Spectroscopic Characterization of LiFePO 4 as Cathode Material for Li-ion Battery Prepared in the Pulse Thermo-acoustic Reactor

: Lithium iron phosphate (LiFePO 4 ) is a cathode material for the rechargeable-lithium batteries. In this paper is presented a novel method of fabrication carbon-coated LiFePO 4 in a pilot reactor built according to the principles of the thermo-acoustic burner of Helmholtz-type. Crystalline powder with a high percentage of LiFePO 4 was synthesized by incomplete combustion, i.e. in the reductive atmosphere, and calcined at 700 °C for 6 h. The obtained samples were characterized by X-ray diffraction, IR and Raman spectroscopy. The aim of this study was to demonstrate the production of the high-quality lithium-ion cathode material by the incomplete combustion. The synthesis of LiFePO 4 is completed during calcination and an ordered structure is attained. Fast synthesis in the reactor (less than 2 s) is achieved due to the reduction in the size of reactant's particles and a huge number of collisions owing to their strong turbulent flow associated with explosive combustion.


Introduction
Battery technology is a core technology for all future generation clean energy vehicles such as fuel cell vehicles, electric vehicles, and plug-in hybrid vehicles. Augmentation of this superior battery technology is essential for deployment of the same in different applications ranging from hybrid electric vehicles to consumer electronics. Improved battery performance depends on the development of materials for various battery components. Rechargeable lithium cell technology is promising for future applications. The lithium exists in its ionic state at the anode and hence the rechargeable lithium battery is called lithium-ion battery. Hu and coauthors has been used powder metallurgy route to fabricate the solid state batteries (SSBs) [1]. SSBs that convert chemical energy into electrical energy have supported power for portable computers, sensors, and electric vehicles.
A large number of different synthetic methods for the preparation of Li-ion cathode materials have been introduced and evaluated. In the case of LiFePO 4 these methods can be divided into two groups and have been analyzed in many review articles [2][3][4]. All solid state or wet processes that have been reported so far for inorganic synthesis were used to make LiFePO 4 , such as solid state reaction starting from all kinds of solid or liquid precursors [5][6][7] or wet chemistry processes including hydrothermal [8], solvo-thermal [9], ion-thermal [10], sol-gel [11], coprecipitation [12], or spray pyrolysis [13]. An interesting technique is also the solution combustion synthesis [14,15].
The LiFePO 4 crystallizes in the orthorhombic system (No. 62) with Pnma space group. It consists of a distorted hexagonal-close-packed (hcp) oxygen network forming 16 octahedral and 32 tetrahedral sites. There are two distinct and differ in size octahedral sites in this material: M1 is occupied by Li, and M2 is occupied by Fe. The M1 sites form linear chains of edge sharing octahedra along the b-axis, while the M2 sites form staggered lines of corner sharing octahedra along the b-axis. The metal atoms can be viewed as occupying bcmetal planes, where the planes are alternatively occupied by each type of metal, i.e., there is a Li-Fe-Li-Fe ordering along the a-axis. It makes possible the two-dimensional Li diffusion between the hcp-oxygen layers [16,17], but b-axis is privileged.
Cations, Li (M1) and Fe (M2), are placed in half the octahedral sites and P ions in one-eighth of the tetrahedral sites [18]. The FeO 6 octahedra are distorted lowering their local cubic-octahedral O h to the C s symmetry. Corner-shared FeO 6 octahedra are linked together in the bc-plane. The tetrahedral PO 4 groups bridge neighboring layers of FeO 6 octahedra by sharing a common edge with one FeO 6 octahedron and two edges with LiO 6 octahedra. The PO 4 bridges provide three-dimensionality to the lattice. Remarkably short O-O bonds at the shared PO 4 and FeO 6 edges screen the cation charges from each other. Structure of LiFePO 4 is illustrated in Fig. 1. The aim of our work was to produce a high-quality LiFePO 4 by a pulse combustion reactor method. The obtained olivine LiFePO 4 was characterized using XRD, Raman and IR spectroscopy.

Materials and Experimental Procedures
The pulse combustion reactor that was used for the synthesis LiFePO 4 is presented in Ref. [19]. The material was synthesized on a reactor setup (Fig. 2), consisting of a Helmholtztype pulse combustor [19].
Reactor is easy to scale, very energy-efficient, can combust stoichiometric fuel and air mixtures and is suitable for the production of powders in nanometric range because of acoustically driven droplet size reduction [20,21]. Up to now, it has been used mainly in the synthesis of oxide materials [22,23], therefore the synthesis of a material which demands a reducing environment is a special challenge. In the LiFePO 4 synthesis, iron has to take the oxidation state of Fe 2+ , wherein the iron in the precursor is in the Fe 3+ state and must be reduced. Reduction is possible in the presence of carbon monoxide at temperatures near 800°C. This means that we need incomplete combustion, which generates monoxide and high enough temperatures for the synthesis. Resulting LiFePO 4 particles for cathode material must also have a coating carbon layer. Two sources of carbon for the formation of the coating are available. The first source is a precursor addition in the form of hydrocarbons, which can't be completely combusted (sugar) and which form a carbonaceous material, increasing the porosity of the particles. Another source is soot generated by the incomplete combustion of acetylene (C 2 H 2 ). The carbon coating enables the electrical current to reach each particle. Carbon should therefore be in the form of graphite, which has a higher conductivity than amorphous carbon. Air is supplied to the combustor by way of a blower through an aerodynamic valve. The flow of air is measured using a thermal mass flow meter. Fuel gas (propane) is supplied to the combustor at a constant flow and pressure using a thermal mass flow meter and controller. The amount of air needed to completely combust the supplied amount of fuel is calculated. According to this calculation a ratio between fuel and air can be set by varying the motor rotating speed of the blower or changing the fuel mass flow. Additionally, propane can be diluted with nitrogen to control the frequency and amplitude of pressure oscillations in the reactor.
In the neck of the Helmholtz combustor there is a secondary gas inlet. This gas inlet is for maintaining or enhancing the reductive atmosphere by injecting acetylene or hydrogen into the stream of flue gases from the chamber of the combustor. The flow of acetylene or hydrogen is controlled with a thermal mass flow meter and controller. The neck of the combustor is coupled to the reactor pipe with a T-section that makes it possible to spray the precursors into the hot zone of the reactor. The positions of thermocouples are presented in Fig. 3.   Fig. 3. The reactor pipe.
The precursor is sprayed with a two fluid nozzle, the spraying gas being 99.9 % nitrogen with a pressure of 1.5 bar and a flow of 45 NL min -1 , measured using a thermal mass flow-meter. The precursor composition is 41,3 g of LiNO 3 , 230,5 g of Fe(NO 3 ) 3 .9H 2 O, 69,0 g of NH 4 H 2 PO 4 , 137,1 g of urea, 78,2 g of sucrose and 46,4 g of NH 4 NO 3 , dissolved in 700 g of deionized water. The reactor is operating under different conditions with the main difference being the frequency and amplitude of pressure oscillations. The sample was synthesized in reductive reactor conditions with low pressure amplitude (10 mbar). XRD analysis show that as prepared material is 71.5 % LiFePO 4 , 19.8 % Li 3 Fe 2 (PO 4 ) 3  Characterization of the sample was carried out by several methods: • X-ray diffraction analysis was performed on X-ray diffractometer (Rigaku Corporation, Japan) at room temperature. CuKα radiation (λ = 0.15418 nm) with a step size of 0.01° in the range of 2θ = 10-60° was used for all samples. The peaks were identified using the Powder Diffraction File (PDF) database created by International Centre for Diffraction Data (ICDD). • The infrared (IR) reflectivity measurements were carried out with a BOMMEM DA-8 Fourier transform (FTIR) spectrometer at ambient temperature. A DTGS pyroelectric detector was used to cover the wave number range from 50 cm -1 to 700 cm -1 .
• The micro-Raman spectra were taken in the backscattering configuration by Jobin Yvon T64000 spectrometer, equipped with nitrogen cooled charged coupled device detector. As an excitation source we used the 532 nm line of Ti: Sapphire laser, with laser power 50 mW. The measurements were performed in the spectrum range 200 cm -1 to 1100 cm -1 .

Results and Discussion
The result of X-ray diffraction measurement is shown in Fig. 4. The diffraction pattern clearly shows that in the tested sample is present only single phase olivine Using the Le Bail profiled refinement and FullProf program lattice parameters for the mono-phase LiFePO 4 are determined and the obtained results are shown in Table I. By the Koalariet computing program [24], using the fundamental parameter approach to the direct determination of microstructural parameters, the size of the crystallites and the microstrain are determined (Table I).
Tab. I Lattice parameters, crystallite size and microstrain.   In an olivine structure, FeO 6 octahedra are linked with PO 4 3− tetrahedron; therefore, the four fundamental modes of PO 4 3− will split into many components due to the correlation effect induced by Fe-O units.
Far-infrared reflectivity spectrum of the investigated sample (open circles), measured at room temperature, is presented in Fig. 5. The solid line in Fig. 5 is the calculated spectrum obtained by the fitting procedure based on 4-parameters, coupled-phonon model for the dielectric function ε(ω) [26][27][28]: Kramers-Krönig (KK) analysis of reflectivity spectra shows ε 2 (ω) and loss function, σ(ω) = -Im(1/ε(ω)). Maximums of ε 2 (ω) correspond to the values of ω TO , and maximums of σ(ω) to ω LO . Values obtained by fitting procedure are in satisfactory accordance with values obtained by KK analysis (except modes at the range edges, where the signal-to-noise ratio is bad). In Table II are given the best fit parameters together with values of ω LO /ω TO obtained by KK analysis for reflectivity spectrum of the sample LiFePO 4 .
Infrared reflectivity spectra of our carbon-coated LiFePO 4 sample, clearly exhibited fundamental modes of PO 4 at 461-495 (ν 2 ) and 572-635 cm -1 (ν 4 ), indicating that the LiFePO 4 was properly crystallized [28]. Below 400 cm -1 are external modes -primarily translation and libration of the PO 4 3ions and translation motions of the Fe 2+ ions. Far-infrared peak at about 225 cm -1 is characteristic of an asymmetric stretching vibration of Li -O bonds. Also, ν 2 modes are a mixture of Li translations and PO 4 bending motions of the same symmetry. Burba and Frech [29] suggest that 506 and 470 cm -1 bands in LiFePO 4 are predominantly Li-ion "cage modes", i.e. Li-translation.
Tab. II Oscillator fit parameters of reflectivity spectra of LiFePO 4 , as prepared in incomplete combustion and resonance mode of reactor and calcined at 700 °C for 6 h.   To explore the surface properties of the LiFePO 4 particles, Raman spectra have been measured; the penetration depth for carbon with Raman spectroscopy is approximately 30 nm [30]. This is one order of magnitude larger than the thickness of the carbon coat deposited at the surface of the LiFePO 4 particles in case of a uniform carbon distribution. Therefore, any screening effect of carbon on the LiFePO 4 spectra is not expected. The penetration depth inside LiFePO 4 is unknown, but it should be small, so that the detector in the Raman experiments collects the signal within the light penetration depth, which basically represents the total amount of carbon and a few per cent of the amount of LiFePO 4 .
Raman spectrum of the investigated sample measured at room temperature in the range of 200-1100 cm -1 is presented in Fig. 6. Strong narrow peaks in the spectrum, especially characteristic peak at about 950 cm -1 , confirm that combustion synthesized nanomaterial is well-crystallized. According to XRD analysis, sample is of pure LiFePO 4 without any trace of impurities (i.e. starting or intermediate compounds). Positions of obtained Raman modes are in agreement with other experimental data [31][32][33].

Conclusion
We have successfully prepared nanosized lithium iron phosphate as Li-ion cathode materials using a pulse combustion reactor method. The new method is fast, environmentally friendly and easy to scale. X-ray powder diffraction (XRD) shows that LiFePO 4 olivine structure is dominant in the form of fine crystallites with average size 142 nm. In the room temperature Raman spectra are observed 12 first-order Raman active modes. An IR reflectivity spectrum is fitted by 18 modes.