CoFe2O4

May 2, 2014 - (a) A schematic illustration of the preparation procedure of the CFO-700 nanocomposite and CFO-900 samples. (b) Crystal structure of...
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High Rate Capability and Long Cycle Stability of Co3O4/CoFe2O4 Nanocomposite as an Anode Material for High-Performance Secondary Lithium Ion Batteries Alok Kumar Rai,∥,† Jihyeon Gim,∥,† Trang Vu Thi,† Docheon Ahn,‡ Sung June Cho,§ and Jaekook Kim*,† †

Department of Materials Science and Engineering, Chonnam National University, 300 Yongbong-dong, Bukgu, Gwangju 500-757, Republic of Korea ‡ Beamline Research Division, Pohang Accelerator Laboratory, Pohang 790-784, Republic of Korea § Department of Applied Chemical Engineering, Chonnam National University, 300 Yongbong-dong, Bukgu, Gwangju 500-757, Republic of Korea ABSTRACT: A facile and cost-effective urea-assisted autocombustion strategy has been designed for the fabrication of Co3O4/CoFe2O4 nanocomposite and pure CoFe2O4 anode materials followed by annealing at 700 and 900 °C for 6 h, respectively. To confirm the exact structure, Rietiveld analysis was performed on the Synchrotron XRD pattern of both the CoFe2O4 samples annealed at 700 and 900 °C. The results clearly depicts the formation of two phases (Co3O4:CoFe2O4) with the ratio of [76.3(5):23.6(3)%] in the sample annealed at 700 °C sample, while single phase CoFe2O4 formation was observed for the sample annealed at 900 °C. It has also been found that the designed nanocomposite sample is composed of small nanoparticles (50−100 nm), while the size of pure CoFe2O4 particles is in the range from 600 nm to 1 μm. When applied as an anode material, the obtained Co3O4/CoFe2O4 nanocomposite electrode exhibits high reversible capacity as well as excellent cycling stability and better rate capability in comparison to a pure CoFe2O4 electrode. The enhanced electrochemical performance of the nanocomposite can be attributed to the intimate interconnection between Co3O4 and CoFe2O4, along with the nanosize range of particles and high surface area, which not only favor fast kinetic properties facilitating electron transportation and Li+ ion insertion/deinsertion but also relieve the stress caused by volume changes during the numerous charge/discharge cycles and suppress the degradation of the material.

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INTRODUCTION Lithium-ion batteries are the most attractive secondary batteries because of their high energy density, long cycling lifetime, and excellent safety. Rechargeable lithium-ion batteries are also considered as potential power sources for hybrid electric vehicles (HEVs) and electric vehicles (EVs). However, in order to meet the high demand of energy storage, devices with high energy and power density has boosted extensive research in designing novel electrode materials with improved energy density, cycle life, cost, and safety. Graphite is the widely used commercial anode material, but its theoretical specific capacity is only 372 mAh g−1, which cannot satisfy the increasing demand for high-performance lithium-ion batteries.1 Transition metal oxides are potentially good anode materials for Li+-ion batteries2−6 because of their larger lithium storage capacities and better safety than commercially used graphite.7 Despite the many advantages, transition metal oxide anodes often exhibit low conductivity and severe pulverization during charge/ discharge processes, which eventually cause rapid anode disintegration under the induced mechanical stress and capacity fading upon cycling,8 reducing the possibility for practical application. In addition to research on the many potential highcapacity anode materials such as transition metal oxides, lithium © 2014 American Chemical Society

alloy (Li−M, M = Sn, Si, Sb, and Al), and tin-based (Co−Sn− C) and silicon (SiO2) based anodes, there is still much research being done to find new options for anode electrode materials with lower cost, safe operation at high-current charge/discharge rates, and improved energy density and rate capability to replace commercial graphite. Recently, great attention has been paid to the nanostructured mixed spinel transition metal oxides for Li+-ion battery applications.9−17 Spinel transition metal oxides with two metal elements provide the feasibility to tune the energy density and working voltage by varying the metal content. Among all the mixed transition metal oxides, CoFe2O4 has been extensively researched as a promising anode material for lithium ion batteries due to its high theoretical capacity of 916 mAh g−1, which is almost two times higher than that of graphite (372 mAh g−1). However, as like other anode materials, CoFe2O4 also suffers from the problems of poor electronic conductivity and large volume change during the Li+ ion insertion/extraction process, thus leading to capacity fading, poor cycling stability, Received: February 26, 2014 Revised: April 30, 2014 Published: May 2, 2014 11234

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Figure 1. (a) A schematic illustration of the preparation procedure of the CFO-700 nanocomposite and CFO-900 samples. (b) Crystal structure of CoFe2O4 based on the Rietveld refinement observation. Rietveld refinement plot for CFO-700 and CFO-900 samples: (c) CFO-700 and (d) CFO900. The Bragg positions with cyan and magenta color sticks indicated the CoFe2O4 and Co3O4 phase, respectively. Above 2θ = 70°, the magnified patterns (×3) are provided for more clarity.

and rate capability performance,17 which limits its potential for application in the high-energy storage system. To solve these problems, many strategies could be employed, such as the fabrication of nanosized particles with unique nanostructures, which offers shorter lithium diffusion length, optimizes the architecture of mixed transition metal oxides to improve the kinetics,18 and allows for the carbon-based nanocomposites to enhance the electrical conductivity.19−21 Recently, hybridization of bicomponent, such as SnO2−CoO reticular structure,22 Co3O4 nanowire/MnO2 core/shell nanobelt array,23 is also capable of exhibiting a strong synergistic effect of high capacity and remarkable rate capability, which are better than those of each individual component.24 In addition, the electrochemical properties of yolk−shell-structured materials with distinctive core@void@shell configurations have also been extensively studied as anode materials for lithium ion batteries.25−27 These yolk−shell structures provide good electrochemical properties as a result of shortened diffusion length and the void spaces accommodating the volume expansion during charge/recharge cycles.25−27 More importantly, some effort has also been directed at the search for hybrid pseudocapacitive materials such as mixed metal oxides28,29 and binary metal oxide/ hydroxides.30,31 Due to the lack of well-defined micro/ nanostructures, electrochemical performance for this kind of electrode material has been largely unsatisfactory, and the possible synergistic effect between individual constituents has

so far not been well-understood. However, it remains a stiff challenge to develop binary mixed metal oxides with smart architecture using a simple and facile approach, in which structural features and electroactivities of each component are effectively established, and the interface/chemical distributions are homogeneous at the nanoscale.24 Therefore, in the present work, we report the fabrication of a novel binary Co3O4/CoFe2O4 nanocomposite, which was designed to improve the electrochemical performance of host CoFe2O4 as an anode material for lithium ion batteries. It is well-known that Co3O4 is electrochemically active. Therefore, it may be possible that the existence of Co3O4 in the nanocomposite sample may accelerate the electrochemical reaction or participates in the electrochemical reaction by the storage of more than eight lithium atoms per formula unit (Co3O4 + 8Li+ + 8e− → 3Co + 4Li2O). In addition, it is also reasonable to suggest that the volume expansion or contraction during charge or discharge process in both the phases is expected to happen sequentially, thus reducing the strain and improving the stability. The designed Co3O4/CoFe2O4 nanocomposite and pure CoFe2O4 were synthesized by a facile ureaassisted autocombustion method combined with subsequent annealing treatment. Urea-assisted autocombustion synthesis is an efficient and convenient method to prepare metal oxide nanoparticles at relatively low temperature. This process produces sub-nanometer-size of metal oxide nanoparticles by 11235

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self-generated heat of reaction with very short reaction time. The advantage of urea is that they can form stable complexes with metal ions to increase solubility and prevent selective precipitation of the metal ions during water removal. In addition, it is also believed that urea-assisted autocombustion synthesis is very effective for the fabrication of various mixed metal oxide nanocomposites composed of different metal cations.4,6 The resultant oxide ash after combustion is generally composed of very fine particles with the desired stoichiometry linked together in a network structure.4,6 The designed Co3O4/ CoFe2O4 nanocomposite exhibits greatly enhanced electrochemical performance with high reversible capacity, excellent cycling performance, and better rate capability in comparison to pure CoFe2O4 and in situ synthesized CoFe2O4−Co rods.18

6Co(NO3)2 + 6Fe(NO3)3 + 25NH 2CONH 2 1/2O2

⎯⎯⎯⎯⎯→ 3CoFe2O4 + Co3O4 + 40N2 + 50H 2O + 25CO2 (1)

3Co(NO3)2 + 6Fe(NO3)3 + 20NH 2CONH 2 → 3CoFe2O4 + 32N2 + 40H 2O + 20CO2

(2)

In the urea-assisted autocombustion synthesis, the nitrate ions act as the oxidizer, while urea acts as the fuel. The reaction products are finely divided metal oxides, and the evolved gases N2, CO2, and H2O. The excess urea also decomposes into ammonia and other gases. Materials Characterization. The crystal structures of the annealed products were characterized by Synchrotron X-ray powder diffraction data, which was collected at the 9B high resolution powder diffraction beamline of the Pohang Light Source, Korea. The incident X-rays were monochromatized to the wavelength of 1.547 Å by a double-bounce Si (111) monochromator. The detector arm of the diffractometer had soller slits with an angular resolution of 2 degrees, as well as a flat Ge{111} crystal analyzer, an antiscatter baffle, and a scintillation detector. To obtain the accurate chemical compositions for the designed nanocomposite, chemical analysis of the synthesized materials was performed with an inductively coupled plasma-atomic emission spectrometer (ICP-AES) on a PerkinElmer Optima 4300 DV model. The surface morphologies and particle size of the obtained powders were examined by field-emission scanning electron microscopy (FE-SEM, S-4700 Hitachi) and field-emission transmission electron microscopy (FE-TEM, Philips Tecnai F20 at 200 kV in KBSI Chonnam National University), along with corresponding selected-area electron diffraction (SAED) patterns. For FETEM, powder samples were ultrasonically dispersed in ethanol, and a few drops were coated onto copper grids. The surface area can be determined based on the nitrogen adsorption and desorption isotherms using the Brunauer−Emmett−Teller method (BET, Micromeritics ASAP2010 Instrument Company, Norcross, GA). Electrochemical Measurements. Electrochemical measurements were performed with a coin-type cell (type 2032) using Li foil as the counter electrode. The working electrodes were fabricated using a slurry of the active materials (CFO-700 nanocomposite and CFO-900), and super-P and poly vinylidene fluoride at a weight ratio of 70:20:10, dissolved in Nmethyl-2-pyrrolidone. The slurry was pasted onto pure copper foil as a current collector, dried under vacuum at 100 °C for 12 h, and then pressed between hot stainless steel twin rollers. The cells were assembled in a glovebox filled with highly pure argon gas. The electrolyte was 1 M LiPF6 dissolved in an equi-volume mixture of ethylene carbonate and dimethylcarbonate (1:1 volume ratio). The cyclic voltammogram (CV) were obtained at a scan rate of 0.1 mV s−1 in the range of 0.0−3.0 V (vs Li/ Li+) by using Bio Logic Science Instrument (VSP 1075). Galvanostatic testing (BTS-2004H, Nagano, Japan) of the coin cells was conducted using a programmable battery tester over the potential range of 0.01−3.0 V versus Li+/Li.

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EXPERIMENTAL SECTION Materials Synthesis. To synthesize Co3O4/CoFe2O4 spinel oxide, urea-assisted autocombustion synthesis was chosen. The detailed preparation procedure can be found in our previous papers.4,6 In a typical synthesis, a nonstoichiometric amount of cobalt nitrate [CoN2O6·6H2O, 98% Aldrich] and iron nitrate [Fe(NO3)3·9H2O, 98% Junsei extra pure] were carefully dissolved in deionized water separately under continuous stirring at room temperature. Fixed equal amounts of metal nitrates (0.01 mol cobalt nitrate + 0.01 mol iron nitrate) were used to design the nanocomposite. Aqueous solutions of cobalt nitrate and iron nitrate were mixed together with a separately prepared aqueous solution of urea (NH2CONH2, 99%, Aldrich), and the ratio between urea and nitrates was maintained at 25:12 to allow for controlled combustion. 6 The stoichiometric amount of urea was determined by calculations based on the valencies of oxidizing and reducing elements, as suggested for the propellant chemistry and also used for the powder preparation of several oxide materials.32,33 Typical valencies were considered for the most common elements, such as +4 for C, +1 for H, and −2 for O, and the valencies for the metallic elements were those present in the nitrates, such as +2 for Co and +3 for Fe. A valency of 0 is ascribed to N. For example, the equivalent valency for Co(NO3)2 is +2 + [0 + (−2 × 3)] × 2 = −10 and for Fe(NO3)3 is +3 + [0 + (−2 × 3)] × 3 = −15 and that for NH2CONH2 is 0 + 2 + 4 + (−2) + 0 + 2 = 6. Hence, the ureato-nitrate ratio needed to obtain targeted product is thus 10:6 and 15:6 or 25:12.6,32 The obtained ternary mixed solution was evaporated on a hot plate using a magnetic stirrer at 350 °C under continuous stirring to remove excess water. During the evaporation, the homogeneously mixed solution turned viscous, eventually becoming a gel. The formed gel slowly foamed, swelled, and finally burned on its own. The whole process was completed within a few seconds. In order to eliminate possible organic residues and to stabilize the microstructure of the Co3O4/CoFe2O4 powder, the as-synthesized powder was subsequently annealed at 700 °C for 6 h in an air atmosphere. For comparison, pure CoFe2O4 was also synthesized by annealing the same as-synthesized powder at 900 °C for 6 h with the same heating rate in an air atmosphere. However, both the samples annealed at 700 and 900 °C is denoted as CFO700 and CFO-900 in the following discussion. A schematic illustration of the preparation procedure is also provided in Figure 1a. The relevant equations 1 and 2 at 700 and 900 °C, respectively, can be expressed as follows based on the reported literatures:6,32

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RESULTS AND DISCUSSION Crystal Structure and Morphology. During the experiment, the authors have used the 0.1 molar ratios for each cobalt nitrate and iron nitrate in spite of the 1:2 ratio to form the targeted CoFe2O4. However, it was believed that the extra 11236

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lattice parameters, positional coordinates, and thermal parameters were varied in the course of refinement. The Rietiveld analysis performed on the Synchrotron XRD pattern of the CFO-700 nanocomposite sample and estimated the fractions of CoFe2O4 and Co3O4 phases to be 76.3(5)% and 23.6(3)%, respectively. On the other hand, the Rietveld analysis performed on the Synchrotron XRD pattern of the CFO-900 sample, which clearly indicate the presence of only the CoFe2O4 phase, as shown in Figure 1d. The final Rietveld refinement plot shown in Figure 1 (panels c and d) also showed high quality of fitting over the entire region. The slight increase in the lattice parameter of CFO-700 sample than that of the CFO-900 sample may be probably due to the chemical composition change in the host matrix of pure CoFe2O4. Since the lattice parameter change is small, it is clearly approving that the amount of secondary phase formed is also small. However, in both the cases, the low Rp, Rwp, and Rexp values, in Table 2, clearly indicate the good quality of fit in determining the structure of the phase constituents. The obtained crystal structure parameters and site occupation factors for both the prepared samples are listed in Table 2. It can be seen that the CFO-700 nanocomposite sample contained two phases (Co3O4 and CoFe2O4) and tends to become a single phase of CoFe2O4 at elevated temperatures of 900 °C. Hence, it is highly probable that the phase segregation of Co3O4 occurred in the CoFe2O4 nanocomposite sample at 700 °C, and the elevated heat treatment at 900 °C may result in the diffusion of Co into the CoFe2O4 phase and thereby lead to the formation of pure CoFe2O4 as the end product. The sharp diffraction peaks and high intensity indicate the good crystallinity of both CFO-700 and CFO-900 prepared samples. Therefore, it can be concluded that the single phase CoFe2O4 could not be obtained at an apparently lower temperature of 700 °C, whereas the formation of single phase CoFe2O4 occurred at 900 °C.34 Figure 2 shows FE-SEM images of the designed CFO-700 nanocomposite and CFO-900 samples. It is found that the CFO-700 nanocomposite sample consists of agglomerated clusters of small spherical nanoparticles, as shown in Figure 2a. The primary particles in the sample have smooth spherelike morphology and size ranging from 50 to 100 nm. Figure 2b displays the FE-SEM image of the CFO-900 sample. The diameter of the particles grows larger (from 600 nm to 1 μm) with nonuniform size distribution because of the fusion between the nanocrystals at a higher annealing temperature. There are some small particles on the surface of the sample annealed at 900 °C, which may have arisen from the decomposition of CoFe2O4 at high temperature.35 It is also reasonable to suggest that higher annealing temperature is favorable for the complete and pure formation of a CoFe2O4 sample. To further understand the morphology and structural characteristics of the designed CFO-700 nanocomposite and CFO-900 particles, FE-TEM and HR-TEM images and their corresponding SAED patterns were also employed, and the results are illustrated in Figure 3. Figure 3a shows a FE-TEM image of the designed CFO-700 nanoparticles. It can be seen that some of the nanoparticles are connected, forming secondary particles with clear smooth surfaces, which confirms the FE-SEM observations. The nanoparticles will have enlarged contact area with the electrolyte, which is helpful for reducing the path length of both lithium ions (Li+) and electrons (e−) during Li+ ion intercalation and deintercalation. Figure 3b shows a FE-TEM image of the microsized CFO-900 sample.

amount of cobalt (Co) will be present as an extra phase within the obtained product in the form of Co3O4. It is well-known that Co3O4 is electrochemically active. Hence, the authors believed that the obtained Co3O4 as an extra phase may be helpful to improve the electrochemical performance of host material. To know the exact composition of Co and Fe in both CFO-700 and CFO-900 annealed samples, ICP-AES analysis was performed and the results are listed in Table 1. For further Table 1. Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES) Analysis Results and Formula Equation Calculated by ICP and Site Occupancy from Rietveld Refinement for Comparisona sample concentration (wt %) molar ratiob formula obtained from ICP calculation

formula obtained from Rietveld refinementc

CFO700 CFO900 CFO700 CFO900

CFO-700

CFO-900

Co 42.5 Fe 35.6 Co 1.592 Fe 1.408 0.24(Co2+1)[Co3+2]O4 · 0.76(Co2+0.2839Fe3+0.717) [Co2+0.717Fe3+1.126Co3+0.157]O4 (Co2+0.374Fe3+0.626) [Co2+0.626Fe3+0.785Co3+0.589]O4 0.24(Co2+1)[Co3+2]O4 · 0.76(Co2+0.305Fe3+0.700) [Co2+0.700Fe3+1.059Co3+0.236]O4 (Co2+0.366Fe3+0.627) [Co2+0.626Fe3+0.796Co3+0.585]O4

42.9 36.1 1.589 1.411

a

The brackets and square brackets with formula represent tetrahedral and octahedral site, respectively. bThe sum of mole values of Fe and Co was assumed to be 3. cThe value is calculated from occupancy of refinement results as linear proportionality.

confirmation of Co and Fe content, Rietveld refinement was also performed using FULLPROF on the synchrotron XRD data of both the samples. To find a good quality of fit in determining the structure and chemical formula of the phase constituents, ICP data was also taken into consideration. It can be seen that the obtained formula from ICP calculations are corresponded approximately with the formula obtained from Rietveld refinement calculations. The crystal data, atomic coordination, lattice parameters, and site occupation factors estimated from the Rietveld analysis for both the samples and the data are summarized in Table 2. On the investigation of the crystal structure using the Rietveld refinement method, the annealed CFO-700 nanocomposite and CFO-900 samples are observed to belong to the same space group (Fd̅3m symmetry). However, CoFe2O4 and Co3O4 phases coexisted in the 700 °C sample, whereas the 900 °C sample possessed a single phase of CoFe2O4. The crystal structures of CoFe2O4 and Co3O4 phases are clearly depicted in Figure 1c. It can be seen that Co2+ and Fe3+ may have random distribution in both the tetrahedral and octahedral sites, while all the low-spin Co3+ only occupies the octahedral sites for both the CFO-700 and CFO-900 samples.34 On the other hand, Co with different oxidation states is also distributed among both tetrahedral and octahedral positions in the obtained secondary Co3O4 phase. However, Figure 1b shows a crystal structure of CoFe2O4 based on the Rietveld refinement observation. The occupancy parameter of the Fe ion was fixed at their nominal composition, while the occupancy parameter of the Co ion was varied (in order to estimate the exact composition) and vice versa. All other parameters, such as scale factor, zero correction, background, half-width parameters, the mixing parameters, 11237

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Table 2. Rietveld Refinement Results of the CFO-700 Nanocomposite and CFO-900 Samples Using Synchrotron X-ray Diffraction Data CFO-700 sample refined composition formula weight fraction (%) crystal symmetry lattice parameter, a (Å) unit cell volume (Å3) Rp Rwp Rexp S sample phase CFO-700

Co1+xFe2−xO4

Co3O4

CFO-900

a

Co1+xFe2−xO4

CFO-900

Co1+xFe2−xO4

Co3O4 − 1925.36 23.6(3) Fd̅3m 8.1307(3) 537.51(1)

atom

Co1.235Fe1.752O4 1877.97 76.3(5) Fd̅3m 8.3640(5) 585.129(2) 5.79 7.44 6.03 1.23 x

y

z

site

Co(II) Fe(III) Co(II) Fe(III) Co(III) O Co(II) Co(III) O Co(II) Fe(III) Co(II) Fe(III) Co(III) O

0.125 0.125 0.5 0.5 0.5 0.255(6) 0.125 0.5 0.264(2) 0.125 0.125 0.5 0.5 0.5 0.257(1)

0.125 0.125 0.5 0.5 0.5 0.255(6) 0.125 0.5 0.264(2) 0.125 0.125 0.5 0.5 0.5 0.257(1)

0.125 0.125 0.5 0.5 0.5 0.255(6) 0.125 0.5 0.264(2) 0.125 0.125 0.5 0.5 0.5 0.257(1)

8a 8a 16d 16d 16d 32e 8a 16d 32e 8a 8a 16d 16d 16d 32e

Co1+xFe2−xO4 Co1.589Fe1.440O4 1891.47 100 Fd̅3m 8.32405(5) 576.772(2) 6.39 8.36 5.91 1.41 occ. B (Å2) 0.303(2) 0.696(2) 0.348(2) 0.526(2) 0.117(2) 1.0a 1.0a 1.0a 1.0a 0.368(4) 0.631(4) 0.315(4) 0.405(4) 0.294(4) 1.0a

0.64(8) 0.64(8) 0.51(5) 0.51(5) 0.51(5) 1.5(1) 0.9(3) 0.6(2) 2.9(7) 0.6(1) 0.6(1) 0.5(1) 0.5(1) 0.5(1) 1.7(2)

Fixed parameter.

planes are matched very well with those of XRD patterns. However, contrary to the clear interfacial boundaries observed in the formation of amorphous domains for similar types of composites reported earlier,36 we believe that there are a few reasons that the transitional areas between the two phases in the present nanocomposite are difficult to detect. First, this may be interpreted in terms of the distribution of the highly crystalline nanoscale Co3O4−CoFe2O4 dual phase over the entire area of nanocomposite sample.37 Hence, it may be practically very hard to observe grain boundaries; rather it might form a vague phase segregation in the mother texture. Second, it may also be possible that the Co3O4 nanoparticles are densely dispersed on the surface of CoFe2O4 and do not form a complete coating, which may not impede the lithium diffusion during cycling while protecting the surface of the material.38 N2 adsorption−desorption studies were also performed to determine the specific surface area of both the CFO-700 nanocomposite and CFO-900 samples. Nitrogen adsorption/ desorption isotherms of both CFO-700 and CFO-900 samples are presented in Figure 4 (panels a and b, respectively). Little variation is observed for the specific surface areas of both the samples. The BET surface area obtained from adsorption isotherms was decreased from 7 m2 g−1 in CFO-700 to 0.3 m2 g−1 in the CFO-900 sample. The significant reduction in the surface area of CFO-900 sample was believed to have been related to the larger particle sizes of the sample. It can be clearly seen that the CFO-700 nanocomposite due to smaller particle size has a higher surface area. With the increase of the annealing temperature, there is a decrease of surface area. The results

Figure 2. FE-SEM images: (a) CFO-700 nanocomposite and (b) CFO-900 samples.

Increasing the annealing temperature increased the diameter of the CoFe2O4 samples. The particles of the CFO-900 sample are not uniformly distributed. The ranges of particle sizes are the same according to FE-SEM observation. To gain structural information, HR-TEM imaging was also performed on the edges of nanoparticles and microparticles (Figures 3, panels c and d). Both products indicate high crystallinity with distinct lattice fringes. The measured neighboring interplanar distances are consistent with the (311) and (220) planes for both the CFO-700 and CFO-900 samples, confirming the XRD analysis. Figure 3 (panels e and f) show the SAED patterns for the CFO700 nanocomposite and CFO-900 samples, respectively. The well-defined points in the patterns indicate the crystalline nature of the samples. Both the patterns are contributed by the characteristic crystal planes of CoFe2O4, and the calculated 11238

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Electrochemical Performances. Figure 5 (panels a and b) presents the CV curves of the designed CFO-700 nanocomposite electrode and CFO-900 electrodes, respectively, for the first five cycles at a scan rate of 0.1 mV s−1. The first CV curve differs from the subsequent CV curves. In the first scan, the main cathodic peak is observed at 0.55 V for the CFO-700 electrode and at 0.47 V for the CFO-900 electrode, corresponding to the reduction of Fe3+ and Co2+ to their metallic states and the formation of Li2O (CoFe2O4 + 8Li+ + 8e− ↔ Co + 2Fe + 4Li2O), accompanied by the decomposition of organic electrolyte to form a solid electrolyte interphase (SEI) layer. In addition, a small cathodic peak at 0.71 V in the first cycle is also observed (Figure 5a), which could be assigned to the formation of a stable intermediate LinCo3O4/CoFe2O4.39 Moreover, in the first charge process, two anodic peaks located at ∼1.71 and 2.01 V for CFO-700 electrode, whereas only one anodic peak at ∼1.75 V for the CFO-900 electrode, can be ascribed to the oxidation reaction of metallic Fe and Co (Co + 2Fe + 4Li2O ↔ CoO + Fe2O3 + 8Li+). However, the mixed transition metal oxide CoFe2O4 stores Li through the reversible formation and decomposition of Li2O. During the successive cycles, only one cathodic peak at ∼0.95 V for both the electrodes is observed, which may be due to the individual reduction processes of Fe3+ and Co2+ merging into one peak.39 The subsequent anodic peaks are slightly shifted to higher potentials of 1.76 and 2.09 V for the CFO-700 electrode and ∼1.79 V for the CFO-900 electrode, which are related to the oxidation reaction of CoO and Fe2O3 metal oxides. The positive shift between the first and the subsequent cycles should correspond to the polarization of the electrode.39 The decrease of the redox peak intensity implies that the capacity is decreased after the first cycle. Notably, the peak intensity and integrated area during the anodic and cathodic polarization process remained unchanged after the first cycle, indicating good electrochemical reversibility and stability of both the CFO-700 and CFO-900 electrodes.40 As shown in Figure 6 (panels a and b), the voltage profiles for the first three cycles of the designed CFO-700 nanocomposite electrode and CFO-900 electrode are displayed between 0.01 and 3.0 V at 0.07 C, respectively. The discharge and charge capacities in the first run are 1353.9 and 983.0 mAh g−1, respectively, for the CFO-700 nanocomposite electrode, which are higher than that of the CFO-900 electrode (1205.5 and 739.5 mAh g−1). In the case of the CFO-700 nano-

Figure 3. FE-TEM, HR-TEM, and their corresponding SAED patterns of CFO-700 nanocomposite (a, c, e) and CFO-900 (b, d, f) samples, respectively.

reveal that the growth of the nanocrystallites in the sample at high temperature causes the diffusion of the smaller particles. It is well-known that a large surface area can provide more locations and channels for fast Li+ ion insertion/extraction into the electrode material. Obviously, the BET specific surface area of the nanocomposite sample is higher than that of the pure sample, which is due to the small range of nanoparticles.

Figure 4. Nitrogen adsorption/desorption isotherms of (a) CFO-700 nanocomposite and (b) CFO-900 samples, respectively. 11239

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Figure 5. Cyclic voltammograms profiles of (a) CFO-700 nanocomposite and (b) CFO-900 electrodes, respectively.

Figure 6. Charge/discharge curves in the initial three cycles and cycling performance vs Coulombic efficiency at a constant current rate of 0.07 C for CFO-700 nanocomposite (a and c) and CFO-900 electrodes (b and d), respectively.

composite electrode (Figure 6a), the first discharge originates from the open circuit voltage (OCV: ∼2.1 V), and then the cell voltage drops down quickly to ∼1.0 V, followed by a large plateau region at ∼0.85 V that extends to a capacity of ∼880 mAh g−1. This capacity corresponds to the consumption of ∼7.7 mol of Li. After the voltage plateau region, the profile shows a gradual sloping region until the end of the first discharge voltage of 0.01 V, with a total initial discharge capacity of 1353.9 mAh g−1, which is associated with the consumption of ∼11.8 mol of Li per mole of CFO-700. This is

all due to the conversion reactions of Fe3+ and Co2+ to their metallic states and the formation of Li2O. More importantly, one additional small voltage plateau at ∼1.0 V can be also seen in the first discharge. The voltage of this plateau is closer to that of cobalt reduction in Co3O4 than to that of iron reduction.41 Moreover, according to the mechanism (CoFe2O4 + 8Li+ + 8e− ↔ Co + 2Fe + 4Li2O), the discharge process should consume 8.0 lithium per CoFe2O4, but the experimental value is ∼11.8 lithium per CFO-700 nanocomposite. The first charge (Liextraction) profile from 0.01 to 3.0 V shows a broad plateau 11240

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region that occurs in the range of 1.5−2.2 V, which can be ascribed to the oxidation reactions of metallic Fe and Co (Co + 2Fe + 4Li2O ↔ CoO + Fe2O3 + 8Li+). The total first charge capacity of 983.0 mAh g−1 corresponds to the extraction of ∼8.6 mol of Li per formula unit. The difference between the discharge and charge capacities in the first cycle, corresponding to the irreversible capacity loss, is thus 27%. Irreversible capacity loss was observed during the first cycle, which is expected for most anodes due to the incomplete conversion reaction and the SEI layer formation at the electrode/ electrolyte interface caused by the decomposition of electrolytes in the voltage range of 0.05−0.8 V. In the case of the CFO-900 electrode (Figure 6b), the voltage plateau regions in the discharge and charge profiles are almost in the same position except a small voltage plateau at ∼1.0 V, indicating that similar redox reactions take place in the electrode during the charge/discharge processes such as the large discharge plateau region at ∼0.8 V, which continues until the discharge capacity of 800 mAh g−1, corresponding to the consumption of 7.0 mol of Li. Another slope observed at the last 0.01 V, with a total initial discharge capacity of 1205.5 mAh g−1, is associated with the consumption of only 10.5 mol of Li per mole of CoFe2O4. In addition, the first charge capacity (Li+ deinsertion) is 739.5 mAh g−1 (6.5 mol of Li). The irreversible capacity loss is almost 39% during the first cycle. Reversible discharge and charge capacities of 943.5 and 934.9 mAh g−1 and 947.5 and 926.4 mAh g−1 were achieved in the second and third cycles, respectively, for the CFO-700 nanocomposite electrode, which are much higher than those of the CFO-900 electrode (710.7 and 643.1 mAh g−1 for the second cycle and 639.9 and 600.7 mAh g−1 for the third cycle). The cycling performance of the designed CFO-700 nanocomposite electrode and CFO-900 electrode at a constant current rate of 0.07 C is given in Figure 6 (panels c and d, respectively). The CFO-700 nanocomposite electrode exhibits significantly excellent cycling stability with nominal capacity fading compared to CFO-900 electrode, possibly revealing an intimate interconnection between Co3O4 and CoFe2O4. The CFO-700 electrode can maintain a charge capacity of 896.4 mAh g−1 after 60 cycles, corresponding to about 96% of the second charge capacity, while for CFO-900 electrode, its charge capacity drops to only 458.2 mAh g−1 after the same number of cycles (corresponding to 71% of the second charge capacity). The much larger particle sizes of the CFO-900 sample may be responsible for its faster capacity fading during cycling, which can be due to the longer lithium ion (Li+) diffusion distance with lower charge/discharge transfer. The reversible capacity obtained for the nanocomposite after 60 cycles is more than two times higher than the reversible capacity of commercial graphite. In addition, the electrochemical discharging and charging capacities match each other very well during cycling, manifesting good reversibility with almost 100% Coulombic efficiency, as shown in the insets of Figure 6 (panels c and d). Another excellent property associated with the designed CFO-700 nanocomposite electrode is its high rate capability. Figure 7 shows the comparative rate capability of the CFO-700 and CFO-900 electrodes at various C-rates (1 C = 916 mA g−1), measured from 0.07 to 6.6 C in ascending order and followed by a return to 0.07 C. The CFO-700 nanocomposite electrode significantly presents better cycling stability at each rate in comparison to the CFO-900 electrode. The reversible charge capacities of the CFO-700 nanocomposite electrode were measured to be 983.0, 853.0, 796.7, 723.8, 640.8, 548.4,

Figure 7. Comparative rate capability of CFO-700 and CFO-900 electrodes at various C-rates.

423.9, and 328.1 mAh g−1 at the current rates of 0.07 C, 0.1 C, 0.3 C, 0.5 C, 1 C, 2.1 C, 4.2 and 6.6 C, respectively. Even at a high current rate of 6.6 C, the charge capacity still retains 35% of the second specific charge capacity (934.9 mAh g−1). When the current rate returns back to the initial value of 0.07 C, the charge capacity can be recovered to 756.5 mAh g−1, indicating that the structure of the nanocomposite was stable during cycling. The CFO-900 electrode exhibits comparatively smaller charge capacities of 739.5, 610.9, 502.4, 447.0, 361.3, 284.6, 218.2, and 169.6 mAh g−1 at the same respective current rates. The enhanced rate and retention capability of the designed CFO-700 nanocomposite electrode can be attributed to its unique structure, nanosize range of particles, and high surface area, which not only favor fast kinetic properties facilitating electron transportation and Li+ ion insertion/deinsertion but also relieve the stress caused by volume changes during the numerous charge/discharge cycles and suppress the degradation of the material. Remarkably, the designed CFO-700 nanocomposite electrode showed charge/discharge capacity, cycle number, and rate capability values that are not much lower than those reported for a pure CoFe2O4 electrode,18,42 but the synthesis adopted in the present study is cost-effective, facile, and scalable.

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CONCLUSIONS In summary, a simple and cost-effective urea-assisted autocombustion synthesis was used to obtain a designed CFO-700 nanocomposite and CFO-900 samples. This process enables the synthesis of a mixed metal oxide nanocomposite. To confirm the exact structure, Rietiveld analysis was performed on the Synchrotron XRD pattern of both the CoFe2O4 samples annealed at 700 and 900 °C. The results clearly depict the formation of two phases (Co3O4:CoFe2O4) with the ratio of [76.3(5):23.6(3)%] in the sample annealed at 700 °C, while single phase CoFe2O4 formation was observed for the sample annealed at 900 °C. More importantly, the resulting CFO-700 nanocomposite was composed of small nanoparticles (50−100 nm), while the particle sizes of the CFO-900 sample was in the range from 600 nm to 1 μm. As a promising anode material, CFO-700 nanocomposite exhibits significantly high lithium storage, excellent cycling performance, and better rate capability. The CFO-700 nanocomposite delivers an initial charge capacity of 983.0 mAh g−1 at a current rate of 0.07 C and maintains a specific capacity of 896.4 11241

dx.doi.org/10.1021/jp502004c | J. Phys. Chem. C 2014, 118, 11234−11243

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Article

mAh g−1 after 60 cycles and a superior C-rate capability of 328.1 mAh g−1 at 6.6 C, while the CFO-900 electrode capacity fades quickly, delivering only 458.2 mAh g−1 after 60 cycles at 0.07 C and a poor rate capability of 169.6 mAh g−1 at 6.6 °C. The enhanced electrochemical performance of the nanocomposite electrode can be attributed to the unique structure with a complementary intimate interconnection effect between Co3O4 and CoFe2O4, as well as the nanosized range of particles, which offers large surface area for providing more sites for Li+ion insertion, and a shorter path for Li+-ion and electron transport. In addition, it may also be probable that the enhanced electrochemical performance of the CFO-700 nanocomposite sample is due to the few other reasons, such as (i) the presence of the extra phase of Co3O4 in the nanocomposite sample may also contribute to improve the electrochemical performance of the parent CoFe2O4 material by the storage of more than eight lithium atoms per formula unit (Co3O4 + 8Li+ + 8e− → 3Co + 4Li2O), (ii) the extra phase of Co3O4 may act as a catalyst in the nanocomposite sample during Li+ ion intercalation and deintercalation and helping in the fast decomposition of Li2O, and (iii) the obtained Co3O4 may act as a pillar to accommodate the strain induced by volume changes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +82-62-530-1703. Fax: +82-62-530-1699. Author Contributions ∥

A.K.R. and J.G. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Global Frontier R&D Program (2013-073298) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning.

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