Synthesis and Crystal Structure Analysis of LiNiSixP1-xO4/C as a Cathode Material for the Lithium-ion Batteries Application

Lithium-ion batteries are currently the most promising storage device. An electronic conductivity is a key factor that affects the performance of Lithium-ion batteries (LiB). In this work, LiNiSixP1-xO4/C, (x = 0.00, 0.05, 0.10, 0.15, and 0.20) as cathode materials were effectively synthesized by a solid-state reaction technique. To enhance the ionic conductivity of Lithium-Ion Battery application, we used sucrose as a carbon source and Si ion as a dopant in LiNiPO4 cathode material. The effect of Si dopant on its crystal structure, composition, and functional groups existing in cathode material LiNiSixP1-xO4/C are investigated analytically. The samples were characterized by using the X-ray Diffractometer (XRD), X-Ray fluorescence (XRF), and Fourier Transform Infrared Spectroscopy (FTIR). The different amount of Si dopant promotes the structural modification and important role to improve the rate performance of LiB.


Introduction
Energy storage, such as batteries and supercapacitors, is a component to produce a sustainable energy system [1]. The development was influenced by several factors including the active constituent material which can affect its electrical and dielectric properties [2,3]. At this time, one of the rechargeable batteries that attracted the most attention of researchers is Lithium-ion Batteries (LiB). LiB technology has been widely developed in various electronic devices over the past 20 years [4] and intensively applied to Hybrid Electric Vehicles (HEV), Plug-in Hybrid Electric Vehicles (PHEV), and Electric Vehicles (EV) [5]. LiB component consists of cathode, anode, electrolyte, and separator material [6]. One component plays an important role in improving the quality of the battery is a cathode. The cathode is a positive electrode that acts as an electric current when the charging and emptying process occurs in the battery. The cathode material with Olivine phosphate structure is very interesting and specifically it has been studied as cathode LiB material with a high potential energy, abundance in nature, low toxicity, environmentally friendly, has good electrochemical performance that is stable when overloading and has good thermal stability during charging and discharging processes [7,8]. In the group olivine orthophosphates structure LiMPO4 (M = Fe, Mn, Co, and Ni), LiCoO2 and LiFePO4 are currently the  [9]. Air stability of LiCoO2 cathode material is high and able to be produced on a large scale. However, it requires high production costs and has high cobalt toxicity. While the LiFePO4 cathode material includes environmentally friendly materials with low production costs and has good air stability and life cycle. However, the capacity is relatively low which is theoretically 170 mAh/g [10].
Based on the research of Cheruku et al., LiMnPO4 has the highest ionic conductivity compared to other types of olivine phosphate materials structure due to weak Li-O bonds resulting from the largest unit cell volume [11]. However, among the four types of lithium phosphate transition metals with olivine structures, LiNiPO4 has the highest operational voltage of ~ 5.1 V vs. Li / Li + and specific energy density of about 800 Wh/kg [12]. The main advantage of LiNiPO4 is its high stability, is inexpensive and environmentally friendly. However, the electronic and ionic conductivity like another olivine structure cathode materials is insufficiently high. This condition allows to reduce a specific capacity during a high-speed discharge. Many efforts have been made to improve the electrochemical performance and eventually to improve electrical properties by manipulating the active materials through the addition of carbon additives to the olivine matrix, coating the surface of particles with a thin layer of carbon, or reducing particle size [13].
In this study, LiNiPO4 cathode material was synthesized by doping Si element into P and Carbon coating (C) with the chemical equation LiNiSixP1-xO4/C using a solid-state reaction method. Here, microstructure analysis of cathode material was carried out to provide a good understanding of the synthesis of cathode LiNiSixP1-xO4/C material to improve the performance of electrochemical properties in LiB.

Methods
The synthesis of cathode material LiNiSixP1-xO4/C (x = 0 ≤ 0.2) was carried out in two stages, namely the synthesis of LiNiSixP1-xO4 and carbon coating (C) on the prepared LiNiSixP1-xO4. The LiNiSixP1-xO4 material was prepared using are precursor consisting of powder LiOH.H2O, Ni, and SiO2.H2O which are mixed and ground in a mortar to be a homogeneous mixture, and then added a certain amount of H3PO4 as a solvent. The process was continued by heating the mixture in the oven and refining it after drying. Calcination was carried out at 700 o C for 2 hours in an inert gas. The calcined material is then smoothed using a ball-milling tool for 2 hours. The sample was sintered at 900 o C for 6 hours. The sintered product was pulverized and followed by sieving with a 400 mesh. The carbon coating process was carried out by adding sucrose (C12H22O11) into LiNiSixP1-xO4 powder with a ratio of 1: 1. The mixture was then ground using a mortar. The coating process was performed by heating the mixture at 700 o C for 20 minutes. We characterized the material FTIR, X-RD, and SEM to determine the functional group, crystal structure, and their morphology. Figures 1 (a) and (b) show the spectra of functional group bonds from the FTIR LiNiSixP1-xO4 and LiNiSixP1-xO4 /C. Figure 1a shows that in each sample the absorbance peak of the functional group  [8,11] . For Si-doped samples, the Si-O at wave peak appears at a number ~ 748-817 cm -1 [14]. The existing of the main peaks showing that the LiNiSixP1-xO4 sample has been successfully formed. Figure 1b shows similar spectra such as the samples without carbon coating. However, there is another absorbance peak, namely C = C located at a wave number ~ 1593-1596 cm -1 [5]. This shows that carbon coatings have been successfully carried out.  The existence of the impurity phase is possible due to the calcination at a temperature of ~ 800 o C [13]. On the other hand, to identify the success of doping the Si element into P, the analysis was carried out as shown in Figure.    The crystal structure of LiNiSixP1-xO4/C can be determined through analysis using Rietica program. The obtained crystal structure parameters are shown in Table 1. It indicates that the addition of Si doping affects the lattice parameter. The lattice parameter increases with increasing of Si, so the crystal volume increases too. This can increase the diffusion rate of Li ions during the intercalation process. The crystal size was calculated using the Scherrer formula, the results obtained are shown in Table 2.  From the SEMs images, we calculated the particle sizes using ImageJ software. As seen in Figure  4a, and 4b, we found that the crystal size is 1.05 µm and 1.17 µm, respectively. As for Figure 4c, the LiNiSi0.1P0.9O4/C material obtained an average particle size of 0.65 µm. This explains that the presence of Si doping increases the particle size. On the other hand, the presence of carbon coatings can cause particle size to decrease.

Conclusion
The LiNiSixP1-xO4/C cathode material has been successfully fabricated. The functional groups, as well as crystal structure analyses, support the appearance of the phases. The material has an orthorhombic crystal structure. The presence of Si doping gives rise to increase their lattice parameter so crystal volume. The substitution of Si in the P site increases the grain size. The carbon coating, in contrast to Si doping, reduces the particle size.