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Construction of a Unique Two-Dimensional Hierarchical Carbon Architecture for Superior Lithium Ion Storage Zhijie Wang, Xiaoliang Yu, Wenhui He, Yusuf Valentino Kaneti, Da Han, Qi Sun, Yan-Bing He, and Bin Xiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12570 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 25, 2016

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ACS Applied Materials & Interfaces

Construction of a Unique Two-Dimensional Hierarchical Carbon Architecture for Superior Lithium Ion Storage Zhijie Wang,1 Xiaoliang Yu,2 Wenhui He,3 Yusuf Valentino Kaneti,2 Da Han,2 Qi Sun,1 Yan-Bing He,2* Bin Xiang1* 1

Department of Materials Science & Engineering, CAS Key Lab of Materials for Energy Conversion, Synergetic Innovation Center of Quantum Information Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China. 2 Engineering Laboratory for the Next Generation Power and Energy Storage Batteries, and Engineering Laboratory for Functionalized Carbon Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen, China. 3 Laboratory of Physics and Chemistry of Nano-Objects (LPCNO), Institut National des Sciences Appliquées de Toulouse (INSA), Paul Sabatier University-Toulouse III, Toulouse, France. *Corresponding author: [email protected] (Bin Xiang); [email protected] (Yan-Bing He).

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Abstract Two-dimensional nanocarbons are intriguing functional materials for energy storage. However, the serious aggregation problems hinder their wide applications. To address this issue, we developed a unique two-dimensional hierarchical carbon architecture (2D-HCA) with ultrasmall graphene-like carbon nanosheets uniformly grown on hexagonal carbon nanoplates. The obtained 2D-HCA shows an interconnected porous structure and abundant hetero-element doping. As anode for lithium ion batteries, it exhibits a high discharge capacity of 748 m Ah g-1 even after 400 cycles at 2 A g-1.

Keywords: nanocarbon materials; 2D hierarchical architecture; 2D subunits; porous structure; lithium ion batteries.

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Two-dimensional (2D) carbon nanomaterials are being extensively explored for next-generation electrochemical energy storage and conversion systems, including supercapacitors,1 secondary batteries2 and electrochemical catalysts.3 When used as lithium-ion battery anodes, their high surface areas provide abundant electrochemically active sites, thus giving rise to high specific capacities. Furthermore, 2D nanostructures can shorten the ion diffusion length and promote high-rate performance.4 In addition, their extraordinary mechanical properties enable facile strain relaxation, hence allowing for good cycling stability.5 Due to the above advantages, 2D carbon nanomaterials also can be used as buffer matrix to build carbon-based composite for lithium-ion battery anodes, such as metal oxide/sulfide-graphene composite5,6 and Si/Ge-graphene composite.7,8 However, owing to the Van der Waals' interaction, two-dimensional carbon nanomaterials materials, such as graphene,9 or graphene-like carbon nanosheets10 often suffer from serious aggregation problems, which greatly reduce the active surface area and hinder smooth lithium ion diffusion.9-11 Therefore, it is critical to prevent 2D carbon nanomaterials from aggregation in order to take full advantage of their large surface area. The construction of hierarchical structures assembled by 2D nanocarbon subunits may provide an effective approach to address the above-mentioned problem.11,12 The most well-known hierarchical structure is 3D graphene micro-assembly fabricated by graphene nanosheets. Zuo et al.13 reported a porous 3D continuous graphene framework, showing a high specific capacity of 932 m Ah g−1 at 100 mA g−1. To further enhance the porosity, Zheng et al.14 designed a 1D hierarchical porous carbon microrods composed of vertically aligned graphene-like nanosheets, which exhibited excellent cycling stability (833 m Ah g-1 after 700 cycles at 1 A g-1). To date, 2D hierarchical structures constructed by 2D carbon substrates and subunit 2D nanocarbons have 3



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rarely been reported. In fact, due to its large surface area, 2D template can serve as a more effective substrate to enable massive and uniform growth of 2D nanocarbon subunit. Therefore, the development of a 2D hierarchical architecture assembled of 2D carbon subunits is of great significance for achieving high electrochemical performance. Furthermore, developing eco-friendly methods to build such hierarchical carbon architectures is highly desirable.15

In this paper, we reporte a unique 2D hierarchical carbon architecture (2D-HCA) through an eco-friendly synthesis process. Specifically, Mg-Al layered double oxide (LDO) nanoplates were used as a template to adsorb organic anionic dyes in waste water. During the adsorption process, 2D hierarchical structure of Mg-Al LDH/organic dye composite was generated. After subsequent heat treatment and template removal processes, 2D-HCA constructed by graphene-like carbon nanosheets uniformly grown on the surface of hexagonal carbon nanoplates can be achieved. The 2D-HCA possesses an interconnected porous structure and abundant hetero-element doping. When used as a lithium-ion battery anode, the 2D-HCA shows a high specific capacity of 748 m Ah g-1 at a high current density of 2 A g-1 even after 400 cycles.

Scheme 1. The preparation route of the 2D-HCA sample. 4


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The preparation route of 2D-HCA is illustrated in Scheme 1. Mg-Al LDO plates derived from Mg-Al layered double hydroxide (LDH) were utilized to adsorb a typical toxic, refractory organic anionic dyes,16 Orange II (OII) in waste water. During the process of OII adsorption, LDO was reconstructed to LDH by rehydration (noted as RLDH) and a 2D hierarchical structure of Mg-Al RLDH/OII composite (RLDH/OII) was gradually generated. After carbonization and removal of inorganic template, a unique two-dimensional hierarchical carbon architecture (2D-HCA) with ultrasmall graphene-like carbon nanosheets uniformly grown on hexagonal carbon nanoplates was achieved.

Fig. 1. SEM images of (A) Mg-Al LDH, (B) Mg-Al LDO, (C) Mg-Al RLDH/OII and (D) 5



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2D-HCA. Fig. 1 shows the scanning electron microscopy (SEM) images of the inorganic template, the intermediate product and the target 2D-HCA. As can be seen in Fig. 1A, Mg-Al LDH exhibit a uniform hexagonal plate-like structure with an average size of ~ 2.1 µm (see top inset of Fig. 1A). The XRD pattern in Fig. S1 reveals that all the diffraction peaks of Mg-Al LDH can be well assigned to Mg4Al2(OH)12CO3·3H2O (Mg2Al-LDH) (PDF # 51-1525). Mg-Al LDH can be structurally described by stacking of positively charged octahedral magnesium/aluminium double hydroxide layers (as illustrated in the bottom inset of Fig. 1A), which are neutralized by intercalated carbonate anions.17 During the calcination process, carbonate anions were decomposed and LDO was obtained. The hexagonal plate-like structure is maintained in LDO (Fig. 1B). LDO consisted of layer of magnesium/aluminium double oxide (see the bottom inset of Fig. 1B), and it can be served as an effective adsorbent to adsorb the organic anionic dye, such as OII (chemical structure see in Fig. S2) in waste water.18 As shown in Fig. S3, when LDO was added into OII solution, the color of the solution gradually faded from red to orange as the adsorption process continued, suggesting that the OII was successfully adsorbed. During this process, OII is intercalated into interlayer spaces and attached onto the surface of LDO. As the adsorption continued, a 2D hierarchical structure of Mg-Al RLDH/OII composite was produced (as shown in Fig. 1C). In order to investigate the formation mechanism of such 2D hierarchical structure, a reference experiment was conducted. LDO was put into water and stirred continuously. Fig. S4 shows the SEM image of the as-prepared product, which demonstrates a similar 2D hierarchical structure with that in Fig. 1C. According to a previous research by Zhao et al.,19 LDO can be reconstructed to RLDH by rehydration in an aqueous environment. It can be inferred that during

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this rehydration process, not only the phase transformation of metal oxide to metal hydroxide, but also the morphology transition from smooth nanoplate to complex 2D hierarchical structure occur. After carbonization and removal of inorganic template, the target product 2D-HCA was obtained. 2D-HCA also shows a very similar 2D hierarchical structure (Fig. 1D). Besides, no obvious aggregation of the 2D-HCA can be observed (Fig. S5) due to the existence of abundant subunit carbon nanosheets. The enlarged SEM image in Fig. S6 presents that the subunit carbon nanosheets are ultrasmall and ultrathin. Furthermore, the adjacent carbon nanosheets are interconnected and formed many macropores with diameters of few tens of nanometers. For comparison, the OII was also directly carbonized and the microstructure morphology of products (C-OII) was shown in Figure S7. It is seen that the C-OII present an irregular blocky morphology with particle sizes ranging from hundreds of nanometers to few tens of micrometers, and no hierarchical structure was observed.

To further characterize the hierarchical structure of 2D-HCA in detail, transmission electron microscope (TEM) observation was conducted and the results are shown in Fig. 2 and Fig. S8. Fig. 2A shows the hexagonal structure of 2D-HCA (outlined in red) observed from top view. And plenty of macropores throughout the hierarchical structure can be seen, which is demonstrated more clearly in Fig. S8. The interconnected macropores can act as reservoirs to store electrolyte and facilitate easy access of electrolyte ions to the electrochemically active sites. The side view TEM image of 2D-HCA clearly verify that the hierarchical structure is constructed by ultrasmall carbon nanosheets grown on the carbon nanoplate matrix (Figure 2B). The thickness of 2D matrix is ~ 110 nm (marked with dash lines) while the thickness of 2D-HCA is ~ 250 nm. High-magnification TEM image taken from the edge of 2D-HCA demonstrates the size of the 7



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subunit carbon nanosheets is ~ 100 nm (Fig. 2C). The high resolution TEM (HRTEM) image in Fig. 2D reveals that the thickness of the subunit carbon nanosheets is in the range of 2.5 ~ 3.2 nm, corresponding to 7 ~ 9 graphitic carbon layers. Such ultrathin carbon nanosheets are advantageous to ultrafast lithium ion diffusion through 2D-HCA.

Fig. 2. TEM images of 2D-HCA. (A) TEM image observed from top view; (B) High magnification TEM image and low magnification TEM image (inset) observed from side view; (C) High magnification TEM image observed from the edge of the 2D-HCA; (D) HRTEM image of the ultra-small carbon nanosheet marked in (C). The XRD pattern of the 2D-HCA in Fig. S9A reveals the existence of two broad diffraction peaks assigned to (002) and (100) reflections of hexagonal graphite, indicating the partially graphitic nature of 2D-HCA,20 which is further verified by Raman spectroscopy (Fig. S9B). The G 8


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band at around 1586 cm-1 is related to the stretching of graphitic carbon and the D band at around 1345 cm-1 is associated with defects.21 The IG/ID ratio of 1.04 also suggests the partially graphitic feature of 2D-HCA.22

Fig. 3. Structural analysis of 2D-HCA. (A) N2 adsorption/desorption isotherms and (B) corresponding pore size distribution curves; High resolution XPS spectra of C 1s (C) and High resolution S 2p (D). Nitrogen adsorption/desorption tests are undertaken to characterize the texture properties of 2D-HCA (Fig. 3). 2D-HCA shows a hybrid of type-I and type-IV isotherms (Fig. 3A), indicating its hierarchical porous structure combining micro-, meso- and macropores. The specific surface area (SSA) of 2D-HCA can be determined to be 1363 m2 g-1. Such a high SSA can provide an ample electrode/electrolyte interface for ion accumulation.23 The pore size distribution (PSD) curve of 2D-HCA in Fig. 3B reveals a multimodal distribution with well-defined micropore peaks

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centered at 0.7 nm and 1.4 nm and obvious mesopore peaks centered at 2 nm, 4 nm and 10 nm, confirming the hierarchical porous feature of 2D-HCA. The developed micropores can be ascribed to the thermal decomposition of organic OII, and the mesopores mainly result from the removal of the inorganic template. The existing micropores can offer abundant active sites for lithium ions storage and the mesopores can promote the fast transportation of electrolyte ions.24 In comparison, C-OII shows a different isotherm with steep uptake in the low pressure region and a small hysteresis loop in the medium-to-high pressure region (Fig. S10A), suggesting its dominantly microporous feature. Its SSA value can be calculated to be 509 m2 g-1, which is much lower than that of 2D-HCA. From the corresponding PSD curve (Fig. S10B), it can be found well-developed micropore peak and a much weaker mesopore peak centered at about 2 nm. The porous structure is further verified by HRTEM image, and plenty of small size pores can be observed in C-OII (Fig. S11). X-ray photoelectron spectroscopy (XPS) measurements are conducted to probe the composition and state of the elements of 2D-HCA. C 1s, O1s, N 1s, and S 2p peaks can be clearly seen from the XPS survey spectrum. The corresponding contents of these elements can be determined to be 86.13 %, 9.83 %, 1.95 % and 2.05 %, respectively (Fig. S12). The high-resolution XPS spectrum of C 1s can be deconvoluted into four peaks (Fig. 3C), i.e., C-C/C=C (284.6 eV), C-N (285.6 eV), C-O (286.9 eV) and C=O (289.1 eV).25 The high resolution S 2p spectrum (Fig. 3D) can be fitted into three peaks located at 163.8 eV, 164.9 eV and 168.1 eV, which is corresponding to C-S bond, C=S bond and SOx group, respectively, respectively.25 The spectrum of N 1s can be fitted into three peaks, including pyridinic nitrogen, pyrrolic nitrogen and quaternary nitrogen (inset of Fig. S12).26,27 The XPS results suggest the N, S dual-doping nature of 2D-HCA. The N and S elements are homogenously distributed in the carbon framework, which is 10


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demonstrated by TEM elemental mapping (Fig. S13). The doped N and S heteroatoms can improve the electrochemical activity by increasing the active sites in the carbon framework.23, 25-28

Considering its unique 2D hierarchical architecture, interconnected hierarchical porous network and abundant hetero-element doping, 2D-HCA could serve as a promising LIB anode material. Here electrochemical performances measurements were conducted in coin-type half cells. Fig. 4A shows the cyclic voltammetry (CV) curves of 2D-HCA, which are similar with those of previously reported

Fig. 4. (A) CV curves of 2D-HCA at a scanning rate of 0.2 mV s-1; (B) Discharge-charge profiles of 2D-HCA for the first three cycles at 200 mA g-1; (C) Rate capabilities of 2D-HCA and C-OII at various current densities; (D) Long-term cyclability tests of 2D-HCA and C-OII anodes at a high current density of 2 A g-1. 2D carbon nanomaterials.11,14,23 In the first cycle, the irreversible redox peak at 0.64 V is related to the formation of solid electrolyte interface (SEI) film, and the redox peak near 0 V is attributed to

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the intercalation of lithium ions into the graphitic carbon layers.14, The oxidation peaks at ~ 0.2 V and 1.2 V correspond to the process of lithium ion de-intercalation from the graphitic carbon layers and the electrochemically active sites of the porous structure, respectively.25 The CV curves in the following cycles almost overlap, suggesting quite similar lithiation/de-lithiation behavior and excellent reversibility. The CV current densities of C-OII anode are much lower than those of 2D-HCA (Fig. S14), suggesting its much lower capacity. Fig. 4B displays the charge-discharge profiles of 2D-HCA at current density of 200 mA g-1. In the first cycle, 2D-HCA shows quite high discharge capacity of 3277.6 m Ah g-1 and charge capacity of 1667.4 m Ah g-1, with an initial Coulombic efficiency of 51 % which is higher than that of N, S doped nanographene sheet trapped with carbon nanotube,11 3D graphene foam,13 1D nanorods composed of carbon nanosheets.14 The large irreversible capacity may result from the SEI film formation, the irreversible insertion of lithium ions into the porous structure and/or the special positions in the vicinity of residual H atoms in 2D-HCA.23 The following discharge-charge curves show no obvious difference, suggesting highly reversible lithiation/de-lithiation process after the first cycle. And the Coulombic efficiency increases to over 91 % in the following cycles. To investigate the rate capability of two anodes, specific capacities at various current densities from 200 mA g-1 to 5000 mA g-1 was tested (Fig. 4C). At 200, 500, 1000 and 2000 mA g-1 the corresponding discharge capacities of the 2D-HCA (measured from the last cycle of each density) are 1475.1, 931.5, 661.7 and 507.7 m Ah g−1, respectively. When the current density increases to 5000 mA g-1, the capacity still maintains 323.8 m Ah g−1, which is very close to the capacity of commercially available graphite anode at low rates. And when the current density returns to 2000, 1000, 500, 200 mA g-1, the capacities can well recover to the initial capacity values with a little 12


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decrease, confirming good reversibility of the electrochemical reaction. While the corresponding capacities of C-OII anode are only 244.8, 169.8, 114.6, 67.7, 30.3, 67.8, 105.3, 151.3 and 226 m Ah g−1, respectively. High-rate cycling performance is also quite important for LIBs due to the increasing demand for high-power applications. Fig. 4D displays the long-term cyclability of the two anodes at a high current density of 2 A g-1. The discharge capacity of 2D-HCA initially fades to 556.6 m Ah g−1 from 2663 m Ah g−1 in the first 45 cycles, and then gradually increases to 748 m Ah g−1 in the 400th cycle. The initial capacity decrease should be ascribed to the formation of SEI layer while the capacity increase in the following cycles may result from the reversible formation of a polymeric gel-like film.29 The corresponding Coulombic efficiency of 2D-HCA at such high current density reaches over 90 % in the initial 3 cycles and over 95 % after 8 cycles (Fig. S15). In comparison, the C-OII anode exhibits a first reversible capacity of 60 m Ah g−1, and after 400 cycles, it only maintains a capacity of 40 m Ah g−1. To unravel the superior lithium storage properties of 2D-HCA, the cycled electrodes are analyzed by SEM and XPS examinations. As shown in Fig. S16, after 20 cycles (at a current density of 200 mA g-1), the 2D structure of 2D-HCA is maintained, which is covered by a SEI film due to the reduction composition of electrolyte. The stable 2D structure contributes to the excellent high rate cycling performance. After 100 cycles (at a current density of 2 A g-1), the N and S elements still exist in the carbon framework (Fig. S17), which proving the stable doping of N, S heteroatoms in 2D-HCA during discharge/charge cycles. The doped N and S heteroatoms can increase the electrochemical activity of 2D-HCA by creating abundant active cites in the carbon framework, which is beneficial to lithium-ion storage. The cycling performance, rate capabilities and long term tests at a high current density verified 13



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the superior lithium storage properties of 2D-HCA. The unique 2D hierarchical structure is responsible for the outstanding lithium-ion storage performance. Firstly, the hierarchical porosity of 2D-HCA nanocomposite provides an interconnected porous network combining macropores, mesopores and micropores. The macropores can serve as ion buffering reservoirs, from which plenty of electrolyte ions can transport to the mesopores. Then the electrolyte ions can diffuse along the mesopore channels and get easy access to the microporous lithium storage sites. Secondly, the 2D structure ensures quite short ion transport length and thus ultrafast ion diffusion throughout 2D-HCA. And the 2D hierarchical structure enables facile strain relaxation, leading to a good cycling stability. Last, the abundant hetero-element doping could further enhance the electrochemical activity. Due to its unique hierarchical structure and large surface area, 2D-HCA also can serve as electrode materials of supercapacitor. Furthermore, the interconnected porous structure can be utilized to support electrochemical catalyst.

In summary, we report a low-cost and eco-friendly synthesis of a two-dimensional hierarchical carbon architecture (2D-HCA). The 2D hierarchical structure was generated simultaneously during the process of Mg-Al LDO adsorbing toxic organic anionic dyes. As-obtained 2D-HCA consists of ultrasmall graphene-like carbon nanosheets uniformly grown on hexagonal carbon nanoplates. Moreover, it shows an interconnected hierarchical porous network and abundant hetero-element doping. These merits enable 2D-HCA to possess superior lithium-ion storage properties. Even at a high current density of 2 A g-1, a high specific capacity of 748 m Ah g−1 was still maintained over 400 cycles. Moreover, the structural advantages of 2D-HCA may also promote other potential applications, such as supercapacitor, gas adsorption, and electro-catalysis.

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ASSOCIATED CONTENT Experimental section and additional XRD patterns, SEM/TEM images, XPS spectra, N2 adsorption/desorption isotherms and CV curves are supplied in Supporting Information.

ACKNOWLEDGEMENTS This work was supported by the National Key Basic Research Program of China (2014CB932400), Youth research funds of Graduate School at Shenzhen, Tsinghua University (QN20150002), and Production-study-research cooperation project of Guangdong province (No. 2014B090901021) and Dongguan City (2015509119213), Shenzhen Basic Research Project (No. ZDSYS20140509172959981 and JCYJ20140417115840246).

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