Local Structure and Magnetism of LiFeSi0.01P0.99O4/C as a Cathode Material on Lithium- Ion Battery

1Department of Physics, Faculty of Science and Data Analytics, Institut Teknologi Sepuluh Nopember, Surabaya 60111, Indonesia 2Synchotron Light Research Institute, Moo 6 University Ave, Muang, Nakhon Ratchasima 30000, Thailand 3Graduate School of Engineering and Science, Shibaura Institute of Technology, Saitama 337-8570, Japan 4Meson Science Laboratory, RIKEN Nishina Center, Wako, Saitama 351-0198, Japan


Introduction
Lithium-ion batteries have stable cycle performance and higher energy density, so promising as power system that have been used in electrical vehicles, portable devices, grid energy storage, etc. [1]. The main component in lithium-ion batteries is cathode material since it is related to the battery capacity, cycle life, and safety. One of cathodes being investigated and commercialized is LiFePO4 that has intrinsic structural and chemical stability so that it is safe and has long cycle life. However, LiFePO4 has poor electronic conductivity (~10 -9 S cm -1 ) and low ionic diffusivity (10 -13 to 10 -16 cm 2 s -1 ) [2]. Furthermore, to improve the performance of LiFePO4, there are some techniques that can be used, such as reducing particle size, carbon coating, and atomic doping [3].
Reducing particle size is one of the effective ways to increase electronic conductivity. It causes smaller particle size, so lithium ions migrate faster in insertion process because of the reduction of transport distance [4]. Carbon coating is one of the most important techniques used to improve conductivity. The carbon source used as coating such as acetylene black, sucrose, and glucose; however, glucose is better to increase specific surface area in LiFePO4 as a composite [5].
Previously, Amin et al. studied that doping Si to site P in single crystal LiFePO4 was thermodynamically possible. There is no abrupt effect of silicon doping to the crystal structure of LiFePO4 [6]. Furthermore, doping Si to site P in LiFePO4 is able to improve the electronic conductivity. The measurement to verify the sample quality and improvement of the performance of cathode LiFePO4 along with doping are very important such as X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Raman spectra, chargedischarge (CD), cyclic voltammetry (CV), and Electrochemical Impedance Spectroscopy (EIS) [7,8]. In further study, X-ray absorption spectroscopy (XAS) is a powerful technique to probe electronic structure, local structure, and identify the presence of small amount doping on LiFePO4. The information about oxidation state is investigated by X-ray Absorption Near Edge Spectroscopy (XANES) and local structure is investigated by Extended X-ray Absorption Fine Structure (EXAFS) [9]. Furthermore, the use of magnetic characterization in cathode battery materials, especially LiFePO4, is also significant to monitor the sample purity and structural defect of materials [10]. In this study, we investigate the local structure and magnetism of LiFePO4 with doping Si = 1% by X-Ray Absorption Spectroscopy (XAS) and SQUID magnetometer.

Experimental Methods
LiFePO4 with doping Si 1% was synthesized via a solid-state reaction. The starting material to obtain LiFeSi0.01P0.99O4 as precursor were Li2CO3 (Merck, 99.9%), Fe2O3 (technical, 99%), (NH4)2HPO4 (Merck, 99.9%), and SiO2 (Merck 99.9%). The precursor crushed into powders with mortar continued by a ball-milling process with zirconia balls (the precursor ratio: zirconia ball is 1:5) for 10 hours at 150 rpm and dried at 80°C. After that, the sample was annealed at 700°C for 10 hours at nitrogen atmosphere. The sample was coated with 11 wt% glucose C6H12O6 (technical, 99%) as carbon coating and continued by the carburation process at 450°C for 2 hours under nitrogen atmosphere so that produced LiFeSi0.01P0.99O4/C (LFP-Si1%). The details of experimental and characterization of XRD, SEM, cyclic voltammetry, and charge/discharge can be referred to the previous work on the reference [8].
In this study, the sample was measured using XAS instruments, including XANES and EXAFS at beamline 8 in Synchrotron Light Research Institute (SLRI) Thailand. Fe K-edge spectra were collected in the range energy 7 keV to 8 keV [11]. The XANES and EXAFS data were analyzed with ATHENA and ARTEMIS programs by IFEFFIT [12]. In addition, to probe magnetic properties of LFP-Si1%, the SQUID measurement was performed with the temperature range of 2-300 K under a magnetic field of 10 kOe [13].

Results and Discussion
The information of oxidation state from XANES data has been processed using ATHENA program. Fig.1(a). displays the normalization absorption Fe K-edge XANES spectra of LFP-Si1% with data standard including Fe foil (0), FeO (+2), Fe2O3 (+3), and FePO4 (+3) as compared with the LFP-Si1%. The regions of XANES data are pre-edge (symbolized (*)) that explain the transition electron from the orbital 1s to the 3d from the Fe absorber and the edge (symbolized (#)), which corresponds to the first derivative normalized, indicating the minimum energy needed to remove the photoelectron from the absorbing showed in Fig.1(b). The pre-edge and edge region can be used to determine the oxidation state of the sample. The increasing of oxidation state is related to the higher absorption edge value. The energy absorption edge (E0) of LFP-Si1% is 7124.92 eV in between FeO (E0 = 7120.02 eV) and Fe2O3 (E0 = 7125.68 eV) that have oxidation state 2+ and 3+, respectively. The prediction of oxidation state can be estimated by interpolation technique using E0 value of LFP-Si1% in comparison with that of standard data Fe foil, FeO, Fe2O3, and FePO4; which result in oxidation state of LFP-Si1% about +2.64.
In addition, to confirm the oxidation state composition of LFP-Si1%, linear combination fitting (LCF) in ATHENA was used. The standard data used in LCF are FeO and Fe2O3 since the fitting using these two compounds obtained the best fitting result. The LCF result of the composition of oxidation state Fe2O3 (3+) is 73.3% and FeO (2+) is 26.7%. The analysis using interpolation exhibits good agreement with LCF result since the oxidation state of multivalence Fe is in between 2+ and 3+ [14][15][16]. The multivalence of Fe, which triggered the existence of a small magnetic polaron, is responsible for the interplay between electronic conductivity and magnetism in LiFePO4 [17]. The analysis to study local structure of LFP-Si1% can determine the nearest neighbor and interatomic distance using Fe K-edge of XAS. The fitting of EXAFS data analysis prefers structure LiFePO4 model taken from crystallography.net with Crystallography Open Database (COD) ID 1101111. The model of LiFePO4 (olivine) doping Si (LFP-Si1%) in EXAFS fitting obtained from the replacement of scattering paths P1 with Si in ARTEMIS program. Fig.2 shows the EXAFS data fitting of LFP-Si1%. In Fig. 2, the upper curve is the magnitude of Fourier Transform (FT) EXAFS data and the lower curve is either real or imaginary part of the FT data to determine radial distance in R-space [15,18]. Based on the analysis of Fourier transform (FT) of EXAFS spectra of the sample presented in k 3 χ(k) in Fig.3, the nearest neighbors in surrounding atom Fe consist of: the main peak with high intensity confirms as Fe-O bond and the second and third peak with lower intensity confirm as Fe-P and Fe-Fe bonds, respectively. The local structure of LFP-Si1% results with output parameters (CN, σ 2 , and R-factor) to obtain the distance between Fe and nearest neighbor are presented in Table 1. The local structure of LFP-Si1% confirms Fe binding six atom O in octahedral sites (FeO6) and minor changes in the interatomic distance through distortion were found compared to that LiFePO4 without doping based on the previous studies [9,19]. The distortion atom of LiFePO4 by Si doping could affect its structural stability, which further influences its electrochemical performance. This can be a reason for the enhancement of electrochemical performance of LiFePO4 with silicon doping, as reported by Zainuri et al. [8].    Figure 4 displays the temperature dependence of inverse magnetic susceptibility, χ -1 , of LiFeSi0.01P0.99O4/C under an applied magnetic field of 10 kOe. There are two anomalies observed from the temperature dependence of χ -1 . One is ~51 K which indicates the antiferromagnetic ordering temperature (TN) of olivine LiFePO4 as also reported by Julien et al. [20], while the other is ~28 K which could correspond to nasicon Li3Fe2(PO4)3 [21]. The existence of Li3Fe2(PO4)3 as a secondary phase was also confirmed from the XRD results in Ref [8]. It has been reported that nasicon Li3Fe2(PO4)3 is the most common impurity in LiFePO4 [10]. Accordingly, the temperature dependence of the magnetic susceptibility was analyzed by using the Curie-Weiss law. The Curie-constant (C), effective magnetic moments (µeff) and Curie-Weiss temperature (Θ) were estimated to be 3.44 emu.K/mol, 5.23 µB and ˗ 88.14 K, respectively. The large value of magnetic moments is attributed to the presence of Fe ions. In olivine LiFePO4, the purest sample has µeff = 4.98 µB, while the value of µeff in such impure LiFePO4 is estimated to be in the range of 4.9˗5.5µB [10]. It is related to the total spin in LiFePO4 with S=2; hence the spin-only value is ~4.9 µB while the presence of orbital contributions (S = 2; L = 2) results in µeff = 5.48 µB. The high-spin in LiFePO4 is able to trigger the incomplete quenching of orbital moment and further result in the orbital contributions which is sensitive to the distortion of octahedral symmetry of FeO6. The silicon doping on LiFePO4 possibly affects further the distortion in relation to its atomic distance as shown in XAS result which could influence its electrochemical performance. The characterization techniques to study magnetic properties on batteries, especially to highlight their sensitivity to the presence of impurity and structural defect, even beyond the XRD technique, is the goal for the next study.

Conclusions
The analysis of LiFeSi0.01P0.99O4/C composites as lithium-ion battery cathode studied by XAS has been done in this study. The multivalence of Fe with oxidation state 2+ and 3+ was obtained in LFP-Si1% confirmed by linear combination fitting (LCF) and interpolation technique. Furthermore, silicon doping in LiFePO4 could generate the changes in interatomic distance of the sample which further affect the electrochemical performance. The nearest neighbor from Fe K-edge as central absorber was found as Fe-O bond with higher intensity and Fe-TN