Effect of Sintering Temperature on Crystal Structure and Conductivity of the CaCO 3 -Doped Li 4 Ti 5 O 12 Anodes from Blood Clam Shells ( Anadara granosa )

ABSTRACT


I. Introduction
The development of lithium-ion (Li-ion) batteries is an interesting research focus as it is very useful in various applications such as mobile phones, computers, and other electronic devices [1].Recently, many efforts have been made to improve its application to Hybrid Electric Vehicles (HEV) and effective energy storage systems [2].The anode is one of the important components that play a role in creating the characteristics of Li-ion batteries [3].
Li4Ti5O12 (Lithium Titanate Oxide) is a potential material as an anode for Li-ion batteries as it has several advantages over commonly used anode materials such as graphite, including during insertion/extraction of Li + ions does not change the structure (zero strain), high operating voltage (1.55 V) ensure safe operation of li-ion batteries and long lifetime [4] [5].However, the poor conductivity of Li4Ti5O12 (< 10 -13 S/cm) is a problem that can limit its rate performance [6].
Many methods have been developed to improve the performance of Li4Ti5O12, including coating with a conductive material and atomic doping such as Ta, N, Br, Ag, Ca, Cu, Zr, and F [7]- [16].Subhan et al. [3] synthesized Ca-doped Li4Ti5O12 using chicken egg shells as Ca source by solid-state method, delivering Li3.9Ca0.1Ti5O12had better electrochemical properties than the Li4Ti5O12 sample.Priyono et al. also prepared Cadoped Li4Ti5O12 with various concentrations of dopant and explored the Ca 2+ doping can significantly improve the electrochemical performance of Li4Ti5O12 [17].In this research, CaCO3 from blood clam shells (Anadara granosa) was used as doping which had the same Ca content as chicken egg shells.It is known that the dominant content in blood clam shells is Ca [18].
The synthesis method and steps will affect the performance of the anode material, leading to various particle sizes and crystal structures [19].Several methods can be used to synthesize Li4Ti5O12 such as microwave, molten salt, hydrothermal, sol-gel, electrospinning, and solid-state method [20]- [29].In this study, the solid-state method was chosen because the process is simple, low-

II. Theory
Anadara granosa or known as blood clams are a type of shellfish in the family Mollusca and are commonly found in Asia, such as Indonesia [30].This shellfish has a high level of productivity and can be processed into various products.In the province of the Bangka Belitung Islands, especially the West Bangka region, it is known that the total production of blood clams was 445.13 tons/year in 2015 [31].The high consumption of blood clams produces a lot of shell wastes.Clam shells are useful in many applications such as adsorbent, catalyst, and hydroxyapatite [32]- [34].In addition, blood clams are natural ingredients that are abundantly available and economical.
Li4Ti5O12 or Lithium Titanate Oxide (LTO) in an anode is known as "zero strain material" because it has negligible structure change during lithium-ion intercalation/deintercalation [35].The structure of Li4Ti5O12 is Face-Centered Cubic (FCC) spinel with lattice parameter sizes ranging from 8.352 to 8.370 Å [36].The performance of Li4Ti5O12 is known to have a good specific capacity and density of 175 mAh/g and 3.5 g/cm 3 , respectively.In addition, it also has a long life cycle of more than 10000 cycles [37].

III. Method Materials
The precursors used in the synthesis of Li4Ti5O12 were LiOH.H2O and TiO2.Blood clam shells or Anadara granosa (see Figure 1) were used as CaCO3 sources for doping.As a binder, the material used was Polyvinylidene Fluoride (PVDF), N-Methyl-2-Pyrrolidone (NMP) was applied as the solvent, and Acetylene Black (AB) was used as the conductive carbon.

Preparation of CaCO3 Powder
First of all, the blood clam shells are cleaned with water and then dried in the sun.After that, clam shells were ground and sieved through a 200 mesh sieve.To ensure that the sample is completely dry, the white powders were heated in an oven at 100 °C for 12 hours.Finally, the CaCO3 powders were obtained and characterized using X-Ray Fluorescence (XRF) analysis to determine the chemical composition of materials and X-Ray Diffraction (XRD) analysis for phase identification.

Synthesis of CaCO3 Powder
CaCO3-doped Li4Ti5O12 were synthesized via the solid-state method.0.1 mol of CaCO3 was used for doping.Firstly, the precursors material which includes LiOH.H2O and TiO2 were grounded to pass through 200 mesh.A mixture of LiOH.H2O, TiO2, and CaCO3 was mixed by mortar until homogeneous.The mixture was calcined at 700 °C with a holding time of 2 hours.Afterward, sintering was performed at temperature variations of 750 °C, 800 °C, and 850 °C with the same holding time for 4 hours.As a result, the CaCO3-doped Li4Ti5O12 was obtained and then characterized using XRD for crystal structure analysis (see Table 1)

Fabrication of CaCO3-doped Li4Ti5O12 Anodes
To fabricate the anodes, CaCO3-doped Li4Ti5O12 powder, PVDF, and AB (80%: 10%: 10%) were uniformly mixed in NMP solvent.The resulting mixture is put into a mold container and heated in an oven at 50 °C until dry.For the EIS measurements, the anode samples were made into squares with a side length of 1.5 cm.Electrochemical Impedance Spectroscopy (EIS) analysis was used to determine the conductivity value of CaCO3-doped Li4Ti5O12.

Characterization of CaCO3 from Blood Clam Shells
XRF and XRD analyses were used to show the characteristics of CaCO3 from blood clam shells.The elemental compositions of prepared CaCO3 from blood clam shells were evaluated using XRF as shown in Table 2.   XRF analysis showed the main component of blood clam shells is Ca, with a percentage content of 97.67%.The amount of impurities (Ag, Sr, Al, and other elements) is very low when compared to the Ca content, which proves that the blood clam shells have high purity.The results of this study were also proven by several studies that showed Ca content of blood clam shells in the range ≥ 90% [38]- [40].
The XRD pattern of CaCO3 from blood clam shells is shown in Figure 2, the analysis using software Match 2! results show that the phase obtained from the sample is aragonite (CaCO3).The major diffraction peaks are observed at 2θ values such as 26.31°, 33.21°, 37.98°, 45.95°, and 52.51° with miller index of (111), ( 012), ( 112 2019) also has a CaTiO3 peak at 2θ of 33.21° [17].Figure 3(b) shows an enlarged (111) plane at different sintering temperatures.It can be observed that the (111) peak shifted to a lower angle with increasing the sintering temperatures, which is indicating an increase in lattice parameters [42].To analyze the effect of sintering temperature on the crystal structure, several crystal parameters were calculated including average crystallite size, lattice strain, lattice parameters, and unit cell volume.The average crystallite size of CaCO3-doped Li4Ti5O12 is calculated by using Debye-Scherrer's equation: Where β is Full Width at Half Maximum (FWHM) and λ is the wavelength of CuKα.
Table 3 shows the increase in sintering temperature will affect the enlargement of the lattice parameters and unit cell volume.This is because, during the sintering process, some ions are converted, leading to an increase in lattice parameters as the sintering temperature increases [43].Furthermore, an increase in the sintering temperatures also leads to a decrease in crystallite size.The sintering temperature is proportional to the amount of energy the atoms receive which affects the crystallite size and atomic bonding [44].At higher temperatures (850 °C), this facilitates diffusion and agglomeration, causing the crystallite size of L-3 samples to become larger than those of L-1 and L-2.The size of crystals gives space for the atoms in the crystal [45].At larger crystallite size, the atoms are close together, so the lattice strain becomes smaller, as shown in Table 3.

Conductivity Analysis of CaCO3-doped Li4Ti5O12
Further analysis of electrochemical properties of CaCO3-doped Li4Ti5O12 was performed by EIS.The EIS measurement aims to determine the conductivity of the anode.In the EIS measurement, using an AC voltage source of 1 V and test range frequency of 4 Hz to 5 MHz. Figure 4 represents the Nyquist plot of the samples and equivalent circuits used for EIS data analysis.The value of charge transfer resistance (Rct) was obtained by fitting the Nyquist plot with the Simplified Randless Cell model using Zview software.
In the Nyquist plot, it can be observed that the spectrum consists of semicircle patterns.The radius of the semicircle indicates the Rct of CaCO3-doped Li4Ti5O12.The smaller the diameter of the semicircle, representing lower Rct, the better the conductivity of samples [46].Figure 4 shows the order of semicircle patterns from smallest to largest for L-2, L-1, and L-3, respectively.This indicates that L-2 has the highest conductivity among the others, which is approved by the conductivity data obtained from the fitting Nyquist plot shown in Table 4.
According to Table 4, the sintering temperature can affect the conductivity of the CaCO3-doped Li4Ti5O12, in samples L-1 and L-2 the conductivity value increases, while in sample L-3 the conductivity value decreases.For higher temperatures (850 °C), agglomeration occurs in the sample so that crystallite size increases.The enlarged size of the crystallite increases the diffusion length and decreases the conductivity value [47].

V. Conclusion
The effect of sintering temperature on crystal structure and conductivity of CaCO3-doped Li4Ti5O12 from blood clam shells was investigated.The smallest crystallite size was obtained at a sintering temperature of 800 °C, which is 46.49nm, which is beneficial for shortening diffusion length and facilitating the electron and ion transport, causing an increase in anode conductivity.The most optimal conductivity was obtained in samples with a sintering temperature of 800 °C with a value of 2.46 x 10 - 4 S/cm.When the sintering temperature is increased to 850 °C, the particles tend to agglomerate and deteriorate the electrochemical properties.

Figure 2 .Figure 3 .
Figure 2. XRD patterns of CaCO3 from blood clam shells ), (221), and (113), respectively.All of the diffraction peaks are well agreed with the reference patterns of CaCO3 (COD No. 4001361).In addition, the highest peaks of 26.31° correspond with the previous study showing the highest peaks at 2θ of 26.22° [18], and 26.10° [41] Crystal Structure Analysis of CaCO3-doped Li4Ti5O12 Figure 3(a) shows the XRD patterns of CaCO3-doped Li4Ti5O12 with sintering temperatures at 750 °C, 800 °C, and 850 °C.The results of analysis using Match 2 software shows that the dominant phase in the sample is Li1.33Ti1.67O4(or spinel of Li4Ti5O12).The planes at (111), (113), and (004) confirmed that Li4Ti5O12 has a cubic structure and perfect accordance with corresponding COD No. 10111098.There is no CaCO3 phase found, which indicates that the doping of Li4Ti5O12 with CaCO3 has been successful.However, there was CaTiO3 phase formation in the samples marked at 2θ values of 33.35° for L-1, 33.33° for L-2, and 33.35° for L-3.The presence of the CaTiO3 phase is caused by Ca 2+ ions that exceed the maximum Saputra, et al.Effect on Sintering Temperature on Crystal Structure …. p-ISSN: 2621-3761 e-ISSN: 2621-2889 doping amount, following the previous study by Priyono et al. (