Mesoporous Iron Phosphonate Electrodes with Crystalline

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Mesoporous Iron Phosphonate Electrodes with Crystalline Frameworks for Lithium-Ion Batteries Malay Pramanik,† Yoshihiro Tsujimoto,† Victor Malgras,† Shi Xue Dou,‡ Jung Ho Kim,*,‡ and Yusuke Yamauchi*,† †

International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Institute for Superconducting and Electronic Materials (ISEM), University of Wollongong (UOW), Innovation Campus, North Wollongong, New South Wales 2500, Australia S Supporting Information *

ABSTRACT: A new family of mesoporous iron phosphonate (FeP) materials has been prepared through cooperative assembly of cetyltrimethylammonium bromide (CTAB), iron nitrate, and nitrilotris(methylene)triphosphonic acid (NMPA). CTAB is used as a structure directing agent, while the other two chemicals are used as precursors for the formation of pore walls. An extraction procedure is employed to remove the template without damaging the as-prepared ordered mesostructure. The obtained mesoporous FeP materials are well characterized by low angle X-ray diffraction (XRD), N2 adsorption isotherms, and transmission electron microscopy. The mesostructural ordering of the obtained materials strongly depends on the synthetic conditions. The morphology and the crystallinity of the pore walls are investigated by scanning electron microscopy and wide-angle XRD measurements, respectively. It is revealed that the FeP framework is crystallized in the tetragonal crystal phase (I41/amd), according to the Rietveld refinement of the XRD patterns through the MAUD program. The unit cell parameters of the obtained crystals are a = b = 5.1963 (3) Å, c = 12.9808 (1) Å (α = β = γ = 90°). Also, the homogeneous distribution of both Fe species and organo-phosphonic acid groups in the mesoporous architectures is confirmed by Fourier transform infrared spectroscopy and elemental mapping. Mesoporous FeP materials with high surface area have great applicability as high performance electrode materials for lithium-ion (Li-ion) batteries, due to several advantages including a large contact area with the electrolyte, high structural stability, and short transport paths for Li+ ions. Mesoporous FeP electrodes exhibit high reversible specific capacity with very good cycling stability and excellent retention of capacity.

1. INTRODUCTION Periodic mesoporous hybrid materials have recently attracted much research interest due to the superior properties arising from the synergy between organic and inorganic constituents.1 The silicon-based sol−gel process has been successfully demonstrated for the preparation of mesoporous organosilica materials.2,3 During the last two decades, many synthetic strategies have been elaborated for various organosilane precursors, and several applications, including adsorbents, catalysts, and ion-exchangers, have been developed.4,5 In most cases, however, the pore walls are amorphous (sometimes, partially crystallized), which restricts their practical utility.6−8 The most exciting breakthrough was reported by Inagaki et al., in which perfectly crystallized mesoporous organosilica material was successfully prepared by a suitable choice of structure directing agent and organosilane precursor under proper reaction conditions.9 Besides the high cost of the organosilane precursors, however, the possible reaction conditions are very limited, which seriously restricts its utility in a wide range of chemical research.9,10 Among the nonsilica-based organic−inorganic hybrid materials, metal phosphonate materials are very attractive, due © XXXX American Chemical Society

to their potential applications in magnetism, catalysis, and heavy metal ion adsorption.11−13 Many reports have focused on the incorporation of various organo-phosphonic acid ligands inside the inorganic network to construct the hybrid frameworks, although the mesoporous structures have not been well controlled so far.14−18 Due to the rapid hydrolysis of inorganic precursor in a highly acidic phosphonic acid medium, it is very difficult to sustain suitable interaction between the micelles and the precursors.19 There have been only a few successful reports on ordered mesoporous metal phosphonate materials. Kimura and Haskouri et al. have prepared various aluminum phosphonate materials by different templating methods using simple alkylene bridge phosphonic acid.20−22 Ma et al. have also prepared several mesoporous titanium phosphonate materials by surfactant assisted methods.23,24 In most cases, however, the pore walls are amorphous, which seriously restricts their applications in various frontier areas: electronics, photonics, and highly selective and active catalysis.25 Received: December 1, 2014 Revised: January 2, 2015

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dissolved in 5 mL of distilled water separately. The synthetic gel was prepared by the dropwise addition of the iron nitrate solution to the phosphonic acid solution, and then the pH of the gel was adjusted to 4 by further addition of TMAOH solution. The stirring of the synthetic mixture was continued for another 4 h, and was finally transferred to a Teflon-lined autoclave and kept at different temperatures (Step 2, Table 1) for 36 h. The autoclave was cooled down to room temperature by decreasing the temperature very slowly (5 °C·h−1). The product was collected by filtration, washed with water several times, and then dried under vacuum at room temperature. The surfactants were completely removed by an extraction process with ethanol solution containing HCl, which was repeated three times.28 Finally, the mesoporous FeP materials were obtained, as shown in Table 1. For comparison, FeP materials without mesopores were prepared without the addition of any surfactants (CTAB) in the reaction medium, as shown in Table 1. Characterizations. Scanning electron microscope (SEM) images were collected with a Hitachi SU-8000 SEM at an accelerating voltage of 15 kV. Transmission electron microscope (TEM) observation was performed using a JEM-2010 TEM system that was operated at 200 kV. Low- and wide-angle powder X-ray diffraction (XRD) patterns were obtained by using a Rigaku RINT 2500X diffractometer using monochromated Cu Kα radiation (40 kV, 40 mA). Nitrogen adsorption−desorption analysis was performed using a Belsorp-mini II Sorption System at 77 K. The specific surface areas were calculated by the Brunauer−Emmett−Teller (BET) method at a relative pressure, P/P0, ranging from 0.05 to 0.30, while the total pore volumes were calculated by the Barrett−Joyner−Halenda (BJH) method. Thermogravimetric analysis (TG) curves were collected using a Rigaku Thermo Mass Photo TG-DTA-PIMS 410/S. Electrochemical Measurements. The electrodes were prepared by coating copper (Cu, 10 μm) foil with slurries containing the active materials (60 wt %), a conducting agent (Super-P, 20 wt %), and a binder, polyvinylidene difluoride (PVdF, 20 wt %), dissolved in deionized water. After the coating, the electrodes were dried at 120 °C for 10 h and pressed at a pressure of 200 kg·cm−2. The loading and density of the electrodes were fixed at 2.3 mg·cm−2 and 1.0 g·cm−3, respectively. The 2032 coin cells were carefully assembled in a dry room. A porous polyethylene (PE) film and Li foil were employed as the separator and the counter electrode, respectively. LiPF6 (1 M) dissolved in a mixed solvent of ethylene carbonate (EC) and ethylmethyl carbonate (EMC) (in the volume ratio of 3:7) was used as electrolyte. The cells were galvanostatically charged and discharged in the voltage range of 0.01−2.00 V vs Li/Li+ with constant current densities of 72 mA·g−1 (formation) and 144 mA·g−1 (cycling).

Here, we focus on mesoporous iron phosphonate (FeP) materials with crystallized pore walls, which have huge practical applicability in different directions of scientific research. Until now there are no reports on the synthesis of this category of materials due to synthetic difficulties related to the mismatch in interaction energies between iron and organo-phosphonic acid around the surfactant assemblies.26,27 In this study, we have successfully synthesized a new family of ordered mesoporous FeP materials with high surface area by fine-tuning of the reaction conditions, for the first time (Scheme 1). Our Scheme 1. Formation of Mesostructured FeP Materials (FeP-1M, FeP-2M, and FeP-3M)

mesoporous FeP materials have great applicability as high performance anode materials for the Li-ion rechargeable battery, due to several advantages, including a large contact area with electrolyte, high structural stability, and short transport paths for Li+ ions. Mesoporous FeP electrodes exhibit high reversible specific capacity with very good cycling stability (30 cycles) and excellent retention of capacity (97%). These properties are superior to those of other iron-based analogues previously reported.

2. EXPERIMENTAL SECTION Materials. Commercially available cetyltrimethylammonium bromide (CTAB), iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O), nitrilotris(methylene)triphosphonic acid (NMPA), and 25 wt % tetramethylammonium hydroxide solution (TMAOH) were purchased from Sigma-Aldrich. Synthesis of Mesoporous FeP Materials. In a typical synthesis of mesoporous FeP materials, 1 mmol of CTAB was dissolved in 15 mL of distilled water at various temperatures (Step 1, Table 1). After 1 h, 1 mmol of NMPA was further added to the solution, and the pH of the synthetic medium was increased up to 5 by adding TMAOH solution. Then, the mixture was magnetically stirred for 2 h at the same temperature. In an another beaker, 4 mmol of Fe(NO3)3 was

3. RESULTS AND DISCUSSION Low-angle XRD patterns were acquired from the as-synthesized samples (FeP-1M, FeP-2M) and the extracted samples after removal of the template (FeP-1ME, FeP-2ME), as shown in Figure 1. The as-synthesized FeP-1M sample shows a wellshaped peak at 2θ = 2.06° (d = 4.28 nm) with higher order peaks, which are assignable to the (100), (110), and (200) peaks of a hexagonally ordered mesostructure.21 After removal of the CTAB template from the as-synthesized sample (FeP1M), the mesoporous structure is slightly contracted. The XRD peak position is shifted to a higher 2θ value (2θ = 2.10°, d100 = 4.20 nm) (Figure 1a). In the case of FeP-2M prepared at higher temperature, the as-synthesized sample shows a broad peak centered at 2θ = 1.42°, d = 6.23 nm). After removal of the template, FeP-2ME shows a peak shift to 2θ = 1.75° (d = 5.04 nm) (Figure 1b). The peak intensity of FeP-2M (or FeP-2ME) is significantly broader than that of FeP-1M (or FeP-1ME). The mesophase in the FeP materials is generated through the cooperative self-assembly of CTAB molecules and FeP precursor.29 For FeP-1M, the clear appearance of low angle diffraction peaks indicates the formation of well-ordered

Table 1. Different Reaction Conditions for the Synthesis of Various FeP Materials synthetic temperature sample name

CTAB

NMPA

FeP-1Ma FeP-2Ma FeP-3Ma FeP-4b FeP-5b FeP-6b

1 mmol 1 mmol 1 mmol -

1 1 1 1 1 1

mmol mmol mmol mmol mmol mmol

iron nitrate nonahydrate 4 4 4 4 4 4

mmol mmol mmol mmol mmol mmol

step 1

step 2

298 323 323 298 298 298

298 393 423 393 423 453

K K K K K K

K K K K K K

a

After removal of templates by the extraction process, the obtained mesoporous FeP samples were designated as FeP-1ME, FeP-2ME, and FeP-3ME, respectively. bFeP-4, FeP-5, and FeP-6 were prepared without CTAB. B

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Figure 1. Low-angle XRD patterns of (a) (black) FeP-1M and (red) FeP-1ME and (b) (black) FeP-2M and (red) FeP-2ME. The higher order peaks of FeP-1M and FeP-1ME are enlarged.

Figure 2. (a and b) TEM images of 2D hexagonally ordered FeP-1ME. (The corresponding selected-area electron diffraction (ED) patterns in the inset to (b) show amorphous pore wall.). (c) TEM image of disordered FeP-2ME, with the upper right inset showing the selected-area ED patterns, and (d) high resolution TEM image of FeP-2ME showing the lattice fringes. (The mesopores are indicated by dotted circles.) (e and f) SEM images of FeP-1ME and FeP-2ME, respectively.

mesophase which originates from the suitable interaction between CTAB, NMPA and iron nitrate.21 As the synthetic temperature is increased, however (i.e., FeP-3M (or FeP3ME)), the crystal growth rate increases as well and the bulk products are formed without mesopores. Thus, the CTBA template clearly does not work as a structure directing agent. (The details are discussed in a later section.) For comparison, the bulk samples (FeP-4, FeP-5, and FeP-6) were prepared

without CTAB at various temperatures. These samples do not show any diffraction peaks in low angle XRD patterns. A TEM image of the mesoporous FeP sample after the removal of CTAB (FeP-1ME) is shown in Figure 2a, indicating the presence of well-ordered hexagonal arrangements of mesopores. As shown in Figure 2b, striped patterns are also observable for FeP-1ME, indicating the presence of onedimensional mesochannels originating from the two-dimenC

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Figure 3. (a) Wide-angle X-ray diffraction patterns of FeP-1ME, FeP-2ME, FeP-3ME, FeP-4, FeP-5, and FeP-6, respectively, and (b) experimental XRD pattern of FeP-6 (black symbols), computed XRD pattern (red solid line), and the difference between the experimental and the computed XRD patterns (blue spectrum at bottom). (c) The obtained crystal parameters of FeP.

sional (2D) hexagonal mesostructure (p6mm). The pore-topore distance is measured to be 4.82 nm, which almost completely matches the value calculated from low-angle XRD data (d100 × 2/√3 = 4.20 nm × 2/√3 = 4.85 nm). A TEM image of FeP-2ME is shown in Figure 2c. Compared to FeP1ME, the arrangement of mesopores is slightly distorted, which coincides with the low-angle XRD data (Figure 1b). Field emission SEM (FE-SEM) images of the obtained samples are shown in Figure 2e,f. From the SEM images, it is found that all the samples have a spherical morphology, but the particle sizes and the surface structures are varied, depending on the reaction conditions, including the applied temperatures. Among the mesoporous FeP samples, FeP-1ME shows very small spherical particles (Figure 2e). With increasing synthetic temperature, the average particle size gradually increases (Figure 2f). The elemental analysis for FeP-1ME (Figure S1 in the Supporting Information) and FeP-2ME (Supporting Information Figure S2) prove the presence of iron, oxygen, phosphorus, nitrogen, and carbon over the entire area. As mentioned above, in the case of FeP-3ME, the structure directing role of CTAB is not prominent, due to the higher synthetic temperature (423 K). Along with small nanoparticles (120−180 nm), larger spheres (∼2 μm) are also formed due to rapid crystal growth through the direct interaction of NMPA and iron nitrate (Supporting Information Figure S3a). In the case of FeP samples prepared without CTAB, much larger spherical particles are mainly observed (Supporting Information Figure S3b-d), compared to the mesoporous FeP samples (Figure 2). The sphere sizes increase gradually with increasing hydrothermal temperature. The sphere sizes are 2−3 μm (for FeP-4), 3−5 μm (for FeP-5), and 4−6 μm (for FeP-6), respectively (Supporting Information Figure S3b-d). On the surface of these spheres, it is observed that nanocrystals are randomly aggregated (Supporting Information Figure S3e-f). As shown in the above high resolution TEM data (Supporting Information Figure S4), all of the nanocrystals are highly crystallized. Figure 3a shows wide-angle X-ray diffraction (XRD) patterns of FeP-1ME, FeP-2ME, FeP-3ME, FeP-4, FeP-5, and FeP-6, respectively. For all the samples except FeP-1ME, the peak positions are exactly the same, although the obtained wideangle XRD patterns do not match any standard JCPDS cards. Therefore, to obtain the details of the crystal structure of our

new family of FeP materials, Rietveld refinement was carried out for the FeP-6 sample with intense diffraction peaks,30 by employing the MAUD program.31 After the final refinement, the crystal phase of the material is assigned as a tetragonal structure with unit cell parameters, a = b = 5.1963 Å and c = 12.9808 Å. The experimental and simulated XRD patterns of the material (FeP-6) are shown in Figure 3b. High resolution TEM images and the corresponding electron diffraction (ED) patterns for FeP-6 are presented in Supporting Information Figure S4. As the electron beam cannot penetrate through the large spheres, the edges are carefully observed. The lattice fringes are oriented in the same direction, suggesting that the material is highly crystallized. The distances between two fringes are in good agreement with the (101) and (112) crystal planes of the tetragonal crystal structure (a = b = 5.1963 Å and c = 12.9808 Å). The selected area ED patterns taken from the edge are also assignable to tetragonal FeP materials with the same unit cell parameters (inset of Supporting Information Figure S4). Thus, here we prepared various types of FeP materials with controlled porous structures and controlled degree of crystallinity. At lower synthetic temperature, we have successfully prepared hexagonally ordered mesoporous FeP materials (FeP-1ME), but the pore walls are amorphous. The selected area ED patterns for FeP-1ME (taken from 10 000 nm2, 100 nm × 100 nm) do not show any patterns (inset of Figure 2b), indicating that the pore walls are totally (or close to) amorphous phase which is in agreement with XRD data (Figure 3a). With increasing synthetic temperature, we have successfully improved the crystallinity in the pore walls (FeP2ME). Hence, FeP-2ME has well-developed mesoporous structure with crystalline pore walls. We have calculated the average crystallite sizes of the FeP-2ME sample from the Sherrer equation (D = 0.9λ/β cos θ), where D is the average crystallite size (Å), λ is the X-ray wavelength (Cu Kα = 1.5406 Å), β is the full width at half-maximum (fwhm) in radians, and θ is the Bragg diffraction angle. The average crystallite size using this equation is ca. 20 nm. On the high resolution TEM image (Figure 2d), the crystal fringes in the pore wall of the material are clearly observable, and they are coherently extended across over the mesopores. The distance between the observed two fringes (Figure 2d) is in good agreement with the (112) crystal plane of the tetragonal crystal structure. The D

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Figure 4. N2 adsorption−desorption isotherms of (a) FeP-1ME, FeP-2ME, and FeP-3ME and (b) FeP-4, Fep-5, and FeP-6. Pore size distribution curves of (c) FeP-1ME, FeP-2ME, and FeP-3ME and (d) FeP-4, FeP-5, and FeP-6. (e) Summary of surface areas and pore volumes for all the samples.

2.9 nm (Figure 4c). This pore diameter is quite similar to those of other mesoporous materials prepared by CTAB.20 The isotherm for FeP-2ME has a small capillary condensation step, due to the distortion of mesopores, as mentioned above (Figures 1b and 2c). FeP-3ME has no capillary condensation step (Figure 4a). With increasing synthetic temperature, the surface area is greatly decreased, as shown in Figure 4e. The FeP-4, FeP-5, and FeP-6 samples prepared without CTAB were also studied by N2 isotherms (Figure 4b). From the pore size distributions, these samples do not have any mesoporosity (Figure 4d). The FeP-1ME has a well-ordered 2D hexagonal structure, but the pore wall is not crystallized (i.e., an amorphous phase), while the mesostructural ordering of FeP-2ME is not so high (i.e., a wormhole-like structure), but the pore wall is highly crystallized. Therefore, assuming that FeP-2ME possesses a well-ordered 2D hexagonal structure (having the wall thickness (3.0 nm) and the cylindrical mesopore size (3.0 nm)), the theoretical surface area can be roughly calculated by considering the density of the crystalline phase (1.24 g·cm−3) in the pore wall. The calculated value is 296 m2·g−1. Their BET surface areas for FeP-1ME and FeP-2ME (Figure 4e) are quite reasonable and slightly higher than the theoretically calculated value due to significant contribution of the rough surface of pore walls. To further understand the framework structure, Fourier transform infrared (FT-IR) spectra were collected for FeP-1ME and FeP-2ME (Supporting Information Figure S5). The peak at 480 cm−1 reflects the Fe−O stretching vibration. A strong band

selected area ED patterns (inset of Figure 2c) with several intense spots also indicates the formation of tetragonal crystal structure. On further increasing the synthetic temperature, the rate of crystallization increases drastically, resulting in the absence of mesoporous structure (FeP-3ME). The absence of CTAB does not prevent the FeP crystallization. Therefore, FeP-4, FeP-5, and FeP-6 materials have no mesoporosity, but their crystallinity is higher. The porosity of our FeP materials was investigated by N2 adsorption−desorption isotherms at liquid nitrogen temperature. Among the mesoporous FeP samples (FeP-1ME, FeP2ME, and FeP-3ME), the most well-ordered mesoporous FeP (FeP-1ME) sample exhibits a type IV isotherm with a hysteresis loop (Figure 4a).32,33 The appearance of a well-defined step is due to the capillary condensation of N2 gas inside the uniformly sized mesopores. The sharpness of the step indicates the uniformity of the mesopores in the material. Such welldeveloped type IV isotherms have been hardly ever seen in phosphonate-based mesoporous materials.20 Lin et al. and Zhu et al. have reported amorphous mesoporous zirconium and cerium phosphonate materials with broad pore size distributions, in which a well-defined capillary condensation step is not observed in the N2 adsorption−desorption isotherms.34,35 Although Kimura, Haskouri et al., and Ma et al. have reported a few mesoporous phosphonate materials with type IV isotherms and well resolved pore size distributions, they are restricted to very common Al- and Ti-based materials.20−24 Interestingly, the pore size distribution curve of the FeP-1ME sample is sharp, and the mesopore size is found to be around E

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Figure 5. Charge−discharge profiles of (a) FeP-2ME and (b) FeP-4 for selected cycles at current density of 144 mA·g−1. (c) Cyclic performances of FeP-2ME (black symbols) and FeP-4 (blue symbols) at 144 mA·g−1.

centered at 1080 cm−1 is probably due to the tetrahedral stretching vibration of the −CPO3 group. A small intense peak around 1434 cm−1 is attributed to the P−C stretching vibration, and the -CH2- bending vibration appears at 1536 cm−1. The peak at 3184 cm−1 is due to the -CH2- stretching vibration in the material. The peaks at 1633 and 2350 cm−1 are due to the bending vibration of surface adsorbed water molecules and the presence of environmental CO2 molecules due to the experimental conditions, respectively. The broad band centered at 3425 cm−1 is also due to the adsorbed water molecule in the material. Thus, the FT-IR data confirms the formation of the FeP framework, even though FeP-1ME is composed of amorphous pore walls.36 The thermal stability of the FeP samples was determined by thermogravimetric analysis (TGA) under a continuous flow of air with a heating rate of 5 °C·min−1. All the samples have almost the same TG profile. The TG profile for FeP-2ME is shown in Supporting Information Figure S6 as a representative mesoporous FeP sample. The first weight loss (11.04 wt %) up to 434 K corresponds to the loss of the surface adsorbed water molecules. The second weight loss (10.40 wt %) starts at 540 K and continues up to 785 K, which is probably due to the burning of carbon (C) and nitrogen (N) from the framework. Thus, the materials show good thermal stability. As mentioned above, the elemental mapping and energy dispersive X-ray spectroscopy (EDX) analysis for FeP-1ME and FeP-2ME show the presence of iron, nitrogen, phosphorus, oxygen, and carbon and their homogeneous distribution throughout the materials (Supporting Information Figures S1 and S2). The elemental distribution for FeP-6 was also investigated (Supporting Information Figure S7). Here we calculated only the relative compositional mole ratio of P/Fe, because light elements such as N, C, and O cannot be precisely counted, and also, their amounts are slightly changed by contamination on the samples. The relative compositional mole ratios of P/Fe are calculated to be 0.43/1 (for FeP-1ME), 0.47/1 (for FeP-2ME), and 0.48/1 (for FeP-6), respectively. These values are almost the same as the initial compositional ratios of P/Fe (0.41/1). Under the present conditions, the interaction (i.e., charge balance) between the surfactant (CTA+) and the FeP building units is critically important for successful synthesis of wellordered mesostructured materials.37 The FeP species as building units are thought to be negatively charged in the reaction medium, due to the very common presence of free  PO− or FeO−. Therefore, there are two possibilities for the charge balance between CTA+ and FeP species; (i) P O− or (ii) FeO−. As can be seen in the FT-IR spectrum (Supporting Information Figure S5), the absence of any bands at 850−950 cm−1, which are associated with the stretching vibration of POH, clearly excludes the probability of

charge balance through PO−.38 Therefore, FeO− is thought to effectively interact with the positively charged CTA+, resulting in the formation of the proper mesostructure through a self-assembly process. Throughout this study, we found that the best compositional ratio of CTAB:NMPA:Fe(NO 3 ) 3 was 1:1:4 for well-ordered mesoporous FeP. Considering the structure of NMPA (as shown in Scheme 1), we can easily observe that there are oxygen atoms (attached to P) which are able to coordinate the positively charged Fe3+ ions. The amount of Fe3+ ions are in excess even after they are coordinated with all the PO− units of NMPA. Thus, there is no possibility for the free PO− to be present in the final mesostructured FeP materials (as supported by FT-IR data). The other remaining Fe3+ ions in the reaction medium are coordinated through nitrogen atoms and the charge and/or coordination number of iron is balanced through the interaction of hydroxide and/or water molecules present in the reaction medium. In our study, we use TMAOH to maintain the pH of the reaction medium. In such a condition, FeO− species can be easily formed and then encounters the positive charge of CTA+ in the formation of mesostructured FeP.20 Recently, there have been high requirements for the development of renewable, clean, and green energy sources that do not emit any hazardous compounds.39−41 In particular, the Li-ion rechargeable battery is the most promising source of clean energy42,43 and has been also recognized as a highly useful power source in portable electronic equipment such as cell phones, laptop computers, and cameras.44,45 Many efforts have been made to increase the battery capacity through sophisticated cells, mainly by designing the electrodes.46−48 Since the discovery of carbon-coated LiFePO4 particles as an electrode material,49−51 various pure and mixed metal oxides, sulfides, and phosphates have been extensively studied. Boyanov et al. also have reported various crystalline nonporous iron phosphide materials as anode materials in Li-ion battery.52−54 Currently, porous organic−inorganic hybrid solids (mainly iron-based) have been demonstrated as rechargeable electrode in the Li-ion battery.55 Although the organic groups have insulating behavior toward electrochemical intercalation, the porous architecture is helpful for successfully hosting the Li cations.56−58 Inspired by this progress on the successful use of Li-free materials as electrode material in the Li-ion battery,59,60 in this study we used our mesoporous FeP materials as an anode material in Li-ion rechargeable batteries to increase the volumetric energy density, through electrochemical reduction of Fe3+ to Fe2+, along with simultaneous insertion of Li+ into the large-sized mesopores. Crystalline materials with mesoporous architectures are able to easily transport lithium ions, which has great utility in the Li-ion rechargeable battery.61 F

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Here, we selected two samples, FeP-2ME (crystalline material with mesoporous structure) and FeP-4 (crystalline material without mesoporous structure), as anode materials for Li-ion batteries (Figure 5). In the case of FeP-2ME (Figure 5a), long voltage plateaus are clearly observed at around 0.8 and 1.0 V during the first discharge process. The first discharge and charge capacities are 790 mA h g−1 and 390 mA h g−1, respectively. The large capacity loss in the first cycle is probably due to irreversible processes such as the formation of solid electrolyte interphase (SEI) layers and electrolyte decomposition.41 After the second cycle onward, surprisingly the discharge and charge capacity of the material increase gradually, and they reach maximum values (after the fifth cycle) of 541 and 506 mA h g−1. Moreover from the sixth cycle the FeP-2ME electrode shows a good cycling stability (Figure 5c). Even at the end of the 30th cycle, a reversible capacity as high as 355 mA h g−1 can still be retained. But for FeP-4, the first discharge and charge capacities are 940 and 245 mA h g−1, showing very poor Coulombic efficiency (Figure 5b). The huge capacity loss of the material (FeP-4) is mainly caused by the nonporous structure, i.e., fading the kinetics of electrochemical reactions between nonporous FeP and lithium ion. However, from the second cycle onward, the FeP-4 electrode shows the fascinating cycling stability. After 30th cycle the reversible capacity retains 195 mA h g−1, but this reversible capacity at the end of the 30th cycle is much lower than that of FeP-2ME. Recently, mesoporous FePO4 materials have been synthesized under various synthetic conditions, but these capacities are not so high (first discharge and charge capacities are around 80−160 mA h g−1).62,63 Despite the presence of insulating organic units inside the pore walls, our mesoporous organic− inorganic hybrid FeP electrodes have very large energy storage capacity. It can be clearly demonstrated that our mesoporous FeP materials exhibit excellent specific capacity, outstanding rate performance, and very good cycling stability as electrode material in the Li-ion battery, which can be attributed to the unique crystalline porous architecture.

AUTHOR INFORMATION

Corresponding Authors

*(J.H.K.) E-mail: [email protected]. *(Y.Y.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSION A new family of mesoporous FeP materials was prepared through cooperative assembly of positively charged CTAB and negatively charged precursors. By proper selection of reaction temperature, the pore walls were effectively crystallized, although the mesostructural ordering was slightly decreased. To the best of our knowledge, this is the first report on the synthesis of ordered mesoporous FeP materials with crystalline pore walls. Due to the unique crystalline porous architectures, our mesoporous FeP materials showed superior electrochemical performance as electrode materials in Li-ion batteries. Compared to other Fe-based materials reported previously, our materials have quite good potential as electrode materials in advanced energy storage systems. We strongly believe that this synthetic strategy opens up a new route for the preparation of various metal phosphonate materials with several organophosphonic acid linkers, which will be important for further increasing the energy storage capacity.

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Figures showing elemental mappings, SEM images, TEM image, FT-IR spectra, and TGA profile. This material is available free of charge via the Internet at http://pubs.acs.org. G

DOI: 10.1021/cm5044045 Chem. Mater. XXXX, XXX, XXX−XXX

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