Preparation of LiFePO4 Mesocrystals Consisting of Nanorods through

Mar 8, 2010 - E-mail: [email protected]. Telephone: +81-45-566-1556. Fax: +81-45-566-1551. Cite this:Cryst. Growth Des. 2010, 10, 4, 1777-1781 ...
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DOI: 10.1021/cg901457t

Preparation of LiFePO4 Mesocrystals Consisting of Nanorods through Organic-Mediated Parallel Growth from a Precursor Phase

2010, Vol. 10 1777–1781

Hiroaki Uchiyama and Hiroaki Imai* Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, 223-8522, Japan Received November 22, 2009; Revised Manuscript Received February 11, 2010

ABSTRACT: Olivine-type LiFePO4 crystals were prepared from a precursor phase Fe3(PO4)2(H2O)8 by a reaction with LiOH under a hydrothermal condition. Particular rod-like units assembled in the same crystallographic orientation were formed in association with L-(þ)-ascorbic acid as a reducing and capping agent. Fagot-like mesocrystals were constructed with the oriented rods through parallel growth by supplying abundant Fe2þ and PO43- ions from the precursor phase. An increase in the concentration of the organic additive decreased the unit size, and sheaf-like mesocrystals of LiFePO4 nanorods with high crystallinity and porous structure were obtained.

*Corresponding author. E-mail: [email protected]. Telephone: þ81-45-566-1556. Fax: þ81-45-566-1551.

of the formation mechanism of the superstructure is required. In particular, bridged nanocrystals, similar to biominerals, are expected to serve as electrode materials with a high rate of performance because of their highly porous structures consisting of interconnected nanocrystals. Olivine-type LiFePO4 is now receiving attention as an alternative cathode material for lithium ion rechargeable batteries because of its high voltage that is enough for application to the cathode (about 3.5 V vs Li/Liþ), high theoretical capacity (ca. 170 mAh/g), and good stability.33-48 However, its practical use is hampered by its poor rate of performance, which is caused by its low electrical conductivity (10-8-10-10 s/cm). A high rate capability of lithium-ion batteries would be achieved with highly porous electrodes because a framework structure provides high accessibility for an electrolyte and allows the efficient charge/discharge cycles of lithium ions.35-40,42,44,45,47 The reduction of the particle size to nanoscale is effective to improve the rate of performance due to a decrease in the electronic conduction path. In recent years, hydrothermal methods are used for the morphological change of LiFePO4 crystals.35-40,44,47,49 The particle size of LiFePO4 was reduced by the addition of polyethylene glycol to the precursor solution.49 A hydrothermal reaction with surfactants led to the preparation of LiFePO4 nanoparticles.44 Thus, organic additives play an important role in controlling the size of LiFePO4 crystals. We reported that specific architectures consisting of SnO nanocrystals were obtained from Sn6O4(OH)4 as a precursor phase.50,51 Therefore, the transformation of a precursor phase in association with organic additives would lead to the formation of superstructures consisting of oriented, connected nanocrystals and, consequently, provide high performance to LiFePO4 electrodes. Here, we address the preparation of LiFePO4 mesocrystals through a two-step solution process. First, Fe3(PO4)2(H2O)8 as a precursor phase was obtained from an aqueous solution containing Fe2þ and PO43- ions. Then, the parallel growth of LiFePO4 nanorods was mediated by the hydrothermal treatment of the precursor phase in an aqueous solution in which LiOH and ascorbic acid were dissolved. The size of the rod units in the resultant architectures was controlled by adjusting the concentration of ascorbic acid. In this work,

r 2010 American Chemical Society

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Introduction Superstructures consisting of oriented nanoscale units are practically important and interesting to many technological and scientific researchers. Periodic and sophisticated nanostructures hold promise for the enhancement of electronic and optical properties. C€ olfen et al. suggested the construction of highly ordered architectures from small building units of inorganic crystals in association with organic molecules and defined the superstructures as mesocrystals.1-7 Iso-oriented crystals are also obtained through the arrangement of primary nanocrystals via oriented attachment, which can form a single crystal upon fusion of the nanoparticles.8 Moreover, specific morphologies composed of nanoscale units are found in typical biominerals, such as nacres, corals, sea urchin spines, and eggshells.9-18 The hierarchical architectures of these biominerals are deduced to be constructed by oriented and bridged nanocrystals. Recently, O’Brien et al. used the term “mesocrystal” to denote a new group of solid materials comprising crystallographically oriented nanocrystals such as classic mesocrystals, iso-oriented crystals, bridged nanocrystals, nanocrystal superlattices, sponge crystals, and porous single crystals.19 The classic mesocrystals defined by C€ olfen are formed through a self-assembly of nanocrystals in association with organic molecules.1-7 On the other hand, a branching growth of inorganic crystals with incorporated organic molecules could lead to the self-organized formation of bridged nanocrystals.9,10,13,14,17,20-24 The formation of mesocrystals is often associated with the assistance of organic agents, such as soluble polymers and an insoluble gel matrix. The presence of a precursor phase including amorphous calcium carbonate (ACC) and amorphous calcium phosphate (ACP) is also involved in the formation of bridged nanocrystals in biominerals.25-32 Mesocrystals prepared through the self-assembly or self-organization of nanocrystals have a much higher crystallinity than polycrystalline materials and, in some cases, exhibit the properties of single crystals. Thus, mesocrystals have a high potential for many applications, such as sensors, catalysts, and solar cells, and understanding

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Figure 1. XRD patterns of precursor (a) and the product prepared at [LiOH] and [ascorbic acid] = 0.1 M by hydrothermal treatment at 180 °C (b).

the effects of the organic additive and the precursor phase are discussed on the basis of the morphological change of LiFePO4 crystals. The highly ordered architecture of rod-like nanounits with a highly porous structure and high crystallinity is expected as a cathode material with a high rate of performance.

Uchiyama and Imai

Figure 2. SEM images of Fe3(PO4)2(H2O)8 as a precursor phase (a) and LiFePO4 prepared at [LiOH] and [ascorbic acid] = 0.05 M (b-c) and 0.1 M (d) by hydrothermal treatment at 160 °C.

Experimental Section An aqueous solution of 0.05 M FeCl2, which was prepared by mixing 40 cm3 purified water with 0.4 g of FeCl2 3 4H2O (Kanto Chemical), was added to another aqueous solution containing 0.26 g of (NH4)2HPO4 (0.05 M) (Kanto Chemical). After the mixture was stirred for 10 min at room temperature and subsequently subjected to centrifugation, a dark-green precipitate as a precursor phase of LiFePO4 was obtained by removal of the solvent. Stock solutions containing 0.05-0.1 M LiOH 3 H2O and L-(þ)-ascorbic acid (Kanto Chemical) were prepared using purified water. The pH value of the stock solutions was adjusted to 8.0 by addition of an HCl solution containing 0.05-0.1 M LiOH 3 H2O and L-(þ)-ascorbic acid. The precursor phase was added to the 40 cm3 stock solution and heated at 160-180 °C using a Teflon-lined stainless steel autoclave for 24 h. To avoid the oxidation of Fe2þ to Fe3þ, the stock solutions were degassed by N2 bubbling for 10 min prior to the hydrothermal treatment. The products were obtained after centrifugation of the precipitates followed by washing with purified water and drying at 60 °C for 24 h. The morphology of the products was investigated with a fieldemission scanning electron microscope (FESEM, FEI Sirion) and a field-emission transmission electron microscope (FETEM, FEI TECNAI F20). X-ray diffraction (XRD) was performed with a Bruker AXS D8 Advance with Cu-Ka radiation, and FT-IR spectra were recorded using a KBr method with a BIO-RAD FTS 60A. The amount of remaining ascorbic acid was estimated from thermogravimetry-differential thermal analysis (TG-DTA, Seiko Instruments, TG-DTA 6200).

Results and Discussion A dark-green precipitate as a precursor phase was rapidly produced by mixing FeCl2 and (NH4)2HPO4 solutions. The product was identified as Fe3(PO4)2(H2O)8 from an XRD pattern (Figure 1a), and it exhibited a flower-like structure consisting of platy units ca. 3 μm in width, 5-10 μm in length, and ca. 100 nm in thickness (Figure 2a). Olivine-type LiFePO4 was produced from the precursor phase in a solution containing LiOH and ascorbic acid at above 160 °C (Figure 1b). The addition of a reducing agent, such as sugar and ascorbic acid, was reported to prevent the oxidation of Fe(II) during hydrothermal treatments and to lead to the formation of LiFePO4.

Figure 3. SEM images of LiFePO4 prepared at [LiOH] and [ascorbic acid] = 0.05 M (a-c), 0.075 M (d-f) and 0.1 M (g-i) by hydrothermal treatment at 180 °C.

In our work, a pure phase of LiFePO4 was obtained in the presence of ascorbic acid. Figure 3 shows the morphological variation of LiFePO4 crystals prepared by the hydrothermal treatment of the precursor at 180 °C. We found various segmented structures in the products. A fagot-like architecture ca. 3 μm in width and ca. 1 μm in thickness was obtained at [LiOH] = 0.05 M and [ascorbic acid] = 0.05 M (Figure 3a-c). The platy shape was

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Figure 4. TEM images and the fast Fourier transform (inset) of a planar section of the sheaf-like LiFePO4 prepared at [LiOH] and [ascorbic acid] = 0.1 M by hydrothermal treatment at 180 °C.

Figure 7. SEM images of LiFePO4 prepared at [LiOH] = 0.05 M and [ascorbic acid] = 0.075 M (a, b) and 0.1 M (c, d) by hydrothermal treatment at 180 °C.

Figure 5. FTIR spectra of LiFePO4 prepared at [LiOH] and [ascorbic acid] = 0.05 M by hydrothermal treatment at 180 °C.

Figure 6. TG analysis of LiFePO4 prepared at [LiOH] and [ascorbic acid] = 0.05 M by hydrothermal treatment at 180 °C.

formed by the arrangement of oriented rod units of ca. 200 nm in width and 1-3 μm in length. Interestingly, the unit size at the edge was smaller than that around the center of the fagot. Increasing the concentrations of LiOH and ascorbic acid led to a decrease in the unit size with surface roughing as shown in Figure 3d-f. Finally, a sheaf-like hierarchical architecture consisting of fine rods of ca. 50 nm in width and ca. 300 nm in length was obtained at [LiOH] and [ascorbic acid] = 0.1 M (Figure 3g-i). Figure 4 shows TEM images of the fine rods in the sheaf-like architecture. The lattice fringes reveal that the nanorods were elongated in the [100] direction and interconnected in the [001] direction. In an FTIR spectrum of the fagot-like crystals (Figure 5), the absorption peak at 1600 cm-1 was attributed to the stretching vibration of -COO-. Moreover, the result of TG analysis of the fagot-like crystals indicated the presence of residual ascorbic acid of ca. 6.0 wt % (Figure 6). These suggest the presence of ascorbic acid on the

Figure 8. Schematic illustrations of a fagot-like architecture (a), a sheaf-like architecture (b), and a general platy shape of LiFePO4 (c).

surface of LiFePO4 crystals. We observed a similar morphological change with an increase in the concentration of ascorbic acid, as shown in Figure 7. Thus, the addition of ascorbic acid was essential for not only the suppression of the oxidation of Fe(II) but also the control of the crystal growth of LiFePO4. The adsorption of ascorbic acid could lead to a decrease in the unit size of LiFePO4 composing the architecture. Figure 8 is a schematic illustration of various architectures of LiFePO4 crystals. The fagot-like and sheaf-like structures were composed of oriented rods elongated in the [100] direction. The stacking of the units in the [001] and [010] directions produced thick plate-like mesocrystals. An increase in the concentration of ascorbic acid decreased the size of the rod-like units elongated in [100]. Thus, the segmented and

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is hardly associated with the assembly of primary nanoparticles because each unit of the fagots had the size depending on the growth stage. The rods at the edge were larger than those around the center of the mesocrystals. This fact could indicate that the LiFePO4 mesocrystals formed through a branching growth, rather than the assembly of primary crystals. Lithium ion in LiFePO4 crystal was reported to diffuse along the [010] direction.38,52 The segmented architectures were composed of rod units exposing the large (010) plane. The mesocrystals of LiFePO4 would be suitable for the application to a cathode material with a high rate of performance. We are currently studying the electrochemical performance of the mesocrystals of LiFePO4. Figure 9. Schematic illustration of the formation of a segmented architecture through organic-mediated parallel growth from precursor matrix.

parallel growth in the [001] and [010] directions would be induced by the presence of the organic agent adsorbing to the specific faces. The intermediate stage of the transformation from Fe3(PO4)2(H2O)8 to the LiFePO4 mesocrystals prepared at relatively low temperature (160 °C) is shown in Figure 2b-d. The fagots of LiFePO4 were formed on/in platy units composing the flower-like structure of the precursor phase with the reaction of Liþ, Fe2þ, and PO43- ions through the partial dissolution of the precursor. The flower-like structure completely changed into the fagot- and sheaf-like architectures with an increase in the solubility of the precursor phase at 180 °C. We previously reported that SnO mesocrystals with oriented units were obtained from Sn6O4(OH)4 as a precursor phase, where the large amounts of precursor phase served as the source of the raw materials and the matrix suppressing the nucleation and the crystal growth.51 Thus, the segmented structures would be produced with parallel growth by supplying raw materials from the precursor phase. Figure 9 shows a schematic model of the parallel growth for the formation of the fagot- and sheaf-like architectures consisting of oriented rods. Thin plates of LiFePO4 exhibiting large, smooth (010) planes were prepared by a conventional hydrothermal process.38 This suggests that the growth rate of the crystal increases in the order of the [001], [001], and [010] directions. Nucleation of LiFePO4 crystals could occur on/in the platy units of Fe3(PO4)2(H2O)8 as the precursor matrix. In the initial stage of the formation of segmented crystals, the adsorption of ascorbic acid suppressed the growth on the (001) and (010) faces, resulting in the formation of rod-like structures elongated in the (001) phase. However, the length of the rods is limited by the size of the precursor matrix. In the successive stage, the parallel growth in the [001] and [010] directions occurs with breaking of the organic barrier due to the excessive supply of ions from the precursor matrix. In this case, a daughter rod is grown on the basal rod under a high degree of the driving force around the center of the rod. The parallel growth of the rods on the side surface of the basal rods results in the formation of the specific segmented architectures similar to fagots and sheaves. A macroscopic platy form would be produced because the growth rate in [001] is originally greater than that in [010]. Finally, the fagot- and sheaf-like architectures of LiFePO4 were produced through branching growth with adsorption of ascorbic acid in the precursor matrix. On the other hand, the formation of the highly ordered architectures consisting of oriented nanorods

Conclusion Mesocrystal of LiFePO4 crystals was prepared in a hydrothermal treatment of Fe3(PO4)2(H2O)8 as a precursor phase in an aqueous solution containing LiOH 3 H2O and L-(þ)-ascorbic acid. Crystal growth in the precursor matrix with inhibition of the crystal growth by adsorption of the organic agent is essential for the formation of highly ordered architectures consisting of oriented nanorods. The concept for the formation of sophisticated and periodic morphologies holds promise for the preparation of superstructures of functional materials.

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