Facile Synthesis of Flowerlike LiFe5O8

Article Cite This: Inorg. Chem. 2017, 56, 14960−14967

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Facile Synthesis of Flowerlike LiFe5O8 Microspheres for Electrochemical Supercapacitors Ying Lin,† Jingjing Dong,† Jingjing Dai,† Jingping Wang,‡ Haibo Yang,*,† and Hanwen Zong† †

School of Materials Science and Engineering and ‡College of Chemistry and Chemical Engineering, Shaanxi University of Science and Technology, 710021 Xi’an, China

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S Supporting Information *

ABSTRACT: Facile synthesis of porous and hollow spinel materials is very urgent due to their extensive applications in the field of energy storage. In present work, flowerlike porous LiFe5O8 microspheres etched for 15, 30, and 45 min (named as p-LFO-15, p-LFO-30, and p-LFO-45, respectively) are successfully synthesized through a facile chemical etching method based on bulk LiFe5O8 (LFO) particles as precursors, and they are applied as electrode materials for highperformance electrochemical capacitors. In particular, the specific surface area of p-LFO-45 reaches 46.13 m2 g−1, which is 112 times greater than that of the unetched counterpart. Therefore, the p-LFO-45 electrode can achieve a higher capacitance of 278 F g−1 at a scan rate of 5 mV s−1 than the unetched counterpart. Furthermore, the p-LFO-45 electrode presents a good cycling stability with 78.3% of capacitive retention after 2000 cycles, which is much higher than that of the unetched LFO particles (66%). Therefore, the flowerlike porous LFO microspheres are very promising candidate materials for supercapacitor applications.

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INTRODUCTION To satisfy urgent needs of the modern electronic industry for sustainable development and renewable energy, massive efforts have been made to develop flexible, lightweight, and environmental-friendly energy storage devices, for example, electrochemical capacitors (ECs)1−4 and batteries.5−8 ECs are a new type of energy storage devices between the traditional capacitors and rechargeable batteries. The capacity of ECs can reach hundreds to thousands of farads. Compared with the traditional capacitors, they have larger capacity, higher power density, wider operating temperature range, and longer service life. Moreover, they have high specific power and no pollution to the environment compared with the storage batteries. Therefore, it can be said that ECs are efficient, practical, environmental-friendly energy storage devices. These excellent features make them to be superior energy storage devices in a variety of applications, for instance, portable electronic devices, large-sized industrial equipments, hybrid vehicles, and so on, in which higher power density and longer cycle-life are extremely desired.9−11 Currently, porous metal oxide materials have aroused great interest, as they are widely used in many fields, for example, lithium ion batteries,12,13 ion exchange,14 and catalysis processes,15 due to their interaction with ions, atoms, and molecules, not only on the surface of materials but also in the interior of materials.16,17 Consequently, converting the structure of materials from the lumpy to the porous offers an effective way to enhance their performance. Lithium ferrites, © 2017 American Chemical Society

especially LiFe5O8, as one of the prospective typical iron-based materials, have the advantages of low cost and environmental friendliness,18 and they have been widely researched for all kinds of technological applications, including microwave devices,19 optoisolators,20 ferrite-core memory systems,21 and cathode/anode materials in lithium ion batteries,22−26 and so on. LFO is a cubic spinel ferrite27 with a high Curie temperature (620 °C),28 where Fe3+ ions occupy sixcoordinated octahedral or four-coordinated tetrahedral sites, and Li+ ions occupy six-coordinated octahedral sites. At present, Kim and Manthiram29 reported that LFO had a discharge capacity of ∼140 mAh g−1 in the range of 1.5−4.3 V, demonstrating an excellent cycling retention. Similarly, Lee et al.30 also fabricated the nanocrystalline LixFeyOz powder containing α-LiFe5O8 and β-LiFe5O8, and the initial discharge capacity of α-LiFeO2 is 215 mAh g−1 with outstanding cyclability. Nevertheless, to the best of our knowledge, the electrochemical performance of the flowerlike porous LFO microspheres as the active materials for ECs has not been reported. Recently, there are many kinds of synthesis methods for LFO, such as the thermal decomposition method,31 the conventional ceramic method,32 the sol−gel autocombustion method,33 etc. Although these methods have many advantages, they only fit to prepare bulk LFO particles. It is necessary to Received: September 2, 2017 Published: December 1, 2017 14960

DOI: 10.1021/acs.inorgchem.7b02257 Inorg. Chem. 2017, 56, 14960−14967

Article

Inorganic Chemistry find a novel method that can fabricate the flowerlike porous LFO particles. The etching method34,35 is an excellent synthesis method, which attracts much attention, because it possesses a simple process, short production cycle, no waste residue, and environmental friendliness. Herein, flowerlike porous LFO microspheres were synthesized through a facile two-step process that involves preparation of bulk LFO particles by a molten salt approach and the subsequent preparation of flowerlike porous LFO microspheres by utilizing the etching method. A simple path was developed to optimize the microsphere morphology by adjusting the etching time, and finally the flowerlike porous LFO microspheres can be achieved. The specific surface area reaches the maximum value, when the bulk LFO particles are etched for 45 min. Particularly, the p-LFO-45 electrode displays an ideal capacitive behavior of 278 F g−1 at a scan rate of 5 mV s−1 in 1 M LiNO3 solution. The flowerlike porous LFO microspheres and the connected porous structures could shorten charge transmission path and provide a continuous pathway for the diffusion of the electrolyte, which is favorable for improving the electrochemical performance of electrode materials.

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the following procedures. Electroactive materials (LFO, 80 wt %), acetylene black (10 wt %), and poly(vinylidene fluoride) (PVDF, 10 wt %) were homogeneously mixed in N-methyl-2-pyrolidene (NMP) to obtain a uniform slurry. The slurry was then coated on a nickel grid (1 cm2) and dried at 80 °C in vacuum for 12 h. The mass load of active material was ∼1−2 mg cm−2. All the electrochemical measurements were performed in 1 M LiNO3 aqueous solution, and the potential window is between −0.8 and −0.3 V. The specific capacitance (C) of the electrode can be evaluated according to the following equation:

C=

I × Δt M × ΔE

(1)

−1

where C (F g ) is the specific capacitance, I (A) is the discharge current, M (g) is the mass of electrode material, Δt (s) is the discharge time, and ΔE (V) is the voltage change after full discharge.

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RESULTS AND DISCUSSION Structure Characterization. The crystalline structure and phase purity of product are investigated by XRD. As shown in Figure 1, all the diffraction peaks in the spectrum can be well-

EXPERIMENTAL SECTION

Reagents and Materials. Reagents including Li2CO3, Fe2O3 (starting materials), and Na2SO4−Li2SO4 (molten salt system) were purchased from Sinopharm Group. Dimethylformamide (DMF), hydrazine (a reducing agent), and methyl mercaptoacetate (a complexing agent) were also obtained from Sinopharm Group. All the chemicals were of analytical grade. Preparation of Bulk LFO Particles and Flowerlike Porous LFO Microspheres. Bulk LFO particles were synthesized via a molten salt method. Li2CO3, Fe2O3, and Na2SO4−Li2SO436−38 were mixed in a molar ratio of [Li]/[Fe] = 1:5 and a molar ratio of LFO/ Na2SO4−Li2SO4 = 1:5. The mixture was ground in ethanol for 4 h. The mixture was transferred to a crucible after drying. Finally, the bulk LFO particles can be obtained after the mixture was calcined at 900 °C for 4 h and then rinsed with deionized water several times to remove the Na2SO4−Li2SO4 salt mixture. Flowerlike porous LFO microspheres were prepared via a one-step etching method. Bulk LFO particles (100 mg) were dispersed ultrasonically into a solvent of DMF (100 mL) and heated in a water bath at 80 °C, and hydrazine (12.5 mL) and methyl mercaptoacetate (5 mL) were added. N2 was used to prevent the reaction between methyl mercaptoacetate and O2. The reaction was terminated by cold ethanol after etching for different times (15, 30, 45 min). The black powders were obtained after washing by ethanol and deionized water several times, respectively, and drying in vacuum for 12 h. Characterization. The phase structure of the samples was analyzed by X-ray diffraction (XRD, D/max-2200, Japan) using Cu Kα radiation (λ = 0.154 18 nm) and Raman spectra at room temperature using a microscopic Raman spectrometer (ALMEGATM, Thermo Nicolet, American). The morphology characterization of all the samples was obtained using field emission scanning electron microscopy (FE-SEM, Hitachi, S-4800, Japan), and transmission electron microscope (TEM, JEOL-2010, Japan). The surface areas of as-prepared samples were determined using a Brunauer−Emmet− Teller (BET, ASAP 2020, Micromeritics, USA), and the pore size distribution was estimated according to Horvath−Kawazoe (HK) theory. The surface chemical compositions and chemical element valence states of as-prepared samples were analyzed via X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, USA). Electrochemical Measurements. The electrochemical characteristics were studied on an electrochemical workstation (CHI660D, China) using a three-electrode system composed of a platinum plate (counter electrode), Ag/AgCl electrode (reference electrode), and working electrode. The working electrodes were prepared according to

Figure 1. XRD patterns of the obtained (a) Sample LFO; (b) Sample p-LFO-15; (c) Sample p-LFO-30; (d) Sample p-LFO-45.

corresponded to the orthorhombic structure of LFO with the lattice parameters of a = 8.337 Å, b = 8.337 Å, and c = 8.337 Å, which is consistent with the standard data (JSPDS card No. 17−0115). The unetched bulk LFO particles prepared by the molten salt method are well-crystallized. Furthermore, the XRD patterns of the etched LFO samples with different etching times show that the phase constitution keeps unchanged, but the diffraction peak intensities significantly decrease with enlarging the etching time, which may be attributed to numerous nanosized fragments in the porous structure of LFO particles.34 When prolonging the etching time to 60 min, two impurity phases of FeS and FeLi1.13S2 generate, as shown in Figure S1 (Supporting Information). The detailed structural information on the as-prepared samples is proved by Raman spectra. The LFO samples have an ordered spinel structure belonging to the X-ray P41 (No. 332) space group with the unit-cell formula of Fe8[Li4Fe12]O32 (a = 8.337 Å). The irreducible representation for the allowed modes of the ordered spinel LiFe5O8 is Γ = 6A1(R) + 14E(R) + 22F2(R) + 20F1(IR), in which (R) and (IR) represent Ramanand infrared-active vibrations, respectively. Raman-active (42 modes) vibrations point group is 6A1(R) + 14E(R) + 22F2(R) and infrared-active vibrations (20 modes) point group is 20F1.39 As shown Figure 2, it can be observed that there are six modes located at 201, 225, 264, 486, 597, and 709 cm−1 in the unetched bulk LFO particles. The peak positions of the etched 14961


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seen that the unetched LFO particles display a typical blocky shape, which is in good agreement with the SEM image in Figure 3a. The TEM image of Sample p-LFO-45, as shown in Figure 4b, further confirms that the flowerlike porous structure is composed of 2D nanosheets. The high-resolution TEM (HRTEM) images of Sample p-LFO-45 are presented in Figure 4c. The d-spacings of ∼2.514 Å are corresponding to the (311) lattices of orthorhombic LFO. The N2 adsorption−desorption isotherms and pore size distribution of as-prepared samples are presented in Figure 5. According to the IUPAC classification, the N2 adsorption− desorption isotherm of the unetched bulk LFO particles displays a type II isotherm, illustrating the characteristics of nonporous adsorbent. Additionally, the isotherms of etched LFO samples with different etching times exhibit Type IV behaviors, indicating the presence of pores.40 The surface area, pore volume, and pore size calculated by using the BET equation are evidently enhanced, as shown in Table 1. It can be concluded that, with increasing the etching time, the specific surface area increases accordingly and reaches the maximum value of 46.13 m2 g−1 after etching for 45 min, significantly larger than that of the unetched bulk LFO particles (0.41 m2 g−1), which indicates that the generation of pore enlarges the surface area effectively. Because of the increased surface area, flowerlike porous LFO microspheres are expected to exhibit enhanced electrochemical performance.41 The XPS patterns of the unetched bulk LFO particles and Sample p-LFO-45 are illustrated in Figure 6. It can be seen from Figure 6a that both the unetched bulk LFO particles and Sample p-LFO-45 mainly include Li, Fe, and O elements. As shown in Figure 6b, the peaks of Li 1s for unetched bulk LFO particles are located at ∼55.4 eV. By comparison, Li 1s peaks of Sample p-LFO-45 move slightly to higher binding energy due to the formation of surface lithium defects. Furthermore, for the unetched bulk LFO particles, the peaks at 710.6 and 724.6 eV, which are, respectively, coincident with Fe 2p3/2 and Fe 2p1/2, corresponding to the signals of Fe3+,42 as shown in Figure 6c. By comparison, the peaks of Fe 2p corresponding to Fe2+ can be found at 709.1 and 722.7 eV in Sample p-LFO-45, indicating that Fe3+ is reduced to Fe2+ by hydrazine. From Figure 6d, the

Figure 2. Raman spectra of the obtained Samples: (a) LFO; (b) pLFO-15; (c) p-LFO-30; (d) p-LFO-45.

LFO samples remain unchanged, while the intensities present a trend of decline with increasing the etching time, corresponding to the above XRD result, which indicates the stabilization of crystal structure and the reduction of crystallinity in porous LFO microspheres. The morphology and microstructure of the as-prepared samples are detected by the SEM, as shown in Figure 3. It can be clearly seen that the unetched LFO particles are produced homogeneously into a blocky structure, and the average grain size is ∼0.2−1 μm. Great change has taken place on the surface of the bulk LFO particles after etching, and the surfaces of the bulk LFO particles become rough after etching for 15 min. With gradually increasing the etching time to 30 min, some nanosheets are generated on the surface of LFO particles. When etching for 45 min, the produced nanosheets become more and more noticeable, and bulk LFO particles are almost etched into large amounts of nanosheets, which gather together and then construct flowerlike porous LFO microspheres. When etched for 60 min, it can be seen that the LFO surface becomes smooth, as shown in Figure S2 (Supporting Information). To further investigate the morphology of flowerlike porous LFO microspheres, the as-prepared samples are collected and investigated by TEM. As shown in Figure 4a, it can be clearly

Figure 3. SEM images of the obtained Samples: (a) LFO; (b) p-LFO-15; (c) p-LFO-30; (d) p-LFO-45. 14962


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Figure 4. TEM images of Samples (a) LFO and (b) p-LFO-45. (c) HRTEM image of Sample p-LFO-45.

with methyl mercaptoacetate to dissolve in DMF, and breaking of Fe−O results in the peeling of the LFO grains. At the same time, LFO ultrathin nanosheets are generated from the composition of LFO in DMF, which are growing on their own parents.35 Electrochemical Characterization. Figure 7 shows the cyclic voltammetry (CV) curves of as-prepared samples at various scan rates ranging from 5 to 100 mV s−1 within a voltage window between −0.8 and −0.3 V in 1 M aqueous LiNO3 solution at room temperature to reflect the electrochemical properties. Obviously, all the CV curves almost present a rectangular shape, demonstrating an ideal capacitive behavior and a rapid charging/discharging process characteristic for all the as-prepared LFO samples. The specific capacitances are summarized in Table 2. Sample p-LFO-45 possesses the greatest capacitance behavior, and the maximum specific capacitance of 278 F g−1 at a scan rate of 5 mV s−1 can be obtained. The much-enhanced capacitance of Sample p-LFO45 might result from the highly porous surface, which brings about lots of active sites, and these active sites can augment the deintercalation and intercalation of electrolyte ions. While for the unetched bulk LFO particles, the specific capacitance is smaller than that of Sample p-LFO-45, demonstrating the increase of porous surface is beneficial to improve the capacitance performance. The surface redox reactions can be related to the formation of Fe−O−Li or Fe−O−H upon Fe3+ to Fe2+ reduction, as it is similar with Mn4+ to Mn3+ reduction in MnO2 electrode.44−46 To examine the electrochemical capacitive properties of the as-prepared samples, the galvanostatic charge/discharge (GCD) curves were further measured at varied current densities, as shown in Figure 8. The specific capacitance of all the samples calculated from Equation 1 is presented in Table 3, and the maximum value of specific capacitance reaches 190 F g−1 for Sample p-LFP-45. The GCD curves show that the discharge time decreases with increasing the current density, indicating that a lower current density could exhibit a higher Coulombic efficiency for the electrode materials arising from the sufficient deintercalation and intercalation of protons during the charge and discharge steps.

Figure 5. N2 adsorption−desorption isotherms and pore size distribution (inset) of Samples LFO, p-LFO-15, p-LFO-30, and pLFO-45.

Table 1. Porous Characteristics of All LFP Samples sample

pore volume (cm3/g)

pore size (Å)

specific surface area (m2/g)

LFO particles p-LFO-15 p-LFO-30 p-LFO-45

0.001 281 0.000 691 0.000 359 0.003 166

336.2220 197.2286 146.8959 75.9685

0.41 6.57 25.17 46.13

O 1s peaks of the unetched bulk LFO particles correspond with three Gaussian curves at 529.5, 531.5, and 533.1 eV, which can be referred to oxygen vacancy, ordered lattice oxygen ions, and surface-absorbed H2O,43 respectively. Besides, the O 1s peaks of Sample p-LFO-45 show a slight shift toward the higher binding energy, revealing a growing number of lattice oxygen ions on Sample p-LFO-45. The results concluded by XPS spectra clarify elaborately the reaction mechanism, in which the partial Fe3+ of LFO particles are reduced to the Fe2+ by hydrazine and subsequently coordinated with methyl mercaptoacetate. A plausible formation scheme of flowerlike porous LFO microspheres based on XPS result is illustrated in Scheme 1. First, Fe3+ of bulk LFO particles can be reduced gradually to Fe2+ by hydrazine. Then, the Fe2+ is immediately coordinated 14963


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Figure 6. XPS spectra of Samples LFO and p-LFO-45: (a) survey; (b) Li 1s; (c) Fe 2p; and (d) O 1s.

Scheme 1. Chemical Reaction Schemes of the Etching Process

Figure 7. CV curves of the obtained Samples: (a) LFO; (b) p-LFO-15; (c) p-LFO-30; (d) p-LFO-45.

samples are presented in Figure 9. The inset of the figure shows an enlarged portion of the high-frequency region. The electrical

Following an overview of electrochemical impedance spectroscopy (EIS) investigations, the Nyquist diagrams of prepared 14964


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Inorganic Chemistry Table 2. Specific Capacitance Values of All LFO Samplesa sample LFO particles p-LFO-15 p-LFO-30 p-LFO-45 a

5 (mV s−1)

10 (mV s−1)

20 (mV s−1)

50 (mV s−1)

100 (mV s−1)

57

23

19

15

15

64 76 278

25 52 201

23 44 173

19 34 135

17 28 105

Table 3. Specific Capacitance Values of All LFO Samples at Different Current Densities sample

0.5 (A g−1)

2 (A g−1)

3 (A g−1)

4 (A g−1)

5 (A g−1)

LFO particles p-LFO-15 p-LFO-30 p-LFO-45

9 10 37 190

7 9 22 142

6 8 14 99

5 7 11 83

5 6 8 73

At different scan rates of 5, 10, 20, 50, and 100 mV s−1.

equivalent circuit (EEC) was selected according to a proper physicochemical model of the prepared electrode, and it fits the experimental EIS data well. These data are fitted with an equivalent circuit model consisting of an equivalent series resistance (Rs) denoting the resistance of electrolyte. R1 and R2 refer to the charge transfer resistance.47 Two constant phase elements CPE1 and CPE2 of pure capacitors represent the capacitances at high and low frequencies, respectively. Warburg diffusion element (W value) was related to the lithium ion diffusion during the deintercalation/intercalation process of the flowerlike porous LFO microsphere electrode materials in LiNO3 electrolyte solution. Low Rs value of 2.998 Ω can be observed for Sample p-LFO-45 compared with the values of 3.553, 4.239, and 3.091 Ω, determined for bulk LFO particles, Sample p-LFO-15, and Sample p-LFO-30, respectively, which indicates the better electrical conductivity of Sample p-LFO-45, because the porous structure shortens the charge transmission path and brings about lots of active sites.48−50 It may be noticed that in the low-frequency region, the supercapacitors made of LFO samples show an inclined line above 45°, which indicates the pseudocapacitive nature of the materials.51 In addition, cycling stability is a significant parameter in the practical application of supercapacitors. So the stability and durability of the unetched bulk LFO particles and Sample pLFO-45 electrodes are also evaluated up to 2000 cycles by the repeated charge−discharge tests at a constant current density of 1 A/g. The results are shown in Figure 10, and the inset plot shows the GCD curves of charge/discharge test from 991 to

Figure 9. EIS curves of the obtained Samples (a) LFO, (b) p-LFO-15, (c) p-LFO-30, and (d) p-LFO-45 and their equivalent circuits.

1000 cycles. It is clearly observed that the Sample p-LFO-45 presents a good cycling stability with 78.3% of capacitive retention after 2000 cycles, which is much higher than that of unetched bulk LFO particles (66%). On the basis of the results above, it is believed that the Sample p-LFO-45 is a promising electrode material for aqueous-based supercapacitor application regarding high specific capacitance and superior cycling property.

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CONCLUSIONS In this work, flowerlike porous LFO microspheres were synthesized via two steps containing a molten salt method and the subsequent etching process. The microstructure of LFO particles is transformed to a flowerlike porous micro-

Figure 8. GCD curves at different current densities of the obtained Samples: (a) LFO; (b) p-LFO-15; (c) p-LFO-30; (d) p-LFO-45. 14965


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Postdoctoral Science Foundation (Grant No. 2016M590916), the Science and Technology Foundation of Weiyang District of Xi’an City (Grant No. 201605), and the Industrialization Foundation of Education Department of Shaanxi Provincial Government (Grant No. 16JF002).

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Figure 10. Cycling performance of Samples (a) LFO and (b) p-LFO45. (inset) GCD curves from 991 to 1000 cycles.

structure by making use of the etching properties with a DMF solution of methyl mercaptoacetate and hydrazine. There is no doubt that flowerlike porous LFO microspheres with higher specific surface areas can be obtained after etching. The results show that the LFO particles etched for 45 min display the largest surface area, and they exhibit a much enhanced electrochemical capacitive performance of 278 F g−1 at scan rates of 5 mV s−1 and a 78.3% capacitance retention after 2000 cycles. The results suggest that the as-synthesized flowerlike porous LFO microspheres might be of potential application for supercapacitor electrode material.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02257. XRD pattern and SEM image of LFO etched for 60 min (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-29-86168688. Fax: +86-29-86168688. E-mail: [email protected]. ORCID

Haibo Yang: 0000-0003-1828-3750 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant No. 51572159), the Chinese 14966


Article

Inorganic Chemistry

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Facile Synthesis of Flowerlike LiFe5O8

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