Effect of Different Calcination Temperatures and Carbon Coating on the Characteristics of LiFePO4 Prepared by Hydrothermal Route

The characteristics of lithium iron phosphate (LiFePO4) prepared via hydrothermal route and calcined at various temperatures have been examined. Calcinations were performed at temperature variations of 500, 600, and 750C for 5 hours. The properties were characterized through thermal decomposition, structure, morphological and electrical properties. Flake-shaped pure LiFePO4 and LiFePO4/C was successfully synthesized with the addition of 5 wt.% carbon black. The results showed that the addition of carbon effectively protected the material from oxidation and grain growth. The optimum calcination temperature was obtained at 750C with flake diameter of 80 nm and average length of 427 nm. The measured conductivity of the carbon coated LiFePO4 (2.23 x 10 S/cm) was much higher than that of the as-synthesized LiFePO4 (5 x 10-7 S/cm). The battery performance was obtained with a stable voltage ranging from 3.3 to 3.4 volts. KeywordCarbon coating. Hydrothermal, LiFePO4, Cathode, Lithium ion battery


II. EXPERIMENTAL
A. Synthesis of LiFePO 4 The synthesis of LiFePO 4 was performed via hydrothermal route. The starting materials LiOH, FeSO 4 ·7H 2 O and NH 4 H 2 PO 4 were purchased from Merck and were used as precursors with no further treatment. The precursors were weighed in an analytical balance with a molar ratio of Li : Fe : P = 2 : 1 : 1. Lithium hydroxide was dissolve to form 0.05 mole aqueous solution and was mixed with 0.025 mole of NH 4 H 2 PO 4 under magnetic stirrer agitation to form a white suspension. A solution of 0.025 mole FeSO 4 ·7H 2 O was added drop wisely into the suspension and stirred for 45 minutes to form a bluish green suspension. This green suspension was quickly transferred into 100 mL Teflon-lined stainless-steel autoclave and was heated at 180°C for 20 hours. The resultant light bluish green precipitation was collected and washed by distilled water for several times and lastly by using deionized water. The precipitates were then dried at 80°C in an oven for 4 hours before further treatment and characterization.

B. Preparation of Carbon Coated LiFePO 4
The as-synthesized LiFePO 4 powder from previous step was mixed with 5 wt.% carbon black (CB) and the mixture was ball milled until homogenized and labelled as LiFePO 4 /C. Some of the mixture were analysed using thermal analyser (Shimadzu DTG-60 Simultaneous Measuring Instrument) for thermal decomposition behaviour. The remain LiFePO 4 /C was subsequently calcined at various temperatures 500, 600 and 750 °C for 5 hours under nitrogen atmosphere. For comparison, the as-synthesized LiFePO 4 was also prepared by the same procedure but with no carbon addition and labelled as LiFePO 4 . X-ray diffraction (XRD, Pan-Analytical) measurements were conducted using Cu-Kα at 2θ 10-70° and the diffractograms were analysed using JCPDS database as a reference. Scanning electron microscope (SEM, FEI Inspect F50) was used to observe particle morphology and size distribution.

C. Battery Preparation
The carbon coated LiFePO 4 from the previous step was used for conductivity measurement and was performed using an electrical impedance spectroscopy (EIS, Hioki LCR 3532-50) from 1 gram of the active material prepared in cylindrical pellet with 0.9 cm diameter pressed at 6 MPa. For the electrochemical test, 80 wt.% of the as-prepared LiFePO 4 was mixed with carbon black and poly-vinylidene fluoride (PVDF) with a ratio of LiFePO 4 : CB : PVDF = 80 : 10 : 10 in N-methyl pyrrolidone (NMP) solvent inside a vacuum mixer. The mixture was applied onto an aluminium sheet as a current collector (MTI) and dried. After drying, the sample was hot-rolled and heated in a vacuum oven at 80°C. The battery was prepared in the forms of a coin using an Li metal anode for half-cell and LiPF 6 was used as an electrolyte. The cell was then tested through charge/discharge performance using an MTI battery analyser.

III. RESULT AND DISCUSSION
At dissolving process during the initial stage, the following reaction may take place, 6 LiOH (l) + 3 NH 4 H 2 PO 4 (l) + 3 FeSO 4 .7 H 2 O (l)  Li 3 PO 4 (s) + Fe 3 (PO 4 ) 2 (s) + 13 H 2 O + 3 Li + + 3 NH 2 -+ 3 HSO 4 -+ 3 H + (1) Visual appearance of the resultant reaction is bluish green suspension as an indication of Li 3 PO 4 and Fe 3 (PO 4 ) 2 . These compounds are metastable and will act as intermediate compounds of LiFePO 4 . The equimolar of Li 3 PO 4 and Fe 3 (PO 4 ) 2 is a key factor to obtain pure LiFePO 4 formation in the precursor either in a solid state or a wet chemical method and could only be achieved under excessive Li + molar condition [25]. In the autoclave, at reaction temperature of 180C for 20 hours, the initial precursor suspension will react to form amorphous LiFePO 4 and H 2 SO 4 (l) according to the expected reaction below: (2) As can be seen in the reaction, other reaction products Li + and NH 2 excess dissolving in strong acid supernatant H 2 SO 4 (l) should be remove through the washing process. The bluish green precipitate of amorphous LiFePO 4 was then collected and dried and subsequently was calcined to obtain pure LiFePO 4 .
The aforementioned reaction behavior could be observed under thermal decomposition. In this work, samples were characterized using a thermogravimetric analyzer at a heating rate of 10°C/minutes from room temperature to 850 °C under nitrogen environment. Visual observation of the results showed that LiFePO 4 /C has a dark colour, whereas LiFePO 4 has red colour as an indication of oxidation. Figure 1 shows thermal behavior curves for both samples.
As can be seen in Fig. 1a, the TG curve of LiFePO 4 sample consists of 3 stages. In the first stage, at temperature 60 -300°C, there is a slight decrease in mass due to water evaporation. This phenomenon is also characterized by an endothermic reaction culminating at 227⁰C. Therefore, it is clear that LiFePO 4 decomposition would not occur in this temperature range. In the second part, at 325-610⁰C, the weight increases around 3.86% with three exothermic reactions at 348⁰C, 540⁰C and 605⁰C. This reaction is an oxide decomposition of amorphous LiFe II PO 4 into Li 3 Fe 2 (PO 4 ) 3 and Fe 2 O 3 [9] according to the following reaction: The next stage is a constant plane with slight weight reduction in order to continues oxidation of LiFePO 4 . This result is in agreement with that obtain by others in which the olivine LiFePO 4 will oxidized into Li 3 Fe 2 (PO 4 ) 3 and Fe 2 O 3 at temperature range of 250-550C [9], [14], [16] without any other substances that protect LiFePO 4, even when 30 mL/min N 2 is still flowing.
Visual observation of LiFePO 4 /C after thermal characterization shows that most all of the samples were black with a little red on the surface as can be seen in Fig. 1b. The red part could be Li 3 Fe 2 (PO 4 ) 3 and Fe 2 O 3 , which occurred as a result of oxidation, whereas the black part would be crystalline LiFePO 4 . Mass change of the LiFePO 4 /C curve can also be divided into 3 stages. In the first part, at temperature of 60-300C, there is a slight decrease in weight due to water evaporation. The second stage occur within the range of 300-510°C. In this stage, the weight increases 1.93%, and two reactions take place. The first reaction is oxidation of LiFePO 4 into Li 3 Fe 2 (PO 4 ) 3 + Fe 2 O 3 and the second is decomposition of amorphous LiFePO 4 into crystalline LiFePO 4 according to the following reaction: The third stage is at 510-750°C, where the weight decreases by 4.8% due to carbon oxidation into CO 2 , leaving the LiFePO 4 coupled with the excess carbons. The carbon content of LiFePO 4 /C was calculated in which the remaining carbon is equal to total carbon added minus the burned carbon [9]. In this case, the carbons remained active in the material are 5%-4.8% = 0.2%. From this decomposition behavior, it is clear that the carbon role in the calcination process would be mainly as an oxidation protection of LiFePO 4 . From the thermal behavior, it can be seen that the formation temperature of LiFePO 4 crystal completes at 610C, followed by oxidation of carbon that completes at 750C. This LiFePO 4 formation temperature is slightly different from that obtained by Frangers [26] at 550 C for 12 hours with the addition of carbon less than 5 wt.%, while Belharouak [9] obtained at 450 C. This difference is expected to be due to the precursors used and the synthesis routes [27].
X-ray diffraction patterns of LiFePO 4 /C at various temperature and LiFePO 4 at 750 C are shown in Fig. 2. The entire diffraction peaks of LiFePO 4 /C at various calcination temperatures are in agreement with that of LiFePO 4 standard indexed to the orthorhombic Pnma space group (JCPDS No.083-2092). Strong and sharp diffraction peaks indicate that LiFePO 4 /C sample is highly crystalline. For the sample with no carbon, the peaks shift slightly and are in agreement with that of LiFePO 4 standard indexed to the orthorhombic Pnmb space group (JCPDS No.019-0721). The peak difference between LiFePO 4 and LiFePO 4 /C is expected to be due to the presence of carbon. There are no other phases nor other impurities are detected in the XRD diffractograms. The variations of unit cell volume (V) and lattice parameters at various calcination temperatures were calculated and the results are summarized in Table 1 and Table 2 for LiFePO 4 /C and LiFePO 4 , respectively. At calcination temperature 500ºC, the lattice parameters are in agreement with that reported by other [28] with JCPDS No. 083-2092; however, when the calcination temperature increased to 600 and 700C, lattice parameters decreased. This could be explained by the facts that at high calcination temperature, more atom carbons will be oxidized forming CO 2 and thus more room for LiFePO 4 crystal to get organized and hence the volume gets smaller. The lattice parameters of LiFePO 4 are in agreement with that of JCPDS No. 019-0721. To understand the effect of carbon addition on the morphology of the LiFePO 4 /C, secondary electron images using SEM were taken for LiFePO 4 and LiFePO 4 /C from the same calcination temperature of 750C and the results are shown in Fig. 3. As seen in the figure, LiFePO 4 has large grains of up to 10 µm whereas LiFePO 4 /C has smaller grains (427 nm in average). The large grains size in LiFePO 4 is expected to be caused by the grain growth during calcination. During calcination process, the heat will cause the small grains to merge with the large one by grain boundaries diffusion. The small grain in LiFePO 4 /C, on the other hand, is expected to be the result of mechanical grinding process, which breaks the precursor grain during the mixing of carbon black and LiFePO 4 . The carbons then cover the LiFePO 4 particles and protect them from grain growth during the calcination process.  Fig. 4. At temperature of 500ºC, LiFePO 4 /C formed in two shapes i.e. flakes and spheres as shown in Fig 4(a). Flake shaped LiFePO 4 /C is basically the original shape of LiFePO 4 coated by carbon, whereas the sphere one is initiated by the carbon excess. EDX analysis of the flake shape showed that the composition of O = 35.88; P = 23.41 and Fe = 40.71 wt.%, which represents LiFePO 4 and sphere composition is dominated by C = 53.72 with the remaining are O = 30.63; P = 05.08; and Fe = 10.57 wt.%. Referring to the LiFePO 4 /C TGA result shown previously, it can be inferred that at 500ºC, LiFePO 4 formation has finished but still contain most of the added carbon. When calcination temperature is increased from 500 to 600ºC, globular shape of carbon disappears because most of the carbons have turned into CO 2 whereas the other carbons agglomerate in the forms of small particles. Furthermore, flake shaped of LiFePO 4 /C is now dominating the grains. At the calcination temperature 750ºC, only flake shaped LiFePO 4 remains coated by carbons on the surface. Image analysis has been performed in order to calculate the particle size distribution, and the result is shown in Table 3. Within temperature increment from 500 to 600ºC, grain growth occurs in LiFePO 4 /C flake and make the size of the flake to become large although it is not as large as pure LiFePO 4 . This can be understood since the grain growth mechanism will occur when two grain boundaries in same phase but different size get into contact each other. In this case, carbon will act as a barrier that keeps LiFePO 4 grain boundary from being contacted. At temperature 750ºC, the grain size is supposed to be larger than that of lower temperature. In this case, however, it turns to be small. This is expected because of the effect of mechanical mixing before calcination, and thus the carbon sphere disappears in this temperature. This confirm the result of DTA-TGA curve where the carbon oxidation has completed at 750ºC. Hydrothermal synthesis followed by calcination process without additional carbon is indeed susceptible to grain growth. This confirms that the carbons act as inhibiting additive for the grain growth [15].  The effect of carbon addition and calcination temperature on the electrical conductivity of crystalline LiFePO 4 was examined from pressed LiFePO 4 /C and LiFePO 4 sample pellets using EIS. The conductivity test result is shown in Table 4. Pure LiFePO 4 crystal is having insulator property, and thus the test result of LiFePO 4 shows only a conductivity in the order of 10 -7 S/cm. This result is in agreement with other results [3], [12]. With the addition of carbon, the conductivity increases. In this instance, carbon will act as a conductive agent, and hence, more carbon will increase the conductivity. Conductivity, however, reduces at increasing calcination temperature as shown in Table 4. As has been mentioned previously, at high calcination temperature, more atom carbon will oxidize to form CO 2 and thus less carbon will be available to cover the LiFePO 4 . The conductivity of active material is also related to the particle size, as shown in Fig.5. With the assumption that the flake size is proportional to the grain size, whereas the electron will take the shortest path to travel, the electrical conductivity will be lower at larger particle size. This can be understood since the larger the particle sizes the greater the distance and thus the lower the conductivity. In order to determine the capacity and performance of LiFePO 4 cathode, charge-discharge cycle tests were performed on LiFePO 4 /C at various rates. Result from LiFePO 4 /C sample showed that the stable active material is the sample with carbon addition and calcined at 750C. The charge-discharge voltage curves for the LiFePO 4 /C-750 in the voltage range of 2.5-4.0 volts and current density of 0.1-0.5C are shown in Fig. 6. A flat profile over 3.3-3.5 volts potential range indicates that the extraction and the insertion reaction of the lithium ions proceeded by the motion of a two-phase interface between FePO 4 and LiFePO 4 . The specific capacity from the testing that yet to be improved is at 11.66 mAh/g for 0.1C. This low value could be due to hygroscopic properties of carbon, which allows it to get very high absorption rate of water vapor. The water content and other contaminants could reduce the capacity and damage the electrolyte solution and the cathode [17], [22]. Nonetheless, despite the low capacity, the trend obtained in this work is promising for the next development.

IV. CONCLUSION
Pure LiFePO 4 and carbon coated LiFePO 4 has been synthesized using hydrothermal route. Thermal decomposition behaviour showed that LiFePO 4 formation was completed at 510C followed by carbon oxidation until 750C. The addition of carbon effectively protected the material from oxidation and grain growth during calcination process. High calcination temperature resulted in large grain particles and reduction of carbon content. The X-ray diffraction diffractograms exhibited well crystallized peaks corresponding to an orthorhombic olivine type structure with Pnma space group. SEM analysis showed that the synthesized powder consists of flake-like shaped grains in the size range of 400-650 nm. The charge-discharge measurements performed at various rates exhibited a good cycling stability under working voltage at 3.3~3.5 volts.