Interconnected CoFe2O4âPolypyrrole Nanotubes as Anode Materials

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Interconnected CoFeO-Polypyrrole Nanotubes as Anode Materials for High Performance Sodium Ion Batteries Qiming He, Kun Rui, Chunhua Chen, Jianhua Yang, and Zhaoyin Wen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12503 • Publication Date (Web): 29 Sep 2017 Downloaded from http://pubs.acs.org on September 30, 2017

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ACS Applied Materials & Interfaces

Interconnected CoFe2O4-Polypyrrole Nanotubes as Anode Materials for High Performance Sodium Ion Batteries a,b Qiming He , Kun Ruia, Chunhua Chenc, Jianhua Yanga,b and Zhaoyin Wena,b,* a CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China. b University of Chinese Academy of Sciences, Beijing 100049, P. R. China c CAS Key Laboratory of Materials for Energy Conversion, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China *Corresponding author Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 DingXi Road, Shanghai 200050, P. R. China Tel: +86-21-52411704 Fax: +86-21-52413903 E-mail: [email protected] Abstract CoFe2O4-coated polypyrrole (PPy) nanotubes (CFO-PPy-NTs) with a three-dimensional (3-D) interconnected network have been prepared through a simple hydrothermal method. The application for sodium ion batteries (SIBs) has been also studied. The finely-crystallized CoFe2O4 nanoparticles (around 5 nm in size) are uniformly grown on the PPy nanotubes. When tested as anode materials for SIBs, the CFO-PPy-NT electrode maintains a discharge capacity of 400 mA h g-1 and a stable coulombic efficiency of 98% after 200 cycles at 100 mA g-1. Even at a higher current density of 1000 mA g-1, the composite can still retain a discharge capacity of 220 mA h g-1 after 2000 cycles. The superior electrochemical performance could be mainly ascribed to the uniform distribution of CoFe2O4 on the 3-D matrix of PPy interconnected nanotubes, which favors the diffusion of sodium ions and electronic transportation and also buffers the large volumetric expansion during charge/discharge. Thereby our study suggests that such CFO-PPy-NTs have great potential as an anode material for SIBs. Keywords: CoFe2O4; Polypyrrole; Nanotubes; Anode; Sodium-ion batteries 1 Introduction Nowadays, large-scale stationary applications of lithium-ion batteries (LIBs) are severely hindered by limited reserves and uneven geographical distribution of lithium resources on the earth which have caused notable rise in the cost of LIBs. Based on the above concern, it is really necessary to develop new kinds of energy storage devices. Lately, sodium-ion batteries (SIBs) have attracted enormous interest as promising alternatives for LIBs owing to the virtually inexhaustible sodium resources which are ubiquitous around the world along with their prominent advantage of cost efficiency.1-4 One of the crucial challenges to realize the future commercialization of SIBs is to develop high performance anode materials of low-cost. Unfortunately, although some research work has made certain improvement,5-7 the discovery of viable anode materials still remains a big challenge, which hampers the overall performance of SIBs. A main reason is Na+ has larger ionic radius (1.02 Å) and molar mass (22.99 g

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mol-1) compared to those of Li+ (0.59 Å, 6.94 g mol-1), which will lead to larger volume changes during charge and discharge, making it hard to find suitable anode materials capable of rapid and stable sodium storage.8-9 Recently iron-based oxide materials have been investigated as anode materials of SIBs due to their low cost, high sodium storage capacity and good conductivity.10-14 Spinel ferrites, with a composition of AFe2O4 (A = Zn, Ni, Co, Mn), are a series of iron-based oxide materials displaying superior performance than simple iron oxides. There are a few reports showing that binary oxides can deliver high capacity originated from the synergetic effect.15-17 The binary metals in this structure can form buffer matrix to reduce the impact of volume change and retain structural stability during charge and discharge process.18-19 This matrix also maintains relatively low activation energy for electron transfer, resulting in higher electrical conductivity and electrochemical activities than single oxides.20-21 Based on the discussion above, the spinel ferrites hold great potentials to be high performance anode materials for SIBs. In recent years, lots of efforts have been made to build three-dimensional (3-D) microstructure composites to solve the problem of volume expansion and shrinkage during cycling, thus to further enhance the cycle stability and rate performance of anode materials, such as anchoring nanoparticles onto graphene frameworks22 or confining them in mesoporous carbon.23 As a type of conducting polymer, polypyrrole (PPy) maintaining a 3-D structure of interconnected nanotubes, can both act as a conducting agent and a supporting matrix to enhance the conductivity and improve the rate performance of the cell.24-25 Apart from good structural stability and fast electron transport paths, this hierarchical interconnected microstructure has enlarged active surface area to ensure good contact with electrolyte, thus providing efficient open channels for rapid Na+ transportation. Here in this work, we prepared interconnected CoFe2O4-PPy nanotubes with a 3-D microstructure using in situ hydrothermal synthesis method. In this composite, the CoFe2O4 nanoparticles are uniformly anchored on the surface of PPy nanotubes. The as-prepared CFO-PPy-NTs showed good cycle performance. After 200 cycles at 100 mA g-1, a reversible capacity of 400 mA h g-1 was achieved, maintaining a coulombic efficiency of 98%. Even when the current density was up to 1000 mA g-1, the capacity still remained 220 mA h g-1 after 2000 cycles. The performance in terms of sodium storage capacity for CoFe2O4 is extraordinary and it exhibits high potential for future application in next-generation SIBs. 2 Experimental 2.1 Material synthesis 2.1.1 Preparation of polypyrrole nanotubes (PPy-NTs) The PPy nanotubes were prepared according to a previous report26 with minor modifications. First, 0.1 g methyl orange (MO) was dissolved into 150 mL deionized water to form a solution. Next, 1.96 g of FeCl3 was added to the above solution under vigorous stirring and a blood-red flocculent precipitate showed up simultaneously. After that, 0.484 g pyrrole monomer was added into the mixture followed by continuous stirring for 24 h at ambient temperature. Finally, the as-synthesized PPy precipitate was washed with deionized water and ethanol for several times and then


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dried under vacuum at 60 °C for 24 h. 2.1.2 Preparation of CoFe2O4-polypyrrole nanotubes To prepare the CFO-PPy-NTs, 0.22 g hexamethylenetetramine (HMTA) and 0.18 g urea were first dissolved into 76 mL ethanol, and then 0.15 g PPy was uniformly dispersed into the above solution by ultrasonication for 1 h. Subsequently, 0.40 g Fe(NO3)3·9H2O and 0.59 g Co(NO3)2·6H2O were added. After Fe3+ and Co2+ salts were dissolved, the final blend was transferred into a 95 mL Teflon-lined autoclave and then heated and maintained at 120 °C for 10 h. The final CFO-PPy-NTs were obtained by washing the precipitates with deionized water and ethanol for several times and dried under vacuum at 60 °C overnight. The CoFe2O4 nanoparticles (CFO-NPs) were also prepared using the same procedure without adding PPy. 2.2 Material Characterization The powder X-ray diffraction (PXRD) patterns were obtained by using a Rigaku Ultima IV diffractometer, with Cu Kα radiation at a scan rate of 6° min-1 (2θ from 10° to 80°) to identify the phase and crystal structure of the as-synthesized samples. The weight ratio of PPy in the composite was characterized by thermogravimetric analysis (TGA) using a NETZSCH STA 449C analyzer between 50 °C and 800 °C at a heating rate of 10 °C min-1 in air. The Fourier transform infrared (FTIR) characterization was conducted on a Bruker Tensor 27 FTIR analyzer. The valence state of the elements in the PPy and the composite was studied by X-ray photoelectron spectroscopy (XPS) using an ESCAlab250 system with a monochromatic Al Kα X-ray source. Field emission scanning electron microscopy (FESEM, SU8200 and Magellan 400) and high resolution transmission electron microscopy (HRTEM, JEM-2100F) were performend to characterize the morphology of the composite. Tristar II 3020 specific surface area analyzer was used to measure the BET surface areas of PPy nanotubes and the composite. 2.3 Electrochemical measurements 1. The electrochemical performance testing was conducted using 2025-type coin cells. Na foil (Sigma-Aldrich) was exploited as counter and reference electrode. Glass fiber was used as the separator. The working electrode was fabricated by ball milling 70 wt% CFO-PPy-NTs, 20 wt% Super-P (carbon black), 5 wt% polymerized styrene butadiene rubber (SBR), and 5 wt% carboxymethyl cellulose (CMC) binder in deionized water. The weight ratio of the solid content is about 20 wt%. After being ball milled for 6h, the slurry was coated on the copper foil substrate using doctor blade. Then the as-prepared electrode was dried under vacuum at 80 °C overnight and cut into round pieces of 12 mm in diameter by cutting machine. The calculation of the mass loading is by cutting out the mass of the copper substrate, which is 12.25 mg in average, and only considering the weight ratio of the CFO-PPy-NTs (70 wt%). Thus the average mass loading of CFO-PPy-NTs is 1.2 ± 0.2 mg cm-2. A solution of 1.0 M NaClO4 in propylene carbonate (PC) with 5 wt% fluoroethylene carbonate (FEC) was employed as electrolyte in this study. All the cells were assembled in a glove box filled with purified argon gas with both moisture and oxygen content below 0.1 ppm and measured at room temperature. Galvanostatic charge and discharge testing was



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conducted on a Land Test System (Wuhan, China) with a cut-off voltage range from 0.01 to 3.0 V (vs. Na/Na+) at various current densities. Cyclic voltammetry test was carried out on an electrochemical workstation from 0.01 V to 3.0 V (vs. Na/Na+) at a scan rate of 0.1 mV s-1. 3 Results and Discussion 3.1 Phase and morphology characterization

Scheme 1 Fabrication of the CFO-PPy-NT. The fabrication of CFO-PPy-NT is schematically illustrated in Scheme 1. The process starts with oriented self-assembly of MO molecules into a tubular supermolecule template. Then the pyrrole monomers are attached onto MO templates followed by further polymerization into PPy nanotubes. With the addition of Fe/Co precursors, CFO-PPy-NT can be obtained after a proper hydrothermal reaction along with post-heat treatment.


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Fig. 1 (a-b) SEM images of the CFO-PPy-NTs. TEM images of (c) PPy-NTs and (d) CFO-PPy-NTs. (e) HRTEM image of the CFO-NPs on CFO-PPy-NTs. (f) Electronic image and EDS elemental mapping analysis of the CFO-PPy-NTs. The morphology characterization of PPy-NTs and CFO-PPy-NTs were conducted using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Fig. S1 and Fig. S2a, an interconnected network of PPy nanotubes can be obtained via the MO assisted-template method. After the introduction of Fe3+ and Co2+ sources, CoFe2O4 nanoparticles are uniformly distributed on the surface of PPy nanotubes (Fig. 1a and b). For comparison, the SEM image of CFO-NPs is also presented in Supporting Information (Fig. S3). As displayed by TEM image in Fig. 1c-d and S2b, the diameters of CFO-PPy-NTs increased a little after loading with CoFe2O4 nanoparticles than pure PPy-NTs (ca. 20 nm). HRTEM image in Fig. 1e further reveals that the roughened surface layer of CFO-PPy-NTs consists of CoFe2O4 nanoparticles in the dimensions of 5 nm. Well defined lattice fringes of (111), (220), (311), (222) and (400), are in accordance with d-spacings of 0.486, 0.297, 0.253, 0.241 and 0.208 nm, respectively. Uniform composition of CFO-PPy-NTs were also confirmed by energy dispersive X-ray



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spectroscopy (EDS) mapping in Fig. 1f. As can be seen, the light points representing Co, Fe and O have wider distribution areas than that of C and N, clearly indicating the homogeneous growth of CoFe2O4 nanoparticles on the surface of PPy-NTs.

Fig. 2 (a) XRD pattern of CFO-PPy-NTs and the corresponding PDF. (b) FTIR analysis of CFO-PPy-NTs. High resolution XPS spectra of (c) C 1s, (d) N 1s, (e) Co 2p and (f) Fe 2p of CFO-PPy-NTs. All the characteristic peaks in the XRD pattern of CFO-PPy-NTs (Fig.2a) can be indexed to the spinel CoFe2O4 structure (PDF No. 03-0864) without any impurity, similar to the pattern of bare CoFe2O4 particles (Fig. S4). Especially, the peak intensity of the (311) plane is obviously the highest, indicating preferred orientation,


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which is also explained in the literature.27 Absence of PPy peaks for CFO-PPy-NTs can be ascribed to the amorphous nature of PPy (Fig. S5). The Fourier transform infrared (FTIR) spectrum of CFO-PPy-NTs is displayed in Fig. 2b. The band at 598 cm-1 is attributed to intrinsic vibration of tetrahedral sites in metal oxides, which is corresponding with the Co/Fe-O bonding. The other bands can be assigned to N-H (3433 cm-1, 1259 cm-1), C-H (2928 cm-1, 1383 cm-1, 806 cm-1), C=C (1622 cm-1), C-C and C-N (1454 cm-1) and quinonoid structure (1103 cm-1), respectively, which are correlated with characteristic groups in PPy. Thus, the FTIR result further substantiate the existence of both PPy and CoFe2O4 in CFO-PPy-NTs. The X-ray photoelectron spectroscopy (XPS) was conducted to study the chemical states of elements in the surface of PPy and CFO-PPy-NTs. The C 1s and N 1s spectra of as-prepared PPy nanotubes (Fig. S6a and S6b) are in good accordance with the reported spectra of PPy.28-30 For comparison, the corresponding spectra of CFO-PPy-NTs are also presented in Fig. 2c and 2d. No obvious difference can be observed between bare PPy and CFO-PPy-NTs in both C1s and N 1s spectra. The binding energies of 285 and 286.8 eV correspond to C=C and C-N, respectively. Previous studies suggest that nitrogen atoms in pyrrole rings coordinated to the metal ions are responsible for enhancing the bonding between CoFe2O4 and PPy matrix.31-32 In addition, Co 2p and Fe 2p spectra of CFO-PPy-NTs are displayed in Fig. 2e and 2f. The fitting results of these two spectra suggest that the Fe in this composite exists as Fe3+,33-35 while Co exists as Co2+.36-39

Fig. 3 (a) TGA curves of PPy-NTs and CFO-PPy-NTs. N2 adsorption-desorption isotherms of (b) PPy-NTs and (c) CFO-PPy-NTs. (d) Pore size distribution of CFO-PPy-NTs.



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Thermogravimetric analysis (TGA) measurements were conducted in air as shown in Fig. 3a. The amount of PPy in the CFO-PPy-NTs was calculated to be 42.70 wt% according to the weight loss caused by the oxidation of PPy. As indicated by nitrogen absorption-desorption isotherm curves of PPy and CFO-PPy-NTs (Fig. 3b and c), Brunauer-Emmett-Teller (BET) specific surface area is enlarged after nanoparticle growth (from 14.04 to 43.39 m2 g-1). It can be seen that the diameters of most pores in PPy-NTs are less than 2 nm (Fig. S7), while the average diameter of pores of CFO-PPy-NTs is about 8 nm (Fig. 3d). The Barrett-Joyner-Halenda (BJH) adsorption-desorption cumulative pore volumes of both PPy-NTs and CFO-PPy-NTs are also obtained through the analyzer, which are 0.03 and 0.12 cm3 g-1, respectively (Fig. S8a-b). Thus the total pore volume of the CFO-PPy-NTs is much larger than that of the PPy-NTs. The larger surface area and pore volume will benefit the electrochemical performance owing to more active sites and contact between electrode and electrolyte. 3.2 Electrochemical performance

Fig. 4 (a) Cycle performance of CFO-PPy-NTs at 100 mA g-1. (b) Cycle performance comparison among CFO-PPy-NTs with different amounts of PPy, bare CoFe2O4


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particles and bare PPy-NTs. (c) Long-term cycle performance of CFO-PPy-NTs at 1000 mA g-1. (d) Comparison between the rate performance of CFO-PPy-NTs and CFO-NPs. (e) EIS study of both CFO-PPy-NTs and CoFe2O4 particles before and after cycling. To investigate the potential application of the CFO-PPy-NTs as anode materials of SIBs, the galvanostatic charge/discharge tests were conducted with a cut-off voltage range from 0.01 to 3.0 V (vs. Na/Na+). When tested at a current density of 100 mA g-1 (Fig. 4a), the initial discharge and charge capacities are 1096 and 614 mA h g-1, respectively, resulting in the low initial coulombic efficiency of 56%, which may be caused by irreversible decomposition of the electrolyte on the surface of the electrode40 and the formation of irreversible Na2O due to the slow kinetics of sodium storage accompanied with the large radius of Na+ (1.02Å).41 After 200 cycles, the CFO-PPy-NTs still retain a discharge capacity of 400 mA h g-1, displaying high sodium storage capability and good cyclability. To identify the contributions of the PPy matrix to the sodium storage capacity of the CFO-PPy-NTs, bare PPy and CoFe2O4 particles were also tested at the same current and voltage conditions. The contribution of PPy to the capacity of the CoFe2O4-PPy composite is negligible (Fig. 4b). Meanwhile, the bare CoFe2O4 can only deliver a capacity around 200 mA h g-1, less than half of the capacity of the CFO-PPy-NTs, thus indicating that the introduction of PPy nanotubes has effectively maximum the conversion reaction of CoFe2O4 without contributing extra capacity. Composites with different proportions of PPy were also tested to study the influence of the amount of PPy to the electrochemical performance. It is noted that the composites with intial PPy amount of 0.1 g and 0.2 g only deliver capacities of about 300 mA h g-1, much lower that of the one with 0.15 g initial PPy. To further study the cycle performance of the composite at higher current density in the long term, the charge/discharge test was also conducted at 1000 mA g-1 (Fig. 4c). Even at such a high current density, the initial discharge capacity still remains at 502 mA h g-1. After 2000 cycles, the capacity remains at 220 mA h g-1, exhibiting good long-term cycle stability, owing to the interconnected conductive network of the PPy nanotubes. Furthermore, the rate performance of the CFO-PPy-NTs was evaluated by increasing the current density from 0.05 A g-1 up to 10 A g-1 and finally turning it back to 0.05 A g-1 reversely. As can be seen in Fig. 4d, the discharge capacity of the composite decreases from 767 (0.05 A g-1), to 542 (0.1 A g-1), 471 (0.3 A g-1), 421 (0.5 A g-1), 370 (1 A g-1), 272 (3 A g-1), 240 (5 A g-1) and 189 mA h g-1 (10 A g-1), respectively. The capacity successfully returned to 656 mA h g-1 when the current density is back to 0.05 A g-1, indicating excellent robustness of this composite. For comparison, the rate performance of CFO-NPs was also tested. It is clear that the rate performance of the CFO-PPy-NTs is far better than that of the CFO-NPs due to enhanced conductivity by introducing PPy nanotubes.42-44 As Fig. S9 is displayed, the electrochemical reactions of the CFO-PPy-NTs were investigated by cyclic voltammetry (CV) at a scan rate of 0.1 mV s-1 in a voltage range from 0.01 to 3 V vs. Na/Na+. During the initial scan, the cathodic peak at 0.38 V is related to the reduction of Fe3+ and Co2+ into metallic Fe and Co,11, 45 while the



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broad anodic peak centered at 1.51 V can be attributed to the oxidation of metallic Fe and Co.45 The curves of the following scans obtain good conformity. In conclusion, the significant difference between the initial and the following scans implies the initial irreversible electrochemical reactions and subsequent favorable reversibility of the composite, indicating good accordance with the cycling performance in Fig. 4a. Electrochemical impedance spectroscopy (EIS) measurement was conducted to further verify the contributions of PPy nanotubes to the conductivity of CFO-PPy-NT electrode. As is shown in Fig. 4e (equivalent circuit in Fig. S10), the semicircle at high frequency is correlated with the charge transfer resistance (Rct), which represents the impedance corresponding with sodium ion migration through the surface film and charge transfer through the electrode/electrolyte interfaces. After 200 cycles, the radius of the depressed semicircle for bare CoFe2O4 has been obviously enlarged compared with the fresh one, corresponding with the increasing Rct from 230 to 511 Ω. As for CFO-PPy-NTs, the Rct increased from 118 to 220 Ω, demonstrating a much-lower Rct before cycling and a smaller increasing after 200 cycles, indicating good accordance with the electrochemical performance discussed above, exhibiting beneficial effect from the highly conductive network of interconnected PPy nanotubes. 3.3 Compositional and morphological characterization of cycled products The composition and morphology of cycled products were characterized to further study the reaction mechanism, elucidating the relationship between the good electrochemical performance and the hierarchical nanostructure. The existence of Co2+ were confirmed by XPS for cycled CFO-PPy-NTs electrode (charged state) in Fig. S11a.46-49 However, an additional peak attributed to Fe2+ at 714.7 eV appears 50 in Fe 2p spectrum (Fig. S11b) apart from three other peaks ascribed to Fe3+.51-53 According to previous reports, the XPS results indicate the existence of CoO, while Fe might exist Fe2O3 and FeO.54-55 And the reaction mechanism is proposed as follows (Eq. 1-4) CoFe2O4 + 8Na → Co + 2Fe + 4Na2O

(1)

Co + Na2O ↔ CoO + 2Na

(2)

Fe + Na2O ↔ FeO + 2Na

(3)

2FeO + Na2O ↔ Fe2O3 + 2Na


(4)

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Fig. 5 FESEM images of cycled electrodes of (a) CFO-PPy-NTs and (b) CoFe2O4 particles. The insets are corresponding images at higher magnification. Both electrodes were cycled at 100 mA g-1 after 200 cycles. (c) TEM, (d) SAED, (e) HRTEM images, (f) Electronic image and EDS elemental distribution of the cycled CFO-PPy-NTs. The cycled electrode of CFO-PPy-NTs retains an interconnected network structure after cycling, with uniform distribution of nanoparticles on the nanotubes (Fig. 5a and the inset, pristine electrode in Fig. S12a and S12b), displaying good structure stability,



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leading to stable long term cyclability. Compared with CFO-PPy-NTs, there are obvious cracks on the cycled electrode of bare CoFe2O4 particles, where many particles detach from each other (Fig. 5b and the inset, pristine electrode in Fig. S13a and S13b), resulting in inferior conductivity and poor cycle stability. The nanotube structure after cycles is further confirmed by TEM images in Fig. 5c. In the selected area electronic diffraction (SAED) image (Fig. 5d), the (220), (200) and (002) plane of FeO and (220), (200) plane of CoO can be clearly observed in the polycrystalline diffraction pattern, while the (440) and (220) plane of Fe2O3 are also detected. This result is in good accordance with the XPS analysis, providing more evidence for the composition of the cycled products. As can be seen from the HRTEM image (Fig. 5e), the corresponding lattice planes in Fig. 5d are also observed. There are no obvious oversized particles on PPy nanotubes, retaining their original sizes (around 5 nm). Thus the initial well-distributed CoFe2O4 nanoparticles on the PPy nanotubes have effectively avoided the severe agglomeration and oversized particle growth which may cause pulverization and decline in the electrical conductivity of the electrode. The EDS elemental mapping displayed in Fig. 5f indicates well distribution of products particles on the surface of PPy nanotubes after cycling. Based on the above post cycle analysis, we can conclude that the PPy nanotube structure favors fast electronic transportation, enhancing the conductivity and thus improving the rate performance. The high specific area provides more active site for Na+. Uniquely, the nanoparticles anchored on the PPy-NTs avoid severe agglomeration and electrode degradation, maintaining good cyclability. 4 Conclusion In summary, CoFe2O4 nanoparticles were successfully coated on the surfaces of the interconnected PPy nanotube network through a facile and environmentally friendly hydrothermal method. The as-prepared CFO-PPy-NT network displays excellent electrochemical performance as the anode material for SIBs, especially with regards to the high capacity and stable cyclability (400 mA h g-1 after 200 cycles at 100 mA g-1 and 220 mA h g-1 after 2000 cycles at 1000 mA g-1), high coulombic efficiency (98%) and good rate capability (189 mA h g-1 at 10 A g-1). These results are owing to the homogeneous growth of CoFe2O4 nanoparticles on the PPy matrix with interconnected 3-D structure, which can greatly enhance the electronic conductivity of the composite and act as a buffer to relieve the strain caused by the volume changes in the electrode during the cycling. At the meantime, the diffusion of sodium ions is also favored. For comparison, the bare CoFe2O4 nanoparticles prepared with the same method delivered much lower capacity. Thus, these results obviously indicate that the coated CoFe2O4 particles and the PPy nanotube network generate great synergetic effect during the cycling, enabling maximum utilization of the sodium storage capability of CoFe2O4 nanoparticles. Based on the excellent performance discussed above, it is really hopeful for such kind of transitional metal oxide-conductive polymer nanocomposites to be promising candidates of anode materials for future sodium ion batteries. Supporting Information SEM images of PPy-NTs, TEM images of PPy-NTs and CFO-PPy-NTs at low


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magnification, SEM image of CoFe2O4 nanoparticles, comparison between the XRD patterns of CFO-NPs and CFO-PPy-NTs, XRD pattern of PPy-NTs, high resolution XPS spectra of PPy-NTs including C 1s and N 1s, pore size distribution of PPy-NTs, BJH adsorption-desorption cumulative pore volumes of PPy-NTs and CFO-PPy-NTs, CV scan curves of the CFO-PPy-NTs, the equivalent circuit for the EIS analysis, high resolution XPS Spectra of Co 2p and Fe 2p of the cycled CFO-PPy-NTs, SEM images of pristine electrode of CFO-PPy-NTs, SEM images of pristine electrode of CoFe2O4 nanoparticles Acknowledgements This work was financially supported by the National Natural Science Foundation of China under Grant no. 51432010 and fundamental research project from the Science and Technology Commission of Shanghai Municipality no. 14JC1493000 and 15DZ2281200. References (1) Palomares, V.; Casas-Cabanas, M.; Castillo-Martínez, E.; Han, M. H.; Rojo, T. Update on Na-Based Battery Materials. A Growing Research Path. Energy Environ. Sci. 2013, 6, 2312-2337. (2) Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S. Research Development on Sodium-Ion Batteries. Chem. Rev. 2014, 114, 11636-11682. (3) Kundu, D.; Talaie, E.; Duffort, V.; Nazar, L. F. The Emerging Chemistry of Sodium Ion Batteries for Electrochemical Energy Storage. Angew. Chem. Int. Ed. 2015, 54, 3431-3448. (4) Gruber, P. W.; Medina, P. A.; Keoleian, G. A.; Kesler, S. E.; Everson, M. P.; Wallington, T. J. Global Lithium Availability. J. Ind. Ecol. 2011, 15, 760-775. (5) Zou, G.; Jia, X.; Huang, Z.; Li, S.; Liao, H.; Hou, H.; Huang, L.; Ji, X. Cube-Shaped Porous Carbon Derived from MOF-5 as Advanced Material for Sodium-Ion Batteries. Electrochim. Acta 2016, 196, 413-421. (6) Fu, L.; Tang, K.; Song, K.; van Aken, P. A.; Yu, Y.; Maier, J. Nitrogen Doped Porous Carbon Fibres as Anode Materials For Sodium Ion Batteries with Excellent Rate Performance. Nanoscale 2014, 6, 1384-1389. (7) Zou, G.; Chen, J.; Zhang, Y.; Wang, C.; Huang, Z.; Li, S.; Liao, H.; Wang, J.; Ji, X. Carbon-Coated Rutile Titanium Dioxide Derived from Titanium-Metal Organic Framework with Enhanced Sodium Storage Behavior. J. Power Sources 2016, 325, 25-34. (8) Palomares, V.; Serras, P.; Villaluenga, I.; Hueso, K. B.; Carretero-González, J.; Rojo, T. Na-Ion Batteries, Recent Advances and Present Challenges to Become Low Cost Energy Storage Systems. Energy Environ. Sci. 2012, 5, 5884-5901. (9) Chevrier, V.; Ceder, G. Challenges for Na-Ion Negative Electrodes. J. Electrochem. Soc. 2011, 158, A1011-A1014. (10) Liu, X.; Chen, T.; Chu, H.; Niu, L.; Sun, Z.; Pan, L.; Sun, C. Q. Fe2O3-Reduced Graphene Oxide Composites Synthesized via Microwave-Assisted Method for Sodium Ion Batteries. Electrochim. Acta 2015, 166, 12-16. (11) Jiang, Y.; Hu, M.; Zhang, D.; Yuan, T.; Sun, W.; Xu, B.; Yan, M. Transition Metal Oxides for High Performance Sodium Ion Battery Anodes. Nano Energy 2014, 5,



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Scheme 1 Fabrication of the CFO-PPy-NT. 402x100mm (149 x 149 DPI)


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Fig. 1 (a-b) SEM images of the CFO-PPy-NTs. TEM images of (c) PPy-NTs and (d) CFO-PPy-NTs. (e) HRTEM image of the CFO-NPs on CFO-PPy-NTs. (f) Electronic image and EDS elemental mapping analysis of the CFO-PPy-NTs. 112x112mm (300 x 300 DPI)



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Fig. 2 (a) XRD pattern of CFO-PPy-NTs and the corresponding PDF. (b) FTIR analysis of CFO-PPy-NTs. High resolution XPS spectra of (c) C 1s, (d) N 1s, (e) Co 2p and (f) Fe 2p of CFO-PPy-NTs . 284x350mm (149 x 149 DPI)


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Fig. 3 (a) TGA curves of PPy-NTs and CFO-PPy-NTs. N2 adsorption-desorption isotherms of (b) PPy-NTs and (c) CFO-PPy-NTs. (d) Pore size distribution of CFO-PPy-NTs. 286x216mm (120 x 120 DPI)



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Fig. 4 (a) Cycle performance of CFO-PPy-NTs at 100 mA g-1. (b) Cycle performance comparison among CFOPPy-NTs with different amounts of PPy, bare CoFe2O4 particles and bare PPy-NTs. (c) Long-term cycle performance of CFO-PPy-NTs at 1000 mA g-1. (d) Comparison between the rate performance of CFO-PPyNTs and CFO-NPs. (e) EIS study of both CFO-PPy-NTs and CoFe2O4 particles before and after cycling. 302x301mm (149 x 149 DPI)


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Fig. 5 FESEM images of cycled electrodes of (a) CFO-PPy-NTs and (b) CoFe2O4 particles. The insets are corresponding images at higher magnification. Both electrodes were cycled at 100 mA g-1 after 200 cycles . (c) TEM, (d) SAED, (e) HRTEM images, (f) Electronic image and EDS elemental distribution of the cycled CFO-PPy-NTs. 131x175mm (300 x 300 DPI)


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