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Tufted NiCo2O4 Nanoneedles Grown on Carbon Nanofibers with advanced electrochemical property for Lithium Ion Batteries
Accepted Manuscript Title: Tufted NiCo2 O4 Nanoneedles Grown on Carbon Nanofibers with advanced electrochemical property for Lithium Ion Batteries Author: Gang Zhou Chen Wu Yuehua Wei Chengchao Li Qingwang Lian Chao Cui Weifeng Wei Libao Chen PII: DOI: Reference:
S0013-4686(16)32545-2 http://dx.doi.org/doi:10.1016/j.electacta.2016.12.001 EA 28471
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
18-8-2016 18-10-2016 1-12-2016
Please cite this article as: Gang Zhou, Chen Wu, Yuehua Wei, Chengchao Li, Qingwang Lian, Chao Cui, Weifeng Wei, Libao Chen, Tufted NiCo2O4 Nanoneedles Grown on Carbon Nanofibers with advanced electrochemical property for Lithium Ion Batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.12.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Tufted NiCo2O4 Nanoneedles Grown on Carbon Nanofibers with advanced electrochemical property for Lithium Ion Batteries Gang Zhou a, 1, Chen Wu a, 1, Yuehua Wei a, Chengchao Li b, *, Qingwang Liana, Chao Cui a, Weifeng Wei a, Libao Chen a, * a
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, 410083,
China b
School of Chemical Engineering and Light Industry, Guangdong University of Technology,
Guangzhou 510006, China. 1
These authors contributed equally to this work.
To whom the proofs and correspondence should be sent. Professor Libao Chen Powder Metallurgy Research Institute Central South University, Changsha, 410083, China E-mail: [email protected] Professor Chengchao Li School of Chemical Engineering and Light Industry Guangdong University of Technology, Guangzhou 510006, China
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E-mail: [email protected]
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Graphical Abstract
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Highlights
Tufted NiCo2O4 nanoneedles grown on carbon nanofibers (TNN/CNFs) were successfully fabricated by facile hydrothermal and annealing procedure.
The composite possesses a unique three-dimensional (3D) hybrid structure and morphology.
The composite delivers a reversible discharge specific capacity up to 1033.6 mAh g-1 even after 250 cycles.
The outstanding electrochemical performance origins from unique 3D hybrid structure.
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Abstract In this article, tufted NiCo2O4 nanoneedles directly grown on carbon nanofibers (CNFs) were successfully fabricated by a facile hydrothermal procedure and subsequent annealing. The morphology of tufted NiCo2O4 nanoneedles/carbon nanofibers (TNN/CNFs) composite appears like clustered acicular leaves on branches, forming a unique three dimensional (3D) hybrid structure. Compared to NiCo2O4 nanoneedles spheres (NNS) without CNFs, the TNN/CNFs material presents a reversible discharge specific capacity up to 1033.6 mAh g-1 at a current density of 200 mA g-1 even after 250 cycles, with the coulombic efficiency of above 98%. The unique 3D hybrid structure formed with CNFs should be responsible for the superior lithium storage capacity and enhanced cycling performance, which provides short distance for ions transport, high specific surface area for large contact areas between electrode and electrolyte, together with the interspace to accommodate volume change and smaller agglomeration tendency. Moreover, the design route proposed offers more possibility to preparation of highperformance anode materials for LIBs. Keywords： Tufted NiCo2O4 nanoneedle; carbon nanofiber; hydrothermal synthesis; lithium ion battery; electrochemical performance 1. Introduction The demands for renewable and sustainable energy resources are dramatically increasing because of the rapid development of economy and society [1, 2]. To meet the demands, many efforts have been devoted to develop high-property energy storage devices during the past
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decades such as lithium ion batteries (LIBs) and supercapacitors [3-6]. Rechargeable LIBs are widely applied to portable electronic devices, electric and hybrid electric vehicles on account of its high working voltage, light weight, high energy density, and environmentally friendly characteristics [7, 8]. At present, the commercial LIBs anode materials are graphite and carbon. However, the limited theoretical specific capacity of 372 mAh g-1 restricts improvement of the energy density of LIBs [7]. Afterwards, many transition-metal oxides have been widely studied to solve this problem due to the lithium storage mechanism based on conversion reaction, which endows them high theoretical special capacity[9-11]. For instance, the redox conversion reaction for NiO in which nickel is reduced to metallic state during discharge process and reoxidized during charge process, exhibits a large theoretical capacity of 716 mAh g-1 [12]. And α-Fe2O3 owns a specific capacity of 1007 mAh g-1 [7, 13] while Co3O4 possesses a specific capacity of 890 mAh g-1 [14-16]. However, some transition metal oxides (e.g. Co3O4, NiO and Fe2O3) suffer from poor conductivity, and Co3O4 is toxic, not friendly to the environment [7, 17]. So binary metal oxides become one of considered anode materials, as they could reversibly extract and insert numerous Li+ and combine superiorities of two different metals, exhibiting more potential prospects [18]. Among most studied materials of binary metal oxides, such as Mn1.8Fe1.2O4 [19], CoMoO4 [20], NiCo2O4 [3, 9], NiMn2O4 [21] and et al, NiCo2O4 stands out due to its high specific capacity (891 mAh g-1), better conductivity as well as low diffusion resistance between Li+ and electrolyte [22, 23]. However, there is a common problem that the structure deterioration (aggregation or pulverization) by large volume change during cycling would result in
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performance degradation [11]. More recently, to ease volume expansion and ameliorate cycling performance as well as rate capability, researchers have been committed to the combination of NiCo2O4 nanomaterial with conductive carbon materials and morphology-tuned synthesis. Moreover, combination with carbon materials and morphology-tuned synthesis (introducing porous or hierarchical structures) are commonly considered as effective ways to buffer volume expansion and ameliorate properties [8, 24-26]. Zhang et al. synthesized two different morphologies of NiCo2O4 nanocomposites on surface of graphene-coated nickel foam which represented flower-type morphology and sheet-type morphology respectively. And the flowertype NiCo2O4 demonstrated the specific capacity over 985 mAh g-1 after 60 cycles, superior to the sheet-type NiCo2O4, which attributed to the larger mesopore volume and surface area [22]. Peng et al. designed core-shell NiCo2O4 nanostructure coated by carbon layers, which could accommodate the volume change [27]. A hybrid structure of NiCo2O4 nanosheets with multiwall carbon nanotubes achieved the specific capacity of 904 mAh g-1 [28]. In the meantime, it is reported that the structure of single NiCo2O4 may be unstable during cycling, while it may be broken or pulverized. For instance, Chen et al. found that pure NiCo2O4 nanosheets suffered the decay of capacity because the volume expansion caused the pulverization of NiCo2O4 [29]. Compared with single structure, hybrid structure is more bearable to the volume changes during cycling. Liu et al. prepared three-dimensional CNTs/NiCo2O4 core/shell composite structure, which exhibited stable cycling properties, no significant decay of capacity even after 200 cycles [30]. So, we consider the combination of NiCo2O4 with carbon materials and design them to
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form 3D hybrid structure to enhance its electrochemical properties. Among so many carbon materials, carbon nanotubes (CNTs), graphene and carbon cloth are most commonly used, because of their better chemical stability, large specific surface area and good electron conductivity [31-38]. However, there is a drawback for CNTs of low loading density, and graphene is inclined to stack together [39]. Carbon nanofibers possess a lot of merits such as large load, high porosity, also, larger specific surface area than carbon cloth, which are chosen as the substrate of NiCo2O4 in this article [39-41]. As far as we know, there are a few literatures reported on NiCo2O4 nanomaterial which use the carbon nanofibers as the substrate with superior cycling properties. Herein, we have adopted a facile hydrothermal process followed by calcination to fabricate TNN/CNFs composite, which the tufted NiCo2O4 nanoneedles were strongly covered on the carbon nanofibers. Meanwhile, the composite demonstrated a superior electrochemical property and a high special capacity of 1033.6 mAh g-1 after 250 cycles was achieved. The outstanding performance is attributed to the unique 3D hybrid structure, including abundant interspace to accommodate volume change, large surface area and the merits of CNFs of high porosity along with the ability to prevent NiCo2O4 from agglomeration. Therefore, it is noteworthy that the TNN/CNFs composite could become a potential candidate of anode materials for LIBs. 2. Experimental 2.1. Preparation of NiCo2O4 on Carbon Nanofibers
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All reagents were of analytical pure and used without any further purification. The carbon nanofibers were firstly fabricated by means of electrospinning which has been reported in our previous literature [39]. 0.3778g Polyacrylonitrile (PAN, Mw =150000, Sigma-Aldrich Co., Ltd, China) and 5 mL N, N - dimethylformide (DMF, Sinopharm Chemical Reagent Co., Ltd, China) were used to form a uniform and transparent solution, and then it was filled in a 5 mL syringe with a needle which diameter is 0.6 mm. Then, a high voltage of 12 kV was applied to needle and collector to create a charged jet and form electrospun nanofibers. The nanofibers were dried in a vacuum, followed by a two-step treatment which included annealing in air and carbonizing in Ar. Subsequently, hydrothermal method and annealing process were used to fabricate NiCo2O4 on carbon nanofibers as follows: 5 mM Ni(NO3)2•6H2O (Sinopharm Chemical Reagent Co., Ltd, China), 10 mM Co(NO3)2•6H2O (Sinopharm Chemical Reagent Co., Ltd, China) and 35 mM urea (Aladdin Industrial Corporation, China) were dissolved in 20 mL of deionized (DI) water. Later, the solution was magnetic stirring for 1 h, and was transferred into a 50 mL Teflonlined stainless steel autoclave with about 10 mg of the carbon nanofibers immersed in the solution to provide the substrate for the growth of precursor. Subsequently, a temperature of 120 ℃ was applied for 12 h. The as-synthesized product was washed several times using DI water and ethanol, followed by drying overnight in air. Finally, an annealing procedure was adopted under the condition of 300 ℃×2 h in Ar with a heating rate of 2 ℃ min-1. And the control sample of bare NiCo2O4 nanoneedles spheres (NNS) followed the same preparation method except that
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the carbon nanofibers were not put into the Teflon-lined stainless steel autoclave in the procedure of hydrothermal reaction. 2.2. Sample characterizations The structure was characterized by X-ray powder diffraction (XRD) analysis, which was carried out on Rigaku Desktop X-ray diffractometer using Cu Kα radiation (λ = 1.54056 Å ) with 2θ ranging from 10° to 85°. Raman spectrum was conducted on LabRAM Aramis (Germany). The thermal gravimetric analysis was studied by a thermogravimetric analyzer (TGA, SHIMADZU DTG-60AH) in air with the temperature ranging from room temperature to 800 ℃ and a heating rate of 10 ℃ min-1. The morphology and nanostructure of the samples were observed by a scanning electron microscope (SEM, NOVA NANO SEM230) and transmission electron microscopy equipped with an energy-dispersive X-ray (EDX) (TEM, TECNAI G2 F20), collectively. X-ray photoelectron spectroscopy (XPS, ESCALAB 250 Xi) was also conducted to obtain surface information. The specific surface area was obtained by Brunauer-Emmett-Teller test (BET, Tristar 3020, America) and pore size distribution curve was acquired by BarrettJoyner-Halenda (BJH) equation. To evaluate electrochemical properties, CR2025 coin-type half-cell was assembled. The slurry was made of active material TNN/CNFs composite (or NNS composite), acetylene black and Carboxyl Methyl Cellulose (CMC) with the mass ratio of 8:1:1. The solvent for CMC is the solution composed of DI water and ethyl alcohol with the volume ration of 3:2. And the copper foil was used for collector. The mass of the active material is 0.45 g cm-2. The half-cell was
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prepared in an argon-filled glove box (M. BRAUN, Germany). The counterpart was Li metal. Electrodes were separated by Celgard 2400 membrane and the electrolyte used LiPF6 in solvent composed of ethylene carbonate (EC), dimethyl carbonate (DMC), and Ethyl Methyl Carbonate (EMC) with volume ratio of 1:1:1. Before examing electrochemical properties, the cells were aged overnight. The charge-discharge curves, cycle stability, and rate performance were tested on a multichannel battery test system (LANHE CT2001A, Wuhan LAND Electronics Co., P. R. China) at 30 ℃ in the voltage window of 0.01 V and 3.0 V. Cyclic voltammetry (CV) measurements were performed via Arbin Workstation (USA) with a scan rate of 0.1 mV/s between 0.01 V and 3.0 V vs Li/Li+. 3. Results and discussion The preparation processes are elucidated as following Fig. 1. Three steps are clearly shown in this illustration, comprising electrospinning, hydrothermal and subsequent annealing. Firstly, CNFs were fabricated by electrospinning, which were used as the support for the growth of precursor. Secondly, hydrothermal method was applied to grow the NiCo-precursor on the CNFs. Finally, TNN/CNFs composite was converted from NiCo-precursor/CNFs by the final annealing process at 300 ℃ for 2 h at the temperature rate of 2 ℃ min-1. Fig. 2 (a) displays diffraction patterns of TNN/CNFs and NNS by X-ray diffraction (XRD). All peaks can be clearly indexed to (111), (220), (311), (400), (422), (511), (440) and (622) lattice planes which coincide well with the standard diffraction pattern of PDF card of NiCo2O4
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(JCPDS no. 20-0781). There are no other peaks detected except the peaks forementioned, betokening high purity of the products. The X-ray Photoelectron Spectroscopy (XPS) was carried on to obtain more surface information about the sample. The survey spectrum in Fig. 2 (b) reveals the presence of elements of Ni, Co, C and O. A typical Gauss Lorentz equation was applied to fit the peaks. Fig. 2 (c) exhibits the high resolution spectrum of Co 2p with two peaks at 780.44 eV and 781.99 eV considered as Co 2p3/2 and another two peaks at 795.58 eV and 797.60 eV indexed as Co 2p1/2 accompanied by two satellite peaks (indicated as “Sat.”). As the previous literatures report, the binding energy of 780.44 eV is ascribed to Co3+, while the binding energy of 781.99 eV is in agreement with Co2+ [42, 43]. The fitted Ni 2p high resolution spectrum demonstrates two peaks located at the binding energy of 855.53 eV and 873.15 eV, which are identified as Ni2+, and another two peaks situated at the binding energy of 856.77 eV and 874.63 eV are deemed to Ni 3+ [44, 45]. There are yet two satellite peaks (indicated as “Sat.”) as displayed. As we know, NiCo2O4 is face-centered-cubic spinel-structure, and Ni atoms are located at octahedron while Co atoms are situated at both octahedron and tetrahedron composed of oxygen atoms [46], which can also be regarded as local substitution of Co2+ with Ni2+ in Co3O4 spinel structure. The peak centered at 529.55 eV in Fig. 2 (e) is commonly considered as the characteristic peak of metaloxygen bonds [47], on account of the formation of Ni-O bond and Co-O bond forementioned. The peak sitting at 531.48 eV represents the defects according to the previous reports, including oxygen vacancies, contaminants, hydroxyls or chemisorbed oxygen [27]. Another peak at 532.66
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eV is derived from multiplicity of physi- and chemi-sorbed water at or near the surface [47]. It is clearly observed that the intensity of the peak at 531.48 eV is higher compared to other two peaks, which gives the fact of the existence of numerous defects.[48] As reported in the literature, oxygen vacancies can act as the electron donors, thus improving the electrical conductivity to some extent [47]. The C 1s high resolution spectrum is assigned to the presence of carbon nanofibers, where the peaks at 284.65 eV, 285.84 eV and 289.52 eV are ascribed to CC, C-O, O-C=C [11, 49-51], respectively. Raman spectroscopy was also conducted. As shown in Raman spectrum (Fig. 3), four characteristic peaks of TNN/CNFs and NNS were observed, which represent F2g (185.3 cm-1), Eg (484.9 cm-1), F2g (538.9 cm-1), and A1g (646.9 cm-1) modes of NiCo2O4 [52-54], respectively. Fig. 4 exhibits different morphologies of CNFs, TNN/CNFs and control sample NNS. The CNFs with a diameter of around 150 nm form a crisscross network as seen from Fig. 4 (a). And the framework made up of CNFs is beneficial to form a unique three dimensional (3D) structure when NiCo2O4 grows on them, offering efficient ion and electron transmission paths [55]. The typical morphology of the TNN/CNFs composite is characterized by scanning electron microscope (SEM) displayed in Fig. 4 (b) and (c). With the support of CNFs, the tufted NiCo2O4 nanoneedles through hydrothermal and annealing procedures form a relative ordered structure, which grow into radial flowers with high densities. Fig. 4 (b) is the panoramic morphology which presents an interlaced structure formed by CNFs covered with NiCo2O4 nanoneedles, and there are much interspace to accommodate the volume variation during cycling and fasten
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penetration of the electrolyte. From the local magnification Fig. 4 (c), we can observe that the length of NiCo2O4 acicular leaf is around 0.5-2 μm with the width less than 100 nm, providing large specific surface area which is beneficial to enlarge the contact areas between anode material and electrolyte, shortening the distance of ions transportation. In our previous report [39], the time-dependent experiments were investigated to display the growth mechanism which could also explain the formation of the precursor of TNN/CNFs during hydrothermal process. In brief, the seeds of Ni-Co precursor sprouted and grew into nanoneedles finally as time went on. The NNS materials in Fig. 4 (d) appear like fluffy balls which have the diameter of 3-4 μm, with the nanoneedles stretching in all directions. Although the configuration could endow the material with more specific surface area, it would be inclined to distribute randomly and agglomerate together without the support of the CNFs. And it is reported that aggregation and pulverization of the structure are nonnegligible issues for transition metal oxides in their application in LIBs [56]. To observe more detailed morphology, transition electron microscope (TEM) was also applied to characterize TNN/CNFs composite and the relevant outcomes are depicted in Fig. 5 (a) and (b). Fig. 5 (a) shows single carbon nanofiber decorated with integral NiCo2O4 nanoneedles, which interweave with each other to form a special 3D structure. The inset in Fig. 5 (a) is the selected area electron diffraction (SAED) of the region marked by red rectangular frame, from which distinct polycrystalline rings composed of superposition of diffraction spots are presented, indicating the good crystallization of NiCo2O4. The high resolution transmission
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electron microscopy (HRTEM) (Fig. 5 (b)) of a single NiCo2O4 nanoneedle further confirms its composition, where the lattice fringe is calculated as 0.287 nm, matching well with the interplanar distance of (220) of NiCo2O4. The inset is the corresponding fast fourier transform algorithm (FFT), which demonstrates a monocrystal structure. The EDS mapping (Fig. 5 (c)) shows the distribution of elements of Co and Ni. Also, a clear contour of nanoneedles can be identified from the mapping, indicating homogeneous distribution of the elements. To compare the specific surface area of TNN/CNFs and NSS, nitrogen isothermal adsorption-desorption measurements were carried out. The two isotherm shown in Fig.6 can be classed as Ⅳ type with a H1 hysteresis loop on a scale of 0.6-1.0 P/P0, manifesting a mesoporous structure [47, 57]. Meanwhile, the BET specific surface area of TNN/CNFs is 134.1 m2 g-1, larger than NNS of 120.5 m2 g-1. The insets describe the distribution of pore size of TNN/CNFs and NNS by the Barrett-Joyner-Halenda (BJH) method from absorption branch, which indicate the pore size of 4.685 nm for TNN/CNFs and 4.696 nm for NNS. These characteristics are beneficial to the diffusion of Li+ and conductive to contact of active material and electrolyte, in favor of better electrochemical properties [57]. Fig. 7 (a) and (b) are the TGA curves of the as-synthesized samples of TNN/CNFs and NNS fabricated on the same condition except the addition of CNFs, measured from room temperature up to 800 ℃ at a temperature rate of 10 ℃min-1 at the atmosphere of O2 with the flow rate of 50 mL min-1. Also, XRD was used to characterize the residue after the same experiment condition as TGA measurements (Fig. 7 (c) and (d)), indicating NiCo2O4 has
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decomposed into NiO and Co3O4. As seen from Fig. 7 (a), the small weight loss of 2.55 % as the temperature up to 250 ℃ results from water evaporation, while the second steep slope represents combustion of carbon and decomposition of NiCo2O4, calculated as 22.75 %. In the same way, we can compute the weight loss of decomposition of NiCo2O4 as 16.29 % in the TGA measurement of NNS. So the mass ration of NiCo2O4 and CNFs are 90.99 % and 6.46 %, respectively. The theoretical specific capacity of TNN/CNFs (C) is calculated as the following formula [58]: C(TNN/CNFs) = C(NiCo2O4) * M(NiCo2O4) % + C(CNFs) * M(CNFs) % = (891 * 90.99 % + 372 *6.46 %) mAh g-1 = 834.75 mAh g-1 where M % is the mass fraction of NiCo2O4 and CNFs, respectively. The electrochemical performances of TNN/CNFs composite and NNS composite were evaluated by cycle voltammetry (CV) test and galvanostatic charge-discharge measurement. Fig. 8 (a) presents the first three cycles of CV plots of the as-prepared TNN/CNFs at a scan rate of 0.1 mV s-1 between 0.01-3.0 V. As we can see, there are two peaks during first cathodic sweep which is different from the subsequent cycles, indicating that two different mechanisms exist. The intense peak at 1.06 V originates from the amorphization of the structure [30]. The second peak appearing at about 0.68 V can be credited to the lithiation reaction of NiCo 2O4 in which Ni2+ and Co3+ are reduced to metal Ni and Co and Li2O is formed, along with the formation of solid electrolyte interface (SEI) film [3, 30, 56, 59]. The main reduction peak shifts to a more positive potential of 0.9 V in the following cycles, which originates from the decomposition of
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NiCo2O4 [22]. During the first anodic sweep, two peaks situated at 1.36 V and 2.25 V exist which derive from the oxidation of Ni0 and Co0 into NiO and Co3O4. There is no obvious change in the position of voltage in the subsequent anodic sweeps. The thorough storage mechanisms of NiCo2O4 have been studied in many previous researches [10, 28, 45]. NiCo2O4 is firstly reduced to metallic Ni and Co, which is an irreversible discharge process, corresponding to the first cathodic sweep of the CV plots. It is reported that the metal particle Ni generated in the irreversible process could yield a catalysis effect on the decomposition of Li2O, reducing the initial irreversible capacity and improving its electrochemical properties [39]. And the subsequent electrochemical redox processes are conducted between metals of Ni and Co and their corresponding oxides. To better understand the reaction mechanism, the charge-discharge profiles were measured at the voltage range between 0.01-3 V at the current density of 200 mA g-1 with lithium metal as counter electrode. As Fig. 8 (b) illustrates, there are two plateaus situated at 1.1-1.3 V and 0.50.9 V in the first lithiation process, along with two plateaus located at 1.2-1.6 V and 2.2-2.5 V in the homologous anodic process, which match well with the CV plots. We can also observe that the initial discharge specific capacity is as high as 1408 mAh g-1, while the corresponding charge specific capacity is 1005 mAh g-1, with the initial Coulombic efficiency of 71.38%. That may derive from the irreversibility of the reaction between NiCo2O4 and Li as well as the formation of SEI film and some Li2O that could be not decomposed [45]. It is also reported that the initially
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extra discharge capacity over the theoretical specific capacity (834.75 mAh g-1) of the composite of NiCo2O4 and CNFs is on account of the formation of SEI film [60]. After the first three cycles, the coulombic efficiency retains above 98%, exhibiting a good reversibility of the redox process. Fig. 8 (c) shows the cycling property of the as-synthesized TNN/CNFs, the control sample of NNS and pure CNFs. The control sample suggests dramatic decay of capacity which reduces to 277.1 mAh g-1 after 50 cycles, which is far below the NiCo2O4 grown on CNFs. The pure CNFs has the specific capacity of 211.9 mAh g-1 at the same current density of 200 mAh g-1. When cycled up to 250th, a discharge capacity of the TNN/CNFs composite up to 1033.6 mAh g-1 is achieved, revealing the well cyclic stability of the TNN/CNFs composite at the relative low rate (200 mA g-1). The extra capacity over the theoretical specific capacity can be partly ascribed to the mechanism of interfacial storage derived from the unique 3D hybrid structure, which provides large surface area to storage more Li [45]. Meanwhile, the cycling performance is slightly increasing as seen from Fig. 8 (c). According to the viewpoint in many reports [61-64], the electrolyte solution (EC, DMC and EMC in this case) decomposed to form a polymeric gel-like layer, which reversibly grew and store Li+, leading to the slight increase of the cyclic performance. The rate performance of the both materials are also studied at various current densities from 200 mA g-1 to 2000 mA g-1 as shown in Fig. 8 (d). As the current density increases, a constant decay of specific capacity occurs on the both materials. But when recovered back to 200 mA g-1, the TNN/CNFs composite still delivers an average discharge specific capacity above 800 mAh g-1 as displayed.
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Based above description and discussion, the improvement of the specific capacity should be mainly credited to the combination with carbon nanofibers and the unique 3D hybrid structure. On the one hand, the crisscross framework of CNFs provides a strong support for NiCo2O4 to grow into 3D structure, fast the migration of electrons and avoid the agglomeration of NiCo 2O4. Meanwhile, the 3D hierarchically structure offers high specific area which increases the contact area between electrode and electrolyte, shortening the transport pathway for lithium ions [65, 66]. On the other hand, the existence of interspace among TNN/CNFs composite could accommodate more volume change and cushion the strain during the charge-discharge process, reducing the irreversible capacity with the assist of metallic Ni particles generated during chargedischarge process.
The different discharge specific capacities in literatures about NiCo2O4
reported before are listed in Table 1. As we can see, the TNN/CNFs composite exhibits a relatively better specific capacity and cyclic property. 4. Conclusions In conclusion, we proposed a facile route including hydrothermal and final calcination to fabricate TNN/CNFs composite which tufted NiCo2O4 nanoneedles grew on interconnected carbon nanofibers, forming a unique 3D hybrid structure. When evaluated as anode active material for LIBs, TNN/CNFs composite achieved a discharge specific capacity up to 1033.6 mAh g-1 even after 250 cycles, far surpassing the single NiCo2O4 nanoneedles sphere. The excellent electrochemical property such as superior cycling property at the current density of 200
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mA g-1 and relative high specific capacity is mainly ascribed to the characteristic structure and the combination with CNFs. Acknowledgements This research work has been financially supported by the National Natural Science Foundation of China (21373081 and 21303047), Hunan Provincial Natural Science Foundation of China (14JJ3067), Program for Shenghua Overseas Talents from Central South University, Project of Innovation-driven Plan in Central South University and Self-established Project of State Key Laboratory of Powder Metallurgy. References [1] F.-X. Ma, L. Yu, C.-Y. Xu, X.W. Lou, Self-supported formation of hierarchical NiCo2O4 tetragonal microtubes with enhanced electrochemical properties, Energy Environ. Sci., 9 (2016) 862-866. [2] A.D. Roberts, X. Li, H. Zhang, Porous carbon spheres and monoliths: morphology control, pore size tuning and their applications as Li-ion battery anode materials, Chem Soc Rev, 43 (2014) 4341-4356. [3] S. Chaudhari, D. Bhattacharjya, J.-S. Yu, Facile Synthesis of Hexagonal NiCo2O4 Nanoplates as High-Performance Anode Material for Li-Ion Batteries, Bulletin of the Korean Chemical Society, 36 (2015) 2330-2336. [4] G. Huang, L. Zhang, F. Zhang, L. Wang, Metal-organic framework derived Fe2O3@NiCo2O4 porous nanocages as anode materials for Li-ion batteries, Nanoscale, 6 (2014) 5509-5515. [5] L. Li, S. Peng, Y. Cheah, P. Teh, J. Wang, G. Wee, Y. Ko, C. Wong, M. Srinivasan, Electrospun porous NiCo2O4 nanotubes as advanced electrodes for electrochemical capacitors, Chemistry, 19 (2013) 5892-5898. [6] D.S. Sun, Y.H. Li, Z.Y. Wang, X.P. Cheng, S. Jaffer, Y.F. Zhang, Understanding the mechanism of hydrogenated NiCo2O4 nanograss supported on Ni foam for enhanced-
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performance supercapacitors, J. Mater. Chem. A, 4 (2016) 5198-5204. [7] Y. Tang, Y. Zhang, W. Li, B. Ma, X. Chen, Rational material design for ultrafast rechargeable lithium-ion batteries, Chemical Society reviews, 44 (2015) 5926-5940. [8] K.X. Wang, X.H. Li, J.S. Chen, Surface and interface engineering of electrode materials for lithium-ion batteries, Advanced materials, 27 (2015) 527-545. [9] J. Liu, C. Liu, Y. Wan, W. Liu, Z. Ma, S. Ji, J. Wang, Y. Zhou, P. Hodgson, Y. Li, Facile synthesis of NiCo2O4 nanorod arrays on Cu conductive substrates as superior anode materials for high-rate Li-ion batteries, CrystEngComm, 15 (2013) 1578. [10] G. Gao, H.B. Wu, X.W.D. Lou, Citrate-Assisted Growth of NiCo2O4 Nanosheets on Reduced Graphene Oxide for Highly Reversible Lithium Storage, Advanced Energy Materials, 4 (2014) 1400422. [11] Guobo Zeng, Nan Shi, Michael Hess, Xi Chen, Wei Cheng, Tongxiang Fan, M. Niederberger, A General Method of Fabricating Flexible Spinel-Type Oxide/Reduced Graphene Oxide Nanocomposite Aerogels as Advanced Anodes for Lithium-Ion Batteries, ACS Nano, 9 (2015) 4227-4235. [12] X. Wang, L. Qiao, X. Sun, X. Li, D. Hu, Q. Zhang, D. He, Mesoporous NiO nanosheet networks as high performance anodes for Li ion batteries, Journal of Materials Chemistry A, 1 (2013) 4173-4176. [13] K.S. Lee, S. Park, W. Lee, Y.S. Yoon, Hollow Nanobarrels of alpha-Fe2O3 on Reduced Graphene Oxide as High-Performance Anode for Lithium-Ion Batteries, ACS applied materials & interfaces, 8 (2016) 2027-2034. [14] C. Yan, G. Chen, X. Zhou, J. Sun, C. Lv, Template-Based Engineering of Carbon-Doped Co3O4 Hollow Nanofibers as Anode Materials for Lithium-Ion Batteries, Advanced Functional Materials, 26 (2016) 1428-1436. [15] D. Wang, Y. Yu, H. He, J. Wang, W. Zhou, H.D. Abruña, Template-Free Synthesis of Hollow-Structured Co3O4 Nanoparticles as High-Performance Anodes for Lithium-Ion Batteries, ACS Nano, 9 (2015) 1775-1781. [16] D. Gu, W. Li, F. Wang, H. Bongard, B. Spliethoff, W. Schmidt, C. Weidenthaler, Y. Xia, D. Zhao, F. Schuth, Controllable Synthesis of Mesoporous Peapod-like Co3O4@Carbon Nanotube Arrays for High-Performance Lithium-Ion Batteries, Angewandte Chemie, 54 (2015) 7060-7064.
21
[17] J. Li, S. Xiong, Y. Liu, Z. Ju, Y. Qian, High Electrochemical Performance of Monodisperse NiCo2O4Mesoporous Microspheres as an Anode Material for Li-Ion Batteries, ACS Applied Materials & Interfaces, 5 (2013) 981-988. [18] Jun Du, Gang Zhou, Haiming Zhang, Chao Cheng, Jianmin Ma, Weifeng Wei, Libao Chen, T. Wang, Ultrathin Porous NiCo2O4 Nanosheet Arrays on Flexible Carbon Fabric for HighPerformance Supercapacitors, ACS Appl Mater Interfaces, 5 (2013) 7405-7409. [19] F. Zheng, D. Zhu, X. Shi, Q. Chen, Metal–organic framework-derived porous Mn1.8Fe1.2O4 nanocubes with an interconnected channel structure as high-performance anodes for lithium ion batteries, J. Mater. Chem. A, 3 (2015) 2815-2824. [20] J. Xu, S. Gu, L. Fan, P. Xu, B. Lu, Electrospun Lotus Root-like CoMoO4@Graphene Nanofibers as High-Performance Anode for Lithium Ion Batteries, Electrochimica Acta, 196 (2016) 125-130. [21] W. Kang, Y. Tang, W. Li, X. Yang, H. Xue, Q. Yang, C.S. Lee, High interfacial storage capability of porous NiMn2O4/C hierarchical tremella-like nanostructures as the lithium ion battery anode, Nanoscale, 7 (2015) 225-231. [22] C. Zhang, J.S. Yu, Morphology‐Tuned Synthesis of NiCo2O4‐Coated 3D Graphene Architectures Used as Binder ‐Free Electrodes for Lithium ‐ Ion Batteries, Chemistry– A European Journal, (2016). [23] Y. Lei, J. Li, Y. Wang, L. Gu, Y. Chang, H. Yuan, D. Xiao, Rapid microwave-assisted green synthesis of 3D hierarchical flower-shaped NiCo2O4 microsphere for high-performance supercapacitor, ACS applied materials & interfaces, 6 (2014) 1773-1780. [24] K.R. Reddy, M. Hassan, V.G. Gomes, Hybrid nanostructures based on titanium dioxide for enhanced photocatalysis, Applied Catalysis A: General, 489 (2015) 1-16. [25] R. Kakarla Raghava, G.G. Vincent, H. Mahbub, Carbon functionalized TiO 2 nanofibers for high efficiency photocatalysis, Materials Research Express, 1 (2014) 015012. [26] K.R. Reddy, B.C. Sin, K.S. Ryu, J. Noh, Y. Lee, In situ self-organization of carbon black– polyaniline composites from nanospheres to nanorods: Synthesis, morphology, structure and electrical conductivity, Synthetic Metals, 159 (2009) 1934-1939. [27] L. Peng, H. Zhang, L. Fang, Y. Bai, Y. Wang, Designed Functional Systems for HighPerformance Lithium-Ion Batteries Anode: From Solid to Hollow, and to Core-Shell NiCo2O4 Nanoparticles Encapsulated in Ultrathin Carbon Nanosheets, ACS applied materials & interfaces,
22
8 (2016) 4745-4753. [28] A.K. Mondal, H. Liu, Z.-F. Li, G. Wang, Multiwall carbon nanotube-nickel cobalt oxide hybrid structure as high performance electrodes for supercapacitors and lithium ion batteries, Electrochimica Acta, 190 (2016) 346-353. [29] Y. Chen, J. Zhu, B. Qu, B. Lu, Z. Xu, Graphene improving lithium-ion battery performance by construction of NiCo2O4/graphene hybrid nanosheet arrays, Nano Energy, 3 (2014) 88-94. [30] W. Liu, C. Lu, K. Liang, B.K. Tay, A High‐Performance Anode Material for Li‐Ion Batteries Based on a Vertically Aligned CNTs/NiCo2O4 Core/Shell Structure, Particle & Particle Systems Characterization, 31 (2014) 1151-1157. [31] M. Hassan, K.R. Reddy, E. Haque, S.N. Faisal, S. Ghasemi, A. I. Minett, V.G. Gomes, Hierarchical assembly of graphene/polyaniline nanostructures to synthesize free-standing supercapacitor electrode, Composites Science and Technology, 98 (2014) 1-8. [32] L. Fritea, A. Le Goff, J.-L. Putaux, M. Tertis, C. Cristea, R. Săndulescu, S. Cosnier, Design of a reduced-graphene-oxide composite electrode from an electropolymerizable graphene aqueous dispersion using a cyclodextrin-pyrrole monomer. Application to dopamine biosensing, Electrochimica Acta, 178 (2015) 108-112. [33] M. Hassan, K.R. Reddy, E. Haque, A.I. Minett, V.G. Gomes, High-yield aqueous phase exfoliation of graphene for facile nanocomposite synthesis via emulsion polymerization, Journal of colloid and interface science, 410 (2013) 43-51. [34] S.H. Choi, D.H. Kim, A.V. Raghu, K.R. Reddy, H.-I. Lee, K.S. Yoon, H.M. Jeong, B.K. Kim, Properties of Graphene/Waterborne Polyurethane Nanocomposites Cast from Colloidal Dispersion Mixtures, Journal of Macromolecular Science, Part B, 51 (2012) 197-207. [35] Y.R. Lee, S.C. Kim, H.-i. Lee, H.M. Jeong, A.V. Raghu, K.R. Reddy, B.K. Kim, Graphite oxides as effective fire retardants of epoxy resin, Macromolecular Research, 19 (2011) 66-71. [36] K.R. Reddy, H.M. Jeong, Y. Lee, A.V. Raghu, Synthesis of MWCNTs-Core/Thiophene Polymer-Sheath Composite Nanocables by a Cationic Surfactant-Assisted Chemical Oxidative Polymerization and Their Structural Properties, Journal of Polymer Science Part A: Polymer Chemistry, 48 (2010) 1477-1484. [37] K.R. Reddy, B.C. Sin, C.H. Yoo, D. Sohn, Y. Lee, Coating of multiwalled carbon nanotubes with polymer nanospheres through microemulsion polymerization, Journal of colloid and interface science, 340 (2009) 160-165.
23
[38] K.R. Reddy, B.C. Sin, C.H. Yoo, W. Park, K.S. Ryu, J.-S. Lee, D. Sohn, Y. Lee, A new onestep synthesis method for coating multi-walled carbon nanotubes with cuprous oxide nanoparticles, Scripta Materialia, 58 (2008) 1010-1013. [39] Y. Wei, F. Yan, X. Tang, Y. Luo, M. Zhang, W. Wei, L. Chen, Solvent-Controlled Synthesis of NiO-CoO/Carbon Fiber Nanobrushes with Different Densities and Their Excellent Properties for Lithium Ion Storage, ACS applied materials & interfaces, 7 (2015) 21703-21711. [40] A. Banerjee, S. Bhatnagar, K.K. Upadhyay, P. Yadav, S. Ogale, Hollow Co(0.85)Se nanowire array on carbon fiber paper for high rate pseudocapacitor, ACS applied materials & interfaces, 6 (2014) 18844-18852. [41] L. Huang, D. Chen, Y. Ding, S. Feng, Z.L. Wang, M. Liu, Nickel-cobalt hydroxide nanosheets coated on NiCo2O4 nanowires grown on carbon fiber paper for high-performance pseudocapacitors, Nano letters, 13 (2013) 3135-3139. [42] T. Choudhury, S. Saied, J. Sullivan, A. Abbot, Reduction of oxides of iron, cobalt, titanium and niobium by low-energy ion bombardment, Journal of Physics D: Applied Physics, 22 (1989) 1185. [43] Y. Liu, J. Xu, H. Li, S. Cai, H. Hu, C. Fang, L. Shi, D. Zhang, Rational design and in situ fabrication of MnO2@NiCo2O4 nanowire arrays on Ni foam as high-performance monolith deNOx catalysts, J. Mater. Chem. A, 3 (2015) 11543-11553. [44] F. Zheng, D. Zhu, Q. Chen, Facile fabrication of porous NixCo3-xO4 nanosheets with enhanced electrochemical performance as anode materials for Li-ion batteries, ACS applied materials & interfaces, 6 (2014) 9256-9264. [45] J. Li, S. Xiong, Y. Liu, Z. Ju, Y. Qian, High electrochemical performance of monodisperse NiCo2O4 mesoporous microspheres as an anode material for Li-ion batteries, ACS applied materials & interfaces, 5 (2013) 981-988. [46] S. Xu, L. Lu, Q. Zhang, H. Zheng, L. Liu, S. Yin, S. Wang, G. Li, C. Feng, Morphologycontrolled synthesis and electrochemical performance of NiCo2O4 as anode material in lithiumion battery application, Journal of Nanoparticle Research, 17 (2015) 1-11. [47] X. Li, L. Jiang, C. Zhou, J. Liu, H. Zeng, Integrating large specific surface area and high conductivity in hydrogenated NiCo2O4 double-shell hollow spheres to improve supercapacitors, NPG Asia Materials, 7 (2015) e165. [48] J. Wang, Z. Wang, B. Huang, Y. Ma, Y. Liu, X. Qin, X. Zhang, Y. Dai, Oxygen vacancy
24
induced band-gap narrowing and enhanced visible light photocatalytic activity of ZnO, ACS applied materials & interfaces, 4 (2012) 4024-4030. [49] G.M. Burke, D.E. Wurster, M.J. Berg, P. Weng-Pedersen, D.D. Schottelius, Surface characterization of activated charcoal by X-ray photoelectron spectroscopy (XPS): correlation with phenobarbital adsorption data, Pharmaceutical Research, 9 (1992) 126-130. [50] Y.-C. Chiang, C.-C. Liang, C.-P. Chung, Characterization of Platinum Nanoparticles Deposited on Functionalized Graphene Sheets, Materials, 8 (2015) 5318. [51] Y. Chen, J. Xu, Y. Yang, Y. Zhao, W. Yang, X. He, S. Li, C. Jia, Enhanced electrochemical performance of laser scribed graphene films decorated with manganese dioxide nanoparticles, Journal of Materials Science: Materials in Electronics, 27 (2016) 2564-2573. [52] C.Z.J.-S. Yu, Morphology-Tuned Synthesis of NiCo2O4 -Coated 3D Graphene Architectures Used as Binder-Free Electrodes for Lithium-Ion Batteries, Chem.Eur.J, 22 ( 2016) 4422–4430. [53] N. Jabeen, Q. Xia, M. Yang, H. Xia, Unique Core-Shell Nanorod Arrays with Polyaniline Deposited into Mesoporous NiCo2O4 Support for High-Performance Supercapacitor Electrodes, ACS applied materials & interfaces, 8 (2016) 6093-6100. [54] W. Li, L. Xin, X. Xu, Q. Liu, M. Zhang, S. Ding, M. Zhao, X. Lou, Facile synthesis of three-dimensional structured carbon fiber-NiCo2O4-Ni(OH)2 high-performance electrode for pseudocapacitors, Scientific reports, 5 (2015) 9277. [55] Y. Tang, Y. Zhang, J. Deng, J. Wei, H. Le Tam, B.K. Chandran, Z. Dong, Z. Chen, X. Chen, Mechanical force-driven growth of elongated bending TiO2 -based nanotubular materials for ultrafast rechargeable lithium ion batteries, Advanced materials, 26 (2014) 6111-6118. [56] Y. NuLi, P. Zhang, Z. Guo, H. Liu, J. Yang, NiCo2O4/C Nanocomposite as a Highly Reversible Anode Material for Lithium-Ion Batteries, Electrochemical and Solid-State Letters, 11 (2008) A64. [57] L. Li, Y. Cheah, Y. Ko, P. Teh, G. Wee, C. Wong, S. Peng, M. Srinivasan, The facile synthesis of hierarchical porous flower-like NiCo2O4 with superior lithium storage properties, Journal of Materials Chemistry A, 1 (2013) 10935. [58] J. Zhu, L. Chen, Z. Xu, B. Lu, Electrospinning preparation of ultra-long aligned nanofibers thin films for high performance fully flexible lithium-ion batteries, Nano Energy, 12 (2015) 339-
25
346. [59] J. Cheng, Y. Lu, K. Qiu, H. Yan, J. Xu, L. Han, X. Liu, J. Luo, J.K. Kim, Y. Luo, Hierarchical Core/Shell NiCo2O4@NiCo2O4 Nanocactus Arrays with Dual-functionalities for High Performance Supercapacitors and Li-ion Batteries, Scientific reports, 5 (2015) 12099. [60] A.K. Mondal, D. Su, S. Chen, X. Xie, G. Wang, Highly porous NiCo2O4 Nanoflakes and nanobelts as anode materials for lithium-ion batteries with excellent rate capability, ACS applied materials & interfaces, 6 (2014) 14827-14835. [61] X. Wang, L. Qiao, X. Sun, X. Li, D. Hu, Q. Zhang, D. He, Mesoporous NiO nanosheet networks as high performance anodes for Li ion batteries, Journal of Materials Chemistry A, 1 (2013) 4173. [62] Y. Shi, B. Guo, S.A. Corr, Q. Shi, Y.-S. Hu, K.R. Heier, L. Chen, R. Seshadri, G.D. Stucky, Ordered Mesoporous Metallic MoO2 Materials with Highly Reversible Lithium Storage Capacity, Nano Lett., 9 (2009) 4215-4220. [63] Y. Feng, R. Zou, D. Xia, L. Liu, X. Wang, Tailoring CoO–ZnO nanorod and nanotube arrays for Li-ion battery anode materials, Journal of Materials Chemistry A, 1 (2013) 9654. [64] F.M. Courtel, H. Duncan, Y. Abu-Lebdeh, I.J. Davidson, High capacity anode materials for Li-ion batteries based on spinel metal oxides AMn2O4 (A = Co, Ni, and Zn), Journal of Materials Chemistry, 21 (2011) 10206. [65] Y. Tang, X. Rui, Y. Zhang, T.M. Lim, Z. Dong, H.H. Hng, X. Chen, Q. Yan, Z. Chen, Vanadium pentoxide cathode materials for high-performance lithium-ion batteries enabled by a hierarchical nanoflower structure via an electrochemical process, J. Mater. Chem. A, 1 (2013) 82-88. [66] Y. Tang, Z. Jiang, G. Xing, A. Li, P.D. Kanhere, Y. Zhang, T.C. Sum, S. Li, X. Chen, Z. Dong, Efficient Ag@ AgCl Cubic Cage Photocatalysts Profit from Ultrafast Plasmon‐Induced Electron Transfer Processes, Advanced Functional Materials, 23 (2013) 2932-2940. [67] G. Gao, H.B. Wu, S. Ding, X.W. Lou, Preparation of carbon-coated NiCo2O4 @SnO2 hetero-nanostructures and their reversible lithium storage properties, Small, 11 (2015) 432-436. [68] A.K. Mondal, D. Su, S. Chen, K. Kretschmer, X. Xie, H.J. Ahn, G. Wang, A microwave synthesis of mesoporous NiCo2O4 nanosheets as electrode materials for lithium-ion batteries and supercapacitors, Chemphyschem, 16 (2015) 169-175.
26
[69] Y. Zhu, C. Cao, A Simple Synthesis of Two-Dimensional Ultrathin Nickel Cobaltite Nanosheets for Electrochemical Lithium Storage, Electrochimica Acta, 176 (2015) 141-148.
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Figure captions
Fig. 1. The illustration of three preparation procedures of TNN/CNFs.
28
Fig. 2. (a) Representative XRD patterns for TNN/CNFs and NNS and XPS spectra (b) survey spectrum, (c) Co 2p, (d) Ni 2p, (e) O 1s, and (f) C 1s for TNN/CNFs.
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Fig. 3. Raman spectrum of TNN/CNFs and NNS
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Fig. 4. SEM images of (a) CNFs (b), (c) TNN/CNFs, (d) NNS
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Fig. 5. TEM (a), HRTEM (b) and EDX-mapping of TNN/CNFs composite. The inset in (a) is SAED of the region marked by red rectangular frame. The inset in (b) is the corresponding FFT image.
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Fig. 6. Nitrogen adsorption-desorption isotherm of (a) TNN/CNFs (b) NNS and the inset is the matching pore size distribution.
33
Fig. 7. TGA curve of (a) TNN/CNFs (b) NNS from room temperature to 800 ℃ at a ramping rate of 10 ℃ min-1 and XRD of the residues after TGA test (c) TNN/CNFs after TGA (d) NNS after TGA
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Accepted Manuscript Title: Tufted NiCo2 O4 Nanoneedles Grown on Carbon Nanofibers with advanced electrochemical property for Lithium Ion Batteries Author: Gang Zhou Chen Wu Yuehua Wei Chengchao Li Qingwang Lian Chao Cui Weifeng Wei Libao Chen PII: DOI: Reference:
S0013-4686(16)32545-2 http://dx.doi.org/doi:10.1016/j.electacta.2016.12.001 EA 28471
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
18-8-2016 18-10-2016 1-12-2016
Please cite this article as: Gang Zhou, Chen Wu, Yuehua Wei, Chengchao Li, Qingwang Lian, Chao Cui, Weifeng Wei, Libao Chen, Tufted NiCo2O4 Nanoneedles Grown on Carbon Nanofibers with advanced electrochemical property for Lithium Ion Batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.12.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Table 1. Comparison of specific capacity of NiCo2O4 as anodes for LIBs Table 1. Current
Reversible Cycle
Material
density
capacity
Ref
number (mA g-1)
(mAh g-1)
NiCo2O4 100
50
729
[46]
40
50
715.8
[56]
100
50
765
[67]
NiCo2O4 nanosheets
100
50
767
[68]
NiCo2O4 nanosheets
200
100
804.8
[69]
NiCo2O4@ NiCo2O4
120
100
830
[59]
60
50
918
[3]
100
60
939
[57]
200
250
1033.6
microspheres NiCo2O4/C nanocomposite NiCo2O4@SnO2@C HSs
Hexagonal NiCo2O4 nanoplates Porous
flower-like
NiCo2O4 TNN/CNFs composite
this work
36
S0013-4686(16)32545-2 http://dx.doi.org/doi:10.1016/j.electacta.2016.12.001 EA 28471
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
18-8-2016 18-10-2016 1-12-2016
Please cite this article as: Gang Zhou, Chen Wu, Yuehua Wei, Chengchao Li, Qingwang Lian, Chao Cui, Weifeng Wei, Libao Chen, Tufted NiCo2O4 Nanoneedles Grown on Carbon Nanofibers with advanced electrochemical property for Lithium Ion Batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.12.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Tufted NiCo2O4 Nanoneedles Grown on Carbon Nanofibers with advanced electrochemical property for Lithium Ion Batteries Gang Zhou a, 1, Chen Wu a, 1, Yuehua Wei a, Chengchao Li b, *, Qingwang Liana, Chao Cui a, Weifeng Wei a, Libao Chen a, * a
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, 410083,
China b
School of Chemical Engineering and Light Industry, Guangdong University of Technology,
Guangzhou 510006, China. 1
These authors contributed equally to this work.
To whom the proofs and correspondence should be sent. Professor Libao Chen Powder Metallurgy Research Institute Central South University, Changsha, 410083, China E-mail: [email protected] Professor Chengchao Li School of Chemical Engineering and Light Industry Guangdong University of Technology, Guangzhou 510006, China
1
E-mail: [email protected]
2
Graphical Abstract
3
Highlights
Tufted NiCo2O4 nanoneedles grown on carbon nanofibers (TNN/CNFs) were successfully fabricated by facile hydrothermal and annealing procedure.
The composite possesses a unique three-dimensional (3D) hybrid structure and morphology.
The composite delivers a reversible discharge specific capacity up to 1033.6 mAh g-1 even after 250 cycles.
The outstanding electrochemical performance origins from unique 3D hybrid structure.
4
Abstract In this article, tufted NiCo2O4 nanoneedles directly grown on carbon nanofibers (CNFs) were successfully fabricated by a facile hydrothermal procedure and subsequent annealing. The morphology of tufted NiCo2O4 nanoneedles/carbon nanofibers (TNN/CNFs) composite appears like clustered acicular leaves on branches, forming a unique three dimensional (3D) hybrid structure. Compared to NiCo2O4 nanoneedles spheres (NNS) without CNFs, the TNN/CNFs material presents a reversible discharge specific capacity up to 1033.6 mAh g-1 at a current density of 200 mA g-1 even after 250 cycles, with the coulombic efficiency of above 98%. The unique 3D hybrid structure formed with CNFs should be responsible for the superior lithium storage capacity and enhanced cycling performance, which provides short distance for ions transport, high specific surface area for large contact areas between electrode and electrolyte, together with the interspace to accommodate volume change and smaller agglomeration tendency. Moreover, the design route proposed offers more possibility to preparation of highperformance anode materials for LIBs. Keywords： Tufted NiCo2O4 nanoneedle; carbon nanofiber; hydrothermal synthesis; lithium ion battery; electrochemical performance 1. Introduction The demands for renewable and sustainable energy resources are dramatically increasing because of the rapid development of economy and society [1, 2]. To meet the demands, many efforts have been devoted to develop high-property energy storage devices during the past
5
decades such as lithium ion batteries (LIBs) and supercapacitors [3-6]. Rechargeable LIBs are widely applied to portable electronic devices, electric and hybrid electric vehicles on account of its high working voltage, light weight, high energy density, and environmentally friendly characteristics [7, 8]. At present, the commercial LIBs anode materials are graphite and carbon. However, the limited theoretical specific capacity of 372 mAh g-1 restricts improvement of the energy density of LIBs [7]. Afterwards, many transition-metal oxides have been widely studied to solve this problem due to the lithium storage mechanism based on conversion reaction, which endows them high theoretical special capacity[9-11]. For instance, the redox conversion reaction for NiO in which nickel is reduced to metallic state during discharge process and reoxidized during charge process, exhibits a large theoretical capacity of 716 mAh g-1 [12]. And α-Fe2O3 owns a specific capacity of 1007 mAh g-1 [7, 13] while Co3O4 possesses a specific capacity of 890 mAh g-1 [14-16]. However, some transition metal oxides (e.g. Co3O4, NiO and Fe2O3) suffer from poor conductivity, and Co3O4 is toxic, not friendly to the environment [7, 17]. So binary metal oxides become one of considered anode materials, as they could reversibly extract and insert numerous Li+ and combine superiorities of two different metals, exhibiting more potential prospects [18]. Among most studied materials of binary metal oxides, such as Mn1.8Fe1.2O4 [19], CoMoO4 [20], NiCo2O4 [3, 9], NiMn2O4 [21] and et al, NiCo2O4 stands out due to its high specific capacity (891 mAh g-1), better conductivity as well as low diffusion resistance between Li+ and electrolyte [22, 23]. However, there is a common problem that the structure deterioration (aggregation or pulverization) by large volume change during cycling would result in
6
performance degradation [11]. More recently, to ease volume expansion and ameliorate cycling performance as well as rate capability, researchers have been committed to the combination of NiCo2O4 nanomaterial with conductive carbon materials and morphology-tuned synthesis. Moreover, combination with carbon materials and morphology-tuned synthesis (introducing porous or hierarchical structures) are commonly considered as effective ways to buffer volume expansion and ameliorate properties [8, 24-26]. Zhang et al. synthesized two different morphologies of NiCo2O4 nanocomposites on surface of graphene-coated nickel foam which represented flower-type morphology and sheet-type morphology respectively. And the flowertype NiCo2O4 demonstrated the specific capacity over 985 mAh g-1 after 60 cycles, superior to the sheet-type NiCo2O4, which attributed to the larger mesopore volume and surface area [22]. Peng et al. designed core-shell NiCo2O4 nanostructure coated by carbon layers, which could accommodate the volume change [27]. A hybrid structure of NiCo2O4 nanosheets with multiwall carbon nanotubes achieved the specific capacity of 904 mAh g-1 [28]. In the meantime, it is reported that the structure of single NiCo2O4 may be unstable during cycling, while it may be broken or pulverized. For instance, Chen et al. found that pure NiCo2O4 nanosheets suffered the decay of capacity because the volume expansion caused the pulverization of NiCo2O4 [29]. Compared with single structure, hybrid structure is more bearable to the volume changes during cycling. Liu et al. prepared three-dimensional CNTs/NiCo2O4 core/shell composite structure, which exhibited stable cycling properties, no significant decay of capacity even after 200 cycles [30]. So, we consider the combination of NiCo2O4 with carbon materials and design them to
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form 3D hybrid structure to enhance its electrochemical properties. Among so many carbon materials, carbon nanotubes (CNTs), graphene and carbon cloth are most commonly used, because of their better chemical stability, large specific surface area and good electron conductivity [31-38]. However, there is a drawback for CNTs of low loading density, and graphene is inclined to stack together [39]. Carbon nanofibers possess a lot of merits such as large load, high porosity, also, larger specific surface area than carbon cloth, which are chosen as the substrate of NiCo2O4 in this article [39-41]. As far as we know, there are a few literatures reported on NiCo2O4 nanomaterial which use the carbon nanofibers as the substrate with superior cycling properties. Herein, we have adopted a facile hydrothermal process followed by calcination to fabricate TNN/CNFs composite, which the tufted NiCo2O4 nanoneedles were strongly covered on the carbon nanofibers. Meanwhile, the composite demonstrated a superior electrochemical property and a high special capacity of 1033.6 mAh g-1 after 250 cycles was achieved. The outstanding performance is attributed to the unique 3D hybrid structure, including abundant interspace to accommodate volume change, large surface area and the merits of CNFs of high porosity along with the ability to prevent NiCo2O4 from agglomeration. Therefore, it is noteworthy that the TNN/CNFs composite could become a potential candidate of anode materials for LIBs. 2. Experimental 2.1. Preparation of NiCo2O4 on Carbon Nanofibers
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All reagents were of analytical pure and used without any further purification. The carbon nanofibers were firstly fabricated by means of electrospinning which has been reported in our previous literature [39]. 0.3778g Polyacrylonitrile (PAN, Mw =150000, Sigma-Aldrich Co., Ltd, China) and 5 mL N, N - dimethylformide (DMF, Sinopharm Chemical Reagent Co., Ltd, China) were used to form a uniform and transparent solution, and then it was filled in a 5 mL syringe with a needle which diameter is 0.6 mm. Then, a high voltage of 12 kV was applied to needle and collector to create a charged jet and form electrospun nanofibers. The nanofibers were dried in a vacuum, followed by a two-step treatment which included annealing in air and carbonizing in Ar. Subsequently, hydrothermal method and annealing process were used to fabricate NiCo2O4 on carbon nanofibers as follows: 5 mM Ni(NO3)2•6H2O (Sinopharm Chemical Reagent Co., Ltd, China), 10 mM Co(NO3)2•6H2O (Sinopharm Chemical Reagent Co., Ltd, China) and 35 mM urea (Aladdin Industrial Corporation, China) were dissolved in 20 mL of deionized (DI) water. Later, the solution was magnetic stirring for 1 h, and was transferred into a 50 mL Teflonlined stainless steel autoclave with about 10 mg of the carbon nanofibers immersed in the solution to provide the substrate for the growth of precursor. Subsequently, a temperature of 120 ℃ was applied for 12 h. The as-synthesized product was washed several times using DI water and ethanol, followed by drying overnight in air. Finally, an annealing procedure was adopted under the condition of 300 ℃×2 h in Ar with a heating rate of 2 ℃ min-1. And the control sample of bare NiCo2O4 nanoneedles spheres (NNS) followed the same preparation method except that
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the carbon nanofibers were not put into the Teflon-lined stainless steel autoclave in the procedure of hydrothermal reaction. 2.2. Sample characterizations The structure was characterized by X-ray powder diffraction (XRD) analysis, which was carried out on Rigaku Desktop X-ray diffractometer using Cu Kα radiation (λ = 1.54056 Å ) with 2θ ranging from 10° to 85°. Raman spectrum was conducted on LabRAM Aramis (Germany). The thermal gravimetric analysis was studied by a thermogravimetric analyzer (TGA, SHIMADZU DTG-60AH) in air with the temperature ranging from room temperature to 800 ℃ and a heating rate of 10 ℃ min-1. The morphology and nanostructure of the samples were observed by a scanning electron microscope (SEM, NOVA NANO SEM230) and transmission electron microscopy equipped with an energy-dispersive X-ray (EDX) (TEM, TECNAI G2 F20), collectively. X-ray photoelectron spectroscopy (XPS, ESCALAB 250 Xi) was also conducted to obtain surface information. The specific surface area was obtained by Brunauer-Emmett-Teller test (BET, Tristar 3020, America) and pore size distribution curve was acquired by BarrettJoyner-Halenda (BJH) equation. To evaluate electrochemical properties, CR2025 coin-type half-cell was assembled. The slurry was made of active material TNN/CNFs composite (or NNS composite), acetylene black and Carboxyl Methyl Cellulose (CMC) with the mass ratio of 8:1:1. The solvent for CMC is the solution composed of DI water and ethyl alcohol with the volume ration of 3:2. And the copper foil was used for collector. The mass of the active material is 0.45 g cm-2. The half-cell was
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prepared in an argon-filled glove box (M. BRAUN, Germany). The counterpart was Li metal. Electrodes were separated by Celgard 2400 membrane and the electrolyte used LiPF6 in solvent composed of ethylene carbonate (EC), dimethyl carbonate (DMC), and Ethyl Methyl Carbonate (EMC) with volume ratio of 1:1:1. Before examing electrochemical properties, the cells were aged overnight. The charge-discharge curves, cycle stability, and rate performance were tested on a multichannel battery test system (LANHE CT2001A, Wuhan LAND Electronics Co., P. R. China) at 30 ℃ in the voltage window of 0.01 V and 3.0 V. Cyclic voltammetry (CV) measurements were performed via Arbin Workstation (USA) with a scan rate of 0.1 mV/s between 0.01 V and 3.0 V vs Li/Li+. 3. Results and discussion The preparation processes are elucidated as following Fig. 1. Three steps are clearly shown in this illustration, comprising electrospinning, hydrothermal and subsequent annealing. Firstly, CNFs were fabricated by electrospinning, which were used as the support for the growth of precursor. Secondly, hydrothermal method was applied to grow the NiCo-precursor on the CNFs. Finally, TNN/CNFs composite was converted from NiCo-precursor/CNFs by the final annealing process at 300 ℃ for 2 h at the temperature rate of 2 ℃ min-1. Fig. 2 (a) displays diffraction patterns of TNN/CNFs and NNS by X-ray diffraction (XRD). All peaks can be clearly indexed to (111), (220), (311), (400), (422), (511), (440) and (622) lattice planes which coincide well with the standard diffraction pattern of PDF card of NiCo2O4
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(JCPDS no. 20-0781). There are no other peaks detected except the peaks forementioned, betokening high purity of the products. The X-ray Photoelectron Spectroscopy (XPS) was carried on to obtain more surface information about the sample. The survey spectrum in Fig. 2 (b) reveals the presence of elements of Ni, Co, C and O. A typical Gauss Lorentz equation was applied to fit the peaks. Fig. 2 (c) exhibits the high resolution spectrum of Co 2p with two peaks at 780.44 eV and 781.99 eV considered as Co 2p3/2 and another two peaks at 795.58 eV and 797.60 eV indexed as Co 2p1/2 accompanied by two satellite peaks (indicated as “Sat.”). As the previous literatures report, the binding energy of 780.44 eV is ascribed to Co3+, while the binding energy of 781.99 eV is in agreement with Co2+ [42, 43]. The fitted Ni 2p high resolution spectrum demonstrates two peaks located at the binding energy of 855.53 eV and 873.15 eV, which are identified as Ni2+, and another two peaks situated at the binding energy of 856.77 eV and 874.63 eV are deemed to Ni 3+ [44, 45]. There are yet two satellite peaks (indicated as “Sat.”) as displayed. As we know, NiCo2O4 is face-centered-cubic spinel-structure, and Ni atoms are located at octahedron while Co atoms are situated at both octahedron and tetrahedron composed of oxygen atoms [46], which can also be regarded as local substitution of Co2+ with Ni2+ in Co3O4 spinel structure. The peak centered at 529.55 eV in Fig. 2 (e) is commonly considered as the characteristic peak of metaloxygen bonds [47], on account of the formation of Ni-O bond and Co-O bond forementioned. The peak sitting at 531.48 eV represents the defects according to the previous reports, including oxygen vacancies, contaminants, hydroxyls or chemisorbed oxygen [27]. Another peak at 532.66
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eV is derived from multiplicity of physi- and chemi-sorbed water at or near the surface [47]. It is clearly observed that the intensity of the peak at 531.48 eV is higher compared to other two peaks, which gives the fact of the existence of numerous defects.[48] As reported in the literature, oxygen vacancies can act as the electron donors, thus improving the electrical conductivity to some extent [47]. The C 1s high resolution spectrum is assigned to the presence of carbon nanofibers, where the peaks at 284.65 eV, 285.84 eV and 289.52 eV are ascribed to CC, C-O, O-C=C [11, 49-51], respectively. Raman spectroscopy was also conducted. As shown in Raman spectrum (Fig. 3), four characteristic peaks of TNN/CNFs and NNS were observed, which represent F2g (185.3 cm-1), Eg (484.9 cm-1), F2g (538.9 cm-1), and A1g (646.9 cm-1) modes of NiCo2O4 [52-54], respectively. Fig. 4 exhibits different morphologies of CNFs, TNN/CNFs and control sample NNS. The CNFs with a diameter of around 150 nm form a crisscross network as seen from Fig. 4 (a). And the framework made up of CNFs is beneficial to form a unique three dimensional (3D) structure when NiCo2O4 grows on them, offering efficient ion and electron transmission paths [55]. The typical morphology of the TNN/CNFs composite is characterized by scanning electron microscope (SEM) displayed in Fig. 4 (b) and (c). With the support of CNFs, the tufted NiCo2O4 nanoneedles through hydrothermal and annealing procedures form a relative ordered structure, which grow into radial flowers with high densities. Fig. 4 (b) is the panoramic morphology which presents an interlaced structure formed by CNFs covered with NiCo2O4 nanoneedles, and there are much interspace to accommodate the volume variation during cycling and fasten
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penetration of the electrolyte. From the local magnification Fig. 4 (c), we can observe that the length of NiCo2O4 acicular leaf is around 0.5-2 μm with the width less than 100 nm, providing large specific surface area which is beneficial to enlarge the contact areas between anode material and electrolyte, shortening the distance of ions transportation. In our previous report [39], the time-dependent experiments were investigated to display the growth mechanism which could also explain the formation of the precursor of TNN/CNFs during hydrothermal process. In brief, the seeds of Ni-Co precursor sprouted and grew into nanoneedles finally as time went on. The NNS materials in Fig. 4 (d) appear like fluffy balls which have the diameter of 3-4 μm, with the nanoneedles stretching in all directions. Although the configuration could endow the material with more specific surface area, it would be inclined to distribute randomly and agglomerate together without the support of the CNFs. And it is reported that aggregation and pulverization of the structure are nonnegligible issues for transition metal oxides in their application in LIBs [56]. To observe more detailed morphology, transition electron microscope (TEM) was also applied to characterize TNN/CNFs composite and the relevant outcomes are depicted in Fig. 5 (a) and (b). Fig. 5 (a) shows single carbon nanofiber decorated with integral NiCo2O4 nanoneedles, which interweave with each other to form a special 3D structure. The inset in Fig. 5 (a) is the selected area electron diffraction (SAED) of the region marked by red rectangular frame, from which distinct polycrystalline rings composed of superposition of diffraction spots are presented, indicating the good crystallization of NiCo2O4. The high resolution transmission
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electron microscopy (HRTEM) (Fig. 5 (b)) of a single NiCo2O4 nanoneedle further confirms its composition, where the lattice fringe is calculated as 0.287 nm, matching well with the interplanar distance of (220) of NiCo2O4. The inset is the corresponding fast fourier transform algorithm (FFT), which demonstrates a monocrystal structure. The EDS mapping (Fig. 5 (c)) shows the distribution of elements of Co and Ni. Also, a clear contour of nanoneedles can be identified from the mapping, indicating homogeneous distribution of the elements. To compare the specific surface area of TNN/CNFs and NSS, nitrogen isothermal adsorption-desorption measurements were carried out. The two isotherm shown in Fig.6 can be classed as Ⅳ type with a H1 hysteresis loop on a scale of 0.6-1.0 P/P0, manifesting a mesoporous structure [47, 57]. Meanwhile, the BET specific surface area of TNN/CNFs is 134.1 m2 g-1, larger than NNS of 120.5 m2 g-1. The insets describe the distribution of pore size of TNN/CNFs and NNS by the Barrett-Joyner-Halenda (BJH) method from absorption branch, which indicate the pore size of 4.685 nm for TNN/CNFs and 4.696 nm for NNS. These characteristics are beneficial to the diffusion of Li+ and conductive to contact of active material and electrolyte, in favor of better electrochemical properties [57]. Fig. 7 (a) and (b) are the TGA curves of the as-synthesized samples of TNN/CNFs and NNS fabricated on the same condition except the addition of CNFs, measured from room temperature up to 800 ℃ at a temperature rate of 10 ℃min-1 at the atmosphere of O2 with the flow rate of 50 mL min-1. Also, XRD was used to characterize the residue after the same experiment condition as TGA measurements (Fig. 7 (c) and (d)), indicating NiCo2O4 has
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decomposed into NiO and Co3O4. As seen from Fig. 7 (a), the small weight loss of 2.55 % as the temperature up to 250 ℃ results from water evaporation, while the second steep slope represents combustion of carbon and decomposition of NiCo2O4, calculated as 22.75 %. In the same way, we can compute the weight loss of decomposition of NiCo2O4 as 16.29 % in the TGA measurement of NNS. So the mass ration of NiCo2O4 and CNFs are 90.99 % and 6.46 %, respectively. The theoretical specific capacity of TNN/CNFs (C) is calculated as the following formula [58]: C(TNN/CNFs) = C(NiCo2O4) * M(NiCo2O4) % + C(CNFs) * M(CNFs) % = (891 * 90.99 % + 372 *6.46 %) mAh g-1 = 834.75 mAh g-1 where M % is the mass fraction of NiCo2O4 and CNFs, respectively. The electrochemical performances of TNN/CNFs composite and NNS composite were evaluated by cycle voltammetry (CV) test and galvanostatic charge-discharge measurement. Fig. 8 (a) presents the first three cycles of CV plots of the as-prepared TNN/CNFs at a scan rate of 0.1 mV s-1 between 0.01-3.0 V. As we can see, there are two peaks during first cathodic sweep which is different from the subsequent cycles, indicating that two different mechanisms exist. The intense peak at 1.06 V originates from the amorphization of the structure [30]. The second peak appearing at about 0.68 V can be credited to the lithiation reaction of NiCo 2O4 in which Ni2+ and Co3+ are reduced to metal Ni and Co and Li2O is formed, along with the formation of solid electrolyte interface (SEI) film [3, 30, 56, 59]. The main reduction peak shifts to a more positive potential of 0.9 V in the following cycles, which originates from the decomposition of
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NiCo2O4 [22]. During the first anodic sweep, two peaks situated at 1.36 V and 2.25 V exist which derive from the oxidation of Ni0 and Co0 into NiO and Co3O4. There is no obvious change in the position of voltage in the subsequent anodic sweeps. The thorough storage mechanisms of NiCo2O4 have been studied in many previous researches [10, 28, 45]. NiCo2O4 is firstly reduced to metallic Ni and Co, which is an irreversible discharge process, corresponding to the first cathodic sweep of the CV plots. It is reported that the metal particle Ni generated in the irreversible process could yield a catalysis effect on the decomposition of Li2O, reducing the initial irreversible capacity and improving its electrochemical properties [39]. And the subsequent electrochemical redox processes are conducted between metals of Ni and Co and their corresponding oxides. To better understand the reaction mechanism, the charge-discharge profiles were measured at the voltage range between 0.01-3 V at the current density of 200 mA g-1 with lithium metal as counter electrode. As Fig. 8 (b) illustrates, there are two plateaus situated at 1.1-1.3 V and 0.50.9 V in the first lithiation process, along with two plateaus located at 1.2-1.6 V and 2.2-2.5 V in the homologous anodic process, which match well with the CV plots. We can also observe that the initial discharge specific capacity is as high as 1408 mAh g-1, while the corresponding charge specific capacity is 1005 mAh g-1, with the initial Coulombic efficiency of 71.38%. That may derive from the irreversibility of the reaction between NiCo2O4 and Li as well as the formation of SEI film and some Li2O that could be not decomposed [45]. It is also reported that the initially
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extra discharge capacity over the theoretical specific capacity (834.75 mAh g-1) of the composite of NiCo2O4 and CNFs is on account of the formation of SEI film [60]. After the first three cycles, the coulombic efficiency retains above 98%, exhibiting a good reversibility of the redox process. Fig. 8 (c) shows the cycling property of the as-synthesized TNN/CNFs, the control sample of NNS and pure CNFs. The control sample suggests dramatic decay of capacity which reduces to 277.1 mAh g-1 after 50 cycles, which is far below the NiCo2O4 grown on CNFs. The pure CNFs has the specific capacity of 211.9 mAh g-1 at the same current density of 200 mAh g-1. When cycled up to 250th, a discharge capacity of the TNN/CNFs composite up to 1033.6 mAh g-1 is achieved, revealing the well cyclic stability of the TNN/CNFs composite at the relative low rate (200 mA g-1). The extra capacity over the theoretical specific capacity can be partly ascribed to the mechanism of interfacial storage derived from the unique 3D hybrid structure, which provides large surface area to storage more Li [45]. Meanwhile, the cycling performance is slightly increasing as seen from Fig. 8 (c). According to the viewpoint in many reports [61-64], the electrolyte solution (EC, DMC and EMC in this case) decomposed to form a polymeric gel-like layer, which reversibly grew and store Li+, leading to the slight increase of the cyclic performance. The rate performance of the both materials are also studied at various current densities from 200 mA g-1 to 2000 mA g-1 as shown in Fig. 8 (d). As the current density increases, a constant decay of specific capacity occurs on the both materials. But when recovered back to 200 mA g-1, the TNN/CNFs composite still delivers an average discharge specific capacity above 800 mAh g-1 as displayed.
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Based above description and discussion, the improvement of the specific capacity should be mainly credited to the combination with carbon nanofibers and the unique 3D hybrid structure. On the one hand, the crisscross framework of CNFs provides a strong support for NiCo2O4 to grow into 3D structure, fast the migration of electrons and avoid the agglomeration of NiCo 2O4. Meanwhile, the 3D hierarchically structure offers high specific area which increases the contact area between electrode and electrolyte, shortening the transport pathway for lithium ions [65, 66]. On the other hand, the existence of interspace among TNN/CNFs composite could accommodate more volume change and cushion the strain during the charge-discharge process, reducing the irreversible capacity with the assist of metallic Ni particles generated during chargedischarge process.
The different discharge specific capacities in literatures about NiCo2O4
reported before are listed in Table 1. As we can see, the TNN/CNFs composite exhibits a relatively better specific capacity and cyclic property. 4. Conclusions In conclusion, we proposed a facile route including hydrothermal and final calcination to fabricate TNN/CNFs composite which tufted NiCo2O4 nanoneedles grew on interconnected carbon nanofibers, forming a unique 3D hybrid structure. When evaluated as anode active material for LIBs, TNN/CNFs composite achieved a discharge specific capacity up to 1033.6 mAh g-1 even after 250 cycles, far surpassing the single NiCo2O4 nanoneedles sphere. The excellent electrochemical property such as superior cycling property at the current density of 200
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mA g-1 and relative high specific capacity is mainly ascribed to the characteristic structure and the combination with CNFs. Acknowledgements This research work has been financially supported by the National Natural Science Foundation of China (21373081 and 21303047), Hunan Provincial Natural Science Foundation of China (14JJ3067), Program for Shenghua Overseas Talents from Central South University, Project of Innovation-driven Plan in Central South University and Self-established Project of State Key Laboratory of Powder Metallurgy. References [1] F.-X. Ma, L. Yu, C.-Y. Xu, X.W. Lou, Self-supported formation of hierarchical NiCo2O4 tetragonal microtubes with enhanced electrochemical properties, Energy Environ. Sci., 9 (2016) 862-866. [2] A.D. Roberts, X. Li, H. Zhang, Porous carbon spheres and monoliths: morphology control, pore size tuning and their applications as Li-ion battery anode materials, Chem Soc Rev, 43 (2014) 4341-4356. [3] S. Chaudhari, D. Bhattacharjya, J.-S. Yu, Facile Synthesis of Hexagonal NiCo2O4 Nanoplates as High-Performance Anode Material for Li-Ion Batteries, Bulletin of the Korean Chemical Society, 36 (2015) 2330-2336. [4] G. Huang, L. Zhang, F. Zhang, L. Wang, Metal-organic framework derived Fe2O3@NiCo2O4 porous nanocages as anode materials for Li-ion batteries, Nanoscale, 6 (2014) 5509-5515. [5] L. Li, S. Peng, Y. Cheah, P. Teh, J. Wang, G. Wee, Y. Ko, C. Wong, M. Srinivasan, Electrospun porous NiCo2O4 nanotubes as advanced electrodes for electrochemical capacitors, Chemistry, 19 (2013) 5892-5898. [6] D.S. Sun, Y.H. Li, Z.Y. Wang, X.P. Cheng, S. Jaffer, Y.F. Zhang, Understanding the mechanism of hydrogenated NiCo2O4 nanograss supported on Ni foam for enhanced-
20
performance supercapacitors, J. Mater. Chem. A, 4 (2016) 5198-5204. [7] Y. Tang, Y. Zhang, W. Li, B. Ma, X. Chen, Rational material design for ultrafast rechargeable lithium-ion batteries, Chemical Society reviews, 44 (2015) 5926-5940. [8] K.X. Wang, X.H. Li, J.S. Chen, Surface and interface engineering of electrode materials for lithium-ion batteries, Advanced materials, 27 (2015) 527-545. [9] J. Liu, C. Liu, Y. Wan, W. Liu, Z. Ma, S. Ji, J. Wang, Y. Zhou, P. Hodgson, Y. Li, Facile synthesis of NiCo2O4 nanorod arrays on Cu conductive substrates as superior anode materials for high-rate Li-ion batteries, CrystEngComm, 15 (2013) 1578. [10] G. Gao, H.B. Wu, X.W.D. Lou, Citrate-Assisted Growth of NiCo2O4 Nanosheets on Reduced Graphene Oxide for Highly Reversible Lithium Storage, Advanced Energy Materials, 4 (2014) 1400422. [11] Guobo Zeng, Nan Shi, Michael Hess, Xi Chen, Wei Cheng, Tongxiang Fan, M. Niederberger, A General Method of Fabricating Flexible Spinel-Type Oxide/Reduced Graphene Oxide Nanocomposite Aerogels as Advanced Anodes for Lithium-Ion Batteries, ACS Nano, 9 (2015) 4227-4235. [12] X. Wang, L. Qiao, X. Sun, X. Li, D. Hu, Q. Zhang, D. He, Mesoporous NiO nanosheet networks as high performance anodes for Li ion batteries, Journal of Materials Chemistry A, 1 (2013) 4173-4176. [13] K.S. Lee, S. Park, W. Lee, Y.S. Yoon, Hollow Nanobarrels of alpha-Fe2O3 on Reduced Graphene Oxide as High-Performance Anode for Lithium-Ion Batteries, ACS applied materials & interfaces, 8 (2016) 2027-2034. [14] C. Yan, G. Chen, X. Zhou, J. Sun, C. Lv, Template-Based Engineering of Carbon-Doped Co3O4 Hollow Nanofibers as Anode Materials for Lithium-Ion Batteries, Advanced Functional Materials, 26 (2016) 1428-1436. [15] D. Wang, Y. Yu, H. He, J. Wang, W. Zhou, H.D. Abruña, Template-Free Synthesis of Hollow-Structured Co3O4 Nanoparticles as High-Performance Anodes for Lithium-Ion Batteries, ACS Nano, 9 (2015) 1775-1781. [16] D. Gu, W. Li, F. Wang, H. Bongard, B. Spliethoff, W. Schmidt, C. Weidenthaler, Y. Xia, D. Zhao, F. Schuth, Controllable Synthesis of Mesoporous Peapod-like Co3O4@Carbon Nanotube Arrays for High-Performance Lithium-Ion Batteries, Angewandte Chemie, 54 (2015) 7060-7064.
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[17] J. Li, S. Xiong, Y. Liu, Z. Ju, Y. Qian, High Electrochemical Performance of Monodisperse NiCo2O4Mesoporous Microspheres as an Anode Material for Li-Ion Batteries, ACS Applied Materials & Interfaces, 5 (2013) 981-988. [18] Jun Du, Gang Zhou, Haiming Zhang, Chao Cheng, Jianmin Ma, Weifeng Wei, Libao Chen, T. Wang, Ultrathin Porous NiCo2O4 Nanosheet Arrays on Flexible Carbon Fabric for HighPerformance Supercapacitors, ACS Appl Mater Interfaces, 5 (2013) 7405-7409. [19] F. Zheng, D. Zhu, X. Shi, Q. Chen, Metal–organic framework-derived porous Mn1.8Fe1.2O4 nanocubes with an interconnected channel structure as high-performance anodes for lithium ion batteries, J. Mater. Chem. A, 3 (2015) 2815-2824. [20] J. Xu, S. Gu, L. Fan, P. Xu, B. Lu, Electrospun Lotus Root-like CoMoO4@Graphene Nanofibers as High-Performance Anode for Lithium Ion Batteries, Electrochimica Acta, 196 (2016) 125-130. [21] W. Kang, Y. Tang, W. Li, X. Yang, H. Xue, Q. Yang, C.S. Lee, High interfacial storage capability of porous NiMn2O4/C hierarchical tremella-like nanostructures as the lithium ion battery anode, Nanoscale, 7 (2015) 225-231. [22] C. Zhang, J.S. Yu, Morphology‐Tuned Synthesis of NiCo2O4‐Coated 3D Graphene Architectures Used as Binder ‐Free Electrodes for Lithium ‐ Ion Batteries, Chemistry– A European Journal, (2016). [23] Y. Lei, J. Li, Y. Wang, L. Gu, Y. Chang, H. Yuan, D. Xiao, Rapid microwave-assisted green synthesis of 3D hierarchical flower-shaped NiCo2O4 microsphere for high-performance supercapacitor, ACS applied materials & interfaces, 6 (2014) 1773-1780. [24] K.R. Reddy, M. Hassan, V.G. Gomes, Hybrid nanostructures based on titanium dioxide for enhanced photocatalysis, Applied Catalysis A: General, 489 (2015) 1-16. [25] R. Kakarla Raghava, G.G. Vincent, H. Mahbub, Carbon functionalized TiO 2 nanofibers for high efficiency photocatalysis, Materials Research Express, 1 (2014) 015012. [26] K.R. Reddy, B.C. Sin, K.S. Ryu, J. Noh, Y. Lee, In situ self-organization of carbon black– polyaniline composites from nanospheres to nanorods: Synthesis, morphology, structure and electrical conductivity, Synthetic Metals, 159 (2009) 1934-1939. [27] L. Peng, H. Zhang, L. Fang, Y. Bai, Y. Wang, Designed Functional Systems for HighPerformance Lithium-Ion Batteries Anode: From Solid to Hollow, and to Core-Shell NiCo2O4 Nanoparticles Encapsulated in Ultrathin Carbon Nanosheets, ACS applied materials & interfaces,
22
8 (2016) 4745-4753. [28] A.K. Mondal, H. Liu, Z.-F. Li, G. Wang, Multiwall carbon nanotube-nickel cobalt oxide hybrid structure as high performance electrodes for supercapacitors and lithium ion batteries, Electrochimica Acta, 190 (2016) 346-353. [29] Y. Chen, J. Zhu, B. Qu, B. Lu, Z. Xu, Graphene improving lithium-ion battery performance by construction of NiCo2O4/graphene hybrid nanosheet arrays, Nano Energy, 3 (2014) 88-94. [30] W. Liu, C. Lu, K. Liang, B.K. Tay, A High‐Performance Anode Material for Li‐Ion Batteries Based on a Vertically Aligned CNTs/NiCo2O4 Core/Shell Structure, Particle & Particle Systems Characterization, 31 (2014) 1151-1157. [31] M. Hassan, K.R. Reddy, E. Haque, S.N. Faisal, S. Ghasemi, A. I. Minett, V.G. Gomes, Hierarchical assembly of graphene/polyaniline nanostructures to synthesize free-standing supercapacitor electrode, Composites Science and Technology, 98 (2014) 1-8. [32] L. Fritea, A. Le Goff, J.-L. Putaux, M. Tertis, C. Cristea, R. Săndulescu, S. Cosnier, Design of a reduced-graphene-oxide composite electrode from an electropolymerizable graphene aqueous dispersion using a cyclodextrin-pyrrole monomer. Application to dopamine biosensing, Electrochimica Acta, 178 (2015) 108-112. [33] M. Hassan, K.R. Reddy, E. Haque, A.I. Minett, V.G. Gomes, High-yield aqueous phase exfoliation of graphene for facile nanocomposite synthesis via emulsion polymerization, Journal of colloid and interface science, 410 (2013) 43-51. [34] S.H. Choi, D.H. Kim, A.V. Raghu, K.R. Reddy, H.-I. Lee, K.S. Yoon, H.M. Jeong, B.K. Kim, Properties of Graphene/Waterborne Polyurethane Nanocomposites Cast from Colloidal Dispersion Mixtures, Journal of Macromolecular Science, Part B, 51 (2012) 197-207. [35] Y.R. Lee, S.C. Kim, H.-i. Lee, H.M. Jeong, A.V. Raghu, K.R. Reddy, B.K. Kim, Graphite oxides as effective fire retardants of epoxy resin, Macromolecular Research, 19 (2011) 66-71. [36] K.R. Reddy, H.M. Jeong, Y. Lee, A.V. Raghu, Synthesis of MWCNTs-Core/Thiophene Polymer-Sheath Composite Nanocables by a Cationic Surfactant-Assisted Chemical Oxidative Polymerization and Their Structural Properties, Journal of Polymer Science Part A: Polymer Chemistry, 48 (2010) 1477-1484. [37] K.R. Reddy, B.C. Sin, C.H. Yoo, D. Sohn, Y. Lee, Coating of multiwalled carbon nanotubes with polymer nanospheres through microemulsion polymerization, Journal of colloid and interface science, 340 (2009) 160-165.
23
[38] K.R. Reddy, B.C. Sin, C.H. Yoo, W. Park, K.S. Ryu, J.-S. Lee, D. Sohn, Y. Lee, A new onestep synthesis method for coating multi-walled carbon nanotubes with cuprous oxide nanoparticles, Scripta Materialia, 58 (2008) 1010-1013. [39] Y. Wei, F. Yan, X. Tang, Y. Luo, M. Zhang, W. Wei, L. Chen, Solvent-Controlled Synthesis of NiO-CoO/Carbon Fiber Nanobrushes with Different Densities and Their Excellent Properties for Lithium Ion Storage, ACS applied materials & interfaces, 7 (2015) 21703-21711. [40] A. Banerjee, S. Bhatnagar, K.K. Upadhyay, P. Yadav, S. Ogale, Hollow Co(0.85)Se nanowire array on carbon fiber paper for high rate pseudocapacitor, ACS applied materials & interfaces, 6 (2014) 18844-18852. [41] L. Huang, D. Chen, Y. Ding, S. Feng, Z.L. Wang, M. Liu, Nickel-cobalt hydroxide nanosheets coated on NiCo2O4 nanowires grown on carbon fiber paper for high-performance pseudocapacitors, Nano letters, 13 (2013) 3135-3139. [42] T. Choudhury, S. Saied, J. Sullivan, A. Abbot, Reduction of oxides of iron, cobalt, titanium and niobium by low-energy ion bombardment, Journal of Physics D: Applied Physics, 22 (1989) 1185. [43] Y. Liu, J. Xu, H. Li, S. Cai, H. Hu, C. Fang, L. Shi, D. Zhang, Rational design and in situ fabrication of MnO2@NiCo2O4 nanowire arrays on Ni foam as high-performance monolith deNOx catalysts, J. Mater. Chem. A, 3 (2015) 11543-11553. [44] F. Zheng, D. Zhu, Q. Chen, Facile fabrication of porous NixCo3-xO4 nanosheets with enhanced electrochemical performance as anode materials for Li-ion batteries, ACS applied materials & interfaces, 6 (2014) 9256-9264. [45] J. Li, S. Xiong, Y. Liu, Z. Ju, Y. Qian, High electrochemical performance of monodisperse NiCo2O4 mesoporous microspheres as an anode material for Li-ion batteries, ACS applied materials & interfaces, 5 (2013) 981-988. [46] S. Xu, L. Lu, Q. Zhang, H. Zheng, L. Liu, S. Yin, S. Wang, G. Li, C. Feng, Morphologycontrolled synthesis and electrochemical performance of NiCo2O4 as anode material in lithiumion battery application, Journal of Nanoparticle Research, 17 (2015) 1-11. [47] X. Li, L. Jiang, C. Zhou, J. Liu, H. Zeng, Integrating large specific surface area and high conductivity in hydrogenated NiCo2O4 double-shell hollow spheres to improve supercapacitors, NPG Asia Materials, 7 (2015) e165. [48] J. Wang, Z. Wang, B. Huang, Y. Ma, Y. Liu, X. Qin, X. Zhang, Y. Dai, Oxygen vacancy
24
induced band-gap narrowing and enhanced visible light photocatalytic activity of ZnO, ACS applied materials & interfaces, 4 (2012) 4024-4030. [49] G.M. Burke, D.E. Wurster, M.J. Berg, P. Weng-Pedersen, D.D. Schottelius, Surface characterization of activated charcoal by X-ray photoelectron spectroscopy (XPS): correlation with phenobarbital adsorption data, Pharmaceutical Research, 9 (1992) 126-130. [50] Y.-C. Chiang, C.-C. Liang, C.-P. Chung, Characterization of Platinum Nanoparticles Deposited on Functionalized Graphene Sheets, Materials, 8 (2015) 5318. [51] Y. Chen, J. Xu, Y. Yang, Y. Zhao, W. Yang, X. He, S. Li, C. Jia, Enhanced electrochemical performance of laser scribed graphene films decorated with manganese dioxide nanoparticles, Journal of Materials Science: Materials in Electronics, 27 (2016) 2564-2573. [52] C.Z.J.-S. Yu, Morphology-Tuned Synthesis of NiCo2O4 -Coated 3D Graphene Architectures Used as Binder-Free Electrodes for Lithium-Ion Batteries, Chem.Eur.J, 22 ( 2016) 4422–4430. [53] N. Jabeen, Q. Xia, M. Yang, H. Xia, Unique Core-Shell Nanorod Arrays with Polyaniline Deposited into Mesoporous NiCo2O4 Support for High-Performance Supercapacitor Electrodes, ACS applied materials & interfaces, 8 (2016) 6093-6100. [54] W. Li, L. Xin, X. Xu, Q. Liu, M. Zhang, S. Ding, M. Zhao, X. Lou, Facile synthesis of three-dimensional structured carbon fiber-NiCo2O4-Ni(OH)2 high-performance electrode for pseudocapacitors, Scientific reports, 5 (2015) 9277. [55] Y. Tang, Y. Zhang, J. Deng, J. Wei, H. Le Tam, B.K. Chandran, Z. Dong, Z. Chen, X. Chen, Mechanical force-driven growth of elongated bending TiO2 -based nanotubular materials for ultrafast rechargeable lithium ion batteries, Advanced materials, 26 (2014) 6111-6118. [56] Y. NuLi, P. Zhang, Z. Guo, H. Liu, J. Yang, NiCo2O4/C Nanocomposite as a Highly Reversible Anode Material for Lithium-Ion Batteries, Electrochemical and Solid-State Letters, 11 (2008) A64. [57] L. Li, Y. Cheah, Y. Ko, P. Teh, G. Wee, C. Wong, S. Peng, M. Srinivasan, The facile synthesis of hierarchical porous flower-like NiCo2O4 with superior lithium storage properties, Journal of Materials Chemistry A, 1 (2013) 10935. [58] J. Zhu, L. Chen, Z. Xu, B. Lu, Electrospinning preparation of ultra-long aligned nanofibers thin films for high performance fully flexible lithium-ion batteries, Nano Energy, 12 (2015) 339-
25
346. [59] J. Cheng, Y. Lu, K. Qiu, H. Yan, J. Xu, L. Han, X. Liu, J. Luo, J.K. Kim, Y. Luo, Hierarchical Core/Shell NiCo2O4@NiCo2O4 Nanocactus Arrays with Dual-functionalities for High Performance Supercapacitors and Li-ion Batteries, Scientific reports, 5 (2015) 12099. [60] A.K. Mondal, D. Su, S. Chen, X. Xie, G. Wang, Highly porous NiCo2O4 Nanoflakes and nanobelts as anode materials for lithium-ion batteries with excellent rate capability, ACS applied materials & interfaces, 6 (2014) 14827-14835. [61] X. Wang, L. Qiao, X. Sun, X. Li, D. Hu, Q. Zhang, D. He, Mesoporous NiO nanosheet networks as high performance anodes for Li ion batteries, Journal of Materials Chemistry A, 1 (2013) 4173. [62] Y. Shi, B. Guo, S.A. Corr, Q. Shi, Y.-S. Hu, K.R. Heier, L. Chen, R. Seshadri, G.D. Stucky, Ordered Mesoporous Metallic MoO2 Materials with Highly Reversible Lithium Storage Capacity, Nano Lett., 9 (2009) 4215-4220. [63] Y. Feng, R. Zou, D. Xia, L. Liu, X. Wang, Tailoring CoO–ZnO nanorod and nanotube arrays for Li-ion battery anode materials, Journal of Materials Chemistry A, 1 (2013) 9654. [64] F.M. Courtel, H. Duncan, Y. Abu-Lebdeh, I.J. Davidson, High capacity anode materials for Li-ion batteries based on spinel metal oxides AMn2O4 (A = Co, Ni, and Zn), Journal of Materials Chemistry, 21 (2011) 10206. [65] Y. Tang, X. Rui, Y. Zhang, T.M. Lim, Z. Dong, H.H. Hng, X. Chen, Q. Yan, Z. Chen, Vanadium pentoxide cathode materials for high-performance lithium-ion batteries enabled by a hierarchical nanoflower structure via an electrochemical process, J. Mater. Chem. A, 1 (2013) 82-88. [66] Y. Tang, Z. Jiang, G. Xing, A. Li, P.D. Kanhere, Y. Zhang, T.C. Sum, S. Li, X. Chen, Z. Dong, Efficient Ag@ AgCl Cubic Cage Photocatalysts Profit from Ultrafast Plasmon‐Induced Electron Transfer Processes, Advanced Functional Materials, 23 (2013) 2932-2940. [67] G. Gao, H.B. Wu, S. Ding, X.W. Lou, Preparation of carbon-coated NiCo2O4 @SnO2 hetero-nanostructures and their reversible lithium storage properties, Small, 11 (2015) 432-436. [68] A.K. Mondal, D. Su, S. Chen, K. Kretschmer, X. Xie, H.J. Ahn, G. Wang, A microwave synthesis of mesoporous NiCo2O4 nanosheets as electrode materials for lithium-ion batteries and supercapacitors, Chemphyschem, 16 (2015) 169-175.
26
[69] Y. Zhu, C. Cao, A Simple Synthesis of Two-Dimensional Ultrathin Nickel Cobaltite Nanosheets for Electrochemical Lithium Storage, Electrochimica Acta, 176 (2015) 141-148.
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Figure captions
Fig. 1. The illustration of three preparation procedures of TNN/CNFs.
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Fig. 2. (a) Representative XRD patterns for TNN/CNFs and NNS and XPS spectra (b) survey spectrum, (c) Co 2p, (d) Ni 2p, (e) O 1s, and (f) C 1s for TNN/CNFs.
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Fig. 3. Raman spectrum of TNN/CNFs and NNS
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Fig. 4. SEM images of (a) CNFs (b), (c) TNN/CNFs, (d) NNS
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Fig. 5. TEM (a), HRTEM (b) and EDX-mapping of TNN/CNFs composite. The inset in (a) is SAED of the region marked by red rectangular frame. The inset in (b) is the corresponding FFT image.
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Fig. 6. Nitrogen adsorption-desorption isotherm of (a) TNN/CNFs (b) NNS and the inset is the matching pore size distribution.
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Fig. 7. TGA curve of (a) TNN/CNFs (b) NNS from room temperature to 800 ℃ at a ramping rate of 10 ℃ min-1 and XRD of the residues after TGA test (c) TNN/CNFs after TGA (d) NNS after TGA
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Accepted Manuscript Title: Tufted NiCo2 O4 Nanoneedles Grown on Carbon Nanofibers with advanced electrochemical property for Lithium Ion Batteries Author: Gang Zhou Chen Wu Yuehua Wei Chengchao Li Qingwang Lian Chao Cui Weifeng Wei Libao Chen PII: DOI: Reference:
S0013-4686(16)32545-2 http://dx.doi.org/doi:10.1016/j.electacta.2016.12.001 EA 28471
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
18-8-2016 18-10-2016 1-12-2016
Please cite this article as: Gang Zhou, Chen Wu, Yuehua Wei, Chengchao Li, Qingwang Lian, Chao Cui, Weifeng Wei, Libao Chen, Tufted NiCo2O4 Nanoneedles Grown on Carbon Nanofibers with advanced electrochemical property for Lithium Ion Batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.12.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Table 1. Comparison of specific capacity of NiCo2O4 as anodes for LIBs Table 1. Current
Reversible Cycle
Material
density
capacity
Ref
number (mA g-1)
(mAh g-1)
NiCo2O4 100
50
729
[46]
40
50
715.8
[56]
100
50
765
[67]
NiCo2O4 nanosheets
100
50
767
[68]
NiCo2O4 nanosheets
200
100
804.8
[69]
NiCo2O4@ NiCo2O4
120
100
830
[59]
60
50
918
[3]
100
60
939
[57]
200
250
1033.6
microspheres NiCo2O4/C nanocomposite NiCo2O4@SnO2@C HSs
Hexagonal NiCo2O4 nanoplates Porous
flower-like
NiCo2O4 TNN/CNFs composite
this work
36