MetalâOrganic Framework-Derived Metal Oxide Embedded in

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Metal–Organic Framework-Derived Metal Oxide Embedded in NitrogenDoped Graphene Network for High-Performance Lithium-Ion Batteries Zhu-Yin Sui, Pei-Ying Zhang, Meng-Ying Xu, Yuwen Liu, Zhi-xiang Wei, and Bao-Hang Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15315 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 18, 2017

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ACS Applied Materials & Interfaces

Metal–Organic Framework-Derived Metal Oxide Embedded in Nitrogen-Doped Graphene Network for High-Performance Lithium-Ion Batteries Zhu-Yin Sui,† Pei-Ying Zhang,†,‡ Meng-Ying Xu,† Yu-Wen Liu,‡ Zhi-Xiang Wei,*,†,§ and Bao-Hang Han*,†,§

†

CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS

Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China ‡

Department of Environment and Chemical Engineering, Yanshan University,

Qinhuangdao 066004, China §

University of Chinese Academy of Sciences, Beijing 100049, China

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ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Metal–organic frameworks (MOFs) are hybrid inorganic–organic materials that can be used as effective precursors to prepare various functional nanomaterials for energy-related applications. Nevertheless, most MOF-derived metal oxides exhibit low electrical conductivity and mechanical strain. These characteristics limit their electrochemical performance and hamper their practical application. Herein, we report a rational strategy for enhancing the lithium storage performance of MOF-derived metal oxide. The hierarchically porous Co3O4@NGN is successfully prepared by embedding ZIF-67-derived Co3O4 particles in a nitrogen-doped graphene network (NGN). The high electrical conductivity and porous structure of the NGN accelerates the diffusion of electrolyte ions and buffers stress resulting from the volume changes of Co3O4. As an anode material, the Co3O4@NGN shows high capacity (1030 mA h g‒1 at 100 mA g‒1), outstanding rate performance (681 mA h g–1 at 1000 mA g‒1), and good cycling stability (676 mA h g–1 at 1000 mA g‒1 after 400 cycles).

Keywords: metal–organic frameworks; nitrogen-doped graphene; aerogel; metal oxide; anode; lithium-ion batteries

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INTRODUCTION High-performance lithium-ion batteries (LIBs) are attracting intensive attention with the continuous development of the emerging portable electronic devices and electric vehicles.1,2,3 However, owing to the low capacity (theoretical value: 372 mA h g–1) of graphite, currently available commercial LIBs have approached their performance limit, thus hampering the further development of advanced LIBs. Transition-metal oxides are highly promising anode materials owing to their outstanding electrochemical behaviors.4,5,6 The commercialization of these materials, however, is hampered by their unsatisfactory rate performance and cycling stability. These properties stem from their low conductivity and drastic volume change during charge–discharge processes. Metal–organic frameworks (MOFs) have caused extensive concern given their numerous outstanding properties, including ultrahigh surface area, well-defined pore structure, tunable surface properties, and excellent adsorption capability. The above features make MOFs attractive for applications in gas adsorption/storage,7,8 catalysis,9,10 energy storage,11,12 drug delivery,13 and water treatment.14 Recently, MOFs have been used as precursors or templates for novel functional materials, such as porous carbons, metal oxides, and metal/carbon composites, with applications in various fields.15,16,17,18 Metal oxides derived from MOFs usually possess low electrical conductivity and/or microporous structures. These properties are conducive for the fast electron transport and mass transfer in LIB anodes. The problem of low electrical conductivity can be addressed 3



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by introducing conductive carbonaceous materials to MOFs. For example, Huang et al. synthesized a multi-walled carbon nanotube/Co3O4 nanohybrid by subjecting multi-walled carbon nanotubes/ZIF-67 precursor to thermal treatment; the nanomaterial showed good electrochemical performance (813 mA h g–1 at 100 mA g‒1) when used as an anode.19 High-performance LIBs can be obtained by extending the characteristic micropores of MOF-derived materials to mesopores and even macropores. Such an endeavor, however, is difficult. Graphene has been widely studied and explored because of its unique structural and physicochemical properties.20,21,22,23 Graphene was also used as a conductive matrix to load MOFs, thus preparing graphene/metal oxide composites for LIBs. Cao et al. prepared a three-dimensional (3D) graphene/Fe2O3 composite by annealing 3D graphene/MIL-88-Fe.24 Ji et al. reported a 3D graphene network/MOF-derived CuO hybrid with excellent electrochemical properties.25 However, these graphene–MOF materials have suboptimal electrochemical performance because metal oxides particles can only be deposited on graphene sheets and cannot enter the space between graphene sheets. Graphene sheets can be constructed into 3D graphene aerogels from a graphene oxide precursor through a hydrothermal process.26,27,28 Aerogels are highly porous solid materials obtained from wet gels and are prepared by replacing gel solvents with air.29,30 Graphene-based aerogels have been studied as electrodes on the basis of their large surface area, high conductivity, and hierarchically porous structure (including micropores, mesopores, and macropores).31,32,33 4


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These excellent properties are beneficial to the high rate of electron transport and mass transfer. Therefore, an excellent anode material for LIBs can be prepared by combining the dual advantages of MOF and graphene-based aerogels. However, work on porous materials derived from MOF@graphene-based aerogels for LIBs has never been reported. Herein, we report the synthesis of MOF@nitrogen-doped graphene aerogel (MOF@NGA) through in situ growth of MOF in NGA pores. Co3O4@NGN, a hierarchically porous composite of Co3O4 and a nitrogen-doped graphene network, is successfully prepared through the calcination treatment of ZIF-67@NGA in air. During calcination, ZIF-67-derived Co3O4 is embedded in an NGA-derived nitrogen graphene network (NGN). The as-synthesized Co3O4@NGN as an anode displays high discharge capacity, excellent cycling behavior, and outstanding rate performance. These properties can be attributed to the hierarchically porous structure of the Co3O4@NGN, as well as the synergistic interaction between NGN and Co3O4. NGN provides excellent electrical conductivity (87 S m–1) and porous structure, while Co3O4 contributes to the high capacity (theoretical value: 890 mA h g‒1).

EXPERIMENTAL SECTION Materials Cobaltous nitrate hexahydrate, zinc nitrate hexahydrate, acetate acid, and zirconium chloride were procured from Sinopharm Chemical Reagent Co. Ltd, China. Terephthalic 5



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acid and 2-methylimidazole were procured from Sigma–Aldrich. All organic solvents including ethanol, methanol, acetone, and tert-butyl alcohol, were obtained from Beijing Chemical works, China. Preparation of ZIF-67@NGA, ZIF-8@NGA, and UiO-66@NGA NGA was obtained by a hydrothermal approach as previous work reported.31 A detailed preparation procedure can be found in the Supporting Information. The general strategy for the preparation of ZIF-67@NGA, ZIF-8@NGA, and UiO-66@NGA is shown as follows. To prepare ZIF-67@NGA, NGA (240 mg) was added to 25 mL of cobaltous nitrate hexahydrate (498 mg) solution in methanol. The mixture was stirred for 1 h at ambient temperature to promote the adsorption of Co2+ onto NGA. Then, 25 mL of 2-methylimidazole (765 mg) solution in methanol was poured into the former mixture. The mixture was then stirred for 2 h and maintained under static conditions for 24 h. The reaction product was then separated from the mixture through centrifugation, repeatedly washed with methanol and tert-butyl alcohol, and freeze-dried for 12 h to yield ZIF-67@NGA. ZIF-67 was synthesized as a control sample through similar steps without the introduction of NGA. To prepare ZIF-8@NGA, NGA (240 mg) was added to 25 mL of zinc nitrate hexahydrate (730 mg) solution in methanol. The mixture was stirred for 1 h at ambient temperature to promote the adsorption of Zn2+ onto NGA. Then, 25 mL of 6


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2-methylimidazole (850 mg) solution in methanol was poured into the former mixture and stirred for 2 h. The mixture was maintained under static conditions for 24 h. The reaction product was then separated from the mixture through centrifugation, repeatedly washed with methanol and tert-butyl alcohol, and freeze-dried for 12 h to yield ZIF-8@NGA. ZIF-8 was synthesized as a control sample through similar steps but without the introduction of NGA. To prepare UiO-66@NGA, NGA (200 mg) was added to 15 mL of zirconium chloride (61 mg) solution containing acetic acid (0.45 mL) in N,N-dimethylformamide. The mixture was stirred for 30 min at ambient temperature to promote the adsorption of Zr4+ onto NGA. Then, terephthalic acid (43 mg) was poured into the former mixture. The mixture was then heated in a Teflon-lined autoclave at 120 °C for 24 h. The reaction product was separated from the mixture through centrifugation, repeatedly washed with methanol and tert-butyl alcohol, and freeze-dried for 12 h to yield UiO-66@NGA. UiO-66 was synthesized as a control sample through similar steps but without the introduction of NGA. Preparation of Co3O4@NGN Composite ZIF-67@NGA was heated to 300 °C at 2 °C min–1 in a horizontal tube furnace and maintained under air atmosphere for 1 h to obtain the Co3O4@NGN composite. For comparison, Co3O4 and NGN materials were also prepared by a calcination treatment procedure of single ZIF-67 and NGA. Furthermore, a specimen without MOF structure was 7



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also prepared. After the adsorption of Co2+ on the NGA, no additional 2-methylimidazole was introduced. The resulting Co2+/NGA was directly calcined to obtain Co3O4/NGA. Instrumental Characterization An S-4800 scanning electron microscope (Hitachi, Japan) was applied to observe SEM images. X-ray diffraction (XRD) tests were conducted with a Philips X’Pert PRO diffractometer (PANalytical B.V., Netherlands). X-ray photoelectron spectroscopy (XPS) analysis was characterized by using an ESCALab220i-XL X-ray photoelectron spectrometer system (VG Scientific Ltd., U.K.). Thermal gravimetric analysis (TGA) curves were acquired from a Pyris Diamond differential scanning calorimeter (Perkin Elmer, U.S.A.) at 5 °C min–1 in air. A TriStar II 3020 instrument (Micromeritics, U.S.A.) was used to analyze the pore parameters. Electrical conductivity was measured using a 4200 semiconductor characterization system (Keithley Instruments, U.S.A.). Electrochemical Measurements The 2032 coin-type batteries were assembled to assess the electrochemical behaviors of the as-prepared samples. Lithium foil, copper foil, Celgard 2400 membrane, and 1.0 M LiPF6 with the volume ratio of ethyl carbonate to diethyl carbonate of 1:1 in the cells were used as the anode, current collector, and separator, and electrolyte, respectively. Active material (70 wt %), carbon black (20 wt %), and polyvinylidene fluoride binder (10 wt %) was mixed to fabricate the working electrode. The addition of carbon black might promote a fast lithium ion transport, thus improving rate performance of the electrode. 34 The 8


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working electrode was dried at 80 °C for 10 h prior to assembly into a 2032-coin cell. Cyclic voltammetry (CV) curves and impedance spectra were acquired on a VMP3 instrument (Bio-Logic Science Instruments, France). Galvanostatic charge/discharge tests were conducted with a CT2001A measurement system (Wuhan LAND, China). Capacity was calculated according to the mass of active materials.

RESULTS AND DISCUSSION The general strategy for preparing Co3O4@NGN composite is schematically displayed in Figure 1. NGA, a novel graphene-based aerogel with numerous unique characteristics, including abundant functional groups, high porosity, hierarchically porous structure, and excellent electrical conductivity, can be prepared through the hydrothermal method.31,35 When NGA and inorganic Co2+ salt are mixed, Co2+ can be adsorbed onto the graphene sheets due to the coordination and/or electrostatic interaction between Co2+ and oxygenated functional groups of NGA. With the addition of 2-methylimidazolate solution in methanol, ZIF-67 microparticles can be formed and anchored on the surface of graphene sheets to prepare ZIF-67@NGA. Eventually, ZIF-67@NGA is transformed into the Co3O4@NGN composite through calcination in air.

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Figure 1. Schematic of the fabrication processes of hierarchically porous Co3O4@NGN composite. ZIF-67@NGA was first synthesized through the in situ growth of ZIF-67 in aerogel pores and then heated in air to yield the Co3O4@NGN composite.

SEM is used to observe the morphologies of NGA and ZIF-67@NGA (Figure 2). It can be seen that the as-prepared ZIF-67@NGA (Figures 2c–f) has retained the 3D architecture of NGA (Figures 2a and 2b) and displays an interconnected porous network with the pore sizes that range from hundreds of nanometers to several micrometers. ZIF-67 particles approximately 600 nm in size are uniformly anchored on graphene sheets and embedded in aerogel pores. This uniformity can be ascribed to that NGA with a large amount of functional groups can easily adsorb metal ions and promote the heterogeneous nucleation to form ZIF-67 particles, highlighting the effectiveness of the present strategy for the in situ growth of ZIF-67 particles in graphene aerogel.

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Figure 2. SEM images of NGA (a and b) and ZIF-67@NGA (c–f) at different magnifications.

The proposed fabrication protocol can be further extended to the preparation of other MOF@NGA hybrids. MOF materials can be synthesized in water and organic solvents or even under hydrothermal/solvothermal conditions. Therefore, the excellent environmental stability of NGA has an important role in the preparation of various MOF@NGA hybrid 11



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materials. As displayed in Figure S1 (Supporting Information), NGA is stable in different solvents. Water or organic solvents can quickly infiltrated into NGA. Even after hydrothermal/solvothermal reaction, there is no obvious change for the porous morphology of NGA (Figure S2, Supporting Information), which indicates that NGA can be used as a host to accommodate different MOFs grown under different preparation conditions. Typically, ZIF-8@NGA and UiO-66@NGA were successfully prepared by using ZIF-8 and UiO-66 instead of ZIF-67 on the basis of their similar reaction mechanism of metal ions bridged by organic ligands. The SEM images (Figure S3, Supporting Information) of ZIF-8@NGA and UiO-66@NGA reveal that ZIF-8 and UiO-66 particles are highly dispersed and embedded within the NGA. The various resultant MOF@NGA hybrid materials are further analyzed by XRD. The diffraction peaks of the obtained MOF@NGA hybrid materials (Figure S4, Supporting Information) are highly consistent with those of the pure MOF materials. This result indicated that crystalline MOF materials have been successfully prepared and introduced into the NGA. The wide peak located at around 25° corresponds to the (002) plane of the NGA. The above analysis results suggested the successful introduction of various MOF materials into the pores of NGA.

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Figure 3. SEM images of the Co3O4@NGN composite under low (a) and high (b) magnifications; (c) XRD pattern of the Co3O4@NGN composite; (d) nitrogen sorption isotherm at 77 K of the Co3O4@NGN; XPS survey spectrum (e) and Co 2p spectrum (f) of the Co3O4@NGN.

The as-prepared MOF@NGA hybrid materials might have great potential for gas 13



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adsorption/separation, water treatment, catalysis, and energy conversion/storage.36,37 It is well known that ZIF-67 can be transformed into Co3O4, which is a promising anode with the high capacity (theoretical value: 890 mA h g‒1).19 Therefore, ZIF-67@NGA was used as a precursor to prepare a high-performance anode material. According to the TGA analysis (Figure S5, Supporting Information), the mass ratio of NGA and ZIF-67 in the ZIF-67@NGA composite is ~2:1. Figures 3a and 3b display SEM images of the Co3O4@NGN composite. The Co3O4@NGN composite (Figure 3a) can well preserve the overall morphology of ZIF-67@NGA even after a calcination process. The magnified observation (Figure 3b) revealed that the surface roughness of the Co3O4@NGN composite increased. This morphology change can be ascribed to the release of water and carbon dioxide during thermal treatment. In addition, a high amount of particles are embedded within the graphene network and are not merely attached to graphene surface, indicating the successful incorporation of ZIF-67-derived Co3O4 in the graphene network. The close contact between graphene and Co3O4 maximizes the performance of the Co3O4@NGN composite. For comparison, ZIF-67 and NGA were treated through a similar calcination process, and their SEM images are presented in Figure S6 (Supporting Information). For single ZIF-67-derived Co3O4, obvious aggregation phenomenon can be observed. Obviously, the morphology of ZIF-67 wasn’t preserved well after the calcination process. This change in the morphology can be ascribed to the decomposition of organic components during thermal treatment in air.19,24 As observed from the XRD pattern of 14


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Co3O4@NGN (Figure 3c), an obvious peak at around 37.1° corresponds to the (311) facet of spinel Co3O4 (JCPDS No.43-1003),19 suggesting the formation of Co3O4 after calcination of ZIF-67@NGA in air. In addition, TEM images (Figure S7) of Co3O4@NGN show that some small particles derived from ZIF-67 have dispersed onto graphene sheets. These particles are about 10 nm in size. The high-resolution TEM image further indicated that the lattice spacing of Co3O4 particles is 0.24 nm, which is assigned to the (311) plane in the Co3O4 phase. These ZIF-67-derived Co3O4 nanoparticles can provide a great amount of surface atoms and accessible surface area, thus shortening the transport path of electrolyte and improving their electrochemical performance. The pore characteristics of the Co3O4@NGN composite were characterized through the nitrogen sorption test. On the basis of the nitrogen sorption isotherm (Figure 3d), the Co3O4@NGN composite reveals a Brunauer–Emmett–Teller specific surface area of 50 m2 g–1 and total pore volume of 0.25 cm3 g–1. These values are lower than those of the ZIF-67@NGA precursor (780 m2 g–1 and 1.21 cm3 g–1) as displayed in Figure S8a (Supporting Information). The decrease in the specific surface area and pore volume may be attributed to the partial collapse of the porous structure of ZIF-67@NGA after calcination in air. However, the porosity parameters of the Co3O4@NGN composite are higher than those of ZIF-67-derived Co3O4 (31 m2 g–1, 0.055 cm3 g–1). The hysteresis and high nitrogen adsorption capacity at P/P0 = 0.8–1.0 suggests the existence of mesopores and macropores. Figure S8b (Supporting Information) gives the pore size distribution (PSD) 15



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of the Co3O4@NGN composite, which shows a hierarchically porous structure with a wide PSD. Taking into account the high density of Co3O4, the Co3O4@NGN composite possesses a relatively high porosity. The moderate surface area is beneficial to the contact between the electrolyte and electrode. Meanwhile, its hierarchically porous structure can not only promote lithium-ion diffusion and electrolyte penetration but also buffer the volume changes. The chemical composition of the Co3O4@NGN composite was determined through XPS measurements. It can be found from Figure 3e that Co3O4@NGN composite possesses carbon, oxygen, nitrogen, and cobalt elements. According to the Co 2p spectrum (Figure 3f), there are two peaks at 780.2 and 795.6 eV, which can correspond to Co 2p3/2 and Co 2p1/2 spin–orbit peaks of Co3O4, further confirming the formation of Co3O4.38 In addition, the Co3O4@NGN composite has high nitrogen content (8.4 atom %). As observed from Figure S9 (Supporting Information), the nitrogen species present in the Co3O4@NGN composite can be fitted into three types of peaks, which include pyridinic (398.5 eV), pyrrolic (399.9 eV), and graphitic (401.1 eV) nitrogen.16,39 The presence of nitrogen species might provide additional active sites and defects, which are beneficial to the storage of lithium ions.32

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Figure 4. (a) Typical CV curves of the Co3O4@NGN composite; (b) charge/discharge curves of the Co3O4@NGN composite at 100 mA g–1; (c) rate capability of the as-prepared NGN, Co3O4, and Co3O4@NGN composite at various charge/discharge rates; (d) cycling stability of NGN, Co3O4, and Co3O4@NGN composite at 200 mA g–1; (e) cycling stability of the Co3O4@NGN composite at 1000 mA g–1.

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behavior of Co3O4@NGN was evaluated in the voltage range of 0.01–3.0 V. CV experiment was performed to explore the lithiation and delithiation process of the Co3O4@NGN composite (Figure 4a). The first scanning curve of Co3O4@NGN composite presents a main peak at ~0.8 V; this peak resulted from the formation of a solid electrolyte interphase film and the reduction of Co3O4 with Li.40,41,42 This peak disappears and is replaced by a broad peak with decreased intensity at around 1.3 V in the following scanning curves, suggesting that structural modification and irreversible transformation have occurred.43,44 Co3O4@NGN composite shows a main oxidation peak at ~2.1 V, which represents the reformation of Co3O4 from Li2O and Co during the charge process. Considering the CV analysis in the present work and previous reports,

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the

electrochemical conversion reactions can be described as Co3O4 + 8 e– + 8 Li+ ↔ 3 Co +4 Li2O. Figure 4b presents the charge and discharge curves of the Co3O4@NGN composite at 100 mA g–1. Its first discharge and charge capacities are 1865 and 976 mA h g–1, corresponding to a Coulombic efficiency of 52.3 %. In the second cycle, its capacities are 1045 (discharge) and 969 (charge) mA h g–1, respectively. In accordance with CV analysis, the initial loss in discharge capacity can be ascribed to the formation of the solid electrolyte interphase layer and the incomplete conversion reactions.46 The Co3O4@NGN composite displays excellent lithium storage capacity from the second cycle and shows a reversible discharge capacity of 1045 mA h g–1 in the tenth cycle. Coulombic efficiency rises to 97 % 18


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in the fifth cycle and maintains above 97 % in the subsequent cycles. This behavior suggests that the solid electrolyte interphase layer remains highly stable during the charge/discharge process. The high Coulombic efficiency indicates the almost reversible insertion/extraction of lithium ions. This characteristic is attributed to the alleviation of the volume expansion effect of Co3O4 by the conductive network of nitrogen-doped graphene. The rate capability of the Co3O4@NGN composite is explored to further evaluate its electrochemical performance. As observed in Figure 4c, the Co3O4@NGN composite can be reversibly charged and discharged. The average discharge capacities of Co3O4@NGN at 100, 200, 400, 600, and 1000 mA g–1 are 1030, 924, 885, 785, and 681 mA h g–1, respectively. Co3O4 and NGN materials were tested under the same conditions. For the obtained NGN material, the average discharge capacities are 855, 657, 534, 477, and 396 mA h g–1, respectively. However, Co3O4 exhibits only 581, 183, 107, 79, and 46 mA h g–1. In addition, a ZIF-67/NGA composite was prepared through the simple mechanical blending of ZIF-67 and NGA. The composite was then calcinated in air to obtain the Co3O4/NGN composite. At 1000 mA g–1, the discharge capacity of the Co3O4/NGN composite is 411 mA h g–1 (Figure S10, Supporting Information), which is obviously lower than that of Co3O4@NGN composite (785 mA h g–1). This result highlights the advantages of the proposed strategy for in situ growth of ZIF-67 particles inside graphene aerogel. Therefore, the rate performance of the Co3O4@NGN composite is superior to those of Co3O4, NGN, and Co3O4/NGN composite materials, indicating the presence of a strong 19



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synergistic interaction between NGN and Co3O4. Importantly, as the current density decreases to 100 mA g–1, the reversible capacity of the Co3O4@NGN composite increases to a value of 1111 mA h g–1, which is higher than the theoretical capacities of graphite (372 mA h g–1) and bulk Co3O4 (890 mA h g–1). These extra discharge capacities might be ascribed to the extensive grain boundary areas of ZIF-67-derived Co3O4 particles with small sizes and/or the large active surface area of nitrogen-doped graphene. The cycle performance of the Co3O4@NGN composite was investigated at 200 mA g–1 (Figure 4d). The Co3O4@NGN composite delivers a high discharge capacity of 966 mA h g–1 at the second cycle. Even after 100 cycles, the composite achieves a discharge capacity of 955 mA h g–1. However, for the as-prepared Co3O4 and NGN materials, their discharge capacities are only 251 and 728 mA h g–1, respectively, after 100 cycles. Although single ZIF-67-derived Co3O4 displays high discharge capacities during the first few cycles, its discharge capacities quickly decrease with increasing cycles. This behavior emphasizes the importance of NGN. The weight percentage of NGN in the Co3O4@NGN composite is estimated to be ~56.6 wt % on the basis of TGA curve of Co3O4@NGN (Figure S5, Supporting Information). The maximum reversible capacities of Co3O4 and NGN are assumed to be 890 (theoretical capacity of Co3O4) and 956 mA h g–1 (the highest capacity of NGN measured), respectively. Therefore, the maximum reversible capacity of Co3O4@NGN is calculated to be about 927 mA h g–1. However, the reversible discharge capacity of Co3O4@NGN is 955 mA h g–1 even after 100 cycles (Figure 4d), indicating that 20


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the strong synergy between NGN and Co3O4 in the Co3O4@NGN composite improves the performance of LIBs. Furthermore, the control sample Co3O4/NGA without MOF structure shows a lower reversible capacity (617 mA h g–1) than Co3O4@NGN (955 mA h g–1) after 100 cycles at 200 mA g–1 (Figure S11, Supporting Information), indicating that the presented fabrication strategy yields composites with advantageous properties. The Co3O4@NGN composite is further tested to analyze the long-term stability at 1000 mA g–1. As observed from Figure 4e, it still retains a high reversible discharge capacity of 676 mA h g–1 after 400 cycles. No obvious capacity decay is observed. The cycling performance of Co3O4@NGN is remarkable compared with that of graphene or MOF-derived materials (Table S1, Supporting Information). These results indicated that the Co3O4@NGN composite displays high reversibility and outstanding cycling performance due to its structural advantage. According to the above-mentioned SEM results, ZIF-67-derived Co3O4 can be successfully embedded in NGN. The hierarchically porous structure of NGN not only can be favorable for the penetration of electrolyte and promote lithium-ion transport, but also would buffer the volume change of Co3O4. The as-prepared Co3O4@NGN composite has potential applications as an anode material. A rough comparison demonstrated that although the reversible capacity of the Co3O4@NGN (1030 mA h g‒1 at 100 mA g‒1) is lower than those of atomically thin mesoporous

Co3O4/graphene

composite

(2015

mA

h

g‒1

at

98

mA

g‒1),41

carbon-encapsulated Co3O4 (1413 mA h g‒1 at 100 mA g‒1), 47 and defect-free 21



Page 22 of 32

graphene/Co3O4 (1200 mA h g‒1 at 100 mA g‒1),48 it is comparable with or higher than those of most previously reported Co3O4-based materials, such as carbon nanofiber/Co3O4 (947 mA h g‒1 at 100 mA g‒1),49 multi-walled carbon nanotube/Co3O4 (813 mA h g‒1 at 100 mA g‒1),19 graphene/Co3O4 (935 mA h g‒1 at 50 mA g‒1; 760 mA h g‒1 at 74 mA g‒1),38,50 and Co3O4 with unique micro/nanostructures (850 mA h g‒1 at 50 mA g‒1).45 The excellent electrochemical behaviors of Co3O4@NGN can be mainly attributed to the following reasons. First, nitrogen-doped graphene sheets possess good electrical conductivity and electronic properties that can decrease the inner resistance of battery.32 As observed in the Nyquist plot (Figure S12, Supporting Information), the Co3O4@NGN composite exhibits a smaller semicircle diameter than NGN and Co3O4, which indicates that the Co3O4@NGN composite possesses a lower charge-transfer impedance. This result demonstrates that the lithium reaction kinetics of the Co3O4@NGN is faster than those of NGN and Co3O4. Second, the introduction of graphene may prevent Co3O4 aggregation and buffer the stress resulting from the volume changes, thus maintaining excellent rate performance and cycling stability.38 Third, the small particle size of ZIF-67-derived Co3O4 nanoparticles is beneficial to shortening the diffusion path of electrolyte, thus improving their electrochemical activity. Fourth, the hierarchically porous structure of the Co3O4@NGN composite provides a large electrolyte/electrode contact area, offers numerous pores for electrolyte penetration, and alleviates the stress induced by the volume changes of Co3O4.51 22


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CONCLUSIONS We report a facile approach for the preparation of ZIF-67@NGA through the in situ growth of ZIF-67 in the pores of the aerogels. This protocol can be extended to the preparation of other MOF@NGA hybrids. ZIF-67@NGA can be converted into a hierarchically porous Co3O4@NGN composite through a simple calcination in air, and ZIF-67-derived Co3O4 can be successfully embedded in NGN after calcination. The as-synthesized Co3O4@NGN as an anode shows an extremely high capacity, excellent rate capability, and outstanding cycling stability. The superior electrochemical properties of the Co3O4@NGN composite can be attributed to its hierarchically porous structure and the synergistic interaction between NGN and Co3O4 in the composite. The presented preparation protocol provides a new approach to the production of various MOF-derived functional materials with various applications, such as energy conversion/storage, gas adsorption/separation, and catalysis.

Associated Content Supporting Information. Preparation procedure of NGA; digital pictures of NGA immersed in water, methanol, and N,N-dimethylformamide; SEM images of NGA, which was treated in hydrothermal or solvothermal conditions; SEM images of ZIF-8@NGA and UiO-66@NGA at different magnifications; XRD patterns of NGA, pristine MOFs, and 23



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MOF-containing NGA; TGA curves of NGA, ZIF-67, ZIF-67@NGA, and Co3O4@NGN in air; Barret–Joyner–Halenda desorption PSD profile of Co3O4@NGN composite; N 1s spectrum of Co3O4@NGN composite; Nyquist Plots of Co3O4@NGN composite, NGN and Co3O4 electrodes. This information is available free of charge via the Internet at http://pubs.acs.org/.

Author Information Corresponding Authors *E-mail: [email protected]. Phone: +86 10 8254 5576. *E-mail: [email protected]. Phone: +86 10 8254 5565. Notes The authors declare no competing financial interest.

Acknowledgements. The authors gratefully acknowledge funding support from the National Natural Science Foundation of China (51602070 and 21574032) and the Ministry of Science and Technology of China (2013CB934200).

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