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Facile Construction of Novel 3-dimensional Graphene/Amorphous Porous Carbon Hybrids with Enhanced Lithium Storage Properties Daming Zhu, Huaqiu Liu, Lixuan Tai, Xiaonan Zhang, Sheng Jiang, Shumin Yang, Lin Yi, Wen Wen, and Xiaolong Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08320 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017

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ACS Applied Materials & Interfaces

Facile Construction of Novel 3-dimensional Graphene/Amorphous Porous Carbon Hybrids with Enhanced Lithium Storage Properties Daming Zhu,a,b Huaqiu Liu,a,b Lixuan Tai,c Xiaonan Zhang,a,b Sheng Jiang,a Shumin Yang,a Lin Yi,e Wen Wen,a,d,* Xiaolong Lia,d,* a

Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese

Academy of Sciences, Shanghai 201204, P. R. China b

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

c

Department of Electronic Engineering, Tsinghua University, Beijing, 100084, P. R. China

d

Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics,

Chinese Academy of Sciences, Jialuo Road 2019, Jiading District, Shanghai, 201800, P. R. China e

School of Physics, Huazhong University of Science and Technology, Wuhan, 430074, P. R. China

*

Corresponding author

E-mail addresses: [email protected] (W. Wen) [email protected] (X. Li)

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Abstract: Nowadays, porous materials become essential to many technological applications. In this account, 3-dimensional skeleton composite materials consisted of a core-shell amorphous porous carbon/multi-layer graphene are synthesized by chemical vapor deposition on Ni foam using a facile one-step growth method. The data suggest that these composites have not only outstanding electrical and mechanical properties of the multi-layer graphene but also the mesoporous characteristics of the amorphous carbon. Moreover, the composited carbon materials perfectly inherit the macroporous structure of Ni foam, and the amorphous carbon core in the skeleton serves as a cushion to buffer the volume variation after the removal of Ni. The carbon composites reveal ultralow density (4.45 mg cm-3) and high conductivity (45 S cm-1), essentially issued from the perfectly preserved structural integrity of graphene. The novel carbon composites can be used as anodes for lithium ion batteries. After these carbon composites are incorporated with NaBiO3, superior electrochemical activities above 2 V can be achieved with a discharge capacity ~300 mAh g-1. Keywords: Amorphous porous carbon; Graphene; Core-shell; Lithium ion batteries; NaBiO3

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Introduction Carbonaceous materials have been extensively studied due to their exceptional physical and

chemical properties, including high electronic conductivity, excellent electrochemical stability and high surface area. Graphene, a two-dimensional carbonaceous material, has attracted tremendous attention due to its unique band structure and fascinating electronic, optical, and mechanical properties.1-3 Graphene showed great potential for various applications, such as nanoelectronic devices,4,5 biosensing,6,7 energy-storage materials,8-10 composites,11,12 among others.13-15 Hybrid structures made of different carbon materials can combine the advantages of the constituting components to further extend the diversity of carbonaceous macrostructures and broaden their potential applications in novel areas. As a result, tremendous efforts have been made to integrate graphene and amorphous porous carbon, so as to achieve superior properties in terms of high porosity, light-weight, and chemical stability.16 Furthermore, the poor conductivity and mechanical characteristics of amorphous porous carbon when compared to graphene could be balanced by incorporating graphene in the hybrid structure. For example, a multi-layer graphene/porous carbon woven fabric film was prepared by Li et al. and found to be relevant for supercapacitor use to deliver a capacitance of 20 µF cm-2.17 Also, Schottky-junction solar cell devices with enhanced power conversion efficiencies were prepared using hybrid films of multilayer graphene and amorphous carbon coupled with n-type Si.18 On the other hand, graphene-based layered porous carbons with impregnated sulfur can be used as cathodes for Li–S batteries to yield better electrochemical capacities and cycle stabilities when compared with graphene/sulfur composites.19 However, as most graphene-amorphous carbon hybrid structures have planar geometries, thus they are intrinsically limited by the small effective 3



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surface area.17-19 Besides, the non-ideal electrical contacts between the different components could reduce the conductivity. In the meantime, most of previously reported graphene/amorphous carbon hybrid systems usually employ complicated synthetic methods made of two or more steps manufacturing procedures.19-22 Therefore, facile fabrication methods of bulk 3-dimensional (3-D) high-quality graphene/amorphous porous carbon hybrid structures with good conductivity are highly desirable. In this study，we report the synthesis of a novel bulk 3-D carbonaceous structure composed of multilayer graphene and amorphous porous carbons using a facile chemical vapor deposition (CVD) process. A 3-D network skeleton with a multilayer graphene film (MGF)/amorphous porous carbon film (APCF) was obtained using a one-step synthesis route. The resulting APCF core in the skeleton served as a cushion to buffer the volume changes after removal of the Ni substrates, which prevented cracks in the MGF during the transfer and assembling process. This allowed both MGF and APCF to retain their characteristics in the 3-D hybrid structure. Electrochemical measurements indicated that the 3-D MGF/APCF hybrid material exhibits potential applications in lithium ion batteries.

2.

Results and discussion The schematic illustrations of the products synthesized by chemical vapor deposition method

on Nickel foam at different conditions are shown in Fig. 1. A large number of studies dealing with graphene growth on Nickel substrates using CVD have been reported.23-25 Under an ambient pressure growth process, the hydrocarbon molecules were decomposed at the nickel catalyst surface, and carbon atoms could instantly diffuse into the Nickel.26 When the concentration of 4


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carbon in Nickel reached a saturation value, the subsequently dissociated carbon atoms began to form graphene.25 In general, graphene layers could grow either by isothermal or precipitation processes during cooling periods.27-29 However, it should be pointed out that C saturation in the Ni catalyst is not mandatory for M-/FLG (multi-/few layer graphene) nucleation. Meanwhile, the precipitation of carbon atoms from Ni may be restricted by two factors: (1) High concentrations of carbon atoms in the reaction tube hindering the precipitation; (2) Rapid cooling process that may result in a quenching effect and reduce the mobility of the dissolved carbon atoms.23,30 Consequently, most of the dissolved carbon atoms could remain in the Nickel substrate after the growth process. In our previous work, amorphous porous carbon films with controllable thickness have been successfully prepared by controlling the amount of carbon atoms dissolved in Nickel foil.31 In this work, by tuning the CVD growth conditions on Nickel foam, 3-D APCF, 3-D MGF and 3-D MGF/APCF hybrid structures were synthesized as schematically displayed in Fig. 1. After the dissolution of the carbon atoms in Nickel foam substrates (Fig. 1(a)), carbon doped Nickel foam can be obtained by CVD growth (Fig. 1(b) and (c)). The 3-D APCF structure shown in Fig. 1(d) and (e) appeared after the Nickel foam was etched away. The carbon atoms issued from CH4 or doped nickel foam were almost exhausted by the graphene assembling, thus resulting in the formation of the 3-D MGF structure. A typical 3-D graphene structure was synthesized according to the procedure reported by Cheng et al (Fig. 1(f), (g), (h), (i)).27 The deposition of large amounts of residual carbon atoms into the Nickel substrate during the growth process of graphene layer by CVD resulted in graphene/carbon doped Ni/graphene foam (Fig. 1(j) and (k)). The latter induced a 3-D MGF/APCF hybrid structure after removal of Nickel (Fig. 1(l) and (m)).

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Fig. 1. Schematic illustration of the synthetic procedures using chemical vapor deposition on 3-D Ni foams: (a) the pristine

Nickel foam, (b) and (c) the carbon doped Ni foam after CVD growth, (d) and (e) the obtained 3-D amorphous porous carbon

after etching the Ni substrate,(f) and (g) the graphene/ Ni / graphene foam after CVD growth, (h) and (i) the obtained 3-D

graphene structure after etching the Ni substrate, (j) and (k) graphene/carbon doped Ni / graphene foam after CVD growth, and (l)

and (m) the obtained novel 3-D graphene/ porous amorphous carbon/ graphene hybrid structure after etching the Ni substrates.

The photographs of the resulting Ni foams samples before and after the CVD growth and the products after Nickel foam removal are displayed in Fig. 2(a). It can be seen that after the CVD growth process, the carbon doped Nickel foam shown in Fig. 2(a)-(2) became slightly brighter compared with the original sample depicted in Fig. 2(a)-(1). This was attributed to an increase in the Ni grain size after annealing at higher temperatures (Fig. S1 in Supporting Information). The color of the MGF/Ni/MGF foam (Fig. 2(a)-(3)) and the MGF/carbon doped Ni/MGF foam (Fig. 2(a)-(4)) changed from shiny white (Fig. 2(a)-(1)) to dark grey (Fig. 2(a)-(3) and (4)), resulting from the coverage of the multilayer graphene. After removal of the Nickel foam, the obtained 3-D APCF (Fig. 2(a)-(5)) displayed a different color compared with the 3-D MGF (Fig. 2(a)-(6)) and 3-D MGF/APCF hybrid materials (Fig. 2(a)-(7)). Raman spectroscopy was used to identify the carbon species present in the materials. The 6


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carbon doped Ni foam showed no Raman signals, indicating the absence of any grown graphene in the samples. These data were consistent with the SEM observation shown in Fig. S2 (a). Raman spectra of the MGF/Ni/MGF foam and MGF/carbon doped Ni/MGF foam showed almost similar features, which exhibited prominent peaks at ~ 1580 cm-1 (G band) and 2724 cm-1 (2D band). 30 The 3-D MGF and 3-D MGF/APCF obtained after removal of the Ni foam exhibited the same Raman feature as the spectra obtained before removal of the Nickel foam. This also corroborated data published in previous reports.27,29 In fact, the intensity ratio of I2D/IG was relatively low, indicating that both materials are made of multilayers graphene.32 No visible D band can be observed, suggesting that the obtained MGFs were of high quality.33 Even for the 3-D MGF/APCF hybrid material, only G and 2D bands were visible, indicating that the APCF core did not influence the top-layer MGF structure, and the MGF shell maintained the high quality. Moreover, ripples and wrinkles-like morphologies could be observed in the graphene films, as depicted by Fig. S2 (b) and (c). For 3-D APCF, the Raman spectrum displayed a prominent broad D band peak at ~ 1370 cm-1, illustrating the amorphous nature of the resulting 3-D APCF.33,34

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Fig. 2. (a) Photographs of: (1) pristine nickel foam, (2) carbon doped Ni foam, (3) MGF/Ni/MGF foam, (4) MGF/carbon doped

Ni/MGF foam after CVD growth, (5) 3-D APCF, (6) 3-D MGF, and (7) 3-D MGF/APCF hybrid structure after Nickel foam

removal. (b) Raman spectra of carbon doped Ni foam, MGF/Ni/MGF foam, and MGF/carbon doped Ni/MGF foam after CVD

growth, 3-D APCF, 3-D MGF, and 3-D MGF/APCF hybrid structure after removal of Nickel foam. (c) Temperature and CH4/H2

ratio constraints for 3-D APCF growth at ambient pressure for 20 min. (d) Temperature and CH4/H2 ratio constraints for 3-D

continuous MGF/APCF hybrid materials growth at ambient pressure for 20 min.

The growth parameters of individual products were also systematically studied. Fig. 2 (c) presents the typical growth temperature and CH4/H2 ratio constraints of the 3-D APCF grown at ambient pressure for 20 min. The data suggested that a narrow parameter window could be utilized for 3-D APCF growth. At lower growth temperatures and higher H2 flows, continuous 3-D APCF structures could be obtained as the graphene nucleation was inhibited.31,35 At temperatures (or CH4/H2 ratios) outside of the parameter window, additional graphene sheets grow on the APCF. The temperature and CH4/H2 ratio constraints of 3-D continuous MGF/APCF hybrid materials grown at ambient pressure for 20 min are displayed in Fig. 2 (d) (details in Fig. S3). It can be seen that high temperatures were necessary for the growth of integrated 3-D MGF/APCF hybrid materials to ensure continuous MGF growth and sufficient residual carbon atoms for formation of APCF.23-25,31 However, growth temperatures exceeding 950 °C induced no APCF under the same experimental conditions. This may result from the high mobility of the carbon atoms at elevated temperatures, where most dissolved carbon atoms precipitated during the graphene growth process.23,36,37 In addition, the pressure was also shown to significantly influence the synthesized products. At growth pressure below 10 Torrs, only 3-D MGF could be synthesized because 8


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insufficient carbon atoms may dissolve in nickel.

Fig. 3. (a) Top view SEM image, (b) high-magnification top view SEM image, (c) cross-sectional view SEM image of 3-D APCF

grown at 650 °C (CH4/H2 flow: 30 sccm/40 sccm for 20 min) under ambient pressure, (d) top view SEM image, (e) high-

magnification top view SEM image, (f) cross-sectional view SEM image of 3-D MGF grown at 900 °C (CH4/H2 flow: 180

sccm/50 sccm for 20 min) and 6.5 Torr, (g) top view SEM image, (h) high-magnification top view SEM image, and (i) cross-

sectional view SEM image of 3-D MGF/APCF hybrid structure grown at 900 °C (CH4/H2 flow: 5 sccm /180 sccm for 20 min) at

ambient pressure.

The surface morphologies of the prepared samples were investigated by SEM. Fig. 3 (a) shows the SEM images of the 3-D APCF grown at 650 °C (CH4/H2 flow: 30 sccm /40 sccm for 20 min) at ambient pressure. The picture depicted only some bulges without the presence of visible macropores on the surface, indicating a serious collapse of the structure after removal of the Nickel foam. The high-magnification top view SEM images shown in Fig. 3 (b) confirmed the 9



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surface mesoporous structure of APCF. The cross-sectional view SEM image of Fig. 3 (c) estimated the thickness of the 3-D APCF to about 86 µm, with the original Nickel foam thickness of 1.5 mm. This large shrinkage in thickness of more than 95% should be attributed to the poor mechanical strength of the individual amorphous porous carbon. Fig. 3 (d) illustrates the top view SEM image of 3-D MGF grown at 900 °C at the CH4/H2 flow of 180 sccm/50 sccm and 6.5 Torr for 20 min. The typical multilayer graphene structures could be identified by high-magnification top view SEM image presented in Fig. 3 (e). The cross-sectional view SEM images shown in Fig. 3 (f) of 3-D MGF estimated the thickness of the sample to about 700 µm, with more than 50% shrinkage compared to that of the original Nickel foam. These data corroborated results obtained from previous reports.27 The top and cross-sectional view SEM images of the 3-D MGF/APCF hybrid structure grown at 900 °C (CH4/H2 flow: 5 sccm /180 sccm for 20 min) under ambient pressure are gathered in Fig. 3(g) and (i). A macroporous structure might clearly be observed with no evident cracks or deformations. The high-magnification top view SEM image presented in Fig. 3 (h) of the 3-D MGF/APCF hybrid structure displayed uneven topography of MGF shell, which could be induced by the core structure of the amorphous porous carbon. In fact, the number of graphene layers in both 3-D MGF and 3-D MGF/APCF hybrid structures were almost the same as confirmed by HRTEM images of Fig. S4 (a) and (b). However, the 3-D MGF/APCF hybrid material depicted almost no shrinkage (~0%, Fig. S5), which perfectly maintained the macroscopic structure of the Ni foam when compared to the 50% shrinkage of 3-D MGF. In general, the 3-D MGF skeleton possesses a hollow internal structure after the removal of the Nickel substrate, which originally supported the graphene sheets. Thus, even in absence of any operations, some cracks and collapses 10


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were already observed in the 3-D MGF (Fig. S6 (a)), due to the hollow internal structure of the graphene skeleton. By contrast, the 3-D MGF/APCF hybrid material maintained the structural integrity without any cracks or collapses (Fig. S6 (b)). This was mainly attributed to the amorphous porous carbon, which served as a cushion to support the graphene sheets. Moreover, the formed thickness of APCF core was proportional to the thickness of the pristine Ni substrate, which can minimize changes in volume caused by Ni removal.31 Due to the resulting perfect structural integrity with no cracks or collapses in the graphene skeleton, the 3-D MGF/APCF hybrid materials depicted ultralow densities and very high electrical conductivities. For example, the 3-D MGF/APCF hybrid structure grown at 900 °C (CH4/H2 flow: 5 sccm /180 sccm for 20 min at ambient pressure) had a density of 4.45 mg cm-3, which was lower than that of the 3-D MGF (5 mg cm-3).27 The electrical conductivity of the 3-D MGF/APCF hybrid structure was measured as ~ 45 S cm-1, which was 4 folds higher than that of the 3-D MGF (~ 10 S cm-1) and ~ 6 orders of magnitude higher than that of the 3-D APCF.

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Fig. 4. (a) TEM image, (b) STEM HAADF image, (c) SAED pattern of the amorphous porous carbon from 3-D APCF, (d) TEM

image, (e) STEM HAADF image, (f) SAED pattern of the graphene sheet from 3-D MGF, (g) TEM image, (h) STEM HAADF

image, and (i) SAED pattern of the core-shell MGF/APCF structure from the 3-D MGF/APCF hybrid materials.

The microstructure of APCF, 3-D MGF and MGF/APCF hybrid structures were further characterized by TEM and the results are displayed in Fig 4 (a), (d), and (g). The morphology of APCF appeared porous and loose as shown in Fig 4 (a), which was distinctly different from the morphology of MGF (Fig 4(d)). Small-angle X-ray scattering (SAXS) was performed to analysis the pore structure of APCF, shown in Fig. S7. The average pore diameter of APCF is approximately 45.6 nm. The pore diameter distributes from 25 to 118 nm, mainly around 35 and 92 nm, coincides with the SEM and TEM observations. The MGF/APCF hybrid exhibited both the characteristics of MGF shell and APCF core (Fig 4 (g)). The X-ray photoelectron spectroscopy 12


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(XPS) was used to analysis the chemical elements of the fresh fracture surface of 3D MGF/APCF composite, shown in Fig. S8. Only C and O can be detected, and there are no detectable Ni and Fe residuals. The small amount of O should be originated from air absorption. The samples were further characterized by STEM in the high-angle annular dark field (HAADF) imaging mode, where the image contrast is directly proportional to mass density and square of the atomic number. Fig 4 (b) shows the high magnification STEM-HAADF images of individual APCF. A uniform porous structure could be observed throughout the film surface. This was consistent with the highmagnification top view SEM image shown in Fig. 3 (b). By contrast, the STEM-HAADF image of MGF showed a compact structure free of porosity (Fig 4 (e)). Meanwhile, the STEM-HAADF image of MGF/APCF hybrid material not only revealed a porous structure of individual amorphous porous carbon but also a compact structure of graphene sheet (Fig 4 (h)) (Fig. S8). Selected-area electron diffraction (SAED) measurements were also performed to determine the microstructure of the samples. The SAED pattern of APCF indicated a typical amorphous structure (Fig 4 (c)). The sharp diffraction spots in Fig 4 (f) were attributed to the good crystalline structure of the 3-D MGF. Since the MGF/APCF hybrid consisted of both the graphene sheet and amorphous porous carbon, the spots and rings observed on both the individual components were found in the SAED pattern of the composite (Fig 4 (i)). These data were consistent with those obtained from TEM and STEM-HAADF observations. The relevant features, including integrated hybrid structure, abundant pores, outstanding electrical and mechanical properties, as well as the connection between MGF and APCF, made the 3-D MGF/APCF hybrid material suitable for applications in energy storage devices, such as Li-ion batteries. As a proof of concept, the as-synthesized 3-D MGF/APCF hybrid was tested for lithium 13



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batteries applications. The mass ratio of MGF and APCF in the hybrid sample is about 0.8 (detailed in Table S1). Fig. 5 (a) and (b) display the electrochemical cycling results using 3-D MGF/APCF hybrid and the 3-D MGF.

Fig. 5. (a) Cycle performance of 3-D MGF and 3-D MGF/APCF composite at a current density of 100 mA g-1 between 3.0 and

0.01 V versus Li+/Li. (b) Rate performance of 3-D MGF and 3-D MGF/APCF composite. (c) In-situ XRD patterns obtained

during the 93rd cycle of 3-D MGF/APCF hybrid vs Li battery, where C (0 0 2) diffraction peak is displayed. (d) C (002) peak area

and 2theta position vs Voltage plot. (X-ray wavelength is 0.6887 Å).

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The electrochemical discharge capacity of the 3-D MGF/APCF hybrid is recorded as ~ 716 mAh g-1 during the 1st discharge process at a current density of 100 mA g-1, and decreased to ~ 512 mAh g-1 during the 2nd discharge process, indicating that 28.5% of the 1st discharge capacity is irreversible. This is mainly due to the formation of a solid electrolyte interface on the 3-D MGF/APCF hybrid electrode surface and the reaction of lithium ion with residual oxygencontained functional groups.38 After that, the discharge capacity remained almost stable and slightly increased to ~ 605 mAh g-1 during the 100th discharge cycle. The voltage profile during the charging process displays approximately 60 % charge capacity at voltages below 0.3 V (shown in Fig. S9). During the discharge process, almost the same 60% discharge capacity at voltages below 0.3 V can be displayed, demonstrating a very good reversibility. We found that the specific capacity of the 3-D MGF/APCF hybrid is higher than the theoretical limit of graphite electrode of 372 mAh g-1. In fact, lithium storage in carbonaceous materials is strongly dependent on the structure of the host material. It is reported that a trend of increasing reversible capacity was observed when the d-spacing of the material is increased.39 For the 3-D MGF/APCF hybrid, due to the size effect of graphene,25 the MGF shell displays a larger d-spacing than that of graphite (shown in Fig. S10), which should mainly contribute to the higher specific capacity. Moreover, the loose structure of APCF core with throughout mesopores (shown in Fig. 4 (g), (h) and S6) may enable additional sites for lithium ions accommodation. Therefore, a higher specific capacity can be rationally achieved for 3-D MGF/APCF hybrid than that of the theoretical limit of graphite. This voltage profile and enhanced specific capacity made the 3-D MGF/APCF hybrid structure a suitable anode for 2nd generation lithium ion batteries applications. As a reference, the electrochemical cycling performance of the 3-D MGF vs Li battery is also 15



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displayed in Fig. 5 (a). A discharge capacity of ~578 mAh g-1 is obtained during the 1st discharge process at a current density of 100 mA g-1. The discharge capacity is declined to ~ 403 mAh g-1 during the 2nd discharge cycle and further declined to ~ 269 mAh g-1 during the 7th discharge cycle. At the 100th cycle, the discharge capacity is slightly increased to ~ 405 mAh g-1. The cycling profiles of 3-D MGF vs Li battery (Fig. S9) also reveal approximately 60 % capacity at voltages below 0.3 V as that for 3-D MGF/APCF hybrid. However, the charge/discharge capacity obtained with the 3-D MGF is about 160-200 mAh g-1 lower than that of the 3-D MGF/APCF hybrid at each cycle. The rate capabilities of 3-D MGF and 3-D MGF/APCF composite are shown in Fig. 5 (b). The 3-D MGF/APCF hybrid delivers a large capacity of ~ 420 mAh g-1 at 100 mA g-1. At the high current density of 500, 1000 and 2000 mA g-1, the corresponding reversible specific capacity can reach 213, 152 and 121 mAh g-1 respectively. When the current density returns back to 100 mA g-1 after 30 cycles, the specific capacity is recovered to 478 mAh g-1, which is even larger than previous value cycled at 100 mA g-1, indicating the good reversibility and rate capability. In the whole rate performance tests, the 3-D MGF/APCF composite displays much higher capacity than that of the 3-D MGF, further demonstrating the superior electrochemical performance of the 3-D MGF/APCF composite. Although 3-D MGF also displays an expanded d-spacing, which could cause a larger specific capacity, however, the evident cracks and deformations in the MGF plane (shown in Fig. 3 (f) and S5) seriously degrade its integrity and conductivity. In the meantime, the absence of APCF core also decreases many sites for lithium ions accommodation (Fig. S11). Therefore, 3-D MGF alone delivers less capacity than that of 3-D MGF/APCF hybrid. These results indicated that the 3-D MGF/APCF hybrid material was much suitable for lithium ion batteries applications due to its superior integrity, electronic conductivity and abundant porous 16


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structure. In-situ XRD experiments (Fig. 5 (c) and (d)) was conducted to monitor the C (002) peak evolution of the 3-D MGF/APCF hybrid material during the 93rd cycle. The reversible structural transformation between C to LixC6 was observed from Fig. 5 (c). After the charge process, the C (002) peak area recovers back same value as that before the discharge process. In the meantime, the 3-D MGF/APCF hybrid remains unchanged at voltage above 1 V, shown in the light blue area in Fig. 5 (d). This structural stability allow the usage of 3-D MGF/APCF hybrid as conducting additive to improve the electronic conductivity and specific surface area of the holistic electrode materials.

Fig. 6. (a) XRD pattern of NaBiO3·2H2O. (b) Galvanostatic cycling profiles of NaBiO3 cathodes prepared using two substrates:

(black) thin Al foil and (red) MGF/APCF. The applied current density was 10 mA g-1.

Though Bi2O3, BiFeO3 and Bi2Fe4O9-CeO240-43 have large discharge capacity and are of great interests for lithium batteries applications, however, with Bi ionic oxidation state of +3, these compounds usually display a discharge voltage lower than 2 V and are only suitable for anode applications. Bi ions in bismuth oxides can have higher oxidation state (+5)44-46 and can enhance the electrochemical discharge. However, these oxides usually exhibit poor electronic conductivity, and limit their application in lithium batteries. As we mentioned above, the 3-D MGF/APCF 17



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hybrid material displays superior electronic conductivity and abundant porous structure, and it was also shows good structural stability at high voltage. When the high oxidation state (+5) bismuth oxides are incorporated with the 3-D MGF/APCF hybrid material, one can expect an enhanced electrochemical performance. As displayed in Fig. 6 (a), sample of NaBiO3·2H2O was adapted for lithium battery applications. The X-ray data is analyzed using CMPR and EXPGUI programs for scaling and wavelength refinement.47,48 The original NaBiO3·2H2O shows lateral size of several hundreds nanometer (100-300 nm) (Fig. S12). Fig. 6 (b) displays the electrochemical cycling profiles of NaBiO3·2H2O vs Li batteries. When NaBiO3·2H2O was painted on the Al foil, there was no electrochemical cycling activity observed at a current density of ~10 mA/g. However, when NaBiO3·2H2O was painted on a 3-D MGF/APCF hybrid, an excellent electrochemical cycling activity was obtained. A charge activity of ~ 4.7 V could be obtained with a capacity of ~200 mAh g-1. The discharge activity consisted of two discharge plateaus located at ~2.6 V and 2.1 V, respectively, where both showed a discharge capacity of ~150 mAh g-1. Compared to previously investigated systems,40-43 the NaBiO3·2H2O/MGF/APCF hybrid displayed both an excellent electrochemical discharge voltage above 2 V and a large capacity of ~300 mAh g-1 for the 1st discharge process (the detailed structural evolutions of the hybrid during electrochemical charge/discharge cycle measurements are shown in the Supporting Information, Fig. S13 and Fig. S14). Here, the superior electrochemical performance of the NaBiO3·2H2O/MGF/APCF hybrid compared with NaBiO3·2H2O/Al foil can be mainly attributed to the following reasons: Firstly, the NaBiO3·2H2O nanosheets decorated on 3-D MGF/APCF greatly promotes the electron transfer capacity and enhance the electronic conductivity of NaBiO3·2H2O. Secondly, the macroporous 18


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structured 3-D MGF/APCF framework can absorb more NaBiO3·2H2O nanosheets, significantly increase the specific surface area of the whole electrode, and enhance the contact with electrolyte. Thus, the hybrid provides excellent ionic transport from electrolyte to NaBiO3·2H2O and electronic transport from the NaBiO3·2H2O to 3-D MGF/APCF. Thirdly, the abundant porous structure of the 3-D MGF/APCF can buffer the volume swing during electrochemical cycling, therefore minimizing the whole electrode destruction from the associated strain. Finally, the NaBiO3·2H2O/MGF/APCF hybrid enhances the volumetric density, electrochemical activity and mechanical stability. Therefore, the microporous structure, high conductivity and mechanical characteristics of MGF/APCF hybrid greatly benefit the electrochemical performance of lithium ion batteries, which could be further explored for practical applications.

3.

Conclusions A facile synthesis of a bulk 3-dimensional (3-D) carbonaceous structure consisting of core-

shell amorphous porous carbon film (APCF) and multi-layer graphene film (MGF) was prepared using a one-step chemical vapor deposition method. The novel 3-D MGF/APCF composite materials combined the excellent electrical properties of MGF and the porous characteristics of APCF. Meantime, the APCF core served as a buffer layer to protect the MGF shell structural integrity, including the macroporous structure, excellent electrical conductivity, and lower density. The synthesized 3-D MGF/APCF hybrid material was found of relevance for various applications, such as Li-ion batteries. The electrochemical measurements indicated that the 3-D MGF/APCF hybrid displayed 2-folds electrochemical lithium storage capacity when compared with values obtained with pure 3-D MGF. Also, the novel NaBiO3·2H2O/MGF/APCF hybrid exhibited 19



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excellent electrochemical activities at voltages above 2 V. This facile synthetic method opens novel routes for preparing 3-D MGF/APCF hybrids in a controlled and scalable manner suitable for lithium batteries applications.

4.

Experimental

Synthesis of 3-D MGF/APCF hybrid materials Nickel foams (~1.5 mm thickness, ~380 g m-2 surface density, Lizhiyuan Battery Co. Ltd., China) were used as 3-D sacrificial templates for the CVD growth of 3-D hybrid multilayer graphene/amorphous carbon structures. The Nickel foams were first cut into pieces of 15 mm×40 mm, which then were ultrasonically cleaned in acetone, alcohol, and deionized water for 10 minutes each. The clean samples were placed in the center of a horizontal quartz tube furnace, and the typical CVD growth process was performed by first pumping the quartz tube furnace system to reach a base pressure of 8×10-3 Torr. A 500 sccm Ar was then introduced into the system to create an inert atmosphere. Next, the nickel foam samples were heated to the growth temperature (6501000 oC) under Ar (300 sccm) and H2 (100 sccm) atmospheres at a heating rate of 20 oC min-1. This was followed by annealing the resulting Nickel foam samples for 30 min at the growth temperature under Ar (180 sccm) and H2 (180 sccm) atmospheres in order to clean the samples surfaces and eliminate any present thin surface oxide layers. During the growth process, a mixed gas of CH4 and H2 was introduced into the chamber for 20 min, and different CH4/H2 ratios with various growth temperatures/pressures were tested to optimize the structure of the resulting hybrid carbon materials. Finally, the temperature was decreased to 600 oC at a cooling rate of ~ 20 oC min-1, and then rapidly quenched to room temperature by quickly moving the Nickel foam out of 20


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the tube furnace hot zone under the same gas flow as the growth process. The resulting Nickel foam samples were then washed using ferric chloride solution, and the obtained 3-D carbon structures were further washed several times with deionized water to remove any left residual ferric chloride. Sample characterization The morphologies of the obtained 3-D carbon materials were characterized by scanning electron microscopy (SEM, Model JSM-7600F, JEOL Ltd.). The microstructure of the 3-D carbon hybrid materials was studied by field emission transmission electron microscopy (FETEM, Tecnai G2 F20 S-TWIN) operating at 200 kV. For TEM, the analyses were acquired using selected area electron diffraction (SAED, the aperture size of 20 µm) and high-angle annular dark-field (HAADF) detector under a scanning transmission electron microscopy (STEM) mode. The carbon hybrid materials were first ultrasonically dispersed in ethanol and then a certain amount was dropped onto a TEM grid for characterization. Raman spectroscopy was obtained by an HORIBA Jobin-Yvon LabRAM Raman spectrometer (473 nm wavelength with a laser spot diameter of about 2 µm). The electrical conductivities of the 3-D carbon hybrid structures were measured using the standard four-point probe technique (Loresta-GP, Mitsubishi Chemical). The X-ray diffraction (XRD) pattern of NaBiO3·2H2O was measured at BL14B1 of the Shanghai Synchrotron Radiation facility (SSRF).49 The powdered sample was loaded into a quartz capillary, which was continuously spinning during X-ray data collection, with the diameter of 0.5 mm. Mythen 1K linear detector was adapted for the data collection in transmission mode.50 The wavelength of the X-ray was calibrated using LaB6 standard from NIST(660b) and was 0.6898 Å. Electrochemical measurements 21



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The synthesized 3-D MGF/APCF hybrids and 3-D MGF were directly tested for lithium batteries application, without the addition of PVDF (Polyvinylidene Fluoride) binder and acetylene black. Lithium foil was used as an anode for the electrochemical cycling tests. NaBiO3·2H2O with or without 3-D MGF/APCF cathodes were also used for the electrochemical measurements. NaBiO3·2H2O (150 mg, Alfa Aesar) was thoroughly mixed with acetylene black and PVDF at weight percentages of 0.75, 0.125 and 0.125. Few drops of NMP (Nmethyl-2-pyrrolidone) were added to the mixture to form uniform slurry, which was then manually painted onto a thin Al foil (3 cm × 3 cm, thickness 15 µm). Finally, the painted Al foil was dried in a vacuum oven at 80 °C overnight. NaBiO3·2H2O cathodes were also made using 3-D MGF/APCF (1 cm × 4 cm) as the substrate according to similar preparation procedures. The galvanostatic electrochemical cycling was performed using an NEWWARE battery cycler, with a current density of ~10 mAg-1. Lithium foil was used as an anode for the electrochemical cycling test. The in situ XRD experiments were carried out at beamline BL14B1 of Shanghai Synchrotron Radiation Facility at a wavelength of 0.6887 Å. A MX225 detector was used to acquire the XRD signals and a typical time for one scan is 630 s. A spectro-electrochemical cell was used for in situ XRD experiments, with an airtight sandwich-type feature and the Mylar windows in both the cathodic and anodic sides.51,52 The middle of Cu current collectors was cut a hole (2 mm in diameter) for X-ray penetrating through the active materials while it is cycling. The synthesized 3D MGF/APCF hybrids were directly tested for lithium batteries application in the spectroelectrochemical cell, without the addition of PVDF (Polyvinylidene Fluoride) binder and acetylene black. Lithium foil was used as an anode for the electrochemical cycling tests. To clearly 22


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display the structure evolution during the cycling, typical XRD patterns with various voltages are shown in Fig. 5(c) and (d).

Notes: The authors declare no competing financial interests. Supporting Information. Surface Morphologies of the original nickel foam and carbon doped nickel foam (Figure S1), Surface Morphologies of the carbon doped Ni foam, MGF/Ni/MGF foam, and MGF/carbon doped Ni/MGF foam after the CVD growth process (Figure S2), Detailed growth parameters for 3-D continuous MGF/APCF hybrid materials growth at ambient pressure for 20 min (Figure S3), HR-TEM images to count the layers of graphene of 3-D MGF and 3-D MGF/APCF (Figure S4), Optical microscopy cross-sectional views of the 3-D MGF /APCF hybrid structures grown at 900 °C (Figure S5), Morphologies of the 3-D MGF and 3-D MGF/APCF hybrid structure (Figure S6), Pore size distribution of the APCF film, calculated from the measured SAXS data (Figure S7), XPS spectrum of the fresh fracture surface of 3D MGF/APCF composite (Figure S8), Voltage profiles of 3-D MGF and 3-D MGF/APCF hybrid (Figure S9)，XRD pattern of 3-D MGF/APCF hybrid materials and highly oriented pyrolytic graphite (HOPG) (Figure S10), Electrochemical performance of APCF (Figure S11), TEM image of the NaBiO3·2H2O (Figure S12), Structure studies of a working NaBiO3•2H2O/MGF/APCF vs Li battery at different stages during the cycling process (Figure S13), Cycle performance of NaBiO3•2H2O/MGF/APCF vs Li battery (Figure S14), Mass ratio of MGF and APCF in the hybrid material (Table S1).

Acknowledgements 23



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The authors thank the staffs at beamline BL08U, BL14W and BL14B1 of the Shanghai Synchrotron Radiation Facility. This work was financially supported by the National Key Research and Development Program of China (No. 2017YFA0402800, 2017YFA0403000 and 2017YFA0403400) and National Natural Science Foundation of China (No. 11225527, 11505275, U1732121, 21303147). The authors thank Renduo Liu for the support in TEM test.

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TOC

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Amorphous

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