Porous Fe2O3 Nanoframeworks Encapsulated within Three-Dimensional Graphene as HighPerformance Flexible Anode for Lithium-Ion Battery Tiancai Jiang,†,‡ Fanxing Bu,† Xiaoxiang Feng,† Imran Shakir,*,§ Guolin Hao,*,‡ and Yuxi Xu*,† †
State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200433, China ‡ Hunan Key Laboratory for Micro-Nano Energy Materials and Devices, Xiangtan University, Hunan 411105, China § Sustainable Energy Technologies Center, College of Engineering, King Saud University, Riyadh 11421, Kingdom of Saudi Arabia S Supporting Information *
ABSTRACT: Integrating nanoscale porous metal oxides into three-dimensional graphene (3DG) with encapsulated structure is a promising route but remains challenging to develop high-performance electrodes for lithium-ion battery. Herein, we design 3DG/metal organic framework composite by an excessive metal-ion-induced combination and spatially confined Ostwald ripening strategy, which can be transformed into 3DG/Fe 2 O 3 aerogel with porous Fe 2 O 3 nanoframeworks well encapsulated within graphene. The hierarchical structure offers highly interpenetrated porous conductive network and intimate contact between graphene and porous Fe2O3 as well as abundant stress buffer nanospace for effective charge transport and robust structural stability during electrochemical processes. The obtained free-standing 3DG/Fe2O3 aerogel was directly used as highly flexible anode upon mechanical pressing for lithium-ion battery and showed an ultrahigh capacity of 1129 mAh/g at 0.2 A/g after 130 cycles and outstanding cycling stability with a capacity retention of 98% after 1200 cycles at 5 A/g, which is the best results that have been reported so far. This study offers a promising route to greatly enhance the electrochemical properties of metal oxides and provides suggestive insights for developing high-performance electrode materials for electrochemical energy storage. KEYWORDS: prussian blue, three-dimensional graphene, porous Fe2O3, anode, lithium-ion battery insertion/deinsertion reactions in metal oxides.11,13 Second, huge volume expansion and contraction during the electrochemical reaction leads to the pulverization and aggregation of metal oxides and thus short cyclic life.12,14 In addition, the lack of continuous conductive network and interpenetrated ion transport pathway throughout the whole electrode usually result in low utilization of the active metal oxides.15,16 Therefore, deliberate structural design of metal oxides and the whole electrode is necessary to achieve desirable electrochemical performance. To this end, great attention has been paid to the morphology controllable synthesis of metal oxides and large amount of advantageous structures have been established. Among them,
H
igh-performance lithium-ion batteries (LIBs) with large power density, energy density, and long cyclic life are key to the development of rapidly upgrading portable electronic devices and newly emerging large scale applications, e.g., electric vehicles and electricity grid storage.1 However, the currently commercialized LIBs cannot meet this demand because they mainly employ graphite as anode, which suffers low capacity (372 mAh/g) and poor rate performance. Thus, great efforts have been devoted to the exploitation of anode materials with higher capacity, such as metal,2−5 metal oxides,6−8 and metal sulfides.9,10 Among them, metal oxides, such as Fe2O3, have attracted extensive attentions due to their high theoretical capacity, as well as their natural abundance and environmental friendliness. Nevertheless, several general problems exist for metal oxides as anode materials for LIBs and largely compromise their electrochemical properties.11−16 First, the poor intrinsic electronic and/or ionic conductivity is harmful for the sufficient and rapid electrochemical Li+ © 2017 American Chemical Society
Received: March 30, 2017 Accepted: April 30, 2017 Published: April 30, 2017 5140
DOI: 10.1021/acsnano.7b02198 ACS Nano 2017, 11, 5140−5147
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Figure 1. Schematic illustration of the preparation procedure of 3DG/Fe2O3. (a) Excessive metal ions induced combination, (b) self-assembly and spatially confined Ostwald ripening, and (c) thermal treatment under air condition.
sandwich-like graphene/metal oxide/graphene structure together, thus holding great potential for electrochemical applications. When directly used as flexible anode for LIBs upon mechanical pressing, the entire free-standing 3DG/Fe2O3 electrode delivered remarkable electrochemical performance, including an ultrahigh capacity of 1129 mAh/g at 0.2 A/g after 130 cycles, excellent rate capability, and superior cycling stability with a capacity retention of 98% after 1200 cycles at 5 A/g, which is the best ever reported result for Fe2O3-based anode materials.
porous metal oxide superstructures composed of nanoscale building block have attracted enormous interests due to their large surface area and high porosity.7,12,14,17 This architecture can not only provide large electrode/electrolyte interfaces and shortened paths for rapid Li+ ions and electrons diffusion, but also alleviate the structural strain caused by repeated Li+ insertion/extraction processes. On the other hand, significant efforts have been devoted to the integration of metal oxides with conductive carbon materials, particularly graphene.11,13,15,16 As a superior two-dimensional conductive material, graphene not only could be used as substrate to load metal oxides to improve the electron transport and prevent the aggregation, but also could be further engineered into freestanding three-dimensional (3D) porous structure to promote electron and ion diffusion in the whole electrode.15,16,18,19 In addition, graphene can be tailored to coat on the surface of metal oxide particles and alleviate the pulverization induced by the volume changes.20,21 However, the above each specific treatment is usually capable of resolving one problem but fail to overcome other problems, thus combining them to create more elaborate structure and exploring the structure−property relationship are of significant importance for the development of high-performance metal oxide-based anode materials for LIBs. Herein, we design and synthesize 3D graphene (3DG)/ Fe2O3 aerogel with porous Fe2O3 nanoframeworks well encapsulated within graphene skeleton by using deliberately designed 3DG/metal organic framework (MOF) aerogel as template. Due to the inorganic−organic hybrid compositions and intrinsic porous structure, MOF have been widely recognized as appropriate precursors for various porous materials including metal oxide and carbon,12,22−25 and thus chosen as the typical precursor of porous metal oxides in this work. The 3DG/MOF aerogel was fabricated by excessive metal ion induced self-assembly and spatially confined Ostwald ripening strategy, which ensured MOF nanoparticles were all wrapped within graphene skeleton rather than deposited on the surface of graphene skeleton, and then the successful construction of targeted 3DG/Fe2O3 aerogel. Distinguished from previous reports where solid metal oxide nanoparticles and/or large porous metal oxide microparticles were simply deposited on the surface of 3DG,15,26−28 the monolithic 3DG/ Fe2O3 aerogel integrates the porous nanoarchitecture of metal oxides, robust 3D porous conductive network of graphene and
RESULTS AND DISCUSSION Prussian blue (PB), one typical and low-cost MOF selfassembled from Fe ions and cyanide ligands, holds great potential as appropriate precursor of Fe2O312,25 and is chosen as the representative precursor for porous metal oxide to demonstrate our envision. The 3DG/Fe2O3 aerogel with porous Fe2O3 well encapsulated within graphene was fabricated by thermal treatment of presynthesized 3DG/PB aerogel. As shown in Figure 1, we first prepared 3DG/PB aerogel by excessive metal ions induced self-assembly and spatially confined Ostwald ripening strategy. Specifically, graphene oxide (GO)/PB composite was obtained by adding excessive Fe3+ ions into GO/[Fe(CN)6]4− solution. By this way, a portion of Fe3+ ions reacted with [Fe(CN)6]4− to form PB and excessive Fe3+ adsorbed onto the surface of PB, ensuring the effective deposition of PB on GO containing carboxylic group by electrostatic attraction and coordination interaction.29,30 Then, the water-dispersible GO/PB composite sheets were selfassembled into 3DG/PB hydrogel by chemical reduction using ascorbate acid as reducing agent.31 Due to the chemical instability of PB in acidic environment induced by ascorbate acid, the PB nanoparticles deposited on the external surface of 3DG framework were dissolved while the PB nanoparticles encapsulated within graphene layers transformed into larger PB nanoparticles by spatially confined Ostwald ripening.32 This lead to the formation of 3DG/PB hydrogel with PB nanoparticles encapsulated within graphene skeleton. After calcination, PB nanoparticles were decomposed into porous Fe2O3 nanoframeworks and 3DG/Fe2O3 aerogel with wellencapsulated porous Fe2O3 nanoframeworks was achieved. As shown in Figure 2a, b, and S1, the water-dispersible GO/ PB composite well-inherited the two-dimensional morphology of GO and PB nanoparticles with an average size of ∼12 nm 5141
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degraded under HRTEM measurement. The larger size of PB nanoparticles in 3DG/PB aerogel and the structural transformation from PB/GO/PB in GO/PB composite to graphene/PB/graphene in 3DG/PB could fully demonstrate the spatially confined Ostwald ripening process. The structural evolution from GO/PB to 3DG/PB aerogel was further confirmed by XRD and Raman spectra. As shown in Figure 3a, the 3DG/PB showed stronger and sharper peaks which correspond to (100), (110), and (200) planes of PB (JPCDS 01-0239) compared to GO/PB, indicating that the size of PB increased. The average size of PB calculated based on Scherrer formula is about 11 and 27 nm for GO/PB and 3DG/ PB, respectively, which is consistent with SEM and TEM results. Raman spectra of GO/PB and 3DG/PB (Figure 3b) display a largely enhanced intensity ratio of the D band to G band (ID/IG) of 3DG/PB (1.58) compared to that of GO/PB (0.99), demonstrating that GO was reduced efficiently. The above results present that the chemical instability of PB in acid reaction environment and special stacking structure of graphene in 3DG leads to the formation of one kind of 3DG/PB composite with most PB wrapped within 3D graphene. After thermal treatment under air, the 3DG/PB aerogel transformed into 3DG/Fe2O3 aerogel, which could be confirmed by XRD, Raman spectra, and X-ray photoelectron spectroscopy (XPS) analysis. First, XRD peaks and Raman shift bands attributed to PB disappeared. Second, four Raman shifts between 200 and 700 cm−1 related to A1g and Eg bands of Fe2O3 appeared.33 In addition, 2p peaks at 712.4 and 726 eV with a satellite peak at 717.0 eV in XPS associated with the characteristic peak of Fe3+ in Fe2O3 were observed (Figure S2).34−36 However, no obvious peaks were observed in the XRD pattern of 3DG/Fe2O3 aerogel, indicating that the obtained Fe2O3 was amorphous or poorly crystallized. Encouraged by the robust structure of the 3DG/Fe2O3 aerogel, SEM and TEM measurements were carried out to characterize its microstructure. As shown in Figure 4a, the 3DG/Fe2O3 aerogel inherited the 3D interconnected porous structure of 3DG/PB aerogel. At the same time, Fe2O3 nanoparticles also maintained the microscopic morphology of PB nanoparticles and were still well encapsulated within graphene (Figure 4b and c). Different from solid PB nanoparticles, many light regions existed in every particle, meaning those Fe2O3 nanoparticles are highly porous nanoframeworks. To fully demonstrate their featured structure, HRTEM measurement was conducted. As shown by arrow in Figure 4d, few layers of graphene around Fe2O3 nanoframeworks were visible, demonstrating the well-defined graphene wrapped structure. Close observation exhibited that the Fe2O3 nanoframeworks were self-assembled from many disorderedly
Figure 2. (a,b) SEM images of GO/PB composite. SEM (c,d) and TEM (e,f) images of the microstructure of 3DG/PB. The inset in (a) shows a stable aqueous dispersion of GO/PB composite and the inset in (c) shows the photograph of a monolithic 3DG/PB aerogel.
distributed on the both sides of GO. After being reduced by ascorbate acid, the GO/PB composite was self-assembled into 3DG/PB hydrogel, which was converted to 3DG/PB aerogel upon lyophilization. The morphology and microstructure of 3DG/PB aerogel were fully characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). As shown in Figure 2c, the monolithic 3DG/PB aerogel has an interconnected 3D porous network with pore sizes ranging from submicrometer to several micrometers. The PB nanoparticles with sizes of 20−40 nm are wrapped in the interior of the graphene framework with good dispersity (Figure 2d and e). Particularly, a large amount of graphene wrinkles around PB nanoparticles were observed, indicating that graphene could conformally coat on the surface of PB nanoparticles. This was confirmed by high-resolution TEM (HRTEM) image in Figure 2f. It could be clearly observed that the PB nanoparticle was tightly wrapped by few-layer graphene sheets. It is to be noted the PB nanoparticles are vulnerable to electron radiation and the crystalline structure of them was
Figure 3. (a) XRD patterns of GO/PB, 3DG/PB, and 3DG/Fe2O3 and (b) Raman spectra of 3DG/Fe2O3, 3DG/PB, and GO/PB. 5142
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The mass loading of Fe2O3 in the 3DG/Fe2O3 aerogel was determined to be 66.7% by thermogravimetric analysis (Figure S7) and could be facilely mediated by changing the content of Fe3+ and [Fe(CN)6]4− in the precursor solution. For example, changing the content of them from 0.15 to 0.1 mL and 0.2 mL can adjust the mass loading of Fe2O3 to be 48.2% and 73.2% (denoted as 3DG/Fe2O3-L and 3DG/Fe2O3−H), respectively. The change of mass loading of Fe2O3 did not disrupt the 3D porous structure of the resulting 3DG/Fe2O3 aerogel (Figure S8a and c), and more dispersed and denser distribution of porous Fe2O3 nanoparticles on graphene were observed for 3DG/Fe2O3-L and 3DG/Fe2O3−H, respectively (Figure S8b and d). N2 sorption analysis (Figure S9a) demonstrated that the BET surface area of 3DG/Fe2O3-L and 3DG/Fe2O3−H are 191.8 and 104.6 m2/g, respectively, which are distributed among those of 3DG, 3DG/Fe2O3 and porous Fe2O3. This is reasonable in consideration of higher BET surface of 3DG than porous Fe2O3. Interestingly, with the increase of mass loading of Fe2O3, the increment of N2 uptake at the starting points of hysteresis loop increased first and then decreased. This indicated that appropriate amount of porous Fe2O3 could prevent the restacking of RGO to a certain degree and increase the mesopores, while excess Fe2O3 would cause relatively serious aggregation of RGO (Figure S8b and d) and thus smaller mesopore size and less mesopores (Figure S9b). With the highly porous structure and decent mechanical stability, the as-prepared 3DG/Fe2O3 with a thickness of ∼4 mm was compressed to form ∼50 μm-thick flexible 3DG/ Fe2O3 film (Figure 5a and b). Owing to the strong interlock of
Figure 4. (a, b) SEM and (c) TEM images of the microstructure of 3DG/Fe2O3 prepared by annealing of 3DG/PB at 250 °C for 2h. (d) HR-TEM image of 3DG/Fe2O3 and (e) the SAED pattern showing the poor crystallinity of Fe2O3. Elemental mapping images of C (f), Fe (g), and O (h) of 3DG/Fe2O3 composite shown in (d).
oriented domains with small size and poor crystallinity as indicated by circles in Figure 4d. The short and curved lattice fringes with a interplanar spacing about 2.5 Å corresponded to (110) plane of α-Fe2O3 (JPCDS 73-0603). Weak diffraction rings ascribed to (113), (110), (012) planes in selected area electron diffraction (SAED) picture (Figure 4e) further verified the obtained Fe2O3 in 3DG/Fe2O3 are hexagonal hematite with a space group of R3̅c (167), which is consistent with the XRD result of pure Fe2O3 derived from aggregated PB powder (Figure S3). Encouragingly, abundant pores existed throughout the whole particles, which could be sufficiently supported by the distribution of C, Fe, and O on the whole particle in elemental mapping (Figure 4f−h) and N2 adsorption− desorption analysis (Figure S4). A drastic N2 uptake in low pressure (P/P0 < 0.1) indicates the existence of micropores in 3DG/Fe2O3 while a hysteresis loop in the range of P/P0 = 0.4− 1.0 means that the 3DG/Fe2O3 contained constricted mesopores,37 as confirmed by dominant pore diameter at 1.1 and 5.2 nm in pore size distribution curve based on DFT calculation (Figure S5). Compared to 3DG and pure Fe2O3, the 3DG/Fe2O3 aerogel exhibited more mesopores with narrower size distribution (Figure S5), which could be mainly contributed by porous Fe2O3 nanoframeworks. In this case, unlike the formation of pure porous Fe2O3 from aggregated PB powders (Figure S5), the uniformly dispersed distribution of PB nanoparticles in 3DG guarantee their full contact with air and largely promote the crystallization process of Fe2O3, leading to the formation of mesoporous Fe2O3. This could be further demonstrated by morphological difference between Fe2O3 nanoframeworks in 3DG/Fe2O3 and pure Fe2O3 (Figure S6). In addition, the existence of Fe2O3 nanoframeworks between graphene sheets could also alleviate the restacking of graphene and created extra pores. The high porosity imparts 3DG/Fe2O3 a high BET surface area of 182.1 m2/g, a little lower than that of 3D graphene (203.5 m2/g) but much higher than that of pure Fe2O3 (50.3 m2/g).
Figure 5. (a) Preparation of a flexible binder-free 3DG/Fe2O3 electrode; (b) SEM image of the side-view of the pressed 3DG/ Fe2O3 film; and (c) high-magnification SEM image of the interior microstructure of 3DG/Fe2O3 film.
graphene sheets in the 3DG/Fe2O3 aerogel, the 3D continuous porous network was well maintained in the 3DG/Fe2O3 film and the Fe2O3 nanoframeworks were still encapsulated within the graphene sheets (Figure 5c). The pressed 3DG/Fe2O3 film with a packing density of 0.76 g/cm3 and an electrical conductivity of 4.2 S/cm could be bent for many times under a bending angle of 180° without any structural or electrical degradation, suggesting that the 3DG/Fe2O3 film was mechanically strong enough to be used directly as flexible anode for LIB without adding any other additives to assemble the cells. For comparison, pure porous Fe2O3 fabricated by 5143
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Figure 6. (a) Discharge/charge profiles of 3DG/Fe2O3 at a current density of 0.2 A/g. The cycling performance at 0.2 A/g (b) and rate performance (c) of 3DG/Fe2O3 and Fe2O3, respectively. (d) Cycling performance of 3DG/Fe2O3 at 5 A/g.
High rate performance and long cycling span of 3DG/Fe2O3 aerogel was further evaluated for their potential application in next generation LIBs. As shown in Figure 6c, the 3DG/Fe2O3 aerogel exhibited discharge capacities of 1056.5, 996.6, 903.4, 761.9, 534.2 mAh/g in the 10th-cycle at 0.2 A/g, 0.5 A/g, 1 A/ g, 2 A/g, and 5A/g, much better than that of pure porous Fe2O3. After rate performance test, the cycling performance of 3DG/Fe2O3 aerogel at a high current density of 5 A/g was examined. Remarkably, the electrode still showed a discharge capacity of 523.5 mAh/g with a very high capacity retention of 98% even after 1200 cycles (Figure 6d). The influence of mass loading of Fe2O3 in the 3DG/Fe2O3 were also examined (Figure S11). It was found that decreasing or increasing the mass loading of Fe2O3 all resulted in worse electrochemical properties, which could be ascribed to that decreasing the content of Fe2O3 decreases the capacity contribution of active material in 3DG/Fe2O3-L while increasing the content of Fe2O3 degrades the well-defined encapsulated structure of the 3DG/Fe2O3−H. Thus, the mass loading of Fe2O3 should be finely controlled to achieve the optimum results. In spite of these factors, 3DG/Fe2O3-L and 3DG/Fe2O3−H still displayed better electrochemical properties than pure porous Fe2O3. The electrochemical performance of 3DG/Fe2O3 is not only much better than the control porous Fe2O3, but also to the best of our knowledge is the best result for Fe2O3-based electrode materials reported thus far (Table S1). To understand the underlying mechanisms, the electrochemical impedance spectroscopy (EIS) analysis of 3DG/Fe2O3 before and after rate performance test were carried out. As demonstrated in Figure 7a, the Nyquist plot of 3DG/Fe2O3 before cycling exhibited a depressed semicircle in the high-frequency region and a sloping line in the low frequency region, corresponding to the charge transfer process and the semi-infinite Warburg diffusion process, respectively. After rate performance test, a new semicircle in the high-frequency section corresponding to the SEI layer appeared.40,41 The diameter is much smaller than that of pure porous Fe2O3, indicating that the 3DG wrapped structure effectively suppresses the formation of excessive SEI layer (Figure 7b). However, the charge-transfer resistances
thermal treatment of PB nanoparticles alone was mixed with conductive carbon black and PVDF binder and then pasted on Cu foil for electrochemical testing. It should be noted that all electrochemical results of the 3DG/Fe2O3 film below were based on the entire electrode while those of pure Fe2O3 were based on Fe2O3 alone. Figure 6a exhibits the typical discharge/charge profiles of 3DG/Fe2O3 at a current density of 0.2 A/g between 0.01 and 3 V (versus Li/Li+). Two voltage plateaus at 1.5 and 0.9 V in the first discharge curves and one voltage plateau at 1.25 V in the first charge curve correspond to the multisteps electrochemical reactions between Fe2O3 and Li+ (Fe2O3 → LixFe2O3 → cubic Li2Fe2O3 ↔ Fe + Li2O). After several discharge/charge cycles, the subsequent discharge/charge curves became stable with a distinct discharge plateau at about 0.9 V, which is ascribed to the reversible reaction and is in good agreement with previous reports.15 The 3DG/Fe2O3 delivered a high initial discharge capacity of 1870.4 mAh/g and a charge capacity of 1174.4 mAh/g, leading to a limited initial Coulombic efficiency of 62.8% due to the degradation of electrolyte and the formation of solid electrolyte interface (SEI). However, the Coulombic efficiency rapidly increased to about 97% after several cycles. Moreover, after the first five cycles, its discharge capacity slightly decreased to 1135.2 mAh/g and was kept almost constant for the following 130 cycles (1129 mAh/g after 130 cycles) (Figure 6b). In contrast, pure porous Fe2O3 only showed a low discharge capacity of 573 mAh/g in the first cycle and its discharge capacity decreased to 238.5 mAh/g fastly. To be noted, the specific capacity of 3DG/Fe2O3 is higher than those of porous Fe2O3 (Figure 6b) and 3DG (Figure S10) as well as the theoretical specific capacity of Fe2O3 (1007 mAh/g), which may be contributed by extra Li+ storage via interfacial reaction due to the charge separation at the metal/Li2O phase boundary38 and/or the formation of polymeric gel-type layer.7,39 The above results fully demonstrated that the integration of porous Fe2O3 into 3DG with wrapped structure largely enhances the electrochemical performance of Fe2O3 as well as stabilizes the structure of Fe2O3. 5144
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CONCLUSIONS In conclusion, we have developed 3DG/Fe2O3 aerogel with porous Fe2O3 nanoframeworks well encapsulated within graphene by employing 3DG/MOF as precursor, which was fabricated by an excessive metal ions induced self-assembly and spatially confined Ostwald ripening strategy. Combining the synergistic interaction of porous Fe2O3 nanoarchitecture, 3DG and encapsulated structure, the obtained 3DG/Fe2O3 aerogel could be directly used as flexible anode for LIBs upon mechanical pressing and showed superior electrochemical performance including an ultrahigh capacity (1129 mAh/g) and excellent rate performance as well as remarkable cyclic life (98% capacity retention after 1200 cycles), which is the best result for Fe2O3-based anodes ever reported. We believe this work provides one versatile method that could be extended for the synthesis of other 3DG-based composites with this advantageous structure for various electrochemical applications and beyond. EXPERIMENTAL SECTION Graphene Oxide (GO) Synthesis and Purification. GO was synthesized by oxidation of natural graphite powder (325 mesh, Aladdin, 99.95% metal basis) according to the modified Hummers’ method. All the other items are purchased at Sinopharm Chemical Reagent Co., Ltd. Preparation of 3DG/Prussian Blue (PB) Composite. The GO/ PB composite was synthesized by two-step centrifugation method. Typically, 0.15 mL of 0.5 mol L−1 potassium ferrocyanide (K4[Fe(CN)6]·3H2O) was dissolved into 1 mL of 2 mg mL−1 graphene oxide suspension in centrifugal tube. Then, the dispersion was centrifuged at 10000 rpm for 15 min. One milliliter deionized water was added after the supernatant was removed. Ferric chloride (FeCl3·6H2O, 0.15 mL of 0.5 mol L−1) was added into the solution. After the solution turned thoroughly blue, the mixture was centrifuged at 7000 rpm for 15 min. The GO/PB composite was obtained by removing the supernatant. Herein, excessive metal ions were necessary for the successful combination of GO and the rapidly formed PB nanoparticles and decreasing the dosage of Fe3+ will lead to the appearance of large amount PB nanoparticles that are not deposited on GO. To obtain the 3DG/PB composite, 0.1 mL of 88 mg mL−1 vitamin C (VC) was first added into GO/PB composite, and the solution volume was subsequently increased to 2 mL by adding deionized water. Finally, the 0.5 mL mixture was added into 5 mL bottle, and sealed into 95 °C oven for 2 h. Soaking in the deionized water to remove other ions for three times, the 3DG/PB composite was obtained by freeze-drying overnight. Preparation of 3DG/Fe2O3 Composite. The 3DG/Fe2O3 was synthesized by one-pot method using 3DG/PB as precursor. Typically, the 3DG/PB was annealed at 250 °C for 2 h in air. Finally, the 3DG/ Fe2O3 can be used as electrode materials without binders. In the control experiment, the different ratios of the Fe2O3 were prepared (3DG/Fe2O3-L, 48.2 wt%; 3DG/Fe2O3−H, 73.2 wt%), the 3DG/ Fe2O3-L was prepared by adding 0.1 mL of 0.5 mol L−1 K4[Fe(CN)6] and 0.1 mL of 0.5 mol L−1 FeCl3, 3DG/Fe2O3−H was prepared by adding 0.2 mL of 0.5 mol L−1 K4[Fe(CN)6] and 0.2 mL of 0.5 mol L−1 FeCl3, other conditions were the same as 3DG/Fe2O3 composite. The Fe2O3 powder obtained by one-step annealing of Prussian blue was used as the electrode. To get the Fe2O3 electrode, the Fe2O3 powder with poly(vinylidene fluoride) (PVDF) binder and conductive carbon black (8:1:1, weight ratio) was coated on Cu foil and dried in a vacuum oven for 12 h. Characterizations. SEM measurements were measured on a Zeiss Ultra-55 field emission scanning electron microscope (FESEM). TEM studies were conducted on a FEI Tecnai G2 20TWIN electron microscope at an operating voltage of 200 kV. XRD analysis was performed on a Rigaku D/Max 2500 X-ray diffractometer with Cu Kα radiation (λ = 1.54 Å) at a generator voltage of 40 kV and a generator
Figure 7. (a) The Nyquist plots of 3DG/Fe2O3 before and after cycling test. (b) The Nyquist plots of 3DG/Fe2O3 and Fe2O3 after cycling test. (c) TEM image of 3DG/Fe2O3 and (d) highermagnification image of a single Fe2O3 nanoframework under delithiation state after cycling test. (e) The schematic of delithiation/lithiation reactions of 3DG/Fe2O3.
before and after cycling have no obvious changes, indicating a tight contact between porous Fe2O3 nanoframeworks and robust conductive graphene network. In addition, as demonstrated by the more slopped curve in the low frequency part, the formation of SEI layer may slow the Li+ diffusion to a certain degree. To examine the structural stability of porous Fe2O3 nanoframeworks, TEM analysis was conducted on 3DG/ Fe2O3 after cycling test (Figure 7c and d). As shown in Figure 7c, intact Fe2O3 nanoframeworks were still well-dispersed on the graphene and the porous structure of Fe2O3 was also maintained (Figure 7d). The robust structure of Fe2O3 nanoframeworks is highly desirable for high rate performance and long cycling lifetime, which could be ensured by synergistic effect of intrinsic porous structure of Fe2O3 nanoframework and graphene wrapped structure. According to the above analysis, the superior electrochemical properties of 3DG/Fe2O3 aerogel could be ascribed to the following reasons (Figure 7e): (i) nanoscale building blocks and high porosity in porous Fe2O3 nanoframeworks shorten the ion diffusion pathway in Fe2O3.12,42 (ii) the robust interpenetrated 3DG network not only provides multidimensional pathways for rapid electron and ion transport for the electrochemical reaction of Fe2O3, but also could guarantee the integrated electron and ion transport network after repeated discharge/charge cycle.43,44 (iii) The porous structure of Fe2O3 nanoframework and its encapsulated structure could synergistically alleviate pulverization and aggregation of Fe2O3 during cycling process.45,46 In addition, the free-standing flexible nature of 3DG/Fe2O3 aerogel avoid the use of current collector, conductive carbon and binder, making them promising anode for flexible and lightweight electronics applications.13,47 5145
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ACS Nano current of 20 mA with a scanning speed of 5° min−1 from 10° to 80°. Raman measurements of the samples were carried out using an Invia/ Reflrx Laser Micro-Raman spectroscope (Horiba Jobin Yvon, French) with excitation laser beam wavelength of 532 nm. Thermogravimetric analysis (TGA) was tested with a Mettler Toledo TGA with a heating rate of 20 °C/min under 20 mL/min of flowing air. X-ray photoelectron spectroscopy (XPS) was measured by PHI 5000C ESCA System. The BET test was tested under an Autosorb IQ Gas Sorption System at 77 K. Electrochemical Characterization. Electrochemical experiments were carried out by using 2016 coin-type cells. The working electrodes are free-standing and binder-free, and the metallic lithium foil was used as the counter electrode. LiPF6 (1 M) in EC:DMC (= 1:1 by volume ratio) was used as the electrolyte, the Cellgard 2400 was used as the separator. For LIB fabrication, the cells were assembled in an argonfilled glovebox with the concentrations of moisture and oxygen below 0.1 ppm. The galvanostatic charge−discharge experiments were tested using a Land battery testing system in the voltage window from 0.01 to 3 V at different current rates. Electrochemical impedance spectroscopy (EIS) was measured by CHI 760D electrochemical workstation under the frequency range 100 kHz to 0.01 Hz.
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ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge via the Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acsnano.7b02198. Additional characterization data, additional electrochemical data, and comparison with previous results (PDF)
AUTHOR INFORMATION Corresponding Authors
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Yuxi Xu: 0000-0003-0318-8515 Author Contributions
T.J. and F.B. contributed equally to this work. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (No. TP2015002), the National Natural Science Foundation of China (51673042), the National Natural Science Foundation of China (Nos. 11404274), Natural Science Foundation of Hunan Province (No. 2015JJ3118), and the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT13093). We also like to extend our sincere appreciation to the Deanship of Scientific Research at the King Saudi University for its funding of this research work through the International Research Group Project No: IRG14-19. REFERENCES (1) Goodenough, J. B. Electrochemical Energy Storage in a Sustainable Modern Society. Energy Environ. Sci. 2014, 7, 14−18. (2) Luo, B.; Wang, B.; Li, X.; Jia, Y.; Liang, M.; Zhi, L. GrapheneConfined Sn Nanosheets with Enhanced Lithium Storage Capability. Adv. Mater. 2012, 24, 3538−3543. 5146
DOI: 10.1021/acsnano.7b02198 ACS Nano 2017, 11, 5140−5147
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DOI: 10.1021/acsnano.7b02198 ACS Nano 2017, 11, 5140−5147
