Conductive Carbon Filament Network for High

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Neuron-Inspired Fe3O4/Conductive Carbon Filament Network for Highly Fast and Stable Lithium Storage Shu-Meng Hao, Qian-Jie Li, Jin Qu, Fei An, Yu-Jiao Zhang, and Zhong-Zhen Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03174 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 9, 2018

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ACS Applied Materials & Interfaces

Neuron-Inspired Fe3O4/Conductive Carbon Filament Network for Highly Fast and Stable Lithium Storage Shu-Meng Haoa,b, Qian-Jie Lia, Jin Qua*, Fei Anb, Yu-Jiao Zhanga, and Zhong-Zhen Yua,b* a

State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and

Engineering, Beijing University of Chemical Technology, Beijing 100029, China. b

Beijing Key Laboratory of Advanced Functional Polymer Composites, Beijing University of

Chemical Technology, Beijing 100029, China. E-mail: [email protected] (J. Qu), [email protected] (Z.-Z. Yu) KEYWORDS: carbon filaments; bio-inspired anodes; Fe3O4; conductive network; lithium-ion batteries ABSTRACT: Construction of a continuous conductance network with fast electron transfer rate is extremely important for high performance energy storage. Owing to the highly efficient mass transport and information transmission, neurons are exactly a perfect model for electron transport, inspiring us to design a neuron-like reaction network for high-performance lithium-ion batteries (LIBs) with Fe3O4 as an example. The reactive cores (Fe3O4) are protected by carbon shells and linked by carbon filaments, constituting an integrated conductance network. Thus, once the reaction starts, the electrons released from every Fe3O4 cores are capable of being transferred rapidly through the whole network directly to the external circuit, endowing the nanocomposite with tremendous rate performance and ultra-long cycle life. After 1000 cycles at current densities as high as 1 and 2 A g-1, the charge capacity of the as-synthesized nanocomposite maintains 971 and 715 mA h g-1, respectively, much higher than those of reported Fe3O4-based anode materials. The 1

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Fe3O4-based

conductive

network

provides

a

new

idea

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for

future

developments

of

high-rate-performance LIBs. 1. INTRODUCTION Large-scale manufacturing of electric vehicles and other portable electronic devices requires electrical energy storage (EES) devices to achieve both high energy density and high power density.1-4 To realize this goal, a prominently electrical conductivity, especially a continuous conductance, is extremely important because only fast electron transport could bring in fast and complete electrochemical reactions.5-8 Ideally, during the electrochemical reaction, electrons generated by active materials could be transferred through covalent bonds to the framework and uninterruptedly transported to external circuit.9 However, most electrode materials that possess remarkable theoretical specific capacities share low electrical conductivities, let alone the conductive networks. Till now, the incorporation of high theoretical-capacity materials with conductive substrates or additives is still the most effective way. Nevertheless, for conventional compounding methods, whether it is coating, supporting or mechanical mixing type, the interfacial contact resistance is usually a problem hindering electron transfer. Even the conductive network is constructed by physical interconnection rather than chemical bonding, thus the electrons still need to conquer a large resistance to hop from one particle or matrix to another. Three-dimensional (3D) material is an alternative offer ascribed to the continuously conductive framework normally built by carbonaceous materials.10,11 However, their unsatisfactory conductivity caused by the pores and defects introduced during the fabrication processes and their inferior volumetric energy density due

2


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to the low tap density are remaining challenges. Therefore, how to construct a highly conductive network to support the continuous electron transport is still on the way. Neurons are very special cells in human body, normally consisting of soma, dendrite and axon (Figure S1a). Every neuron resembles a biochemical reactor. They connect to each other constituting a large information processing network. Once a neuron gets stimulated, the signal would be collected by dendrites and transferred through axon in electrical impulse at an amazing speed up to 120 m s-1 to the following neurons and finally reaching the effector through the whole neuron network, which is exactly a perfect model for efficient mass transport and information transmission. Therefore, we expect to construct a neuron-like electrode material by using high theoretical-capacity material and highly conductive material, which could exhibit terrific cycling and rate performances for energy storage. The active materials are coated by conductive shells to constitute tons of electrochemical reactors, and then linked by conductive filaments into a reaction network. For every neuron, the nucleus and cytoplasm of a soma have a synergic relationship that the nucleus carries genes as the basis of the whole cell while cytoplasm protects nucleus and simultaneously supplies nutrition for nucleus. Similarly, the active material in the core, as the electrochemical reaction center, could be protected by the conductive shell to suspend the volume expansion during charge/discharge processes. In addition, the electrons released from the active material could be collected and transferred immediately by the conductive shells, and more importantly, the function of conductive filaments looks like dendrites and axons. Thus, once the electrochemical reaction starts, the filaments could receive electrons from the abovementioned

3



reactors and transport them immediately to following ones, and then directly to external circuit by the conductive network. Based on above assumption, we propose a neuron-like continuously conductive network assembled by amorphous carbon and Fe3O4 by a facile way with a potentially high tap density for high-performance lithium ion batteries (LIBs). As shown in Figure S1b, Fe3O4 nanodisks are coated by amorphous carbon to inhibit the volume expansion and agglomeration of the Fe3O4 nanodisks during cycling and facilitate the electron transfer around Fe3O4, constituting numerous electrochemical nanoreactors. More importantly, different from conventional coating materials, at an optimal carbon content, those nanoreactors are able to be linked together by carbon filaments, forming a continuous electrochemical reaction network. Therefore, once the redox reaction starts, every electron released from the nanoreactor core (Fe3O4) could be transferred through oxygen bridges to the carbon shell and then rapidly transported through carbon filaments to other nanoreactors until reaching the external circuit, and so does the reverse process. Besides, reduced graphene oxide (RGO) is also added into the network to further enhance the conductivity. The specially continuous reaction network endows the Fe3O4@C/RGO nanocomposite with extraordinary rate and long cycle performances. Its charge capacities remain 971 and 715 mA h g-1 after 1000 cycles at 1 and 2 A g-1, respectively, to the best of our knowledge, which are much higher than those of previously reported Fe3O4-based electrode materials. 2. EXPERIMENTAL SECTION Materials. Analytical grade iron chloride (FeCl3·6H2O), sodium silicate (Na2SiO3·9H2O), glucose (C6H12O6) were supplied by Sinopharm Chemical Reagent Co., Ltd. (China). Graphite 4


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flakes were supplied by Huadong Graphite Factory (China) with an average diameter of 13 µm. KMnO4, H2O2 (30 %), HCl (36 %) and H2SO4 (98 %) were purchased from Beijing Chemical Factory (China). All of the chemicals were used as received. Synthesis of Fe3O4@C nanocomposites. α-Fe2O3 was produced as reported.12 For the nanocomposite, 300 mg of α-Fe2O3 powder was dispersed in 15 mL deionized water by ultrasonication as suspension A. Glucose (600, 1200 or 1800 mg) was dissolved in 15 mL deionized water and 10 mL ethanol as solution B. The suspension and the solution were then mixed and transferred into a 50 mL autoclave at 190 oC for 15 h. The resultant black α-Fe2O3@C product was collected by centrifugation, rinsed with deionized water and ethanol several times, and dried in an oven at 80 oC for 12 h. By calcinating the α-Fe2O3@C under Ar atmosphere at 600 oC for 12 h, Fe3O4@C nanocomposite was fabricated and donated as Fe3O4@C-x, where x is the dosage of glucose (mg). Neat Fe3O4 nanodisks were also synthesized by calcinating α-Fe2O3 under H2/Ar atmosphere at 600 oC for 12 h. Synthesis of Fe3O4@C/RGO nanocomposites. Graphite oxide was synthesized by oxidizing graphite with a modified Hummers method.13 RGO was prepared by thermally annealing the graphite oxide at 600 oC for 12 h. After 10 mg of RGO was dispersed in 10 mL of ethanol, 70 mg of as-prepared Fe3O4@C-1200 powder was added and stirred for 1 h, and the resultant was slowly dried at 50 °C in an oven. Synthesis of α-Fe2O3@C-1200 cubes, spindles and nanoplates. As control experiments, α-Fe2O3 cubes14, spindles15 and nanoplates16 were prepared according to the literature, and α-Fe2O3@C-1200 cubes, spindles and nanoplates were also prepared using the same procedures as 5



that of the α-Fe2O3@C-1200 nanocomposite by replacing α-Fe2O3 nanodisk to α-Fe2O3 cubes, spindles, and nanoplates. Characterization. Microstructures and morphologies were observed with a Hitachi S4700 field-emission scanning electron microscope (SEM), a JEOL JEM-1011 transmission electron microscope (TEM) and a JEOL JEM-3010 high resolution TEM (HRTEM). X-ray diffraction (XRD) were tested by a Rigaku D/Max 2500 diffractometer with Cu Kα radiation (λ = 0.154 nm) at a generator voltage of 40 kV and a generator current of 200 mA. α-Fe2O3, Fe3O4, Fe3O4@C nanocomposite, GO and RGO were characterized by a Thermo VG RSCAKAB 250X high resolution X-ray photoelectron spectroscopy (XPS) and a Renishaw Raman microscope (Britain). Thermal stability was characterized on a TA Instruments Q50 thermogravimetric analyzer (TGA) at a heating rate of 10 oC min-1 in air. Zeta potentials were measured with a Malvern Nano-ZS zetasizer. Electrochemical Measurements. Electrochemical performances were measured using coin-type cells assembled in an Argon-filled glove box. The working electrode was fabricated by casting the slurry of 70 wt.% active material, 20 wt.% super-P and 10 wt.% polyvinylidene fluoride (PVDF) onto a piece of nickel foam. Lithium foil and Whatman GF/D glass fabric were used as the counter electrode and the separator membrane, respectively. The electrolyte was purchased from Hefei Kejing Materials Technology Co., Ltd. (China), which was prepared by dissolving 1 M LiPF6 in a mixture of ethylene carbonate, dimethyl carbonate, and diethyl carbonate with a weight ratio of 1:1:1. Cycling performances and galvanostatic charge/discharge curves were obtained using a Land CT 2001A electrochemical workstation at different current densities and voltage range from 0.005 6


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to 3.0 V (vs. Li+/Li). Electrochemical impedance spectroscopy (EIS) and cyclic voltammograms (CV) were characterized with a CHI 660E electrochemical workstation. CV curves were recorded at a scanning rate of 0.1 mV s-1 within the voltage window of 0.005-3.00 V; while for EIS measurement, the AC modulation amplitude is 10 mV and the frequency range is 10 kHz - 0.1 Hz. 3. Results and discussion The neuron-like Fe3O4@C nanocomposite is prepared by a facile way with α-Fe2O3 as the precursor. α-Fe2O3 exhibits a uniform disk-like morphology with ~200 nm in diameter and ~50 nm in thickness (Figure 1a), which is constituted by face-to-face stacking of α-Fe2O3 platelets with clear edges (inset of Figure 1a). After calcinating under H2/Ar atmosphere, α-Fe2O3 becomes Fe3O4 nanodisks with smooth edges (Figure 1b). Fe3O4@C nanocomposite is fabricated by coating glucose on α-Fe2O3 nanodisks with a solvothermal process, followed by calcination under Ar atmosphere. During the calcination process, glucose is carbonized by removing its H and O atoms, and simultaneously the C atoms reduce α-Fe2O3 to Fe3O4 by a carbothermal reaction. Similar to that of Fe3O4, the edges of the nanodisks become smooth in the Fe3O4@C nanocomposite (inset of Figure 1c), implying the reduction of α-Fe2O3 to Fe3O4. Moreover, the dark Fe3O4 nanodisk is well encapsulated by low-contrast carbon with clear boundary for each nanodisk, confirming the successful fabrication of electrochemical nanoreactors. XRD patterns (Figure 1d) demonstrate that, despite the difference in carbon contents, all the peaks of the three nanocomposites could be determined to be Fe3O4 (JCPDS No. 19-0629) without visible α-Fe2O3 residues. The distance of lattice fringes in the HRTEM image (Figure 1e) is ~0.59 nm, corresponding to the (220) planes of Fe3O4. Additionally, the selected area electron diffraction (SAED) pattern (Figure 1f) clearly 7



presents six groups of ordered diffraction spots, which can be well indexed to (110), (211), (220), (422), (511) and (440) planes of Fe3O4.

Figure 1. SEM images and TEM images (insets) of (a) α-Fe2O3 nanodisks, (b) Fe3O4 nanodisks and (c) Fe3O4@C nanocomposite. (d) XRD patterns of α-Fe2O3, Fe3O4, and Fe3O4@C nanocomposite. (e) HRTEM image and (f) selected area electron diffraction pattern of Fe3O4@C nanocomposite. As mentioned above, constructing a continuously conductive network is the key for high-performance energy storage devices. Many achievements have been made to build the network by using graphene or carbon cloth to enhance lithium and sodium storage performances.17-20 The aim of this work is to find a facile way to fabricate a unique carbon-connected Fe3O4 network. As shown in the TEM images (Figure 2a), neat Fe3O4 nanodisks have a terrific monodispersity with distinct boundaries between two nanodisks without any gel-like structure. With the increase of carbon content, the carbon shells are gradually formed to encapsulate the Fe3O4 nanodisks. When 8


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the dosage of glucose reaches 600 mg (Figure 2b), the carbon coating on the Fe3O4 nanodisk is as thin as ~5 nm. However, it is noted that the carbon content is too low to link the nanodisks into a network, leading to isolated Fe3O4@C nanodisks. When the dosage of glucose increases to 1200 mg (Figure 2c), the carbon shell increases to ~10 nm. More importantly, all the Fe3O4@C nanodisks are linked by carbon filaments, constituting a neuron-like conductive network. Each Fe3O4@C nanodisk has multiple carbon filaments to connect surrounding Fe3O4@C nanodisks. Besides, most carbon filaments are generated at the edges of α-Fe2O3 nanodisks rather than at the surfaces and hence connect adjacent nanodisks by linking their edges, implying that the edge is beneficial for the formation of carbon filaments. Nevertheless, when the glucose content increases to 1800 mg (Figure 2d), the carbon filaments disappear. Instead, carbon layers are adhered to each other into large areas of carbon matrices, in which Fe3O4 nanodisks with carbon shells as thick as ~15 nm are distributed. These results demonstrate that an appropriate carbon content is critical to form the conductive network.

Figure 2. TEM images and schematics of (a, e) Fe3O4, (b, f) Fe3O4@C-600, (c, g) Fe3O4@C-1200 and (d, h) Fe3O4@C-1800. Note that the yellow core represents Fe3O4 and the blue shell stands for carbon. 9



To explain why the carbon coating is capable of constructing such a special conductive network at certain carbon contents, a series of contrast tests are carried out. Three types of α-Fe2O3 with different morphologies (cube, spindle, and nanoplate) are synthesized to compare with α-Fe2O3 nanodisks. As presented in Figure 3, both α-Fe2O3 cubes and spindles have excellent dispersities, while α-Fe2O3 nanoplates are easily aggregated, most likely due to their higher surface energy. After coating carbon with the same glucose dosage and procedure as those of Fe3O4@C-1200, α-Fe2O3 nanoplates are trapped and stuck together and the aggregation shares the same carbon shell, implying the importance of the dispersity of α-Fe2O3 precursor. Similarly, α-Fe2O3@C-1200 spindles also show an aggregated morphology although α-Fe2O3 spindles have an excellent dispersity. Only α-Fe2O3@C-1200 cubes maintain the same dispersity as their corresponding α-Fe2O3 cubes. The different dispersities of these α-Fe2O3 nanomaterials could be explained by comparing their zeta potentials (Table 1). Both α-Fe2O3 nanodisks and cubes have high absolute values of zeta potential, demonstrating their superior dispersity in water. However, the low zeta potential absolute values of α-Fe2O3 spindles and nanoplates imply that the repulsive forces are not enough to support their stable dispersion in water. Thus, during the solvothermal process for carbon coating, α-Fe2O3 spindles and nanoplates are aggregated first and then encapsulated by glucose. In addition, although α-Fe2O3@C-1200 cubes have well dispersities, the coated carbon could not form any filaments to connect them into a network, which may be attributed to the relatively less edges per unit mass than that of α-Fe2O3 nanodisks, because the edge is the key to the formation of carbon filaments as mentioned before. Apparently, the construction of the conductive network is mainly influenced by the factors below: (1) the zeta potential of the precursor should be far away from zero 10


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point in the coating solvent; (2) the size and morphology of the precursor should be appropriate so that enough edges per unit mass could be exposed to generate carbon filaments; and (3) optimized carbon content is significant.

Figure 3. TEM images of (a) α-Fe2O3 and (d) α-Fe2O3@C-1200 cubes, (b) α-Fe2O3 and (e) α-Fe2O3@C-1200 spindles, (c) α-Fe2O3 and (f) α-Fe2O3@C-1200 nanoplates.

Table 1. Zeta potentials of α-Fe2O3 nanomaterials with different morphologies Nanomaterials

α-Fe2O3 nanodisks

α-Fe2O3 cubes

α-Fe2O3 spindles

α-Fe2O3 nanoplates

Zeta potential (mV)

-15.1

-14.3

-9.8

8.9

To further investigate the influence of carbon contents on constructing a conductive network, Figure 4 shows TGA curves and XPS O 1s spectra of Fe3O4@C nanocomposites. After heated to 11



700 oC at 10 oC/min under air, Fe3O4 and C are oxidized to Fe2O3 and CO2, respectively (Figure 4a). Based on the final mass ratios of Fe3O4 and Fe3O4@C nanocomposites, the carbon contents in Fe3O4@C-600, Fe3O4@C-1200 and Fe3O4@C-1800 are calculated to be ~5, ~23 and ~70 wt%, respectively. By analyzing the O 1s states of the nanocomposites (Figure 4b), it is noticed that all the spectra could be deconvoluted into three peaks at 530.4 eV for Fe-O, 531.7 eV for Fe-O-C, and 533.2 eV for C-O with different fractions of Fe-O-C bond and Fe-O-C/C-O ratio (Table S1). The existence of Fe-O-C bond demonstrates the covalent bonding of the carbon shell with Fe3O4 cores, which is beneficial for the electron hopping from Fe3O4 nanodisk to the carbon shell and then being transferred through the carbon network. In other words, it is the basis of realizing a continuous conductive network. To some extent, the Fe-O-C/C-O ratio represents the ratio of the carbon connected with Fe3O4 to that unconnected with Fe3O4. With the increase of carbon contents, the Fe-O-C/C-O ratio decreases from 2.44 to 1.98 and 0.48. It means that, as the dosage of carbon rises, more and more carbon could not be bonded with Fe3O4. However, Fe3O4@C-1200 nanocomposite enjoys the highest fraction of Fe-O-C (29.8 %), almost the same as the Fe3O4@C-600 (29.0 %), higher than that of Fe3O4@C-1800 (24.8 %), revealing that, when the glucose content increases to 1200 mg, the fraction of Fe-O-C bond tends to reach a maximum. Based on the fact that the quantity of Fe-O bond in practice is constant with the increase of carbon because the dosages of α-Fe2O3 precursor remain the same for the three nanocomposites. Despite the limitation of the probing depth for XPS, it is still reasonable that the quantity of the Fe-O-C bond rises with a slower rate than that of C-O bond and then realizes its saturation when glucose content increases from 600 mg to 1200 mg. 12


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Figure 4. (a) TGA curves of Fe3O4 and Fe3O4@C nanocomposites. (b) XPS O 1s and (d) Fe 2p spectra of Fe3O4@C nanocomposites. (c) Raman spectra of α-Fe2O3, Fe3O4 and Fe3O4@C nanocomposites. If the glucose content continues growing to 1800 mg, the fraction of Fe-O-C bond declines due to the enlarged whole weight of the nanocomposite. It is now clear that, at the glucose dosage of 600 mg, most Fe3O4 nanodisks are encapsulated by carbon but the content is not sufficient to construct a conductive network. With 1200 mg of glucose, all the Fe3O4 nanodisks are well covered 13



with carbon contributing to a superior fraction of Fe-O-C bond, and the excessive carbon is able to build filaments to link Fe3O4@C nanodisks. Nevertheless, 1800 mg of glucose leads to the disappearance of the filaments due to the adhesion of carbon. Although more carbon is involved in constructing the reaction network, the carbon shells may be too thick to transport ions. Besides, the XPS C 1s results (Figure S2) present most of the oxygen groups of glucose are removed during the calcination process. Raman spectra (Figure 4c) show that there are three typical peaks for α-Fe2O3 at 219, 283 and 398 cm-1.21 After reduction by heating α-Fe2O3 under H2 atmosphere, those peaks vanish while a broad peak at ~680 cm-1 appears, which is ascribed to the A1g mode of Fe3O4, demonstrating the reduction of α-Fe2O3 to Fe3O4.22-26 The same phenomena are observed for all three Fe3O4@C nanocomposites, confirming the successful fabrication of Fe3O4 by carbothermal reduction. Besides, XPS Fe 2p results of Fe3O4@C nanocomposites (Figure 4d) also show typical characteristics of Fe3O4 with two peaks at 710.9 and 724.5 eV, corresponding to the Fe 2p3/2 and 2p1/2 states, respectively.23, 27-29 The absence of the satellite peaks indicates the fabrication of Fe3O4 rather than γ-Fe2O3.23, 28 The continuous conductive network, in which the reactive cores (Fe3O4 nanodisks) are protected by amorphous carbon shells and connected with carbon filaments into a highway of electrons, is expected to suppress volume expansion, transport electrons rapidly, and finally be a highly efficient anode material for energy storage with remarkable rate capability and long-cycle life. Thus, coin-type Li half-cell is fabricated to measure the electrochemical performances of the nanocomposites. As shown in Figure 5a, neat Fe3O4 experiences a severe decay in capacity from 14


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650 mA h g-1 of the initial charge capacity to 331 mA h g-1 after 100 cycles, ascribed to the pulverization of Fe3O4 and collapse of the structure. After coated and connected with optimized carbon, the cycling performance of the electrode material is dramatically improved. The Fe3O4@C-1200 nanocomposite has the best cycling performance with 900 mA h g-1 remained after 100 cycles, followed by 771 mA h g-1 of Fe3O4@C-1800 and 467 mA h g-1 of Fe3O4@C-600. The small amount of carbon in the Fe3O4@C-600 endows the nanocomposite with nearly complete redox reaction and thus highest charge capacity (860 mA h g-1) with 60 % of Coulombic efficiency in the initial cycle among three nanocomposites. However, the carbon is insufficient to preserve the whole cycling process, leading to a rapid drop in capacity because the carbon shell is too thin to prevent the fracture of Fe3O4 nanodisks caused by the volume expansion and the absence of a conductive network could not transport electrons efficiently. As the carbon content increases, the initial charge capacity decreases due to a lower theoretical capacity of carbon than Fe3O4, while the impacts of carbon on stabilizing structure and transferring electrons are gradually evident. For Fe3O4@C-1200 nanocomposite, although its initial charge capacity decreases to 728 mA h g-1 with 55 % of Coulombic efficiency, the carbon shell is finally strong enough to confront the volume expansion. More importantly, the conductive network is constructed to transport electrons, indicating that during the redox reaction, the electrons released by Fe3O4 cores could be transferred to their carbon shells through Fe-O-C bonds and then transported by the carbon filaments to current collector rapidly. Therefore, Fe3O4@C-1200 nanocomposite has a superior cycling performance and the highest specific capacity after 100 cycles. For Fe3O4@C-1800 nanocomposite, however, its excessive carbon content drastically reduces its theoretical capacity and then its initial charge 15



capacity (606 mA h g-1). Moreover, the thick carbon shell and the structure of burying of reactive cores in large areas of carbon matrices lengthens the diffusion pathway of lithium ions and decreases the electrode reaction rate. Thus, the charge capacity of Fe3O4@C-1800 is inferior to that of Fe3O4@C-1200 nanocomposite after 100 cycles. Additionally, the charge/discharge curves (Figure S3) also prove the satisfactory reversibility for the fabricated conductive network. Neat Fe3O4 experiences a severe drop of capacity that nearly 45 % of capacity is lost after 5 cycles. In contrast, Fe3O4@C-1200 nanocomposite still retains 85 % of its first charge capacity over 5 cycles and remains constant before the capacity gain.

Figure 5. (a) Cycling performances of Fe3O4 nanodisks and Fe3O4@C nanocomposites at the current density of 0.1 A g-1. (b) Cycling performances of Fe3O4@C/RGO nanocomposite at the 16


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current densities of 0.1 and 1 A g-1. (c) Electrochemical impedance plots of Fe3O4, Fe3O4@C-1200, and Fe3O4@C/RGO nanocomposites and (d) rate performances of Fe3O4, Fe3O4@C/RGO, and Fe3O4@C nanocomposites at varied current densities from 0.1 to 2 A g-1. To further enhance the conductivity of the reaction network, RGO is introduced into the Fe3O4@C-1200 nanocomposite. RGO is prepared by heating graphite oxide at 600 oC under Ar atmosphere for 12 h. The Raman spectrum and the XPS C 1s spectrum of RGO (Figure S4 and S5) indicate that most of its oxygen groups are removed while its defects still exist. Figure 5b shows that the initial charge capacity of Fe3O4@C/RGO nanocomposite significantly rises to 1003 mA h g-1 with 62.5% of Coulombic efficiency which reaches 93.3% in the second cycle and then maintains over 98 % during the following cycles. After 100 cycles, the charge capacity of the Fe3O4@C/RGO nanocomposite remains as high as 1278 mA h g-1 at 0.1 A g-1, and 789 mA h g-1 at 1 A g-1. Based on the electrochemical catalytic conversion mechanism, during the charging process, Fe metal reduced from Fe3O4 catalyzes the decomposition of SEI layer, contributing to the charge capacity as well.30-32 A complete redox reaction under low current density and high electronic conductivity accelerates that process. Therefore, the capacity of the Fe3O4@C/RGO nanocomposite rises upon cycling and higher than the theoretical capacity of Fe3O4 (924 mA h g-1). TEM image of the Fe3O4@C/RGO after 100 cycles at 1 A g-1 is provided in Figure S6. It is evident that, although the complete electrode reaction and large stress during lithium insertion/extraction reduce the crystallinity of Fe3O4 nanodisks, the disk-like morphology is reserved. More importantly, the carbon shells and filaments still glue Fe3O4 nanodisks and connect them into a network. The Nyquist plots also confirm the basically maintained conductivity by the similar semicircle of 17



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Fe3O4@C/RGO after 100 cycles to that of its fresh one (Figure S7). Furthermore, the Fe3O4@C/RGO nanocomposite has a much more prominent reversibility that the charge capacity achieves stability since the second cycle with as high as 96 % of the initial capacity retained (Figure S8). The cyclic voltammetry (CV) curves of neat Fe3O4, Fe3O4@C-1200 and Fe3O4@C/RGO nanocomposites (Figure S9) all show typical cathodic and anodic processes of Fe3O4.22,

28, 33

Electrochemical impedance plots (Figure 5c) verify the improvement in conductivity of the Fe3O4@C-1200 and Fe3O4@C/RGO nanocomposites. All the plots of the three materials are composed of a semicircle in the high-to-medium frequency region and a straight line at low frequencies, which represent the charge-transfer resistance and Warburg impedance, respectively. The diameter of the semicircle for Fe3O4@C-1200 electrode is remarkably smaller than that of neat Fe3O4, demonstrating the tremendous effect of the designed conductive network on promoting electron transport. In addition, Fe3O4@C/RGO exhibits a smaller diameter of the semicircle and a steeper slope of the line than those of Fe3O4@C-1200, confirming the improvement in both ion and electron transports by introducing RGO sheets. Fast transfer rates of ions and electrons always signify an outstanding kinetic property, finally resulting in terrific rate performances. At rates of 0.1, 0.2, 0.5, 1 and 2 A g-1, the charge capacities of Fe3O4@C-1200 nanocomposite reach 641, 646, 560, 463, and 342 mA h g-1, whereas those of neat Fe3O4 are only 257, 177, 112, 74, and 49 mA h g-1, respectively (Figure 5d). When the current density returns to 0.1 A g-1, a charge capacity of 849 mA h g-1 is still available for Fe3O4@C-1200 nanocomposite. Moreover, after introducing RGO, the reversible capacities of the nanocomposite 18


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rise to 972, 943, 831, 694 and 518 mA h g-1, and finally reverse to 1111 mA h g-1, proving the enhancement of its kinetic performances. Not only the rate performance, but also a prominent ultra-long cycle life is obtained for Fe3O4@C/RGO nanocomposite at high current densities (Figure 6). After charging/discharging as long as 1000 cycles, the reversible capacities of Fe3O4@C/RGO nanocomposite are still capable of achieving 971 and 715 at the current densities as high as 1 and 2 A g-1, respectively, much higher than other Fe3O4-based anode materials reported in the literature (Figure S10 and Table S2) to the best of our knowledge, including yolk-shell structure,34-35 core-shell structure,36-37 hollow structure,28,

38-40

carbon supporting structure,27,

33, 41-46

carbon coating structure,47-51 carbon

supporting and coating structure52-56 and 3D structure.9, 23, 57 Even after 1000 cycles at 8 A g-1, the charge capacity of the Fe3O4@C/RGO nanocomposite still reaches 206 mA h g-1. Clearly, the neuron-like conductive network is proved to be highly efficient for LIBs. The conductive material links reactive cores into a continuous reaction network and hence once the redox reaction starts, the electrons released from the reactive cores could be directly transferred by the conductive network to external circuit. At an optimal content, the amorphous carbon is not only capable of linking reactive material (Fe3O4) to transport electrons rapidly, but also controlling the volume expansion and protecting Fe3O4 from fracture. The carbon content exhibits an optimal value with the maximum Fe-O-C bonds in the structure and the introduction of RGO further enhances the conductive network. Therefore, the resultant Fe3O4@C/RGO nanocomposite exhibits superior kinetics performance, excellent rate performance and ultra-long cycle life.

19



Figure 6. Long-cycle performances of Fe3O4@C/RGO nanocomposite at high current densities of 1, 2 and 8 A g-1. 4. Conclusions Inspired by the highly efficient mass transport and information transmission of neurons, a neuron-like conductive network is designed and assembled by taking Fe3O4 as a model for high-performance LIBs. As the reactive cores, Fe3O4 nanodisks are encapsulated and protected by amorphous carbon to form tons of electrochemical nanoreactors, which are connected by carbon filaments for electron transport, constituting a conductive network at an optimized carbon content. Hence, once the electrochemical reaction starts, the electrons released from the Fe3O4 core could be transferred immediately through the oxygen bridges to the carbon shell, and be transported rapidly by the carbon filaments, directly reaching the current collector. Additionally, RGO is introduced into the network to further enhance the conductivity of the nanocomposite. High zeta potential absolute value, appropriate morphology and size of precursor, and optimized carbon content are all 20


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important for constructing the conductive network. The as-synthesized Fe3O4@C/RGO network presents tremendous rate performances and long-cycle life. At the current densities as high as 1 and 2 A g-1 after 1000 cycles, the charge capacities of the Fe3O4@C/RGO nanocomposite still reach 971 and 715 mA h g-1, respectively, much higher than reported Fe3O4-based anode materials, demonstrating the high efficiency of the fabricated conductive network for electron transport. This work provides a new idea for fabricating high-rate-performance LIBs and could be extended to a wide variety of functional nanomaterials. ASSOCIATED CONTENT Supporting Information Schematics of neurons and neuron-like conductive network; C 1s of XPS results of Fe3O4@C nanocomposites; Charge/discharge curves and cyclic voltammograms of Fe3O4, and Fe3O4@C, Fe3O4@C/RGO nanocomposites; Raman and XPS C 1s spectra; TEM image and electrochemical impedance plot of Fe3O4@C/RGO after 100 cycles; Comparison of cycling performance of Fe3O4@C/RGO with the literature; Fraction of Fe-O-C bond and Fe-O-C/C-O ratio of Fe3O4@C nanocomposites; Reported performance of Fe3O4-based anode materials. Supporting Information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding author: E-mail: [email protected] (J. Qu), [email protected] (Z.-Z. Yu) ACKNOWLEDGMENTS

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Financial support from the National Natural Science Foundation of China (51402012, 51533001, 51521062), and the Fundamental Research Funds for the Central Universities (YS201402, PT1613-03) is gratefully acknowledged. REFERENCES 1. Sun, H.; Mei, L.; Liang, J.; Zhao, Z.; Lee, C.; Fei, H.; Ding, M.; Lau, J.; Li, M.; Wang, C.; Xu, X.; Hao, G.; Papandrea, B.; Shakir, I.; Dunn, B.; Huang, Y.; Duan, X. Three-Dimensional Holey-Graphene/Niobia Composite Architectures for Ultrahigh-Rate Energy Storage. Science 2017, 356, 599-604. 2. Xu, Z. L.; Huang, J. Q.; Chong, W. G.; Qin, X.; Wang, X.; Zhou, L.; Kim, J. K. In Situ TEM Study of Volume Expansion in Porous Carbon Nanofiber/Sulfur Cathodes with Exceptional High-Rate Performance. Adv. Energy Mater. 2017, 7, 1602078. 3. Yan, J.; Fan, Z.; Sun, W.; Ning, G.; Wei, T.; Zhang, Q.; Zhang, R.; Zhi, L.; Wei, F. Advanced Asymmetric Supercapacitors Based on Ni(OH)2/Graphene and Porous Graphene Electrodes with High Energy Density. Adv. Funct. Mater. 2012, 22, 2632-2641. 4. Wang, J. G.; Jin, D.; Liu, H.; Zhang, C.; Zhou, R.; Shen, C.; Xie, K.; Wei, B. All-Manganese-Based Li-Ion Batteries with High Rate Capability and Ultralong Cycle Life. Nano Energy 2016, 22, 524-532. 5. Lukatskaya, M. R.; Dunn, B.; Gogotsi, Y. Multidimensional Materials and Device Architectures for Future Hybrid Energy Storage. Nat. Commun. 2016, 7, 12647. 6. Yan, Y.; Yin, Y. X.; Guo, Y. G.; Wan, L. J. A Sandwich-Like Hierarchically Porous Carbon/Graphene Composite as a High-Performance Anode Material for Sodium-Ion Batteries. Adv. 22


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Conductive Carbon Filament Network for High

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