Bacteria-inspired Fabrication of Fe3O4-Carbon

Electrochimica Acta 223 (2017) 39–46

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Bacteria-inspired Fabrication of Fe3O4-Carbon/Graphene Foam for Lithium-Ion Battery Anodes Ning Zhanga , Chong Chena , Xiaohui Yana , Yuan Huanga , Jia Lia , Jianmin Mab,* , Dickon H.L. Nga,* a b

Department of Physics, The Chinese University of Hong Kong, Hong Kong, China Key Laboratory for Micro-/Nano-Optoelectronic Devices, School of Physics and Electronics, Hunan University, Changsha 410082, China

A R T I C L E I N F O

Article history: Received 1 September 2016 Received in revised form 30 November 2016 Accepted 2 December 2016 Available online 3 December 2016 Keywords: Bacteria Fe3O4 Graphene foam Lithium-ion battery Electrochemical properties

A B S T R A C T

Although lithium-ion batteries are commonly used to our daily life, achieving superior properties in lowcost is still our current challenge. Here we report the fabrication of a bacteria-inspired, micro-/ nanostructured Fe3O4-carbon/graphene foam hybrid material for lithium-ion battery anodes. The process employing biological adsorption is featured with low-cost and can have mass-production. Attributed to the graphene foam substrate, the fabricated micro-/nanostructure can be directly employed as a binder-free LIB anode without the need of complex treatments. The product used as an anode delivers a high reversible capacity of 1112 mAh g1 at the current density of 100 mA g1 even after 200 cycles, and exhibits good rate performance. These results demonstrate fabrication and electrochemical properties of a bacteria-inspired Fe3O4-carbon/graphene foam, suggesting a facile method for making anodes to be used in high-performance lithium-ion batteries. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Rechargeable lithium-ion batteries (LIBs) are considered as a promising power source for the portable electronic devices in our daily life [1–5]. Energy stored in LIBs is basically by the shuttle of lithium ions (Li+) between cathode and anode. Some practical applications have been achieved, but critical issues such as cost, stability, energy capacity and rate performances for electrode materials are still needed to be addressed [6–8]. Many interests are drawn to the development of anode materials based on transition metal oxides: MOx, where M can be Fe, Co, Ni, etc [5–10]. Their discharge/charge in LIBs is actually a reversible conversion reaction with Li+, including formation and decomposition of lithium oxide, accompany with the reduction and oxidation of MOx [6–8]. This mechanism endows them with high theoretical capacity reaching to 500–1000 mAh g1, much higher than 372 mAh g1 of conventional graphite anode material [9–12]. Magnetite (Fe3O4), an MOx, possessing relatively high theoretical capacity (926 mAh g1), low-cytotoxicity nature and low-cost for large-scale production, is extensively studied as an anode for

* Corresponding authors. E-mail addresses: [email protected] (J. Ma), [email protected] (D.H.L. Ng). http://dx.doi.org/10.1016/j.electacta.2016.12.006 0013-4686/© 2016 Elsevier Ltd. All rights reserved.

LIBs [12–15]. However, the large volume variation, particle agglomeration, and intrinsic kinetic limitations exist of Fe3O4 during discharge/charge processes, have been reported resulting in its irreversible capacity loss and poor stability [17–22]. Hybridization with carbon-based materials e.g., activated carbon, carbon nanotube, and graphene at nanoscale offers one promising strategy to overcome these challenges of Fe3O4 [22–26]. Due to small dimensions and relatively high specific surface area, nanoscale Fe3O4 can shorten effective diffusion length of ions and electrons, thus provides abundant active sites for Li+ storage, and accommodates volume variation caused by Li+ insertion/extraction [23– 30]. Meanwhile, the carbon-based component can act as a buffer to reduce or even remove the aggregation and volume effects, as well as a conductive media to improve electron transport of Fe3O4 [31– 35]. Of various carbon-based materials, three-dimensional (3-D) graphene foam (GF) constituting sheets of graphene is of excellent electron transport and many other superior properties e.g., light weight and chemical resistance, representing one of powerful substrate candidates for LIBs [36–40]. So far, several works have reported hybrid Fe3O4/carbon-based LIB anodes, Li et al., had successfully fabricated a bio-inspired hierarchical nanofibrous Fe3O4-TiO2-carbon composite by employing the natural cellulose. Such composite showed a significant improvement in stability and rate capability while using as the anode for LIBs [33]; Luo et al., used atomic layer deposition (ALD) to synthesize hierarchical

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N. Zhang et al. / Electrochimica Acta 223 (2017) 39–46

porous Fe3O4/VOx/graphene nanowires. The product exhibited high Coulombic efficiency and outstanding reversible specific capacity [34]; Hu et al., had developed a supercritical carbon dioxide (scCO2) method to anchor Fe3O4 nanoparticles onto the graphene foam, and the composite could be able to deliver excellent capacity [35]. Bacteria, micro-organism widely distributed in nature, exhibit unique structures and functionalities [41–49]. To our interest, they provide abundant biomass for the batch fabrication of micro-/ nanostructures with controlled size, structure, and functionality in a low-cost manner. In this study, we developed a route utilizing biological adsorption to synthesize bacteria-inspired Fe3O4-carbon/GF for LIB anodes. The Escherichia coli (E. coli)-based fabrication is demonstrated as an example. The fabricated Fe3O4-carbon/GF being of hierarchical structures can be directly employed as a binder-free LIB anode without any complex treatments. Results from electrochemical measurements reveal that this kind of anode with a high reversible capacity, long cycle stability and good rate performance. The fabrication of this bacteria-inspired LIB anode takes advantage of biological adsorption, suggesting a versatile and facile method for the low-cost production of high-performance LIBs. 2. Experimental methods 2.1. Pure GF preparation. The pure GF was prepared via chemical etching process of nickel foam supported graphene sheets, which was purchased from Shenzhen 6 carbon technology Co., Ltd (China). We utilized the FeCl3 purchased from Sigma-Aldrich (USA) to remove the nickel backbone and get the pure GF as follows. The nickel foam supported graphene sheets was firstly cut into small pieces of 80 80 mm2, and then immersed into the 80 C 2 M FeCl3 solution for 1 h. The nickel was etched away while the foam-like graphene sheets remained. After several times of washing with deionized (DI) water and ethanol, the as-obtained GF was stored in the DI water for further using. 2.2. Cultivation of E. coli onto the GF. We selected E. coli (strain HCB1737-a derivative of wild-type E. coli AW405 from Howard C. Berg Lab, Harvard University, USA) as our biomass. The E. coli was cultivated overnight in a 3 ml LB (LuriaBertani) medium containing 1% Bactotryptone, 0.5% yeast extract, and 0.5% NaCl at 30 C. During cultivation, they were reproduced from single-colony isolates under soft shaking reached to a stationary phase. 0.5 ml of the E. coli solution was injected into a vented tube containing 3 ml of LB solution and the as-obtained pure GF. After incubation at 30 C over 16 h, the E. coli cells were attached to the surface of the GF.

2.3. Fabrication of the Fe3O4-carbon/GF. The E. coli attached GF (i.e., E. coli/GF) was firstly washed for several times with DI water to exclude the residual cultivating solution. Then, the as-obtained E. coli/GF was treated with ethanol for improving cells' permeability, retaining cell morphology, and dissolving lipid layer on E. coli membrane [46–49]. The E. coli/GF was then placed into a 0.1 M FeCl3 solution for 3 h to allow E. coli adsorb Fe3+ ions (i.e., Fe3+-E. coli/GF) at room temperature. Followed by vacuum-drying at 60 C, the Fe3+-E. coli/GF was subject to annealing at argon for 1 h at 550 C, generating Fe3O4carbon/GF hybrid sample, and its thickness was measured to be 1.25 mm. We have also prepared some control samples: (1) pure GF; (2) carbon/GF: using E. coli/GF without Fe3+ ion adsorption but subject to the same annealing condition (550 C for 1 h in argon) to obtain carbon/GF; (3) Fe3O4/GF: pure GF subject to the 0.1 M FeCl3 solution adsorbing for 3 h and post annealing at same condition; (4) Fe3O4 NPs: commercial Fe3O4 nanoparticles purchased from Sigma-Aldrich (USA). 2.4. Structural characterizations. The structural and composition of the Fe3O4-carbon/GF hybrid sample were carried out by various techniques. The X-ray diffraction (XRD) was performed with RU300 (SmartLab, Rigaku) using Cu Ka radiation (l = 0.1540598 nm) to identify crystalline phases in the samples. Raman scattering spectrum was performed using a Micro-Raman spectrometer (RM- 1000, Renishaw Co., Ltd), equipped with a 10 mW, 514 nm helium-neon laser to identify radical groups in the samples. X-ray photoelectron spectroscopy (XPS) was collected with Al Ka radiation on a PHI Model 5802 (calibrated with C1s at 284.8 eV) to perform an elemental analysis. The Brunauer–Emmmett–Teller (BET) test and Barrett-JoynerHalenda (BJH) analyses were conducted by the ASAP 2000 to obtain the surface area of the samples. Thermogravimetric analysis (TGA) was performed from ambient to 930 C with a TGA Q600 at a heating rate of 10 C min1 to study the weight ratio of the components in the sample. Field-emission scanning electron microscope (FE-SEM) imaging and element analysis were carried out using FEI Quanta 200 equipped with an energy-dispersive Xray spectrometer (EDX). Transmission electron microscopy (TEM) images were captured with a Tecnai F20 microscopy operating at 200 kV. 2.5. Electrochemical characterizations. The working electrode of the LIBs in this work used as-prepared Fe3O4-carbon/GF without any binder or conductive agents. Same procedures were also applied to the control samples (1) pure GF, (2) carbon/GF, and (3) Fe3O4/GF mentioned in Section 2.3. In preparing working electrode of control sample (4) Fe3O4 NPs, its needs a combination of Fe3O4 NPs, conductive carbon black, and the PVDF (polyvinylidene fluoride) in the weight ratio of 80:10:10. The as-prepared mixture was placed onto a copper foil and went

Fig. 1. Schematic illustration of the fabracation approach of Fe3O4-carbon/GF sample.


through 90 C vacuum-drying for 12 h. The standard CR2032 cointype cells were fabricated in an Ar-filled glove box. For the counter electrode, a piece of 1.5 mm thick lithium foil was used. The electrolyte consisted of 1.0 M LiPF6 dissolved in 1: 1 v/v ethylene carbonate (EC) and diethyl carbonate (DMC) (Novolyte Co.). The cyclic voltammetry (CV) test was performed between voltage range of 0.1 and 3.0 V at 0.1 mV s1 at the CHI760 electrochemical working station (Shanghai CH Instrument Co., Ltd.). The galvanostatic charging/discharging cycles (GCD) and cycling stability measurements were conducted by the CT2001A multichannel battery test system (Wuhan Kingnuo Electronic Co., Ltd.). All of the electrochemical characterizations were carried out at room temperature. 3. Results and discussion 3.1. Fe3O4-carbon/GF Fig. 1 schematically illustrates the fabrication process of Fe3O4carbon/GF in three stages i.e., cultivating, dipping and annealing. Firstly, the rod-shaped E. coli bacteria cells are directly anchored on the surface of pure GF networks. generating the E. coli/GF. Secondly, the E. coli/GF is dipped into a FeCl3 solution. In this stage, the E. coli attributed to its negatively-charged surfaces has the ability to spontaneously attract Fe3+ ions and together forming the Fe3+-E. coli/GF precursor [46–49]. Thirdly, the precursor is annealed in an Ar-filled tube furnace, resulting in the formation of micro-/ nanostructured Fe3O4-carbon/GF. Collectively, this fabrication process employs biological adsorption and is thus of low-cost and mass-production, and the as-synthesized Fe3O4-carbon/GF can be directly utilized as a binder-free electrode in the LIBs. 3.2. Structural characterizations The crystalline structure and chemical composition in the Fe3O4-carbon/GF sample were analyzed. Fig. 2a shows the XRD patterns of the Fe3O4-carbon/GF after annealing. It is evident that the sample is a mixture of magnetite (Fe3O4) and GF. The diffraction peaks centered at 30.1, 35.5 , 43.1, 57.0 and 62.6 attribute, respectively, to the (220), (311), (400), (511), and (440) planes of spinal magnetite (Fe3O4: JCPDS 19-0629) [15–20]. Two typical peaks located at 26.5 and 54.6 correspond to (002) and (004) planes of the graphitic phase, respectively (JCPDS 75-1621). No other obvious peaks are observed indicating the purity of Fe3O4 and the well carbonaceous after annealing. Fig. 2b shows the Raman scattering spectra of the Fe3O4-carbon/GF sample and pure GF sample, in red and black, respectively. Two typical peaks of graphene at 1581 cm1 and 2720 cm1 attributed to the G band and 2D band in both two curves. This also indicates the single-layer to multi-layer graphene sheets structure of the as-obtained GF [35– 40]. Moreover, the Raman spectra of pure GF exhibits a suppressed

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related D band, confirming the high quality with well-ordered graphitic structure. Consequently, such graphene layered GF would ideally to be anchored by E. coli and become excellent substrate to support electroactive materials. After Fe3+ dipping and annealing processes, the micro-/nanostructured Fe3O4-carbon/GF is obtained, and two additional typical peaks centered at 667 cm1 and 1365 cm1 appear in the Raman spectra, which reveals the A1g mode of Fe3O4 and the incorporated of the Fe3O4-carbon units onto the GF structure [30–35]. Fig. 3 shows the X-ray photoelectron spectroscopy (XPS) results of the Fe3O4-carbon/GF sample. In the survey spectra of Fig. 3a, three main signals present Fe-2p, O-1s, and C-1s states, suggesting the presence of Fe, O, C in the sample composite [22–25]. In the enlarged range of the Fe-2p spectrum (Fig. 3b), the presence of Fe3+ and Fe2+ in the composite can be assigned in both of the Fe 2p3/2 and 2p1/2 states. For the Fe 2p3/2 component, a strong peak located at 711.0 eV corresponds to the FeII 3/2 state, and the peak located at 714.3 eV is attributing to FeIII 3/2 configuration [24–28]. For the other component of Fe 2p1/2, two peaks appear at the binding energies of 724.5 and 725.2 eV, illustrating the present of Fe2+ and Fe3+. Besides, the additional two peaks at 719.1 and 731.9 eV attribute to the Fe shakeup satellite peaks. For the O-1s spectrum shown in Fig. 3c, the peak located at 530.1 eV corresponds to the oxygen bonded with iron in the Fe3O4 crystal lattice [34,35]. The two high binding energy peaks at 530.9 and 532.4 eV are owing to the chemisorbed oxygen from the surface OH groups [35]. The C1s spectrum of the as-prepared sample is shown in Fig. 3d, the C¼C bond of sp2-hybridized carbon can be represented by the peak at the binding energy of 284.8 eV; and the peak at 286.4 and 288.5 eV attribute to the CO bond and C¼O band, respectively. The nitrogen adsorption/desorption isotherms of the assynthesized Fe3O4-carbon/GF is shown in Fig. 4a. The isotherms, without a distinct hysteresis loop, belong to the IUPAC type III isotherms. The Brunauer-Emmet-Teller (BET) specific surface area of the sample is calculated to be 25.3 m2 g1. In order to evaluate the content of Fe3O4 in the Fe3O4-carbon/GF, thermogravimetric analysis (TGA) is conducted, and the result is shown in Fig. 4b. When the sample is annealed in air, the carbon component is oxidized to CO2 (C + O2 ! CO2), while the Fe3O4 is converted to Fe2O3 (4Fe3O4 + O2 ! 6Fe2O3). Based on the remaining weight ratio of Fe2O3 (77.7%), the mass of Fe3O4 is calculated to be around 75.1% of the total mass of the composite, and the carbon content in the Fe3O4-carbon/GF sample is 25%. The morphologies of as-synthesized hybrid material are characterized by scanning electron microscopy (SEM). In general, the pure GF shows layered graphene sheets comprising a continuously interconnected 3-D network, which inherit the well-organized nickel foam porous microstructures (Fig. S1a, Supporting Information). More importantly, Fig. S1b illustrates the image of carbon/GF, which is obtained after annealing of E. coli/GF. It is evident that foam-like layered-graphene GF can provide

Fig. 2. (a) XRD pattern, and (b) Raman spectra of Fe3O4-carbon/GF sample.

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Fig. 3. (a) XPS survey spectra of the Fe3O4-carbon/GF sample. High-resolution XPS spectra of (b) Fe-2p state, (c) O-1s state, and (d) C-1s state.

sufficient space and active sites for E. coli bacteria cells to anchor on. Fig. 5a–d shows the SEM images of Fe3O4-carbon/GF at different magnifications. It is seen in Fig. 5a and b, the Fe3O4carbon units are smoothly anchored on the surface of GF after the cultivation of E. coli, Fe3+ adsorption and post-annealing processes. The rod-like interconnected channels can be also identified in the low-magnification image Fig. 5c. Fig. 5d illustrates the detailed SEM image of the sample showing the originally smooth surface of bacteria cell that has been transformed to rough surface and becoming hierarchically. Such feature evolution corresponds to the bacteria-inspired adsoption of the Fe3+ reactant on the surface of E. coli [45–49]. Hence, the unique micro-/nanostructure of the Fe3O4carbon/GF can facilitate the shortening of diffusion pathway of Li+ and electrons, and enhance the area of electrode-electrolyte contact interface. Moreover, the carbon and GF substrate can accommodate the volume variation during the Li+ ion insertion (lithiation) and extraction (delithiation) processes. The energydispersive X-ray (EDX) analysis identify that the chemical composition of the as-synthesized material having Fe 75 wt%

and C 14 wt% with a ratio of 5:1 (Fig. S3a, Supporting Information). This result is in good agreement with the TGA result shown Fig. 4b. Fig. 6a shows the TEM image of the carbon/GF (i.e. the annealed of E. coli/GF). We can observe the smooth surface of carbon cell derived from bacteria. In the contrary, the Fe3O4-carbon/GF shown in Fig. 6b exhibits a micro-/nanostructure consisting of the nanorods distributed on the surface. This illustrates the surface evolution from E. coli/GF to Fe3O4-carbon/GF. It can be seen in Fig. 6c, the diameters of nanorod antennas are approximate between 20 to 50 nm. The HR-TEM image of the Fe3O4-carbon/GF is presented in the insert of Fig. 6c. The crystallized nanorods are found to be attached onto the carbon surface. The lattice fringe measured in the HR-TEM image illustrates the lattice d-spacing lattice of 0.25 nm, corresponding to the (311) plane of Fe3O4. The elemental mapping of the Fe3O4-carbon/GF is shown in Fig. 6d and e. From the surface distribution of the Fe and C, we can deduce that the nanorods are Fe-rich (Fig. 6d) and the dead cells are C-rich (Fig. 6e). The HR-TEM and the element mapping analysis further confirms that the Fe3O4 nanorods are grown on the graphitized-

Fig. 4. (a) Nitrogen adsorption/desorption isotherm, and (b) TGA curve of Fe3O4-carbon/GF sample.


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Fig. 5. (a)-(d) SEM images of Fe3O4-carbon/GF sample at different magnifications.

like carbon cells and such unit are well-attached on the surface of GF, which is in agreement with the XRD result. 3.3. Electrochemical measurements The Fe3O4-carbon/GF was assembled to the coin-type cells as anode, and its electrochemical performance is evaluated. Fig. 7a shows the initial four cycles of cyclic voltammetry (CV) curves in the voltage range of 0.01 to 3.00 V (vs Li/Li+) at the scan rate of 0.1 mV s1. A strong reduction peak centered at 0.46 V is found in the first cathodic scan, which corresponds to the reduction of Fe3O4 (Fe2+/Fe3+) to Fe0 and the formation of Li2O via a conversion reaction (Fe3O4 + 8Li+ + 8e ! 3Fe + 4Li2O). In the following anodic scan, there is a relative weak anodic peak centered at 1.63 V. It attributes to the reversible oxidation of Fe0 to Fe2+/Fe3. The occurrence of these peaks was also reported in the previous studies for the Fe3O4-based anodes [25–35]. In the subsequent cycles shown in Fig. 7a, the distinct similar-shape pair of reduction/ oxidation peaks appear at 0.95 V and 1.75 V, corresponding to the reversible reaction (Fe3O4 $ Fe0), accompanying with lithiation and delithiation processes. Apparently, the CV curves for subsequent cycles are different from the first cycle, neither in position nor in shape, which indicates the irreversible reaction and capacity loss caused by the formation of the solid electrolyte interphase (SEI) layer in the first cycle [34,35]. Interestingly, the CV curves

exhibit two additional peaks centered at 0.21 and 0.24 V, which represent the lithiation/delithiation of GF [35–39]. Moreover, the low intensity of the GF peaks compared with Fe3O4 peaks reveals the low weight ratio of graphene in the Fe3O4-carbon/GF sample [34]. The galvanostatic charge/discharge (GCD) analysis of the Fe3O4carbon/GF anode was performed in the voltage range of 0.01 to 3.00 V at the current density of 100 mA g1. The result is shown in Fig. 7b. It can be seen that, in the first cycle, the anode delivers a discharge specific capacity of 1330 mAh g1, while the following charge capacity goes down to 891 mAh g1. The irreversible capacity loss and low Coulombic efficiency of 66.9% for the first discharge/charge cycle are owing to the formation of SEI layer [20– 25]. After the first cycle, the Coulombic efficiency rapidly increases to 99%, indicating a highly reversible of the as-synthesized anode. A discharge voltage plateaus at 0.75 V in the first cycle, which is different from other cycles. It further confirms the irreversible reaction only occurs in the initial cycle [20–25]. It should be noticed that the Fe3O4-carbon/GF anode delivers a high discharge specific capacity of 1112 mAh g1 after 200 cycles, which is higher than the theoretical capacity of bulk Fe3O4 of 926 mAh g1. Such superior phenomenon mainly due to (1) the additional Li+ ion storage in both of the interfacial of micro-/nanostructure and the active sites, that have been released with the increasing number of cycles. (2) the reaction between the oxygen functional groups with

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Fig. 6. (a) TEM image of carbon/GF. (b) TEM images of the Fe3O4-carbon/GF. (c) TEM image of the Fe3O4 nanorods on the surface of carbon cell, insert shows the HRTEM image of an individual nanorods. Elemental mapping of (d) Fe and (e) C of the Fe3O4-carbon/GF sample.

Li+ ions on the GF, and (3) the presence of SEI layer formed by the decomposition of electrolyte could provide excess Li+ ion storage by the so-called “pseudo-capacitance” behavior [17–23]. Fig. 7c shows the rate performance of the Fe3O4-carbon/GF anode material at different current densities. It is seen that the assynthesized anode material has a good reversible performance at various discharge/charge rate. A high reversible capacity of 935 mAh g1 can be delivered when the current density returns back from 2 A g1 to 100 mA g1. In order to evaluate the long-term stability and elaborate the advantage of micro-/nanostructure by the bacteria-inspired synthesis approach of Fe3O4-carbon/GF anode, the cycling performances of the samples were investigated, and the results are shown in Fig. 7d. The micro-/nanostructured Fe3O4-carbon/GF anode exhibits a superior electrochemical performance when compared with the other four control samples. In detail, when compared with control samples (1) pure GF and (2) carbon/GF, the better capacity of the Fe3O4-carbon/GF indicates the Fe3O4introduced capacity enhancement; For the control sample (3)

Fe3O4/GF, it shows a smaller specific capacity of 450 mAh g1, compared with Fe3O4-carbon/GF anode, which highlights the advantages of bacteria-inspired approach. Specifically, without the use of E. coli, the Fe3+ ion adsorption by directly dipping for Fe3O4/ GF will be limited. The EDX analysis (Fig. S3b, Supporting Information) further confirms the weight ratio of Fe and C in the Fe3O4/GF is about 1.35 to 1, which is smaller than that of Fe3O4carbon/GF of 5 to 1, indicating a low adsorption of Fe3+ without the presence of E. coli. Moreover, from the SEM images of Fe3O4/GF (Figs. S2a-b, Supporting Information), we can identify that it shows a rough surface with random bulk Fe3O4 crystalline structure distributed on the surface of GF, unlike the well-ordered Fe3O4carbon/GF. Such sample without any obviously organized structures would impede the reaction kinetics thus reduce the capacity. Finally, the control sample (4) Fe3O4 NPs anode delivers a relatively high initial capacity of 1072 mAh g1. Unfortunately, it degenerates rapidly to 90 mAh g1 in 200 cycles. The reason for the dramatic degradation is probably due to the exposure of the Fe3O4 NPs (Figs. S2c-d, Supporting Information). Such exposure would have a


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Fig. 7. (a) CV cureves of the Fe3O4-carbon/GF anode at a potential range of 0.01 to 3.0 V (vs Li+/Li) at the scan rate of 0.1 mV s1. (b) Disharge/charge curves of the Fe3O4-carbon/ GF anode at the current density of 100 mA g1. (c) Rate performances of Fe3O4-carbon/GF anode at a various current densities. (d) Cycling stability of the Fe3O4-carbon/GF and other control samples at the current density of 100 mA g1.

large volume variation and surface aggregation, resulting in the capacity loss during lithiation and delithiation cycles. While, the bacteria-inspiration of Fe3O4-carbon/GF has micro-/nanostructured Fe3O4-carbon units anchored on the surface of GF. Such unique structure can not only provide a multilevel interconnected channel architecture that reduces the Li+ ions and electrons diffusion length, but reinforces the contact between electrode/ electrolyte and accommodate the volume variation. At the same time, the well-attached GF can effectively reduce the aggregation of Fe3O4 and to some extent, increase the conductivity and mechanical stability, benefits the electrochemical performance. 4. Conclusions We report a bacteria-inspired method featured with low-cost and large-scale production to construct micro-/nanostructured Fe3O4-carbon on GF. As an example, E. coli-based fabrication is demonstrated. The fabricated structures, owing to the use of GF, can be directly employed as a binder-free lithium-ion battery anode. This anode shows good electrochemical properties including a high reversible capacity, long cycling stability, and good rate performance. This report provides a low-cost, mass-production route for high-performance lithium-ion battery manufacturing. Acknowledgements This work was supported by the HKSAR Government RGC-GRF Grant (CUHK14303914) and by the Direct Grant (Project Code: 3132731) from the Faculty of Science, The Chinese University of Hong Kong.

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