Carbon Nanotube


Dec 1, 2017 - Division of Chemistry and Biological Chemistry, School of Physical and ... macroporous network by homogeneously weaving V2O5 nanowires ...
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Scalable Synthesis of Honeycomb-Like V2O5/Carbon Nanotube Networks as Enhanced Cathodes for Lithium-Ion Batteries Xin Yao, Guilue Guo, Pei-Zhou Li, Zhong-Zhen Luo, Qingyu Yan, and Yanli Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15136 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 1, 2017

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Scalable Synthesis of Honeycomb-Like V2O5/Carbon Nanotube Networks as Enhanced Cathodes for Lithium-Ion Batteries Xin Yao,†,§ Guilue Guo,‡,§ Pei-Zhou Li,† Zhong-Zhen Luo,‡ Qingyu Yan,*,‡ Yanli Zhao*,†,‡ †

Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore



School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore *Email: [email protected] (Y. Z.); [email protected] (Q. Y.)

Abstract: A green and scalable route to form a honeycomb-like macroporous network by homogeneously weaving V2O5 nanowires and carbon nanotubes (CNTs) was developed. The intertwinement between V2O5 nanowires and CNTs not only integrates nanopores into the macroporous system, but also elevates the collection and transfer of charges through the conductive network. The unique combination of V2O5 nanowires and CNTs renders the composite monolith with synergic properties for substantially enhancing electrochemical kinetics of lithiation/delithiation when used as a lithium-ion battery (LIB) cathode. This work presents a useful approach for a large-scale production of cellular monoliths as high-performance LIB cathodes.

Keywords: carbon nanotubes, lithium-ion battery cathode, monolithic network, scalable synthesis, V2O5 nanowires

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Smart and ordered arrangement of nanoscale building blocks allows the construction of threedimensional (3D) bulky assemblies for required applications, and further imparts novel physical and mechanical properties for the resultant assemblies. Enhanced specific surface, spatial conductivity, and mechanical strength can be achieved by 3D porous lattice.1,2 Those intriguing properties originated from the 3D network can be exploited by a wide range of metallic, ceramic, polymeric and carbonaceous materials, which are useful in various applications such as exhaust purification, catalysis support, energy conversion/storage, smart filters, controlled release systems and biotechnologies.3 In regard to lithium-ion batteries, large specific surface and numerous multi-dimensional pore channels enabled by the 3D assembly could significantly shorten the diffusion length for lithium ions to access active sites, and also prevent the agglomeration of lithium-active phases.4 In recent studies, 3D porous materials have raised considerable interests as advanced lithium-ion battery (LIB) electrodes, with noticeable enhancements on gravimetric energy density and rate capability.4,5 The 3D cellular networks coupled with electrochemically active phases are potentially promising in LIB industry if their large-scale production could be achieved with practicable and cost-effective methods. Hydrothermal method is an available approach to construct 3D networks from metal oxide nanowires (NWs) and carbon nanotubes (CNTs),6-8 but the harsh hydrothermal conditions would inevitably lead to the agglomeration of CNTs (details shown in Figure S1). In this regard, a cryogenic process is more applicable, since ice can confine the mobility of CNTs and further prevent their agglomeration. As reported previously, the V2O5 electrode materials resulted from cryogenic process showed comparable LIB performance to that of hydrothermal products.9,10 On the other hand, homogenous blending between V2O5 and CNTs was not achieved, and thus higher weight ratio of CNTs is demanded for the optimum performance of hybrids. In need of

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scale-up synthesis and good homogeneity of products, we decided to further explore an improved cryogenic approach for the synthesis of V2O5/CNT hybrid materials (see experimental details in the Supporting Information). We first oxidized CNTs under HNO3 reflux, by which the water dispersity of CNTs was substantially changed without the usage of any surfactants (Figure S2). V2O5 precursors were aged in a mild stirring condition to give rise to the NWs instead of nanosheets. The V2O5 precursors and CNT suspension were mixed and stirred until homogenous blending was achieved. As a result of their homogenous mixture, the intertwinement between the NWs and CNTs was observed, and the optimum weight ratio of CNTs was significantly lowered to be only 8.8%. A unique honeycomb-like architecture would be shaped from the rapid cryogenic solidification of viscous aqueous suspension. In this work, a 3D honeycomb-network collaboratively woven from V2O5 NWs and CNTs was successfully fabricated from a controllable and scalable route, which was named V2O5/CNTs network (VCN). A macroporous monolith of the ordered network was produced as a result of the specific formation mechanism termed ice-segregation-induced self-assembly.3 It is noteworthy that the intriguing incorporation of one-dimensional (1D) V2O5 NWs and 1D CNTs enables unique intertwinement throughout the whole scaffold, which substantially facilitates the transfer and collection of charges within the LIB electrode. This mutual intertwinement between the two 1D building blocks further integrates nanopores into this macroporous assembly, which not only significantly expands the electrode/electrolyte contact, but also provides effective shortcuts for the lithium diffusion.

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Figure 1. (a,b) Optimal image of the aerogels with different CNT contents before the calcination. (c) Powder XRD pattern of VCN. (d) XPS spectra of VCN. Inset shows high-resolution spectrum for vanadium. (e) N2 adsorption/desorption isotherm of the VCN aerogel. Inset shows pore size distribution calculated by DFT model.

The primary component of the aerogel (Figure 1a,b) obtained under direct cryodessication was the hydrated V2O5.11-13 A further calcination process was applied in order to transmute the hydrated precursor into highly crystalline and dehydrated V2O5 for the fabrication into electrodes. The density of the aerogel and the CNT content in the products are highly controllable using our method. Characterizations of the material were based on the sample with 8.8 wt% CNTs that showed the best electrochemical performance. An ultralow density of 6.4 ×

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10-3 g cm-3 (calculated) could be achieved (CNTs = 18.6 wt%) using low concentrated solution mixture (Figure S3), which is in the same order of magnitude as the air density (1.225 × 10-3 g cm-3). More importantly, this facile method involving only solution processing and cryodessication could be easily scaled at industrial level, where an acceptable cost efficiency is secured by the usage of commercial V2O5 powder as the starting material, and the minimized weight ratio of CNTs. Only water was drained out from the overall procedures as the by-product, rendering this method for a green synthesis. To give an example, the one-pot fabrication of a large amount of monolith with ~2 L volume was achieved (Figure S4). The phase purity and chemical composition of the calcined VCN was respectively examined by several techniques (Figures S5 and S6) including powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). As indicated by Figure 1c, the powder XRD pattern displays a series of well-defined diffraction peaks that can be readily indexed to the orthogonal V2O5 phase (JCPDS 41-1426), indicative of the good crystallinity of V2O5 in VCN. As shown in Figure 1d, the existence of vanadium, carbon and oxygen was certified from the XPS spectrum of VCN aerogel. From the high-resolution spectrum in the region of 510-535 eV as demonstrated in the inset of Figure 1d, three peaks at 517.5, 525.0 and 530.2 correspond to the binding energy of V 2p3/2, V 2p1/2, and O 1s electrons, respectively. The 7.5 eV difference between V 2p3/2 and V 2p1/2 electrons could be indexed to +5 electronic state of vanadium.14 The N2 adsorption/desorption isotherm was used to probe the surface area and porous property of the VCN (Figure 1e). According to the Brunauer-Emmett-Teller (BET) model, the surface area of the material was calculated to be 71.7 m2 g-1. As shown in the inset of Figure 1e, the majority of pore diameters was distributed in the vicinity of 2-4 nm according to the Density Functional

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Theory (DFT) model, which should be attributed to the intertwinement between those 1D NWs and CNTs.

Figure 2. (a,b) SEM images for the vertical and parallel view of the VCN product directly obtained from cryodesiccation. (c) SEM image of the macroporous channels on VCN. (d,e) SEM images of the 2D nanosheets in the 3D network. Red arrows mark the nanopores. (f) SEM image of the V2O5/CNTs aerogel after thermal calcination. Yellow arrows indicate the CNTs. (g,h) TEM images of the VCN directly obtained from cryodesiccation.

It could be seen from scanning electron microscopy (SEM) images that the calcination process did not obviously change the morphology of the hybrid. As shown in Figure 2a,b, SEM images displayed a honeycomb-like cellular lattice with hierarchical open pores and continuous pore walls. The cells on this honeycomb were relatively huge in terms of size, on micrometer scale, and the cell walls seemed very thin and holey (Figure 2c). At closer observation (Figure 2d,e), bent walls were present as two-dimensional (2D) holey nanosheets. In this monolith, the 2D nanosheets that assembled the 3D scaffold were primarily fabricated from 1D building blocks,

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namely the mutually intertwined V2O5 NWs and CNTs. On account of loosely packed nature of NWs and NTs, numerous nanotextured holes were spotted on the nanosheets, which showed good consistency with the pore size distribution results. As a typical example revealed in Figure 2f, CNTs were homogeneously distributed on the 2D nanosheets, without any observed aggregation. The transmission electron microscopy (TEM) images further provided more concrete evidence for the homogeneous mixture of NWs and CNTs. As shown in Figure 2g,h, the NWs were revealed as light colored and relatively straight lines, while the curved CNTs entangled around them in different manners like knots, helixes, lassos and so on. Similar sizes of these two 1D building blocks and their homogenous blending enable this unique intertwinement. In further investigations, energy-dispersive X-ray spectroscopy (EDX) was conducted to distinguish the vanadium component from CNTs, as shown in Figure S7. V2O5 is considered as a potential candidate for high-performance LIB cathode.15-18 The theoretical capacity of V2O5 reached 294 mA h g-1 with two Li ions intercalated, which is much higher than that of LiFePO4 (170 mA h g-1). Herein, we employed the obtained VCN monolith as the LIB cathode to demonstrate its excellent electrochemical properties in terms of energy storage. In order to provide a balanced and comparative perspective, we also tested battery electrodes made from pure V2O5 networks (VN) and commercial V2O5 powders as the control groups. We first compared the rate capabilities of VCN, VN and commercial V2O5 (Figure 3a). Prior to the comparison, we optimized the weight ratio of CNTs for VCN (Figures S8-S11), and it was confirmed that the sample with 8.8% CNT possessed the best lithium-storage property (Figures S12-S14). For the first cycle at 100 mA g-1, the VCN electrode delivered a specific capacity of

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291 mA h g-1, and a remarkable columbic efficiency of 91%, of which the later indicates significantly buffered initial irreversibility.19 The reversible discharge capacities of VCN achieved 241, 202, 170, and 145 mA h g-1 at respective current densities of 100, 200, 500, and 1000 mA g-1, respectively. In comparison, the relevant specific capacities delivered by VN under the same current densities were only 189, 127, 92, and 48 mA h g-1. At a high current density of 1000 mA g-1, the specific capacity delivered by VCN was three times higher than VN, which implied that the integration of CNTs to the assembly showed more pronounced advantage at relatively higher charge/discharge rates. Meanwhile, the commercial V2O5 electrode only delivered specific discharge capacities of 170, 95, 40, and 4.4 mA h g-1 at corresponding current densities. The cycling performance of the three samples was first evaluated at 100 mA g-1. As shown in Figure 3b, after 50 discharge/charge cycles at 100 mA g-1, the VCN electrode still retained a capacity of 165 mA h g-1, which was slightly higher than 139 mA h g-1 from the VN electrode. The commercial V2O5 powders showed a quick depletion of capacity, which continuously reduced to only 44 mA h g-1 at the 50th cycle (Figure S15). As shown in Figure 3c, an initial capacity of 208 mA h g-1 was reached for the VCN electrode at 1000 mA g-1, and after 200 cycles, 114 mA h g-1 was still remained. Meanwhile, the VN electrode only delivered a half of the VCN’s capacity at the 200th cycle. Although the cycling performance of VCN and VN may not show significant difference at 100 mA g-1, the integration of CNTs did substantially improve the high rate performance for the 3D assembly. The coulombic efficiency of VCN kept over 98% for majority of the cycles at current densities of 100 and 1000 mA g-1, indicative of good stability of these discharge/charge cycles.

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Figure 3. (a) Rate capabilities of VCN, VN and commercial V2O5 electrodes. (b) Cycling performance of the VCN, VN and commercial V2O5 electrodes at a constant current density of 100 mA g-1. Hollow scatters show the coulombic efficiency of the VCN electrode. (c) Cyclability of the VCN and VN electrodes at relatively high current density of 1000 mA g-1 for 200 cycles. Hollow scatters show the coulombic efficiency of the VCN electrode.

Electrochemical impedance spectroscopy (EIS) is essential to shed a light on the electrochemical mechanisms underlying the compelling lithium-storage properties of the VCN electrode. The comparative tests were carried out with electrodes made from VCN, VN and V2O5 commercial powders. As plotted in Figure 4a, these three Nyquist curves could be simulated to a depressed semicircle at high-medium frequency region along with an inclined line at low

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frequency region, which are associated with the charge-transfer resistance (Rct) and Warburg element (Zw), respectively. The corresponding Randles equivalent circuits for the electrode/electrolyte interface were plotted in Figure 4b. The electrolyte resistance (Re) of these three materials did not show much difference, but Rct of the 3D networks (VCN and VN) was apparently smaller than the commercial powders. Meanwhile, Rct of VCN is again smaller than that of VN, due to the improved electrical conductivity after the integration with CNTs. Therefore, it could be confirmed from the EIS results that the electrochemical kinetics was substantially promoted through the structure layout as 3D monolithic network, and further facilitated through the incorporation of CNTs, as a result of charge transfer acceleration enabled by the intertwined V2O5-CNT network. On the comprehension of the characteristics inherent in the favorable composition and structuration of the present material, we were able to interpret the impressive LIB performance from the following aspects (Figure 4c). Firstly, compared with rough powders, the 3D honeycomb-like structure efficiently enhanced the accessibility for electrolyte, and the structural layout as an interconnected framework was also preferable for rapid transportation of electrons.20 Secondly, the smart combination of multidimensional nanopores and macropores in the same hierarchical framework radically increased the specific surface area, and thus enabled easier and faster diffusion of lithium ions across the bulky electrode.21 Last but not least, both V2O5 and CNTs we used were crafted into 1D geometries associated with beneficial physiochemical properties.22,23 The uniform distribution of CNTs and their homogenous entanglement with V2O5 NWs throughout the entire network provide “highways” to gather and deliver charge carriers from nearby regions, which significantly lowered the optimum weight ratio of CNTs in the hybrid. As a comprehensive result of all these characteristic advantages, the 3D hybrid was

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capable of providing decent specific capacity and high rate capability. Among those state-of-theart V2O5/CNTs cathodes,8,15,24,25 the lithium-storage property of the present material presents a solid development for scalable synthesis, achieved by a controllable, versatile and commercially viable process. It is the outcome balanced between large capacity and high efficiency, with a great promise for large-scale application.

Figure 4. (a) Nyquist plots of VN, VCN and commercial V2O5 powders. Inset shows an enlarged graph. (b) Randles equivalent circuit of the electrode/electrolyte interface. Re: electrolyte resistance; CPE: constant phase element; Rct: charge-transfer resistance; Zw: Warburg element. (c) Scheme of the electron transfer and lithium diffusion within the 3D network of the intertwined V2O5/CNTs. Brown belts stand for V2O5 NWs and navy ones for CNTs.

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As a summary, a rational design to construct a 3D network from 1D V2O5 NWs and CNTs was realized under a green and scalable synthesis, which afforded a lightweight monolith enriched with hierarchical macropores and nanopores. In a proof-of-concept application, the VCN product demonstrated durable lithium-storage properties as a cathode, especially at high charge/discharge rates. The unique integration of V2O5 NWs and CNTs renders their composite monolith the synergic

properties

that

greatly

enhanced

the

electrochemical

kinetics

during

lithiation/delithiation processes. This remarkable electrochemical reactivity could be useful for a variety of applications such as supercapacitors,26,27 catalysis,28 and sensors.29,30 While V2O5 may have the toxicity to human, this issue caused by the disposal of LIBs could be minimized by recycling technologies. This green and scalable synthetic approach we developed could potentially be employed to realize the general fabrication of 3D cellular monoliths consisting of inorganic NWs and CNTs, and therefore it deserves further explorations.

ACKNOWLEDGMENTS This work is supported by the Singapore Academic Research Fund (No. RG112/15, RG19/16, and RG121/16), Singapore Agency for Science, Technology and Research (A*STAR) AME IRG grant (No. A1783c0007), and Singapore NRF under CREATE programme-EMobility in Megacities. The authors would like to convey gratitude to Prof. Yizhong Huang for the approval of accessing their electrochemical workstation. The authors are also grateful for the contributions from Miss Yu Zhang, Miss Yuanyuan Guo and Miss Yan Lu.

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Supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. Experimental details on the 3D V2O5-CNT composites prepared from hydrothermal method; characterization of materials by a digital camera, TEM, SEM, TGA, powder XRD and EDX; complementary electrochemical measurements. Competing interests. The authors declare no competing financial interests. Author contributions. §

XY and GG contributed equally to this work.

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