MOF-derived transition metal oxide encapsulated in carbon layer as stable lithium ion battery anodes

Journal of Alloys and Compounds 797 (2019) 83e91

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

MOF-derived transition metal oxide encapsulated in carbon layer as stable lithium ion battery anodes Jie Zhang a, Ruixia Chu a, Yanli Chen b, Heng Jiang b, Yibo Zeng a, Xin Chen a, Ying Zhang c, **, Nay Ming Huang c, Hang Guo a, * a b c

Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, 361005, Xiamen, People's Republic of China College of Materials, Xiamen University, 361005, Xiamen, People's Republic of China Xiamen University Malaysia, 43900, Sepang, Selangor Darul Ehsan, Malaysia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 January 2019 Received in revised form 11 March 2019 Accepted 15 April 2019 Available online 9 May 2019

Transition metal oxide (TMO) is an important type of conversion reaction anode for lithium ion batteries. Carbon encapsulated zinc oxide and cobalt oxide (ZnO@C, Co3O4@C) were prepared via a MOF-derived strategy. MOF precursors were firstly coated with polypyrrole (PPy) layer and then subjected to subsequent thermal treatment. Benefiting from the synergetic effect of conductive coating layer and 3D porous structure, both anodes showed attractive electrochemical performance. The ZnO@C and Co3O4@C delivered a reversible capacity of 526 and 721 mAh∙g
1 after 500 cycles at 250 mA g
1. With attractive rate performance, the ZnO@C and Co3O4@C have an average capacity of 301 and 306 mAh∙g
1 at 2.0 A g
1. Kinetic analysis revealed that lithium ion storage in both ZnO@C and Co3O4@C were dominated by a surface controlled pseudo-capacitive process. In addition, ZnO@C and Co3O4@C could even stably cycle for 1000 times at a high current density of 2.0 A g
1. © 2019 Elsevier B.V. All rights reserved.

Keywords: MOF Pseudo-capacitance Lithium ion batteries

1. Introduction Nowadays, there is an increasing demand on mobile electronic devices and electric vehicles. This trend has greatly spurred the innovations in energy conversion and storage technology. Due to attractive energy density and high safety, lithium ion battery (LIB) has been the most widely used energy storage device [1e3]. Limited by a sluggish kinetics, the power density of LIBs is far from satisfying [4]. In addition, the low theoretical capacity of graphite anode has also hinder the energy density. Thus, improving kinetics and developing high capacity electrode become the major challenges in the research of LIBs. Currently, fabrication of carboncoated composites is the mostly used approach to improve the kinetic of electrode materials [5e8]. The introduction of conductive carbon coating or matrix could not only enhance the electrical conductivity but also protect active species from pulverization [9]. In the previous decades, researchers have successfully developed a series of high capacity electrode materials [10e13]. Alloying

* Corresponding author. ** Corresponding author. E-mail address: [email protected] (H. Guo). https://doi.org/10.1016/j.jallcom.2019.04.162 0925-8388/© 2019 Elsevier B.V. All rights reserved.

electrodes such as silicon (Si) have ultra-high theoretical lithium storage capacity, but the huge volume expansion is an inevitable challenge [13,14]. Recently, conversion reaction electrodes have captured much attention due to high capacity and moderate volume expansion ratio [15e18]. Transition metal oxide (TMO) is one of the most common conversion type electrode materials. Due to low cost and easy preparation, Zinc oxide (ZnO) and Cobalt oxide (Co3O4) have been widely studied in the past several years. The theoretical capacity of ZnO and Co3O4 is reported to be 981 and 890 mAh∙g
1, respectively [19,20]. Although ZnO and Co3O4 have attractive capacity, actual performance is still limited by large volume expansion and poor conductivity [21e25]. Thus, a series of micro/nano structure from zero dimension to three dimension have been designed to alleviate the above issues [26e36]. For example the Gan group have prepared N-doped carbon coated ZnO nanorods anode via a solvent-free method [27]. The composite ZnO@C anode could stably cycle for 200 cycles at 200 mA g
1, delivering a high reversible capacity of 1011 mAh∙g
1. The Wang group selected a template-free approach to fabricate Co3O4 nanotube anode for LIBs [28]. Experimental results showed that the Co3O4 could have a high capacity of 1081 mAh∙g
1 at 100 mA g
1 after 50 cycles. Conductive carbon coating is also an effective strategy to improve the electrochemical performance of TMO anodes. The

84

J. Zhang et al. / Journal of Alloys and Compounds 797 (2019) 83e91

introduction of carbon coating could not only prevent active species from easy pulverization but also improve the conductivity. The Mai group have successfully fabricated Co3O4/C composite anodes for stable lithium and sodium storage [8]. Due to existence of protective carbon coating layer, the Co3O4/C anode could stably deliver a high reversible capacity of 1100 mAh∙g
1 at a current density of 200 mA g
1 after 120 cycles. The Mei group also prepared sandwiched porous C/ZnO/porous carbon nanosheet anode [32]. The outer carbon coating with mechanical robustness could suppress the expansion of the inner oxides, resulting in a stable SEI layer. Besides, the porous structure and conductive carbon could also facilitate Liþ/electron transfer in the inner oxides, leading to enhanced rate performance. Metal organic frameworks (MOFs) and their derivatives have rich porosity, controllable chemical compositions and diverse structures, which enable them as ideal electrode materials for rechargeable batteries [37]. The Li group applied a MOF-derived method to fabricated flower-like Co3O4/C hybrid, which possessed a capacity of 671 mAh∙g
1 at a high current density of 1.0 A g
1 [38]. The Yoon team designed ZIF-8-derived Ni@ZnO/carbon nanofiber freestanding composite anode for stable lithium storage [30]. The hierarchical structure of Ni@ZnO/CNFs is reported to contribute to a high capacity retention. Herein, ZnO@C and Co3O4@C were prepared via a MOF-derived strategy. The PPy-coated MOF precursors were converted into carbon-encapsulated metal oxides after a two-step thermal treatment. Lithium storage kinetics were investigated by in-depth analysis of CV curves collected at various scan rates. Due to unique core-shell structure and pseudo-capacitive nature the ZnO@C and Co3O4@C have attractive electrochemical performance.

rinsed with methanol and vacuum dried at 60  C for 12 h. In the next step, 200 mg ZIF-8 powder was dispersed in 50 mL deionized water (DI H2O) under stirring in the ice bath. A volume of 400 mL Pyrrole was then added into the dispersion. Aqueous ammonium persulfate (APS) solution (0.8043 g in 10 mL DI H2O) was added into the above mixture under constant stirring. During the polymerization process, initial white dispersion gradually turned into gray and finally a black mixture was obtained. The PPy-coated ZIF-8 was annealed in nitrogen (N2) at 700  C for 2 h with a ramping rate of 5  C∙min
1. After cooling down to room temperature, a mixture of N2:O2 (v:v ¼ 4:1) was introduced into the furnace. The furnace was then kept at 300  C for 30 min before cooling down to room temperature. Finally, fluffy black powder labelled as ZnO@C was obtained after initial PPy coating and following two-step thermal treatment. The Co3O4@C was prepared via the same procedure, in which Zn(NO3)2 was replaced with cobalt nitrate (Co(NO3)2). The as-prepared ZnO@C and Co3O4@C were grinded into powder and vacuum dried at 60  C for 12 h before further test and characterization.

2. Experimental section

The ZnO@C and Co3O4@C powder were ball-milled into slurry with Super P and poly-vinylidene fluoride (PVDF) with a mass ratio of 6:2:2 using N-methyl-2-pyrrolidone (NMP) as solvent. After a 6 h ball milling at 330 rpm, the slurry was blade-coated on copper foil with a thickness of 50 mm by a thin film applicator. Slurry-coated Cu foil was vacuum dried at 120  C for 12 h and cut into circular discs with a diameter of 1.6 cm CR2016 coin cells were assembled in an argon-filled glove box, with metallic lithium foil as counter electrode and Celgard 2500 polypropylene membrane as separator. Lithium hexafluorophosphate (LiPF6 1.0 mol L
1) in a mixture of ethylene carbonate, dimethyl carbonate, and ethyl methyl

2.1. Preparation of carbon encapsulated metal oxide The carbon layer encapsulated metal oxides were prepared via a MOF- derived strategy. In the first step, 1.0 mmol Zinc nitrate (Zn(NO3)2) and 4.0 mmol 2-methylimidazole (2-MIM) were dissolved in 50 mL methanol, respectively. The 2-MIM solution was quickly poured into the Zn(NO3)2 solution under magnetic stirring. After a 5-min stirring, the mixture was incubated at room temperature for 24 h. The white ZIF-8 intermediate was repeatedly

2.2. Characterization X-ray diffraction (XRD) patterns were collected on a Rigaku Ultima IV Diffractometer with a Cu Ka source. Field emission scanning electronic microscope (FE-SEM) images were obtained on a Carl ZEISS SUPRA 55 microscope. Transmission electron microscopy (TEM), high resolution transmission electronic microscope (HR-TEM) images were measured on JEM 2100 microscope. 2.3. Electrochemical measurements

Fig. 1. Schematic illustration for typical preparation procedure of carbon encapsulated metal oxide via a MOF-derived strategy.

J. Zhang et al. / Journal of Alloys and Compounds 797 (2019) 83e91

85

Fig. 2. (a) Optical photograph of samples prepared at different reaction stages; (b) XRD patterns of the as-prepared ZnO@C and Co3O4@C.

Fig. 3. FE-SEM and TEM images of the as-prepared carbon encapsulated transition metal oxide at different magnification. (a)e(b) FE-SEM images of ZnO@C; (c)e(d) FE-SEM images of Co3O4@C; (e)e(f) TEM images of ZnO@C; (g)e(h) TEM images of Co3O4@C.

carbonate (EC, DMC, EMC v/v/v ¼ 1:1:1) was used as the electrolyte. Galvanostatic charge discharge (GCD) tests were performed on Neware battery test systems with a potential window from 0.01 to 3.0 V. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were carried out on a CHI660E electrochemical workstation. All CV curves were obtained in a potential range from 0.01 to 3.0 V. EIS curves were plotted in a frequency range of 0.01 Hze100 kHz with a 10 mV voltage amplitude. 3. Results and discussion The carbon layer encapsulated metal oxides were prepared via a MOF- derived strategy. Fig. 1 shows the schematic illustration for preparation of carbon encapsulated transition metal oxides. In the first step, transition metal ions (Zn2þ or Co2þ) in the methanol were coordinated with organic ligands to produce ZIF-8 and ZIF-67. Metal organic framework (MOF) precursors were then coated with a layer of PPy via the polymerization of pyrrole (Py) monomer in ice bath. During the subsequent annealing in N2, MOF precursor and PPy coating layer would be converted into metal oxide and amorphous carbon layer, respectively. According to

previous report, metal oxide generated from MOF precursor would simultaneously be reduced into metallic nanoparticles by the PPyderived carbon via a carbon-thermal reduction process [39]. Under a relative low temperature, combustion of carbon layer was avoided, while these metallic nanoparticles could be oxidized into ultra-fine metal oxide. Eventually, the carbon encapsulated metal oxides were obtained. Pronounced change in color could be observed during the preparation. As shown in Fig. 2a, white ZIF-8 precursor would turn into black after the PPy coating. The purple ZIF-67 would change into black PPy@ZIF-67 after the polymerization in ice bath. In the subsequent thermal treatment the change in color is much less obvious than that in the PPy coating step. Fig. 2b shows XRD patterns of final products derived from PPy-coated ZIF-8 and ZIF-67. The Zn-MOF was converted into ZnO@C after PPy coating and consecutive thermal treatment, as all these characteristic diffraction peaks could be indexed to wurtzite ZnO (JCPDS 36-1451) [40]. A series of peaks well-fitted with (111), (220), (311), (222), (400), (422), (511) and (440) planes of cubic spinel Co3O4 (JCPDS NO. 421467) were detected in the Co-MOF derived sample. Thus, ZIF67@PPy was eventually converted into cubic spinel Co3O4@C after

86

J. Zhang et al. / Journal of Alloys and Compounds 797 (2019) 83e91

Fig. 4. Electrochemical performance test of ZnO@C and Co3O4@C anode. (a) CV curves of ZnO@C; (b) CV curves of Co3O4@C; (c) Initial GCD curves of ZnO@C anode; (d) Initial GCD curves of Co3O4@C anode; (e) Rate capabilities of ZnO@C and Co3O4@C; (f) Cycling performance curves of ZnO@C and Co3O4@C at a current density of 250 mA g
1.

thermal treatment. Sharp characteristic diffraction peaks in the ZnO@C and Co3O4@C indicates high crystallinity of these metal oxides. Previous reports have proven that the PPy derived carbon is generally amorphous in nature [41]. Thus, these weak diffraction peaks of carbon were shielded in those intense peaks of crystalline metal oxides. In addition, relative low content of carbon in the composites may also result in an ignorable carbon-related XRD signals. Morphology of the as-prepared ZnO@C and Co3O4@C were characterized by FE-SEM and TEM. As shown in Fig. 3a and b, polyhedrons with regular shape could be observed. Average size of these ZnO@C polyhedrons is around 200 nm. FE-SEM images of Co3O4@C is featured by polyhedrons with certain extent of shrinkage. Partial deformation of these polyhedrons may originate from thermal stress during annealing treatment. TEM images shown in Fig. 3e revealed the ZnO@C polyhedron is constructed by

large number of nanoparticles. The average size of these particles is about 15 nm. Detected lattice spacing of 0.247 nm is in agreement with the (101) plane of ZnO. The Co3O4@C is also found to be built by nano-sized subunits. High resolution TEM image showed that these particles are about 10 nm in size. The average inter-planar distance was measured to be 0.246 nm, corresponding to (311) planes of cubic spinel Co3O4 [28]. As for ZnO@C and Co3O4@C anode, introduction of conductive carbon coating layer could not only facilitate electron transfer but also protect active species, resulting in improved rate and cycling performance [25]. Nano-sized active species could provide short Liþ ions diffusion pathway. In addition, three-dimensional porous structure can effectively improve electrolyte-electrode interfacial contact and buffer lithiation-related stress. Thus, ZnO@C and Co3O4@C could theoretically function as ideal anode materials for high-performance LIBs.

J. Zhang et al. / Journal of Alloys and Compounds 797 (2019) 83e91

87

Fig. 5. Cycling stability test of ZnO@C and Co3O4@C anodes at higher current densities. (a) 1.0 A g
1; (b) 2.0 A g
1.

The electrochemical performance of ZnO@C and Co3O4@C anode were measured by CV and GCD tests. As shown in Fig. 4a, an obvious peak at 0.53 V and a weak one at 0.81 V could be observed in the first cathadic scan. The weak peak at 0.81 V is related to the reduction of ZnO to Zn and the peak at 0.53 V is corresponding to formation of LiZn alloy [42]. In the first anodic scan, two peaks originated from de-alloying of LiZn and oxidation of metallic Zn appeared at 0.35 and 1.22 V. The reduction peak shifted to 0.93 V and remained unchanged in the following scans. By contrast, oxidation peak at 0.35 V shifted to lower potential. The almost overlapped CV curves in following scans indicated attractive

electrochemical stability of ZnO@C anode [43]. Fig. 4b shows CV curves of Co3O4@C in initial five cycles at a scan rate of 0.25 mV s
1. There is a weak peak at 1.35 V and a pronounced one at 0.80 V in the first cathadic scan, corresponding to reduction of Co3O4 to CoO and conversion of CoO to metallic Co [24]. A pair of peaks at 1.25 and 2.13 V related to oxidation of metallic Co to CoO could be found in the initial anodic scan. Judging from the almost overlapped CV curves, Co3O4@C may also be a stable anode material. GCD curves of ZnO@C and Co3O4@C were shown in Fig. 4c and d. In consistent with the CV curves, both ZnO@C and Co3O4@C showed pronounced discharge plateau only in the first cycle. It is worth noting that

Fig. 6. Kinetic analysis of electrochemical behavior of the ZnO@C anode. (a) CV curves of ZnO@C and at different scan rates; (b) Determination of b value via linear fitting of peak current and scan rate in Fig. 6(a); (c) Separation of capacitive and diffusion current in ZnO@C at a scan rate of 0.5 mV s
1; (d) Contribution ratios of capacitance controlled and diffusion controlled charge at various scan rates.

88

J. Zhang et al. / Journal of Alloys and Compounds 797 (2019) 83e91

Fig. 7. Kinetic analysis of electrochemical behavior of Co3O4@C anode. (a) CV curves of Co3O4@C and at different scan rates; (b) Determination of b value via linear fitting of peak current and scan rate in Fig. 7(a); (c) Separation of capacitive and diffusion current in Co3O4@C at a scan rate of 0.5 mV s
1; (d) Contribution ratios of the capacitance controlled and diffusion controlled charge at various scan rates.

Fig. 8. Nyquist plots of the ZnO@C and Co3O4@C anodes before and after 500-cycle GCD test.

charging branch of these GCD curves is featured by sloping line, indicating severe voltage hysteresis in electrode materials [44]. Absence of charging plateau in the GCD curves implied there might be another lithium storage mechanism. Generally, the existence of voltage hysteresis would result in low energy efficiency. The initial specific discharge capacity and charge capacity of ZnO@C is

measured to be 1106.2 and 665.8 mAh∙g
1. Initial Coulombic efficiency (ICE) of ZnO@C is calculated to be about 60% at a current density of 250 mA g
1. The nearly 40% capacity loss is generally attributed to formation solid electrolyte interphase (SEI) film, which result from side reaction between electrode and electrolyte [44]. The Co3O4@C anode delivers a specific discharge capacity of 1112 mAh∙g
1 and a charge capacity of 645 mAh∙g
1 in the first cycle, corresponding to an initial Coulombic efficiency of 58%. Both ZnO@C and Co3O4@C irreversibly consume certain amount of Liþ ions during the first discharge process. It is previously reported that incorporation of carbon layer would improve rate performance of metal oxide anodes [33]. Rate capacities of ZnO@C and Co3O4@C were measured at current densities from 0.25 to 2.0 A g
1. As shown in Fig. 4e, both ZnO@C and Co3O4@C possessed attractive rate performance. The ZnO@C anode has an average reversible capacity of 510, 436, 362 and 301 mAh∙g
1 at 0.25, 0.5, 1.0 and 2.0 A g
1, respectively. When the current density was switched back to 0.25 A g
1, the capacity of ZnO@C could recover to 519 mAh∙g
1. Average capacity of Co3O4@C was measured to be 502, 430, 368 and 306 mAh∙g
1 at 0.25, 0.5, 1.0 and 2.0 A g
1. And the capacity could reach up to 537 mAh∙g
1 as the current density went back to 0.25 A g
1. Attractive rate performance of ZnO@C and Co3O4@C may arise from carbon coating layer, which could significantly improve the conductivity of ZnO and Co3O4 [45,46]. Fig. 4f shows cycling stability test curves of ZnO@C and Co3O4@C anode at 0.25A∙g
1. Both reversible capacity of ZnO@C and Co3O4@C decreased in initial cycles due to insufficient electrolyte penetration. After slight capacity fading in initial stage, ZnO@C anode quickly got stabilized. A reversible capacity of 526 mAh∙g
1 could

J. Zhang et al. / Journal of Alloys and Compounds 797 (2019) 83e91

89

Fig. 9. Schematic illustration for the possible mechanism behind superior performance of ZnO@C and Co3O4@C.

be delivered by ZnO@C after 500 cycles. Although initial capacity loss ratio of Co3O4@C is slightly higher than ZnO@C, the reversible capacity gradually went up during the cycling test after initial fading. After a 500-cycle test at 250 mA g
1, the Co3O4@C anode had a reversible capacity of 721 mAh∙g
1. Judging from the curves shown in Fig. 4e, both ZnO@C and Co3O4@C had attractive rate performance even at high current densities. In order to further compare the performance of ZnO@C and Co3O4@C, both anodes were tested at higher current densities. Detailed results of the long-term cycling stability test of ZnO@C and Co3O4@C were shown in Fig. 5. The ZnO@C had an initial reversible capacity of 379 mAh∙g
1 at a current density of 1.0 A g
1. After a 500-cycle test, it could maintain a reversible capacity of 297 mAh∙g
1. The capacity retention of ZnO@C anode was calculated to be about 78.36%. Although the initial capacity of Co3O4@C is slightly lower than that of ZnO@C, it would slowly increased and then gradually decreased during the cycling test. And a reversible capacity of 301 mAh∙g
1 could be maintained after 500 cycles, with a corresponding retention of 80.91%. Fig. 5b shows cycling performance curves of the ZnO@C and Co3O4@C at a high current density of 2.0 A g
1. Both anodes showed attractive stability in the 1000cycle test. The ZnO@C could have a reversible capacity of 238 mAh∙g
1, with a capacity retention of 86.54%. The Co3O4@C could also maintain a reversible capacity of 215 mAh∙g
1 after 1000 cycles at 2.0 A g
1. Although ZnO@C and Co3O4@C did not have high lithium storage capacities at high current densities, outstanding stability still enable them to be promising anodes. Previous reports have proven that kinetic analysis plays an important role in unveiling the lithium storage mechanism [8,47]. By in-depth analysis of CV curves collected at different scan rates, the lithium storage kinetics could be uncovered. Thus, in order to investigate lithium storage kinetics, CV curves of both anodes at various scan rates were collected and analyzed. Fig. 6 shows the kinetic analysis related curves of ZnO@C anode. With gradual increase of scan rate, intensity of CV curves gradually increased accordingly. The redox peaks were also found to shift as the scan rate varied. Based on the following assumption that current (i) obeys a power-law relationship with scan rate (v), as shown in Eq (1) [9].

i ¼ avb

(1)

where a is a constant. b value could be obtained from linear

relationship of log i and log v, as shown in Eq (2).

log i ¼ log a þ b log v

(2)

It is generally accepted that b value reflects the energy storage features of electrode materials. There are two well-defined conditions: b ¼ 0.5 and b ¼ 1.0. In the case of b ¼ 0.5, electrochemical energy was stored via a faradaic intercalation process. In the case of b ¼ 1.0, current exhibits a capacitive response [48]. By plotting log i vs log v, b values of cathodic and anodic peaks in ZnO@C anode were calculated to be 0.942 and 0.814, respectively. Both b values fall in a range close to 1.0, indicating a capacitive-dominated lithium storage mode. Thus, the Liþ ions storage kinetic in ZnO@C was proven to be dominated by a capacitance controlled process. Relationship in Eq (1) could be divided into two parts corresponding to capacitive (k1v) and diffusion-controlled effects (k2 v1=2 ), as follows.

i ¼ k1 þ k2 v1=2

(3)

where k1 and k2 are constants for a given potential. In order to simplify analytical process, Eq (4) was rearranged as follows.

iðVÞ

.

v1=2 ¼ k1 v1=2 þ k2

(4)

By plotting i/v1/2 vs v1/2, k1 is determined as slope, and k2 is determined as intercept; thus capacitive and diffusion contributions can be obtained. As can be observed in Fig. 6c, about 66.58% of total capacity is capacitance controlled at a scan rate of 0.5 mV s
1. As scan rate gradually levelled up, an increasing capacitive contribution ratio could be observed (See Fig. 6d). A maximum contribution ratio of 87.15% could be reached at 2.0 mV s
1. Thus, electrochemical charge-discharge process mainly occurs on surface of active species [49]. In addition, attractive rate performance of ZnO@C anode could also be attributed to its pseudocapacitive nature. The CV curves of Co3O4@C anode collected at various scan rates were also systematically analyzed. As shown in Fig. 7a, CV curves of Co3O4@C exhibited a similar broaden trend with those of ZnO@C. By plotting log i vs log v, b values of cathodic and anodic peaks in Co3O4@C anode were calculated to be 0.922 and 0.844, respectively. Thus, the lithium storage in Co3O4@C was also dominated by the capacitive process. Separation of capacitive and diffusion current in

90

J. Zhang et al. / Journal of Alloys and Compounds 797 (2019) 83e91

Co3O4@C indicated that 52.13% of the lithium storage capacity is related to the capacitive process. At a scan rate of 2.0 mV s
1, the capacitive current could make a maximum contribution of 70.76% to the total capacity. Nyquist plots were collected to compare the resistance before and after GCD test (See Fig. 8). Typically, Nyquist plots consisted of a semicircle in the high frequency region and a slipe line in lowfrequency region [26]. The high frequency semicircle (Rct) is related to charger transfer resistance and double-layer capacitance. The low frequency slope line represents Liþ ions diffusion process in electrode materials. Generally, Liþ ion diffusion resistance is negatively correlated with slope of the line in low frequency region [32]. The ZnO@C is observed to have a slightly larger semicircle than Co3O4@C in the high frequency region, indicating a higher Rct value. Relative lower Rct of Co3O4@C could be attributed to a higher conductivity of Co3O4. The almost parallel slope line in low frequency revealed a similar Liþ ion diffusion resistance in ZnO@C and Co3O4@C. Based on analysis of above characterization and test, a possible mechanism behind the attractive performance was put forward. The corresponding mechanism schematic illustration is shown in Fig. 9. As shown in Fig. 9, introduction of conductive carbon layer could not only facilitate electron transfer but also prevent the active species from losing [50]. Three-dimensional porous structure is beneficial for electrolyte infiltration and provide sufficient space to buffer volume expansion during the lithiation process. Thus, benefiting from synergetic effect of conductive coating layer and 3D porous stricture, ZnO@C and Co3O4@C showed attractive rate capacities and cycling stability.

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17] [18]

[19]

[20]

4. Conclusion In conclusion, carbon encapsulated zinc oxide and cobalt oxide (ZnO@C and Co3O4@C) were prepared via a MOF-derived approach. Benefiting from synergetic effect of conductive coating and 3D porous stricture, both anodes showed attractive electrochemical performance. The ZnO@C and Co3O4@C could deliver a reversible capacity of 526 and 721 mAh∙g
1 at 250 mA g
1 for 500 cycles. At a higher current density of 2.0 A g
1, both anodes could cycle for 1000 times. Surface-controlled pseudocapacitive process is found to play a dominant role in the lithium ion storage in both ZnO@C and Co3O4@C. Thus, ZnO@C and Co3O4@C could function as promising anodes for LIBs. Acknowledgement This work was supported by the New Century Talent Support Plan of the Ministry of Education of China [Grant No. 2007NCET-070723], and the National Natural Science Foundation of China [Grant No. 60936003].

[21]

[22]

[23]

[24]

[25]

[26]

[27]

References [28] [1] Y.Y. Liu, G.M. Zhou, K. Liu, Y. Cui, Design of complex nanomaterials for energy storage: past success and future opportunity, Acc. Chem. Res. 50 (2017) 2895e2905. [2] J.B. Goodenough, K.S. Park, The Li-ion rechargeable battery: a perspective, J. Am. Chem. Soc. 135 (2013) 1167e1176. [3] S. Chu, A. Majumdar, Opportunities and challenges for a sustainable energy future, Nature 488 (2012) 294e303. [4] L. Zhao, L. Wang, P. Yu, C.G. Tian, H. Feng, Z.W. Diao, H.G. Fu, Hierarchical porous NiCo2O4 nanosheet arrays directly grown on carbon cloth with superior lithium storage performance, Dalton Trans. 46 (2017) 4717e4723. [5] C.F. Zhang, 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) 4422e4430. [6] R.J. Zou, Z.Y. Zhang, M.F. Yuen, M.L. Sun, J.Q. Hu, C.S. Lee, W.J. Zhang, Three-

[29]

[30]

[31]

[32]

dimensional-networked NiCo2S4 nanosheet array/carbon cloth anodes for high-performance lithium-ion batteries, NPG Asia Mater. 7 (2015) 195e202. Y.D. Mo, Q. Ru, X. Song, L.Y. Guo, J.F. Chen, X.H. Hou, S.J. Hu, The sucroseassisted NiCo2O4@C composites with enhanced lithium-storage properties, Carbon 109 (2016) 616e623. Y.Z. Wu, J.S. Meng, Q. Li, C.J. Niu, X.P. Wang, W. Yang, W. Li, L.Q. Mai, Interfacemodulated fabrication of hierarchical yolk-shell Co3O4/C dodecahedrons as stable anodes for lithium and sodium storage, Nano Res. 7 (2017) 2364e2376. J. Zhang, R.X. Chu, Y.L. Chen, H. Jiang, Y. Zhang, N.M. Huang, H. Guo, Electrodeposited binder-free NiCo2O4@carbon nanofiber as a high performance anode for lithium ion batteries, Nanotechnology 29 (2018) 125401. N. Liu, H. Wu, M.T. Mc Dowell, Y. Yao, C. Wang, Y. Cui, A yolk-shell design for stabilized and scalable Li-ion battery alloy anodes, Nano Lett. 12 (2012) 3315e3321. Z. Lu, N. Liu, H.W. Lee, J. Zhao, W. Li, Y. Li, Y. Cui, Nonfilling carbon coating of porous silicon micrometer-sized particles for high-performance lithium battery anodes, ACS Nano 9 (2015) 2540e2547. F.F. Wu, J. Bai, J.K. Feng, S.L. Xiong, Porous mixed metal oxides: design, formation mechanism, and application in lithium ion batteries, Nanoscale 7 (2015) 17211e17230. G.X. Guo, H.B. Wu, X.W. Lou, Citrate-assisted growth of NiCo2O4 nanosheets on reduced graphene oxide for highly reversible lithium storage, Adv. Energy Mater. 4 (2014) 1400422. X. Su, Q. Wu, J. Li, X. Xiao, A. Lott, W. Lu, B.W. Sheldon, J. Wu, Silicon-based nanomaterials for lithium-ion batteries: a review, Adv. Energy Mater. 4 (2014) 1300882. M.S. Ko, S.J. Chae, J.Y. Ma, N.H. Kim, H.W. Lee, Y. Cui, J. Cho, Scalable synthesis of silicon nanolayer embedded graphite for high-energy lithium-ion batteries, Nat. Energy 1 (2016) 16113. Y. Zhao, X.F. Li, B. Yan, D.B. Xiong, D.J. Li, S. Lawes, X.L. Sun, Recent developments and understanding of novel mixed transition metal oxides as anodes in lithium ion batteries, Adv. Energy Mater. 6 (2016) 1502175. S.H. Yu, S.H. Lee, D.J. Lee, Y.E. Sung, T. Hyeon, Conversion reaction-based oxide nanomaterials for lithium ion battery anodes, Small 16 (2016) 2146e2172. L.L. Peng, P. Xiong, L. Ma, Y.F. Yuan, Y. Zhu, D.H. Chen, X.Y. Luo, L. Lu, K. Amine, G.H. Yu, Holey two-dimensional transition metal oxide nanosheets for efficient energy storage, Nat. Commun. 8 (2017) 15139. Y.H. Song, Y.Q. Chen, J.F. Wu, Y.Y. Fu, R.R. Zhou, S.H. Chen, L. Wang, Hollow metal organic frameworks-derived porous ZnO/C nanocages as anode materials for lithium-ion batteries, J. Alloys Compd. 694 (2017) 1246e1253. K. Jang, D.K. Hwang, F.M. Auxilia, J. Jang, H. Song, B.Y. Oh, Y. Kim, J. Nam, J.W. Park, S. Jeong, S.S. Lee, S. Choi, I.S. Lim, W.B. Kim, J.M. Myoung, M.H. Ham, Sub-10-nm Co3O4 nanoparticles/graphene composites as high performance anodes for lithium storage, Chem. Eng. J. 309 (2018) 15e21. T. Kim, H. Kim, J.M. Han, J. Kim, ZnO-embedded N-doped porous carbon nanocomposite as a superior anode material for Lithium-Ion batteries, Electrochim. Acta 253 (2017) 190e199. J.Y. Wang, N.L. Yang, H.J. Tang, Z.H. Dong, Q. Jin, M. Yang, D. Kisailus, H.J. Zhao, Z.Y. Tang, D. Wang, Accurate control of multi-shelled Co3O4 hollow microspheres as high-performance anode materials in lithium ion batteries, Angew. Chem. 125 (2013) 6545e6548. G.L. Wu, Z.R. Jia, Y.H. Cheng, H.X. Zhang, X.F. Zhou, H.J. Wu, Easy synthesis of multi-shelled ZnO hollow spheres and their conversion into hedgehog-like ZnO hollow spheres with superior rate performance for lithium ion batteries, Appl. Surf. Sci. 464 (2019) 472e478. Z.Z. Liu, W.W. Zhou, S.S. Wang, W. Du, H.L. Zhang, C.Y. Ding, Y. Du, L.J. Zhu, Facile synthesis of homogeneous core-shell Co3O4 mesoporous nanospheres as high performance electrode materials for supercapacitor, J. Alloys Compd. 774 (2019) 137e144. Y.N. Men, X.C. Liu, F.L. Yang, F.S. Ke, G.Z. Cheng, W. Luo, Carbon encapsulated hollow Co3O4 composites derived from reduced graphene oxide wrapped metal organic frameworks with enhanced lithium storage and water oxidation properties, Inorg. Chem. 57 (2018) 10649e10655. J. Park, J.B. Ju, W. Choi, S.K. Kim, Highly reversible ZnO@ZIF-8-derived nitrogen-doped carbon in the presence of fluoroethylene carbonate for high-performance lithium-ion battery anode, J. Alloys Compd. 773 (2019) 960e969. Q.M. Gan, K.M. Zhao, S.Q. Liu, Z. He, Solvent-free synthesis of N-doped carbon coated ZnO nanorods composite anode via a ZnO support-induced ZIF-8 insitu growth strategy, Electrochim. Acta 253 (2017) 292e301. J.X. Wang, C. Wang, M.M. Zhen, Template-free synthesis of multi-functional Co3O4 nanotubes as excellent performance electrode materials for superior energy storage, Chem. Eng. J. 356 (2019) 1e10. X. Li, X.D. Tian, T. Yang, Y. Song, Z.J. Liu, Hierarchically multi-porous carbon nanotube/Co3O4 composite as an anode material for high performance lithium-ion batteries, Chem. Eur. J. 24 (2018) 14477e14483. B. Joshi, E. Samuel, Y.I. Kim, M.W. Kim, H.S. Jo, M.T. Swihart, W.Y. Yoon, S.S. Yoon, Hierarchically designed ZIF-8-derived Ni@ ZnO/carbon nano-fiber freestanding composite for stable Li storage, Chem. Eng. J. 351 (2018) 127e134. A. Li, M. Zhong, W. Shuang, C.P. Wang, J. Liu, Z. Chang, X.H. Bu, Facile synthesis of Co3O4 nanosheets from MOF nanoplates for high performance anodes of lithium-ion batteries, Inorg. Chem. Front. 5 (2018) 1602e1608. Y.T. Zhao, G.S. Huang, D.R. Wang, Y. Ma, Z.Y. Fan, Z.H. Bao, Y.F. Mei, Sandwiched porous C/ZnO/porous C nanosheet battery anodes with a stable solid-

J. Zhang et al. / Journal of Alloys and Compounds 797 (2019) 83e91

[33]

[34]

[35]

[36]

[37] [38]

[39]

[40]

[41]

electrolyte interphase for fast and long cycling, J. Mater. Chem. A 6 (2018) 22870e22878. C.S. Yan, C. Chen, X. Zhou, J.X. Sun, C.D. Lv, Template-based engineering of carbon-doped Co3O4 hollow nanofibers as anode materials for lithium-ion batteries, Adv. Funct. Mater. 26 (2016) 1428e1436. E. Samuel, B. Joshi, M.W. Kim, Y.I. Kim, S. Park, T.G. Kim, M.T. Swihart, W.Y. Yoon, S.S. Yoon, Zeolitic imidazolate framework-8 derived zinc oxide/ carbon nanofiber as freestanding electrodes for lithium storage in lithium-ion batteries, J. Power Sources 395 (2018) 349e357. A. Numan, N. Duraisamy, F.S. Omar, Y.K. Mahipal, K. Ramesh, S. Ramesh, Enhanced electrochemical performance of cobalt oxide nanocube intercalated reduced graphene oxide for supercapacitor application, RSC Adv. 6 (2016) 34894e34902. T. Yang, Y.G. Liu, Z.H. Huang, J.W. Liu, P.J. Bian, C.D. Ling, H. Liu, G.X. Wang, R.K. Zheng, In situ growth of ZnO nanodots on carbon hierarchical hollow spheres as high-performance electrodes for lithium-ion batteries, J. Alloys Compd. 375 (2018) 1079e1087. R. Zhao, Z.B. Liang, R.Q. Zou, Q. Xu, Metal organic frameworks for batteries, Joule 2 (2018) 2235e2259. J.B. Li, D. Yan, S.J. Hou, T. Lu, Y.F. Yao, L.K. Pan, Metal-organic frameworks converted flower-like hybrid with Co3O4 nanoparticles decorated on nitrogen-doped carbon sheets for boosted lithium storage performance, Chem. Eng. J. 354 (2018) 172e181. S.K. Park, J.K. Kim, Y.C. Kang, Excellent sodium-ion storage performances of CoSe2 nanoparticles embedded within N-doped porous graphitic carbon nanocube/carbon nanotube composite, Chem. Eng. J. 328 (2017) 546e555. L. Xiao, E.W. Li, J.Y. Yi, W. Meng, S.Y. Wang, B.H. Deng, J.P. Liu, Enhancing the performance of nanostructured ZnO as an anode material for lithium-ion batteries by polydopamine-derived carbon coating and confined crystallization, J. Alloys Compd. 764 (2018) 545e554. H.L. An, R.K. Zhang, Z.H. Li, L. Zhou, M.F. Shao, M. Wei, Highly efficient metalfree electrocatalysts toward oxygen reduction derived from carbon nanotubes

91

@ polypyrrole core-shell hybrids, J. Mater. Chem. A. 46 (2016) 18008e18014. [42] J.F. Zhang, T.Z. Tan, Y. Zhao, N. Liu, Preparation of ZnO nanorods/graphene composite anodes for high performance lithium-ion batteries, Nanomaterials 8 (2018) 966e974. [43] T. Xie, J. Min, J. Liu, J.J. Chen, D.J. Fu, R.Z. Zhang, K.J. Zhu, M. Lei, Synthesis of mesoporous Co3O4 nanosheet assembled hollow spheres towards efficient electrocatalytic oxygen evolution, J. Alloys Compd. 754 (2018) 72e77. [44] H. Kim, H. Kim, H. Kim, J. Kim, G. Yoon, K. Kim, W.S. Yoon, K. Kang, Understanding origin of voltage hysteresis in conversion reaction for Na rechargeable batteries: the case of cobalt oxides, Adv. Funct. Mater. 26 (2016) 5042e5050. [45] Y.H. Song, Y.Q. Chen, J.F. Wu, Y.Y. Fu, R.H. Zhou, S.H. Chen, L. Wang, Hollow metal organic frameworks derived porous ZnO/C nanocages as anode materials for lithium-ion batteries, J. Alloys Compd. 694 (2017) 1246e1253. [46] H.B. Geng, Y.Y. Guo, X.G. Ding, H.W. Wang, Y.F. Zhang, X.L. Wu, J. Jiang, J.W. Zheng, Y.G. Yang, H.W. Gu, Porous cubes constructed by cobalt oxide nanocrystals with graphene sheet coatings for enhanced lithium storage properties, Nanoscale 8 (2016) 7688e7694. [47] J. Zhang, R.X. Chu, Y.L. Chen, H. Jiang, Y. Zhang, N.M. Huang, H. Guo, In-situ grown hierarchical ZnCo2O4 nanosheets on nickel foam as binder-free anode for lithium ion batteries, Ceram. Int. 44 (2018) 16219e16226. [48] C. Liao, S.P. Wu, Pseudo-capacitance behavior on Fe3O4-pillared SiOx microsphere wrapped by graphene as high performance anodes for lithium-ion batteries, Chem. Eng. J. 355 (2019) 805e814. [49] G.Y. Zhao, L. Tang, L. Zhang, X. Chen, Y.C. Mao, K.N. Sun, Well-developed capacitive-capacity of metal-organic framework derived Co3O4 films in Li ion battery anodes, J. Alloys Compd. 746 (2018) 277e284. [50] J.Y. Chen, X.F. Wu, Y. Liu, Y. Gong, P.F. Wang, W.H. Li, S.P. Mo, Q.Q. Tan, Y.F. Chen, Hierarchically-structured hollow NiO nano-spheres nitrogen-doped graphene hybrid with superior capacity retention and enhanced rate capability for lithium-ion batteries, Appl. Surf. Sci. 425 (2017) 461e469.