3D Graphene Nanostructure Composed of Porous Carbon Sheets and

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3D Graphene Nanostructure Composed of Porous Carbon Sheets and Interconnected Nanocages for High-Performance Lithium-Ion Battery Anodes and Lithium-Sulfur Batteries Xiao Zhu, Junwei Ye, Yunfeng Lu, and Xilai Jia ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00564 • Publication Date (Web): 17 May 2019 Downloaded from http://pubs.acs.org on May 19, 2019

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ACS Sustainable Chemistry & Engineering

3D Graphene Nanostructure Composed of Porous Carbon Sheets and Interconnected Nanocages for High-Performance Lithium-Ion Battery Anodes and Lithium-Sulfur Batteries Xiao Zhu,†,‡ Junwei Ye,§ Yunfeng Lu,‡ and Xilai Jia*,† †Beijing

Key Laboratory of Function Materials for Molecule & Structure Construction,

Department of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, P. R. China. ‡Department

of Chemical and Biomolecular Engineering, University of California,

Los Angeles, CA 90095, USA. §State

Key Laboratory of Fine Chemicals and Faculty of Chemical, Environmental &

Biological Science and Technology, Dalian University of Technology, Dalian, 116024, P. R. China. Corresponding Author *E-mail: [email protected]

Abstract: Three-dimensional (3D) graphene materials are attractive in energy storage, but they mostly have disordered microstructures and suffer from weak strength. Here, inspired by nature plant, philodendron hederaceum, a hierarchically structured 3D graphene nanostructure composed of vertically aligned porous graphene nanosheets and interconnected nanocages (denoted as PHG) is designed and prepared by a chemical vapor deposition (CVD) method in a fluidized bed reactor. Compared to in-plane graphene, the hierarchical pores of PHG facilitate the infiltration of electrolyte and transport of ions, as well as improve the charge storage capability; the

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integrated conductive networks of the graphene nanoarchitecture accelerate the transportation of electrons; and the in-situ formed connections between the units of the nanocages are robust for durable electrochemical properties. When used as anode materials of lithium ion batteries, the PHG exhibits a reversible capacity of 1560 mAh g-1 at 0.1 A g-1, more importantly, high-rate capacities (160 mAh g-1 at 4 A g-1), and stable cycling performance. Moreover, for lithium-sulfur batteries, this hierarchically structured 3D graphene could be loaded with sulfur at a higher mass loading, which deliver high specific capacity (1640 mAh g-1 at 0.1 C), and maintain stable performance. Considering to the structural properties, as-prepared 3D graphene will have wide applications in energy storage. Keywords: porous graphene, fluidized bed chemical vapor deposition, lithium-ion battery, energy storage.


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■ INTRODUCTION As a clean and sustainable energy supply, electrochemical energy storage is essential for a broad range of applications such as electronics, electric vehicles and large-scale power grids. Therefore, developing better energy storage devices with a higher energy/power density as well as a longer lifetime is always a research hotspot.1 The realization of this goal relies on rational design and synthesis of effective electrode materials with optimized electronic/ionic conductive pathways and stable structure of high strength.2 As a unique nanocarbon material, graphene featured with two-dimensional (2D) morphology has shown great potential in energy storage applications due to its combination of large theoretical specific surface area (2630 m2 g-1) for fast ion reactions, high conductivity for efficient electron transports, excellent mechanical properties, and chemical stability.3 To overcome the aggregation problems of 2D graphene, synthesis of three-dimensional (3D) graphene nanostructures has been proposed based on the assembly of the dispersed monolayer nanosheets of reduced graphene oxide or exfoliated graphene sheets,

4, 5

which usually leads to enhanced

porous networks that can facilitate the infiltration of electrolyte for better performance. However, those post-processing nanostructures usually display weak mechanical strength, which may easily collapse in the organic electrolytes and result into increased resistance and poor cycling stability. In addition, such graphene architectures also possess a very low tap density, which inhibit their actual applications in battery electrodes.6



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As an alternative approach, chemical vapor deposition (CVD) of carbon sources on metal substrates or catalytic oxides can in-situ form 3D porous graphene.7,

8

For

example, 3D graphene have been prepared on nickel foams,9 magnesium oxide,10, 11 and zeolite,12 etc. The network structure of such 3D graphene are more stable due to the formation of sp2-sp3 carbon bonds, which usually makes them more effective for rapid charge transport as compared to in-plane graphene and the 3D graphene nanostructures made from dispersed graphene nanosheets. However, most of the current 3D graphene prepared by CVD method always display disordered porous distribution and random microstructure,3, 13 which inevitably limits the transportation of electrons and ions, as well as the charge storage capability. Beyond conventional 3D graphene materials, we propose a more systematically structural design of graphene for energy storage by learning from natural plants that have multiorganized structure to harvest the energy from sunshine. Here, inspired by the philodendron hederaceum, a kind of tropical plant with dense and orderly leaves grown on stem, of which the large leaves facilitate the energy absorption from sunshine, the stem facilitates energy transportation in the whole system, a new hierarchically structured graphene assembled with vertically aligned, ordered mesoporous nanosheets is designed and synthesized for energy storage in both lithium ion batteries and lithium-sulfur batteries.


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Figure 1. Schematic synthesis of the PHG based on (I) CH4-CVD using nanostructured MgO catalyst and (II) the following removal of the MgO component by acid washing, with the enlarged view beside prep.

Figure 1 illustrated the synthesis of the PHG based on the CVD of methane (CH4) on as-synthesized nanostructured MgO catalyst (step I) in a fluidized bed reactor, followed by removal of the catalysts using acid washing (step II). We started with the synthesis of nanostructured MgO catalyst composed of vertical-aligned MgO nanosheet. After careful control of CVD, uniform graphene nanosheets were in-situ formed on the catalyst and resulted in an integrated nanostructure; the microstructure of the nanosheets is curved graphene that are tightly interconnected with each other. The branched graphene nanosheets of as-obtained PHG have ordered mesoporous microstructures, just like the cells of the leaves of the philodendron hederaceum, which can not only facilitate the electrolyte infiltration, but also increase the charge storage capability; the central branch supporting the vertical gephene nanosheets, just like the stem of the philodendron hederaceum, endows the PHG with effective electron transport; which highly boosts the electrochemical performance. Moreover, it



is worth mentioning that the preparation method based on the fluidized bed reactor here is easy to scaled-up for mass production of this material. ■ EXPERIMENTAL SECTION Catalyst synthesis. 60 mL of ethanol was added into a 250 mL three-necked flask, into which added 100 mL of MgCl2 and Na2CO3 mixed aqueous solution (1:1 in mole ratio) and the mixture was vigorously stirred. Then, a white precipitation was formed and collected, filtered off, and washed with water, then dispersed in 120 mL of deionized water in a 250 mL three-necked flask. Subsequently, the suspension was vigorously stirred at the 60-80 °C for 1 h. After that, the white powder was centrifuged and washed with ethanol three times, and dried in blast drying oven. The product was magnesium carbonate hydroxide. After further calcination at 500°C for 1 h, MgO catalyst was obtained. Synthesis of graphene. The synthesis of graphene was based on a fluid CVD of CH4 on the MgO cayalyst. Typically, fluidized bed reactor was heated to 950 oC under an argon flow of 800 mL min-1. 4.5 g of as-prepared MgO catalyst was transferred into the reactor, and heated for 2 min. Then, CH4 was introduced into the reactor at 400 mL min-1 at reaction time of 2, 5, 10, and 20 min respectively. After that, CH4 was turned off, and then the furnace was cooled to room temperature under an Ar flow of 100 ml-1. The obtained products were washed using 18 wt.% HCl aqueous solution to remove MgO at 80 oC. As-obtained products were filtered, washed, freeze-dried and labeled as PHG-2, PHG-5, PHG-10 and PHG-20 for use. Material Characterizations. SEM experiments were conducted on a JEOL


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JSM-6700 FE-SEM operated at 3.0 kV. TEM experiments were conducted on a FEI T12 instrument operated at 120 kV. Nitrogen sorption isotherms were measured at 77 K with a Micromeritics ASAP 2020 analyzer. The specific surface areas (SBET) were calculated by the Brunauer-Emmett-Teller (BET) method using adsorption branch in a relative pressure range from 0.04 to 0.25. The pore size distributions were derived from the adsorption branch of isotherms using the Barrett-Joyner-Halenda (BJH) model. X-ray diffraction (XRD) was conducted on a Panalytical X’Pert Pro x-ray powder diffractometer using Cu-Ka radiation (λ=1.54 Å). Raman spectra were performed on a Horiba Jobin Yvon Lab RAMHR800 Raman spectrometer with He-Ne laser excitation at 633 nm. Thermalgravimetric analysis (TGA) of S/PHG-5 nanocomposites was conducted on a TGA Q50 instrument. Electrochemical characterizations. Coil-type cells (2025) were constructed for electrochemical characterizations of PHGs as the anode materials of LIBs and the S/PHG-5 nanocomposite for Li-S batteries. The electrode consisted of 85 wt% PHG-5 (or PHG-10; PHG-20), 5 wt.% carbon black, and 10 wt.% polyvinylidene fluoride (PVDF) for anodes of LIBs and the electrolyte was a 1 M LiPF6 in 1:1 ethyl carbonate (EC)/dimethyl carbonate (DEC). As for the Li-S cathodes, the electrode were made from 80 wt.% of as-synthesized S/PHG-5 nanocomposite, 10 wt.% carbon black, and 10 wt.% PVDF. The electrolyte solution of Li-S batteries was a 1 M lithium bis-(trifluoromethanesulfonyl) imide (LiTFSI) in 1,3 dioxolane (DOL) and dimethoxy ethane (DME) (1:1 in volume). The CV and EIS measurements were carried out on a Princeton Applied Research workstation. The galvanostaic charge/discharge



measurements were carried out by LAND CT2000 battery tester. ■ RESULTS AND DISCUSSION

Figure 2. (a, b) SEM images of as-prepared MgO, displaying the nanostructured morphology. (c, d) TEM images of the MgO, displaying the inside continuous pores. (e) XRD pattern of MgO as compared to its precursor of Mg5(CO3)4(OH)2·8H2O. (f) Nitrogen-sorption isotherms and BJH pore size distribution (inset of Figure 2f) of the nanostructured MgO.

Synthesis and structural characterization of PHGs. Nanostructured MgO catalyst used in this work was synthesized by a precipitation method, followed by a thermal treatment (details see catalyst synthesis of Experimental Section). It was


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selected as the catalyst rather than metal substrates (e.g., porous Au14) for graphene synthesis because of its low cost and facile control of a desired nanostructure. Figure 2a and b show the morphology of as-prepared MgO catalyst. It displays 3D morphology that was composed of interconnected MgO nanoﬂakes. The nanoflakes are mainly vertically located on the branch as observed. Their sizes are distributed around several microns with the thickness of tens of nanometers. The close structure of the catalyst was observed under transmission electron microscopy (TEM, Figure 2c, d). It shows that MgO ﬂakes are composed of crystalline (size mainly around 4-6 nm) networks with continuous porous channels. Those porous channels should be produced from decomposition of the precipitated magnesium carbonate hydroxide octahydrate (Mg5(CO3)4(OH)2·8H2O), which was the precursor and was converted into MgO after the thermal treatment in catalyst preparation (confirmed by XRD, Figure 2e). The porous structure of MgO nanoflakes was then confirmed by N2-sorption isotherms, as shown in Figure 2f. A closer type-IV isotherm is observed and suggests a hierarchically porous network of the nanostructured MgO catalyst. The Brunauer-Emmett-Teller (BET) surface area of the catalyst is 181.8 m2 g-1, while the pore diameter is averaged at 3.2 nm, much larger than that of the methane molecule (0.38 nm in diameter). Thus, the continuous porous structure of catalyst could facilitate mass transports of CH4 within the catalyst, endowing graphene formation throughout the whole catalyst.



Figure 3. (a) Low-magnification and (b) high-magnification SEM images of PHG/MgO nanocomposite at a CVD time of 5 min (denoted as PHG/MgO-5, inset is one piece of the PHG/MgO-5 nanosheet). (c) Low-magnification and (d) high-magnification SEM images of PHG-5 after removing MgO component, and inset is one piece of the PHG-5 nanosheet. (e) SEM and (f) TEM images of the interconnections between the nanosheet units of PHG, showing the continuous integrated morphology.

It is important that the 3D structure of the catalyst can be maintained during CH4-CVD process in fluid-bed reactor, since it will determine the shape of the final


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graphene product. As increasing the CVD time from 2 to 20 min, all products displayed similar nanostructured morphology with vertically assembled nanosheets such as the product after 5 min CVD (denoted as the PHG/MgO-5, Figure 3a; other products see Figure S1), similar to that of the catalyst. The enlarged view of Figure 3b has clearly showed that the nanosheets of PHG/MgO-5 are self-supported with each other and interconnected into an integrated structure. Although the catalyst abrasion was usually serious in fluid-bed reactors,15 the results confirm the structure integrity of the catalyst, since they keep the nanostructured morphology in the gas flow of the reactor, even under a longer reaction time of 20 min. After removing MgO, the morphology of the products was observed under SEM. Figure 3c showed that the morphology of the PHG-5 (CVD time of 5 min) well maintains the hierarchically branched structure. It needs to point out that when the CVD time was 2 min (PHG-2), the leaves-assembled nanostructure collapsed unexpectedly after acid washing (Figure S2a), with some cracks and aggregations clearly observed on the backbone. Therefore, to maintain the hierarchical structure, a sufficient reaction time of, ca. more than 5 min is required to form the integrated nanostructure. In Figure 3d, the enlarged image of the PHG-5 also shows that the porous graphene display ordered vertical topography, which are not stacked together nor collapsed after removing the residual MgO. The single porous graphene nanosheet is as thin as a cicada's wing, and is very like the leaves of philodendron hederaceum. SEM and TEM images (Figure 3e and f) of the contacts between the graphene nanosheets shows that they are



chemically bonded together, and forming continuous nanostructure. This is important to form long-range conductivity and maintain the structural integrity.

Figure 4. SEM and TEM images of (a-c) PHG-5, (d-f) PHG-10, and (g-i) PHG-20, respectively, showing the changes of the pore size and graphene layer with increasing CVD time.

Figure 4 compares the microstructure of PHGs made at various CVD time, including the graphene layers, the porous structure, and the specific surface areas (SSA). It shows that the microstructure of the products is highly affected by the CVD time; that is, a longer reaction time will lead to increased thickness of graphene layers, and decrease the surface areas in turn. Besides, all PHGs display that the graphene


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nanosheets are consisting of mesoporous graphene nanocages, similar to the leaves in nature to some degree. Even though the 3D nanostructure of the PHG-2 collapsed, its enlarged microstructure still has a porous structure that are disordered, and composed of 1-2 layers of graphene (Figure S2b). As increasing the reaction time to 5, 10, and 20 minutes, the produced PHG-5 (Figure 4a, b), PHG-10

(Figure 4d, e), and

PHG-20 (Figure 4g, h) all displayed hierarchically porous structure. However, the thickness of the graphene layers increased with the CVD time. As can be seen, the PHG-5 consists of 2-3 graphene layers (Figure 4c); the PHG-10 are composed of 3-4 stacked graphene layers (Figure 4f); while for the PHG-20 (Figure 4i), more than 5 layers are observed in some regions. It should be pointed out that the curved structure between graphene layers increases the inter-graphene spacing at the edges, which can promote the ion transfer into the graphitic layers.16 But such curved porous structure make the PHGs display low crystallinity as compared with in-plane graphene, therefore the X-ray diffraction patterns of the PHGs display broad and weak peaks at 22.6 and 43.8o (Figure 5a). This was similar to the porous carbonaceous materials with low crystalline of graphitic walls.17, 18 Raman spectroscopy analysis shows that all the PHG samples exhibit a disorder-induced D-band (1320 cm-1), as well as an in-plane vibrational G-band (1591 cm-1, Figure 5b).19, 20 It indicates that increasing the reaction time from 2 to 20 min leads to higher degree of graphitic ordering of the graphene, as indicated by the increased integral intensity ratio of IG/ID (Table S1).



Figure 5. (a) XRD patterns, (b) Raman spectra, (c) nitrogen-sorption isotherms, and (d) the BJH pore size distribution of the PHG-2, PHG-5, PHG-10, and PHG-20.

Nitrogen gas sorption measurements of the PHGs were performed to investigate the porous structure. Figure 5c shows the characteristic isotherms, with inset displaying their resultant pore size distributions. The calculated BET surface areas of the PHG-2, PHG-5, PHG-10, and PHG-20 are 928.7, 1460.9, 1273.6, and 1073.3 m2 g-1, respectively. It is reasonable that PH-5 display the largest SSA, since the multiorganized structure of PHG-2 has collapsed, while for PHG-10 and PHG-20, the increased reaction time has resulted in increased graphitic layers as observed. Note that most graphene materials show limited surface areas, the PHGs possess much


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higher value. The pore sizes of all the samples are uniform in a certain degree, and mainly distribute around 4-5 nm (Figure 5d), consistent with the size of the MgO nanocrystals. Such pore networks are much ordered than those of the porous carbons made by KOH activation21 or nanometal templating.7 The slight increase of the pore size with the increasing reaction time was due to the growing of the MgO crystals when they were placed in a high temperature (950 oC). Obviously, this PHGs could effectively prevent the stacking of graphene and maintain a hierarchically ordered structure, important for high-power battery electrodes.22 The formation of the leaves-assembled graphene has been discussed based on CVD time-dependent morphology evolution. In the CVD process, CH4 would transport into MgO catalyst through the porous channels and decompose on the surfaces of MgO nanocrystals. With prolonging reaction times, the 3D porous structure of PHGs was in-situ formed. Such in-situ-formed graphene has much better electronic conductivity than typical reduced graphene oxide (Figure S3), due to the formation of integrated structure in CVD process. In addition, it is worth mentioning that this in-situ-formed nanostructure is robust, since it still remains the nanostructured morphology even after mechanical stirring of them in organic solvent (e.g., NMP, a solvent commonly used in battery manufacturing, Figure S4). This integrated robust nanostructure is critical for stable electrode materials in repeated charge-discharge cycles. PHGs as Li-ion battery anode materials. As we all know, graphite has been used as the anode material for LIBs, but it displays a limit capacity and more seriously low rate capabilities. Here, PHGs were developed as high-rate electrode materials for



LIBs. Figure 6a shows the electrochemical behavior of the PGH-5 as the anode material for LIBs by cyclic voltammogram (CV) scanning. After the pre-lithium treatment of the electrode, it becomes highly reversible, consistent with the Galvanostatic charge-discharge profiles of the electrode (Figure 6b). The charge-discharge profiles display gradually reduced or increased voltages, indicating a gradual ion reaction, with no obvious platforms before 0.5 V. The charge/discharge capacities are 1828 and 1296 mAh g-1 in the voltage range of 0.001-3 V. Meanwhile, the charge-discharge profiles of the PHG-10 and PHG-20 electrodes display similar shape to that of PHG-5 electrode (Figure 6c), but they display smaller capacities, which should be related with the increased graphene layers.

Figure 6. (a) CV curves of the PHG-5 electrode at 0.1 mV s-1 in 0.001-3.0 V. (b) Charge-discharge profiles of the PHG-5 electrode in 0.001-3 V at 0.1 C. (c)


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Charge-discharge profile of the PHG-5 compared with that of PHG-10 and PHG-20 electrodes. (d) Rate capacities of the PHG-5, -10, and -20 electrodes at increasing current densities ranging from 0.1 to 4 A g-1. (e) Cycling stability of the PHG-5, -10, and -20 electrodes at 0.5 A g-1. (f) Nyquist plot of PHG-5, -10, and -20 electrodes. (g) CV curves of PHG-5 electrode at increasing scan rate from 1.0 to 10.0 mV s-1. (h) Capacitive contribution shown in the shaded area of the CV curve of PHG-5 electrode at a scan rate of 5.0 mV s-1. (i) Column graphs represent capacitive (the black area) and intercalated (the shaded area) capacity contribution for the PHG-5 electrode at the scan rate of 1, 3, 5, 7, 9, and 10 mV s-1 respectively.

Figure 6d shows the rate capacities of the PHG-5 electrode at increasing current densities. Thanks to the integrated structure for fast electron transfer, and hierarchically porous structure of the graphene for fast ion transfer, the PHG-5 exhibits a high rate performance. The specific charge/discharge capacities of the PHG-5 electrode are gradually reduced from 1560 to 500 mA g-1 with the increasing current densities from 0.1 to 1.5 A g-1. Even when the current density is 4 A g-1, the electrode still discharges a specific capacity of 160 mAh g-1 in 5 minutes. Moreover, the rate capacities of PHG-5 electrode was compared with the electrode made from isolated porous graphene nanosheeets (Figure S5), which obviously shows reduced capacity, especially for the high-rate performance. This suggests the importance of the integrated 3D nanostructure. Table S2 compares the rate capacities of PHG-5 with the typical anode materials, which shows the electrochemical performance of the graphene presented here can deliver high-rate reversibile capacity, and compares to the best reported carbon anodes,23-33 and other anode materials like silicon,34 tin



metal,35 metal oxides,36 and silicon-graphite composites.37-39 The rate capacities of the PHG-10 and PHG-20 electrodes are also compared, but both display lower capacities as compared to the PHG-5 electrode, which should be ascribed to the increased graphitic layers of them. The Nyquist plot (Figure 6f) of the PHG-5 presents a much smaller semicircle in high-middle frequency and inclined line closer to vertical axis in low frequency. Simulated resistances support that PHG-5 has faster charge transfer and ionic diffusion in respect to the PHG-10 and PHG-20 (Figure S6). Apart from the high-rate capacities, cycling stability of the PHGs electrode is also obtained and shown in Figure 6e. Except from the initial cycles, the capacity maintains steadily during 100 cycles at a lower current density of 0.5 A g-1 for all the three electrodes, with the best is the PHG-5 as expected. The initial coulombic efficiency (ICE) of the PHG electrodes are ~85%, which is much higher than that of the pristine porous graphene electrodes (e.g.,

3D Graphene Nanostructure Composed of Porous Carbon Sheets and

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