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Metal-Organic Framework Derived Porous Hollow Co3O4/N-C Polyhedron Composite with Excellent Energy Storage Capability Wenpei Kang, Yu Zhang, Lili Fan, Liangliang Zhang, Fangna Dai, Rongming Wang, and Daofeng Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15000 • Publication Date (Web): 13 Mar 2017 Downloaded from http://pubs.acs.org on March 14, 2017

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ACS Applied Materials & Interfaces

Metal-Organic Framework Derived Porous Hollow Co3O4/N-C Polyhedron Composite with Excellent Energy Storage Capability Wenpei Kang‡, †,Yu Zhang‡,†, Lili Fan‡, Liangliang Zhang‡, Fangna Dai‡, Rongming Wang‡*, Daofeng Sun‡*

‡ State Key Laboratory of Heavy Oil Processing, College of Science, China University of Petroleum (East China), Qingdao, Shandong 266580, China.

† These authors contributed equally.

KEYWORDS: Co3O4/N-C, metal-organic frameworks, lithium-ion battery, sodium-ion battery, long-term cycling performance

ABSTRACT: Metal-organic frameworks (MOFs) derived transition metal oxides exhibit enhanced performance in energy conversion and storage. In this work, porous hollow Co3O4 with N-doped carbon coating (Co3O4/N-C) polyhedrons have been prepared using cobalt-based MOFs as a sacrificial template. Assembled from tiny nanoparticles and N-doped carbon coating, Co3O4/N-C composite shortens the diffusion length of Li+/Na+ ions and possesses an enhanced conductivity. And the porous and hollow structure is also beneficial for tolerating volume

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changes in the galvanostatic discharge/charge cycles as lithium/sodium battery anode materials. As a result, it can exhibit impressive cycling and rating performance. At 1000 mA g-1, the specific capacities maintaine stable values of ~620 mAh g-1 within 2000 cycles as anodes in lithium ion battery, while the specific capacity keeps at 229 mAh g-1 within 150 cycles as sodium ion battery anode. Our work shows comparable cycling performance in lithium ion battery but even better high-rate cycling stability as sodium ion battery anode. Herein, we provide a facile method to construct high electrochemical performance oxide/N-C composite electrode using new MOFs as sacrificial template.

1. INTRODUCTION As there is urgent need of efficient, environmentally benign and cost-effective tools to storage energy, rechargeable batteries including sodium ion batteries (SIBs) and lithium ion batteries (LIBs) are supposed to be one of the ideal tools to provide power sources attributed to the light weight, high energy density and ultra-long cycle life.1-3 It is well known that electrode materials play a key role on improving rechargeable battery capability, so new anode materials with lower cost, higher capacity, better durability and higher rate capability are urgently needed to increase their energy and power densities.4-6 Transition metal oxides are widely explored as an emerging anode candidate because of their extraordinary specific capacities (~1000 mAh g-1) which are almost three times as high as that of commercial graphite (370 mAh g-1).7-9 Among the anodes of transition metal oxides, Co3O4 has been regarded to be a potential electrode active material for secondary batteries owing to the high theoretical capacity and facile preparation.10-16 However, the practical application is still limited by some factors, such as poor kinetics, large volume variation upon cycling, as well as unstable solid electrolyte interface (SEI) layer


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originated from the iterative volume expansion and shrinkage in the successive cycles.17,18 In order to conquer these limitations, some methods such as nanostructure engineering19,20 and hybridization with carbon-based materials21-23 have been adopted, which demonstrated to effectively tolerate the volume change and increase its conductivity. Among them, N-doped carbon coating is proved to well enhance the electrochemical performance. Because N-doping can generate extrinsic defects, leading to improve the reactivity and conductivity of carbon. Thus, the doped N atoms could drastically alter the electrochemical performance, offer more active sites to react with Li+/Na+ ions, and enhance the interaction between the carbon and lithium/sodium in LIBs and SIBs.24-26 Metal-organic frameworks (MOFs) including organic bridging ligands and metal ions, possess well-defined pore structures, large surface area and adjustable physical chemistry properties.27 Numerous endeavors have been devoted in order to explore new MOFs because of their potential applications in the fields of catalysis, gas storage, drug delivery, adsorptive separation and sensors.28-32 Recently, MOFs with specific structures were widely explored as precursors and sacrificial templates to synthesize metal oxides, metal sulfides and porous carbon materials, expecting to enhance performance in their specific applications.33-35 For example, ZIF67 derived hollow structures prepared by Co(NO3)2·6H2O and 2-methylimidazole have shown impressive

electrochemical

performances

as

electrode

materials

in

LIBs/SIBs

or

supercapacitors.36-39 Co3O4@N-C composite derived from ZIF-67 can keep a capacity retention as high as 67% after 1100 cycles in SIBs, which is attributed to the in situ N-doped carbon coating.38 Furthermore, ZIF-67 can be combined with graphene and transformed into Co3O4/graphene composite.39 And this composite manifested high rate capability (877 mAh g-1 at 5000 mA g-1) and long-term cycle stability (714 mAh g-1 within 200 galvanostatic


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discharge/charge cycles). Hu’s group reported that the mesoporous Co3O4 derived from MOF-71 exhibited a large lithium storage capacity attributed to the impressive specific surface area.40 Also,

shale-like

Co3O4

derived

from

a

new

metal-organic

compounds

(Na[Co3(CN)6(H2O)•2(C2O4)]•H2O) can exhibit outstanding cycling and rate performances in both SIBs and LIBs.14 Generally, cobalt oxides derived from MOFs can exhibit outstanding performance especially when evaluated as energy storage and conversion materials41,42, which benefits from high surface area with numerous active sites to contact with the electrolyte sufficiently in the LIBs/SIBs and supercapacitors. However, the cycling stability and rate performance reported for the Co3O4 electrode materials are still limited.37, 40 As a result, the welldesigned electrodes derived from new Co-MOFs with high conductivity and stable nanostructures are highly desired to endow a high power and energy densities as well as ultralong cycling life of the LIBs or SIBs. In this work, a stable cobalt based MOF ([Co6O(TATB)4]·(H3O+)2·Py , named Co-TATB) was synthesized through a facile solvothermal reaction and selected to be the sacrificial template to fabricate porous hollow Co3O4 polyhedron with N-doped carbon coating composite (Co3O4/NC) through a following annealing process. As anode material, it obtained high and stable capacity values at 620 and 229 mAh g-1 within 2000 and 150 cycles at 1000 mA g-1 for the LIBs and SIBs, respectively. The Co3O4/N-C composite can show impressive electrochemical performance when used as electrode materials in rechargeable batteries, which benefited from a synergistic effect between the porous hollow structure and ultrafine nanoparticles with N-doped carbon coating. High specific capacity, excellent conductivity, stable SEI, and robust cycling stability can be obtained for the Co-MOFs derived Co3O4/N-C electrode owing to their structure features.


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As expected, Co3O4/N-C composite was proved to be an ideal electrode material with outstanding power density as well as long cycling life for the LIBs and SIBs. 2. EXPERIMENTAL SECTION 2.1 Synthesis of [Co6O(TATB)4]•(H3O+)2•Py (Co-TATB) MOF and Co3O4/N-C composite. The Co-TATB MOFs were synthesized according to procedures previously reported.43 Briefly, Co(NO3)2·6H2O (11.4 mg, 0.039 mmol), 2,4,6-tris(4-carboxyphenyl)-1,3,5triazine (TATB, 6.0 mg, 0.014 mmol) and pyridine (Py, 0.01 mL) were added in H2O (12.0 mL). The solution was reacted at 180 oC and kept for 4 days resulted into purple crystals of Co-TATB (yield: 45%). The crystals were washed using deionized water, after that they are dried at 60 oC. Co3O4/N-C composite was synthesized via calcination of the obtained Co-TATB MOFs at 700 oC for 3 h under a vacuum condition using a slow heating rate (2 oC min-1). The resulting Co3O4/N-C composite was collected as black powder. And the schematic representation of the synthesis mechanism for Co3O4/N-C composite is shown in Figure 1. 2.2 Materials Characterization. Crystallinity and phase purity of the Co-TATB were examined with a Super Nova diffractometer equipped with Mo-Kα radiation (λ = 0.71073 Å) at 25 oC. PerkinElmer 240 elemental analyzer was used to measured elementals (C, H, and N). Xray photoelectron spectroscopy (XPS) was conducted on VG ESCALAB 220i-XL equipped with Al Kα X-ray source. Thermogravimetric analysis (TGA) was carried out using PerkinElmer TGA 7 in an air atmosphere (10 oC min-1). Bruker VERTEX-70 spectrometer was used to measure IR spectra within 4000−400 cm-1. Bruker D2 phaser diffractometer (Cu Kα line) was used to perform the X-ray diffraction (XRD) measurements of Co3O4/N-C composite. Philips XL30 FEG SEM with an operation voltage of 5 kV was used to conduct the scanning electron


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microscopy (SEM) measurements. And transmission electron microscopy (TEM) images were obtained from a Philips FEG TEM CM200 operated at 200 kV. 2.3 Electrochemical Measurements. To evaluate the energy storage capability of Co3O4/NC composite, the as-prepared sample was mixed with carbon black, and then sodium alginate was added using H2O as solvent (weight ratio of 6: 2: 2) to form a slurry and coated on copper foil in order to prepared working electrodes. The coated foil was placed on a hotplate (80 oC), pressed on a rolling machine and cut into discs in a size of 16 mm. The active material loading was estimated to be 1.5 mg cm−2. And also the packing density of active material was calculated to be ~2.0 g cm-3. The working electrode were thoroughly dried in vacuum oven and directly transferred into a glove box with Ar atmosphere. LiPF6 (1.0 mol L−1) dissolved into a mixture of ethylene carbonate and dimethyl carbonate (v/v=1 : 1) was used as electrolyte for the LIBs. For the SIBs, NaClO4 (1.0 mol L−1) dissolved into propylene carbonate with 5% (in volume) fluoroethylene carbonate was selected as the electrolyte. Galvanostatic discharge/charge cycling were measured using Neware-5V10mA system (Shenzhen Xinwei) at constant 25 oC. Cyclic voltammetry (CV) curves were measured on the CHI 660E electrochemical workstation (0.0-3.0 V) at 0.1 mV s−1. ZAHNER-elektrik IM6 system was used to measure the electrochemical impedance spectroscopy (EIS) with frequencies of 0.1 MHz to 10 mHz. 3. RESULTS AND DISCUSSION

Figure 1. Schematic diagram for the preparation of Co3O4/N-C composite.


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The schematical synthesis procedure of Co3O4/N-C composite is shown in Figure 1. The formation of Co-TATB MOFs was checked by XRD and FTIR measurements as shown in Figure S1. Co-TATB exhibits a 3D framework in an orthorhombic space group of Fd-3,43 and the hourglass secondary building units (SBUs) are composed of two Td-Co atoms and one Oh-Co bridged by six carboxylate groups. And every µ4-O atom bridges four hourglass SBUs forming an infinite diamondoid network as shown in Figure 1. Accordingly, the obtained Co-TATB was calcined at 700 °C to ensure complete conversion into the final Co3O4/N-C product. After calcination, the peaks of the ligand disappear and two new peaks occurr at 567 cm-1 and 865 cm-1 in the FTIR spectrum, indicating the formation of Co3O4, as shown in Figure S1b. 44

Figure 2. (a) XRD patterns, (b) Raman spectra and (c) TGA for the Co3O4/N-C composite; (d) Comparison of N2 adsorption–desorption isotherms curves for Co3O4/N-C and Co-TATB.


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The phase of the calcination sample was measured by XRD (Figure 2a), and all the peaks in the XRD pattern are well matched with the standard XRD pattern of cubic Co3O4 (JCPDS No. 42-1467). There is no observable impurity found in the calcination sample. The sample was further measured by Raman and the spectrum is shown in Figure 2b. Five characteristic peaks at 196, 479, 517, 617, and 690 cm−1 are identified in the spectrum, which correspond to F2g, Eg, F2g1, F2g2, and A1g modes of the crystalline Co3O4, respectively.45 In the magnified part of the Raman spectrum, the broad bands of 1350 cm−1 and 1565 cm−1 are the typical D-band and Gband of graphitization carbon, respectively.46 N-doped C was further proved by FTIR (Figure S1). In the FTIR spectrum, The C=N bond (~1615 cm-1) and C-N bond (~1380 cm-1) based on nitrogen doping can be clearly observed.47 The band of 2347 cm-1 is evidence of nitrile (C≡N) group formation, which indicates the existence of N-doped C through the decomposition of the ligands in the Co-TATB.48,49 The carbon content can be determined to be 2.5% based on the TGA analysis. Brunauer–Emment–Teller (BET) analysis is used to estimate the specific surface of Co-TATB MOF to be 665 m2 g−1 (Figure 2d), which decreases to be 30 m2 g−1 after calcination due to the structure collapse during heat-treatment along with reduction of pore sizes as shown in Figure S2. This should be able to provide enough active sites for the electrolyte contact when used as electrode materials in secondary battries.


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Figure 3. XPS spectra for the Co3O4/N-C: (a) Co 2p, (b) C 1s, and (c) N 1s level spectra.

XPS measurements were used to obtain more detailed information about elementals and chemical states in the Co3O4/N-C (Figure 3). From the Co2p core level peaks in Figure 3a, it can be concluded that two major peaks located at 780.2 and 794.9 eV should be attributed to Co3+2p3/2 and Co3+2p1/2, respectively. And another two peaks at 781.9 and 796.3 eV are originated from Co2+2p3/2 and Co2+2p1/2, respectively.50,51 Compared with Co2p spectrum for the Co-TATB (Figure S3), the satellite structure is weak, indicating the co-existence of Co2+ and Co3+ in Co3O4/N-C composite.51 After fitting the peaks, C 1s spectrum displays four types of C species (Figure 3b): C=C, C=N & C–O, C=O & C–N and C–O–C, locating at 284.5, 285.4, 285.8, and 286.8 eV, respectively, further proving the existence of carbon atoms connected to N. The deconvolution of the N 1s spectrum (Figure 3c) shows the existence of two kinds of nitrogen species associated with carbon atoms. Combined the XPS and TGA results, the N content can be evaluated to be 0.5 wt %.


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Figure 4. SEM images of Co3O4/N-C composite (a) overview, (b) magnified view, (c) broken sample and (d) the inner face.

Typical SEM images of the Co3O4/N-C composite obtained through Co-TATB thermal decomposition are shown in Figure 4. It reveals that the morphology is polyhedron structure and the size is about tens of microns (Figure 4a), which almost inherits from the Co-MOFs as shown in Figure S4. In a magnified SEM image, the rough surface of the polyhedron can be observed, which is composed of small particles in nano size (Figure 4b). In the SEM image of a broken polyhedron, the hollow structure of the polyhedron is proved (Figure 4c) and the inner interface is observed in 3D-network like structure (Figure 4d). And the hollow polyhedron structures can be kept in the electrodes as shown in Figure S5. The microstructure is further measured by TEM as shown in Figure 5. A fragment of the polyhedron is shown in Figure 5a, which reveals a mesoporous structure Co3O4/N-C composite. This porous structure was further confirmed in a magnified TEM image as shown in Figure S6. The amorphous carbon surrounding some small crystalline particles can be observed in the HRTEM images (Figure 5b). In Figure 5c, the lattice fringes with a spacing of 0.433 and 0.596 nm can be observed, which are agree with the (111) and (110) planes for the lattice of Co3O4, respectively. Figure 5d presents the corresponding electron energy loss spectroscopy mapping of the Co3O4/N-C composite, showing that the composite include these four Co, O, C and N elements and they are uniformly distributed throughout the particles.


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Figure 5. (a) TEM, (b, c) HRTEM, and (d) EELS elemental mapping images for Co3O4/N-C composite.

The successful fabrication of Co3O4/N-C composite for a superior energy storage and conversion material is evidently seen from the excellent electrochemical performance as the LIB and SIB anodes. Figure 6a shows the initial six CV profiles of the Co3O4/N-C anode (0.0 and 3.0 V). Obviously, the curve for the first cycle is distinct from those for the following cycles, especially for the cathodic process. In the first reduction procedure, a weak peak appears between 1.2 and 1.6 V, which is explained as the generation of Li2O. Simultaneously, a strong and sharp peak locates at 0.75 V, corresponding to the Co3O4 reduction into metallic cobalt, and the side reactions on the interfaces of electrodes owing to SEI film formation.36-39 Furthermore, the stronger reduction peak intensity in the first cycle indicates the existence of the unavoidable irreversible reactions. Meanwhile, after first cycle the reduction peaks move to higher voltage because of an activation process in the continuous cycles. In the oxidation process, a peak located at ~2.1 V is ascribed to the reversible transformation of cobalt into cobalt oxide. From the second cycle onwards, all the CV curves are almost same and overlapped. This indicates the


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cycling stability and the reversibility of the as-prepared Co3O4/N-C composite electrode is quite good. Figure 6b exhibits the typical discharge/charge profiles at 500 mA g-1 (0.01 and 3.0 V). The voltage curves are almost similar to those which were previously described for Co3O4 anodes.36-39 The long plateau appeared at about 1.0 V in the first cycle changed into a slope in the subsequent cycles, which is in accordance with the CV results, showing typical features of potential changes for Co3O4 anodes. It can be obtained that the first discharge capacity is 1062 mAh g−1 with an initial coulombic efficiency of 75.0%. And its cycling specific capabilities are shown in Figure S7. The high irreversible capacity in the 1st cycle is related to some side reactions, including unavoidable SEI layer formation on the surfaces and interfaces of electrode and the electrolyte decomposition.52 Rate capability of this Co3O4/N-C composite electrode was studied, and the results were summarized in Figure 6c. When cycled at 0.1, 0.5, 1.0, 2.0 and 5.0 A g-1, it delivers average capacities of 818, 815, 753, 671 and 529 mAh g-1, respectively. And the Co3O4/N-C composite electrode maintains capacity retention as high as 65% when current raises from 0.1 to 5.0 A g-1. This reveals that the rate capability of the Co3O4/N-C composite anode is excellent as anode material in LIBs. This exceptional lithium ion and electron mobility can be ascribed to the distinctive structure derived from the Co-MOFs, which combines the porous and hollow structure with N-doped carbon coating on the tiny primary Co3O4 particle surface. Importantly, the specific capacity can be almost recovered up to the original value when the current gradually decreases to 0.1 A g-1. Following the rate cycling, the electrode was continuously cycled at high current of 2.0 A g-1. The stable capacities can be maintained up to 507 mAh g-1 within 1000 cycles. Long life is of pivotal importance for many applications of secondary batteries. So the long-term cycling performance was also studied at 1000 mA g-1, and the result is shown in


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Figure 6d. The Co3O4/N-C electrode delivers stable reversible capacities in the initial 200 cycles and then gradually decreases in the following 200 cycles. The gradually decreased impedance (Figure S8) indicates that the Co3O4/N-C composite electrode has a good cycling stability. After 2000 cycles, the specific capacity keeps a stable value of ~620 mAh g-1 with capacity retention up to 86.4% against to the value in the 2nd cycle, which is much more stable than that of Co3O4 electrodes (Figure S9). This further indicates N-C in the Co3O4/N-C composite electrodes can enhance the cycling stability especially at high current rate.

Figure 6. (a) CV curves, (b) typical charge-discharge profiles at a 500 mA g−1, (c) rate capability, and (d) long-term cycling performance at 1000 mA g−1 for Co3O4/N-C composite anode in LIBs.


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The sodium storage behaviours of the Co3O4/N-C anode were also investigated as shown in Figure 7. The SIB is an attractive rechargeable energy storage device based on the low-cost solution. However, more severe volume expansion and much lower kinetics during conversion reactions will occur due to much larger radius of the Na+ ions.53 Therefore, the strategies including nanostructure engineering and carbon hybridization used in the transitional metal oxide anode in LIB are expected to extend to the SIBs and achieve an ideal electrochemical performance. The sodium storage mechanism is similar to that in lithium storage. The CV of the electrode was measured at the same condition for the lithium ion battery as shown in Figure 7. In the first cathodic procedure, broad peaks located at 0.46 V and 0.92 V are observed, which are because of the electrochemical reduction reaction of Co3O4 with Na, producing metal Co, Na2O and SEI layer.54,55 The following anodic peaks (0.88 and 1.64 V) are related to the Co3O4 oxidation conversion. The galvanostatic discharge and charge curves are exhibited in Figure 7b. These curves are almost analogous to those in the lithium half cells except that the potential of the platform is lower. The first discharge/charge specific capacities are 712 and 468 mAh g−1 (Figure 7b), respectively, giving 34.3% irreversible capacity, which results from the formation of SEI similar to other transition metal oxide anode materials. The subsequent coulombic efficiency quickly increases to 92% during the 2nd cycle and 97-99% in the following cycles.


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Figure 7. (a) CV, (b) typical charge-discharge profiles at a 100 mA g−1, (c) cycling performance, and (d) rate capability −1 for Co3O4/N-C composite as SIB anode material. Figure 7c exhibits the cycling test results. After 50 cycles, the Co3O4/N-C anode in SIBs can maintain capacities up to 368 and 276 mAh g-1 at 100 and 500 mA g-1, respectively. Furthermore, the rate capability was evaluated with 5 discharge/charge cycles at each current density between 50 and 2000 mA g-1 (Figure 7d). The specific capacities are 673, 615, 520, 432, 385 and 326 mAh g-1 at 50, 100, 200, 500, 1000 and 2000 mA g-1, respectively. The specific discharge capacity is remained at 503 mAh g-1 after 40 cycles when the current density decreased to be 50 mA g-1. This reserve capacity is slightly lower than that in the initial 5 cycles also at 50 mA g-1. This indicates that the Co3O4/N-C anode can well tolerant ion or electron mobility even during high current rate cycling process. And a discharge capacity of 326 mAh g-1 is kept even at high current (2000 mA g-1) cycling, which is higher compared with that of the hard carbon anodes


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investigated for SIBs.56 After the rate cycling, the electrode was cycled at high current of 1000 mA g-1, and a specific capacity of 229 mAh g-1 can be kept in the 150th cycle. Such a superior cycling stability and rate capability of Co3O4/N-C anode is comparable or even better than other MOF-derived cobalt oxide electrodes reported (Tab. S1) for the lithium/sodium storage. This excellent energy storage performances are because of the advantages of the artful structures of the Co3O4/N-C anode: (1) the hollow porous polyhedron structures are aggregated by the nanocrystals with numerous of pores and voids, which is beneficial to the liquid electrolyte penetration into the electrode materials and provides sufficient space to tolerate volume expansion and shrinkage during the discharge-charge process; (2) nanoparticles assembled in Co3O4 polyhedron significantly shorten the migration length of Na+ ions; (3) N doped carbon can increase active sites for electrons and increase the conductivity of the composite electrode. 4. Conclusions In summary, porous hollow Co3O4/N-C polyhedron composite was successfully prepared using Co-TATB as sacrificial template. Due to the synergistic effect between the assembled tiny primary particles and the N-doped C coating, this material exhibites good electrochemical properties as active electrode materials for LIBs or SIBs. In the LIBs, this Co3O4/N-C anode can reach a high capacity retention of 65% in the current range 0.1 to 5.0 A g−1 and deliver almost stable capacities of ~620 mAh g−1 at 1000 mA g-1 within 2000 cycles. As anode material in SIBs, high reversible capacity of 468 mAh g−1 can be obtained at 100 mA g-1 and a specific capacity of 229 mAh g-1 can be maintained in the 150th cycle at 1000 mA g-1. Our results suggest Co3O4/NC composite have potential applications to be used for long life and high-power secondary batteries.


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ASSOCIATED CONTENT

Supporting Information. XRD, FTIR, XPS results of different samples, electrochemical performances of different samples for comparison. Tables on the electrochemical performance comparison for Co-MOFs derived anodes in LIBs and SIBs. Corresponding Authors * E-mail: [email protected], * E-mail: [email protected] Author Contributions Wenpei Kang and Yu Zhang gave equal contribution to this work. All the listed authors contributed to manuscript preparation and gave approval to the manuscript. Funding Sources This work was supported by the NSFC (Grant No. 21571187, 21371179, 21271117), NCET11-0309, Taishan Scholar Foundation (ts201511019), and the Fundamental Research Funds for the Central Universities (13CX05010A, 14CX02158A, 15CX02069A).

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