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A Coral-inspired Nanoengineering Design for Longcycle and Flexible Lithium-ion Battery Anode Yangyong Sun, Cheng Wang, Yinghui Xue, Qin Zhang, Rafael G. Mendes, Linfeng Chen, Tao Zhang, Thomas Gemming, Mark H. Rümmeli, Xinping Ai, and Lei Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02011 • Publication Date (Web): 22 Mar 2016 Downloaded from http://pubs.acs.org on March 23, 2016

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ACS Applied Materials & Interfaces

A Coral-inspired Nanoengineering Design for Long-cycle and Flexible Lithium-ion Battery Anode Yangyong Sun,a,† Cheng Wang,a,† Yinghui Xue,a Qin Zhang,a Rafael G. Mendes,b Linfeng Chen,a Tao Zhang,a Thomas Gemming,b Mark H. Rümmeli,b,c Xinping Aia and Lei Fua* a

College of Chemistry and Molecular Science, Wuhan University, Wuhan, 430072, China IFW Dresden, P.O. Box 270116, 01069 Dresden, Germany c College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China † These authors contributed equally to this work *E-mail: [email protected] b

ABSTRACT: Conversion reaction electrode materials (CREMs) have gained significant interest in lithium-ion batteries (LIBs) owing to their high theoretical gravimetric capacity. However, traditional CREMs-based electrodes, with large strain arising from Li+ intercalation/deintercalation causes pulverization or electrical breakdown and cracking of the active materials which leads to structural collapse, limiting performance. Therefore, in order to construct electrodes with a strong tolerance to the strain incurred during the conversion reaction process, we design a coral-like three-dimensional (3D) hierarchical heterostructure by using crosslinked nanoflakes (NFs) interspersed with nanoparticles (NPs) standing vertically on graphene foam (GF). The coral-like 3D hierarchical heterostructures can efficiently disperse the strain from both internal and external forces as well as increase the specific surface area for enhanced electrochemical reactions. These features lead to long-cycle stability and excellent flexibility in LIBs. Fe3O4 NPs and CoO NFs are utilized as a model system to demonstrate our strategy. The as-prepared coral-like hierarchical electrode is studied as an anode in LIBs for the first time and is shown to deliver a high reversible specific gravimetric capacity of ~1200 mAh g–1 at rate of 0.5 A g–1 for 400 cycles. In addition, our batteries can even power a green light-emitting diode (LED) when bent to high degrees confirming the excellent flexibility of the material. KEYWORDS: Coral-like, strain dispersion, long-cycle, flexible, lithium-ion battery anode 1 Environment ACS Paragon Plus


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INTRODUCTION Rechargeable lithium-ion batteries (LIBs) have become the pre-eminent power sources for portable electronic devices. In order to meet the ever increasing demand for future flexible and wearable electronic devices, the development of advanced electrodes with high electrochemical performance and flexibility is urgently needed. Conversion reaction electrode materials (CREMs), with high theoretical gravimetric capacity, are promising candidates for LIBs anodes.1–5 However, the applications of CREMs in practical LIBs are seriously hindered by their relatively large initial irreversible loss and poor capacity retention over extended cycling, owing to their low electrical conductivity and large volume change during the process of insertion/deinsertion cycles of lithium ions.6–10 Thus, numerous research efforts have been directed to enhance CREMs-based LIBs performance in recent years. Most efforts focus on compositing CREMs with good electrical conductivity substrates such as carbon11, graphen12 and carbon nanotubes13 to enhance performance. Wu et al. reported a facile strategy to synthesize a nanocomposite of Co3O4 nanoparticles (NPs) anchored on conducting graphene as an advanced anode material for high performance LIBs.14 However, Co3O4/graphene nanocomposites as an electrode show only 30 efficient cycles of the charge-discharge process and the graphene seems to contribute significant additional gravimetric capacity to the LIBs. Moreover, the inadequate release of the strain associated with Li+ intercalation/deintercalation leads to breakage of the NPs as well as the graphene, such that eventually pulverization of the active materials leads to their structural collapse, thus restricting the enhancement of LIBs performance, as shown in Scheme 1a.15–16 Therefore, it is necessary to construct electrodes with a strong tolerance to the strain induced during the conversion reaction process. Reasonable three-dimensional (3D) hierarchical heterostructures for electrodes, for instance, CREMs with a specific structure loading on the surface of substrates, have been investigated because they can disperse the strain effectively for a high energy capacity.12,17–19 Kong et al. reported a highly ordered 3D Co3O4@MnO2 hierarchical nanoneedle array on nickel foam for electrochemical energy storage.19 The outer MnO2 shell can sustain strain so as to help maintain the 2 Environment ACS Paragon Plus

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structure during repeated cycling enabling a better electrochemical performance to be achieved. However, it should be noted that the design of the nanoneedle array on nickel foam may not be comparable with other structural designs.20 Nickel foam exhibits deficiencies i.e. as a rigid substrate, it cannot effectively disperse the strain from both internal and external forces. In addition, the strain easily collects at the tip and the bottom interface of the nanoneedles which causes individual nanoneedles to fracture or fall off from the substrates during the electrochemical reaction process, as shown in Scheme 1b.21 Herein, we gain inspiration from corals found in nature in which the tidal forces from seawater can be dispersed through the crosslinked backbone of corals and the outer coral polyps are in full contact with the seawater and protect the inner backbone to maintain the morphology of the corals. Here we report a smart hybridization strategy for coral-like 3D hierarchical heterostructures by anchoring vertically interspersed CREMs crosslinked nanoflakes on graphene foam (GF) for use in LIBs for the first time. Magnetite (Fe3O4)-cobalt (II) oxide (CoO) was used as a model system to demostrate our strategy, i.e. crosslinked CoO nanoflakes (NFs) interspersed with Fe3O4 NPs standing vertically on GF (coral-like magnetite@cobalt (II) oxide@graphene foam, coral-MCG). Through such a coral-inspired nanoengineering process, we are able to fabricate hierarchical heterostructures without the use of any surfactants, in which GF serves as the “seabed”, the crosslinked CoO NFs being the “backbone of corals” and the thin layer of Fe3O4 NPs working as the “coral polyps”. In this smart electrode design, flexible GF substrates can effectively disperse the strain associated with Li+ intercalation/deintercalation as well as ensure fast mass and electron transport kinetics, and can be directly used as a current collector to load active materials without the need of a binder.22–24 The crosslinked NFs play a dual role of serving as the backbone for transferring and dispersing the strain between NPs-NFs, NFs-NFs and NFs-GF substrates, and secondly, as a conductive connection between the NPs. Moreover, the specific surface area can be enhanced. The thin layer of NPs can also increase the contact area with the electrolyte, enabling fast redox reactions as well as protecting the inner structure.19 Therefore, similar to the process that corals adopt to release thrust forces from the seawater they live in, the strong contact 3 Environment ACS Paragon Plus


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established in coral-like NPs-NFs-GF hierarchical heterostructures can disperse the strain from both internal and external forces to maintain structural integrity so well that one can achieve long-cycle and flexible LIB performace. In addition, the structure helps prevent the aggregation of active materials. These features are shown in Scheme 1c. This innovatively designed anode delivers a high reversible specific gravimetric capacity of ~1200 mA h g–1 at a rate of 0.5 A g–1 for 400 cycles, showing excellent performance for LIBs. In addition, our batteries can even power a green light-emitting diode (LED) despite a strong external force being applied to cause mean curvature of around 1.0 rad cm–1, highlighting the excellent flexibility property of this designed material. In short, the coral-like, binder-free, multifunctional 3D hierarchical heterostructures deliver excellent flexibility, long-cycle life and high reversible gravimetric capacity in LIBs, which suggests their development and application as an advanced anode material for next generation LIBs is very promising. RESULTS AND DISCUSSIONS As shown in Figure 1a, flexible GF serves as the conducting matrix and current collector due to its good structural stability, large specific surface area, freestanding ability and excellent conductivity.9 The homogeneous dispersed CoO NFs are assembled on GF by a facile solvothermal and annealing process without adding any binders or metal substrates. At the same time, Fe3O4 NPs are interdispersed on the CoO NFs to form coral-like 3D hierarchical heterostructure (Figure 1b). This architecture provides several functions. Firstly, flexible, conducting and self-supported GF was introduced to release the strain associated with Li+ intercalation/deintercalation as well as ensure high mobility for electrons. Secondly, the crosslinked CoO NFs not only disperse strain from both internal and external forces but also augment the specific surface area. Thirdly, the thin layer of Fe3O4 NPs contributes to increasing the area in contact with the electrolyte as well as protecting the inner structure. During the electrochemical reactions a large strain associated with Li+ intercalation/deintercalation exists. When applying force in our designed coral-like 3D hierarchical heterostructures, due to the crosslinked CoO NFs and the interconnected GF, most of the applied force is dispersed in all directions. 4 Environment ACS Paragon Plus

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Force can transfer from the Fe3O4 NPs to CoO NFs, from the CoO NFs to adjacent crosslinked CoO NFs and from the CoO NFs to the flexible GF substrates to cause deformation while the overall structure is well maintained (Figure 1c, d). In general, this design can effectively disperse strain, accommodate the volume change of active materials without negative consequences and allow the electrolyte to contact fully with active materials for fully-converted electrochemical reactions for long-cycle and flexible LIB functions (Figure 1e). The Raman spectra of the Coral-MCG and GF are shown in Figure 2a. There are two typical peaks in the black curve corresponding to the G band and 2D band of graphene, which are centered at 1580 and 2720 cm–1, respectively. The lack of the trace of the defect-related D band at 1350 cm–1 indicates overall high quality of the GF, which is in accordance with previous works.24–26 Also, the Raman intensity of 2D band is lower than the G band, which points to the multilayer character of the GF. This helps make the GF sufficiently rigid to sustain active materials.26 The red curve in Figure 2a shows the Raman spectra of Coral-MCG. The ID/IG value of GF is 0.298, which indicates that few defects are introduced to the GF after the coral-MCG formation process. We consider the reasons should be ascribed to the interaction between graphene and transition metal oxides.9,26 The peaks centered at 466 and 670 cm–1 can be attributed to the Eg and A1g modes of CoO.27 The peaks centered at 305 and 580 cm–1 can be attributed to Fe3O4, and a peak at 670 cm–1 which overlaps a peak from CoO.28 X-ray diffraction (XRD) was also used to characterize the Coral-MCG. As shown in Figure 2b, the black curve corresponds to the XRD spectra of graphene, in which two typical diffraction peaks observed at 26.4° and 54.5° can be assigned to the (002) and (004) reflections of GF (JCPDS card 41–1487).29 The XRD spectra of Coral-MCG is shown in red in Figure 2b. Besides these two peaks from graphene, one can also observe the (111), (200) and (311) reflections from CoO (JCPDS Card 48–1719).30 In addition, the (220), (311), (511) and (440) reflections from Fe3O4 (JCPDS card 19–0629) are also present.9 In order to further confirm chemical composition of the composite, we also performed X-ray photoelectron spectroscopy (XPS) to analyse the Coral-MCG. We should mention that all of the binding 5 Environment ACS Paragon Plus


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energies in this paper have been corrected with respect to the C 1s peak which is set at 284.6 eV. XPS analysis of Coral-MCG can be observed in Figures 2c, 2d and Supporting Information, Figure S1. The full survey XPS spectrum, as displayed in Supporting Information, Figure S1c, shows clear signatures for carbon, oxygen, cobalt and iron. As shown in Figure 2c, two obvious shake-up satellite peaks in 758.8 eV and 802.8 eV together with the Co 2p3/2 (780.8 eV) and Co 2p1/2 peaks (796.6 eV) can be observed, which confirm the presence of a CoO phase.7 The Fe 2p spectrum (Figure 2d) demonstrates the existence of Fe3+ and Fe2+, which agrees well with the Fe 2p spectrum in previous work.31 The peak located at 710.7 eV corresponds to the Fe3+ 2p3/2 configuration, while the small peak at 712.9 eV is attributed to a Fe2+ 2p3/2 configuration.9 The binding energy 724.7 eV is another spin-orbit component of Fe 2p1/2, which is the average value for the binding energies of the Fe3+ 2p1/2 and Fe2+ 2p1/2 states.32 The broad peak centered around 717.9 eV is the Fe2+ shakeup satellite peak.31 By analyzing the experimental Raman spectra, XRD and XPS data, we can conclude the successful formation of Coral-MCG. The coral-like 3D hierarchical characteristics of the as-synthesized Coral-MCG was also investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as for example depicted in Figure 3. In Figure 3a, it can be seen that crosslinked CoO NFs stand vertically on the GF. The SEM image in Figure 3b shows that a thin layer of Fe3O4 NPs are interdispersed on the surface of the CoO NFs, thus forming the coral-like structures. These crosslinked CoO NFs can disperse the strain incurred during electrochemical reactions and increase the specific surface area to expose more electrochemical reaction sites. Moreover, this designed structure enables electrolyte infiltration and maintain good contact between the GF and active materials, which is beneficial for fast electron and lithium ion transport. The corresponding TEM image is shown in Figure 3c. Fe3O4 NPs are interspersed on the surface of CoO NFs, which is in accord with the insert SEM image in Figure 3b. The lattice fringe spacings of 0.48 nm and 0.20 nm can be assigned to the (111) and (200) planes of Fe3O4 (Figure 3I) and CoO (Figure 3II), respectively, which are in accordance with the XRD analysis.30,33 In order to observe the homogeneity of the distribution of Fe3O4 and CoO on the GF, 6 Environment ACS Paragon Plus

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we used energy dispersive X-ray (EDX) mapping to evaluate the presence of Co (Figure 3e), Fe (Figure 3f), and O (Figure 3g). The elemental analysis provided in Figure 3h shows that the main elements in our materials are C, O, Fe and Co. And it can be seen that Co, Fe and O are evenly distributed over the entire structure. In order to evaluate the electrochemical performance of the as-synthesized Coral-MCG anode material, the composite was assembled into coin cells with lithium foil as the reference electrode. Figure 4a shows the first three cyclic voltammetry (CV) curves from a Coral-MCG electrode, which were obtained at a scan rate of 0.2 mV s–1 over a potential range from 0.01–3.00 V vs. Li/Li+. The CV curves coincide well with previously reported CoO/graphene and Fe3O4@graphene anodes.6,34 As can be seen, a weak and broad reduction peak centered at 1.21 V (marked as 3) can be attributed to a structure transition which is caused by the insertion of Li+ into Fe3O4 (Fe3O4 + xLi+ + xe− → LixFe3O4). The peak (marked as 2) which appears at 0.88 V corresponds to the reduction of CoO and the formation of Li2O (CoO + 2Li++ 2e− → Li2O + Co).6 In addition, the major reduction peak located at 0.57 V with a shoulder (marked as 1) appears in the first cycle and disappears in subsequent cycles, can be attributed to the formation of a solid electrolyte interface (SEI) layer and the transformation of LixFe3O4 to Fe0 by the reaction: LixFe3O4 + (8−x)Li+ + (8−x)e− → 4Li2O + 3Fe.35 In this reaction, amorphous Li2O is formed to exacerbate the irreversible reaction.36 In addition, the current intensity of the two reduction peaks at lower potentials become weaker and these two peaks are almost overlapped, which indicates that the SEI layer is well formed.37 The reversible reduction peak at 0.18 V (marked as 8) appears in all three cycles indicating a phase transformation of LixC during the electrochemical reaction. The appearance of a reduction peak at 0.01 V indicates the intercalation of lithium ions into GF.38 During the second and third scans, due to the structure pulverization and cathodic polarization, the peaks at ~0.57 V, ~0.88 V and ~1.21 V shift positively to ~0.80 V, ~1.01 V (marked as 4) and ~1.62 V. These three peaks could be assigned to the transformation of LixFe3O4 to Fe0, the reduction of CoO, and the insertion of Li+ into Fe3O4, respectively. The three CV curves from the anodic scans repeat well indicating good reversibility of the Coral-MCG electrode. The reversible oxidation peak at 0.23–0.27 V corresponds to 7 Environment ACS Paragon Plus


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deintercalation of lithium ions into GF.38 What is more, the broad oxidation peaks at 1.58 V (marked as 7) and 1.86 V (marked as 6) can be explained as the oxidation of Fe0 to Fe2+ and Fe2+ to Fe3+, respectively. The small oxidation peak at 2.11 V (marked as 5) can be assigned to the oxidation of CoO.6 From the CV measurement, it can be inferred that the Coral-MCG anode provides high electrochemical activity and stability. Figure 4b shows the discharge-charge voltage-capacity curves for the Coral-MCG anode between 0.01 and 3.00 V. The experiment was carried out with a current density of 0.5 A g–1. The discharge and charge capacities of the electrode were 1551.2 mAh g–1 and 1201.3 mAh g–1 in the first cycle, respectively. The loss of capacity can be ascribed to the irreversible formation of an SEI layer due to the decomposition of the electrolyte. The discharge voltage plateaus at ~0.65 V only appears in the first cycle, which confirms the CV data, indicating that the irreversible reaction only occurs in the first cycle. Discharge-charge measurements were performed at a 0.5 A g–1 rate for 400 cycles in ambient conditions. The mass of active materials was based on the whole weight of CoO and Fe3O4 (see Supporting Information, Figure S2). As can be seen in Figure 5g, the Coral-MCG anode shows a high discharge capacity in the first cycle of 1551.2 mAh g–1 and the coulombic efficiency is around 77.4% at a 0.5 A g–1 rate. The formation of an SEI layer and some undecomposed Li2O phase may contribute to the low coulombic efficiency.36 Due to the CoO NFs there is an increase in the specific surface area so more reaction sites are activated, and thus one can predict that the cycling performance will be good. Moreover, the coral-like 3D hierarchical structure of the material can disperse the strain from internal and external forces effectively and facilitate the effective accommodation of the volume change incurred during the cycling process, thus providing structural integrity which provides long-cycle stability LIBs using this material. It can be seen that the discharge capacity decreases slowly to ~700 mAhg–1 over the first 46 cycles and then increases to ~1200 mAh g–1 and remains stable from 270 to 400 cycles. The phenomenon of a gradual capacity increase during cycling is common in many metal oxide/sulfide electrodes.13,39–41 The phenomenon can be attributed to the following reasons. First, the pulverization of active materials will further enlarge the specific surface area for more active sites. 8 Environment ACS Paragon Plus

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Second, the kinetically activated electrolyte decomposition may also form a polymeric gel-like SEI layer. Both these aspects will contribute to increasing capacity.26,42–43 Note that the coulombic efficiency remains stable at ~99.3% indicating the excellent cycling stability of the Coral-MCG electrode. For comparison, a random structure of Magnetite-Cobalt (II) oxide-Carbon (Random-MCC) was prepared to determine its electrochemical capacity. The serious agglomeration of the Fe3O4-CoO is observed (see Supporting Information, Figure S3). In Figure 5g, the capacity of Random-MCC drops sharply and remains ~250 mAh g–1. The low capacity of Random-MCC can be ascribed to the lack of organization, huge strain and agglomeration of the powders. The superior discharge capacity of our binder-free Coral-MCG anode is in agreement with the electrochemical impedance spectroscopy (EIS) analysis in Supporting Information, Figure S4, which can be attributed to the following: On the one hand, that high conductivity of flexible GF and coral-like structures enable fast ion and electron transportation. The active materials (MCG) can be fully exposed to the electrolyte, which is similar to the coral polyps full immersing in the seawater. On the other hand, similar to the process that tidal forces from seawater can be dispersed through the crosslinked backbone of corals, our coral-like structures can also effectively disperse strain to provide structural integrity over repeated cycling, the crosslinked NFs can serve as the backbone for transferring and dispersing the strain between NPs-NFs, NFs-NFs and NFs-GF substrates (Figure 5f). Also, as can be seen that the structures of Coral-MCG after cycling in Supporting Information, Figure S5 maintain well, the layer of NPs are interdispersed tightly on the surface of NFs, which can futher illustrate the sructural stability of our Coral-MCG electrode. For Random-MCC, huge strain will lead to drastic electrical breakdown and cracking of the active materials, as well as agglomeration. These factors can cause permanent electrode failure. Discharge-charge measurement was also performed on Co3O4 NFs@GF (see Supporting Information, Figures S6) and Fe3O4 NPs@GF (see Supporting Information, Figures S7) for comparison, as can be seen in Supporting Information, Figures S8 and S9, the discharge capacity of Co3O4 NFs@GF and Fe3O4 NPs@GF were ~310 mAh g–1 and ~510 mAh g–1 at 100 cycles, respectively, which are much lower than the electrochemical performance of Coral-MCG. In addition, discharge-charge measurement 9 Environment ACS Paragon Plus


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was also performed on pure GF (see Supporting Information, Figure S10). The discharge capacity of pure GF was ~160 mAh g–1 at 100 cycles, it is obvious that the capacity contribution of graphene in Coral-MCG is small. Moreover, different current densities were performed on a Coral-MCG anode and Random-MCC anode to test the charging and discharging capability (Figure 4c), the capacity for the Coral-MCG anode was higher than Random-MCC anode at different current densities, which shows the superior rate performance of Coral-MCG in contrast to Random-MCC. In addition, an external force was applied to the Coral-MCG anode to investigate its structural stability (Figure 5a–d). Our Coral-MCG electrodes and lithium metal electrodes were encapsulated by silica gel together to form a flexible battery. Note that the mean curvature is calculated by the equation: K=

∆α ∆s

where K is mean curvature, ∆s stands for the length of two points on the arc, while ∆α is the rotation angle of the tangents to the two points. When different external forces were applied to cause a mean curvature from about 0 to 1.0 rad cm–1, the green LED could be lightened all the time (see Supporting Information, Table S1). Even when a strong external force is applied to provide a mean curvature about 1.0 rad cm–1, our LIB was still able to power a green LED (Figure 5d). In addition, cycling performance of Coral-MCG anode at flat and bent states was also tested at 1 A g–1 rate (see Supporting Information, Figure S11). It can be seen that the capacity could be maintained at bent states, highlighting its flexibility feature which makes it promising for the applications in flexible batteries. Also, for our magnetic active materials, Supporting Information, Figure S12 illustrates that the Fe3O4-CoO powder can be attracted by a magnet, which is beneficial to collect and recycle the active materials. CONCLUSION In summary, inspired by corals found in nature, we have successfully fabricated coral-like 3D hierarchical heterostructures using Fe3O4-CoO as a model system. In this structure, Fe3O4 NPs are 10 Environment ACS Paragon Plus

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interdispersed on crosslinked CoO NFs which stand vertically on GF. Similar to the process that corals adopt to release thrust forces from the seawater they live in, this designed structure can effectively disperse the strain from both internal and external forces as well as increase the specific surface area for enhanced sites during electrochemical reactions. The reversible specific capacity of our Coral-MCG anode reaches ~1200 mAh g–1 at rate of 0.5 A g–1 for a long cycle of 400 cycles which demonstrate the excellent capacity of Coral-MCG systems. In addition, our batteries can even power a green LED when bent with high curvature. We believe our coral-like, binder-free, multifunctional 3D hierarchical heterostructures offering long-cycle and flexible LIB performance will greatly facilitate future energy storage applications. EXPERIMENTAL SECTION Synthesis of the graphene foam (GF). Graphene foam was directly grown on a nickel foam substrate (1.65 mm thick, Alantum Advanced Technology Materials, China) under ambient pressure chemical vapor deposition (APCVD), and the nickel foam was cut into small disks (R = 12 mm). The growth procedure was conducted in a horizontal quartz tube fixed inside a high-temperature furnace (HTF 55322C Lindberg/Blue M). The growth protocol consisted of four steps: (1) the nickel foam was heated to 1000 °C in 30 min under an atmosphere of Ar (500 sccm) and H2 (200 sccm), then annealed at 1000 °C for 10 min without changing the gas flow; (2) a nominal amount of CH4 (5 sccm) was brought into the reaction tube at ambient pressure for 5 min; (3) the samples were cooled to room temperature naturally with Ar (500 sccm); and (4) the nickel backbone was etched by HNO3 solution (1M) for 12 h. The GF was then washed by ultrapure water and ethanol. Preparation of Co3O4 NFs@GF. Co3O4 NFs@GF was fabricated by adopting a solvothermal strategy as follows. First, 145 mg Co(NO3)26H2O and 150 mg CO(NH2)2 were dissolved in ethanol to form a transparent solution. At the same time, two pieces of GFs were put into the solution and then transferred to an autoclave. The autoclave was heated to 140 °C for 17 h and then cooled to room temperature. The obtained product was washed by ultrapure water and ethanol for at least 5 times and 11 Environment ACS Paragon Plus


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dried under vacuum. In this way, Co(CO3)0.5(OH)0.11H2O precursors were obtained by the process (see Supporting Information, Figure S13). Finally, the composites were annealing at 350 °C in air for 2–4 h to get Co3O4 NFs@GF. Bare Co3O4 powder was obtained in the same way without adding GF. Supporting Information, Figures S6, S14 and S15 are the analyses of Co3O4 NFs@GF. Synthesis of Coral-MCG. Coral-MCG was synthesized by putting one piece of Co3O4@GF into 20 mL ethanol with 100 mg Fe(NO3)39H2O adding in. Then, the solution was transferred to the autoclave and heated at 140 °C for 7 h. After cooling to room temperature, the product was washed by ethanol several times. Later, it was dried and then calcined at 450 °C for 2–4 h with a gas flow of 100 sccm Ar. During the annealing process, the Co3O4 NFs was reduced to CoO NFs. Random Fe3O4-CoO powder was obtained via the same procedure by putting Co3O4 powder in the solution. Characterization. Raman spectroscopy was performed with a laser micro-Raman spectrometer (Renishaw inVia, Renishaw, 532 nm excitation wavelength). Scanning electron microscopy (SEM) images were obtained by Hitachi-S4800. X-ray photoelectron spectroscopy (XPS) measurements were conducted with a Thermo Scientific, ESCALAB 250Xi. The measuring spot size was 500 µm, and the binding energies were calibrated by referencing the C 1s peak (284.6 eV). X-Ray diffraction (XRD) measurement was performed with LabX XRD–6000 using Cu-Kα radiation over the range of 2θ = 10~80°. Thermogravimetric analysis (TGA) was conducted on a TGA Q600 (Thermal Analysis Instrument, Burlington) in air with a heating rate of 10 °C/min from room temperature to 900 °C. Electrochemical Measurements. Electrochemical measurements were conducted in standard CR2016 cell. The cells were assembled in an Ar-filled glovebox by directly using the as-prepared Coral-MCG as working electrodes without adding any binders or conductive agents and lithium metal circular foil (1.5 mm thick) as the counter electrode. The electrolyte used was a 1.0 M LiPF6 solution in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (7:3 by volume). For comparison, the Random-MCC anode was prepared by mixing random Fe3O4-CoO powder, amorphous carbon and polyvinylidene difluoride (PVDF, Alfa Aesar) in a weight ratio of 4:4:2 (corresponding to the weight percent of Fe3O4-CoO in the composite) in N-methyl-2-pyrrolidone (NMP, Sigma-Aldrich) to form a 12 Environment ACS Paragon Plus

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slurry that then was coated on a copper foil substrate. Galvanostatical charge−discharge tests were conducted at various current densities at the voltage range from 0.01 to 3.00 V with a multichannel battery tester (LAND CT 2001A, Wuhan LAND Electronics Co., Ltd.). The cyclic voltammetry (CV) tests were measured using electrochemical workstation (CHI610E, Chenhua, Shanghai) at a sweeping rate of 0.2 mV s–1 with the voltage ranging from 0.01 to 3.00 V. Electrochemical impedance spectroscopy (EIS) were measured in the frequency range from 100 kHz to 0.01 Hz on a electrochemical workstation (Im6e, Zahner). ASSOCIATED CONTENT Supporting Information XPS analyses of Coral-MCG, TGA of Coral-MCG electrode, SEM image of random Fe3O4-CoO powder, EIS of Coral-MCG and Random-MCC, SEM image of Coral-MCG after cycling, SEM images of Co3O4 NFs@GF, SEM image of Fe3O4 NPs@GF, cycling performance of Co3O4 NFs@GF at 0.5 A g–1 rate, cycling performance of Fe3O4 NPs@GF at 0.5 A g–1 rate, cycling performance of pure GF at 0.5 A g–1 rate, cycling performance of Coral-MCG at flat states and bent states, photograph of random Fe3O4-CoO powder dispersed in ethanol and it was attracted by a magnet, XRD pattern of Co(CO3)0.5(OH)0.11H2O precursor, Co3O4 powder and random Fe3O4-CoO powder, raman and XRD analyses of Co3O4 NFs@GF, XPS analyses of Co3O4 NFs@GF, the status of LED when external forces were applied to Coral-MCG anode. This information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions

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L.F. developed the concept and conceived the experiments. Y.Y.S. and C.W. carried out the experiments. L.F., Y.Y.S. and C.W. wrote the manuscript. L.F., Y.Y.S., C.W., Y.H.X.; Q.Z.; R.G.M.; L.F.C.; T.Z.; T.G.; M.H.R. and X.P.A. and contributed to data analysis and scientific discussion. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The research was supported by the Natural Science Foundation of China (Grants 51322209, 21473124) and the Sino-German Center for Research Promotion (Grants GZ 871).


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Scheme 1. Morphological changes of structures in the electrode causing by strain associated with Li+ intercalation/deintercalation during electrochemical reactions. (a) Nanoparticles on graphene. (b) Nanoneedles on rigid substrates. (c) Coral-like structures on graphene foam.



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Figure 1. Scheme illustrating Coral-MCG. (a) Schematic illustration of the 3D structure of Coral-MCG. (b) Photograph of coral. Scheme illustration of (c) the typical CoO NFs and Fe3O4 NPs assembled on the surface of GF, (d) during electrochemical reaction, and (e) after discharge. The arrows in (d) indicate that forces can be dispersed in all directions while the arrows in (c) indicate the transfer of electrons.


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Figure 2. (a) Raman spectra of Coral-MCG and GF, (b) XRD of Coral-MCG and GF, the intensity of (002) reflections of GF in Coral-MCG and GF are divided by 2 and 3, respectively. (c and d) XPS of Coral-MCG, in which (c) corresponds to the Fe 2p states and (d) corresponds to Co 2p states.



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Figure 3. Low-magnification SEM images of (a) Coral-MCG. (b) higher magnification images of Coral-MCG. The inset picture of single Coral-MCG with further increased magnification. (c) High-resolution TEM image of Coral-MCG. (I and II) are the enlarged areas of regions I and II in Figure 3c. (e, f and g) EDX analysis of (e) Co, (f) Fe, (g) O in (d) Coral-MCG. (h) EDX pattern of (d).


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Figure 4. (a) CV curves form a Coral-MCG electrode at a scanning rate of 0.2 mV s–1 ranging from 0.01 to 3.0 V. (b) The 1st, 2nd, 100th, 200th and 400th discharge-charge voltage-capacity curves of Coral-MCG electrode. (c) Rate capabilities of Coral-MCG and Random-MCC at rate of 0.5 A g–1, 1 A g–1, 2 A g–1, 5 A g–1 and 0.5 A g–1.



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Figure 5. (a) Photograph of flexible Coral-MCG being bent (R = 0.6 cm). (b, c and d) Lighting a green LED device before (b), after (c) and under extremely bending (d). (e) SEM image of Coral-MCG. (f) Schematic illustration of the 3D structure of Coral-MCG. The arrows in (f) indicate that forces can be dispersed in all directions. (g) Cycling performance of Coral-MCG and Random-MCC at 0.5 A g–1 rate.


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TOC A Coral-inspired Nanoengineering Design for Long-cycle and Flexible Lithium-ion Battery Anode


Coral-Inspired Nanoengineering Design for Long ... - ACS Publications

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