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• Since the lithium-ion batteries installed in devices are usually used as single functional modules in the systems such as smartphones in actual applications, resulting in an unnecessary increase of the whole weight. Thus, the SLIBs have been proposed [1-4], which consider the batteries as the power supply and part of the structure in the system. Besides, in recent years, the SLIBs have acquired much attention. For example, in an unmanned air vehicle, prismatic batteries were served as parts of the two wings, leading to a significant increase of flight time [5].

  As one of the anode materials in the SLIBs, carbon fiber (CF) has gained widely attention recently, which has excellent mechanical properties and good electrical conductivity [6]. It exhibits a higher tensile strength than that of other carbonaceous materials and possesses a much lighter weight, deciding its wide use in an electric device [7]. However, there are still some disadvantages in its application as a structural anode material, in which the low reversible capacity during fabricating process need to be resolved urgently. For example, most of the carbon fibers exhibit a reversible capacity about 100 mAh/g after 10 cycles at 100 mA/g, which cannot completely meet the increasing demands of anode materials [8]. Thus, it is necessary to introduce functional groups into the carbon fiber surface such as carbonyl groups to improve the surface activity so as to synthesize carbon fiber composites [9, 10].

  In recent years, metal-organic frameworks (MOFs) with the large surface area have been continuously attracted tremendous interest in research of energy storage materials [11] and ternary conversion anodes such as ZnCo₂O₄, have been synthesized via MOFs as a precursor. Meanwhile, the ZnCo₂O₄ also has attracted much attention as a promising anode material because of its high electrochemical capacities (theoretical capacity 1300~1400 mA h/g) [12, 13]. Thus, for structural anode materials, it is of great value to synthesize a composite of MOF-derived ZnCo₂O₄/C fixed on the carbon fiber, owing to the potential possibility that it may have significant improvement on the electrochemical properties of carbon fiber. Meanwhile, it is noticed that carbon fiber was usually considered as a totally conductive material or supporting material in most of the previous studies [14, 15]. Besides, Deng et al. [16] developed the coconut-like monocrystalline SnS/C nanospheres, exhibiting quite high reversible capacities. Wang et al. [17] synthesized MoS₂/C on carbon cloth, achieving a good electrochemical performance for flexible batteries. Deng et al. [18] fabricated the macroporous MoS₂@C nanostructure on carbon cloth, exhibiting a high electrochemical performance and excellent cycling stability for lithium-ion batteries. Thus, the unique structural feature can improve the electrochemical performance of anode materials. So, it is of great value to synthesize ZnCo₂O₄/C@CF composite with a unique nanostructure so as to have a potential application on SLIBS.

  Herein, based on the above discussion, we designed and fabricated a MOF-derived ZnCo₂O₄/C@CF composite via a simple method and subsequent annealing treatment in this paper. Meanwhile, this structural anode material exhibited good electrochemical properties.

  The acidified carbon fiber was synthesized according to our previous work [10]. The coating of ZnCo₂O₄/C on the CF was sythensized by a facile method and subsequent annealing treatment under N₂ atmosphere. Briefly, the surfaces of acidified CFs serve as nucleation centers for the formation of MOFs and then annealing treatment under N₂ atmosphere. In a typical process, 0.583 g Co(NO₃)₂·6H₂O and 0.297 g Zn(NO₃)₂·6H₂O were dissolved into the 60 mL methanol to form a homogeneous solution. Then, 200 mg acidified CF was homogeneously soaked in the above solution, followed by vigorous agitation for 60 min. Subsequently, 0.984 g 2-methylimidazolate in 20 mL methanol solution was added to allow for heterogeneous nucleation of MOFs. After mild stirring for 24 h at room temperature, the CF was taken out and washed with methanol. Next, in a tube furnace, the ZnCo₂O₄/C@CF was obtained by annealing the as-synthesized sample at 400 ℃ for 3 h under a N₂ flow.

  The electrochemical measurements were carried out by 2032 coin-type cell sassembled in an Ar-filled glovebox. The active material, acetylene black and polyvinylidene difluoride were mixed in N-methyl-2-pyrrolidine solvent at a weight ratio of 8:1:1, and then they were pressed onto a copper foil and dried in a vacuum oven at 120 ℃ for 12 h. The mass of electrode in each cell is about 1.20 mg. The galvanostatic charge-discharge tests of fibers were carried out at a voltage range from 0.01 V to 3.0 V (versus Li/Li⁺) and a current densities of 50 mA/g on a LAND CT2001 A at room temperature. The rate performances were also evaluated on a LAND CT2001 A. The cyclic voltammetry (CV) curves were recorded on CHI620D electrochemical station at a scan rate of 0.5 mV/s from 0.01 V to 3.0 V.

  The synthesis strategy of ZnCo₂O₄/C@CF is schematically depicted as shown in Scheme 1. After acid treatment, the surface of carbon fiber is functionalized with the carboxyl and other acid functionalization groups. Subsequently, when the CF is added, some Zn²⁺ and Co²⁺ ions can be easily absorbed onto CF due to the electrostatic interactions. Subsequently, nucleation and growth of MOF takes place on the surface of CF with the introduce of 2-methylimidazole, leading to the formation of MOF deposited on the CF surface. Thus after annealing the MOF@CF at 400 ℃ for 3 h under N₂, resulting in the formation of ZnCo₂O₄/C@CF.

  Scheme 1

  

  Scheme 1.  Schematic illustration of the formation of the ZnCo₂O₄/C@CF structure.

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  Figs. 1a and b display the typical SEM images of CF, it is observed that the surface morphology of carbon fiber is filled with grooves. Figs. 1c and d show the typical SEM image and magnified one of MOF@CF, respectively. It is observed that the MOF coating in Fig. 1c is uniformly wrapped on the surface of CF. From the low magnification image (Fig. 1e) and the high magnification image (Fig. 1f), it is noticed that the fiber is uniformly coated with the ZnCo₂O₄/C after annealing at N₂ atmosphere.

  Figure 1

  

  Figure 1.  SEM images (a and b) of acidified CF, (c and d) MOF@CF, (e and f) the ZnCo₂O₄/C@CF after annealing treatment.

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  The elemental mappings (EDS) have been carried out to determine the Zn, Co, O and C distribution on the surface of CF. From Figs. 2a-e, the Zn, Co, O and C elements on the surface of CF match well with the SEM image, indicating that the ZnCo₂O₄ and C layers are uniformly coated on the surface of CF. As seen in the low magnification TEM image (Figs. 2f and g), it can further confirm the porous structure of ZnCo₂O₄/C coating, which consists of interconnected nanoparticles with an average size of 25 nm. Interestingly, a carbon layer is observed, which is derived from the pyrolysis of the N-containing organic ligand 2-methylimidazole. The lattice fringes are clearly observed in the HRTEM image (Fig. 2h) with the d-spacing of 0.24 nm corresponding to the (311) interplanar spacing of ZnCo₂O₄. Moreover, the corresponding fast Fourier transforms (FFT) of the lattice image shown in the inset of Fig. 2h confirms that there exists a large number of the polycrystalline ZnCo₂O₄ in the coating.

  Figure 2

  

  Figure 2.  SEM image and EDS mappings (a-e) of ZnCo₂O₄/C@CF, (f and g) TEM images of the ZnCo₂O₄/C coating; (h) HRTEM image of the ZnCo₂O₄/C coating. The inset shows the corresponding FFT pattern image.

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  Thermogravimetry (TG) analysis is used to acquire the actual contents of ZnCo₂O₄ in the CF composite. The TG curves in Fig. S1a (Supporting information) exhibits the thermal behavior of CF and ZnCo₂O₄/C @CF, respectively. It can be observed that the weight of ZnCo₂O₄/C@CF has lost a little below 400 ℃, which may be ascribed to the oxidation of carbon layers in the ZnCo₂O₄/C coatings. Obviously, when the temperature is higher than 600 ℃, the weight declines which is related to the oxidation of CF and the whole weight is almost lost when it is up to 900 ℃. Since the fact that ZnCo₂O₄ is stable upon calcination in air, the proportion of ZnCo₂O₄ is measured to be almost 15.1 wt% based on its TGA curve. The XRD pattern of ZnCo₂O₄/C@CF can be seen in Fig. S1b (Supporting information). It is noticed that all the XRD pattern show broad peaks at about 23°, indicating graphitized carbon in the fiber, while other characteristic peaks match well with those of ZnCo₂O₄ (JCPDS No. 23-1390).

  Fig. 3a presents the CV curves of the ZnCo₂O₄/C@CF anode performed at a scan rate of 0.5 mV/s from 0.0 V to 3.0 V. In the first cathodic scanning, two peaks at about 0.6 V are irreversible reduction peaks, which are attributed to the formation of the solid electrolyte interphase (SEI) layer and the reduction of ZnCo₂O₄ into Zn and Co. A broad cathodic peak between 0.0 V and 0.4 V is due to the formation of Li_XC₆. Meanwhile, a corresponding anodic peak at about 0.4 V is ascribed to the deinsertion of Li ion from carbon networks in carbon materials and two broad anodic peaks at around 1.7 V and 2.2 V are related to the oxidation of Zn and Co. In the following two cycles, the CV curves were mostly overlapped, indicating the good stability of capacity for these fiber anodes. Thus, the main electrochemical reactions could be discribed as follows:

  Figure 3

  

  Figure 3.  (a) The CV curves of the first three cycles of the ZnCo₂O₄/C@CF anode at a scan rate of 0.5 mV/s. (b) The galvanostatic discharge-charge profiles of ZnCo₂O₄/C@CF anode at a current density of 50 mA/g. (c) The cycling performance of CF and ZnCo₂O₄/C@CF anodes. (d) Rate capability of the ZnCo₂O₄/C@CF anode. (e and f) The morphology of the ZnCo₂O₄/C@CF anode after cycles.

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  Fig. 3b depicts the galvanostatic discharge-charge profiles of the ZnCo₂O₄/C@CF anode at a current density of 50 mA/g. In the initial discharge curve, the potential had a rapid drop. And then it can be observed that a long plateau starting from about 1.0 V and stabilizing between 0.5 V and 0.7 V, which is mainly attributed to the reduction of ZnCo₂O₄ and the formation of the SEI layer. Subsequently, there is another long plateau from 0.01 V to 0.4 V, which is assigned to the formation of Li_xC₆. It is noticed that there is an obvious initial irreversible capacity loss, which is mostly assigned to the irreversible redox reaction of the anode such as the formation of SEI layer. In the following 2^nd and 3^rd cycles, the curves are almost overlapped, indicating that the results are consistent with those of the above CV curves.

  The cycling performance of CF and ZnCo₂O₄/C@CF anodes were tested at a current density of 50 mA/g, as displayed in Fig. 3c. It is observed that both anodes maintain very good reversible capacity after the second cycle. Besides, after 100 cycles, the ZnCo₂O₄/C@CF anode acquires a reversible capacity of 463 mA h/g with a 0.04% capacity fading per cycle and the CF anode exhbits a reversible capacity of 230 mA h/g. In addition, the initial discharge capacities of CF and ZnCo₂O₄/C@CF anodes are 623 and 733 mA h/g, respectively, while the initial corresponding charge capacities of CF decrease to 283 and 482 mA h/g. So the initial Coulombic efficiency (CE) of CF and ZnCo₂O₄/C@CF are 0.45 and 0.66, respectively, suggesting that the ZnCo₂O₄/C can enhance the initial CE of fibers.

  To further investigate the electrochemical performance of ZnCo₂O₄/C@CF anode, the rate capability of the anode was performed varying the current densities from 50 mA/g to 1000 mA/g. As shown in Fig. 3d, for the ZnCo₂O₄/C@CF anode, the reversible charge capacities of about 465, 416, 362 and 308 mA h/g are achieved at current densities of 50, 200, 500 and 1000 mA/g, respectively. While those of about 236, 196, 153 and 98 mA h/g for CF anode are achieved at current densities of 50, 200, 500 and 1000 mA/g, respectively. Besides, After 140 cycles, when current density is again recovered to the initial state, the reversible charge capacity of ZnCo₂O₄/C@CF increases to about 460 mA h/g. Meanwhile, the morphology of this anode materials after cycles has been observed as shown in Figs. 3e and f. It can be observed that although the large ZnCo₂O₄/C coatings have changed into smaller size after cycles, the CF still maintains a fibrous structure and the porous ZnCo₂O₄/C coatings also exists, respectively. Thus, under the conditions that carbon layers protect the porous structure of the ZnCo₂O₄/C coatings during cycles, perhaps like the similar structure reported in the previous literature [16-20], the micro-/nanostructure of this material may change to be much more suitable for the lithiation/delithiation process with the steady increase of the current density.

  Based on the above analysis, ZnCo₂O₄/C@CF has been repeatedly verified. Thus, the related mechanism can be proposed. The possible electrons transport, Li ions diffusion process are as follows. The Li ions reach to carbon fibers, and absorb on the carbon layers or insert into the carbon network structure. Then, the carbon layers deliver the electrons, and the Li ions continue to insert into and store in the carbon network structure. The carbon layers and carbon fibers not only act as the key intermediary of the Li ions diffusion and providing electrons transport, but also a kind of lithium storage materials. In addition, the enhanced stability and high capacity may be assigned to the unique structure of ZnCo₂O₄/C@CF.

  In summary, a novel composite formed by ZnCo₂O₄/C uniformly wrapped carbon fiber within multichannel conductive networks constructed by the carbon layers and carbon fiber has been acquired via a facile and new method. Compared with recently reported CF-based structural anode materials, the ZnCo₂O₄/C@CF in this work exhibited considerably enhanced capacity, excellent cycling stability. Besides, after 100 cycles, the ZnCo₂O₄/C@CF can exhibit a high reversible capacity of 463 mA h/g at 50 mA/g, corresponding to a 0.04% capacity fading per cycle. Besides, surface engineering such as MOF derived ternary metal oxides can open up new possibilities for the design and fabrication of high capacity structural anodes that do not suffer from huge volume expansion.

  Appendix A. Supplementary data

  Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2018.06.024.

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  Scheme 1  Schematic illustration of the formation of the ZnCo₂O₄/C@CF structure.

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  Figure 1  SEM images (a and b) of acidified CF, (c and d) MOF@CF, (e and f) the ZnCo₂O₄/C@CF after annealing treatment.

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  Figure 2  SEM image and EDS mappings (a-e) of ZnCo₂O₄/C@CF, (f and g) TEM images of the ZnCo₂O₄/C coating; (h) HRTEM image of the ZnCo₂O₄/C coating. The inset shows the corresponding FFT pattern image.

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  Figure 3  (a) The CV curves of the first three cycles of the ZnCo₂O₄/C@CF anode at a scan rate of 0.5 mV/s. (b) The galvanostatic discharge-charge profiles of ZnCo₂O₄/C@CF anode at a current density of 50 mA/g. (c) The cycling performance of CF and ZnCo₂O₄/C@CF anodes. (d) Rate capability of the ZnCo₂O₄/C@CF anode. (e and f) The morphology of the ZnCo₂O₄/C@CF anode after cycles.

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