Nano Energy 26 (2016) 57–65

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MOFs nanosheets derived porous metal oxide-coatedthree-dimensional substrates for lithium-ion battery applications

Guozhao Fang a, Jiang Zhou a,b,n, Caiwu Liang a, Anqiang Pan a,b, Cheng Zhang a, Yan Tang a,b,Xiaoping Tan a, Jun Liu a,b, Shuquan Liang a,b,n

a School of Materials Science and Engineering, Central South University, Changsha 410083, Hunan, Chinab Key Laboratory of Nonferrous Metal Materials Science and Engineering, Ministry of Education, Central South University, Changsha 410083, Hunan, China

a r t i c l e i n f o

Article history:Received 15 December 2015Received in revised form28 March 2016Accepted 3 May 2016Available online 7 May 2016

Keywords:NanosheetsMetal-organic frameworksMetal oxidesPorous materialsLithium-Ion batteries

x.doi.org/10.1016/j.nanoen.2016.05.00955/& 2016 Elsevier Ltd. All rights reserved.

esponding authors at:School of Materials Scieniversity, Changsha 410083, Hunan, China.ail addresses: zhou_jiang@csu.edu.cn (J. Zhou)

a b s t r a c t

Porous nanosheet-structured materials have received great attention because of their promising appli-cations in energy field. Construction of porous metal oxides nanosheets on three-dimensional (3D)electro-conductive substrates is an effective way to further enhance electrochemical performance ofenergy storage devices. Herein, porous transition metal oxides (i.e. ZnO, Co3O4) nanosheets derived fromMOFs coated 3D substrates (i.e. 3D nickel foam, carbon fiber) are successfully synthesized by a facileliquid-phase deposition method with subsequent calcination. The growth mechanism of MOFs withnanosheet morphology coated 3D substrates is investigated. As a proof of concept application, theCo3O4/3DNF hybrid, possessing the advantages of porous nanosheet-structured and 3D electro-con-ductive substrates, is used as a binder-free anode material for lithium-ion battery, which exhibits high-rate capability and long-term cyclic stability. High discharge capacities of 1135, 1268, 1226, 1130, 923, 751,and 543 mA h g�1 are obtained at the current densities of 0.2, 0.5, 1, 2, 5, 10, and 20 A g�1, respectively.Even measured at 25 A g�1, it still retains a desired discharge capacity of 364 mA h g�1. Besides, thelong-term cyclic stability up to 2000 cycles can be obtained at 5 A g�1 and 20 A g�1.

& 2016 Elsevier Ltd. All rights reserved.

1. Introduction

Metal-organic frameworks (MOFs), a class of organic-inorganichybrid materials, have received great interest over the past dec-ades [1–3]. Due to their ultrahigh porosity, high internal surfacearea, exceptional thermal and chemical stability, MOFs hold greatpromise as porous materials for a variety of applications such asseparation, gas storage, and catalysis [4–7]. In addition, MOFs areproved to be ideal self-sacrificial templates to fabricate poroustransition metal oxides, such as ZnO [8], Fe2O3 [9], and Co3O4 [10–12], and they demonstrate a good utilization efficiency in energyfield. However, although great efforts have been made, the lowelectrical conductivity of MOFs or MOF-derived materials greatlylimits their application. In this respect, to explore a viable ap-proach to overcome this defect is necessary.

An effective way to improve electrical conductivity of MOFs orMOF-derived materials is to combine them with other materialssuch as graphene-based nanosheets, metal nanoparticles, MoS2nanosheet, and so on [13–16]. In the meantime, MOFs/three-

nce and Engineering, Central

, lsq@csu.edu.cn (S. Liang).

dimensional (3D) graphene networks was reported by Zhang'sgroup and their derived hybrids demonstrate excellent perfor-mances in photo-catalysis and lithium-ion battery applications [8].It is believed that MOFs or MOF-derived materials combined with3D electro-conductive substrates like 3D grapheme networks,nickel foams, and carbon fibers can gain the maximum efficienciesbecause 3D substrates possess not only large internal surface areafor anchoring nanomaterials, but also high-efficiency electricalconductivity for electron transfer [17–21]. Therefore, rational de-sign and synthesis of MOFs or MOF derived materials coating 3Dsubstrates may represent a promising direction for efficient energystorage. However, there is very few work involved in this field.Specially, to the best of our knowledge, there has been no reportconcerning the preparation of MOF nanosheets-derived porousmetal oxide-coated 3D substrates with great promise for variousapplications.

Porous nanosheet-structured materials have been widely in-vestigated and proven to be critical role for many applications inenergy field due to their large specific surface area and abundantactive sites for chemical reactions and their porous structures thatease strain of materials [22–27]. Herein, we have successfullyfabricated Zn/Co-MOF nanosheets coated on 3D nickel foam(3DNF) or carbon fiber (CF) and their grown mechanism wasstudied. In particular, Zn/Co-MOF nanosheets can directly derived

G. Fang et al. / Nano Energy 26 (2016) 57–6558

into highly porous metal oxides (i.e. ZnO, Co3O4) nanosheets bycalcining in air. As a proof-of-concept application of the hybrids,Co-MOF derived porous Co3O4 nanosheets on 3D nickel foam, re-ferred to as Co3O4/3DNF, is used as a binder-free anode electrodefor lithium-ion batteries (LIBs). The hybrids show excellent elec-trochemical performances with superior long-term cycling per-formance up to 2000 cycles and high rate capability at 25 A g�1. Tothe best of our knowledge, lithium storage performances ofCo3O4/3DNF in this work are the best among the Co3O4-basedanode materials, demonstrating a potential prospect of practicalapplication.

2. Experimental section

Chemicals: Cobalt nitrate hexahydrate (Co(NO3)2 �6H2O), zincnitrate hexahydrate (Zn(NO3)2 �H2O), and 2-methylimidazole(Hmim) are of analytical grade and used as received without anypurification process. The Ni foam substrate (2�2 cm2) wascleaned and etched with 6 M HCl for 30 min to remove the surfaceNiO layer, and then rinsed with deionized water and absoluteethanol for 3 times respectively. Carbon fiber (2�2 cm2) wascleaned for 30 min of successive sonication cleaning using acetone,ethanol, and deionized water, respectively.

Synthesis of porous ZnO nanosheets grown on Ni foam/Carbonfiber: In a typical procedure, 0.595 g of Zn(NO3)2 �6H2O and 1.3 gof 2-methylimidazole (Hmim) were dissolved in 40 mL deionizedwater, respectively, and following 30 min magnetic stirring atroom temperature. Then the Zn(NO3)2 �6H2O solution was pouredinto the Hmim solution quickly and a piece of cleaned Ni foam/

Fig. 1. (a) XRD patterns and (b) FT-IR spectrums of Zn-MOF/3DNF and Co-MOF/3DNF, Vagrown for 1 h.

carbon fiber (2�2 cm2) was placed in above mixing solution im-mediately. Being kept still for a period of time, Ni foam/carbonfiber was washed with deionized water and dried in an oven at60 °C. Finally, the as-grown Ni foam/carbon fiber was placed in amuffle furnace and then heated to 300 °C with a ramping rate of1 °C min�1 for 1 h to get porous ZnO nanosheets grown on Nifoam or Carbon fiber.

Synthesis of porous Co3O4 nanosheets grown on Ni foam/Car-bon fiber: The synthetic procedures are similar to that of porousZnO nanosheets grown on Ni foam/Carbon fiber, except that weused 0.582 g of Co(NO3)2 �6H2O to replace Zn(NO3)2 �6H2O.

Synthesis of porous Co3O4 powder: The synthetic procedure issimilar to that of porous Co3O4 nanosheets grown on Ni foam/Carbon fiber, but without adding Ni foam or Carbon fiber. Thereaction time was kept still for 1 h. And then the Co-MOF powderwas washed and centrifuged with deionized water three times anddried in an oven at 60 °C. Finally, the as-obtained powder wasplaced in a muffle furnace and then heated to 300 °C with a rampof 1 °C min�1 for 1 h to get porous Co3O4 nanosheets.

Electrochemical measurements of Co3O4/3DNF and Co3O4

powder: The electrochemical properties were carried out viastainless-steel coin cells (CR 2016). A solution of 1 M LiPF6 inethylene carbonate/dimethyl carbonate (EC/DMC) (1:1v/v) wasused as the electrolyte, and lithium foil was used as both thecounter and the reference electrode. The Co3O4/3DNF hybrid wasused directly as the working electrode, without adding any con-ductive materials and binders. For the Co3O4 powder, workingelectrode was obtained with 80% as-synthesized Co3O4 powder,10% acetylene black, and 10%polyvinylidene fluoride (PVDF) bin-der. Cyclic voltammetry (CV) of Co3O4/3DNF-based coin cell was

rious magnification SEM images of (c and d) Zn-MOF/3DNF, (e and f) Co-MOF/3DNF

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tested using an electrochemical workstation (CHI660C, China) at ascan rate of 0.1 mV s�1 in the voltage range of 0.01–3 V (vs. Li/Liþ).The galvanostatic charge/discharge experiments were studied in apotential range of 0.01–3 V (vs. Li/Liþ) using a multichannel bat-tery testing system (Land CT 2001 A) at room temperature. Theelectrochemical impedance spectrometry (EIS) was performed ona ZAHNER-IM6ex electrochemical workstation (ZAHNER Co., Ger-many) in the frequency range of 100 kHz to 10 m Hz on a cell.

Characterization: The crystallographic phases of the sampleswere performed by X-ray diffraction (XRD, Rigaku D/max 2500)and FT-IR spectrometer (Nicolet 6700). The morphologies and si-zes were characterized by scanning electron microscopy (SEM,Quanta FEG 250). Transmission electron microscope (TEM) images,high-resolution transmission electron microscope (HRTEM) image,and selected area electron diffraction (SAED) image were recordedby using JEOL JEM-2100F transmission electron microscope.

3. Results and discussion

The synthetic procedure for MOFs derived ZnO or Co3O4 na-nosheets coated three-dimensional substrates includes two mainsteps. Recently, Chen et al. reported a new 2D zeolitic imidazolateframework with unique cushion-shaped cavities and leaf-likecrystal morphology at room temperature [28]. The synthesismethod of MOFs in this work is similar to the approach developedby Chen et al. [28]. Firstly, Zn- or Co-MOF were directly grown on3D substrates via a facile liquid-phase deposition method at roomtemperature. Fig. 1a shows the X-ray diffraction (XRD) patterns ofZn- or Co-MOF/3DNF synthesized for 1 h. It can be obviously seenthat the XRD pattern of Co-MOF matches well with the pattern of

Fig. 2. XRD patterns and various magnification SEM images

Zn-MOF, indicating Co-MOF has an isostructure with Zn-MOF. Boththe XRD patterns are consistent well with the XRD patterns of Zn/Co-MOF powder (shown in Fig. S1 in Electronic Supporting In-formation (ESI†)) and also match well with the previous work [28],confirming the successfully preparation of Zn- and Co-MOF on Nifoam, which is further proved by FT-IR spectrums (Fig. 1b). Thecorresponding FT-IR spectrums of Zn- and Co-MOF exhibit thesimilar shape, showing nine main vibration modes between 400and 1600 cm�1, which correspond well to the previous study [29].Taking the FT-IR spectrum of Zn-MOF for example, the peak at424 cm�1 corresponds to Zn-N stretching, while the peak around1148 and 1304 cm�1 belongs to the C-H vibrations, 1566 cm�1 toC¼N vibration, respectively. This class of MOFs is structurally two-dimensional, which shows 2D layer network along the ab planeand these layers are stacked along the c direction [28]. The mor-phology of Zn-MOF/3DNF (Fig. 1c and d) and Co-MOF/3DNF(Fig. 1e and f) is investigated by the field emission scanning elec-tron microscopy (FE-SEM). The SEM images indicate that 3DNF iscovered with uniform nanosheets for both samples. The surface ofthe nanosheets is smooth. The thickness of each nanosheet for Zn-MOF is 50–100 nm, while for Co-MOF is 100–150 nm. The ZnO/3DNF and Co3O4/3DNF can be prepared by calcinating Zn- and Co-MOF/3DNF at 300 °C in air (step 2). Identification about the phasepurity and crystal structure of the final products are provided byXRD. As is shown in Fig. 2a and d, except for three typical peaks ofnickel, other diffraction peaks can be well indexed to ZnO phase[JCPDS Card No. 43–1003] and Co3O4 phase [JCPDS Card No. 43–1003], respectively, indicating the complete conversion of Zn- andCo-MOF. The representative SEM images of ZnO/3DNF andCo3O4/3DNF are shown in Fig. 2b-c and Fig. 2e-f, respectively. Asshowed in Fig. 2b, the ZnO nanosheets are connected with each

of (a-c) ZnO/3DNF and (d-f) Co3O4/3DNF grown for 1 h.

Fig. 3. SEM images of (a) pure nickel foam and Co-MOF/3DNF grown for varies time: (b) 2 min, (c) 5 min, (d) 15 min, (e) 30 min, and (f) 2 h.

Scheme 1. Schematic illustration of detailed synthetic process of Co3O4/3DNF hybrid.

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other to form a nanoarray network. Leaf-like nanosheets mor-phology can be clearly observed for Co3O4/3DNF (Fig. 2e). More-over, the nanosheets exhibit ultrahigh porosities as depicted inhigh-magnification SEM images (Fig. 2c and f). Similar porousstructures can also be found in other type of MOFs derived ma-terials [7–9,11,12].

Besides ZnO/3DNF and Co3O4/3DNF, other hybrids, such as ZnOcoated carbon fiber (ZnO/CF), Co3O4 coated carbon fiber(Co3O4/CF) can also be successfully fabricated and their phases andmorphologies are characterized by XRD and SEM (see Fig. S2 and

S3 in ESI†). Generally, our method could be extended to fabricateother hybrids by the appropriate choice of a specific MOF. There-fore, understanding the mechanism of MOFs with nanosheetmorphology grown on the surface of 3D substrates is necessaryand important. Based on this consideration, we chose Co-MOF/3DNF as research object and exposed its growth process.

We propose the nucleation growth mechanism for the synth-esis of Co-MOF/3DNF. The growth process of Co-MOF/3DNF arecharacterized by XRD and SEM. Firstly, cobalt ions and 2-methy-limidazole molecules are adsorbed on the surface of 3DNF to form

Fig. 4. (a) TEM images, (b) HRTEM image and (c) SAED pattern of Co3O4 nanosheets grown for 1 h.

Fig. 5. (a) The initial three cyclic voltammetry curves at a scan rate of 0.1 mV s�1, (b) The initial five charge/discharge profiles of Co3O4/3DNF electrode at 0.5 A g�1, and(c) Cycling performances and coulobic efficiency at 0.5 A g�1 of Co3O4/3DNF electrode and Co3O4 powder.

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a thin film at a short time of 2 min, as shown in Fig. 3b. Then thesample is forming nucleus with large amounts of small particlescoated on 3DNF (Fig. 3c), which was the early stage of crystal-lization of Co-MOF. With long reaction time, nucleus grows up(Fig. 3d and e) and acts as buds for crystal growth to form uniformleaf-like nanosheets, as discussed earlier. However, if the growthtime is too long, some leaf-like nanosheets aggregate together(Fig. 3f). Fig. S4 in ESI† showed the corresponding XRD pattern ofCo-MOF grown at different time. It can be clearly seen that Co-MOF grown for 5 min exhibited some week diffraction peaks,confirming the initial crystallization of Co-MOF. The diffractionpeaks become much more obvious when prolonging the reactiontime, indicating that the increasing crystallinity with the reactiontime. Porous Co3O4 nanosheets coated 3DNF is obtained after an-nealing Co-MOF/3DNF at 300 °C in air and its detailed formationprocess is illustrated in Scheme 1. It is worth noting that porousCo3O4 nanosheets are composed of many infinitesimal nanosheets.

The connected nanosheets can improve electronic conductivity ofelectrode material. The highly porous structure of Co3O4 na-nosheets would enlarge the contact area between electrode andelectrolyte and facilitate Liþ ion diffusion.

The transmission electron microscopy (TEM) image in Fig. 4afurther confirms that annealed products demonstrate the porousleaf-like nanosheet structure. High-resolution transmission elec-tron microscopy (HRTEM) showed in Fig. 4b reveals that the na-nosheets are composed of many interconnected nanoparticles, andeach nanoparticle has a diameter of 5–10 nm. Importantly, nu-merous nanopores around 1–3 nm in diameter between nano-particles are observed. The nanoparticles are randomly oriented,and the representative interplanar spacings of ∼4.67 Å and 2.43 Åare match well with the (111) and (311) planes of Co3O4 phase[JCPDS Card No. 43–1003], respectively. Electron diffraction ringsdepicted in Fig. 4c have been indexed, further indicating thepolycrystalline structure of the as-prepared leaf-like Co3O4

Fig. 6. (a) Rate performance and (b) Long-term cycling performances at 5 A g�1 and 20 A g�1 of Co3O4/3DNF electrodes, SEM images of Co3O4/3DNF at 20 A g�1 after (c) 1stcycle, (d) 10th cycle, and (e) 50th cycle.

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nanosheets. The electrochemical performances of porousCo3O4/3DNF hybrids as anode electrodes for lithium ion storageare evaluated in the following section.

Electrochemical behavior of Co3O4/3DNF as anode for LIB isevaluated by cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) testing. The CV profiles are recorded for the initialthree cycles in the voltage window of 0.01–3.0 V (vs Li/Liþ) at ascan rate of 0.1 mV s�1. As shown in Fig. 5a, the first cathodic scanexhibits an irreversible reduction peak at around 0.95 V, which ismainly attributed to the reduction of cobalt-based species to me-tallic cobalt and the formation of the solid electrolyte interphase(SEI) film [30]. It should be noted that there is one slight reductionpeak located at 0.7 V following the reduction peak at 0.95 V, which

is in accordance with the previous report [31–34]. Two anodicpeaks, observed at around 1.25 V and 2.05 V, are associated withthe multistep oxidation reactions of Co0 to CoO. According to theprevious reports, the peak at 1.25 V may be ascribed to an inter-mediate state CoOx (0oxo1) between Co and CoO [12]. In thesubsequent cycles, the oxidation peaks are almost unchanged, butthe reduction peak shifts to 1.15 V, suggesting the existence of thepolarization effect after first cycle. The nearly overlapped CVcurves after first cycle demonstrate excellent reversibility andstability of Co3O4/3DNF anodes. Fig. 5b displays the discharge/charge profiles of the initial five cycles at a constant currentdensity of 0.5 A g�1. The initial discharge curve depicts a distinctdischarge plateau at around 1.0 V, following by a slight plateau at

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around 0.7 V, which is consistent well with the first CV curve. Thedischarge voltage plateaus of subsequent cycles are higher, cor-responding well to the previous reports on the typical lithium-ionintercalation behavior for Co3O4 anodes [12,30,34–37]. The largeoverlap of the following four charge/discharge curves suggests thegood structural reversibility of Co3O4/3DNF after initial cycle.Fig. 5c gives an intuitive instruction of cycling performance ofCo3O4/3DNF electrode at 0.5 A g�1. In the initial 60 cycles, thedischarge capacity demonstrates the rising trend, which may beascribed to the formation of a gel-like surface film during the in-itial activation process and the improvement of Liþ accessibility[11]. In the subsequent cycles, the capacity of the electrode be-comes quite stable. High discharge capacity of 1217 mA h g�1 canbe maintained at 150th cycle. Even after 300 cycles, the electrodecan also deliver a stable capacity of 1204 mA h g�1, indicatingexceptional cyclic stability of the Co3O4/3DNF hybrid. Furthermore,the coulombic efficiency is around 99%, illustrating the excellentreversible Liþ insertion/extraction of electrode.

In order to illustrate the advantage of the hybrid, cycling per-formance of Co3O4 powder derived from Co-MOF (Crystal phaseand morphology of Co3O4 powder are shown in Fig. S5 in ESI†) at0.5 A g�1 is tested. The discharge capacity of Co3O4 powder is872 mA h g�1 at second cycle and then drops rapidly to138 mA h g�1 at 20th cycle, showing serious capacity fading uponcycling. This result indicates that the cycling performance can beremarkably enhanced by using 3D Ni foam substrate to growCo3O4 nanosheets, which is confirmed by the electrochemicalimpedance spectroscopy (EIS) data shown in Fig. S6 and Table S1in ESI†. It can be seen that the charge transfer resistance (Rct) ofCo3O4/3DNF is 53.93Ω and much smaller than that of Co3O4

powder (373.3Ω), suggesting that 3DNF can effectively enhancethe kinetics of electronic transportation in electrodes resulting inan excellent electrochemical performances for LIBs.

Fig. 6a shows the rate performance of Co3O4/3DNF evaluated atvarious current densities from 0.2 to 25 A g�1. It is found that thedischarge capacity increased in the first 20 cycles (at 0.2 and 0.5 Ag�1) from 1135 mA h g�1 at 2nd cycle and then reversible capa-cities of 1226, 1130, 923, 751, and 543 mA h g�1 can be maintainedwith increasing the current densities to 1, 2, 5, 10, and 20 A g�1,respectively. After the high-rate measurement, the electrode isable to recover a high specific capacity of 1192 mA h g�1 at 1 Ag�1. Furthermore, when the current densities are reset to highrates again after 100 cycles, it still retain desired discharge capa-cities at various current densities and a high capacity of364 mA h g�1 is observed even at 25 A g�1, highlighting the re-markable rate capability of Co3O4/3DNF hybrid. When reset to 1 Ag�1, it shows no capacity fading after 400 cycles. Notably, theresult reveals outstanding rate capability when compared to theprevious reported high rate values of Co3O4-based anode materi-als, such as Co3O4@carbon nanotube arrays (408 mA h g�1 at 5 Ag�1) [35], snowflake-shaped Co3O4 (977 mA h g�1 at 3 A g�1)[38], multiwalled carbon nanotubes/Co3O4 nanocomposites(514 mA h g�1 at 1 A g�1) [11], and Co3O4-carbon nanosheet hy-brid (390 mA h g�1 at 10 A g�1) [39]. More comparisons are listedin the Table S2 in ESI†. As is known to all, rate capability is one ofthe most critical requirements for LIBs. Hence, Co3O4/3DNF ob-tained in our work will have a potential prospect of practicalapplication.

Besides the high rate performance, long-term cyclic stability,particularly under heavy current charge/discharge, is another cri-tical requirement for practical applications. As depicted in Fig. 6b,the Co3O4/3DNF hybrid shows superior cyclic stability over 2000cycles at the current density of 5 and 20 A g�1. When evaluated at5 A g�1, the hybrid exhibits a high discharge capacity of896 mA h g�1 at the second cycle, keeping about 80% of that at thefirst cycle. It can maintain a reversible capacity of 976 mA h g�1

after 500 cycles, showing no capacity fading based on the secondcycle. Even after 2000 cycles, the reversible capacity of theCo3O4/3DNF hybrid can still reach about 600 mA h g�1, corre-sponding to a capacity fading rate of 0.016% per cycle. Furthermeasured at a higher rate of 20 A g�1, Co3O4/3DNF hybrid alsodemonstrates excellent cycling performance. In spite of the rapiddecrease of capacity from 630 mA h g�1 at the second cycle, it canfortunately maintain a reversible capacity of 366 mA h g�1 at the50th cycle. And then, Co3O4/3DNF hybrid keeps a relatively stablecapacity up to 2000 cycles with a remarkable discharge capacity of300 mA h g�1. The capacity fading rates are 0.026% and 0.009% percycle based on the capacity of 2nd cycle and 50th cycle, respec-tively. In order to illustrate the capacity fading at 20 A g�1 before50 cycles, the SEM images of electrode materials after 1, 10, and 50cycles are displayed. The drastic capacity fading may result fromlithiation-induced mechanical degradation and the formation ofunstable solid electrolyte interphase (SEI) [40–42]. As depicted inFig. 6c, a thick layer of SEI film was formed and covered on Co3O4

nanosheets after first cycle. However, the SEI film became thinnerupon cycling (see SEM image after 10 cycles in Fig. 6d). Sun et al.have demonstrated the evolution of the SEI film during cycling,concluding that the thick SEI film was formed in capacity fadingprocess, but evolved into stable thin SEI film in capacity stabili-zation [43]. Fig. 6e shows that the surface of Co3O4 nanosheetsafter 50 cycles is clean and smooth, indicating the SEI films tend tobe stable, which lead to good cyclic stability in the followingcycles.

According to the analysis above, Co3O4/3DNF hybrid exhibitsboth ultrahigh rate capacities and outstanding cyclic stability. Tothe best of our knowledge, the electrochemical results obtained inthis work are the best among the Co3O4-based anode materials.Among all the binder-rich electrodes, mesoporous Co3O4 hollowspheres prepared by Sun et al. exhibit the best performance with along-term cyclic stability of 7000 cycles at 5 A g�1 [43]. However,the electrodes suffer from low specific capacity and need to ex-perience a complex reactivation process. Wang et al. reportedtriple-shelled Co3O4 hollow microsphere which exhibited a ultra-high specific capacity of 1615.8 mA h g�1, but the cycling was onlyup to 30 cycles [44]. For most of the binder-free Co3O4-basedelectrodes, they are subjected to either poor cycling performance(e.g. Co3O4 nanobelt array [45], Co3O4/3D graphene [46]), or in-ferior rate capability (e.g. Co3O4 nanowire arrays [20], Co3O4 na-noparticles films [47]). None of them can reach a long-term life of2000 cycles and high rate of 25 A g[32,48–50]. The superior electro-chemical performance of Co3O4/3DNF hybrid could probably beattributed to the optimized structure and mechanical properties. i)2D porous leaf-like Co3O4 nanosheets not only possess large sur-face areas with the maximum exposure of active sites for elec-trochemical reactions, but also shorten Li ion diffusion length andeffectively buffer the volume expansion during cycling [51]; ii) 3Dnickel foam with outstanding electrical conductivity facilitateselectron transportation and improves the poor electrical con-ductivity of the MOF-derived leaf-like Co3O4 nanosheets, whichdecreases the inner resistance of LIBs [8]; iii) The synergistic effectof 2D porous Co3O4 nanosheets and 3D nickel foam lead to ex-cellent cycling stability and ultrahigh rate capability.

4. Conclusions

In summary, we have demonstrated that an advanced hybridmaterial based on MOF nanosheets derived porous transitionmetal oxide-coated 3D substrates can successfully be prepared. Asa proof of concept application, the Co3O4/3DNF hybrid is applied asanode material for rechargeable lithium-ion batteries. Combinedwith the advantages of porous nanosheet-structured and 3D

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electro-conductive substrates, the hybrids exhibit excellent elec-trochemical performances, including long-term stability up to2000 cycles at 5 A g�1 and 20 A g�1, and ultrahigh rate capabilityat 25 A g�1. We believe that these hybrids could be extended toother field of applications, such as sodium-ion battery, super-capacitor, catalysis, photocatalysis, gas storage, etc.

Acknowledgments

This work was supported by National High Technology Re-search and Development Program of China (863 Program) (Grantno. 2013AA110106), National Natural Science Foundation of China(Grant no. 51374255 and 51572299) and the Fundamental Re-search Funds for the Central Universities of Central South Uni-versity (Grant no. 2015zzts174).

Appendix A. Supplementary material

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.nanoen.2016.05.009.

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Guozhao Fang received his Bachelor's degree in Ma-terials science and Engineering from Central SouthUniversity (CSU) in 2014. He is now a pH.D. candidatein School of Materials science and Engineering, CentralSouth University supervised by Prof. Shuquan Liang. Hisresearch focuses on the synthesis of nanocomposites,hierarchical mirco/nano-structured materials, hetero-structures and their applications in energy storage,such as lithium ion batteries, and sodium ion batteries,and supercapacitors.

Jiang Zhou received his B. E. (2001) and pH.D. (2015)degrees in Materials Physics and Chemistry from Cen-tral South University. He worked in Prof. Hua Zhang'sgroup at Nanyang Technological University as an ex-change pH.D. student (2014–2015). After graduated, hejoined Central South University as a Research Associatein Prof Shuquan Liang's group. His current interests arethe controllable synthesis of nanostructured materialsand their applications in energy storage and conversiondevices, such as lithium ion batteries, supercapacitors,electrocatalysis, etc.

Caiwu Liang is now an undergraduate in School ofMaterials Science and Engineering, Central South Uni-versity. His research focuses on the synthesis and theapplications of nanocomposites and micro/nanos-tructured functional materials for energy storage, suchas lithium ion batteries and sodium ion batteries.

G. Fang et al. / Nano Energy 26 (2016) 57–65 65

Anqiang Pan received his B. E. (2005) and D. Phil.(2011) degrees in Materials Physics and Chemistry fromCentral South University. He worked in Prof. GuozhongCao's group at University of Washington as an exchangestudent (2008–2009). Then, he worked in PNNL as avisiting scholar in Dr. Ji-Guang Zhang and Dr. Jun Liu'sgroup (2009–2011). He joined Prof. Xiongwen (David)Lou's group at Nanyang Technological University as aresearch fellow (2011–2012). He joined Central SouthUniversity as a Sheng-Hua Professor in 2013. His cur-rent interests are on lithium ion batteries, andsupercapacitors.

Cheng Zhang received his Bachelor's degree in Mate-rials science and Engineering from Central South Uni-versity (CSU) in 2015. He is now a master in School ofMaterials science and Engineering, Central South Uni-versity supervised by Prof. Shuquan Liang. His researchfocuses on the synthesis of nanomaterials and theirapplications in lithium/sodium ion batteries.

Yan Tang received a pH.D. degree in Materials Physicsand Chemistry in Central South University in 2011. Shewas employed as an associate professor of CentralSouth University in 2013. She is now engaging in theteaching and scientific research in Materials Physics.Her current interests focus on the synthesis of highenergy micro/nanomaterials and their applications inlithium ion batteries.

Xiaoping Tan received a pH.D. degree in MaterialsPhysics and Chemistry from Central South University in2007. Since March in 2008, she has worked in the ex-periment center in School of materials science andengineering. In 2012, she was employed as a seniorexperimentalist. Her current interests focus on thesynthesis and the applications of nanomaterials.

Jun Liu received a pH.D. degree in condensed matterphysics from the Institute of Physics, Chinese Academyof Sciences, in 2009. He was an associate research fel-low in Institute for Superconducting and ElectronicMaterials (ISEM), University of Wollongong, Australia.In 2011, he joined the School of Materials Science andEngineering in Central South University, where he iscurrently a professor of material physics. He has pub-lished more than 30 papers in referred journals. Hiscurrent research interests include two-dimensionalnanosheets, hierarchical mirco/nano-structured mate-rials, heterostructures and the application of these

materials for energy devices.

Prof Shuquan Liang received his pH.D. degree fromCentral South University (P.R. China) in 2000. He hasbeen the Dean of School of Materials Science and En-gineering in Central South University (CSU) since 2010.He is the winner of Monash University Engineering SirJohn Medal. He hosted 5 state research projects in-cluding national 973 sub-project and national 863project. He has published more than 60 papers infrontier journals. Currently, his main research interestsinclude micro/nanostructured functional materials,nanocomposites and their energy storage and conver-sion devices.