Synthesis and Evaluation of Olivine Material Coated with Carbon

. Lithium iron phosphate, LiFePO 4 , was synthesized by hydrothermal process and subsequently coated with carbon by the thermal decomposition of acetylene gas. The products were characterized by XRD, SEM, and TG-DTA. As-synthesized LiFePO 4 was submicron sized plate-like particle. After heating at various temperature in nitrogen atmosphere, the particle size and the crystalline sizes were grown with increasing the heating temperature. Above 700 ° C, the grain growth was remarkably. Carbon coating temperature was set at 500-600 ° C because of fine particle and good crystallinity. As a carbon raw material, acetylene gas was flowing to the as-synthesized LiFePO 4 in nitrogen atmosphere, and the LiFePO 4 /C composite was obtained. TG curves showed weight loss above 500 ° C, which was thought to be associated with carbon layer composition.


Introduction
LiCoO 2 has been used as a cathode material for lithium ion batteries. Usage of cobalt prohibits to reduce the material costs because of scarcity. The amount of needed lithium ion battery increases significantly for the application of EV and energy storage system. Then, the development of Co-free cathode material is quietly important. Lithium iron phosphate, LiFePO 4 , is the material, in which iron having high number of Clark is rich in resources, and is inexpensive. Padhi et al. reported that polyanion material like as LiFePO 4 and Li 2 FeSiO 4 , showed the charge-discharge property as cathode for LIB.
[1] LiFePO 4 shows 3.5 V v.s. Li of reversible potential and 170 mAhg -1 of theoretical capacity. These values are the energy density comparable to LiCoO 2 . LiFePO 4 has orthorhombic crystal olivine type structure. The crystal structure of LiFePO 4 is showed in Figure 1. FeO 6 octahedra and PO 4 tetrahedra have the structure that a ridge shared one side, and Li arranges it in a b-axis. PO 4 tetrahedra is strongly connected by covalent bond, and it restrains the oxygen decomposition. The structure stability of produce for LiFePO 4 the safety. On the other hand, ionic conduction for LiFePO 4 is difficult because a Li ion in structure is localized electronically. The discharge reaction of LiFePO 4 is showed in the following equation.
In the discharge reaction, FePO 4 phase is formed by deintercalation of the lithium ion. With the biphase reaction of LiFePO 4 and FePO 4 , equilibrium potential is seen in around 3.5V. Both structure is similar, and cycle capability with charge-discharge reaction is good because the symmetricalness of both does not change. As a model of these reaction, domino-cascade model is proposed. [2] In a border between LiFePO 4 and FePO 4 with the disorder of the atom placement, electronic conductivity is relatively high, and it is thought that Li + is deintercalated from there. The conductivity of 10 -8 -10 -9 S/cm for LiFePO 4 , however, is very lower than that of 10 -2 S/cm for LiCoO 2 . [3] Therefore, the problem for commercial use is this low conductivity, and the improvement of conductivity can lead to the good rate capability. In order to improve the conductivity, many researchers have been tried. First, focused on the material composition, it reported that the multi cation doped LiFePO 4 showed the good rate capability. It was because that the doped cations, such as Mg 2+ , Al 3+ , Zr 4+ , Nb 5+ and W 6+ , reduce the volumetric resistance. [4] On the other hand, the microstructure for the electrode materials is also important. If the electrode material makes a fine particle, its reactivity increases due to the improvement of the supecific surface area. In addition, the diffusion length of the ion is shortened by microparticulation, and this is quietly advantageous. There were the reports focused on the morphology with high performance. [5] In order to obtain the finer particle, a various synthesis method has been examined. A. Yamada reported that the optimization of solid state reaction conditions produced a fine particle, and it showed a good performance. [6] However, the diffusion necessary for a complete reaction needs the condition at high temperature for long-time. As the synthesis method at lower temperature, many methods such as hydrothermal method [7], sol-gel method [8,9], and splay pyrolysis method [10] are reported. In this study, using the hydrothermal process, the synthesis of LiFePO 4 composed of a fine particle was attempted.
In order to cope with high rate charge-discharge reaction, LiFePO 4 needs not only the microparticulation but also the composition with conductive assistant. Coating on LiFePO 4 particle surfaces is suitable to make a little conductive assistant in the composition methods. The conduction layer has two kinds of an electronic conduction and the ionic conduction. B. Kang reported the ionic conduction layer. [11] The amorphous Li 4 P 2 O 7 phase as the ionic conduction layer was precipitated by making non-stoichiometric composition. This amorphous phase prohibit the grain growth, and finer particle would be obtained. As the result, the obtained active material showed good property with 130 mAh/g of the discharging capacity at high rate. On the contrary, as the electronic conduction layer, carbon coating has been examined.
There are various kinds of raw carbon material and that composition methods. Then, the LiFePO 4 /C composite produced by heating the mixture of the active material and ractose in inert atmosphere [12], and depositing carbon from heat decomposition of propylene gas. [13] The products by wet method are easy to be affected to the crystalinity and the grain growth among heating process. In this research, LiFePO 4 was synthesized by hydrothermal method, and the products was coated with carbon by the thermal decomposition of acetylene gas. The influence of carbon coating process to LiFePO 4 was examined.

Synthesis of LiFePO 4 by hydrothermal process
LiOH.H 2 O, (NH 4 ) 2 HPO 4 and FeSO 4 .7H 2 O (Wako Chemical) were used as starting materials. Each starting materials were weighted to become a molar ratio of Li: P: Fe = 2: 1: 1. After measurement, they were put into distilled water in Teflon vessel and vigorously mixed for a few minutes. Teflon vessel with mixed solution was encapsulated into autoclave and was hydrothermally treated at 200°C for 24 hours. Obtained slurry was filtrated and then dried at 50°C for overnight.

Heat treatment of LiFePO 4
LiFePO 4 synthesized by hydrothermal treatment was heated in nitrogen atmosphere. 0.5g of LiFePO 4 was measured, and it was put in carbon crucible. Carbon crucible was moved in a furnace. After the furnace was evacuated slowly, nitrogen gas was carried out with flow 5L/min for 30min. Then, the flow quantity of nitrogen gas was controlled 0.2L/min. The furnace was raised up to target temperature (T=400 -800°C) with heat rate of 4°C/min and kept it for 2 hours.

LiFePO 4 /C composites by thermal decomposition
Rotary kiln (Takasago Industry Co., Ltd.) showed in Figure 2 was used for the manufacture of the LiFePO 4 /C composite. 5g of LiFePO 4 was measured, and it was put in carbon capsule for product. Capsule was rolled at the rate of 1rpm. In order to reduce the oxygen concentration in the capsule, N 2 gas was carried out with flow of 5L/min for 15min. Then, 0.2 L/min of acetylene gas was flowing to the capsule through a pipe of the alumina. The capsule was heated externally at 550°C and 600°C for 1 hour, respectively.

Evaluation
The crystal phase of samples was indexed by XRD (UltimaIV, Rigaku Co., Japan). Scans were performed at 2 = 10-40°with scan rate of 4°/min using Cu-K radiation. In order to estimate the lattice parameter of samples, internal reference method was used using pure Si powder. The morphology was observed by FE-SEM (s-4500, Hitachi, Japan) with applied voltage of 15 kV. Thermal gravimetric analysis was carried out under air atmosphere between room temperature and 700°C, and the flow rate of the synthetic air was 2 mL/min.

Synthesis of LiFePO 4 by hydrothermal process
LiFePO 4 was synthesized by hydrothermal process at 200°C for 24h. XRD patterns of the products were showed in Figure 3. The diffraction peaks attributed to olivine-type structure (JCPDS#83-2092) were shown, and the other peaks considered the secondary phase like Li 3 PO 4 and Fe 2 P were not confirmed. So, the products by hydrothermal method was identified as olivine-type CMPSE2017 structure. The lattice parameter estimated by inter reference method was a=10.336Å, b=6.002Å, c=4.695Å. This result was good agreement with JCPDS value (a=10.3340Å, b=6.0100Å, c=4.6930Å). In the case of hydrothermal process, it was reported that the lattice parameter of LiFePO 4 depended on the reaction condition, especially reaction temperature. [14] At higher reaction temperatures, the unit cell volume was closer to the value of 290.5Å 3 . The unit cell volume of the products by hydrothermal process was 291.3Å 3 . As this difference was thought to be case by the crystal purity, the products by hydrothermal process in this study would have the relatively high crystallinity. In addition, the crystalline from (311) plane was 35 nm. Therefore, using of this assynthesized LiFePO 4 , the heat-treatments for LiFePO 4 and the subsequent synthesis of LiFePO 4 /C composite was attempted.

Heat treatments of LiFePO 4
The as-synthesized LiFePO 4 was heated at 400-800°C in nitrogen atmosphere. Heating process is affected to the crystalline and the grain growth, so that is important role to influence the last products. The influence of heating process was examined. In Figure 4, the crystal phase of the products heated at various temperature in N 2 atmosphere was identified by XRD analysis. After heating, no diffraction peaks of the secondary phases except the peaks attributed olivine-type phase was appeared. In addition, the peak intensity increased with the higher temperature. Therefore, it was thought that the decomposition of LiFePO 4 or the phase transformation due to heating process was not occurred, after heattreatments, but the crystallinity was well done. Next, the crystalline sizes were estimated by Scherar equation with the following, and the result was plotting in Figure 5.
The peaks belonged to nine lattice plane, (200), (101), (210), (201), (020), (301), (311), (121) and (410), were used for analysis. In (311) plane, the crystalline size was 41nm at 400°C larger than that of as-synthesized LiFePO 4 . The crystalline size tended to be increased linearly with heating temperature. At 800°C, 84nm of the crystalline size was estimated. Similarly, the same tendency was seen with the other diffraction peaks. In addition, the remarkable difference was appeared in the range of 600°C to 700°C.
In Figure 5, SEM images of the heated products was shown. The particle size of the as-synthesized LiFePO 4 was about 0.5m and the morphology was plate-like particle. Finer particles were also observed in a part. There was no effect of particle size on longer reaction time, so no growth particle was generated during heattreatments. Next, the size of samples at 400°C was about 0.5m similar to no-heated samples and the shape became the round particle without finer particles. In addition, the particle size increased remarkably with heated temperature above 600°C. At 800°C, the coarse particle was observed, and the particle size was up to 1m. From these observation, it was thought that the grain growth occurred above 600°C. As a result, heating process led to the improvement of the crystallinity. Below 600°C, amorphous phase or amorphous layer on particle surface was crystalized and the grain growth was not remarkable. Above 600°C, the grain growth was occurred, and it led to crystalline improvement.

LiFePO 4 /C composites by thermal decomposition
The LiFePO 4 /C composite was synthesized by the thermal decomposition of acetylene gas. With flowing of acetylene gas, the as-synthesized LiFePO 4 was heated at various temperature for 1 hour. The reaction temperature was settled at 550°C and 600°C which the crystallinity was improved and no grain growth occurred. XRD patterns of the products thermal-decomposed at 550°C and 600°C with flow of acetylene gas were showed in Figure 6. The diffraction peaks were attributed to olivinetype structure, and the secondary phase were not appeared. During this thermal decomposition, no oxidation was occurred for obtained olivine. SEM images were shown in Figure 7. The particle size was about 0.6m. At both 550°C and 600°C, no grain growth during thermal decomposition was occurred remarkably. But, whereas the round particle was observed for samples at 550°C, the square shape was appeared partially at 600°C. The thermal decomposition of acetylene gas was exothermal reaction of 228kJ/mol. The morphology change of samples that morphology of squared shapes was similar to that at higher reaction temperature was thought to be related with exothermal reaction.
Thermal stability characterization of the obtained LiFePO 4 /C composite was examined in air condition. TG-DTA curves were draw in Figure 8. In air, LiFePO 4 was oxidized with following the reaction.
[15] Estimated total weight gain is 4.2%. In case of the assynthesized LiFePO 4 , after initial weight loss associated to loss of water under 250°C, TG curves showed weight gain of 2.8%. From DTA curves, the exothermic peak was observed at around 300°C, and the oxidation would be occurred. On the other hand, initial weight loss was not appeared for LiFePO 4 /C products. Then, weight gain associated to the oxidation started at around 300°C and weight loss started at around 500°C. This weight loss was thought to be caused by the burning of carbon phase. Therefore, the LiFePO 4 /C composite were successfully synthesized. In addition, the oxidation starting temperature was different from the LiFePO 4 /C heated at 550°C in that heated at 600°C. Thus, it was thought that the oxidation reaction was dependent on coated carbon layer on LiFePO 4 /C composites.

CMPSE2017
In this study, LiFePO 4 was synthesized by hydrothermal method, and the products was coated with carbon by the thermal decompositionof acetylene gas. The influence of carbon coating process to LiFePO 4 was examined. The obtained LiFePO 4 samples was crystaline phase and lattice parameter was a=10.336Å, b=6.002 Å, c=4.695Å. Heating process was influenced to the microstructure and the crystallinity. Below 600°C, amorphous phase or amorphous layer on particle surface was crystalized. Above 600°C, the grain growth was occured, and it led to crystalline improvement. Carbon coating temperature settled at 500-600°C because of fine particle and good crystalinity. TG-DTA curves of the LiFePO 4 /C products showed weight loss above 500°C, and it thought to be associated with carbon layer composition.