Unique three-dimensional Co3O4@N-CNFs derived from ZIFs and bacterial cellulose as advanced anode for sodium-ion batteries

Applied Surface Science 508 (2020) 145295

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Unique three-dimensional Co3O4@N-CNFs derived from ZIFs and bacterial cellulose as advanced anode for sodium-ion batteries ⁎

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Ling Lia, Qiming Wanga, Xinyu Zhanga, Lide Fangb, , Xiaoting Lib, , Wenming Zhanga,

T

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a

National-Local Joint Engineering Laboratory of New Energy Photoelectric Devices, College of Physics Science and Technology, Hebei University, Baoding, Hebei 071002, China b National & Local Joint Engineering Research Center of Metrology Instrument and System, College of Quality and Technical Supervision, Hebei University, Baoding, Hebei 071002, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Metal-organic frameworks Three-dimensional Porous structure Sodium ion batteries Anode material

A novel strategy is exploited herein to fabricate zeolitic-imidazolate frameworks (ZIFs) and bacterial cellulosederived Co3O4 nanoparticles anchored on three-dimensional nitrogen-doped carbon nanofibers (Co3O4@NCNFs) network by simply growing ZIFs on bacterial cellulose and then applying a two-step annealing process. When utilized as an anode for sodium ion batteries, the as-fabricated Co3O4@N-CNFs electrode exhibits a high capacity (864.2 mA h g−1 for the first discharge and 450.2 mA h g−1 after 50 cycles at 100 mA g−1), excellent rate performance, and an ultralong cycling life stability (220.1 mA h g−1 at 1.6 A g−1 after 1000 cycles), which are better than those of other ZIF-derived cobalt-based oxide composites. The excellent capability can be ascribed to the synergistic effect between the Co3O4 polyhedrons and carbon nanofibers network, in which the unique interconnected nanostructures can decrease the ion diffusion route and enhance the conductivity and structural stability. These results indicate that the as-fabricated Co3O4@N-CNFs can be a promising anode material for high-performance sodium ion batteries. The present strategy for Co3O4@N-CNFs architectures can provide a promising approach for other metal-organic framework-derived materials for high-performance energy storage equipment.

1. Introduction With the rapid development of science and technology, the following influences, such as environmental contamination and continuously increasing demand for energy, promote the innovation of renewable energy technologies that are low cost and environmentally friendly [1–7]. As the current primary energy storage technology, lithium ion batteries (LIBs) dominate the portable devices market, and are one of the most reliable choices among various energy storage technologies. However, the high cost and resource limitations of LIBs have hindered their expanded application into large-scale energy devices. Although lithium-sulfur batteries have a low cost and high energy density, the poor cycle stability of lithium-sulfur batteries hinders their further development [8,9]. Sodium ion batteries (SIBs) have attracted enormous attention due to the widespread availability and low cost of sodium sources [10–15]. Unfortunately, the ionic radius of sodium is larger than that of lithium, and it is more difficult for sodium to intercalate into/be extracted from the electrode material. The appropriate electrode materials used in LIBs cannot support SIBs with a high

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capacity and structural stability. Thus, exploring uniquely and ingeniously designed anode materials with an excellent capacity and structural stability is critical for the development of advanced SIBs [16–18]. Metal–organic frameworks (MOFs) consist of metal ions/clusters and organic ligands and are characterized by the formation of dedicated porous structures; they have triggered significant interest and have gradually become a very promising type of materials [19–21]. In recent years, the use of MOFs as templates to synthesize electrode materials (e.g. metal oxide/carbons, metal sulfides/carbons, and porous carbons) has rapidly expanded [22–28]. Recently, zeolitic-imidazolate frameworks (ZIFs) have been reported, a species of MOFs that can be directly pyrolyzed to generate a transition metal energy storage material [29,30]. For instance, ZIF-8-derived N-doped porous carbon exhibited a capacity of 2150 mA h g−1 during the second cycle at 100 mA g−1 in LIBs [31]. Fan et al. synthesized an amorphous carbon nitride composite based on ZIF-8, which demonstrated good long-term cycling life as anodes in SIBs with a capacity retention of 175 mA h g−1 after 2000 cycles at 1.67 A g−1 [32]. Shen and co-workers fabricated a porous

Corresponding authors. E-mail addresses: [email protected] (L. Fang), [email protected] (X. Li), [email protected] (W. Zhang).

https://doi.org/10.1016/j.apsusc.2020.145295 Received 9 October 2019; Received in revised form 31 December 2019; Accepted 4 January 2020 Available online 07 January 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.

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Co., Ltd. Methanol and 2-Methylimidazole (2-MeIM) were supplied by Shanghai Macklin Biochemical Co., Ltd. Cobalt nitrate hexahydrate (Co (NO3)2·6H2O) and Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) were purchased from Sigma-Aldrich. All reagents were used as received without further purification.

starfish-like Co3O4@nitrogen-doped carbon based on a Co-MOF which displayed a remarkable discharge capacity of 795 mAh·g−1after 300 cycles at 0.5 A·g−1 as an anode for LIBs [33]. Despite the excellent performance of ZIF-derived materials, the carbon framework in the ZIFs aggregates during the pyrolysis process, which reduces the surface area and obstructs diffusion of the electrolyte into the anode material, hindering access to the active sites. Single structures in the ZIFs cause less favourable electronic conductivity. Although the individual granules are highly graphitized, the granules are isolated from each other (i.e., the granules are not interconnected to a high degree). Thus, the intervals between the granules severely obstruct the transport of electrons, thus reducing the electrical conductivity [34,35]. Bacterial cellulose (BC) is a three-dimensional network structure material, compared with other three-dimensional structure materials, such as graphene aerogel, has the advantages of low-cost, eco-friendly, and abundant natural resource [36,37]. BC possesses abundant hydrogen bonds and a high Young’s modulus, and it is a typical biomass material that can be fabricated at an industrial scale through microbial fermentation technology [38]. Notably, the three-dimensional nanofiber network persists during BC calcination and can provide anchoring sites to immobilize active nanoparticles [39]. In addition, the porous structure of the nanofiber network has the potential to inhibit the volume expansion of the electrode material during the Na insertion-extraction processes that occur during electrochemical cycling [40]. The novel structure and characteristics of BC inspired us to explore BC-derived electrode materials with resilient three-dimensional carbon nanofiber networks [41]. For instance, Wang and co-workers demonstrated that SnO2 nanoparticle-decorated pyrolyzed bacterial cellulose can exhibit an outstanding capacity of 600 mA h g−1 after 100 cycles at 100 mA g−1 in LIBs [40]. The excellent cycling performance of a bacterial cellulose/polypyrrole-derived composite anode material was reported by Zhang, and the composite electrode in SIBs showed a capacity of 240 mA h g−1 after 100 cycles at 100 mA g−1 [42]. However, there have been very limited studies on the direct preparation of ZIFs and bacterial cellulose derived composite materials applied to sodium ion batteries. Therefore, in recent studies, a unique three-dimensional porous electrode comprising Co3O4@N-CNFs was first synthesized by pyrolyzing and oxidizing of ZIF-8/ZIF-67 and BC composites. Composite materials with unique structures have many advantages. (i) The threedimensional interconnected carbon nanofibers (CNFs) derived from BC can prevent the aggregation of carbon frameworks and increase an electrically diffusion path, which provides the possibility for excellent electrochemical performance. (ii) In particular, the unique structure of Co3O4 nanoparticles is encapsulated in carbon nanofibers and a threedimensional porous carbon framework, which can restrain the volume expansion of the electrode material. (iii) The Co3O4@N-CNFs derived from ZIF-8 and ZIF-67 bimetallic composites have a synergistic effect, which can increase the specific surface area of composites, and a large specific surface area could increase the number of active sites for efficient insertion/extraction of Na+. As a result, when utilized as an advanced anode electrode material for SIBs, this novel Co3O4@N-CNFs electrode exhibits an excellent reversible capacity of 450.2 mA h g−1 (at 100 mA g−1 after 50 cycles) and exhibits a superior long cycling performance (220.1 mA h g−1 over 1000 cycles at 1.6 A g−1) which represents a leading position among recently reported ZIF-derived cobalt based oxides. This effective method inspired us to develop other metal oxides with unique structures and excellent electrochemical performance.

2.2. Synthesis of CNFs CNFs were fabricated with a general freeze-drying method followed by calcination. In brief, bacterial cellulose was immersed for 12 h in deionized water to remove impurities. Next, it was frozen with liquid nitrogen and freeze-dried for 24 h to produce BC aerogels. The obtained BC aerogels were annealed to 800 °C in a N2 atmosphere at a heating rate of 5 °C min−1 and retained at this temperature for 1 h to obtain the CNFs. 2.3. Synthesis of pure Co3O4 In a typical synthesis of pure Co3O4, Co(NO3)2·6H2O (2 mmol), Zn (NO3)2·6H2O (1 mmol) and 0.41 g of 2-methylimidazole (2-MeIM) were dispersed in a 15 mL methanol solution under magnetic stirring. After being stirred for 10 h at 20 °C, the precipitate was obtained, rinsed, centrifuged with methanol three times, and subsequently dried in an oven at 70 °C. In a typical procedure, to convert the ZIF-8/ZIF-67 to pure Co3O4, two steps were involved. The as-obtained precipitates were first placed in a tube furnace and annealed at 800 °C for 1 h under N2 atmosphere. Then, the as-prepared black precipitates were subjected to an oxidation treatment by heating them at 350 °C for 2 h in air to obtain pure Co3O4. 2.4. Synthesis of Co3O4@N-CNFs The Co3O4@N-CNFs we fabricated by an in-situ growth method with Zn/Co-ZIFs decorated bacterial cellulose. The as-obtained bacterial cellulose was immersed in a Co(NO3)2·6H2O (2 mmol) and Zn (NO3)2·6H2O (1 mmol) solution in 15 mL methanol for 12 h. After cleaning with methanol, the samples were transferred into a 15 mL methanol solution with 2-MeIM (0.41 g) for 8 h. Afterwards, the bacterial cellulose was treated with liquid nitrogen and freeze-dried for 24 h. After that, the samples were placed in a quartz boat and annealed at 800 °C for 1 h under nitrogen atmosphere followed by further thermal calcining in air at 350 °C for 2 h to obtain the Co3O4@N-CNFs. 2.5. Material characterization The morphology of the CNFs, pure Co3O4, and Co3O4@N-CNFs samples were investigated by field-emission scanning electron microscopy (FEI, Nova NanoSEM 450) at 15 kV and transmission electron microscopy (FEI, Tecnai G220). X-ray photoelectron spectroscopy (XPS) characterization was obtained on an Escalab 250Xi instrument. The phase composition and crystal structure clarification of three samples were characterized by XRD using a D8 ADVANCE (3 kW) with Cu Kα irradiation (λ = 1.5418 Å). The nitrogen adsorption and desorption isotherms of three samples were measured by using a NOVA 2200 system. 2.6. Electrochemical measurements The electrochemical measurements of the CNFs, pure Co3O4, and Co3O4@N-CNFs were obtained by assembling CR2032 coin-type cells in an argon filled glove box (O2 < 0.1 ppm and H2O < 0.1 ppm). For the sodium storage measurements, the synthetic active materials were combined with acetylene black and poly (vinyl difluoride) (PVDF) with a mass ratio of 7:2:1 to form slurry in N-methyl pyrrolidinone (NMP). The slurry was coated on a copper foil and used as the working electrode. Then, the electrodes were pressed, cut into 14 mm diameter disks

2. Experimental 2.1. Materials and reagents Bacterial cellulose (BC) was provided by Guilin Qi hong Technology 2

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Scheme 1. Schematic illustration for the prepared process of the porous Co3O4@N-CNFs composites.

and mass loading was controlled at 1.4 mg cm−2. A 1 M NaClO4 solution in propylene carbonate (PC) with 2 wt% fluoroethylene carbonate (FEC) was used as an electrolyte. A sodium foil was used as the counter/reference electrode, and a Whatman glass fiber filter (Whatman, GF/D) was used as the separator. The galvanostatic cycling profiles were recorded on a LANHE battery tester. The cyclic voltammetry (CV) and Nyquist plots were obtained using a CHI600E electrochemical analyser.

398.5 eV is usually consistent with pyridinic N. The peak observed at 400.2 eV is representative of pyrrolic N [44–48]. It has been reported that N-doping in carbon networks can enhance electron transport efficiency [17,49]. Fig. 1e shows an O 1s peak located at 529.6 eV, which can be assigned to the oxygen species from the Co3O4 crystal lattice [50]. The peak at 531.5 eV can be attributed to the hydroxyl groups (OH) [45]. For the high-resolution Co 2p XPS spectrum (Fig. 1f), two characteristic peaks at 780.1 eV (Co 2p3/2) and 795.3 eV (Co 2p1/2) with a 15.2 eV energy difference can be seen, and these are typically indicative of spinel Co3O4 [16,51]. In addition, the other two peaks can be attributed to the satellite peaks. The typical XPS spectrum confirmed the presence of Co3O4 in the composite sample and agrees with the XRD results [17]. The specific surface area and microstructures of the samples were characterized by Brunauer-Emmett-Teller (BET) gas-sorption measurements and the nitrogen adsorption-desorption isotherms are shown in Fig. S6a in the Supporting Information. The isotherm matches well with the type VI isotherm of the IUPAC classification, confirming the mesoporous structure of the Co3O4@N-CNFs, which promoted the reaction between the electrolyte and electrode. Additionally, the Co3O4@NCNFs isotherm types correspond to the H-3 loop according to the modern categorization of hysteresis loops [6,16,17]. The pore size distribution in the Co3O4@N-CNFs was measured by the Barrett-JoynerHalenda (BJH) method (Fig. S6b). Most of the data is centered at 3.82 nm, which indicates the presence of a mesoporous structure that enhances electrochemical performance. From the BET analysis, the specific surface area and pore volume of the Co3O4@N-CNFs were calculated to be 103 m2 g−1 and 0.334 cm3 g−1, respectively [52]. In addition, the BET measurement results of the pure Co3O4 and CNFs are also presented in Fig. S6. The single structure led to a relatively low surface area for the pure Co3O4 and CNFs. However, the specific surface area of the Co3O4@N-CNFs relatively increased, which means that the three-dimensional porous structure and the special configuration of the ZIF precursor improved the surface area. The large specific surface area and rich mesoporous structures can facilitate electrolyte infiltration and promote the participation of sodium ions in electrochemical reactions. The morphology of the obtained Co3O4@N-CNFs was determined by scanning and transmission electron microscopy (SEM and TEM). The SEM image (Fig. 2a and b) clearly shows the morphology of the nanofiber network structure derived from the bacterial cellulose. Its porous structure provided space for the Co3O4 particles and facilitated electron transport [38]. As shown in Fig. 2c, the bacterial cellulose was first activated and incubated in a methanol solution of Co2+/Zn2+ followed by treatment with 2-methylimidazole. The ZIF-8/ZIF-67 threedimensional network displayed a uniform polyhedral morphology with an average size of 3 µm. The as-obtained ZIF-coated bacterial cellulose was annealed at 800 °C for 2 h under N2 atmosphere (Fig. 2d). The Co particles inherited the contour of the ZIF-67 but shrank to 2 µm. At the same time, the organic ligands were transformed to a carbon framework that linked Co particles (Co-NC). Subsequently, the three-dimensional Co-NC was annealed at 350 °C in air for 2 h (Fig. 2e). The Co particles

3. Results and discussion The Co3O4@N-CNFs was fabricated via a two-step calcination process as exhibited in Scheme 1. The BC was dipped into the methanol solution comprising Co2+/Zn2+ and 2-methylimidazole for 6 h. The ZIF-8 and ZIF-67 were composed of Zn2+/Co2+ ions bonded with the organic ligands in 2-methylimidazole. The bacterial cellulose, as a stable support, supplied many hydroxyl groups to nucleate and anchor ZIF-67 particles. Afterwards, the as-obtained bacterial cellulose was treated with liquid nitrogen and freeze-dried for 24 h. During the calcination step, which occurred under a nitrogen atmosphere, the bacterial cellulose was converted into a three-dimensional CNF framework, and the cobalt ions transformed into small metallic Co particles. Subsequently, the ZIF-8 particles decomposed to form a porous structure during the oxidation process and Co particles were gradually oxidized to Co3O4 combined with CNFs to form a three-dimensional network (Scheme 1). Thus, Co3O4 nanoparticles embedded in a three-dimensional carbon network were successfully fabricated. The crystal structure and phase purity of the Co3O4@N-CNFs, pure Co3O4 and CNFs were determined by X-ray diffraction (XRD) (Fig. 1a). Diffraction peaks clearly appeared at approximately 31.3°, 36.9°, 44.8°, 59.4°, and 65.2° for the Co3O4@N-CNFs and pure Co3O4 samples, all the peaks matched well with the (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0) Co3O4 planes, respectively (JCPDS no. 42-1467). The results demonstrated the successful conversion of Co2+ in ZIF-67 to Co3O4. In addition, no diffraction peaks from other phases were detected, indicating the high purity of the samples [2]. Moreover, the CNFs had a wide C (1 1 0) diffraction peak at approximately 29.7°, which revealed that the carbon diffraction peak was derived from the pyrolyzed BC [17]. XPS also indicated the chemical composition details and surface component of the Co3O4@N-CNFs composite. Fig. 1b-f indicates the existence of Co, C, N, and O in the Co3O4@N-CNFs sample, demonstrating the facile formation of Co3O4 and simultaneous nitrogen doping [2]. Fig. 1b indicates that the C 1s, N 1s, O 1s, and Co 2p peaks occurred in the Co3O4@N-CNFs sample. The C 1s high-resolution spectrum (Fig. 1c) indicates the presence of carbon, which was absent in the XRD pattern because of its amorphous nature. The peak located at 284.8 eV corresponds to sp2-hybridized CeC [43], and the other two peaks at 285.6 and 286.7 eV can be assigned to CeO/CeN and C]C, respectively, which proves that doping of N into the CNFs occurred [16]. Fig. 1d shows the high-resolution N 1s XPS spectrum, and the N 1s spectrum contains two peaks at 398.5 eV and 400.2 eV. Significantly, the peak at 3

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Fig. 1. (a) XRD patterns of the Co3O4@N-CNFs, pure Co3O4, and CNFs samples. (b) XPS spectra for the Co3O4@N-CNFs; (c) C 1s, (d) N 1s, (e) O 1s, (f) Co 2p level spectra.

Fig. 2. SEM images of (a) bacterial cellulose and (b) an magnified SEM of (a); (c) SEM of ZIF-8/ZIF-67@BC frozen-dried by liquid nitrogen; (d) SEM of Co@NC; (e) SEM of Co3O4@N-CNFs; (f) the high-resolution TEM (HR-TEM) image of Co3O4@N-CNFs; (g, h) corresponding HR-TEM image of the red rectangular in (f); (i–m) The dark-field TEM image and the corresponding elemental mappings images of Co (fluorescent green), O (purple), N (green) and C (red), respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 4

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Fig. 3. (a) CV curves of Co3O4@N-CNFs for the initial three cycles at a scan rate of 0.1 mV s−1; (b) galvanostatic charge/discharge profiles of Co3O4@N-CNFs; (c) cycling performance of Co3O4@N-CNFs, pure Co3O4 and CNFs at 100 mA g−1; (d) rate capability of Co3O4@N-CNFs, pure Co3O4 and CNFs; (e) long-term cycling performance of all samples at a constant current density of 1.6 A g−1.

0.92 and 0.46 V and were ascribed to the intercalation of sodium. The first peak at 0.92 V was attributed to the reaction of Co3+ to Co2+ and the reaction to generate Na2O (Eq. (1)); the peak at approximately 0.46 V was ascribed to the reduction of Co2+ to metallic Co and the generation of a solid electrolyte interface (SEI) film [17,27]. Subsequently, the anodic peaks (0.87 and 1.24 V) were correlated with the Co3O4 oxidation conversion [21]. The second and third CV curves are very consistent with each other, revealing highly reversible Na ion insertion/extraction behaviour [54–57]. Fig. 3b shows the first, second and fifth galvanostatic charge/discharge voltage curves of the Co3O4@ N-CNFs electrode at a current density of 100 mA g−1. Obviously, the first discharge and charge specific capacities reached 860 and 532 mA h g−1, respectively, and the first coulombic efficiency reached 62%. Combined with the CV results, the initial capacity degradation can be attributed to the generation of an SEI film and the decomposition of the electrolyte. Afterwards, the coulombic efficiency rapidly rose to 93% during the 5th cycle [58]. An obvious voltage plateau in the discharge curve appeared at approximately 1.3 V followed by a reduction in the voltage to 0.19 V, which could be attributed to the transformation of Co3O4 to CoO, the reduction of metals and sodium insertion. Fig. 3c shows the cycling performances of the Co3O4@N-CNFs, pure Co3O4 and CNFs at 100 mA g−1. The discharge and charge capacities of

were steadily oxidized to Co3O4. The carbon framework also survived during this short anneal [2]. Furthermore, the TEM image (Fig. 2f) further reveals that the Co3O4 nanoparticles were uniformly connected by the carbon framework. The dark dots in the field are Co3O4 nanoparticles and the grey part is the interconnected nitrogen-doped carbon nanofibers (N-CNFs). The typical high-resolution TEM (HRTEM) images shown in Fig. 2g and h, the images show a unidirectional fringe pattern, indicating the highly crystalline nature of all samples. The lattice fringe spacing of 0.243 and 0.285 nm are in accord with the spacing of the (3 1 1) and (2 2 0) planes of Co3O4, respectively, which agrees with the XRD results presented in Fig. 1a [53]. The corresponding energy dispersive spectroscopy (EDS) mappings (Fig. 2i–m) display the distribution of elemental carbon (red), nitrogen (green), oxygen (purple) and cobalt (bright yellow), which were all uniformly distributed. The elements, including the CNFs, were mostly carbon and nitrogen, and the cobalt particles were concentrated in dense fiber centers, revealing that the Co3O4 nanoparticles and CNFs were connected. To characterize the electrochemical Na-storage capability of the Co3O4@N-CNFs as active electrodes, we assembled them into standard 2032 half-cells. Fig. 3a shows the first three CV profiles of the Co3O4@ N-CNFs sample at a scan rate of 0.1 mV s−1. During the first cathodic procedure, two obvious broad peaks were discovered at approximately 5

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Fig. 4. (a) Kinetics investigation of Co3O4@N-CNFs: (a) CV curves at different scan rates; (b) corresponding ln (peak current) versus ln (scan rate) plots at different redox states; (c) CV curve of the pseudocapacitive shown by the shadow region at a scan rate of 0.4 mV s−1; (d) bar chart showing the percent of pseudocapacitive contribution at different scan rates.

current densities of 387, 281, 225, 183, 157, 138 mA h g−1 and 184, 149, 134, 120, 109, 102 mA h g−1, respectively, under identical condition. Even when the current density was returned to 100 mA g−1, the Co3O4@N-CNFs manifested a considerable capacity of 412 mA h g−1. These results reveal that the stability of the three-dimensional network structure improved the rate capability of the electrodes [59]. To further investigate the cycling stability of the Co3O4@N-CNFs electrode, the long-term cycling life was investigated at a high current density of 1.6 A g−1 (Fig. 3e). Impressively, the Co3O4@N-CNFs presented excellent long-term stability with a reversible discharge capacity of 223 mA h g−1 over 500 cycles. In stark contrast, the capacities of the pure Co3O4 and CNFs were 81 mA h g−1 (over 200 cycles) and 60 mA h g−1 (over 200 cycles), respectively. Combined with the previous electrochemical performance analysis, the increased sodium storage capacity of the Co3O4@N-CNFs compared to that of the pure Co3O4 and CNFs can be ascribed to the unique three-dimensional network structure [49,55,56]. Combined with the above studies, the Co3O4@NCNFs sodium storage mechanism can be illustrated the following equations:

Fig. 5. EIS curves and the corresponding equivalent circuit (inset) of the electrodes.

the Co3O4@N-CNFs electrode were 864 and 577 mA h g−1 in the first cycle, respectively. In addition, the Co3O4@N-CNFs maintained an excellent reversible capacity of 329 mA h g−1 after 200 cycles, which was ascribed to their unique three-dimensional porous structure. Furthermore, a significant capacity decline can be obviously noted from the pure Co3O4 discharge curve, indicating poor capacity retention [2,6,7]. The cycling performances of the Co3O4@N-CNFs and CNFs electrodes were comparatively steady. The rate capability of the Co3O4@N-CNFs, pure Co3O4 and CNFs electrodes was determined to have current densities of 100–200, 400, 800, 1600, and 3200 mA g−1 followed by a return to 100 mA g−1. As illustrated in Fig. 3d, the Co3O4@N-CNFs demonstrated average discharge capacities of 520, 414, 333, 267, 234, and 205 mA h g−1. The pure Co3O4 and CNFs electrodes displayed

Co3 O4 +2Na++2e− ↔ Na2O+3CoO

(1)

3CoO+6Na++6e− ↔ 3Na2O+3Co

(2)

To research the comprehensive electrochemical performance of the Co3O4@N-CNFs electrode, CV measurements were carried out at various scan rates, which is an effective way to investigate the reaction kinetics of Co3O4@N-CNFs, as shown in Fig. 4a. One peak was observed during the reduction processes, marked as A, and the other marked as B appeared during the oxidation processes. The power-law relationship between the measured current (i) and the scan rate (ν) used to evaluate the electrochemical reaction is illustrated in Eqs. (3) and (4), where a and b are constants [4,14]. The value of b was between 0.5 (diffusioncontrolled behaviour) and 1.0 (capacitive behaviour) and was 6

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Fig. 6. Structural evolution of the Co3O4@N-CNFs electrode during the electrochemical reacted. (a) Typical galvanostatic charge-discharge curve. Points A, B, C, D, and E represent the pristine state, the discharged state at 0.6 V and 0.01 V, and the charged state at 1.24 V and 3.0 V, respectively; (b) Ex situ XRD patterns corresponding to these points.

(discharged to 0.6 V), C (discharged to 0.01 V), D (charged to 1.24 V) and E (charged to 3.0 V), were selected to study the aspects of the structural evolution of the electrode material during the electrochemical reaction. The XRD patterns from the Co3O4@N-CNFs electrode in the different states were measured and are shown in Fig. 6b. The Cu peaks in each pattern were from the current collectors. In the fresh electrode, the Co3O4 phase can be clearly recognized. After being discharged to 0.6 V, the CoO phase was captured, and the Co3O4 phase intensity attenuation, however, can still be observed, revealing the presence of a decreasing amount of Co3O4. After being fully discharged, the Co3O4 phase completely disappeared and a new phase of Co gradually appeared, which is consistent with Eq. (2). The reverse oxidation trend appeared during the following charging process: both CoO and Co3O4 were observed at 1.24 V, and only Co3O4 was retained at 3.0 V, indicating that the Na+ insertion and extraction reactions were reversible. Additionally, the XRD patterns displayed no indication of the Na2O phase, which could be ascribed to its small particle size or poor crystallization.

determined by the slope of the log (i) versus log (ν) plot. According to Fig. 4b, the b value was calculated to be 0.88 for the cathode (peak A), revealing improved capacitive kinetics for the Co3O4@N-CNFs electrode [60,61]. To further quantitatively distinguish the diffusion-controlled and surface-controlled behaviours, Dunn and co-workers divided Equation (3) into two parts, as shown in Eq. (5), where i is the total current response, k1v is the capacitive behaviour and k2ν1/2 is the diffusion-controlled behaviour. Equation (6) can be acquired by dividing both sides of Eq. (5) by ν1/2. Then by plotting i/ν1/2 vs. ν1/2, the values of k1 and k2 can be determined [29]. Utilizing Eq. (6), the contribution percentages for the capacitive behaviour at the scan rate of 0.4 mV s−1 were determined and are exhibited in Fig. 4c. The capacitive behaviour contribution accounted for approximately 44.2% of the total capacity [62]. As showed in Fig. 4d, the percentage of the capacitive contribution steadily increased as the scan rate increased from 0.4 to 1 mV s−1 (reaching 75% at 1 mV s−1). The results revealed that the majority of the total capacity in the Co3O4@N-CNFs electrode was a pseudocapacitive process specifically at high scan rates, which was beneficial for rapid charge storage and long-term cycling capability [63,64].

i = av b

(3)

logi = b × logv + loga

(4)

i = k1 v + k2 v1/2

(5)

i/ v1/2 = k1 v1/2 + k2

(6)

4. Conclusions In summary, we demonstrated the fabrication of a three-dimensional porous electrode material consisting of Co3O4 nanoparticles connected with carbon nanofibers networks. The Co3O4@N-CNFs was synthesized by controlled pyrolysis after simple immersion. The reasonably designed Co3O4@N-CNFs exhibit excellent electrochemical performance, which can be attributed to the capacitive contribution and the full conversion of Co3O4 to metallic Co during the sodiation/ desodiation processes. Importantly, as a result of these novel structural characteristics, the Co3O4@N-CNFs presented a high capacity (450.2 mA h g−1 after 50 cycles at 100 mA g−1) and ultralong cycling life stability (220.1 mA h g−1 at 1.6 A g−1 after 1000 cycles), which is superior to most of the ZIF-derived cobalt-based oxide materials reported thus far. The excellent sodium storage capacity indicates that Co3O4@N-CNFs can be used to produce an outstanding anode material for SIBs. Moreover, this facile and scalable process can provide an approach to produce various metal oxides with unique structures and excellent performance for energy storage devices and a novel strategy for the optimization of MOF architectures.

Electrochemical impedance spectroscopy (EIS) data was obtained to determine the electrochemical performance of three samples as SIBs (Fig. 5). All the plots consisted of a semicircle in the high frequency region followed by a line in the low frequency region, revealing that charge transfer and Na-ion diffusion both participated in the electrochemical process. In addition, the inset in Fig. 5 shows the equivalent circuit model for the EIS. The measurements displayed that all samples had a similar diffusion resistance (Rs); however, the value of the charge transfer impedance (Rct) for the Co3O4@N-CNFs electrode (362 Ω) was lower than that of the other samples (548 Ω for the pure Co3O4 and 908 Ω for the CNFs), suggesting that the Co3O4@N-CNFs electrode possessed excellent electronic and ionic conduction. The decreased Rct can promote the kinetics and consequently ameliorate the rate capability. To further analyse the sodium storage mechanism of the Co3O4@NCNFs half cells, the discharge-charge profile from the first cycle was measured by ex situ XRD, and the results are displayed in Fig. 6. Fig. 6a displays the discharge-charge profile from the first cycle for the Co3O4@N-CNFs half-cells. The plateaus at 0.6 V in the discharge process and 1.24 V in the charging process are in accordance with the CV results [2,16]. Specifically, five voltage plateaus, A (fresh electrode), B

CRediT authorship contribution statement Ling Li: Conceptualization, Writing - original draft, Supervision, Funding acquisition. Qiming Wang: Methodology, Investigation. Xinyu Zhang: Investigation, Formal analysis. Lide Fang: Supervision, Funding acquisition. Xiaoting Li: Funding acquisition, Project administration, Validation. Wenming Zhang: Conceptualization, Writing 7

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review & editing, Resources, Funding acquisition.

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