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Metal-organic frameworks derived porous CuO/Cu2O composite hollow octahedrons as high performance anode materials for sodium ion batteries Xiaojie Zhang, Wei Qin, Dongsheng Li, Dong Yan, Bingwen Hu, Zhuo Sun, Likun Pan
DOI: 10.1039/x0xx00000x www.rsc.org/
Porous CuO/Cu2O composite hollow octahedrons were synthesized simply by annealing Cu-based metal-organic frameworks templates. When evaluated as anode materials for sodium ion batteries, they exhibit a high maximum reversible capacity of 415 mAh g-1 after 50 cycles at 50 mA g-1 with excellent cycling stability and good rate capability. Over the past several decades, the increasing consumption of fossil fuel resources has caused severe energy crisis and environmental problems. Renewable sources of energy, such as wind and solar power, have received intensive attention as the most promising substitutes, but their irregular energy output requires energy storage systems.1 Rechargeable batteries, owing to their flexibility, high energy conversion efficiency, have been considered as the promising devices for successful development of energy storage and conversion.2-4 Currently, lithium ion batteries (LIBs) are the leading candidate for such utilization. However, mass production of huge LIBs would rapidly deplete the limited and unevenly distributed Li resources in earth’s crust. Recently, sodium ion batteries (SIBs) have aroused enormous attention as potential alternatives to LIBs for large-scale energy storage systems because of the relatively low cost, the abundance of sodium resources and the similar chemistry between sodium and lithium.3, 5, 6 Unfortunately, sodium ion has larger radius compared with lithium ion. Therefore, typical graphite material, which have been used as commercial anode for LIBs, is not a suitable candidate for SIBs because its interlayer distance is too small to accommodate sodium ion.7 Therefore, exploring advanced anode materials for SIBs to achieve superior electrochemical performance is urgently desirable but remains a great challenge. Up to date, many attempts have been tried to find suitable anode materials for SIBs, including various carbonaceous materials,8 metal nitrides,9 metal oxides,10 phosphor,11 and alloys.12 Among various anode materials, transition metal oxides have been proven to be possible candidates because of their high theoretical capacities, and widespread availability. In particular, Cu-based oxides, such as CuO and Cu2O, have attracted much attention due to their abundance, affordable price, chemical stability, and nontoxic nature.9, 13 However, similar to the other metal oxides, practical application of Cu-based oxides for SIBs is still hindered by large volume change during cycling due to conversion reaction mechanism, which leads to dramatic capacity fading and
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poor rate performance. Huang et al. demonstrated that CuO could be employed as an anode material for SIBs with a high capacity in the initial few cycles, but the cycling stability and rate performance were still unsatisfied and needed to be improved.14 Klein et al. reported that the Cu2O used for SIBs exhibited a high capacity of ~ 600 mAh g-1 at 0.1 C but it decreased quickly to 300 mAh g-1 at the fifth cycle.6 To alleviate the volume change during cycling, an effective method is to fabricate hollow or porous structure materials, which can accommodate the volume change and facilitate the contact between active materials and electrolyte due to their interior space. To date, CuO or Cu2O with porous or hollow structures, such as porous nanorods, mesoporous nanoparticles, hollow nanospheres, have been fabricated using various methods.15-18 Despite continuous efforts that have been made, the rational design and synthesis of anisotropic hollow structures with complex hierarchical architectures remain a big challenge. Metal-organic frameworks (MOFs), a novel class of porous materials consisting of metal ions or clusters coordinated to organic molecules, have attracted much attention due to their diverse structural topologies, tunable functionalities and versatile applications in gas storage and separation,19, 20 catalysis,21 biomedicine,22 sensing,23 and energy storage and conversion.24-26 Recently, MOFs are proved to be ideal precursors or templates for the synthesis of porous hierarchical metal oxides27 and carbon nanostructures28-30 via thermal decomposition under different atmospheres or chemical etching16 due to their unique structure, high specific surface area, porosity and nanosized cavities. Studies have shown that porous metal oxides such as Fe2O3,31, 32 CuO18 and Co3O433 constructed from MOFs precursors exhibited excellent lithium storage properties. Nevertheless, as excellent members of metal oxides, hollow CuO/Cu2O nanostructures derived from MOFs have been seldom reported for SIBs applications by now. Herein, we report a simple and convenient method to synthesize porous structured CuO/Cu2O composite hollow octahedrons (CHO) using Cu-MOFs ([Cu3(btc)2]n, btc = benzene-1,3,5-tricarboxylate) as the template. The experimental details can be referred to ESI†. The Cu-MOFs have an octahedral structure with a size of several micrometers (Fig. S1) and their X-ray diffraction (XRD) pattern (Fig. S2) is in agreement with that of [Cu3(btc)2]n.34, 35 The CHO samples obtained from pyrolysis of the MOFs at 300 °C, 350 °C and
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structure inside CHO-1. It can be observed from Fig. 2d that the measured lattice fringe distances are 0.27 nm and 0.32 nm, corresponding to the (002) and (111) lattice planesView of Article CuOOnline and 10.1039/C5CC06924F selected area electron Cu2O, respectively. The bright spots in DOI: diffraction pattern of CHO-1 (Fig. S5) indicates good crystallinity of CHO-1 nanostructure. When the calcination temperature is increased to 350 °C (CHO-2), the octahedron porous structure collapses to form cluster structure (Fig. S6a). At a higher temperature of 400 °C, the octahedral structure entirely disappears and non-regular agglomerates are obtained (Fig. S6b). The porous structure of CHO was further characterized by nitrogen adsorption/desorption isotherm measurements (Fig. S7). It is found that the CHO samples display a broad pore-size distribution (inset in Fig. S7) and a moderate specific surface area, which is similar to the previous report.36 The specific surface areas, pore volumes and mean CHO-3 pore diameters of the samples are listed in Table S1. CHO-1 exhibits a largest surface area and pore volume, which can facilitate the sodium ion insertion and rapid diffusion during the electrochemical reactions. The surface areas and pore volumes of CHO-2 and CHO-3 are observed to decrease, which should be ascribed to more pores CHO-2 closed at higher calcination temperature. The electrochemical properties of porous CHO electrodes for SIBs were investigated by using the standard half-cell configuration. Fig. 3a shows the first three cyclic voltammetry (CV) curves of CHO-1 CHO-1 in a range of 0.005-3 V at a scan rate of 0.2 mV s-1. It is found that there is a broad and irreversible cathodic peak located at 0.84 V in the first discharge cycle, which corresponds to the decomposition of CuO 65-2309 the organic electrolyte and the formation of a solid electrolyte Cu2O 05-0667 interface (SEI) layer.17, 37 In the subsequence cycles, the cathodic 10 20 30 40 50 60 70 80 peaks at 1.83, 0.54, and 0.12 V during the discharging process involve three reversible electrochemical reactions: the formation of 2-Theta (degree) Cu(II)1-xCu(I)xO1-x/2, further reduction of Cu2O into Cu and Na2O, Fig. 1 XRD patterns of CHO samples. respectively.38 Furthermore, in the charge processes, the anodic peaks at 1.3 and 2.18 V correspond to the process 2Cu + Na2O → Cu2O + 2Na and the oxidation of Cu2O into CuO, respectively.39 Notably, after the first cycle, CV curves exhibit good repeatability, indicating high reversibility of the subsequent reactions.17 Galvanostatic charge/discharge test was carried out to investigate the electrochemical performances of CHO electrodes. Fig. S8a-c show the charge/discharge profiles at 1st, 2nd, and 10th cycles in a voltage range of 0.005-3 V at a current density of 50 mA g-1 for CHO-1, CHO-2, and CHO-3, respectively. Large irreversible capacity is observed in the first cycle for all electrodes, which is typical for such conversion type anodes.6 The irreversible capacity loss is consistent to CV measurements, in which the decrease of area during cycling is noticed due to decomposition of the electrolyte and subsequent formation of SEI.17, 37 In the subsequent cycles, the charge/discharge curves of all electrodes overlap well, indicating high reversibility of the materials. However, CHO-2 and CHO-3 show an unsatisfied sodium storage capacity due to the collapse of the hollow nanostructure and their relative low specific surface area. The cycling performance together with the Coulombic efficiency (CE) of CHO electrodes is shown in Fig. 3b at a current density of 50 mA g1 . The initial CE of ~ 45% is relatively low, which may be attributed to the formation of SEI layer on the CHO surface. Remarkably, after several cycles, the CHO electrodes exhibit excellent cyclic stability and their CE remains more than 98%. The specific charge/discharge Fig. 2 (a) Low- and (b) high-magnification FESEM, (c) low- and (d) highcapacity of CHO-1 maintains well with the increased cycles and a magnification TEM images of CHO-1. reversible capacity of 415 mAh g-1 can be obtained after 50 cycles. only display reversible As shown in field-emission scanning electron microscopy However, CHO-2 and CHO-3 samples -1 (FESEM) images of CHO-1 (Fig. 2a and b), CHO-1 exhibits a capacities of 248 and 185 mAh g after 50 cycles, respectively, typical porous octahedron structure that consists of clustered much lower than that of CHO-1. Electrochemical impedance (EIS) was employed to investigate the kinetics of CHO nanoparticles with size of tens of nanometers. The transmission spectroscopy 40 electron microscopy (TEM) image (Fig. 2c) shows the hollow electrodes. Fig. S9 show the EIS curves of the samples measured at
400 °C were named as CHO-1, CHO-2 and CHO-3, respectively. Fig. 1 shows the XRD patterns of CHO-1, CHO-2 and CHO-3. It can be seen that the peaks in the patterns of CHO-1, CHO-2 and CHO-3 are ascribed to CuO (JCPDS card no. 65-2309) and Cu2O (JCPDS card no. 05-0667), indicating the existence of CuO/Cu2O composite in CHO. Further measurement using X-ray photoelectron spectroscopy (Fig. S4) shows that the binding energy of Cu 2p3/2 can be assigned to Cu2+ (934.1 eV) and Cu+ (932.4 eV), and the ratios of Cu2+ to Cu+ are about 4.43, 5.41 and 11.32 for CHO-1, CHO-2 and CHO-3, respectively.
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different temperatures. The Nyquist plots can be fitted and interpreted well based on the equivalent electric circuit.40, 41 The large semicircle is related to charge transfer resistance (Rct) and constant phase element (CPE2). Another indistinct small highfrequency semicircle stemming from Rf and CPE1 is ascribed to the SEI layer. Table S2 lists the fitted values of Rct. The Rct is found to decrease when the temperature increases, indicating that better electrochemical kinetics is obtained at higher temperature. It can be seen that CHO-1 exhibits a smallest Rct among all samples measured at the same temperatures, showing that electrochemical reaction occurs easier in CHO-1. The apparent activation energy (Ea) for the intercalation of sodium can be calculated by the following equations:42, 43
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i0 = RT/nFRct
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i0 = Aexp(Ea/RT) where i0 is the exchange current. A is the temperature-independent coefficient. R is the gas constant. T is the absolute temperature. n is the number of transferred electrons and F is the Faraday constant. Fig. S9d shows the corresponding Arrhenius plots of –ln(T/Rct) versus 1000/T. Ea is equal to the slope of the fitting straight line multiplied by R. The calculated values of Ea are 35.73, 50.46 and 66.75 kJ mol-1 for CHO-1, CHO-2 and CHO-3 samples, respectively. The lowest Ea value of CHO-1 indicates the best sodium intercalation ability in CHO-1, which further confirms the superior capacity of CHO-1 for sodium ions storage. The reason should be ascribed to the following: (i) The more porous structure of CHO-1 is beneficial to the increase of contact between the electrode and electrolyte, which facilitates the intercalation of Na ions into the active materials and accelerates their diffusion velocity. (ii) The hollow structure can efficiently enhance structural integrity with voids for buffering stresses caused by the volume change during cycling. (iii) The synergistic effect between CuO and Cu2O contributes to the high capacity. 44-46 Fig. 3c shows the rate performance of CHO-1 electrode at different current densities from 50 to 2500 mA g-1. The corresponding charge/discharge curves of CHO-1 electrode are displayed in Fig. 3d. It is clearly seen that the CHO-1 exhibits the specific capacities of 440, 395, 349.3, 273.5, 217.2 and 153.8 mAh g-1 at 50, 100, 250, 500, 1000 and 2500 mA g-1, respectively. The specific capacity recovers to ~ 410 mAh g-1 when the current density returns back to 50 mA g-1, indicating that the unique porous hollow structure of CHO-1 can preserve the integrity of the electrode material. To evaluate the long cycling performance, the CHO-1 electrode was measured for more than 1000 cycles at high current densities. Fig. 3e displays the cyclability of CHO-1 after the tests for the first 30 cycles at 50 mA g-1, 1 A g-1 for the subsequent 400 cycles and 2 A g-1 for the final 600 cycles. Clearly, the porous CHO-1 shows an excellent performance with a capacity of ~212 mAh g-1 when cycled at 1 A g-1 after 400 cycles. The capacity can retain ~165 mAh g-1 at 2 A g-1 after 1000 cycles. During the cycling, the CE of ~99 % is achieved after the first several cycles, further demonstrating the superior stability for sodium storage capability of porous CHO electrode.25 In summary, CHO samples were successfully synthesized via thermal decomposition of Cu-MOFs and used as anode materials for SIBs for the first time. They exhibit excellent sodium storage capacity of 415 mAh g-1 at 50 mA g-1 after 50 cycles and superior cyclability at higher current density as well as good rate capability, which can be attributed to synergistic effect between CuO and Cu2O and excellent stability of their porous hollow structure. The CHO should be promising anode materials for SIBs.
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Fig. 3 (a) CV curves of CHO-1 and (b) cycling performance of CHO electrodes at a current density of 50 mA g-1. (c) Rate capabilities at different current densities, (d) typical discharge/charge curves at different current densities and (e) long cycling performance of CHO-1.
Acknowledgements
Financial support from Basic Research Project of Shanghai Science and Technology Committee (No. 14JC1491000) is gratefully acknowledged.
Notes and references
Engineering Research Center for Nanophotonics & Advanced Instrument, Ministry of Education, Shanghai Key Laboratory of Magnetic Resonance, Department of Physics, East China Normal University, Shanghai 200062, China. E-mail: [email protected]; Fax: +86 21 62234321; Tel: +86 21 62234132. †Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/c000000x/
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This article can be cited before page numbers have been issued, to do this please use: X. Zhang, W. Qin, D. Li, D. Yan, B. Hu, Z. Sun and L. Pan, Chem. Commun., 2015, DOI: 10.1039/C5CC06924F.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Received 00th January 2012, Accepted 00th January 2012
Metal-organic frameworks derived porous CuO/Cu2O composite hollow octahedrons as high performance anode materials for sodium ion batteries Xiaojie Zhang, Wei Qin, Dongsheng Li, Dong Yan, Bingwen Hu, Zhuo Sun, Likun Pan
DOI: 10.1039/x0xx00000x www.rsc.org/
Porous CuO/Cu2O composite hollow octahedrons were synthesized simply by annealing Cu-based metal-organic frameworks templates. When evaluated as anode materials for sodium ion batteries, they exhibit a high maximum reversible capacity of 415 mAh g-1 after 50 cycles at 50 mA g-1 with excellent cycling stability and good rate capability. Over the past several decades, the increasing consumption of fossil fuel resources has caused severe energy crisis and environmental problems. Renewable sources of energy, such as wind and solar power, have received intensive attention as the most promising substitutes, but their irregular energy output requires energy storage systems.1 Rechargeable batteries, owing to their flexibility, high energy conversion efficiency, have been considered as the promising devices for successful development of energy storage and conversion.2-4 Currently, lithium ion batteries (LIBs) are the leading candidate for such utilization. However, mass production of huge LIBs would rapidly deplete the limited and unevenly distributed Li resources in earth’s crust. Recently, sodium ion batteries (SIBs) have aroused enormous attention as potential alternatives to LIBs for large-scale energy storage systems because of the relatively low cost, the abundance of sodium resources and the similar chemistry between sodium and lithium.3, 5, 6 Unfortunately, sodium ion has larger radius compared with lithium ion. Therefore, typical graphite material, which have been used as commercial anode for LIBs, is not a suitable candidate for SIBs because its interlayer distance is too small to accommodate sodium ion.7 Therefore, exploring advanced anode materials for SIBs to achieve superior electrochemical performance is urgently desirable but remains a great challenge. Up to date, many attempts have been tried to find suitable anode materials for SIBs, including various carbonaceous materials,8 metal nitrides,9 metal oxides,10 phosphor,11 and alloys.12 Among various anode materials, transition metal oxides have been proven to be possible candidates because of their high theoretical capacities, and widespread availability. In particular, Cu-based oxides, such as CuO and Cu2O, have attracted much attention due to their abundance, affordable price, chemical stability, and nontoxic nature.9, 13 However, similar to the other metal oxides, practical application of Cu-based oxides for SIBs is still hindered by large volume change during cycling due to conversion reaction mechanism, which leads to dramatic capacity fading and
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poor rate performance. Huang et al. demonstrated that CuO could be employed as an anode material for SIBs with a high capacity in the initial few cycles, but the cycling stability and rate performance were still unsatisfied and needed to be improved.14 Klein et al. reported that the Cu2O used for SIBs exhibited a high capacity of ~ 600 mAh g-1 at 0.1 C but it decreased quickly to 300 mAh g-1 at the fifth cycle.6 To alleviate the volume change during cycling, an effective method is to fabricate hollow or porous structure materials, which can accommodate the volume change and facilitate the contact between active materials and electrolyte due to their interior space. To date, CuO or Cu2O with porous or hollow structures, such as porous nanorods, mesoporous nanoparticles, hollow nanospheres, have been fabricated using various methods.15-18 Despite continuous efforts that have been made, the rational design and synthesis of anisotropic hollow structures with complex hierarchical architectures remain a big challenge. Metal-organic frameworks (MOFs), a novel class of porous materials consisting of metal ions or clusters coordinated to organic molecules, have attracted much attention due to their diverse structural topologies, tunable functionalities and versatile applications in gas storage and separation,19, 20 catalysis,21 biomedicine,22 sensing,23 and energy storage and conversion.24-26 Recently, MOFs are proved to be ideal precursors or templates for the synthesis of porous hierarchical metal oxides27 and carbon nanostructures28-30 via thermal decomposition under different atmospheres or chemical etching16 due to their unique structure, high specific surface area, porosity and nanosized cavities. Studies have shown that porous metal oxides such as Fe2O3,31, 32 CuO18 and Co3O433 constructed from MOFs precursors exhibited excellent lithium storage properties. Nevertheless, as excellent members of metal oxides, hollow CuO/Cu2O nanostructures derived from MOFs have been seldom reported for SIBs applications by now. Herein, we report a simple and convenient method to synthesize porous structured CuO/Cu2O composite hollow octahedrons (CHO) using Cu-MOFs ([Cu3(btc)2]n, btc = benzene-1,3,5-tricarboxylate) as the template. The experimental details can be referred to ESI†. The Cu-MOFs have an octahedral structure with a size of several micrometers (Fig. S1) and their X-ray diffraction (XRD) pattern (Fig. S2) is in agreement with that of [Cu3(btc)2]n.34, 35 The CHO samples obtained from pyrolysis of the MOFs at 300 °C, 350 °C and
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structure inside CHO-1. It can be observed from Fig. 2d that the measured lattice fringe distances are 0.27 nm and 0.32 nm, corresponding to the (002) and (111) lattice planesView of Article CuOOnline and 10.1039/C5CC06924F selected area electron Cu2O, respectively. The bright spots in DOI: diffraction pattern of CHO-1 (Fig. S5) indicates good crystallinity of CHO-1 nanostructure. When the calcination temperature is increased to 350 °C (CHO-2), the octahedron porous structure collapses to form cluster structure (Fig. S6a). At a higher temperature of 400 °C, the octahedral structure entirely disappears and non-regular agglomerates are obtained (Fig. S6b). The porous structure of CHO was further characterized by nitrogen adsorption/desorption isotherm measurements (Fig. S7). It is found that the CHO samples display a broad pore-size distribution (inset in Fig. S7) and a moderate specific surface area, which is similar to the previous report.36 The specific surface areas, pore volumes and mean CHO-3 pore diameters of the samples are listed in Table S1. CHO-1 exhibits a largest surface area and pore volume, which can facilitate the sodium ion insertion and rapid diffusion during the electrochemical reactions. The surface areas and pore volumes of CHO-2 and CHO-3 are observed to decrease, which should be ascribed to more pores CHO-2 closed at higher calcination temperature. The electrochemical properties of porous CHO electrodes for SIBs were investigated by using the standard half-cell configuration. Fig. 3a shows the first three cyclic voltammetry (CV) curves of CHO-1 CHO-1 in a range of 0.005-3 V at a scan rate of 0.2 mV s-1. It is found that there is a broad and irreversible cathodic peak located at 0.84 V in the first discharge cycle, which corresponds to the decomposition of CuO 65-2309 the organic electrolyte and the formation of a solid electrolyte Cu2O 05-0667 interface (SEI) layer.17, 37 In the subsequence cycles, the cathodic 10 20 30 40 50 60 70 80 peaks at 1.83, 0.54, and 0.12 V during the discharging process involve three reversible electrochemical reactions: the formation of 2-Theta (degree) Cu(II)1-xCu(I)xO1-x/2, further reduction of Cu2O into Cu and Na2O, Fig. 1 XRD patterns of CHO samples. respectively.38 Furthermore, in the charge processes, the anodic peaks at 1.3 and 2.18 V correspond to the process 2Cu + Na2O → Cu2O + 2Na and the oxidation of Cu2O into CuO, respectively.39 Notably, after the first cycle, CV curves exhibit good repeatability, indicating high reversibility of the subsequent reactions.17 Galvanostatic charge/discharge test was carried out to investigate the electrochemical performances of CHO electrodes. Fig. S8a-c show the charge/discharge profiles at 1st, 2nd, and 10th cycles in a voltage range of 0.005-3 V at a current density of 50 mA g-1 for CHO-1, CHO-2, and CHO-3, respectively. Large irreversible capacity is observed in the first cycle for all electrodes, which is typical for such conversion type anodes.6 The irreversible capacity loss is consistent to CV measurements, in which the decrease of area during cycling is noticed due to decomposition of the electrolyte and subsequent formation of SEI.17, 37 In the subsequent cycles, the charge/discharge curves of all electrodes overlap well, indicating high reversibility of the materials. However, CHO-2 and CHO-3 show an unsatisfied sodium storage capacity due to the collapse of the hollow nanostructure and their relative low specific surface area. The cycling performance together with the Coulombic efficiency (CE) of CHO electrodes is shown in Fig. 3b at a current density of 50 mA g1 . The initial CE of ~ 45% is relatively low, which may be attributed to the formation of SEI layer on the CHO surface. Remarkably, after several cycles, the CHO electrodes exhibit excellent cyclic stability and their CE remains more than 98%. The specific charge/discharge Fig. 2 (a) Low- and (b) high-magnification FESEM, (c) low- and (d) highcapacity of CHO-1 maintains well with the increased cycles and a magnification TEM images of CHO-1. reversible capacity of 415 mAh g-1 can be obtained after 50 cycles. only display reversible As shown in field-emission scanning electron microscopy However, CHO-2 and CHO-3 samples -1 (FESEM) images of CHO-1 (Fig. 2a and b), CHO-1 exhibits a capacities of 248 and 185 mAh g after 50 cycles, respectively, typical porous octahedron structure that consists of clustered much lower than that of CHO-1. Electrochemical impedance (EIS) was employed to investigate the kinetics of CHO nanoparticles with size of tens of nanometers. The transmission spectroscopy 40 electron microscopy (TEM) image (Fig. 2c) shows the hollow electrodes. Fig. S9 show the EIS curves of the samples measured at
400 °C were named as CHO-1, CHO-2 and CHO-3, respectively. Fig. 1 shows the XRD patterns of CHO-1, CHO-2 and CHO-3. It can be seen that the peaks in the patterns of CHO-1, CHO-2 and CHO-3 are ascribed to CuO (JCPDS card no. 65-2309) and Cu2O (JCPDS card no. 05-0667), indicating the existence of CuO/Cu2O composite in CHO. Further measurement using X-ray photoelectron spectroscopy (Fig. S4) shows that the binding energy of Cu 2p3/2 can be assigned to Cu2+ (934.1 eV) and Cu+ (932.4 eV), and the ratios of Cu2+ to Cu+ are about 4.43, 5.41 and 11.32 for CHO-1, CHO-2 and CHO-3, respectively.
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different temperatures. The Nyquist plots can be fitted and interpreted well based on the equivalent electric circuit.40, 41 The large semicircle is related to charge transfer resistance (Rct) and constant phase element (CPE2). Another indistinct small highfrequency semicircle stemming from Rf and CPE1 is ascribed to the SEI layer. Table S2 lists the fitted values of Rct. The Rct is found to decrease when the temperature increases, indicating that better electrochemical kinetics is obtained at higher temperature. It can be seen that CHO-1 exhibits a smallest Rct among all samples measured at the same temperatures, showing that electrochemical reaction occurs easier in CHO-1. The apparent activation energy (Ea) for the intercalation of sodium can be calculated by the following equations:42, 43
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DOI: 10.1039/C5CC06924F
i0 = RT/nFRct
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i0 = Aexp(Ea/RT) where i0 is the exchange current. A is the temperature-independent coefficient. R is the gas constant. T is the absolute temperature. n is the number of transferred electrons and F is the Faraday constant. Fig. S9d shows the corresponding Arrhenius plots of –ln(T/Rct) versus 1000/T. Ea is equal to the slope of the fitting straight line multiplied by R. The calculated values of Ea are 35.73, 50.46 and 66.75 kJ mol-1 for CHO-1, CHO-2 and CHO-3 samples, respectively. The lowest Ea value of CHO-1 indicates the best sodium intercalation ability in CHO-1, which further confirms the superior capacity of CHO-1 for sodium ions storage. The reason should be ascribed to the following: (i) The more porous structure of CHO-1 is beneficial to the increase of contact between the electrode and electrolyte, which facilitates the intercalation of Na ions into the active materials and accelerates their diffusion velocity. (ii) The hollow structure can efficiently enhance structural integrity with voids for buffering stresses caused by the volume change during cycling. (iii) The synergistic effect between CuO and Cu2O contributes to the high capacity. 44-46 Fig. 3c shows the rate performance of CHO-1 electrode at different current densities from 50 to 2500 mA g-1. The corresponding charge/discharge curves of CHO-1 electrode are displayed in Fig. 3d. It is clearly seen that the CHO-1 exhibits the specific capacities of 440, 395, 349.3, 273.5, 217.2 and 153.8 mAh g-1 at 50, 100, 250, 500, 1000 and 2500 mA g-1, respectively. The specific capacity recovers to ~ 410 mAh g-1 when the current density returns back to 50 mA g-1, indicating that the unique porous hollow structure of CHO-1 can preserve the integrity of the electrode material. To evaluate the long cycling performance, the CHO-1 electrode was measured for more than 1000 cycles at high current densities. Fig. 3e displays the cyclability of CHO-1 after the tests for the first 30 cycles at 50 mA g-1, 1 A g-1 for the subsequent 400 cycles and 2 A g-1 for the final 600 cycles. Clearly, the porous CHO-1 shows an excellent performance with a capacity of ~212 mAh g-1 when cycled at 1 A g-1 after 400 cycles. The capacity can retain ~165 mAh g-1 at 2 A g-1 after 1000 cycles. During the cycling, the CE of ~99 % is achieved after the first several cycles, further demonstrating the superior stability for sodium storage capability of porous CHO electrode.25 In summary, CHO samples were successfully synthesized via thermal decomposition of Cu-MOFs and used as anode materials for SIBs for the first time. They exhibit excellent sodium storage capacity of 415 mAh g-1 at 50 mA g-1 after 50 cycles and superior cyclability at higher current density as well as good rate capability, which can be attributed to synergistic effect between CuO and Cu2O and excellent stability of their porous hollow structure. The CHO should be promising anode materials for SIBs.
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Fig. 3 (a) CV curves of CHO-1 and (b) cycling performance of CHO electrodes at a current density of 50 mA g-1. (c) Rate capabilities at different current densities, (d) typical discharge/charge curves at different current densities and (e) long cycling performance of CHO-1.
Acknowledgements
Financial support from Basic Research Project of Shanghai Science and Technology Committee (No. 14JC1491000) is gratefully acknowledged.
Notes and references
Engineering Research Center for Nanophotonics & Advanced Instrument, Ministry of Education, Shanghai Key Laboratory of Magnetic Resonance, Department of Physics, East China Normal University, Shanghai 200062, China. E-mail: [email protected]; Fax: +86 21 62234321; Tel: +86 21 62234132. †Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/c000000x/
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DOI: 10.1039/C5CC06924F
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