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Co1−xFe2+xO4 (x = 0.1, 0.2) anode materials for rechargeable lithium-ion batteries
Solid State Sciences 36 (2014) 1e7
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Co1xFe2þxO4 (x ¼ 0.1, 0.2) anode materials for rechargeable lithium-ion batteries Alok Kumar Rai, Trang Vu Thi, Jihyeon Gim, Vinod Mathew, Jaekook Kim* Department of Materials Science and Engineering, Chonnam National University, 300 Yongbong-dong, Bukgu, Gwangju 500-757, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 April 2014 Received in revised form 30 June 2014 Accepted 3 July 2014 Available online 15 July 2014
A cobalt-poor or iron rich bicomponent mixture of Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 anode materials have been successfully prepared using simple, cost-effective, and scalable urea-assisted autocombustion synthesis. The threshold limit of lower cobalt stoichiometry in CoFe2O4 that leads to impressive electrochemical performance was identified. The electrochemical performance shows that the Co0.9Fe2.1O4/Fe2O3 electrode exhibits high capacity and rate capability in comparison to a Co0.8Fe2.2O4/Fe2O3 electrode, and the obtained data is comparable with that reported for cobalt-rich CoFe2O4. The better rate performance of the Co0.9Fe2.1O4/Fe2O3 electrode is ascribed to its unique stoichiometry, which intimately prefers the combination of Fe2O3 with Co1xFe2þxO4 and the high electrical conductivity. Further, the high reversible capacity in Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 electrodes is most likely attributed to the synergistic electrochemical activity of both the nanostructured materials (Co1xFe2þxO4 and Fe2O3), reaching beyond the well-established mechanisms of charge storage in these two phases. © 2014 Elsevier Masson SAS. All rights reserved.
Keywords: CoFe2O4 Anode Rate capability Lithium-ion battery
1. Introduction Recently, new candidates for anode materials using iron-based spinel oxides with a general formula of AFe2O4 (A ¼ Co, Zn, Ni, Mn) have been reported for use in lithium ion batteries (LIBs) due to their high gravimetric specific capacities [1]. Spinel transitionmetal oxides with two metal elements make it feasible to tune the energy density and working voltage by varying the metal content [2]. Among these, cobalt ferrite (CoFe2O4) has received particular attention due to its high theoretical capacity of 916 mAh g1, which is two times higher than that of graphite (372 mAh g1). In general, the lithium storage mechanism in CoFe2O4 is attributed to a redox conversion reaction (CoFe2O4 þ 8Liþ þ 8e 4 Co þ 2Fe þ 4Li2O), wherein it is reduced to metals nanograins dispersed in to Li2O matrix upon lithiation and then reversibly restored to their initial oxidation states during delithiation. On the other hand, among the other alternative anode materials, Fe2O3 has always been regarded as very appealing transition metal oxide because of its much higher capacity (1007 mAh g1) than that of conventional graphite (372 mAh g1), as well as nontoxicity, high corrosion resistance and low processing
* Corresponding author. Tel.: þ82 62 530 1703; fax: þ82 62 530 1699. E-mail address: [email protected] (J. Kim). http://dx.doi.org/10.1016/j.solidstatesciences.2014.07.002 1293-2558/© 2014 Elsevier Masson SAS. All rights reserved.
cost. The lithium storage mechanism of Fe2O3 is based on the assumption of the reversible reduction of the oxide into metallic Fe (Fe2O3 þ 6Liþ þ 6e 4 2Fe þ 3Li2O). However, similar to other anodes, both the above said materials suffer from common issues such as poor cycling performance, a large initial irreversible capacity loss, and poor rate capability due to material pulverization and slow charge diffusion during the charging and discharging processes [3e5]. The present study attempts to focus on improving the performance of spinel based CoFe2O4 oxides. In fact, great efforts have been made to improve the electrochemical performance of these iron-based oxides [6,7]. The most commonly used strategy is to design hybrid nanocomposites such as metal oxide/carbon composite [8,9]. However, carbon containing hybrids generally exhibit relatively low specific capacity compared to pure metal oxides owing to the presence of carbon, as well as its low theoretical capacity. On the other hand, optimizing the architecture of mixed transition-metal oxides at the nano-scale to improve the kinetics has been also one of the main approaches. Due to the small dimensions, the nanoparticles can tolerate the strain associated with expansion/contraction much better. In addition, recently, M. Jia et al. (2013) and M. Zhang et al. (2013) designed new cobalt-rich CoFe2O4-Co rods, which showed better electrochemical performance than pure CoFe2O4 electrodes [3,10]. Further, synthesizing bicomponent metal oxides has been also proven to be an efficient
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route to improve capacity and rate capability in comparison to each individual component since bicomponent metal oxides often integrates two types of functional materials for a synergistic effect that can improve the intrinsic properties of each component such as electrical/ionic conductivity, electrochemical reactivity, and mechanical stability [11e13]. Therefore, it is still a great challenge to develop a simple and low cost method for synthesizing novel nanostructured bicomponent electrode architectures with improved electrochemical performances. Herein, we report on an interesting strategy to combat the effects of a drastic volume change more effectively by combining two phases that react with lithium efficiently and improve the capacity. It is also believed that during the charge or discharge process in such a bicomponent mixture electrode, the volume expansion or contraction in the two phases is expected to happen sequentially, thus reducing the strain and improving the stability [14]. It is well-known that Fe2O3 has higher theoretical capacity than that of spinel-type CoFe2O4. However, to utilize the high capacity and minimize the structure instability of Fe2O3, we describe the preparation of bicomponent architecture comprised of nanostructured CoFe2O4 and Fe2O3 and their electrochemical performances were also tested as anode materials for secondary LIBs. The threshold limit of lower cobalt stoichiometry in CoFe2O4 that leads to impressive electrochemical performance was also identified. The cobalt-poor Co1xFe2þxO4 (x ¼ 0.1) electrode demonstrated better properties among the prepared stoichiometric compositions in Co1xFe2þxO4 (x ¼ 0.1, 0.2). The high reversible capacity of the obtained bicomponent mixtures of Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 can be attributed to the synergistic electrochemical activity of both the nanostructured material (Co1xFe2þxO4 and Fe2O3), reaching beyond the well-established mechanisms of charge storage in these two phases. 2. Experimental
2.2. Materials characterization The crystal structure and morphology of the obtained products were investigated using a Shimadzu X-ray diffractometer with Cu Ka radiation (l ¼ 1.5406 Å), as well as field-emission scanning electron microscopy (FE-SEM, S-4700 Hitachi) and field-emission transmission electron microscopy (FE-TEM, Philips Tecnai F20 at 200 kV). For FE-TEM characterization, the samples were first dipped in ethanol and dispersed by ultrasonic vibration before coating onto copper grids. The chemical compositions of transition metals in the annealed bicomponent mixtures were analyzed by an inductively coupled plasma (ICP) emission spectroscopy (Perkin Elmer OPTIMA 4300 DV). 2.3. Electrochemical measurements The synthesized products were uniformly mixed with super-P and PVDF in a weight ratio of 70:20:10 in N-methyl-2pyrrolidone solvent to form a slurry. The resulting slurry was coated onto a copper foil current collector and dried under vacuum at 80 C overnight. The slurry was punched into circular electrodes after pressing between stainless steel twin rollers, in order to improve the contact between the active material and copper foil. The 2032 coin-type cells were assembled in a glove box under a dry argon atmosphere using lithium metal as a reference electrode, and a polymer membrane together with glass fiber as a separator. The electrolyte used was 1 M LiPF6 dissolved in a 1:1 (volume ratio) mixture of ethylene carbonate and dimethyl carbonate. The cell was assembled in a glove box filled with argon gas. The cells were cycled between 0.01 and 3.0 V at different C-rates (BTS-2004H, Nagano, Japan). Cyclic voltammetry (CV) measurements were carried out on a Bio Logic Science Instrument (VSP 1075) over the potential range 0.0e3.0 V at a scanning rate of 0.1 mV s1. Electrochemical impedance spectroscopy (EIS) measurements of the electrodes were also carried out using a Bio Logic Science Instrument (VSP 1075) after the cell was cycled for 5 cycles.
2.1. Materials synthesis 3. Results and discussion Nanosized cobalt-poor or iron rich mixtures of Co1xFe2þxO4 (x ¼ 0.1, 0.2) were synthesized by facile and cost-effective ureaassisted auto-combustion synthesis [15,16]. 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 self-generated heat of reaction within a very short reaction time. The advantage of urea is that it can form stable complexes with metal ions to increase solubility and prevent selective precipitation of the metal ions during water removal. In addition, the resultant oxide ash after combustion is generally composed of very fine particles with the desired stoichiometry linked together in a network structure [15,16]. The calculated amounts 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. The obtained aqueous nitrate solutions were mixed together and subsequently an aqueous solution of urea ((NH2CONH2, 99%, Aldrich) was added. The urea-to-nitrate molar ratio (urea : cobalt nitrate ¼ 10:6 and urea : iron nitrate ¼ 15:6) was maintained to facilitate controlled combustion [15,16]. The obtained turbid solution was evaporated on a hot plate using a magnetic stirrer at 300 C, which finally turned into gels and burnt on its own. In order to eliminate possible organic residues and to stabilize the microstructure of the samples, the as-synthesized powders were subsequently annealed at 700 C for 6 h in air atmosphere.
3.1. Structural and morphological analysis XRD patterns of Co1xFe2þxO4 (x ¼ 0.1, 0.2) powders annealed at 700 C for 6 h are shown in Fig. 1(a) and (b). Both the samples show a major phase of spinel CoFe2O4 (JCPDS No. 22-1086, Fd-3m (227)) along with the expected Fe2O3 (JCPDS No. 89e0599) as a secondary phase. It can be clearly observed that the intensity of Fe2O3 peaks increases with the decrease of the amount of cobalt. The sharp diffraction peaks and high intensity indicate good crystallinity in both samples. No peaks of any other phases or any other impurities were detected. To synthesize the bicomponent mixture electrode, the authors have reduced the amount of Co or increased the amount of Fe during the experiment in spite of the 1:2 ratio to form the targeted compound Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3. It is believed that the extra amount of Fe present in the form of Fe2O3 as an extra phase can be helpful to enhance the electrochemical performance of host material. However, to know the exact chemical composition of Co and Fe in both the obtained bicomponent mixtures, ICP-AES analysis was performed and the results are displayed in Table 1. The results showed that the chemical compositions of both the samples are nearly equal to the stoichiometric ratio as targeted. In addition, ICP data was also taken into consideration to roughly calculate the phase fraction ratio of CoFe2O4 and Fe2O3 in both the obtained mixtures assuming that the CoFe2O4 and Fe2O3 are perfectly stoichiometric and the detailed results are displayed in Table 1. The phase fraction ratio of CoFe2O4
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synthesis probably due to their magnetic behavior. In addition, Fig. 2(e) and (f) shows typical HR-TEM images of Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 samples, revealing that the particles are structurally uniform with a lattice spacing of about 2.5 Å, which corresponds to the (311) lattice plane of the CoFe2O4. Furthermore, the insets of Fig. 2(e) and (f) shows the corresponding Fast-Fourier transform (FFT) patterns of Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/ Fe2O3 samples, respectively. The FFT images were taken from the marked red square regions in the HR-TEM images. The measured lattice spacings in the FFT patterns for both the samples also tend to support the formation of CoFe2O4. 3.2. Electrochemical properties
Fig. 1. XRD patterns of Co1xFe2þxO4 (a) x ¼ 0.1 and (b) x ¼ 0.2.
and Fe2O3 is 89% and 11% for Co0.9Fe2.1O4/Fe2O3 and 79% and 21% for Co0.8Fe2.2O4/Fe2O3 samples, respectively. Fig. 2(a) and (b) shows FE-SEM images of the annealed bicomponent mixture of Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 samples, respectively. The FE-SEM image displays that the powders consist of spherical nanoparticles with faceted and slightly agglomerated grain-packed morphology. Inhomogeneous particle growth was also observed. The image also demonstrates that the composition variation process has very little influence on the morphology, but has a major influence on the electrochemical performance (discussed in the later section). Fig. 2(c) and (d) illustrates the FE-TEM images of annealed bicomponent mixtures of Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3. In both the samples, the particles have spherical shape with different distributions of diameter. The particles sizes are in the range of 50e100 nm. It appears that the nanoparticles were reunited together during the
Table 1 Chemical composition and phase fraction analysis of spinel type Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 materials. Target composition
ICP concentrations (wt%) Co
Fe
Co0.9Fe2.1O4 Co0.8Fe2.2O4
25.4 23.1
53.9 55.7
Practical mole composition (normalized to Fe) Co0.93Fe2.1O4 Co0.87Fe2.2O4
Phase fraction calculation of CoFe2O4 and Fe2O3 Target ICP calculation Phase calculation composition Composition Phase Extra Element ICP value Practical fraction (wt%) molar ratio remained (normalized mol to Co as 1) Co0.9Fe2.1O4 Co Fe Co0.8Fe2.2O4 Co Fe
25.4 53.9 23.1 55.7
1 2.239397 1 2.544599
e 0.239397 e 0.544599
CoFe2O4 Fe2O3 CoFe2O4 Fe2O3
0.89 0.11 0.79 0.21
Fig. 3(a) and (b) presents the CV curves of the Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 electrodes, for the initial three cycles at a scan rate of 0.1 mV s1 between 0.0 and 3.0 V (vs. Li/Liþ), respectively. It can be obviously seen that the first CV curve differs from the subsequent CV curves. In the first scan, both the electrodes show a small cathodic peak at 0.84 V, which could be assigned to the formation of stable intermediate LinCo1xFe2þxO4 (x ¼ 0.1, 0.2) phase [17]. Furthermore, the following large and sharp cathodic peak at 0.56 V corresponds to the reduction of Fe3þ and Co2þ to their metallic states and the formation of Li2O, accompanied by the decomposition of organic electrolyte to form a solid electrolyte interphase (SEI) layer [17,18]. In the charge process of both the electrodes, the anodic peaks at 1.67 V and 1.92 V are ascribed to the oxidation of the metallic iron and cobalt to Fe3þ and Co2þ, respectively. In the second scan, the shift in the cathodic and anodic peaks to 0.94 V and 1.50 V and 1.72 V and 1.89 V for both the electrodes can be attributed to the reversible reduction and oxidation reaction of Fe2O3 and CoO to Fe and Co metals and vice versa, respectively. The decrease of the redox peak intensity implies that the capacity is decreased after the first cycle. In addition, the cathodic and anodic peaks positions for both the electrodes in the following third cycle are also almost at same position corresponding to second cycle such as 0.94 V and 1.49 V and 1.72 V and 1.90 V, respectively, which indicate good structural stability and electrochemical reversibility. Fig. 4(a) and (b) shows the charge/discharge curves of the bicomponent electrodes, namely, Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 for the 1st, 2nd, and 5th cycles at a current rate of 0.1C (1C ¼ 916 mA g1). It can be clearly observed that the curve area and voltage plateau regions in the discharge and charge profiles of both the electrodes are almost in the same position, indicating that similar redox reactions take place in the electrodes. In addition, the electrodes also demonstrate typical chargeedischarge behavior for transition-metal anode materials [19]. The first discharge curve of both electrodes exhibited a distinctly long flat potential plateau at about 0.85 V followed by a gradual sloping until the deep discharge limit of 0.01 V. This behavior is explained well by the conversion reactions of Fe3þ and Co2þ to their metallic states and the formation of Li2O (CoFe2O4 þ 8Liþ þ 8e 4 Co þ 2Fe þ 4Li2O), respectively [3]. One additional small voltage plateau at ~1.0 V can be also seen in the first discharge of both the samples. The voltage of this plateau is closer to that of cobalt reduction in Co3O4 than to that of iron reduction [20]. The first charge curves show a steady and smooth voltage increase, indicating a different electrochemical mechanism from the first discharge such as oxidation reactions of metallic Fe and Co (Co þ 2Fe þ 4Li2O 4 CoO þ Fe2O3 þ 8Li) [3]. The potential plateau is shifted upward at around 1.0 V in the subsequent discharge curves with a more sloping profile accompanied by a gradual increase of Coulombic efficiency. The discharge and charge capacities in the first run are 1273.1 and 910.8 mAh g1 and 1157.8 and 830.9 mAh g1 for the Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/
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Fig. 2. FE-SEM image (a, b); FE-TEM images (c, d); and HR-TEM images (e, f) of the Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 samples, respectively. Inset of (e) and (f) is their corresponding indexed FFT patterns.
Fe2O3 electrodes respectively, with similar Coulombic efficiencies (~71%). The irreversible capacity loss for the first cycle is probably due to the incomplete conversion reaction and the solid electrolyte interface (SEI) layer formation at the electrode/electrolyte interface caused by the reduction of electrolyte in the voltage range of 0.05e0.8 V [20]. The specific capacities obtained for the Co0.9Fe2.1O4/Fe2O3 composition is higher than that of Co0.8Fe2.2O4/ Fe2O3 composition over all the measured cycle numbers (Fig. 4c) and even all the investigated current rates (Fig. 4d), probably due to the higher electrical conductivity in the former electrode and the preferable stoichiometric composition. For example, the 2nd and 5th reversible discharge capacities obtained for the Co0.9Fe2.1O4/ Fe2O3 electrode (917.8 and 862.5 mAh g1) are higher than those of the Co0.8Fe2.2O4/Fe2O3 electrode (827.9 and 768.8 mAh g1).
Fig. 4(c) shows the cycling stability of both the bicomponent electrodes (Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3) at a constant current rate of 0.1C. It is interesting to observe that the capacity of both the electrodes decreases in the first 30 cycles, and then gradually increases as the cycle number increases, which could be attributed to the activation process in cobalt ferrite electrodes [21,22]. Two aspects can be taken into account for the present observation: (i) during first discharge, a large amount of lithium ions inserted into the active materials layers induces a sudden volume expansion and simultaneously may also block further transfer of Liþ ions from electrode to electrolyte. Therefore, it is possible that the conversion reaction may not be fully reversible after the first charge and thereby result in capacity loss during extended cycling. This factor may also be responsible for the low
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Fig. 3. Cyclic voltammetry (CV) plots of (a) Co0.9Fe2.1O4/Fe2O3 and (b) Co0.8Fe2.2O4/Fe2O3 electrodes.
coulombic efficiency in the first cycle. (ii) After repeated Liþ ions insertion/de-insertion, both the bicomponent mixture electrodes become more expanded and porous to some extent, as demonstrated by Ex-situ FE-TEM images (Fig. 6). The porous feature could effectively aid the electrolyte to access the inner part of the active materials and thus, the conversion reaction tend to become more reversible and release Liþ ions. Meanwhile, the enhanced electrode/ electrolyte contact area may also contribute to improve the storage capacities [21]. The Co0.9Fe2.1O4/Fe2O3 electrode delivers an initial charge capacity of 910.8 mAh g1 and retains a capacity of 758.2 mAh g1 after 80 cycles, corresponding to ~83% of the initial charge capacity, while the Co0.8Fe2.2O4/Fe2O3 electrode decays quite rapidly from the initial charge capacity of 830.9 to 635.8 mAh g1 after same number of cycles (corresponding to 77%
of the initial charge capacity). The initial Coulombic efficiency is 71%, and in the following cycles, the Coulombic efficiency of both the electrodes gradually increases along with cycling number, keeping above ~97% in the whole cycling, indicating good reversibility of the electrodes. Clearly, the Co0.9Fe2.1O4/Fe2O3 nanoparticle electrode has better cycling performance. However, the obtained values of electrochemical performance of both the bicomponent electrodes are comparable to those reported for nano-sized CoFe2O4 and Fe2O3 electrodes, but the synthesis strategy adopted in the present study is cost-effective, simple, and scalable unlike previously reported syntheses [3,10,23e25]. To evaluate the rate capability, both the bicomponent electrodes (Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3) were cycled at various current rates, and the results are shown in Fig. 4(d). Both the
Fig. 4. Discharge/charge profiles of (a) Co0.9Fe2.1O4/Fe2O3 and (b) Co0.8Fe2.2O4/Fe2O3 electrodes. (c) Cycling performance at constant rate of 0.1C and (d) rate capability at different current rates between 0.1 and 6.4C.
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Fig. 6. Ex-situ FE-TEM images of cycled electrodes (a) Co0.9Fe2.1O4/Fe2O3 and (b) Co0.8Fe2.2O4/Fe2O3.
Fig. 5. EIS plots of Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 electrodes.
electrodes exhibit good rate performance; Co0.9Fe2.1O4/Fe2O3 showing the highest specific capacity at high current rate. Precisely, the Co0.9Fe2.1O4/Fe2O3 electrode demonstrates better rate performance with a charge capacity of 328.9 mAh g1 at a high current rate of 6.4C, whereas only 229.3 mAh g1 is delivered by the Co0.8Fe2.2O4/Fe2O3 electrode at the same current rate. Furthermore, between 80 and 90% of the charge capacity can be recovered when the current is returned back to 0.1C, which is indicative of the better structural stability. However, the obtained bicomponent mixture electrodes exhibit capacities comparable to that of original single CoFe2O4 component; the enhancement is not simply as a result of introducing of a higher capacity component. Instead, it more likely originates from the unique heterostructure of the mixture
electrode, which is elaborated as follows. It is noted that reversible formation and decomposition of the Li2O nanomatrix could be electrochemically driven by the metal nanoparticles formed insitu. Precisely, the nanoparticles of Fe can probably make the extra Li2O reversibly convert to lithium ion if there is any extra Li2O present. The irreversibility during the initial electrochemical reaction indicates that the CoFe2O4 nanoparticles may facilitate extra Li2O formation during discharge. The presence of Fe nanoparticles may thus make the extra Li2O (provided by CoFe2O4) reversibly convert to lithium ion, giving the electrodes higher reversible capacity [14,26,27]. Hence, the presence of Fe nanoparticles at the interface between Co1xFe2þxO4 and Fe2O3 may improve the reversibility of the reaction and further result in a higher reversible capacity. In addition, it is highly possible that the high surface area and the nanoparticles size of both the samples enable better contact between active materials and the electrolyte and thereby reduce the traverse time for both electrons and lithium ions. Fig. 5 presents typical EIS plots of both the bicomponent mixture (Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3) electrodes. Both electrodes were investigated after 5 charge/discharge cycles and display similar behavior. In both the cases, it can be seen that the impedance curves consist of two semicircles in the high and medium frequency regions and an approximately 45 inclined line in the low frequency region. Generally, the semicircle in the highfrequency region is related to Liþ-ion migration through the SEI film covered on the surface of the electrodes, the semicircle in the middle-frequency region is attributed to charge transfer through the electrode/electrolyte interface, and the steep sloping line is assigned to solid-state diffusion of the Liþ-ions into the bulk of the
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electrode material matrix [15,16]. The impedance fitting was performed using EC-lab software and the corresponding equivalent circuits are also shown in the insets of Fig. 5. The circuit parameters R1, R2, R3 and Zw correspond to the electrolyte resistance, the diffusion resistance of Li ions through SEI layer, the charge transfer resistance and the Warburg impedance, respectively. From the fitting results, it can be seen that the charge transfer (R3) value of Co0.9Fe2.1O4/Fe2O3 electrode is slightly smaller than that of Co0.8Fe2.2O4/Fe2O3 electrode, indicating better electronic conductivity in the former electrode. Furthermore, the values of R1, R2 and Zw of the Co0.9Fe2.1O4/Fe2O3 electrode are also smaller than those of the Co0.8Fe2.2O4/Fe2O3 electrode. Fig. 6(a) and (b) show the ex-situ FE-TEM images of Co0.9Fe2.1O4/ Fe2O3 and Co0.8Fe2.2O4/Fe2O3 electrodes after being cycled. In brief, the cycled electrode was initially dissociated from the cell in an argon-filled glove box. The electrode was then washed with dimethyl carbonate solvent, followed by sonication treatment, and then dried overnight before FE-TEM observation. Both the electrodes exhibited similar morphology. It can be observed that the Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 nanoparticles reveal deteriorated crystallinity probably due to the repeated lithium insertion/extraction process being accompanied by volume expansion/contraction of the Co1xFe2þxO4 structure. In addition, it is also observed that the nanoparticles are being embedded on the carbon shell from the ketjen black used for electrode fabrication. However, the expansion and contraction of the nanoparticles associated with lithium ion insertion and extraction may be contained by carbon acting as an alternative buffer and thereby preserving the structural integrity of the electrode during cycling [28]. More importantly, it is seen that the nanoparticles anchored onto the carbon shell and did not drop out even after the ultrasonication treatment before performing TEM measurement. 4. Conclusions In summary, cobalt-poor or iron rich bicomponent mixtures of Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 have been fabricated as an anode material for LIBs via simple urea-assisted auto-combustion synthesis. The threshold limit of lower cobalt stoichiometry in CoFe2O4 that leads to the impressive electrochemical performance was identified. As compared to Co0.8Fe2.2O4/Fe2O3 electrode, the Co0.9Fe2.1O4/Fe2O3 electrode exhibited higher lithium storage capacity, better cyclic stability, and good rate capability, which can be ascribed to the high electrical conductivity or the preferable stoichiometric composition in the latter electrode. Furthermore, the high reversible capacity in Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/ Fe2O3 electrodes is most likely attributed to the synergistic electrochemical activity of both the nanostructured materials (Co1xFe2þxO4 and Fe2O3), reaching beyond the well-established mechanisms of charge storage in these two phases.
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Acknowledgments This work was supported by the Global Frontier R&D Program (2013e073298) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning. This research was also supported by the Ministry of Science, ICT & Future Planning (MSIP), Korea, under the Convergence Information Technology Research Center (C-ITRC) support program (NIPA-2013H0301-13-1009) supervised by the National IT Industry Promotion Agency (NIPA). References [1] Z.H. Li, T.P. Zhao, X.Y. Zhan, D.S. Gao, Q.Z. Xiao, G.T. Lei, Electrochim. Acta 55 (2010) 4594e4598. [2] C.T. Cherian, M.V. Reddy, G.V. Subba Rao, C.H. Sow, B.V.R. Chowdari, J. Solid State Electrochem. 16 (2012) 1823e1832. [3] M. Zhang, Y. Jin, Q. Wen, C. Chen, M. Jia, Appl. Surf. Sci. 277 (2013) 25e29. [4] N.Y. Nuli, P. Zhang, Z.P. Guo, H.K. Liu, J. Electrochem. Soc. 155 (2008) A196eA200. [5] Y.-M. Lin, P.R. Abel, A. Heller, C.B. Mullins, J. Phys. Chem. Lett. 2 (2011) 2885e2891. [6] Y.H. Jin, S.D. Seo, H.W. Shim, K.S. Park, D.W. Kim, Nanotechnology 23 (2012) 125402. [7] M.X. Li, Y.X. Yin, C.J. Li, F.Z. Zhang, L.J. Wan, S.L. Xu, D.G. Evans, Chem. Commun. 48 (2012) 410e412. [8] L. Wu, Q. Xiao, Z. Li, G. Lei, P. Zhang, L. Wang, Solid State Ionics 215 (2012) 24e28. [9] Z. Zhang, Y. Wang, M. Zhang, Q. Tan, X. Lv, Z. Zhong, F. Su, J. Mater. Chem. A 1 (2013) 7444e7450. [10] M. Zhang, M. Jia, Y. Jin, Q. Wen, C. Chen, J. Alloys Compd. 566 (2013) 131e136. [11] J. Jiang, Y. Li, J. Liu, X. Huang, C. Yuan, X. Lou, Adv. Mater. 24 (2012) 745e748. [12] X.J. Zhu, Z.P. Guo, P. Zhang, G.D. Du, R. Zeng, Z.X. Chen, S. Li, H.K. Liu, J. Mater. Chem. 19 (2009) 8360e8365. [13] J. Liu, J. Jiang, C. Cheng, H. Li, J. Zhang, H. Gong, H.J. Fan, Adv. Mater. 23 (2011) 2076e2081. [14] M.M. Rahman, A.M. Glushenkov, T. Ramireddy, T. Tao, Y. Chen, Nanoscale 5 (2013) 4910e4916. [15] A.K. Rai, J. Gim, L.T. Anh, J. Kim, Electrochim. Acta 100 (2013) 63e71. [16] A.K. Rai, L.T. Anh, J. Gim, V. Mathew, J. Kang, B.J. Paul, J. Song, J. Kim, Electrochim. Acta 90 (2013) 112e118. [17] Z. Xing, Z. Ju, J. Yang, H. Xu, Y. Qian, Electrochim. Acta 102 (2013) 51e57. [18] S. Liu, J. Xie, C. Fang, G. Cao, T. Zhu, X. Zhao, J. Mater. Chem. 22 (2012) 19738e19743. [19] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000) 496e499. [20] J. Liu, H. Xia, L. Lu, D. Xue, J. Mater. Chem. 20 (2010) 1506e1510. [21] W. Ai, L. Xie, Z. Du, Z. Zeng, J. Liu, H. Zhang, Y. Huang, W. Huang, T. Yu, Sci. Rep. 3 (2013) 2341e2345. [22] R. Yi, J. Feng, D. Lv, M.L. Gordin, S. Chen, D. Choi, D. Wang, Nano Energy 2 (2013) 498e504. [23] H.D. Oh, S.W. Lee, S.O. Kim, J.K. Lee, J. Power Sources 244 (2013) 575e580. [24] H. Liu, G. Wang, J. Park, J. Wang, H. Liu, C. Zhang, Electrochim. Acta 54 (2009) 1733e1736. [25] Y. Wang, L. Yang, R. Hu, L. Ouyang, M. Zhu, Electrochim. Acta 125 (2014) 421e426. [26] X-Yu Xue, Z-Hui Chen, L-Li Xing, S. Yuan, Y-Jin Chen, Chem. Commun. 47 (2011) 5205e5207. [27] W. Zhou, C. Cheng, J. Liu, Y.Y. Tay, J. Jiang, X. Jia, J. Zhang, H. Gong, H.H. Hng, T. Yu, H.J. Fan, Adv. Funct. Mater. 21 (2011) 2439e2445. [28] A.K. Rai, L.T. Anh, J. Gim, V. Mathew, J. Kim, Electrochim. Acta 109 (2013) 461e467.
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Co1xFe2þxO4 (x ¼ 0.1, 0.2) anode materials for rechargeable lithium-ion batteries Alok Kumar Rai, Trang Vu Thi, Jihyeon Gim, Vinod Mathew, Jaekook Kim* Department of Materials Science and Engineering, Chonnam National University, 300 Yongbong-dong, Bukgu, Gwangju 500-757, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 April 2014 Received in revised form 30 June 2014 Accepted 3 July 2014 Available online 15 July 2014
A cobalt-poor or iron rich bicomponent mixture of Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 anode materials have been successfully prepared using simple, cost-effective, and scalable urea-assisted autocombustion synthesis. The threshold limit of lower cobalt stoichiometry in CoFe2O4 that leads to impressive electrochemical performance was identified. The electrochemical performance shows that the Co0.9Fe2.1O4/Fe2O3 electrode exhibits high capacity and rate capability in comparison to a Co0.8Fe2.2O4/Fe2O3 electrode, and the obtained data is comparable with that reported for cobalt-rich CoFe2O4. The better rate performance of the Co0.9Fe2.1O4/Fe2O3 electrode is ascribed to its unique stoichiometry, which intimately prefers the combination of Fe2O3 with Co1xFe2þxO4 and the high electrical conductivity. Further, the high reversible capacity in Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 electrodes is most likely attributed to the synergistic electrochemical activity of both the nanostructured materials (Co1xFe2þxO4 and Fe2O3), reaching beyond the well-established mechanisms of charge storage in these two phases. © 2014 Elsevier Masson SAS. All rights reserved.
Keywords: CoFe2O4 Anode Rate capability Lithium-ion battery
1. Introduction Recently, new candidates for anode materials using iron-based spinel oxides with a general formula of AFe2O4 (A ¼ Co, Zn, Ni, Mn) have been reported for use in lithium ion batteries (LIBs) due to their high gravimetric specific capacities [1]. Spinel transitionmetal oxides with two metal elements make it feasible to tune the energy density and working voltage by varying the metal content [2]. Among these, cobalt ferrite (CoFe2O4) has received particular attention due to its high theoretical capacity of 916 mAh g1, which is two times higher than that of graphite (372 mAh g1). In general, the lithium storage mechanism in CoFe2O4 is attributed to a redox conversion reaction (CoFe2O4 þ 8Liþ þ 8e 4 Co þ 2Fe þ 4Li2O), wherein it is reduced to metals nanograins dispersed in to Li2O matrix upon lithiation and then reversibly restored to their initial oxidation states during delithiation. On the other hand, among the other alternative anode materials, Fe2O3 has always been regarded as very appealing transition metal oxide because of its much higher capacity (1007 mAh g1) than that of conventional graphite (372 mAh g1), as well as nontoxicity, high corrosion resistance and low processing
* Corresponding author. Tel.: þ82 62 530 1703; fax: þ82 62 530 1699. E-mail address: [email protected] (J. Kim). http://dx.doi.org/10.1016/j.solidstatesciences.2014.07.002 1293-2558/© 2014 Elsevier Masson SAS. All rights reserved.
cost. The lithium storage mechanism of Fe2O3 is based on the assumption of the reversible reduction of the oxide into metallic Fe (Fe2O3 þ 6Liþ þ 6e 4 2Fe þ 3Li2O). However, similar to other anodes, both the above said materials suffer from common issues such as poor cycling performance, a large initial irreversible capacity loss, and poor rate capability due to material pulverization and slow charge diffusion during the charging and discharging processes [3e5]. The present study attempts to focus on improving the performance of spinel based CoFe2O4 oxides. In fact, great efforts have been made to improve the electrochemical performance of these iron-based oxides [6,7]. The most commonly used strategy is to design hybrid nanocomposites such as metal oxide/carbon composite [8,9]. However, carbon containing hybrids generally exhibit relatively low specific capacity compared to pure metal oxides owing to the presence of carbon, as well as its low theoretical capacity. On the other hand, optimizing the architecture of mixed transition-metal oxides at the nano-scale to improve the kinetics has been also one of the main approaches. Due to the small dimensions, the nanoparticles can tolerate the strain associated with expansion/contraction much better. In addition, recently, M. Jia et al. (2013) and M. Zhang et al. (2013) designed new cobalt-rich CoFe2O4-Co rods, which showed better electrochemical performance than pure CoFe2O4 electrodes [3,10]. Further, synthesizing bicomponent metal oxides has been also proven to be an efficient
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route to improve capacity and rate capability in comparison to each individual component since bicomponent metal oxides often integrates two types of functional materials for a synergistic effect that can improve the intrinsic properties of each component such as electrical/ionic conductivity, electrochemical reactivity, and mechanical stability [11e13]. Therefore, it is still a great challenge to develop a simple and low cost method for synthesizing novel nanostructured bicomponent electrode architectures with improved electrochemical performances. Herein, we report on an interesting strategy to combat the effects of a drastic volume change more effectively by combining two phases that react with lithium efficiently and improve the capacity. It is also believed that during the charge or discharge process in such a bicomponent mixture electrode, the volume expansion or contraction in the two phases is expected to happen sequentially, thus reducing the strain and improving the stability [14]. It is well-known that Fe2O3 has higher theoretical capacity than that of spinel-type CoFe2O4. However, to utilize the high capacity and minimize the structure instability of Fe2O3, we describe the preparation of bicomponent architecture comprised of nanostructured CoFe2O4 and Fe2O3 and their electrochemical performances were also tested as anode materials for secondary LIBs. The threshold limit of lower cobalt stoichiometry in CoFe2O4 that leads to impressive electrochemical performance was also identified. The cobalt-poor Co1xFe2þxO4 (x ¼ 0.1) electrode demonstrated better properties among the prepared stoichiometric compositions in Co1xFe2þxO4 (x ¼ 0.1, 0.2). The high reversible capacity of the obtained bicomponent mixtures of Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 can be attributed to the synergistic electrochemical activity of both the nanostructured material (Co1xFe2þxO4 and Fe2O3), reaching beyond the well-established mechanisms of charge storage in these two phases. 2. Experimental
2.2. Materials characterization The crystal structure and morphology of the obtained products were investigated using a Shimadzu X-ray diffractometer with Cu Ka radiation (l ¼ 1.5406 Å), as well as field-emission scanning electron microscopy (FE-SEM, S-4700 Hitachi) and field-emission transmission electron microscopy (FE-TEM, Philips Tecnai F20 at 200 kV). For FE-TEM characterization, the samples were first dipped in ethanol and dispersed by ultrasonic vibration before coating onto copper grids. The chemical compositions of transition metals in the annealed bicomponent mixtures were analyzed by an inductively coupled plasma (ICP) emission spectroscopy (Perkin Elmer OPTIMA 4300 DV). 2.3. Electrochemical measurements The synthesized products were uniformly mixed with super-P and PVDF in a weight ratio of 70:20:10 in N-methyl-2pyrrolidone solvent to form a slurry. The resulting slurry was coated onto a copper foil current collector and dried under vacuum at 80 C overnight. The slurry was punched into circular electrodes after pressing between stainless steel twin rollers, in order to improve the contact between the active material and copper foil. The 2032 coin-type cells were assembled in a glove box under a dry argon atmosphere using lithium metal as a reference electrode, and a polymer membrane together with glass fiber as a separator. The electrolyte used was 1 M LiPF6 dissolved in a 1:1 (volume ratio) mixture of ethylene carbonate and dimethyl carbonate. The cell was assembled in a glove box filled with argon gas. The cells were cycled between 0.01 and 3.0 V at different C-rates (BTS-2004H, Nagano, Japan). Cyclic voltammetry (CV) measurements were carried out on a Bio Logic Science Instrument (VSP 1075) over the potential range 0.0e3.0 V at a scanning rate of 0.1 mV s1. Electrochemical impedance spectroscopy (EIS) measurements of the electrodes were also carried out using a Bio Logic Science Instrument (VSP 1075) after the cell was cycled for 5 cycles.
2.1. Materials synthesis 3. Results and discussion Nanosized cobalt-poor or iron rich mixtures of Co1xFe2þxO4 (x ¼ 0.1, 0.2) were synthesized by facile and cost-effective ureaassisted auto-combustion synthesis [15,16]. 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 self-generated heat of reaction within a very short reaction time. The advantage of urea is that it can form stable complexes with metal ions to increase solubility and prevent selective precipitation of the metal ions during water removal. In addition, the resultant oxide ash after combustion is generally composed of very fine particles with the desired stoichiometry linked together in a network structure [15,16]. The calculated amounts 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. The obtained aqueous nitrate solutions were mixed together and subsequently an aqueous solution of urea ((NH2CONH2, 99%, Aldrich) was added. The urea-to-nitrate molar ratio (urea : cobalt nitrate ¼ 10:6 and urea : iron nitrate ¼ 15:6) was maintained to facilitate controlled combustion [15,16]. The obtained turbid solution was evaporated on a hot plate using a magnetic stirrer at 300 C, which finally turned into gels and burnt on its own. In order to eliminate possible organic residues and to stabilize the microstructure of the samples, the as-synthesized powders were subsequently annealed at 700 C for 6 h in air atmosphere.
3.1. Structural and morphological analysis XRD patterns of Co1xFe2þxO4 (x ¼ 0.1, 0.2) powders annealed at 700 C for 6 h are shown in Fig. 1(a) and (b). Both the samples show a major phase of spinel CoFe2O4 (JCPDS No. 22-1086, Fd-3m (227)) along with the expected Fe2O3 (JCPDS No. 89e0599) as a secondary phase. It can be clearly observed that the intensity of Fe2O3 peaks increases with the decrease of the amount of cobalt. The sharp diffraction peaks and high intensity indicate good crystallinity in both samples. No peaks of any other phases or any other impurities were detected. To synthesize the bicomponent mixture electrode, the authors have reduced the amount of Co or increased the amount of Fe during the experiment in spite of the 1:2 ratio to form the targeted compound Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3. It is believed that the extra amount of Fe present in the form of Fe2O3 as an extra phase can be helpful to enhance the electrochemical performance of host material. However, to know the exact chemical composition of Co and Fe in both the obtained bicomponent mixtures, ICP-AES analysis was performed and the results are displayed in Table 1. The results showed that the chemical compositions of both the samples are nearly equal to the stoichiometric ratio as targeted. In addition, ICP data was also taken into consideration to roughly calculate the phase fraction ratio of CoFe2O4 and Fe2O3 in both the obtained mixtures assuming that the CoFe2O4 and Fe2O3 are perfectly stoichiometric and the detailed results are displayed in Table 1. The phase fraction ratio of CoFe2O4
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3
synthesis probably due to their magnetic behavior. In addition, Fig. 2(e) and (f) shows typical HR-TEM images of Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 samples, revealing that the particles are structurally uniform with a lattice spacing of about 2.5 Å, which corresponds to the (311) lattice plane of the CoFe2O4. Furthermore, the insets of Fig. 2(e) and (f) shows the corresponding Fast-Fourier transform (FFT) patterns of Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/ Fe2O3 samples, respectively. The FFT images were taken from the marked red square regions in the HR-TEM images. The measured lattice spacings in the FFT patterns for both the samples also tend to support the formation of CoFe2O4. 3.2. Electrochemical properties
Fig. 1. XRD patterns of Co1xFe2þxO4 (a) x ¼ 0.1 and (b) x ¼ 0.2.
and Fe2O3 is 89% and 11% for Co0.9Fe2.1O4/Fe2O3 and 79% and 21% for Co0.8Fe2.2O4/Fe2O3 samples, respectively. Fig. 2(a) and (b) shows FE-SEM images of the annealed bicomponent mixture of Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 samples, respectively. The FE-SEM image displays that the powders consist of spherical nanoparticles with faceted and slightly agglomerated grain-packed morphology. Inhomogeneous particle growth was also observed. The image also demonstrates that the composition variation process has very little influence on the morphology, but has a major influence on the electrochemical performance (discussed in the later section). Fig. 2(c) and (d) illustrates the FE-TEM images of annealed bicomponent mixtures of Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3. In both the samples, the particles have spherical shape with different distributions of diameter. The particles sizes are in the range of 50e100 nm. It appears that the nanoparticles were reunited together during the
Table 1 Chemical composition and phase fraction analysis of spinel type Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 materials. Target composition
ICP concentrations (wt%) Co
Fe
Co0.9Fe2.1O4 Co0.8Fe2.2O4
25.4 23.1
53.9 55.7
Practical mole composition (normalized to Fe) Co0.93Fe2.1O4 Co0.87Fe2.2O4
Phase fraction calculation of CoFe2O4 and Fe2O3 Target ICP calculation Phase calculation composition Composition Phase Extra Element ICP value Practical fraction (wt%) molar ratio remained (normalized mol to Co as 1) Co0.9Fe2.1O4 Co Fe Co0.8Fe2.2O4 Co Fe
25.4 53.9 23.1 55.7
1 2.239397 1 2.544599
e 0.239397 e 0.544599
CoFe2O4 Fe2O3 CoFe2O4 Fe2O3
0.89 0.11 0.79 0.21
Fig. 3(a) and (b) presents the CV curves of the Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 electrodes, for the initial three cycles at a scan rate of 0.1 mV s1 between 0.0 and 3.0 V (vs. Li/Liþ), respectively. It can be obviously seen that the first CV curve differs from the subsequent CV curves. In the first scan, both the electrodes show a small cathodic peak at 0.84 V, which could be assigned to the formation of stable intermediate LinCo1xFe2þxO4 (x ¼ 0.1, 0.2) phase [17]. Furthermore, the following large and sharp cathodic peak at 0.56 V corresponds to the reduction of Fe3þ and Co2þ to their metallic states and the formation of Li2O, accompanied by the decomposition of organic electrolyte to form a solid electrolyte interphase (SEI) layer [17,18]. In the charge process of both the electrodes, the anodic peaks at 1.67 V and 1.92 V are ascribed to the oxidation of the metallic iron and cobalt to Fe3þ and Co2þ, respectively. In the second scan, the shift in the cathodic and anodic peaks to 0.94 V and 1.50 V and 1.72 V and 1.89 V for both the electrodes can be attributed to the reversible reduction and oxidation reaction of Fe2O3 and CoO to Fe and Co metals and vice versa, respectively. The decrease of the redox peak intensity implies that the capacity is decreased after the first cycle. In addition, the cathodic and anodic peaks positions for both the electrodes in the following third cycle are also almost at same position corresponding to second cycle such as 0.94 V and 1.49 V and 1.72 V and 1.90 V, respectively, which indicate good structural stability and electrochemical reversibility. Fig. 4(a) and (b) shows the charge/discharge curves of the bicomponent electrodes, namely, Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 for the 1st, 2nd, and 5th cycles at a current rate of 0.1C (1C ¼ 916 mA g1). It can be clearly observed that the curve area and voltage plateau regions in the discharge and charge profiles of both the electrodes are almost in the same position, indicating that similar redox reactions take place in the electrodes. In addition, the electrodes also demonstrate typical chargeedischarge behavior for transition-metal anode materials [19]. The first discharge curve of both electrodes exhibited a distinctly long flat potential plateau at about 0.85 V followed by a gradual sloping until the deep discharge limit of 0.01 V. This behavior is explained well by the conversion reactions of Fe3þ and Co2þ to their metallic states and the formation of Li2O (CoFe2O4 þ 8Liþ þ 8e 4 Co þ 2Fe þ 4Li2O), respectively [3]. One additional small voltage plateau at ~1.0 V can be also seen in the first discharge of both the samples. The voltage of this plateau is closer to that of cobalt reduction in Co3O4 than to that of iron reduction [20]. The first charge curves show a steady and smooth voltage increase, indicating a different electrochemical mechanism from the first discharge such as oxidation reactions of metallic Fe and Co (Co þ 2Fe þ 4Li2O 4 CoO þ Fe2O3 þ 8Li) [3]. The potential plateau is shifted upward at around 1.0 V in the subsequent discharge curves with a more sloping profile accompanied by a gradual increase of Coulombic efficiency. The discharge and charge capacities in the first run are 1273.1 and 910.8 mAh g1 and 1157.8 and 830.9 mAh g1 for the Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/
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Fig. 2. FE-SEM image (a, b); FE-TEM images (c, d); and HR-TEM images (e, f) of the Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 samples, respectively. Inset of (e) and (f) is their corresponding indexed FFT patterns.
Fe2O3 electrodes respectively, with similar Coulombic efficiencies (~71%). The irreversible capacity loss for the first cycle is probably due to the incomplete conversion reaction and the solid electrolyte interface (SEI) layer formation at the electrode/electrolyte interface caused by the reduction of electrolyte in the voltage range of 0.05e0.8 V [20]. The specific capacities obtained for the Co0.9Fe2.1O4/Fe2O3 composition is higher than that of Co0.8Fe2.2O4/ Fe2O3 composition over all the measured cycle numbers (Fig. 4c) and even all the investigated current rates (Fig. 4d), probably due to the higher electrical conductivity in the former electrode and the preferable stoichiometric composition. For example, the 2nd and 5th reversible discharge capacities obtained for the Co0.9Fe2.1O4/ Fe2O3 electrode (917.8 and 862.5 mAh g1) are higher than those of the Co0.8Fe2.2O4/Fe2O3 electrode (827.9 and 768.8 mAh g1).
Fig. 4(c) shows the cycling stability of both the bicomponent electrodes (Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3) at a constant current rate of 0.1C. It is interesting to observe that the capacity of both the electrodes decreases in the first 30 cycles, and then gradually increases as the cycle number increases, which could be attributed to the activation process in cobalt ferrite electrodes [21,22]. Two aspects can be taken into account for the present observation: (i) during first discharge, a large amount of lithium ions inserted into the active materials layers induces a sudden volume expansion and simultaneously may also block further transfer of Liþ ions from electrode to electrolyte. Therefore, it is possible that the conversion reaction may not be fully reversible after the first charge and thereby result in capacity loss during extended cycling. This factor may also be responsible for the low
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Fig. 3. Cyclic voltammetry (CV) plots of (a) Co0.9Fe2.1O4/Fe2O3 and (b) Co0.8Fe2.2O4/Fe2O3 electrodes.
coulombic efficiency in the first cycle. (ii) After repeated Liþ ions insertion/de-insertion, both the bicomponent mixture electrodes become more expanded and porous to some extent, as demonstrated by Ex-situ FE-TEM images (Fig. 6). The porous feature could effectively aid the electrolyte to access the inner part of the active materials and thus, the conversion reaction tend to become more reversible and release Liþ ions. Meanwhile, the enhanced electrode/ electrolyte contact area may also contribute to improve the storage capacities [21]. The Co0.9Fe2.1O4/Fe2O3 electrode delivers an initial charge capacity of 910.8 mAh g1 and retains a capacity of 758.2 mAh g1 after 80 cycles, corresponding to ~83% of the initial charge capacity, while the Co0.8Fe2.2O4/Fe2O3 electrode decays quite rapidly from the initial charge capacity of 830.9 to 635.8 mAh g1 after same number of cycles (corresponding to 77%
of the initial charge capacity). The initial Coulombic efficiency is 71%, and in the following cycles, the Coulombic efficiency of both the electrodes gradually increases along with cycling number, keeping above ~97% in the whole cycling, indicating good reversibility of the electrodes. Clearly, the Co0.9Fe2.1O4/Fe2O3 nanoparticle electrode has better cycling performance. However, the obtained values of electrochemical performance of both the bicomponent electrodes are comparable to those reported for nano-sized CoFe2O4 and Fe2O3 electrodes, but the synthesis strategy adopted in the present study is cost-effective, simple, and scalable unlike previously reported syntheses [3,10,23e25]. To evaluate the rate capability, both the bicomponent electrodes (Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3) were cycled at various current rates, and the results are shown in Fig. 4(d). Both the
Fig. 4. Discharge/charge profiles of (a) Co0.9Fe2.1O4/Fe2O3 and (b) Co0.8Fe2.2O4/Fe2O3 electrodes. (c) Cycling performance at constant rate of 0.1C and (d) rate capability at different current rates between 0.1 and 6.4C.
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Fig. 6. Ex-situ FE-TEM images of cycled electrodes (a) Co0.9Fe2.1O4/Fe2O3 and (b) Co0.8Fe2.2O4/Fe2O3.
Fig. 5. EIS plots of Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 electrodes.
electrodes exhibit good rate performance; Co0.9Fe2.1O4/Fe2O3 showing the highest specific capacity at high current rate. Precisely, the Co0.9Fe2.1O4/Fe2O3 electrode demonstrates better rate performance with a charge capacity of 328.9 mAh g1 at a high current rate of 6.4C, whereas only 229.3 mAh g1 is delivered by the Co0.8Fe2.2O4/Fe2O3 electrode at the same current rate. Furthermore, between 80 and 90% of the charge capacity can be recovered when the current is returned back to 0.1C, which is indicative of the better structural stability. However, the obtained bicomponent mixture electrodes exhibit capacities comparable to that of original single CoFe2O4 component; the enhancement is not simply as a result of introducing of a higher capacity component. Instead, it more likely originates from the unique heterostructure of the mixture
electrode, which is elaborated as follows. It is noted that reversible formation and decomposition of the Li2O nanomatrix could be electrochemically driven by the metal nanoparticles formed insitu. Precisely, the nanoparticles of Fe can probably make the extra Li2O reversibly convert to lithium ion if there is any extra Li2O present. The irreversibility during the initial electrochemical reaction indicates that the CoFe2O4 nanoparticles may facilitate extra Li2O formation during discharge. The presence of Fe nanoparticles may thus make the extra Li2O (provided by CoFe2O4) reversibly convert to lithium ion, giving the electrodes higher reversible capacity [14,26,27]. Hence, the presence of Fe nanoparticles at the interface between Co1xFe2þxO4 and Fe2O3 may improve the reversibility of the reaction and further result in a higher reversible capacity. In addition, it is highly possible that the high surface area and the nanoparticles size of both the samples enable better contact between active materials and the electrolyte and thereby reduce the traverse time for both electrons and lithium ions. Fig. 5 presents typical EIS plots of both the bicomponent mixture (Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3) electrodes. Both electrodes were investigated after 5 charge/discharge cycles and display similar behavior. In both the cases, it can be seen that the impedance curves consist of two semicircles in the high and medium frequency regions and an approximately 45 inclined line in the low frequency region. Generally, the semicircle in the highfrequency region is related to Liþ-ion migration through the SEI film covered on the surface of the electrodes, the semicircle in the middle-frequency region is attributed to charge transfer through the electrode/electrolyte interface, and the steep sloping line is assigned to solid-state diffusion of the Liþ-ions into the bulk of the
A.K. Rai et al. / Solid State Sciences 36 (2014) 1e7
electrode material matrix [15,16]. The impedance fitting was performed using EC-lab software and the corresponding equivalent circuits are also shown in the insets of Fig. 5. The circuit parameters R1, R2, R3 and Zw correspond to the electrolyte resistance, the diffusion resistance of Li ions through SEI layer, the charge transfer resistance and the Warburg impedance, respectively. From the fitting results, it can be seen that the charge transfer (R3) value of Co0.9Fe2.1O4/Fe2O3 electrode is slightly smaller than that of Co0.8Fe2.2O4/Fe2O3 electrode, indicating better electronic conductivity in the former electrode. Furthermore, the values of R1, R2 and Zw of the Co0.9Fe2.1O4/Fe2O3 electrode are also smaller than those of the Co0.8Fe2.2O4/Fe2O3 electrode. Fig. 6(a) and (b) show the ex-situ FE-TEM images of Co0.9Fe2.1O4/ Fe2O3 and Co0.8Fe2.2O4/Fe2O3 electrodes after being cycled. In brief, the cycled electrode was initially dissociated from the cell in an argon-filled glove box. The electrode was then washed with dimethyl carbonate solvent, followed by sonication treatment, and then dried overnight before FE-TEM observation. Both the electrodes exhibited similar morphology. It can be observed that the Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 nanoparticles reveal deteriorated crystallinity probably due to the repeated lithium insertion/extraction process being accompanied by volume expansion/contraction of the Co1xFe2þxO4 structure. In addition, it is also observed that the nanoparticles are being embedded on the carbon shell from the ketjen black used for electrode fabrication. However, the expansion and contraction of the nanoparticles associated with lithium ion insertion and extraction may be contained by carbon acting as an alternative buffer and thereby preserving the structural integrity of the electrode during cycling [28]. More importantly, it is seen that the nanoparticles anchored onto the carbon shell and did not drop out even after the ultrasonication treatment before performing TEM measurement. 4. Conclusions In summary, cobalt-poor or iron rich bicomponent mixtures of Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 have been fabricated as an anode material for LIBs via simple urea-assisted auto-combustion synthesis. The threshold limit of lower cobalt stoichiometry in CoFe2O4 that leads to the impressive electrochemical performance was identified. As compared to Co0.8Fe2.2O4/Fe2O3 electrode, the Co0.9Fe2.1O4/Fe2O3 electrode exhibited higher lithium storage capacity, better cyclic stability, and good rate capability, which can be ascribed to the high electrical conductivity or the preferable stoichiometric composition in the latter electrode. Furthermore, the high reversible capacity in Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/ Fe2O3 electrodes is most likely attributed to the synergistic electrochemical activity of both the nanostructured materials (Co1xFe2þxO4 and Fe2O3), reaching beyond the well-established mechanisms of charge storage in these two phases.
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Acknowledgments This work was supported by the Global Frontier R&D Program (2013e073298) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning. This research was also supported by the Ministry of Science, ICT & Future Planning (MSIP), Korea, under the Convergence Information Technology Research Center (C-ITRC) support program (NIPA-2013H0301-13-1009) supervised by the National IT Industry Promotion Agency (NIPA). References [1] Z.H. Li, T.P. Zhao, X.Y. Zhan, D.S. Gao, Q.Z. Xiao, G.T. Lei, Electrochim. Acta 55 (2010) 4594e4598. [2] C.T. Cherian, M.V. Reddy, G.V. Subba Rao, C.H. Sow, B.V.R. Chowdari, J. Solid State Electrochem. 16 (2012) 1823e1832. [3] M. Zhang, Y. Jin, Q. Wen, C. Chen, M. Jia, Appl. Surf. Sci. 277 (2013) 25e29. [4] N.Y. Nuli, P. Zhang, Z.P. Guo, H.K. Liu, J. Electrochem. Soc. 155 (2008) A196eA200. [5] Y.-M. Lin, P.R. Abel, A. Heller, C.B. Mullins, J. Phys. Chem. Lett. 2 (2011) 2885e2891. [6] Y.H. Jin, S.D. Seo, H.W. Shim, K.S. Park, D.W. Kim, Nanotechnology 23 (2012) 125402. [7] M.X. Li, Y.X. Yin, C.J. Li, F.Z. Zhang, L.J. Wan, S.L. Xu, D.G. Evans, Chem. Commun. 48 (2012) 410e412. [8] L. Wu, Q. Xiao, Z. Li, G. Lei, P. Zhang, L. Wang, Solid State Ionics 215 (2012) 24e28. [9] Z. Zhang, Y. Wang, M. Zhang, Q. Tan, X. Lv, Z. Zhong, F. Su, J. Mater. Chem. A 1 (2013) 7444e7450. [10] M. Zhang, M. Jia, Y. Jin, Q. Wen, C. Chen, J. Alloys Compd. 566 (2013) 131e136. [11] J. Jiang, Y. Li, J. Liu, X. Huang, C. Yuan, X. Lou, Adv. Mater. 24 (2012) 745e748. [12] X.J. Zhu, Z.P. Guo, P. Zhang, G.D. Du, R. Zeng, Z.X. Chen, S. Li, H.K. Liu, J. Mater. Chem. 19 (2009) 8360e8365. [13] J. Liu, J. Jiang, C. Cheng, H. Li, J. Zhang, H. Gong, H.J. Fan, Adv. Mater. 23 (2011) 2076e2081. [14] M.M. Rahman, A.M. Glushenkov, T. Ramireddy, T. Tao, Y. Chen, Nanoscale 5 (2013) 4910e4916. [15] A.K. Rai, J. Gim, L.T. Anh, J. Kim, Electrochim. Acta 100 (2013) 63e71. [16] A.K. Rai, L.T. Anh, J. Gim, V. Mathew, J. Kang, B.J. Paul, J. Song, J. Kim, Electrochim. Acta 90 (2013) 112e118. [17] Z. Xing, Z. Ju, J. Yang, H. Xu, Y. Qian, Electrochim. Acta 102 (2013) 51e57. [18] S. Liu, J. Xie, C. Fang, G. Cao, T. Zhu, X. Zhao, J. Mater. Chem. 22 (2012) 19738e19743. [19] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000) 496e499. [20] J. Liu, H. Xia, L. Lu, D. Xue, J. Mater. Chem. 20 (2010) 1506e1510. [21] W. Ai, L. Xie, Z. Du, Z. Zeng, J. Liu, H. Zhang, Y. Huang, W. Huang, T. Yu, Sci. Rep. 3 (2013) 2341e2345. [22] R. Yi, J. Feng, D. Lv, M.L. Gordin, S. Chen, D. Choi, D. Wang, Nano Energy 2 (2013) 498e504. [23] H.D. Oh, S.W. Lee, S.O. Kim, J.K. Lee, J. Power Sources 244 (2013) 575e580. [24] H. Liu, G. Wang, J. Park, J. Wang, H. Liu, C. Zhang, Electrochim. Acta 54 (2009) 1733e1736. [25] Y. Wang, L. Yang, R. Hu, L. Ouyang, M. Zhu, Electrochim. Acta 125 (2014) 421e426. [26] X-Yu Xue, Z-Hui Chen, L-Li Xing, S. Yuan, Y-Jin Chen, Chem. Commun. 47 (2011) 5205e5207. [27] W. Zhou, C. Cheng, J. Liu, Y.Y. Tay, J. Jiang, X. Jia, J. Zhang, H. Gong, H.H. Hng, T. Yu, H.J. Fan, Adv. Funct. Mater. 21 (2011) 2439e2445. [28] A.K. Rai, L.T. Anh, J. Gim, V. Mathew, J. Kim, Electrochim. Acta 109 (2013) 461e467.