4 Zr 2 Si 2.4 P 0.6 O 12 and its crystal structure is rhombohedral according dengna JCPDF 00-036-0351. In addition to the mole fraction x = 2, variable mole fraction x = 2.5 also changes the chemical formula of sodium and phosphorus concentrations decline and the increase in the concentration of zirconia and silica. But the system did not change the crystal structure of monoclinic structure. In this variable there were silica and zirconia which have reacted yet.

Morphology of NASICON
Scanning electron microscopy (SEM) analysis was conducted on the powder NASICON that has heated and compacted to form pellets. The Figure 3.shown that the surface of the pellet did not have many changes in morphologies of before and after calcinating at 5000x magnification. However, the pellet surface becomes smoother and clearer morphology. On silica mole fraction x = 1.5, NASICON particles were unevenly distributed, not uniform and it was bigger than a fraction x = 2.0 and x = 2.5 which is smaller in size and uniform across the surface of the pellet. The mole fraction x = 1.5 is still appeared a raw material that did not react perfectly to the shape of particles obtained NASICON was no difference, irregular shape and are elongated fiber. Figure 4 shown the shape of NASICON surface with x = 2.0 was impresive. The shape like a small sheet with diameter 500 nm. There were difference between before and after calcinating, from unregular shape to be sheet shape [7]. The figure was taken on magnification 75.000x.

Ionic Conductivity of NASICON
To proof of NASICON that was made is Superconductor, we must characterized with LCR meter. The result of this analysis shown in Table 1. The table shown that the higher conductivity of NASICON which is obtained on fraction of mol silica x = 2.0 as big as 1.142x10 -8 S/m [8]. From the surface in this variable (Figure 4.) we could know that the transfer of ions sodium could move smoothly in charge and discharge. Figure 5 shown that the nyquist plot of NASICON with three variable. The optimum charge transfer was took placed in x = 2.0 with 2 semicircle. There were two phenomenom in this figure, is chrage transfer on semicircle graph and diffusion on the body of NASICON on line straight graph.

Conclusion
The addition of silica on the structure of NASICON can be impact on improving ion conductivity and uniformity of the surface structure. Silica was well prepared by sol-gel method had amorphous structure and the phase of NASICON were monoclicnic structure. They were the result of x-ray diffraction analysis. The greatest value of ionic conductivity was 1.142x10 -8 S/m from x=2.0 from LCR analysis. The stable form from the effect of silica addition is Na 3 Zr 2 Si 2 PO 12 by x=2.0.