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rGO nanospheres as high-performance anodes for lithium ion batteries
Accepted Manuscript Facile synthesis of hierarchical lychee-like Zn3V3O8@C/rGO nanospheres as high-performance anodes for lithium ion batteries Hun Xue, Yixing Fang, Lingxing Zeng, Xiaotong He, Fenqiang Luo, Renpin Liu, Junbin Liu, Qinghua Chen, Mingdeng Wei, Qingrong Qian PII: DOI: Reference:
S0021-9797(18)31033-6 https://doi.org/10.1016/j.jcis.2018.08.110 YJCIS 24043
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
8 July 2018 28 August 2018 29 August 2018
Please cite this article as: H. Xue, Y. Fang, L. Zeng, X. He, F. Luo, R. Liu, J. Liu, Q. Chen, M. Wei, Q. Qian, Facile synthesis of hierarchical lychee-like Zn3V3O8@C/rGO nanospheres as high-performance anodes for lithium ion batteries, Journal of Colloid and Interface Science (2018), doi: https://doi.org/10.1016/j.jcis.2018.08.110
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Facile synthesis of hierarchical lychee-like Zn3V3O8@C/rGO nanospheres as high-performance anodes for lithium ion batteries
Hun Xuea, c, Yixing Fanga, Lingxing Zeng*a, c, Xiaotong Hea, Fenqiang Luoa, Renpin Liua, Junbin Liua, Qinghua Chena, c, Mingdeng Wei,b and Qingrong Qian*a, c a
Engineering Research Center of Polymer Green Recycling of Ministry of Education, College of Environmental Science and Engineering, Fujian Normal University, Fuzhou, Fujian 350007, China. b Institute of Advanced Energy Materials, Fuzhou University, Fuzhou, Fujian 350002, China. c Fujian Key Laboratory of Pollution Control & Resource Reuse, Fuzhou, Fujian 350007, China
Abstract In the present work, the hierarchical Zn3V3O8@C/rGO composite with a unique lychee-like architecture was fabricated by a simple one-pot ethanol thermal reduction strategy. When used as an anode material, it exhibited outstanding electrochemical performance for lithium-ion batteries (LIBs). For instance, the Zn3V3O8@C/rGO composite delivers high reversible capacities (1012 mAh g−1 at 0.1 A g−1 after 200 cycles) and high rate stability (448 mAh g−1 at 4 A g−1 after 1000 cycles). This outstanding performance can be attributed to the synergistic effect of the diverse structural virtues, effective interface and dual-spatially hybrid carbon network. Significantly, this one-pot simple strategy can be extended to fabricating highly stable and high rate performance of vanadates or other anode materials for LIBs. Keywords: Zn3V3O8@C/rGO; Lychee-like architecture; Ethanol thermal reduction; Lithium-ion batteries; Anode
*
Corresponding authors. Tel. & Fax: +86-591 8346 5158 [email protected] (L. Zeng), [email protected] (Q. Qian) 1
1. Introduction With the development of energy demand for societies of the world, and increasingly serious environmental pollution brought by the traditional energy, exploiting clean and renewable energy technology is becoming a hotspot of world[1]. Among various energy storage technologies, lithium-ion batteries (LIBs) is one of the most important renewable energy storage devices in recent years[2]. The anode in commercial LIBs is mainly made of graphite, which possesses a lower theoretical capacity (372 mAh g-1), insufficient to meet the increasing demand for high power density and long cycle life of LIBs[3, 4]. Therefore, tremendous efforts have been devoted to design novel high performance anode material for LIBs, which are desired for practical applications in future[5-11]. Vanadates have been investigated as the electrode materials for LIBs due to their high theoretical capacities and extraordinary morphology[12, 13]. For instance, Luo et al. reported Co 3V2O8 interconnected hollow microspheres were fabricated by hydrothermal reaction, which exhibited a stable capacity of 424 mAh g-1 at 10 A g-1 after 300 cycles[14]. Wang’s group synthesized the quasi-cuboidal CoV2O6 and it delivered a good capacity of ca. 702 mAh g-1 (200 mA g-1, 200 cycles)[15]. Among these vanadates, zinc vanadium oxides with variable states of vanadium and rich source of zinc, have gradually attracted much attention as anode materials of LIBs. they possess impressive electrochemical performance due to the variable states of vanadium stimulating flexible architecture[16, Z3V2O8[18], ultralong
monoclinic ZnV2O6 2
17]
. For example, porous sheet-like
nanowires[19],
Zn3V2O7(OH)2•2HO
microflowers[20], spinel ZnV2O4 nanoparticles[21] were explored and so on[22, 23]. In our previous work, we already have displayed the hierarchical ZnV2O4 microspheres[24] and ZnV2O4-CMK nanocomposite[25] , which exhibited large reversible capacity and good cycling stability. However, the large volume changes and phase transitions cause severe pulverization of zinc vanadium oxides electrode during Li+ intercalation process, resulting in capacity fading, especially after long-term cycling process. To exalt the electrochemical performance of zinc vanadium oxides, many strategies have been conducted including fabricating zinc vanadium oxides −carbon composite and the formation of novel nanostructure[26, 27]. Recently, the reduced graphene oxide (rGO) has been proven to be an effective carbon matrix for enhancing the stability of electrode[28-32]. In the meantime, hierarchical carbon composite with porous structure not only buffers the strain and volume changes during cycling, but also possesses high-efficiency electron transport and Li+ diffusion[33-36]. However, to the best of our knowledge, these is rarely report on combination of graphene with hierarchical Zn3V3O8 porous spheres with high capacity and robust stability for LIBs. In the present work, the hierarchical Zn3V3O8@C/rGO composite with a unique lychee-like architecture was prepared by a simple one-pot ethanol thermal reduction strategy, with regulating different amount of CTAB utilization and graphene oxides supported. The unique lychee-like Zn3V3O8@C/rGO composite prepared in this study could endow Zn3V3O8@C particles with hierarchical structure and combine reduced graphene oxide sheets (RGO) with Zn3V3O8@C sphere simultaneously. Due to this unique structure, the hierarchical Zn3V3O8@C/rGO composite with a unique 3
lychee-like architecture exhibits a remarkable reversible capacity, high-rate performance and superior long-term cycling stability (over 1000 cycles) as the anode material for LIBs.
2. Experimental Section 2.1 Synthesis of Zn3V3O8@C/rGO. The hierarchical Zn3V3O8@C/rGO (ZVO@C/G) nanospheres were synthesized by one step ethanol thermal reduction method. In a typical synthesis procedure, 0.3 mmol of zinc nitrate hexahydrate, 0.3 mmol of ammonium vanadate and 0.15 g of cetyltrimethylammonium bromide (CTAB) were dissolved in 12 mL of ethanol. Subsequently, a proper amount of graphene oxide (GO) (2 mg/ml, 18 ml) ethanol solution was added into the above solution with vigorous magnetic stirring for 5 min and then dispersed by ultrasonication for 20 min. Then, the obtained mixture was transferred to a 50 mL Teflon-lined autoclave and maintained at 200 °C for 5 days. After cooling naturally to room temperature, the product was collected and washed three times with ethanol, then was lyophilized for 12 h. As a result, the lychee-like ZVO@C/G was obtained. For a comparison, the effect of different amounts (0.075 g, 0.15 g or 0.3 g) of CTAB on the structures and morphologies of samples (denoted as ZVO@C/G-1, ZVO@C/G-2, ZVO@C/G-3) were investigated. 2.2 Structural characterization. The crystal structures of the products were investigated by X-ray diffraction (XRD) recorded on a Bruker D8 diffractometer using filtered Cu-Kα radiation. equipped with Cu-Kα radiation (λ=0.15406 nm) in the range of 10-80°. The morphologies and 4
microstructures of the samples were observed by SEM (Hitachi 4800) and TEM (FEI F20 S-TWIN). N2 adsorption–desorption measurements were performed on a BELSORP-mini type BET analyzer. The X-ray photoelectron spectra (XPS) were performed with an ESCALAB MARK II spherical analyzer using an aluminum anode (Al 1486.6 eV) X-ray source. 2.3 Electrochemical Measurements. To prepare the electrode, the active materials (ZVO@C/G-1, ZVO@C/G-2, ZVO@C/G-3), a conductive agent (acetylene black) and a binder (PVDF) with mass ratio of 70:20:10 in N-methyl-2-pyrrolidone (NMP) were mixed, followed by posting on copper foil. The electrolyte for LIBs was 1 M LiPF6 composed of EC, EMC and DMC with a ratio of 1:1:1 (v/v/v) and the metallic lithium foil served as reference electrodes. Subsequently, the standard CR2025 coin cells were assembled in an argonfilled glove box. Tests of constant galvanostatic charge-discharge curves and cycling performance were measured with Land CT 2001A tester. The cyclic voltammograms (CV) and electrochemical impedance spectroscopy (EIS) were achieved on an electrochemical workstation at room temperature (Ivium, Netherlands).
3. Results and discussion Fig. 1 shows the XRD patterns of the three ZVO@C/G-1, ZVO@C/G-2, ZVO@C/G-3 samples. The samples displayed clear diffraction peaks that could be indexed well to pure phase of Zn3V3O8 (JCPDS, no. 031-1477). The primary diffraction peaks at 2θ =18.4°, 30.1°, 35.5°, 43.1°, 53.4° and 56.9° can be attributed to 5
the (111), (220), (311), (400), (422) and (511). No characteristic peaks were observed for other impurities, indicating the high purity of the samples. The ZVO@C/G-1, ZVO@C/G-2 and ZVO@C/G-3 samples were prepared by a simple one-pot ethanol thermal reduction strategy with regulating different amount of CTAB
utilization.
The
morphologies
of the
ZVO@C/G-1,
ZVO@C/G-2,
ZVO@C/G-3 samples were characterized by SEM measurement (Fig. 2). Fig. 2a-b shows the SEM images of the ZVO@C/G-1, it was composed of staked particles with size about 100 nm. Adding appropriate amount of CTAB leads to form uniform and dispersed hierarchical lychee-like spheres which were self-embedded in the wrinkled rGO nanosheets (Fig. 2c-d). Each hierarchical Zn3V3O8 sphere maintains spherical morphology (diameter is ca. 400 nm) while its surface is coarse and with porous structure. The ZVO@C/G-3 consists of closely packed particles of spheres with diameter ranging ca. 900 nm (Fig 2e-f). The hierarchical ZVO@C/G-2 composite was characterized by TEM measurement (Fig. 3). According to Fig. 3a-b, the hierarchical ZVO@C/G-2 microspheres are well anchored on the thin rGO layers with some folding on the edge. As demonstrated in Fig. 3c, the lattice fringes with spacing of 0.482 and 0.252 nm are the characteristics of (111) and (311) planes for Zn3V3O8, respectively. Furthermore, we can also clearly observe that the thin carbon layers were formed on the surface of Zn3V3O8 nanoparticles which formed by CTAB as a surfactant encapsulates the internal structure (Fig. 3c). EDS element mapping result (Fig. 3d) of the ZVO@C/G-2 evidently confirmed that the Zn, V, O and C are uniformly distributed throughout the 6
whole composite. On the basis of the above measurements, it is evident that the hierarchical Zn3V3O8@C/rGO composite with a unique lychee-like architecture was fabricated by a facile ethanol thermal reduction strategy, with appropriate amount of CTAB utilization and graphene oxides supported. X-ray photoelectron spectra (XPS) analysis was carried out to characterize the surface chemical information and the valence changes of the products. The ZVO@C/G-2 sample is consist of Zn, V, O and C without anther impurities in Fig. 4a. As depicted in Fig. 4b, the peaks at 1021.4 and 1044.5 eV observed in the XPS spectrum of the Zn2+ can be attributed to Zn 2p3/2 and Zn 2p1/2, respectively. For V 2p XPS spectra (Fig. 4c), whereas the two fitting peaks at 516.3 and 523.3 eV are assigned to 2p3/2 and 2p1/2 of V3+, respectively. The two fitting peaks at 517.2 and 524.6 eV corresponding to 2p3/2 and 2p1/2 of V4+, respectively[37, 38]. The main O 1s peaks at 530.2 and 531.4 eV can be assigned to the lattice-oxygen and adsorbed-oxygen[26,
39]
. As shown in Fig. 4d, here reveals the existence of three
carbon states at 284.5, 285.1 and 288.3 eV, which is due to the presence of carbon in different environments in the sample. The peak at 284.5 eV represents sp2-bond carbon (C=C) signal, which was a feasible rearrangement by carbonizing CTAB during the synthetic process. The remaining peaks at 285.1 and 288.3 eV could be assigned to the C-O and C=O bonds, which could be due to carbon deposited on the surface[40, 41]. The porous character of ZVO@C/G-2 composite was confirmed by the N2 adsorption-desorption isotherms (Fig. 5). As shown in Fig. 5, the ZVO@C/G-2 7
composite displays a typical type IV adsorption-desorption curve, corresponding to mesoporous feature. The BET specific surface area is 20 m2 g-1 for ZVO@C/G-2 composite, accompanied by the pore volume of 0.168 cm3 g-1 . It can also be found that a continuous pore distribution in ZVO@C/G-2 composite, which corresponds well with the results of the SEM and TEM measurements. We investigated the electrochemical performances of different ZVO@C/G-1, ZVO@C/G-2 and ZVO@C/G-3 samples to validate our hypothesis (see the introduction section). The cyclic voltammetry (CV) curves of ZVO@C/G-2 electrodes are shown in Fig. 6a, which tested by CV measurement for initial 5 cycles at scanning rate of 0.5 mV s-1 between 0.01 and 3.0 V. During the first cathodic scanning, we can find two obvious irreversible peak at approximately 0.05 and near 0.540 V, which can be ascribed to the transformation of Zn3V3O8 accompanied by the formation of ZnO and the reduction of Zn2+ to Zn and form Li-Zn, respectively. However, the two reduction peaks disappeared in the following cycles, which most likely due to the formation of a solid electrolyte interface (SEI) layer and electrolyte decomposition by an irreversible reaction[42]. The broad peak at 1.54 V can be attributed to the reduction process of vanadium ions[43]. And in the anodic process, three oxidation peaks at 0.25, 1.16 and 2.35 V were recorded. The first oxidation peak was attributed the de-lithiation of Li-Zn alloys. And the second peak can be attributed to oxidation of V2+ into V3+, in the meantime, accompanied by Zn into Zn2+. During following cycles, the intensities and integral areas of peaks were almost the same, or even slightly increased, suggesting the good reversibility of lithium insertion and 8
extraction reactions[44-47]. The reaction mechanism of Zn3V3O8 as a Li-ion anode material may be described by multiple steps as follows[48-51]: VOx + yLi+ + ye- → LiyVOx
(1)
ZnO + 2Li+ + 2e- → Zn + Li2O
(2)
Zn + xLi+ + xe- → LixZn
(3)
Fig. 6b displays the charge-discharge profiles of ZVO@C/G-2 at 0.5 A g-1 within a voltage range of 0.01-3.0 V (versus Li/Li+). The first cycle delivers a discharge capacity of the hierarchical lychee shell-like ZVO@C/G-2 electrode are presented that the initial discharge capacity is about 810 mAh g-1 at the current density of 0.5 A g-1 within a voltage range of 0.01-3.0 V (versus Li/Li+). Particularly, as the number of loops increases, the capacity of ZVO@C/G-2 electrode also rises from the 2nd cycle. It corresponds well with the results of the CV measurement, indicating the highly reversible performance of ZVO@C/G-2 electrode[52]. To gain further insight into the reason of carbon coating and rGO addition for the high capacity and outstanding rate performance of ZVO@C/G-2 composite electrode, the CV profiles of the electrode from 0.1 to 1.0 mV·s−1 were carried out (Fig. 7a). The obtained storage system could be evaluated using the following two equations [53, 54]: i = a·ʋb
(1)
log i = b·log ʋ + log a
(2)
where i is the measured current, ʋ is the scan rate, and both a and b are adjustable constant parameters. If b value is equal to 0.5, the lithium storage process represents diffusion controlled; whereas the b value approaches 1.0, lithium storage process is 9
dominated by pseudocapacitance. In the present work, the relationship between log i and log ʋ of the ZVO@C/G-2 electrodes (Fig. 7b), the calculated b values were 0.89 (peak-1), 0.83 (peak-2), and 0.86 (peak-3), respectively. In a word, the electrochemical
behaviors
of
ZVO@C/G-2
electrode
originate
from
the
pseudocapacitive effect and lithium storage process, which leads to the high capacity of the ZVO@C/G-2 electrode. In addition, to study deep contribution of carbon coating and rGO, the capacitor capacity was evaluated quantitatively according to the following equation[55, 56]: i(ʋ) =k1 ʋ + k2·ʋ1/2
(3)
where i(ʋ) is the measured current response at a fixed potential V, ʋ is the scan rate, and k1 and k2 are adjustable constant parameters. In the case of the capacitive effects, i is proportional to the scan rate ʋ, while for the diffusion-controlled reactions, i is proportion to k2·ʋ1/2. The total capacity contribution at a given scan rate could be distinguished into two parts (Fig. 7c). As the scan rate ʋ increased (from 0.1 to 1.0 mV s-1), the capacitive contributions of the ZVO@C/G-2 electrode are 51%, 57%, 65%, 77% and 84%, respectively. These results strongly indicate that rGO addition and carbon coating layer can effectively improve electrochemical performance of the ZVO@C/G-2 electrode. The cycling performances of the different ZVO@C/G-1, ZVO@C/G-2 and ZVO@C/G-3 samples at 0.1 A g-1 and 0.5 A g-1 are described in Fig. 8a-b. No capacity fading is observed during cycling for hierarchical ZVO@C/G-2 electrode. It delivers high capacities of 1012 mAh g-1 and 727 mAh g-1 at 0.1 A g-1 and 0.5 A g-1 10
for 200 cycles and 400 cycles, respectively. In contrast, the ZVO@C/G-1 and ZVO@C/G-3 electrodes show relatively poor cycling performance, and the discharge capacities reach to 248 and 430 mAh g-1 at 0.5 A g-1 after 400 cycles, as shown in Fig. 8b. Although the phases of ZVO@C/G-1 and ZVO@C/G-3 composites are similar to that of ZVO@C/G-2, the structure and morphology effects play more important role in elevating the electrochemical performance. As previous depicted in Fig. 2, ZVO@C/G-1 and ZVO@C/G-3 composites show more unwrapped and agglomerate ZVO particles or spheres, resulting in the degradation of electrochemical properties. We further investigated the rate capabilities of ZVO@C/G-1, ZVO@C/G-2 and ZVO@C/G-3 samples at different current densities from 0.1 to 2 A g-1 (Fig. 8c). It demonstrates that the reversible capacities of ZVO@C/G-2 composite are 609, 606, 577, 514 and 441 mAh g-1 at the current densities of 0.1, 0.2, 0.5, 1 and 2 A g-1, respectively, indicating the superior rate performance of ZVO@C/G-2. In addition, the ZVO@C/G-2 even maintains an increasing capacity of 765 mAh g-1 when current density is restored to 0.1 A g-1, implying great reversibility and cyclic stability. In addition, for ZVO@C/G-1 and ZVO@C/G-3, they deliver irreversible capacities of 464, 431, 388, 334, 278 mAh g-1 and 525, 546, 488, 439, 387 mAh g-1 at the current densities of 0.1, 0.2, 0.5, 1 and 2 A g-1, respectively. The long-term cycling performance of ZVO@C/G-2 electrode is conducted at high current density of 4 A g-1 (Fig. 8d). The ZVO@C/G-2 composite deliver a high reversible capacity of 448 mAh g-1 after 1000 cycles. Impressively, the Coulombic
11
efficiency is close to 100% after the initial few cycles, which implying the endearing long-term cycling stability performance. To understand the underlying mechanisms, EIS measurements were carried out. The spectrum of the ZVO@C/G-1, ZVO@C/G-2 and ZVO@C/G-3 composites were displayed (Fig. 9). The simplified equivalent circuit in the inset of Fig. 9 was used for fitting the raw results. The EIS plots of the three electrodes are constituted by a depressed semicircle in the medium/high frequency region and a linear Warburg part in low frequency region. The circuit elements consist of the electrolyte resistance(Re); the semicircle at the medium/high frequency region is ascribed to the impedance (R f) of the SEI film, and the charge-transfer impedance (Rct) of the electrode/electrolyte interface. In medium/high frequency region, the diameter of the semicircle of ZVO@C/G-2 composite is smaller than those of the ZVO@C/G-1 and the ZVO@C/G-3. As seen from Table 1, the Rct value of the ZVO@C/G-2 (26.18Ω) is lower than those of ZVO@C/G-1 (36.71Ω) and ZVO@C/G-3 (61.73Ω), suggesting that lowest Rct of the ZVO@C/G-2 among the three anodes. Besides, in the low frequency region, the ZVO@C/G-2 displayed highest slope angle of the linear Warburg parts, reflecting enhanced electrochemical reaction kinetics of the ZVO@C/G-2 composite[57]. The results indicate that the hierarchical ZVO@C/G-2 composite with a unique lychee-like architecture and diverse carbon coating (rGO and thin carbon layer) can facilitate electrolytic infiltration and diffusion of Li+. As graphically illustrated in Scheme 1, the superior high capacity and long-term cycling stability of hierarchical Zn3V3O8 @C/rGO composite might be mainly 12
attributed to the following factors, hierarchical spheres with porous structure, small nano-crystal size, continuous carbon substrate and tightly wrapped reduced graphene oxide layer. The hierarchical spheres with porous structure ensures a aggregation-free property during lithiation and delithiation, alleviates the volume change during the electrochemical reaction as well as favours the electrolyte penetration and electron transportation. The small nano-crystal size can offer the short diffusion path for lithium-ion intercalation, thus enhancing the rate capability. The continuous carbon substrate and tightly wrapped reduced graphene oxide layer ensure expedite electron transfer paths, and act as a buffer that inhibits the volume change of electrode during cycling, leading to a excellent cycling stability. As seen from Table 2, the electrochemical performance of the hierarchical Zn3V3O8@C/rGO composite electrode was comparable with other previous reported vanadium-based electrodes for LIBs.
4. Conclusions In summary, the hierarchical Zn3V3O8@C/rGO composite with a unique lychee-like architecture was successfully fabricated by a facile one-pot solvothermal method, with regulating different amount of CTAB utilization and graphene oxides supported. The hierarchical Zn3V3O8@C/rGO composite with porous structure prepared in this study could endow Zn3V3O8@C particles with hierarchical structure and combine reduced graphene oxide sheets (rGO) with Zn3V3O8@C sphere simultaneously. When used as an anode material for LIBs, it exhibits a remarkable reversible capacity, high-rate performance and superior long-term cycling stability 13
(over 1000 cycles). We believe that the present work will open an effective avenue to exploit advanced hierarchical nanocomposite for energy application, not only in lithium-ion batteries but also in other areas such as sodium-ion batteries, supercapacitors, electrocatalysts, etc.
Acknowledgements This work was financially supported by National Natural Science Foundation of China (NSFC 51502036, U1505241, 21407025 and 21307012), National Key Research and Development Program of China (2016YFB0302303), the Outstanding Youth Research Training Program of University of Fujian Province and Natural Science Foundation of Fujian Province (2016J05116).
Conflict of interests Authors declare no conflict of interests.
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Figure captions Fig. 1 The XRD patterns of the three ZVO@C/G-1, ZVO@C/G-2, ZVO@C/G-3 samples. Fig. 2 The SEM images of the (a, b) ZVO@C/G-1, (c, d) ZVO@C/G-2 and (e, f) ZVO@C/G-3 samples with regulating different amount of CTAB utilization. Fig. 3 The (a, b) TEM and (c) HR-TEM images of the ZVO@C/G-2 microspheres, as well as (d) the corresponding elemental mapping results for Zn (green), V (red), O (purple) and C (blue). Fig. 4 (a) XPS survey of the ZVO@C/G-2 microspheres. High resolution XPS spectra of (b) Zn 2p, (c) V 2p, (d) O 1s, (e) C 1s. Fig. 5 N2 adsorption-desorption isotherms and the corresponding pore size distribution (inset) of the ZVO@C/G-2. Fig. 6 (a) Cyclic voltammentry curves of the ZVO@C/G-2 at a scan rate of 0.5 mV s-1. -1
(b) The charge-discharge profiles of ZVO@C/G-2 at 0.5 A g within a voltage range of 0.01-3.0 V. Fig. 7 (a) CV curves of the ZVO@C/G-2 electrode at different scan rates from 0.1 to −1
1.0 mV s ; (b) log i vs. log v plots for the oxidation and reduction states of the ZVO@C/G-2 electrode; (c) normalized contribution ratio of capacitive and diffusion-controlled capacities of the ZVO@C/G-2 electrode at different scan rates. Fig. 8 The cycling performance of ZVO@C/G-1, ZVO@C/G-2 and ZVO@C/G-3 electrodes at the current density of (a) 0.1 A g-1 and (b) 0.5 A g-1, as well as (c) the rate capability at different current densities between 0.1 and 2 A g-1. (d) Long-term cycling performance and Coulombic efficiency of the ZVO@C/G-2 electrode at high current density of 4 A g-1.
23
Fig. 9 Electrochemical impedance spectra (EIS) of ZVO@C/G-1, ZVO@C/G-2 and ZVO@C/G-3 electrodes. Inset: the equivalent circuit used to fit the original data. Scheme 1 Schematic illustration of the excellent structure stability in hierarchical Zn3V3O8@C/rGO composite during the electrochemical charge-discharge processes.
Fig. 1
24
Fig. 2
25
Fig. 3
26
Fig. 4
Fig. 5
27
Fig. 6
Fig. 7
28
Fig. 8
29
Fig. 9
Scheme 1
30
Table 1 Impedance parameters calculated from an equivalent circuit model.
Sample
Re (Ω)
Rf (Ω)
Rct (Ω)
ZVO@C/G-1
8.85
24.65
36.71
ZVO@C/G-2
6.93
19.96
26.18
ZVO@C/G-3
3.53
18.40
61.73
Table 2 Comparisons of selected performance metrics of vanadates LIBs electrodes.
Electrode materials
Specific capacity (mAh g-1)
Current density (mAh g-1)
Number of cycles
Ref.
Hierarchical LiZnVO4@C
547
150
150
[16]
Zn3V2O8 porous sheets
1228
300
200
[18]
Zn3V2O7(OH)2·•2H2O microflowers
298
5000
1400
[20]
ZnV2O4 nanophase
660
50
200
[21]
Clewlike ZnV2O4 hollow spheres
524
50
50
[22]
Ultrathin Zn2(OH)3VO3 nanosheets
545
1000
500
[23]
Hierarchical ZnV2O4 microspheres
638
100
200
[24]
ZnV2O4–CMK nanocomposite
575
100
200
[25]
Three-dimensional Zn3V3O8/carbon fiber cloth composites
455
2000
500
[26]
Zn3V2O8/GNPs
488
3200
400
[27]
Hierarchical Zn3V3O8/C composite microspheres
912
400
200
[35]
Zn3V3O8 microspheres
1019
100
100
[48]
31
Zn3V3O8 nanosheets
537
120
200
[49]
Ultrathin ZVO nanosheets
602
1000
980
[51]
Hierarchical lychee-like Zn3V3O8@C/rGO microspheres
1012 448
100 4000
200 1000
This work This work
32
Graphical abstract
The hierarchical Zn3V3O8@C/rGO composite with a unique lychee-like architecture was fabricated by a simple one-pot ethanol thermal reduction strategy. When used as an anode material for lithium-ion batteries, it exhibited high reversible capacities (1012 mAh g−1 at 0.1 A g−1 after 200 cycles) and high rate stability (448 mAh g−1 at 4 A g−1 after 1000 cycles).
33
S0021-9797(18)31033-6 https://doi.org/10.1016/j.jcis.2018.08.110 YJCIS 24043
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
8 July 2018 28 August 2018 29 August 2018
Please cite this article as: H. Xue, Y. Fang, L. Zeng, X. He, F. Luo, R. Liu, J. Liu, Q. Chen, M. Wei, Q. Qian, Facile synthesis of hierarchical lychee-like Zn3V3O8@C/rGO nanospheres as high-performance anodes for lithium ion batteries, Journal of Colloid and Interface Science (2018), doi: https://doi.org/10.1016/j.jcis.2018.08.110
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Facile synthesis of hierarchical lychee-like Zn3V3O8@C/rGO nanospheres as high-performance anodes for lithium ion batteries
Hun Xuea, c, Yixing Fanga, Lingxing Zeng*a, c, Xiaotong Hea, Fenqiang Luoa, Renpin Liua, Junbin Liua, Qinghua Chena, c, Mingdeng Wei,b and Qingrong Qian*a, c a
Engineering Research Center of Polymer Green Recycling of Ministry of Education, College of Environmental Science and Engineering, Fujian Normal University, Fuzhou, Fujian 350007, China. b Institute of Advanced Energy Materials, Fuzhou University, Fuzhou, Fujian 350002, China. c Fujian Key Laboratory of Pollution Control & Resource Reuse, Fuzhou, Fujian 350007, China
Abstract In the present work, the hierarchical Zn3V3O8@C/rGO composite with a unique lychee-like architecture was fabricated by a simple one-pot ethanol thermal reduction strategy. When used as an anode material, it exhibited outstanding electrochemical performance for lithium-ion batteries (LIBs). For instance, the Zn3V3O8@C/rGO composite delivers high reversible capacities (1012 mAh g−1 at 0.1 A g−1 after 200 cycles) and high rate stability (448 mAh g−1 at 4 A g−1 after 1000 cycles). This outstanding performance can be attributed to the synergistic effect of the diverse structural virtues, effective interface and dual-spatially hybrid carbon network. Significantly, this one-pot simple strategy can be extended to fabricating highly stable and high rate performance of vanadates or other anode materials for LIBs. Keywords: Zn3V3O8@C/rGO; Lychee-like architecture; Ethanol thermal reduction; Lithium-ion batteries; Anode
*
Corresponding authors. Tel. & Fax: +86-591 8346 5158 [email protected] (L. Zeng), [email protected] (Q. Qian) 1
1. Introduction With the development of energy demand for societies of the world, and increasingly serious environmental pollution brought by the traditional energy, exploiting clean and renewable energy technology is becoming a hotspot of world[1]. Among various energy storage technologies, lithium-ion batteries (LIBs) is one of the most important renewable energy storage devices in recent years[2]. The anode in commercial LIBs is mainly made of graphite, which possesses a lower theoretical capacity (372 mAh g-1), insufficient to meet the increasing demand for high power density and long cycle life of LIBs[3, 4]. Therefore, tremendous efforts have been devoted to design novel high performance anode material for LIBs, which are desired for practical applications in future[5-11]. Vanadates have been investigated as the electrode materials for LIBs due to their high theoretical capacities and extraordinary morphology[12, 13]. For instance, Luo et al. reported Co 3V2O8 interconnected hollow microspheres were fabricated by hydrothermal reaction, which exhibited a stable capacity of 424 mAh g-1 at 10 A g-1 after 300 cycles[14]. Wang’s group synthesized the quasi-cuboidal CoV2O6 and it delivered a good capacity of ca. 702 mAh g-1 (200 mA g-1, 200 cycles)[15]. Among these vanadates, zinc vanadium oxides with variable states of vanadium and rich source of zinc, have gradually attracted much attention as anode materials of LIBs. they possess impressive electrochemical performance due to the variable states of vanadium stimulating flexible architecture[16, Z3V2O8[18], ultralong
monoclinic ZnV2O6 2
17]
. For example, porous sheet-like
nanowires[19],
Zn3V2O7(OH)2•2HO
microflowers[20], spinel ZnV2O4 nanoparticles[21] were explored and so on[22, 23]. In our previous work, we already have displayed the hierarchical ZnV2O4 microspheres[24] and ZnV2O4-CMK nanocomposite[25] , which exhibited large reversible capacity and good cycling stability. However, the large volume changes and phase transitions cause severe pulverization of zinc vanadium oxides electrode during Li+ intercalation process, resulting in capacity fading, especially after long-term cycling process. To exalt the electrochemical performance of zinc vanadium oxides, many strategies have been conducted including fabricating zinc vanadium oxides −carbon composite and the formation of novel nanostructure[26, 27]. Recently, the reduced graphene oxide (rGO) has been proven to be an effective carbon matrix for enhancing the stability of electrode[28-32]. In the meantime, hierarchical carbon composite with porous structure not only buffers the strain and volume changes during cycling, but also possesses high-efficiency electron transport and Li+ diffusion[33-36]. However, to the best of our knowledge, these is rarely report on combination of graphene with hierarchical Zn3V3O8 porous spheres with high capacity and robust stability for LIBs. In the present work, the hierarchical Zn3V3O8@C/rGO composite with a unique lychee-like architecture was prepared by a simple one-pot ethanol thermal reduction strategy, with regulating different amount of CTAB utilization and graphene oxides supported. The unique lychee-like Zn3V3O8@C/rGO composite prepared in this study could endow Zn3V3O8@C particles with hierarchical structure and combine reduced graphene oxide sheets (RGO) with Zn3V3O8@C sphere simultaneously. Due to this unique structure, the hierarchical Zn3V3O8@C/rGO composite with a unique 3
lychee-like architecture exhibits a remarkable reversible capacity, high-rate performance and superior long-term cycling stability (over 1000 cycles) as the anode material for LIBs.
2. Experimental Section 2.1 Synthesis of Zn3V3O8@C/rGO. The hierarchical Zn3V3O8@C/rGO (ZVO@C/G) nanospheres were synthesized by one step ethanol thermal reduction method. In a typical synthesis procedure, 0.3 mmol of zinc nitrate hexahydrate, 0.3 mmol of ammonium vanadate and 0.15 g of cetyltrimethylammonium bromide (CTAB) were dissolved in 12 mL of ethanol. Subsequently, a proper amount of graphene oxide (GO) (2 mg/ml, 18 ml) ethanol solution was added into the above solution with vigorous magnetic stirring for 5 min and then dispersed by ultrasonication for 20 min. Then, the obtained mixture was transferred to a 50 mL Teflon-lined autoclave and maintained at 200 °C for 5 days. After cooling naturally to room temperature, the product was collected and washed three times with ethanol, then was lyophilized for 12 h. As a result, the lychee-like ZVO@C/G was obtained. For a comparison, the effect of different amounts (0.075 g, 0.15 g or 0.3 g) of CTAB on the structures and morphologies of samples (denoted as ZVO@C/G-1, ZVO@C/G-2, ZVO@C/G-3) were investigated. 2.2 Structural characterization. The crystal structures of the products were investigated by X-ray diffraction (XRD) recorded on a Bruker D8 diffractometer using filtered Cu-Kα radiation. equipped with Cu-Kα radiation (λ=0.15406 nm) in the range of 10-80°. The morphologies and 4
microstructures of the samples were observed by SEM (Hitachi 4800) and TEM (FEI F20 S-TWIN). N2 adsorption–desorption measurements were performed on a BELSORP-mini type BET analyzer. The X-ray photoelectron spectra (XPS) were performed with an ESCALAB MARK II spherical analyzer using an aluminum anode (Al 1486.6 eV) X-ray source. 2.3 Electrochemical Measurements. To prepare the electrode, the active materials (ZVO@C/G-1, ZVO@C/G-2, ZVO@C/G-3), a conductive agent (acetylene black) and a binder (PVDF) with mass ratio of 70:20:10 in N-methyl-2-pyrrolidone (NMP) were mixed, followed by posting on copper foil. The electrolyte for LIBs was 1 M LiPF6 composed of EC, EMC and DMC with a ratio of 1:1:1 (v/v/v) and the metallic lithium foil served as reference electrodes. Subsequently, the standard CR2025 coin cells were assembled in an argonfilled glove box. Tests of constant galvanostatic charge-discharge curves and cycling performance were measured with Land CT 2001A tester. The cyclic voltammograms (CV) and electrochemical impedance spectroscopy (EIS) were achieved on an electrochemical workstation at room temperature (Ivium, Netherlands).
3. Results and discussion Fig. 1 shows the XRD patterns of the three ZVO@C/G-1, ZVO@C/G-2, ZVO@C/G-3 samples. The samples displayed clear diffraction peaks that could be indexed well to pure phase of Zn3V3O8 (JCPDS, no. 031-1477). The primary diffraction peaks at 2θ =18.4°, 30.1°, 35.5°, 43.1°, 53.4° and 56.9° can be attributed to 5
the (111), (220), (311), (400), (422) and (511). No characteristic peaks were observed for other impurities, indicating the high purity of the samples. The ZVO@C/G-1, ZVO@C/G-2 and ZVO@C/G-3 samples were prepared by a simple one-pot ethanol thermal reduction strategy with regulating different amount of CTAB
utilization.
The
morphologies
of the
ZVO@C/G-1,
ZVO@C/G-2,
ZVO@C/G-3 samples were characterized by SEM measurement (Fig. 2). Fig. 2a-b shows the SEM images of the ZVO@C/G-1, it was composed of staked particles with size about 100 nm. Adding appropriate amount of CTAB leads to form uniform and dispersed hierarchical lychee-like spheres which were self-embedded in the wrinkled rGO nanosheets (Fig. 2c-d). Each hierarchical Zn3V3O8 sphere maintains spherical morphology (diameter is ca. 400 nm) while its surface is coarse and with porous structure. The ZVO@C/G-3 consists of closely packed particles of spheres with diameter ranging ca. 900 nm (Fig 2e-f). The hierarchical ZVO@C/G-2 composite was characterized by TEM measurement (Fig. 3). According to Fig. 3a-b, the hierarchical ZVO@C/G-2 microspheres are well anchored on the thin rGO layers with some folding on the edge. As demonstrated in Fig. 3c, the lattice fringes with spacing of 0.482 and 0.252 nm are the characteristics of (111) and (311) planes for Zn3V3O8, respectively. Furthermore, we can also clearly observe that the thin carbon layers were formed on the surface of Zn3V3O8 nanoparticles which formed by CTAB as a surfactant encapsulates the internal structure (Fig. 3c). EDS element mapping result (Fig. 3d) of the ZVO@C/G-2 evidently confirmed that the Zn, V, O and C are uniformly distributed throughout the 6
whole composite. On the basis of the above measurements, it is evident that the hierarchical Zn3V3O8@C/rGO composite with a unique lychee-like architecture was fabricated by a facile ethanol thermal reduction strategy, with appropriate amount of CTAB utilization and graphene oxides supported. X-ray photoelectron spectra (XPS) analysis was carried out to characterize the surface chemical information and the valence changes of the products. The ZVO@C/G-2 sample is consist of Zn, V, O and C without anther impurities in Fig. 4a. As depicted in Fig. 4b, the peaks at 1021.4 and 1044.5 eV observed in the XPS spectrum of the Zn2+ can be attributed to Zn 2p3/2 and Zn 2p1/2, respectively. For V 2p XPS spectra (Fig. 4c), whereas the two fitting peaks at 516.3 and 523.3 eV are assigned to 2p3/2 and 2p1/2 of V3+, respectively. The two fitting peaks at 517.2 and 524.6 eV corresponding to 2p3/2 and 2p1/2 of V4+, respectively[37, 38]. The main O 1s peaks at 530.2 and 531.4 eV can be assigned to the lattice-oxygen and adsorbed-oxygen[26,
39]
. As shown in Fig. 4d, here reveals the existence of three
carbon states at 284.5, 285.1 and 288.3 eV, which is due to the presence of carbon in different environments in the sample. The peak at 284.5 eV represents sp2-bond carbon (C=C) signal, which was a feasible rearrangement by carbonizing CTAB during the synthetic process. The remaining peaks at 285.1 and 288.3 eV could be assigned to the C-O and C=O bonds, which could be due to carbon deposited on the surface[40, 41]. The porous character of ZVO@C/G-2 composite was confirmed by the N2 adsorption-desorption isotherms (Fig. 5). As shown in Fig. 5, the ZVO@C/G-2 7
composite displays a typical type IV adsorption-desorption curve, corresponding to mesoporous feature. The BET specific surface area is 20 m2 g-1 for ZVO@C/G-2 composite, accompanied by the pore volume of 0.168 cm3 g-1 . It can also be found that a continuous pore distribution in ZVO@C/G-2 composite, which corresponds well with the results of the SEM and TEM measurements. We investigated the electrochemical performances of different ZVO@C/G-1, ZVO@C/G-2 and ZVO@C/G-3 samples to validate our hypothesis (see the introduction section). The cyclic voltammetry (CV) curves of ZVO@C/G-2 electrodes are shown in Fig. 6a, which tested by CV measurement for initial 5 cycles at scanning rate of 0.5 mV s-1 between 0.01 and 3.0 V. During the first cathodic scanning, we can find two obvious irreversible peak at approximately 0.05 and near 0.540 V, which can be ascribed to the transformation of Zn3V3O8 accompanied by the formation of ZnO and the reduction of Zn2+ to Zn and form Li-Zn, respectively. However, the two reduction peaks disappeared in the following cycles, which most likely due to the formation of a solid electrolyte interface (SEI) layer and electrolyte decomposition by an irreversible reaction[42]. The broad peak at 1.54 V can be attributed to the reduction process of vanadium ions[43]. And in the anodic process, three oxidation peaks at 0.25, 1.16 and 2.35 V were recorded. The first oxidation peak was attributed the de-lithiation of Li-Zn alloys. And the second peak can be attributed to oxidation of V2+ into V3+, in the meantime, accompanied by Zn into Zn2+. During following cycles, the intensities and integral areas of peaks were almost the same, or even slightly increased, suggesting the good reversibility of lithium insertion and 8
extraction reactions[44-47]. The reaction mechanism of Zn3V3O8 as a Li-ion anode material may be described by multiple steps as follows[48-51]: VOx + yLi+ + ye- → LiyVOx
(1)
ZnO + 2Li+ + 2e- → Zn + Li2O
(2)
Zn + xLi+ + xe- → LixZn
(3)
Fig. 6b displays the charge-discharge profiles of ZVO@C/G-2 at 0.5 A g-1 within a voltage range of 0.01-3.0 V (versus Li/Li+). The first cycle delivers a discharge capacity of the hierarchical lychee shell-like ZVO@C/G-2 electrode are presented that the initial discharge capacity is about 810 mAh g-1 at the current density of 0.5 A g-1 within a voltage range of 0.01-3.0 V (versus Li/Li+). Particularly, as the number of loops increases, the capacity of ZVO@C/G-2 electrode also rises from the 2nd cycle. It corresponds well with the results of the CV measurement, indicating the highly reversible performance of ZVO@C/G-2 electrode[52]. To gain further insight into the reason of carbon coating and rGO addition for the high capacity and outstanding rate performance of ZVO@C/G-2 composite electrode, the CV profiles of the electrode from 0.1 to 1.0 mV·s−1 were carried out (Fig. 7a). The obtained storage system could be evaluated using the following two equations [53, 54]: i = a·ʋb
(1)
log i = b·log ʋ + log a
(2)
where i is the measured current, ʋ is the scan rate, and both a and b are adjustable constant parameters. If b value is equal to 0.5, the lithium storage process represents diffusion controlled; whereas the b value approaches 1.0, lithium storage process is 9
dominated by pseudocapacitance. In the present work, the relationship between log i and log ʋ of the ZVO@C/G-2 electrodes (Fig. 7b), the calculated b values were 0.89 (peak-1), 0.83 (peak-2), and 0.86 (peak-3), respectively. In a word, the electrochemical
behaviors
of
ZVO@C/G-2
electrode
originate
from
the
pseudocapacitive effect and lithium storage process, which leads to the high capacity of the ZVO@C/G-2 electrode. In addition, to study deep contribution of carbon coating and rGO, the capacitor capacity was evaluated quantitatively according to the following equation[55, 56]: i(ʋ) =k1 ʋ + k2·ʋ1/2
(3)
where i(ʋ) is the measured current response at a fixed potential V, ʋ is the scan rate, and k1 and k2 are adjustable constant parameters. In the case of the capacitive effects, i is proportional to the scan rate ʋ, while for the diffusion-controlled reactions, i is proportion to k2·ʋ1/2. The total capacity contribution at a given scan rate could be distinguished into two parts (Fig. 7c). As the scan rate ʋ increased (from 0.1 to 1.0 mV s-1), the capacitive contributions of the ZVO@C/G-2 electrode are 51%, 57%, 65%, 77% and 84%, respectively. These results strongly indicate that rGO addition and carbon coating layer can effectively improve electrochemical performance of the ZVO@C/G-2 electrode. The cycling performances of the different ZVO@C/G-1, ZVO@C/G-2 and ZVO@C/G-3 samples at 0.1 A g-1 and 0.5 A g-1 are described in Fig. 8a-b. No capacity fading is observed during cycling for hierarchical ZVO@C/G-2 electrode. It delivers high capacities of 1012 mAh g-1 and 727 mAh g-1 at 0.1 A g-1 and 0.5 A g-1 10
for 200 cycles and 400 cycles, respectively. In contrast, the ZVO@C/G-1 and ZVO@C/G-3 electrodes show relatively poor cycling performance, and the discharge capacities reach to 248 and 430 mAh g-1 at 0.5 A g-1 after 400 cycles, as shown in Fig. 8b. Although the phases of ZVO@C/G-1 and ZVO@C/G-3 composites are similar to that of ZVO@C/G-2, the structure and morphology effects play more important role in elevating the electrochemical performance. As previous depicted in Fig. 2, ZVO@C/G-1 and ZVO@C/G-3 composites show more unwrapped and agglomerate ZVO particles or spheres, resulting in the degradation of electrochemical properties. We further investigated the rate capabilities of ZVO@C/G-1, ZVO@C/G-2 and ZVO@C/G-3 samples at different current densities from 0.1 to 2 A g-1 (Fig. 8c). It demonstrates that the reversible capacities of ZVO@C/G-2 composite are 609, 606, 577, 514 and 441 mAh g-1 at the current densities of 0.1, 0.2, 0.5, 1 and 2 A g-1, respectively, indicating the superior rate performance of ZVO@C/G-2. In addition, the ZVO@C/G-2 even maintains an increasing capacity of 765 mAh g-1 when current density is restored to 0.1 A g-1, implying great reversibility and cyclic stability. In addition, for ZVO@C/G-1 and ZVO@C/G-3, they deliver irreversible capacities of 464, 431, 388, 334, 278 mAh g-1 and 525, 546, 488, 439, 387 mAh g-1 at the current densities of 0.1, 0.2, 0.5, 1 and 2 A g-1, respectively. The long-term cycling performance of ZVO@C/G-2 electrode is conducted at high current density of 4 A g-1 (Fig. 8d). The ZVO@C/G-2 composite deliver a high reversible capacity of 448 mAh g-1 after 1000 cycles. Impressively, the Coulombic
11
efficiency is close to 100% after the initial few cycles, which implying the endearing long-term cycling stability performance. To understand the underlying mechanisms, EIS measurements were carried out. The spectrum of the ZVO@C/G-1, ZVO@C/G-2 and ZVO@C/G-3 composites were displayed (Fig. 9). The simplified equivalent circuit in the inset of Fig. 9 was used for fitting the raw results. The EIS plots of the three electrodes are constituted by a depressed semicircle in the medium/high frequency region and a linear Warburg part in low frequency region. The circuit elements consist of the electrolyte resistance(Re); the semicircle at the medium/high frequency region is ascribed to the impedance (R f) of the SEI film, and the charge-transfer impedance (Rct) of the electrode/electrolyte interface. In medium/high frequency region, the diameter of the semicircle of ZVO@C/G-2 composite is smaller than those of the ZVO@C/G-1 and the ZVO@C/G-3. As seen from Table 1, the Rct value of the ZVO@C/G-2 (26.18Ω) is lower than those of ZVO@C/G-1 (36.71Ω) and ZVO@C/G-3 (61.73Ω), suggesting that lowest Rct of the ZVO@C/G-2 among the three anodes. Besides, in the low frequency region, the ZVO@C/G-2 displayed highest slope angle of the linear Warburg parts, reflecting enhanced electrochemical reaction kinetics of the ZVO@C/G-2 composite[57]. The results indicate that the hierarchical ZVO@C/G-2 composite with a unique lychee-like architecture and diverse carbon coating (rGO and thin carbon layer) can facilitate electrolytic infiltration and diffusion of Li+. As graphically illustrated in Scheme 1, the superior high capacity and long-term cycling stability of hierarchical Zn3V3O8 @C/rGO composite might be mainly 12
attributed to the following factors, hierarchical spheres with porous structure, small nano-crystal size, continuous carbon substrate and tightly wrapped reduced graphene oxide layer. The hierarchical spheres with porous structure ensures a aggregation-free property during lithiation and delithiation, alleviates the volume change during the electrochemical reaction as well as favours the electrolyte penetration and electron transportation. The small nano-crystal size can offer the short diffusion path for lithium-ion intercalation, thus enhancing the rate capability. The continuous carbon substrate and tightly wrapped reduced graphene oxide layer ensure expedite electron transfer paths, and act as a buffer that inhibits the volume change of electrode during cycling, leading to a excellent cycling stability. As seen from Table 2, the electrochemical performance of the hierarchical Zn3V3O8@C/rGO composite electrode was comparable with other previous reported vanadium-based electrodes for LIBs.
4. Conclusions In summary, the hierarchical Zn3V3O8@C/rGO composite with a unique lychee-like architecture was successfully fabricated by a facile one-pot solvothermal method, with regulating different amount of CTAB utilization and graphene oxides supported. The hierarchical Zn3V3O8@C/rGO composite with porous structure prepared in this study could endow Zn3V3O8@C particles with hierarchical structure and combine reduced graphene oxide sheets (rGO) with Zn3V3O8@C sphere simultaneously. When used as an anode material for LIBs, it exhibits a remarkable reversible capacity, high-rate performance and superior long-term cycling stability 13
(over 1000 cycles). We believe that the present work will open an effective avenue to exploit advanced hierarchical nanocomposite for energy application, not only in lithium-ion batteries but also in other areas such as sodium-ion batteries, supercapacitors, electrocatalysts, etc.
Acknowledgements This work was financially supported by National Natural Science Foundation of China (NSFC 51502036, U1505241, 21407025 and 21307012), National Key Research and Development Program of China (2016YFB0302303), the Outstanding Youth Research Training Program of University of Fujian Province and Natural Science Foundation of Fujian Province (2016J05116).
Conflict of interests Authors declare no conflict of interests.
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Figure captions Fig. 1 The XRD patterns of the three ZVO@C/G-1, ZVO@C/G-2, ZVO@C/G-3 samples. Fig. 2 The SEM images of the (a, b) ZVO@C/G-1, (c, d) ZVO@C/G-2 and (e, f) ZVO@C/G-3 samples with regulating different amount of CTAB utilization. Fig. 3 The (a, b) TEM and (c) HR-TEM images of the ZVO@C/G-2 microspheres, as well as (d) the corresponding elemental mapping results for Zn (green), V (red), O (purple) and C (blue). Fig. 4 (a) XPS survey of the ZVO@C/G-2 microspheres. High resolution XPS spectra of (b) Zn 2p, (c) V 2p, (d) O 1s, (e) C 1s. Fig. 5 N2 adsorption-desorption isotherms and the corresponding pore size distribution (inset) of the ZVO@C/G-2. Fig. 6 (a) Cyclic voltammentry curves of the ZVO@C/G-2 at a scan rate of 0.5 mV s-1. -1
(b) The charge-discharge profiles of ZVO@C/G-2 at 0.5 A g within a voltage range of 0.01-3.0 V. Fig. 7 (a) CV curves of the ZVO@C/G-2 electrode at different scan rates from 0.1 to −1
1.0 mV s ; (b) log i vs. log v plots for the oxidation and reduction states of the ZVO@C/G-2 electrode; (c) normalized contribution ratio of capacitive and diffusion-controlled capacities of the ZVO@C/G-2 electrode at different scan rates. Fig. 8 The cycling performance of ZVO@C/G-1, ZVO@C/G-2 and ZVO@C/G-3 electrodes at the current density of (a) 0.1 A g-1 and (b) 0.5 A g-1, as well as (c) the rate capability at different current densities between 0.1 and 2 A g-1. (d) Long-term cycling performance and Coulombic efficiency of the ZVO@C/G-2 electrode at high current density of 4 A g-1.
23
Fig. 9 Electrochemical impedance spectra (EIS) of ZVO@C/G-1, ZVO@C/G-2 and ZVO@C/G-3 electrodes. Inset: the equivalent circuit used to fit the original data. Scheme 1 Schematic illustration of the excellent structure stability in hierarchical Zn3V3O8@C/rGO composite during the electrochemical charge-discharge processes.
Fig. 1
24
Fig. 2
25
Fig. 3
26
Fig. 4
Fig. 5
27
Fig. 6
Fig. 7
28
Fig. 8
29
Fig. 9
Scheme 1
30
Table 1 Impedance parameters calculated from an equivalent circuit model.
Sample
Re (Ω)
Rf (Ω)
Rct (Ω)
ZVO@C/G-1
8.85
24.65
36.71
ZVO@C/G-2
6.93
19.96
26.18
ZVO@C/G-3
3.53
18.40
61.73
Table 2 Comparisons of selected performance metrics of vanadates LIBs electrodes.
Electrode materials
Specific capacity (mAh g-1)
Current density (mAh g-1)
Number of cycles
Ref.
Hierarchical LiZnVO4@C
547
150
150
[16]
Zn3V2O8 porous sheets
1228
300
200
[18]
Zn3V2O7(OH)2·•2H2O microflowers
298
5000
1400
[20]
ZnV2O4 nanophase
660
50
200
[21]
Clewlike ZnV2O4 hollow spheres
524
50
50
[22]
Ultrathin Zn2(OH)3VO3 nanosheets
545
1000
500
[23]
Hierarchical ZnV2O4 microspheres
638
100
200
[24]
ZnV2O4–CMK nanocomposite
575
100
200
[25]
Three-dimensional Zn3V3O8/carbon fiber cloth composites
455
2000
500
[26]
Zn3V2O8/GNPs
488
3200
400
[27]
Hierarchical Zn3V3O8/C composite microspheres
912
400
200
[35]
Zn3V3O8 microspheres
1019
100
100
[48]
31
Zn3V3O8 nanosheets
537
120
200
[49]
Ultrathin ZVO nanosheets
602
1000
980
[51]
Hierarchical lychee-like Zn3V3O8@C/rGO microspheres
1012 448
100 4000
200 1000
This work This work
32
Graphical abstract
The hierarchical Zn3V3O8@C/rGO composite with a unique lychee-like architecture was fabricated by a simple one-pot ethanol thermal reduction strategy. When used as an anode material for lithium-ion batteries, it exhibited high reversible capacities (1012 mAh g−1 at 0.1 A g−1 after 200 cycles) and high rate stability (448 mAh g−1 at 4 A g−1 after 1000 cycles).
33