DOI: 10.1002/cssc.201403420
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Porous Co3O4/CuO Composite Assembled from Nanosheets as High-Performance Anodes for Lithium-Ion Batteries Qin Hao, Dianyun Zhao, Huimei Duan, and Caixia Xu*[a] Upon dealloying a carefully designed CoCuAl ternary alloy in NaOH solution at room temperature, a Co3O4/CuO nanocomposite with an interconnected porous microstructure assembled by a secondary structure of nanosheets was successfully fabricated. By using the dealloying strategy, the target metals directly grew to form uniform bimetallic oxide nanocomposites. Owing to the unique hierarchical structure and the synergistic effect of both active electrode materials, the Co3O4/CuO nanocomposite exhibits much enhanced electrochemical per-
formance with higher capacities and better cycling stability compared to anodes of pure Co3O4. Moreover, it performs excellently in terms of cycle reversibility, Coulombic efficiency, and rate capability, at both low or high current rates. With the advantages of unique performance and ease of preparation, the as-made Co3O4/CuO nanocomposite demonstrates promising application potential as an advanced anode material for lithium-ion batteries.
Introduction Lithium-ion batteries (LIBs) have found widespread application and are among the most popular energy storage devices in recent years, owing to their advantages such as a high energy density, the absence of memory effects, and a long cycle life.[1, 2] The anodes in commercial LIBs are currently made of graphite materials, which have a relatively low theoretical capacity of 372 mA h g¢1. This makes it difficult to meet the everincreasing demands made of LIBs in new applications such as electric vehicles.[3] Consequently, considerable efforts have been dedicated towards exploring other suitable candidate materials for anodes. Pizot et al.[4] studied the conversion reaction mechanism of simple binary oxides MO (M = Co, Ni, Fe, Cu). Among them, Co3O4 represents a particular class of anode material because of its high theoretical capacity of 890 mA h g¢1.[5] However, owing to the inferior intrinsic electrical conductivity, poor ion transport kinetics, and large volume changes during discharge–charge processes, Co3O4 anodes will suffer large initial irreversible capacity losses and show poor capacity retention over extended cycling, which leads to unsatisfactory cycling stability. Therefore, there is an urgent need to ensure a large capacity accompied by high reversibility for Co3O4 anodes. To overcome its drawbacks and enhance its electrochemical performances, various nanostructured Co3O4 materials have been designed, aimed at shortening Li + diffusion distances, improving the electroactivity, and decreasing the absolute
[a] Q. Hao, D. Zhao, H. Duan, C. Xu School of Chemistry and Chemical Engineering University of Jinan Jinan, 250022 (PR China) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201403420.
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volume variations resulting from Li + extraction and insertion.[6–8] However, bulky electrode materials also have non-negligible advantages compared to nanometer-scale electrodes. For example, they can be packed more densely on the current collector, preventing collapse, and are less susceptible to agglomeration.[9] As a consequence, it is essential to achieve an optimized performance of Co3O4 anodes by designing its structure in such a way that the advantages of micro- and nanoscaled materials are coupled. Moreover, novel hybrid electrode materials with advanced architectures have also been designed in order to achieve synergetic properties by modifying two or more components with each other,[10–15] which is not possible for single-component materials. For example, Fe2O3/Co3O4 composite anodes demonstrated much higher capacities and better cycling stabilities than pure Co3O4 electrodes owing to an elegant synergy effect between both active materials.[14, 15] Inspired by the aforementioned statement and recent work, we focus herein on designing Co3O4/CuO composites with a bimodal micro- and nanostructure, considering that CuO offeres the advantages of high theoretical capacity, nontoxicity, and abundance.[16, 17] At present, various synthetic methods have been proposed to prepare metal oxides, such as sol–gel methods, chemical vapor deposition, hydrothermal/solvothermal methods, and others.[18–20] However, these methods always require high temperatures, capping or organic agents, or other conditions. Therefore, it is desirable to develop a simple method that allows high throughput under mild conditions. Our group developed a dealloying method to prepare nanostructured metal oxides.[21–24] The dealloying mainly involves alloying of the targeted transition metal with a more reactive metal, followed by selective leaching of the more reactive metal in an appropriate corrosion medium, and spontaneous oxidation of the remaining transition metal atoms at the metal/electrolyte interface to form metal oxides. This strategy
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Full Papers has evident advantages: simple processing, nearly absolute yield, and flexibility for large-scale synthesis. In this work, we report the fabrication of a type of unique Co3O4/CuO nanocomposite with a controllable component by directly dealloying CoCuAl alloy in NaOH solution. The as-prepared sample has a porous microstructure that itself is formed by a secondary structure of nanosheets. Owing to the unique hierarchical structure and a synergistic effect between both active electrode materials, the Co3O4/CuO nanocomposite exhibits a much-enhanced electrochemical performance compared to pure Co3O4 anode, including high capacities, stable cycling performance, excellent rate capability, etc. With the advantages of unique performance and easy preparation, the Co3O4/CuO nanocomposite demonstrates promising application potential as an advanced anode material for LIBs.
Results and Discussion Selective etching of active species from a well-designed alloy is well-known to produce very useful porous metal nanostructures, such as nanoporous gold.[25] In addition, this simple dealloying method can be extended to prepare important transition-metal oxides due to the spontaneous oxidation of the transition metal atoms at the metal/electrolyte interface during the dealloying process. Examples are Co3O4 and Mn3O4.[21–24] In this work, we considered that aluminum is rich in supply with a relatively low price, and thus would allow to greatly reduce an alloy’s fabrication cost. Hence, CoCuAl ternary alloy was selected as the source alloy. Meanwhile, in order to achieve the preparation of composite oxides with high porosity and an appropriate ratio of cobalt to copper, the source alloy was designed in the component ratio Co13Cu2Al85. Figure S1a (Supporting Information) shows a powder X-ray diffraction (XRD) pattern of the CoCuAl precursor alloy, which suggests that the alloy comprises an Al13Co4 alloy phase (JCPDS 50-0786) and a aluminum-like face center cubic (fcc) structure. Notably, no diffraction peak can be indexed to metallic copper, which indicates that the copper atoms may be distributed in the crystal lattice of the aluminum-like fcc structure and/or the Al13Co4 alloy phase. Figure S1b shows a field emission scanning electron microscopy (SEM) image of a typical as-prepared alloy foil, indicating the smooth and dense surface structure. The inset also shows its silvery lustrous features. Energy-dispersive X-ray spectrometer (EDS) measurements confirmed the composition of the CoCuAl alloy. As indicated in Figure S1c, the atomic percentage of Co/Cu/Al is 12.76:2.03:85.21, which is close to the initial feeding ratio in the refining process. In addition, to verify the influence of the incorporation of CuO on the electrochemical properties of Co3O4 anode material, we prepared pure Co3O4 material through dealloying CoAl binary alloy, as described in our previous research.[22] Considering the amphoteric property of aluminum, NaOH solution was selected as the electrolyte to selectively dissolve aluminum atoms as well to protect cobalt and copper from corrosion and dissolution. SEM was used to investigate the structure of the dealloyed products. Figure 1 a is a typical SEM ChemSusChem 2015, 8, 1435 – 1441
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Figure 1. The (a, b) SEM images, (c) EDS compositional analysis, and element mapping of (d) Co, (e) Cu, (f) O from the fresh Co3O4/CuO samples prepared through dealloying CoCuAl alloy in 2 m NaOH solution for 10 h.
image of the fresh product after dealloying CoCuAl in 2 m NaOH solution for 10 h at room temperature. Selective corrosion of aluminum successfully generates a bulky porous structure. Figure 1 b is a higher-magnification SEM image, showing a detailed structure. Interestingly, the abundant uniform nanosheets with a thickness about 30 nm interconnect with each other and assemble into flower-like substructures, which further form a first-order porous structure on the micrometer scale. Element mapping results of the flower structures are presented in Figure 1 c–e. As demonstrated, the three elements cobalt, copper, and oxygen are dispersed homogenously, indicating uniform intercrossed growth of both Co3O4 and CuO in the dealloyed product. Finally, EDS was carried out to confirm the composition of the freshly dealloyed sample. As presented in Figure 1 f, the atomic ratio between cobalt and copper upon dealloying is 6.43:1, which is agreement with the initial feeding ratio between cobalt and copper in Co13Cu2Al85 alloy precursor, indicating good control over the resulting components. Transmission electron microscopy (TEM) experiments were aimed at further understanding this structure. Figure 2 a shows some agglomerations that are interconnected and form a porous morphology, and many nanosheets are found at the edges of those agglomerations. This is further visible in a higher-magnification image (Figure 2 b). These structural
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Figure 3. The XRD patterns of (a) fresh Co3O4/CuO sample prepared through etching CoCuAl alloy in 2 m NaOH solution for 10 h, (b) annealed product obtained by annealing the fresh Co3O4/CuO at 450 8C for 5 h under N2 ; (c,d) the standard XRD patterns of pure Co3O4 and CuO; XPS spectra of (e) Co 2p and (f) Cu 2p for the fresh Co3O4/CuO.
Figure 2. The (a,b) TEM, and (c) HRTEM images of the fresh Co3O4/CuO sample prepared through dealloying CoCuAl alloy in 2 m NaOH solution for 10 h.
characteristics agree well with the SEM observations. Moreover, Figure 2 c shows a high-resolution TEM (HRTEM) image of one nanosheet in which no clear lattice fringes are resolved, indicating the poor crystallinity of the fresh Co3O4/CuO sample. We reported earlier[22] that once CoAl binary alloy is immersed in NaOH solution, aluminum is selectively etched by OH¢ , and the remaining cobalt atoms undergo spontaneous oxidation at the metal/electrolyte interface to form Co3O4. XRD was used to investigate the crystal structure obtained after immersing CoCuAl alloy in 2 m NaOH solution for 10 h at room temperature. As shown in Figure 3 a, a set of weak diffraction ChemSusChem 2015, 8, 1435 – 1441
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peaks can be indexed to CuO phase but no peaks that can be assigned to the Co3O4 phase are found. To further understand its structural composition, the sample was annealed at 450 8C for 5 h under an atmosphere of N2. As demonstrated in Figure 3 b, the annealed sample comprises two phases, Co3O4 and CuO, indicating the presence of an amorphous state of Co3O4 in the fresh sample, which agrees with the results described in our earlier work.[22] We employed X-ray photoelectron spectrometry (XPS), a more sensitive technique, to examine the surface structures and properties of the freshly dealloyed sample. Figure 3 e displays the XPS spectra for the Co 2p core level region, revealing strong Co 2p peaks at around 780.0 and 795.4 eV and weak shoulder peaks at higher binding energies, in agreement previous reports on Co3O4.[26] Figure 3 f further shows the Cu 2p spectra, indicating the presence of both CuO and metallic copper,[27] which can be attributed to incomplete oxidation. However, no diffraction peak corresponding to metallic copper is detected in the XRD pattern, implying a low content of metallic copper. According to this analysis, the freshly dealloyed sample can be characterized as mainly Co3O4/CuO composite. To give insight into the formation and evolution of this Co3O4/CuO structure, we employed SEM measurements to monitor the product at different corrosion stages during the dealloying process. As shown in Figure 4 a, an initial porous structure is formed with ultrathin sheets on the surface of walls of the pores, which further self-assembles into regular nanosheets as the reaction time reaches 5 h. As a consequence, the product comprises uniform porous Co3O4/CuO product comprising interlaced nanosheets when etching for 10 h (Figure 4 b). Once the etching time is extended to 15 h (Figure 4 c),
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Figure 5. Schematic fabrication of the porous Co3O4/CuO structure.
Figure 4. The SEM images of the products obtained through etching Co13Cu2Al85 alloy in 2 m NaOH solution for (a) 5, (b) 10, (c) 15, (d) 24 (e) 30 h; (f) SEM image of the fresh Co3O4 through dealloying Co15Al85 alloy in 2 m NaOH solution for 24 h.
the structure of the product changes slightly, and small amounts of irregular particles appear among the nanosheets. Eventually, the porous structure is replaced by a structure of coexisting nanosheets and nanoparticles when etching for 24 h (Figure 4 d). When the reaction was further extended to 30 h, the product exhibited a more disordered structure (Figure 4 e), indicating that overlong corrosion is detrimental to the formation of a uniform structure. To better understand the effect of introducing copper on the structures formed when dealloying CoCuAl, Figure 4 f shows an SEM image of the product upon dealloying CoAl in 2 m NaOH solution for 24 h. The product consists of uniform nanosheets with a thickness of ca. 100 nm. However, the Co3O4 nanosheets are in close contact with each other, rather than having assembled into a porous structure. According to our earlier research results,[30] dealloying CuAl binary alloy in NaOH solution should generate a nanoporous structure. Therefore, the formation of the porous structure of the as-made Co3O4/ CuO composite should be mainly attributed to the diffusion of copper in the electrolyte during the corrosion process. However, the copper atoms combine with OH¢ and/or oxygen species to form oxides because the freshly dealloyed copper atoms are highly reactive. Consequently, dealloying ternary CuCoAl alloy results in a porous Co3O4/CuO nanocomposite assembled by interconnected nanosheets. ChemSusChem 2015, 8, 1435 – 1441
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The synthesis procedure of porous Co3O4/CuO structure is illustrated in Figure 5, based on the observation described above. Firstly, bulk metals cobalt, copper, and aluminum with the designed atomic ratio were used to prepare CoCuAl alloy foils (Figure 5 a and b), which were further immersed in NaOH solution to selectively dissolve aluminum atoms. Meanwhile, the remaining cobalt and copper atoms were simultaneously oxidized by OH¢ and/or oxygen species at the metal/electrolyte interface to form Co3O4 and CuO, respectively. When etching for about 5 h, a porous structure is formed by assembly of multinuclei aggregates, which serve as the initial central sites for the growth of ultrathin nanosheets (Figure 5 c). As the reaction proceeds, the nanosheets further continue to grow and assemble to a flower architecture (Figure 5 d) based on the porous skeleton. Such a growth process was similar to that of the similar flower-like materials.[28, 29] However, with the dealloying process is further prolonged, more and more irregular particles appear among the nanosheets (Figure 5 e), and finally the porous structure disappears (Figure 5 f). To clarify the effects of hybridizing CuO on the lithium storage properties of Co3O4, the as-made fresh Co3O4/CuO sample was used as an anode material with the pure Co3O4 sample included for comparison. Figure 6 a presents the sustainable cycling performance of both electrodes at 400 mA g¢1 between 0.01 and 3 V. As indicated in Figure 6 a, the Co3O4/CuO electrode delivered an initial capacity of 1145.2 mA h g¢1, which was slightly higher than that of pure Co3O4 (1140.0 mA h g¢1). During the top 25 cycles, both samples exhibited the similar slow declining tendencies in capacity. However, such the capacity degradation persistently existed in the whole tested period for pure Co3O4 electrode, and its residual capacity was only 671.6 mA h g¢1 at 200th cycle, indicating a low retention of 58.9 %. In contrast, the Co3O4/CuO electrode demonstrated a much higher electrochemical reversibility, which held an essentially constant capacity about 780 mA h g¢1 from 25th to 125th cycles. It is interesting to note that the capacity exhibited an increasing trend from ~ 125th cycle upwards, and remained at ~ 810 mA h g¢1 during the rest cycles, corresponding to a capacity retention of 70.0 % at 200th cycle against the initial value. Such a capacity rising phenomenon is normally observed for transition metal oxides and should attributed to the reversible growth of a polymeric gel-like film resulted from the kinetically activated electrolyte degradation. This has been widely reported and is well documented throughout the literature.[31–33] For the direct verification of the electrochemical superiority of Co3O4/CuO electrode compared with pure Co3O4 electrode,
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Figure 7. Cycling behaviour and coulombic efficiency of the fresh Co3O4/ CuO sample tested at (a) 100 and (b) 1000 mA g¢1 between 0.01 and 3 V. Figure 6. (a) Cycling behaviour [rate: 400 mA g¢1] and (b) Nyquist plot of the fresh Co3O4/CuO and Co3O4 samples.
electrochemical impedance spectroscopy (EIS) measurement for both electrodes was measured at an open circuit voltage state using fresh cells. As illustrated in the Nyquist profiles (Figure 6 b), each plot consists of a depressed semicircle in the high-frequency region that represents the charge-transfer impedance of the cell, and a sloping line in the low-frequency region that attributed to the mass transfer of Li + .[14] It is evident that the Co3O4/CuO electrode displays a relatively smaller diameter, indicating a lower impedance value of the charge transfer. In other words, the charge transfer of Co3O4/CuO porous electrode is easier than that of pure Co3O4 nanosheets, thus leading to a better electrochemical performance. This is in accordance with the results of the reported similar materials.[13] To express the unique superiority of the nanosheets assembled porous structure and the hybridization of CuO with Co3O4, the galvanostatic cycling performance of the as-made porous Co3O4/CuO nanocomposite was tested at a low rate of 100 mA g¢1 and a high rate of 1000 mA g¢1, respectively. Figure 7 a firstly presents its discharge capacities and coulombic efficiencies (CE, the ratio between the charge and discharge capacities) at 100 mA g¢1 during the tested 150 cycles. It is found that its initial capacity reached 1151.2 mA h g¢1, and the first CE was calculated to be 76.9 %. Subsequently, the capacity decreased to 863.7 mA h g¢1 at the second cycle, while the CE increased sharply to 96.4 %. In the following cycles, the Co3O4/ CuO composite exhibited a high reversible capacity about 830 mA h g¢1 and maintained the CEs between 97–100 %. After ChemSusChem 2015, 8, 1435 – 1441
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150 cycles, a capacity of 838.9 mA h g¢1 was reserved, indicating a retention of 72.9 % against the capacity at 1 st cycle and a retention up to 99.6 % against the capacity at 3rd cycle. An excellent cycling stability and long cycling life at high current density is an important property for an advanced electrode material. Therefore, the galvanostatic cycling performance of the Co3O4/CuO sample was further evaluated at a high rate of 1000 mA g¢1 over 400 cycles. As illustrated in Figure 7 b, the capacity of the Co3O4/CuO electrode was highly stable at this fast charge-discharge rate and over the ultralong cycling process. In the first cycle, it delivered a high initial capacity close to 1000 mA h g¢1, and exhibited a CE of 75.6 %. Subsequently, capacity degradation was observed during the early 30 cycles. However, the electrode performed an essentially constant capacity about 600 mA h g¢1 during the rest long cycling period from then to 400th cycle, exhibiting a capacity retention of 59.0 % at the test ending. These results manifest the outstanding capacity stability of the porous Co3O4/CuO composite both at low and high current densities, implying the great value of our Co3O4/CuO anode for LIBs. The rate performance of the fresh Co3O4/CuO sample was investigated in order to further evaluate its power capability. Figure 8 a firstly gives its electrochemical performance measured in discrete steps from 50 to 500 mA g¢1. It is clearly observed that the as-made Co3O4/CuO composite showed very good cycling stability at each current density. When the current rates were increased from 50 to 100, 200, 300 and 500 mA g¢1 in stages, the Co3O4/CuO anode held the stable capacities of about 860, 830, 770, 710 and 640 mA h g¢1, respectively. When
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Full Papers Co3O4/CuO composite has a micro-sized porous flower-like structure assembled by a first-order structure of nanosheets. Compared with the independent nanosheets, Co3O4/CuO micro-composites can be packed densely on the current collector against collapse to ensure the electric connectivity, while which are less susceptible to agglomeration.[9] In addition, compared with the Co3O4 nanosheets anode, the porosity in the porous structured Co3O4/CuO composite can allow the free expansion of electrode material with alleviative mechanical constrain, as well facilitate the transport behaviors of electrons and Li + .[34] Finally, the decreased charge transfer impedance should lead to an efficient electron conducting pathway.
Conclusions
Figure 8. The rate performance of the fresh Co3O4/CuO and pure Co3O4 sample [rate: (a) 50–500 and (b) 500–2500 mA g¢1].
the current rate was set back to 50 mA g¢1, a capacity of ~ 830 mA h g¢1 was well recovered, which reached up to 96 % of the capacity at the previous rate of 50 mA g¢1. Moreover, the rate performance of the Co3O4/CuO composite was also evaluated at much higher current rates from 500 to 2500 mA g¢1, with the pure Co3O4 sample included for comparison. (Figure 8 b). As expected, the capacities of Co3O4/CuO composite were stable at each rate. Even at 2000 and 2500 mA g¢1, the Co3O4/CuO anode could deliver the steady reversible capacities of about 560 and 500 mA h g¢1, respectively. Such a high current rate performance is a very attractive feature of our porous Co3O4/CuO composite. Upon altering the current density back to 500 mA g¢1, an average capacity of 740 mA h g¢1 was recovered, which was close to the capacity at 10th cycle (763.0 mA h g¢1, the initial rate of 500 mA g¢1). In contrast, the pure Co3O4 performed unsatisfactorily. As shown in Figure 8 b, the capacity of Co3O4 electrode degrade gradually with each test rate, especially at high current rates. For example, the pure Co3O4 anode only delivered a low capacities of 372.0 mA h g¢1 at 41th cycle and 287.1 mA h g¢1 at 50th cycle when the rate was 2500 mA g¢1. When the current rate was set back to 500 mA g¢1, a capacity of 588 mA h g¢1 was recovered at 51st cycle, which was far lower than the capacity at 10th cycle (703.1 mA h g¢1, the initial rate of 500 mA g¢1). These results indicated the superiority of Co3O4/CuO composite in high power capability, and visualized again the enhanced electrochemical performance due to the hybridization with CuO, which may be attributed to the following reasons. First, the ChemSusChem 2015, 8, 1435 – 1441
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We successfully achieve the hybridization of Co3O4 with CuO in a well-dispersed naocomposite by dealloying a CoCuAl alloy in NaOH solution at room temperature. The resulting Co3O4/CuO composite has a porous microstructure assembled by secondary nanosheets. Owing to the structural advantage coupled with the synergistic effect of both active electrode materials, the Co3O4/CuO nanocomposite exhibits higher capacities and a much-enhanced cycling stability towards lithium storage compared to a pure Co3O4 anode. More importantly, the Co3O4/CuO electrode shows a long-term cycle life at high current rates and shows excellent rate capabilities. With the advantages of unique performance and ease of preparation, the as-made Co3O4/CuO composite shows great application potential as an advanced anode material.
Experimental Section Sample preparation All reagents were obtained from Shanghai Sinopharm Chemical Reagent Ltd. Co of China and used as-received with analytical purity. Pure (> 99.99 %) Co, Cu, and Al with the designed atomic percentages were refined in an arc-furnace, followed by melt-spinning at 1600 rpm under a protective Ar atmosphere to prepare Co13Cu2Al85 and Co15Al85 alloy foils. The as-made alloy foil had a thickness of 50 mm and a width of approximately 0.6 cm. Subsequently, Co13Cu2Al85 and Co15Al85 were selectively etched in 2 m NaOH solutions at room temperature for a certain time. Finally, the products were washed several times with ultra-pure water (18.2 m Ohm) and dried at room temperature in air.
Characterization XRD patterns were collected on a Bruker D8 advanced X-ray diffractometer using Cu Ka radiation at a step rate of 0.04 s¢1. The morphology and structure of the product were analyzed by using SEM (JEOL JSM-7600F) with an EDS for compositional analysis. The elemental mapping was obtained using a FEI QUANTA FEG250 scanning electron microscope equipped with an INCA Energy XMAX-50 X-ray spectroscopy analyzer. Surface structure properties of the Co3O4/CuO product were analyzed using an XPS (ESCALab250), employing a monochromatized MgKa X-ray as the excitation source and choosing C1 s (284.60 eV) as the reference line. TEM images were taken on a Hitachi H7650 microscope. HRTEM
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Electrochemical tests The electrochemical measurements were carried out by using cointype cells (2032), which used a cellgard 2300 as separator, a lithium metal foil as reference electrode. The electrolyte was composed of 1 mol l¢1 LiPF6 dissolved in ethylene carbonate (EC)-dimethyl carbonate (DMC)-ethylene methyl carbonate (EMC) with the volume ratio of 1:1:1. The working electrodes were prepared by mixing Co3O4/CuO or Co3O4 powders, acetylene black and carboxymethyl cellulose (CMC) in a weight ratio of 7:1.5:1.5 in ultrapure water. The as-made slurry was then coated onto a piece of Cu foil and dried under vacuum at 80 8C for 8 h. The cells were assembled in an Ar filled humidity-free glove box and cycled galvanostatically between 0.01 and 3 V using a NEWARE BTS 5 V-5 mA computercontrolled galvanostat (Shenzhen, China) at different rates at 25 8C. EIS was carried out using a Princeton Applied Research spectrometer by applying an alternating current voltage of 10 mV in the frequency range from 0.01 to 100 kHz.
Acknowledgements This work was also supported by the National Science Foundation of China (21401074, 21271085) and Shandong Province (ZR2014BP013). Keywords: cobalt · copper · lithium-ion batteries · metal oxides · porous materials [1] [2] [3] [4] [5] [6] [7] [8] [9]
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Received: December 15, 2014 Revised: January 17, 2015 Published online on March 31, 2015
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Porous Co3O4/CuO Composite Assembled from Nanosheets as High-Performance Anodes for Lithium-Ion Batteries Qin Hao, Dianyun Zhao, Huimei Duan, and Caixia Xu*[a] Upon dealloying a carefully designed CoCuAl ternary alloy in NaOH solution at room temperature, a Co3O4/CuO nanocomposite with an interconnected porous microstructure assembled by a secondary structure of nanosheets was successfully fabricated. By using the dealloying strategy, the target metals directly grew to form uniform bimetallic oxide nanocomposites. Owing to the unique hierarchical structure and the synergistic effect of both active electrode materials, the Co3O4/CuO nanocomposite exhibits much enhanced electrochemical per-
formance with higher capacities and better cycling stability compared to anodes of pure Co3O4. Moreover, it performs excellently in terms of cycle reversibility, Coulombic efficiency, and rate capability, at both low or high current rates. With the advantages of unique performance and ease of preparation, the as-made Co3O4/CuO nanocomposite demonstrates promising application potential as an advanced anode material for lithium-ion batteries.
Introduction Lithium-ion batteries (LIBs) have found widespread application and are among the most popular energy storage devices in recent years, owing to their advantages such as a high energy density, the absence of memory effects, and a long cycle life.[1, 2] The anodes in commercial LIBs are currently made of graphite materials, which have a relatively low theoretical capacity of 372 mA h g¢1. This makes it difficult to meet the everincreasing demands made of LIBs in new applications such as electric vehicles.[3] Consequently, considerable efforts have been dedicated towards exploring other suitable candidate materials for anodes. Pizot et al.[4] studied the conversion reaction mechanism of simple binary oxides MO (M = Co, Ni, Fe, Cu). Among them, Co3O4 represents a particular class of anode material because of its high theoretical capacity of 890 mA h g¢1.[5] However, owing to the inferior intrinsic electrical conductivity, poor ion transport kinetics, and large volume changes during discharge–charge processes, Co3O4 anodes will suffer large initial irreversible capacity losses and show poor capacity retention over extended cycling, which leads to unsatisfactory cycling stability. Therefore, there is an urgent need to ensure a large capacity accompied by high reversibility for Co3O4 anodes. To overcome its drawbacks and enhance its electrochemical performances, various nanostructured Co3O4 materials have been designed, aimed at shortening Li + diffusion distances, improving the electroactivity, and decreasing the absolute
[a] Q. Hao, D. Zhao, H. Duan, C. Xu School of Chemistry and Chemical Engineering University of Jinan Jinan, 250022 (PR China) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201403420.
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volume variations resulting from Li + extraction and insertion.[6–8] However, bulky electrode materials also have non-negligible advantages compared to nanometer-scale electrodes. For example, they can be packed more densely on the current collector, preventing collapse, and are less susceptible to agglomeration.[9] As a consequence, it is essential to achieve an optimized performance of Co3O4 anodes by designing its structure in such a way that the advantages of micro- and nanoscaled materials are coupled. Moreover, novel hybrid electrode materials with advanced architectures have also been designed in order to achieve synergetic properties by modifying two or more components with each other,[10–15] which is not possible for single-component materials. For example, Fe2O3/Co3O4 composite anodes demonstrated much higher capacities and better cycling stabilities than pure Co3O4 electrodes owing to an elegant synergy effect between both active materials.[14, 15] Inspired by the aforementioned statement and recent work, we focus herein on designing Co3O4/CuO composites with a bimodal micro- and nanostructure, considering that CuO offeres the advantages of high theoretical capacity, nontoxicity, and abundance.[16, 17] At present, various synthetic methods have been proposed to prepare metal oxides, such as sol–gel methods, chemical vapor deposition, hydrothermal/solvothermal methods, and others.[18–20] However, these methods always require high temperatures, capping or organic agents, or other conditions. Therefore, it is desirable to develop a simple method that allows high throughput under mild conditions. Our group developed a dealloying method to prepare nanostructured metal oxides.[21–24] The dealloying mainly involves alloying of the targeted transition metal with a more reactive metal, followed by selective leaching of the more reactive metal in an appropriate corrosion medium, and spontaneous oxidation of the remaining transition metal atoms at the metal/electrolyte interface to form metal oxides. This strategy
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Full Papers has evident advantages: simple processing, nearly absolute yield, and flexibility for large-scale synthesis. In this work, we report the fabrication of a type of unique Co3O4/CuO nanocomposite with a controllable component by directly dealloying CoCuAl alloy in NaOH solution. The as-prepared sample has a porous microstructure that itself is formed by a secondary structure of nanosheets. Owing to the unique hierarchical structure and a synergistic effect between both active electrode materials, the Co3O4/CuO nanocomposite exhibits a much-enhanced electrochemical performance compared to pure Co3O4 anode, including high capacities, stable cycling performance, excellent rate capability, etc. With the advantages of unique performance and easy preparation, the Co3O4/CuO nanocomposite demonstrates promising application potential as an advanced anode material for LIBs.
Results and Discussion Selective etching of active species from a well-designed alloy is well-known to produce very useful porous metal nanostructures, such as nanoporous gold.[25] In addition, this simple dealloying method can be extended to prepare important transition-metal oxides due to the spontaneous oxidation of the transition metal atoms at the metal/electrolyte interface during the dealloying process. Examples are Co3O4 and Mn3O4.[21–24] In this work, we considered that aluminum is rich in supply with a relatively low price, and thus would allow to greatly reduce an alloy’s fabrication cost. Hence, CoCuAl ternary alloy was selected as the source alloy. Meanwhile, in order to achieve the preparation of composite oxides with high porosity and an appropriate ratio of cobalt to copper, the source alloy was designed in the component ratio Co13Cu2Al85. Figure S1a (Supporting Information) shows a powder X-ray diffraction (XRD) pattern of the CoCuAl precursor alloy, which suggests that the alloy comprises an Al13Co4 alloy phase (JCPDS 50-0786) and a aluminum-like face center cubic (fcc) structure. Notably, no diffraction peak can be indexed to metallic copper, which indicates that the copper atoms may be distributed in the crystal lattice of the aluminum-like fcc structure and/or the Al13Co4 alloy phase. Figure S1b shows a field emission scanning electron microscopy (SEM) image of a typical as-prepared alloy foil, indicating the smooth and dense surface structure. The inset also shows its silvery lustrous features. Energy-dispersive X-ray spectrometer (EDS) measurements confirmed the composition of the CoCuAl alloy. As indicated in Figure S1c, the atomic percentage of Co/Cu/Al is 12.76:2.03:85.21, which is close to the initial feeding ratio in the refining process. In addition, to verify the influence of the incorporation of CuO on the electrochemical properties of Co3O4 anode material, we prepared pure Co3O4 material through dealloying CoAl binary alloy, as described in our previous research.[22] Considering the amphoteric property of aluminum, NaOH solution was selected as the electrolyte to selectively dissolve aluminum atoms as well to protect cobalt and copper from corrosion and dissolution. SEM was used to investigate the structure of the dealloyed products. Figure 1 a is a typical SEM ChemSusChem 2015, 8, 1435 – 1441
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Figure 1. The (a, b) SEM images, (c) EDS compositional analysis, and element mapping of (d) Co, (e) Cu, (f) O from the fresh Co3O4/CuO samples prepared through dealloying CoCuAl alloy in 2 m NaOH solution for 10 h.
image of the fresh product after dealloying CoCuAl in 2 m NaOH solution for 10 h at room temperature. Selective corrosion of aluminum successfully generates a bulky porous structure. Figure 1 b is a higher-magnification SEM image, showing a detailed structure. Interestingly, the abundant uniform nanosheets with a thickness about 30 nm interconnect with each other and assemble into flower-like substructures, which further form a first-order porous structure on the micrometer scale. Element mapping results of the flower structures are presented in Figure 1 c–e. As demonstrated, the three elements cobalt, copper, and oxygen are dispersed homogenously, indicating uniform intercrossed growth of both Co3O4 and CuO in the dealloyed product. Finally, EDS was carried out to confirm the composition of the freshly dealloyed sample. As presented in Figure 1 f, the atomic ratio between cobalt and copper upon dealloying is 6.43:1, which is agreement with the initial feeding ratio between cobalt and copper in Co13Cu2Al85 alloy precursor, indicating good control over the resulting components. Transmission electron microscopy (TEM) experiments were aimed at further understanding this structure. Figure 2 a shows some agglomerations that are interconnected and form a porous morphology, and many nanosheets are found at the edges of those agglomerations. This is further visible in a higher-magnification image (Figure 2 b). These structural
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Figure 3. The XRD patterns of (a) fresh Co3O4/CuO sample prepared through etching CoCuAl alloy in 2 m NaOH solution for 10 h, (b) annealed product obtained by annealing the fresh Co3O4/CuO at 450 8C for 5 h under N2 ; (c,d) the standard XRD patterns of pure Co3O4 and CuO; XPS spectra of (e) Co 2p and (f) Cu 2p for the fresh Co3O4/CuO.
Figure 2. The (a,b) TEM, and (c) HRTEM images of the fresh Co3O4/CuO sample prepared through dealloying CoCuAl alloy in 2 m NaOH solution for 10 h.
characteristics agree well with the SEM observations. Moreover, Figure 2 c shows a high-resolution TEM (HRTEM) image of one nanosheet in which no clear lattice fringes are resolved, indicating the poor crystallinity of the fresh Co3O4/CuO sample. We reported earlier[22] that once CoAl binary alloy is immersed in NaOH solution, aluminum is selectively etched by OH¢ , and the remaining cobalt atoms undergo spontaneous oxidation at the metal/electrolyte interface to form Co3O4. XRD was used to investigate the crystal structure obtained after immersing CoCuAl alloy in 2 m NaOH solution for 10 h at room temperature. As shown in Figure 3 a, a set of weak diffraction ChemSusChem 2015, 8, 1435 – 1441
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peaks can be indexed to CuO phase but no peaks that can be assigned to the Co3O4 phase are found. To further understand its structural composition, the sample was annealed at 450 8C for 5 h under an atmosphere of N2. As demonstrated in Figure 3 b, the annealed sample comprises two phases, Co3O4 and CuO, indicating the presence of an amorphous state of Co3O4 in the fresh sample, which agrees with the results described in our earlier work.[22] We employed X-ray photoelectron spectrometry (XPS), a more sensitive technique, to examine the surface structures and properties of the freshly dealloyed sample. Figure 3 e displays the XPS spectra for the Co 2p core level region, revealing strong Co 2p peaks at around 780.0 and 795.4 eV and weak shoulder peaks at higher binding energies, in agreement previous reports on Co3O4.[26] Figure 3 f further shows the Cu 2p spectra, indicating the presence of both CuO and metallic copper,[27] which can be attributed to incomplete oxidation. However, no diffraction peak corresponding to metallic copper is detected in the XRD pattern, implying a low content of metallic copper. According to this analysis, the freshly dealloyed sample can be characterized as mainly Co3O4/CuO composite. To give insight into the formation and evolution of this Co3O4/CuO structure, we employed SEM measurements to monitor the product at different corrosion stages during the dealloying process. As shown in Figure 4 a, an initial porous structure is formed with ultrathin sheets on the surface of walls of the pores, which further self-assembles into regular nanosheets as the reaction time reaches 5 h. As a consequence, the product comprises uniform porous Co3O4/CuO product comprising interlaced nanosheets when etching for 10 h (Figure 4 b). Once the etching time is extended to 15 h (Figure 4 c),
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Figure 5. Schematic fabrication of the porous Co3O4/CuO structure.
Figure 4. The SEM images of the products obtained through etching Co13Cu2Al85 alloy in 2 m NaOH solution for (a) 5, (b) 10, (c) 15, (d) 24 (e) 30 h; (f) SEM image of the fresh Co3O4 through dealloying Co15Al85 alloy in 2 m NaOH solution for 24 h.
the structure of the product changes slightly, and small amounts of irregular particles appear among the nanosheets. Eventually, the porous structure is replaced by a structure of coexisting nanosheets and nanoparticles when etching for 24 h (Figure 4 d). When the reaction was further extended to 30 h, the product exhibited a more disordered structure (Figure 4 e), indicating that overlong corrosion is detrimental to the formation of a uniform structure. To better understand the effect of introducing copper on the structures formed when dealloying CoCuAl, Figure 4 f shows an SEM image of the product upon dealloying CoAl in 2 m NaOH solution for 24 h. The product consists of uniform nanosheets with a thickness of ca. 100 nm. However, the Co3O4 nanosheets are in close contact with each other, rather than having assembled into a porous structure. According to our earlier research results,[30] dealloying CuAl binary alloy in NaOH solution should generate a nanoporous structure. Therefore, the formation of the porous structure of the as-made Co3O4/ CuO composite should be mainly attributed to the diffusion of copper in the electrolyte during the corrosion process. However, the copper atoms combine with OH¢ and/or oxygen species to form oxides because the freshly dealloyed copper atoms are highly reactive. Consequently, dealloying ternary CuCoAl alloy results in a porous Co3O4/CuO nanocomposite assembled by interconnected nanosheets. ChemSusChem 2015, 8, 1435 – 1441
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The synthesis procedure of porous Co3O4/CuO structure is illustrated in Figure 5, based on the observation described above. Firstly, bulk metals cobalt, copper, and aluminum with the designed atomic ratio were used to prepare CoCuAl alloy foils (Figure 5 a and b), which were further immersed in NaOH solution to selectively dissolve aluminum atoms. Meanwhile, the remaining cobalt and copper atoms were simultaneously oxidized by OH¢ and/or oxygen species at the metal/electrolyte interface to form Co3O4 and CuO, respectively. When etching for about 5 h, a porous structure is formed by assembly of multinuclei aggregates, which serve as the initial central sites for the growth of ultrathin nanosheets (Figure 5 c). As the reaction proceeds, the nanosheets further continue to grow and assemble to a flower architecture (Figure 5 d) based on the porous skeleton. Such a growth process was similar to that of the similar flower-like materials.[28, 29] However, with the dealloying process is further prolonged, more and more irregular particles appear among the nanosheets (Figure 5 e), and finally the porous structure disappears (Figure 5 f). To clarify the effects of hybridizing CuO on the lithium storage properties of Co3O4, the as-made fresh Co3O4/CuO sample was used as an anode material with the pure Co3O4 sample included for comparison. Figure 6 a presents the sustainable cycling performance of both electrodes at 400 mA g¢1 between 0.01 and 3 V. As indicated in Figure 6 a, the Co3O4/CuO electrode delivered an initial capacity of 1145.2 mA h g¢1, which was slightly higher than that of pure Co3O4 (1140.0 mA h g¢1). During the top 25 cycles, both samples exhibited the similar slow declining tendencies in capacity. However, such the capacity degradation persistently existed in the whole tested period for pure Co3O4 electrode, and its residual capacity was only 671.6 mA h g¢1 at 200th cycle, indicating a low retention of 58.9 %. In contrast, the Co3O4/CuO electrode demonstrated a much higher electrochemical reversibility, which held an essentially constant capacity about 780 mA h g¢1 from 25th to 125th cycles. It is interesting to note that the capacity exhibited an increasing trend from ~ 125th cycle upwards, and remained at ~ 810 mA h g¢1 during the rest cycles, corresponding to a capacity retention of 70.0 % at 200th cycle against the initial value. Such a capacity rising phenomenon is normally observed for transition metal oxides and should attributed to the reversible growth of a polymeric gel-like film resulted from the kinetically activated electrolyte degradation. This has been widely reported and is well documented throughout the literature.[31–33] For the direct verification of the electrochemical superiority of Co3O4/CuO electrode compared with pure Co3O4 electrode,
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Figure 7. Cycling behaviour and coulombic efficiency of the fresh Co3O4/ CuO sample tested at (a) 100 and (b) 1000 mA g¢1 between 0.01 and 3 V. Figure 6. (a) Cycling behaviour [rate: 400 mA g¢1] and (b) Nyquist plot of the fresh Co3O4/CuO and Co3O4 samples.
electrochemical impedance spectroscopy (EIS) measurement for both electrodes was measured at an open circuit voltage state using fresh cells. As illustrated in the Nyquist profiles (Figure 6 b), each plot consists of a depressed semicircle in the high-frequency region that represents the charge-transfer impedance of the cell, and a sloping line in the low-frequency region that attributed to the mass transfer of Li + .[14] It is evident that the Co3O4/CuO electrode displays a relatively smaller diameter, indicating a lower impedance value of the charge transfer. In other words, the charge transfer of Co3O4/CuO porous electrode is easier than that of pure Co3O4 nanosheets, thus leading to a better electrochemical performance. This is in accordance with the results of the reported similar materials.[13] To express the unique superiority of the nanosheets assembled porous structure and the hybridization of CuO with Co3O4, the galvanostatic cycling performance of the as-made porous Co3O4/CuO nanocomposite was tested at a low rate of 100 mA g¢1 and a high rate of 1000 mA g¢1, respectively. Figure 7 a firstly presents its discharge capacities and coulombic efficiencies (CE, the ratio between the charge and discharge capacities) at 100 mA g¢1 during the tested 150 cycles. It is found that its initial capacity reached 1151.2 mA h g¢1, and the first CE was calculated to be 76.9 %. Subsequently, the capacity decreased to 863.7 mA h g¢1 at the second cycle, while the CE increased sharply to 96.4 %. In the following cycles, the Co3O4/ CuO composite exhibited a high reversible capacity about 830 mA h g¢1 and maintained the CEs between 97–100 %. After ChemSusChem 2015, 8, 1435 – 1441
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150 cycles, a capacity of 838.9 mA h g¢1 was reserved, indicating a retention of 72.9 % against the capacity at 1 st cycle and a retention up to 99.6 % against the capacity at 3rd cycle. An excellent cycling stability and long cycling life at high current density is an important property for an advanced electrode material. Therefore, the galvanostatic cycling performance of the Co3O4/CuO sample was further evaluated at a high rate of 1000 mA g¢1 over 400 cycles. As illustrated in Figure 7 b, the capacity of the Co3O4/CuO electrode was highly stable at this fast charge-discharge rate and over the ultralong cycling process. In the first cycle, it delivered a high initial capacity close to 1000 mA h g¢1, and exhibited a CE of 75.6 %. Subsequently, capacity degradation was observed during the early 30 cycles. However, the electrode performed an essentially constant capacity about 600 mA h g¢1 during the rest long cycling period from then to 400th cycle, exhibiting a capacity retention of 59.0 % at the test ending. These results manifest the outstanding capacity stability of the porous Co3O4/CuO composite both at low and high current densities, implying the great value of our Co3O4/CuO anode for LIBs. The rate performance of the fresh Co3O4/CuO sample was investigated in order to further evaluate its power capability. Figure 8 a firstly gives its electrochemical performance measured in discrete steps from 50 to 500 mA g¢1. It is clearly observed that the as-made Co3O4/CuO composite showed very good cycling stability at each current density. When the current rates were increased from 50 to 100, 200, 300 and 500 mA g¢1 in stages, the Co3O4/CuO anode held the stable capacities of about 860, 830, 770, 710 and 640 mA h g¢1, respectively. When
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Full Papers Co3O4/CuO composite has a micro-sized porous flower-like structure assembled by a first-order structure of nanosheets. Compared with the independent nanosheets, Co3O4/CuO micro-composites can be packed densely on the current collector against collapse to ensure the electric connectivity, while which are less susceptible to agglomeration.[9] In addition, compared with the Co3O4 nanosheets anode, the porosity in the porous structured Co3O4/CuO composite can allow the free expansion of electrode material with alleviative mechanical constrain, as well facilitate the transport behaviors of electrons and Li + .[34] Finally, the decreased charge transfer impedance should lead to an efficient electron conducting pathway.
Conclusions
Figure 8. The rate performance of the fresh Co3O4/CuO and pure Co3O4 sample [rate: (a) 50–500 and (b) 500–2500 mA g¢1].
the current rate was set back to 50 mA g¢1, a capacity of ~ 830 mA h g¢1 was well recovered, which reached up to 96 % of the capacity at the previous rate of 50 mA g¢1. Moreover, the rate performance of the Co3O4/CuO composite was also evaluated at much higher current rates from 500 to 2500 mA g¢1, with the pure Co3O4 sample included for comparison. (Figure 8 b). As expected, the capacities of Co3O4/CuO composite were stable at each rate. Even at 2000 and 2500 mA g¢1, the Co3O4/CuO anode could deliver the steady reversible capacities of about 560 and 500 mA h g¢1, respectively. Such a high current rate performance is a very attractive feature of our porous Co3O4/CuO composite. Upon altering the current density back to 500 mA g¢1, an average capacity of 740 mA h g¢1 was recovered, which was close to the capacity at 10th cycle (763.0 mA h g¢1, the initial rate of 500 mA g¢1). In contrast, the pure Co3O4 performed unsatisfactorily. As shown in Figure 8 b, the capacity of Co3O4 electrode degrade gradually with each test rate, especially at high current rates. For example, the pure Co3O4 anode only delivered a low capacities of 372.0 mA h g¢1 at 41th cycle and 287.1 mA h g¢1 at 50th cycle when the rate was 2500 mA g¢1. When the current rate was set back to 500 mA g¢1, a capacity of 588 mA h g¢1 was recovered at 51st cycle, which was far lower than the capacity at 10th cycle (703.1 mA h g¢1, the initial rate of 500 mA g¢1). These results indicated the superiority of Co3O4/CuO composite in high power capability, and visualized again the enhanced electrochemical performance due to the hybridization with CuO, which may be attributed to the following reasons. First, the ChemSusChem 2015, 8, 1435 – 1441
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We successfully achieve the hybridization of Co3O4 with CuO in a well-dispersed naocomposite by dealloying a CoCuAl alloy in NaOH solution at room temperature. The resulting Co3O4/CuO composite has a porous microstructure assembled by secondary nanosheets. Owing to the structural advantage coupled with the synergistic effect of both active electrode materials, the Co3O4/CuO nanocomposite exhibits higher capacities and a much-enhanced cycling stability towards lithium storage compared to a pure Co3O4 anode. More importantly, the Co3O4/CuO electrode shows a long-term cycle life at high current rates and shows excellent rate capabilities. With the advantages of unique performance and ease of preparation, the as-made Co3O4/CuO composite shows great application potential as an advanced anode material.
Experimental Section Sample preparation All reagents were obtained from Shanghai Sinopharm Chemical Reagent Ltd. Co of China and used as-received with analytical purity. Pure (> 99.99 %) Co, Cu, and Al with the designed atomic percentages were refined in an arc-furnace, followed by melt-spinning at 1600 rpm under a protective Ar atmosphere to prepare Co13Cu2Al85 and Co15Al85 alloy foils. The as-made alloy foil had a thickness of 50 mm and a width of approximately 0.6 cm. Subsequently, Co13Cu2Al85 and Co15Al85 were selectively etched in 2 m NaOH solutions at room temperature for a certain time. Finally, the products were washed several times with ultra-pure water (18.2 m Ohm) and dried at room temperature in air.
Characterization XRD patterns were collected on a Bruker D8 advanced X-ray diffractometer using Cu Ka radiation at a step rate of 0.04 s¢1. The morphology and structure of the product were analyzed by using SEM (JEOL JSM-7600F) with an EDS for compositional analysis. The elemental mapping was obtained using a FEI QUANTA FEG250 scanning electron microscope equipped with an INCA Energy XMAX-50 X-ray spectroscopy analyzer. Surface structure properties of the Co3O4/CuO product were analyzed using an XPS (ESCALab250), employing a monochromatized MgKa X-ray as the excitation source and choosing C1 s (284.60 eV) as the reference line. TEM images were taken on a Hitachi H7650 microscope. HRTEM
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Electrochemical tests The electrochemical measurements were carried out by using cointype cells (2032), which used a cellgard 2300 as separator, a lithium metal foil as reference electrode. The electrolyte was composed of 1 mol l¢1 LiPF6 dissolved in ethylene carbonate (EC)-dimethyl carbonate (DMC)-ethylene methyl carbonate (EMC) with the volume ratio of 1:1:1. The working electrodes were prepared by mixing Co3O4/CuO or Co3O4 powders, acetylene black and carboxymethyl cellulose (CMC) in a weight ratio of 7:1.5:1.5 in ultrapure water. The as-made slurry was then coated onto a piece of Cu foil and dried under vacuum at 80 8C for 8 h. The cells were assembled in an Ar filled humidity-free glove box and cycled galvanostatically between 0.01 and 3 V using a NEWARE BTS 5 V-5 mA computercontrolled galvanostat (Shenzhen, China) at different rates at 25 8C. EIS was carried out using a Princeton Applied Research spectrometer by applying an alternating current voltage of 10 mV in the frequency range from 0.01 to 100 kHz.
Acknowledgements This work was also supported by the National Science Foundation of China (21401074, 21271085) and Shandong Province (ZR2014BP013). Keywords: cobalt · copper · lithium-ion batteries · metal oxides · porous materials [1] [2] [3] [4] [5] [6] [7] [8] [9]
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Received: December 15, 2014 Revised: January 17, 2015 Published online on March 31, 2015
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