Synthesis and Evaluation of Olivine Nanosheets from Layered Ammonium Iron Phosphate Monohydrate
Received Date: Nov 27, 2017 / Accepted Date: Nov 29, 2017 / Published Date: Dec 10, 2017
The synthesis of novel microstructured LiFePO4 with advantageous nanosheets for Li ion conductivity was attempted. Using layered NH4FePO4•H2O as raw material, LiFePO4 nanosheet was synthesized by the hydrothermal process in LiCl solution. Prepared NH4FePO4•H2O was several tens micrometer sized sheet with about 200 nm in thickness. As Li ion resource, various LiCl solution like deionized water, ethanol, and ethylene glycol were prepared through subsequent hydrothermal process and the effect of a kind of solvents for LiCl solution on the microstructure of products treated by the hydrothermal process was investigated for LiFePO4 nanosheets synthesis. The products of LiFePO4 nanosheet were characterized by XRD, SEM, TEM, FT-IR and ICP. Regardless of a kind of solvents, LiFePO4 nanosheet was composed of arranged nano-blocks, although the size and morphology of nano-blocks was different in each solvent.
In order to put the sustainable society into practice, environmentfriendly material has been taken strong interest in. Lithium iron phosphate as the material not using rare metal like cobalt, LiFePO4, has the characters like the plateau redox potential, the good cycle performance and the unique stability, compared to other materials like LiCoO2. LiFePO4 shows 3.5 V vs. Li of redox potential and 170 mAhg-1 of theoretical capacity. The discharge reaction of LiFePO4 is showed in the following equation.
LiFePO4 → Li1-xFePO4 + xLi+ + xe-
LiFePO4 has olivine-type structure and belongs to poly-anion group . Olivine-type material, that composition expressed in LiMePO4 (Me=Fe, Mn, Co. Ni), is composed of edge-shearing PO4 tetrahedral and MeO6 octahedral. LiMnPO4 has 4.1 V vs. Li higher potential than that of LiFePO4 . Some groups researched LiMnxFe1-xPO4 of having two-plateau potential [3-5]. The more energy density than that of single composition LiFePO4 would be expected.
On the other hand, the conductivity is very small because olivinetype material is the ionic crystal of XO4- and cations . The material with more conductivity can shows the enough discharging capacity at high charging rate. Doping elements, composite with conductive material and morphology control had been investigated in order to improve the conductivity. Chung reported olivine material doped with the multi cation, such as Mg2+, Al3+, Zr4+, Nb5+ and W6+, had the good rate capability . Also, for conductivity improvement, LiFePO4/C composite was produced by heating the mixture of the active material and lactose in inert atmosphere  and depositing carbon from heat decomposition of propylene gas . With increasing the amount of the conductive additive, however, the active material density in electrode decreases. Therefore, the importance of nano-sized material increases for these LiFePO4 materials.
The nano-sized materials also grow in importance for positive electrode materials for lithium ion battery. If the electrode material makes a finer particle, its reactivity increases due to the improvement of the specific surface area. In addition, the ion diffusion length for nano-sized materials is short, and this is quietly advantageous to battery materials. In addition, for the electrode material, morphology is also the important factor for improvements in the reactivity and cycle performance as well as the particle size. Many studies on LiFePO4 synthesis by hydrothermal reaction has been reported [10-12], and the morphology and particle size are well-known to be affected under synthetic parameters such as pH of reaction solution, reaction temperature, and reaction time. LiFePO4 has one-dimensional Li+ conduction pass, and the domino-cascade conduction model was suggested . Therefore, recently the platelet/sheet structured LiFePO4 materials have attracted much attention for use of more efficient to Li+ conduction. Zhao reported about LiFePO4 nanosheets synthesis and electrical property . They obtained LiFePO4 nanosheets by the exfoliation of bulk LiFePO4 powders.
In recent decades, many attempts for nanosheets synthesis had been also reported using layered oxide compounds such as TiO2, MnO2, and Nb2O5 [15-17]. Mostly, it is reported that various nanosheets are obtained by the exfoliation of layered compound. Layered ammonium iron phosphate monohydrate, NH4FePO4•H2O, was synthesized by hydrothermal process as a starting material for nanosheets and then LiFePO4 sheets was exfoliated by the subsequent hydrothermal reaction for this layered NH4FePO4•H2O in Li solution. The synthesis condition of LiFePO4 nanosheets was investigated by hydrothermal process and the microstructure were evaluated.
Materials And Methods
Preparation of layered NH4FePO4•H2O
Layered NH4FePO4 was prepared by hydrothermal process. As reaction solution, 0.5 M FeSO4 solution and 0.5 M NH4H2PO4 solution were prepared. Fe resources were added in P resources with an equal molar ratio and the pH of mixed solution was maintained at pH 10 by adding aqueous ammonia. Then, the result solution was transferred in Teflon vessels, and they were kept at 150°C for 24 hours. The products were filtrated, washed, and dried at 50°C overnight.
Synthesis LiFePO4 nanosheets from layered NH4FePO4•H2O compounds
Obtained NH4FePO4 was used as raw material for nanosheets. LiCl as lithium resource was dissolved in each solvent, such as deionized water, ethanol, and ethylene glycol. Effect of a kind of solvents for LiCl solution on the microstructure of products treated by the subsequent hydrothermal process was investigated for LiFePO4 nanosheets synthesis. 0.5 g of NH4FePO4 was added in LiCl solution. Mixed solution was transferred in Teflon vessels, and they were kept at 150°C for 24 hours. Then, the products were filtrated, washed and dried at 50°C overnight. The effect of alcohol on LiFePO4 nanosheets microstructures was also investigated.
The crystal phase of samples was indexed by XRD (UltimaIV, Rigaku Co., Japan). Scans were performed at 2θ=10-60° with scan rate of 4°/min. using Cu-Ka radiation. Morphology was observed by FE-SEM (S-4500, Hitachi, Japan) with applied voltage of 15 kV. TEM observation was used by JEM-2000FX at 200 kV. The composition of the sample was analyzed by ICP (PS-7800, Hitachi Co.). The sample was dissolved in 0.1 M nitric acid, and its solution was measured. The vibrational structure was identified by FT-IR (ALPHA-OPT, Bruker Co.) at wave vector range 400-4000 cm-1. For FT-IR measurements, the sample was grinded with KBr, and the powder was pressed in a mechanical press to form a translucent pellet.
Results And Discussion
Preparation of layered NH4FePO4•H2O
XRD results of the products obtained from FeSO4 solution and NH4H2PO4 solution were showed in Figure 1A. The diffraction peaks were attributed to layered NH4FePO4•H2O. The (200) peak at 10° was depended on layered structure. Figure 1B shows SEM images for the obtained products. From SEM observation in Figure 1B, sheetlike particle was observed for samples obtained at pH 10 and it was thought to be layered NH4FePO4. The particle size was about several ten micrometers on a side, and the thickness was about 200 nm for layered NH4FePO4•H2O. From ICP analysis for layered NH4FePO4•H2O, Fe/P ratio was 1.02, and iron concentration in layered NH4FePO4 was 5 mmol/g, which was the estimated Li quantity necessary for reaction. Thus, it was found that large layered NH4FePO4•H2O with several tens μm on a side and about 0.2 μm in thickness was obtained.
Synthesis LiFePO4 nanosheets from layered NH4FePO4•H2O compounds
Synthesis of LiFePO4 nanosheets by the subsequent hydrothermal process were attempted in LiCl aqueous solution with a few kinds of concentration from layered NH4FePO4•H2O as a starting material. The addition of dissolved LiCl solution was changed from 1 mmol to 20 mmol. An XRD result of sample treated by hydrothermal process was showed in Figure 2. With LiCl addition less than 5 mmol, the diffraction peaks were attributed to olivine-type LiFePO4 and in part NH4FePO4•H2O residue. The peak at 10° derived from the layered structure was disappeared with increasing LiCl addition, and single phase of LiFePO4 was obtained in LiCl solution more than 5 mmol. Compared with XRD patterns reported by past report , (200) peak of LiFePO4 at 17°C was larger for obtained products. It was thought that obtained LiFePO4 was highly oriented. SEM images were showed in Figure 3. In case of products obtained with 1 mmol LiCl addition, nanosheet-like products derived from NH4FePO4•H2O was remained and many nano-blocks with some tens nanometer on a side were precipitated on the surface of NH4FePO4•H2O sheets. With increasing LiCl addition, the number of nano-blocks tended to increased. Considering XRD results, it was thought that the nano-blocks were corresponding to LiFePO4 phase. A FT-IR spectrum was showed in Figure 4. Layered NH4FePO4•H2O has the IR active species, NH4+, H2O, and PO43-. From the spectra of the raw material, the broad absorption bands of H2O were existed around 3300 cm-1, those of NH4+ were at 2400 cm-1, and those of PO43- were around 1300-400 cm-1. With increasing LiCl addition, the absorption bands at 2400 cm-1 and around 3300 cm-1 decreased, and it was showed that NH4+ and H2O in interlayer was released. In addition, the absorption bands of PO43- were separating and sharpening, and it was thought that LiFePO4 structure was crystalized. Although the reaction mechanism is not cleared yet, it was thought that hydrothermal reaction was progressing from surface of layered NH4FePO4•H2O. From ICP results, NH4+ and H2O were released from interlayer while reaction from NH4FePO4•H2O to LiFePO4.
Influence of a kind of solvents for LiCl solution through the hydrothermal process
Effect of a kind of solvents for LiCl solution on the microstructure of products treated by the hydrothermal process was investigated for LiFePO4 nanosheets synthesis. Ethanol and ethylene glycol were used as the solvent for LiCl solution through the hydrothermal process for LiFePO4 nanosheets. LiCl was added 5 mmol in ethanol and 10 mmol in ethylene glycol. XRD results of the products in various LiCl solutions were showed in Figure 5. In the case of ethanol solvents, the diffraction peaks were attributed to olivine-type LiFePO4 and Li3PO4. It was thought that dissolution of NH4FePO4 was faster in ethanol, and consequently Li3PO4 was precipitated in Li rich condition. In the case of ethylene glycol, the diffraction peaks were attributed to olivinetype LiFePO4. Compared with deionized water solvents, the peaks of obtained LiFePO4 were broad, and the intensity of (200) peak was not high. TEM images and diffraction pattern of NH4FePO4•H2O as raw material and sample with LiCl addition in each solvent were showed in Figure 6. About NH4FePO4•H2O, large sheet-like particle was observed, and it was thought to be single crystal from diffraction pattern. Then, the samples with LiCl addition in deionized water had large sheet-like morphology, and it was composed of arranged nano-blocks. The size of nano-blocks was estimated 100 nm in width and 180 nm in length from high magnification images. In addition, diffraction pattern showed a part of samples was oriented. In case of ethanol and ethylene glycol solvents, large sheet-like particles were observed. From high magnification images, the size of nano-block in ethanol was 180 nm in width and 280 nm in length, and in ethylene glycol, it was less 50 nm in width and more 200 nm in length. In case of ethylene glycol, LiFePO4 nanosheets were composed of thin and long nanoblocks, although samples obtained in ethanol and in deionized water were composed of cubic nano-blocks. However, diffraction pattern of the samples in ethanol and ethylene glycol were spot pattern as same as that of the samples in deionized water, showing the same crystallinity for LiFePO4 nanosheets sample synthesized in the different solvents. It was obvious that the microstructure of LiFePO4 nanosheets was dependent on the solvents in LiCl solution through the hydrothermal process. The detailed analysis of microstructure and growth mechanism is under investigation.
NH4FePO4•H2O were prepared by hydrothermal process. Obtained NH4FePO4•H2O were composed of sheet-like particle with several tens micrometer. And then LiFePO4 nanosheets were synthesized by the subsequent hydrothermal process at pH6 in various LiCl solution from layered NH4FePO4•H2O as a raw material. In deionized water solvent, the morphology of LiFePO4 was large sheets composed of arranged nano-blocks with several hundred nanometers on a side. With LiCl addition in alcohol solvent, LiFePO4 nanosheets were also composed of nano-blocks, but the shapes of nano-blocks were different. The size of nano-blocks in ethanol was estimated 180 nm in width and 280 nm in length as well as ones in deionized water, and the length of nano-blocks in ethylene glycol increased.
We thank for Grant-in-Aid for Scientific Research on Innovative Areas “Nano Informatics” (Grant No. 16H00892) from JSPS.
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Citation: Togo M, Nakahira A (2017) Synthesis and Evaluation of Olivine Nanosheets from Layered Ammonium Iron Phosphate Monohydrate. J Material Sci Eng 6: 403. Doi: 10.4172/2169-0022.1000403
Copyright: © 2017 Togo M, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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