polypyrrole as an anode for sodium-ion batteries

Carbon 95 (2015) 552e559

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Core-sheath structured porous carbon nanofiber composite anode material derived from bacterial cellulose/polypyrrole as an anode for sodium-ion batteries Zhian Zhang*, Juan Zhang, Xingxing Zhao, Fuhua Yang School of Metallurgy and Environment, Central South University, Changsha, Hunan 410083, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 May 2015 Received in revised form 27 July 2015 Accepted 21 August 2015 Available online 28 August 2015

A core-sheath structured carbon nanofiber@nitrogen-doped porous carbon (CNF@NPC) composite is synthesized as a promising anode material for sodium-ion batteries (SIBs). Particularly, the eco-friendly bacterial cellulose (BC) is used as both template and precursor for the synthesis of CNF@NPC through the carbonization of bacterial cellulose@polypyrrole (BC@PPy). Due to the particular core-sheath structure and the synergetic effect between the CNF core layer and the NPC outer layer, the CNF@NPC exhibit outstanding electrochemical performance, including a high reversible specific capacity (240 mAh g
1 at 100 mA g
1 over 100 cycles), excellent rate performance (146.5 mAh g
1 at 1000 mA g
1), and superior cycling stability (148.8 mAh g
1 at 500 mA g
1 over 400 cycles). The application of environmentally friendly electrode material may open a new situation for SIBs. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Sodium-ion batteries Carbon nanofiber Bacterial cellulose Nitrogen-doped porous carbon

1. Introduction During the last three decades, the rapid development of lithiumion batteries (LIBs) has made them seize the portable electronics market [1,2]. However, the limitation and uneven distribution of lithium minerals can't meet the higher demands for LIBs [3,4]. The exploration of new cost-effective energy storage devices has been intensively sought. Currently, sodium-ion batteries (SIBs) have captured worldwide attention and been regarded as an alternative to LIBs, because of the abundant supply and low cost of Na mineral salts [5,6]. Indeed, the previous study of LIBs plays an important role in development of electrode materials for SIBs. However, a slice of ideal anode materials of LIBs such as graphitic carbon material exhibited very poor storage properties for SIBs [7]. Two critical factors account for the unfavorable performance. One of the factors is that the size of the Naþ is 55% larger than Liþ in radius, which make Naþ more difficult to insert into and extract from host materials reversibly. Besides the kinetic issue, the larger Naþ radius is also relevant for possible structural change during insertion or extraction [8]. Thus, great improvements are imminently needed in seeking appropriate * Corresponding author. School of Metallurgy and Environment, Central South University, No.932, Lushan Road, Changsha, Hunan 410083, China. E-mail address: [email protected] (Z. Zhang). http://dx.doi.org/10.1016/j.carbon.2015.08.069 0008-6223/© 2015 Elsevier Ltd. All rights reserved.

electrode materials for SIBs. Many electrode materials, including carbonaceous materials [9e11], metal oxides [12,13], sodium ternary compounds [14,15], metal nitrides [16], alloys [17] and phosphorous [18,19] have been reported as anode materials for SIBs. Among these, carbonaceous materials, especially disordered carbons demonstrate huge advantages for their relatively high electrical conductivity and the larger interlayer space, in favor of Naþ insertion and extraction [20]. Guan et al. [21] reported a series of mesoporous carbons, which were obtained by pyrolyzing gelatin and magnesium citrate between 600 and 900  C under flowing nitrogen. Tang et al. [9] demonstrated that hollow carbon nanospheres prepared by a facile hydrothermal method possess delivered the specific capacity of 223 mAh g
1 at 50 mA g
1. Bai et al. [22] synthesized highperformance hard carbon from pyrolyzing polyvinyl chloride (PVC) nanofibers, which achieved a high reversible specific capacity of 271 mAh g
1 and retained 211 mAh g
1 over 150 cycles. Although much progress has been realized, above-mentioned carbon materials are commonly obtained by using simple method and single carbon source. It is still a crucial issue to develop suitable anode materials for application in SIBs. Nitrogen-doped porous carbons (NPC) with large surface area have attracted much interest for application in SIBs. The porous structured carbon with large surface area will adequately contact

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to the electrolyte, facilitate good transport of electrons/sodium ion, and facile strain relaxation during the chargeedischarge process [23]. N-doped can generate extrinsic defects and improve the electrochemical performance and the electric conductivity of the carbon material [24e26]. Wang et al. [23] described a 2D porous N-doped carbon sheets, which exhibited a high reversible capacity and good rate capability. Fu et al. [27] reported that Ndoped porous carbon fibers exhibited a high capacity of 296 mAh g
1 at 50 mA g
1. These studies clearly demonstrated that N-doped porous carbons are attractive anode constituents for SIBs. While NPC has above-mentioned advantages, its numerous pores lead to less mechanical strength and low electrical conductivity [28]. Cellulosic substances are promising candidates for the application of anode material due to their low cost and abundance [29]. As a special kind of cellulose, the eco-friendly bacterial cellulose has attracted more and more attention as a bioscaffold owing to its unique ultrafine matrix networks structure and its nature nanofiber structure. Moreover, carbon nanofiber pyrolyzed from BC possess well electrical conductivity [30]. To date, some literature have explored the application of BC in the different fields, such as supercapacitors [31,32], adsorbing toxic organic dyes [33] and medical fields [34]. Chen et al. [35] reported a three-dimensional (3D) N-doped activated nanofiber network derived from pyrolyzed BC for flexible all-solid-state supercapacitor. Wang et al. [36] reported a 3D activated-pyrolyzed BC nanofiber networks as anode material in LIBs, which delivered a reversible capacity of 857.6 mAh g
1 over 100 cycles at 100 mA g
1. To the best of our knowledge, BC hasn't been put into use for SIBs. Compared with other carbon materials, the low surface area and limited pores of the bacterial cellulose and the pyrolyzed BC limit their capability. Inspired by these previous studies, we try to combine the advantages of carbon nanofiber pyrolyzed from BC and N-doped porous carbons material to fabricate the core-sheath hybrid architecture of CNF@NPC composite with abundant porous structure, good electronic conductivity, high surface area and appropriate N-doped, which could serve as a valid way to enhance the electrochemical property of SIBs. Herein, we report on CNF@NPC core-sheath structured composite obtained by carbonizing bacterial cellulose@polypyrrole (BC@PPy) precursor for SIBs. The inner CNF from BC not only guarantee the high electronic conductivity, but also offer a firm carbon matrix to stabilize the anode structure. The outer NPC provide abundant porous structure, high surface area and appropriate N-doped to improve the reversible specific capacity. Benefiting from the synergies of CNF and NPC, the CNF@NPC exhibit a high reversible capacity, including a high reversible capacity (240 mAh g
1 at 100 mA g
1 over 100 cycles), excellent rate capability (146.5 mAh g
1 at 1000 mA g
1), and superior cycling stability (148.8 mAh g
1 at 500 mA g
1 over 400 cycles). 2. Experimental 2.1. Preparation of carbon nanofiber@nitrogen-doped porous carbon (CNF@NPC) To fabricate the CNF@NPC, firstly, the bacterial cellulose@polypyrrole (BC@PPy) were prepared as the precursor. The BC@PPy were synthesized via oxidative polymerization of pyrrole in situ self-assembled onto BC nanofibers. Detail processes were as follows [37]. The BC pellicles (Hainan Yide Foods Co. Ltd, China) were washed by abundant deionized water and cut into small cube, then pulped with proper deionized water by using a mechanical homogenizator for 20 min to prepare BC nanofibers suspension. Subsequently, 100 mL of homogeneous BC nanofibers

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suspension was added into a 100 mL hydrochloric acid (2.0 M) and 20 mM ferric chloride solution, followed by 15 min of sonication. Then 0.7 mL pyrrole was added into the mixture after cooling the reaction temperature to 1e5  C and stirred for another 15 min. The reaction was proceeded for 5 h at 1e5  C. The resultant precipitate was filtered off and sequentially washed several times with distilled water. The as-prepared BC@PPy was formed in a vacuum freeze-drying machine (freeze dryer) overnight. For reference, pure BC was also freeze-dried under the same condition. After heat treatment of the as-prepared BC@PPy and pure BC at 700  C for 3 h with a heating rate of 5  C min
1 under an argon atmosphere, the CNF@NPC and CNF were finally obtained. 2.2. Material characterization Scanning electron microscopy (SEM, Nova SEM 230) and transmission electron microscopy (TEM, Tecnai G2 20ST) were applied to investigate the material morphology. The structure of the material was characterized using X-ray diffraction (XRD, Rigaku3014). Nitrogen adsorption/desorption measurements were performed by using Quantachrome instrument (Quabrasorb SI3MP). Surface functional groups and bonding characterization were conducted using X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB250xi). The fitting errors of XPS test results are within ± 1%. 2.3. Electrochemical measurements To conduct electrochemical measurements of the CNF@NPC electrode, a CR-2025 type coin cell was fabricated. Sodium pellet and a celgard 2400 film membrane were served as counter electrode and separator, respectively. A mixed slurry of CNF@NPC, Super P and sodium alginate (SA) (8:1:1, in wt.%) in deionized water was spread onto a copper foil. The coating copper foil was first dried in air, then transferred into a oven at 80  C for 12 h. The assembly of the tested cells was carried out in a glove box under an argon atmosphere. The electrolyte used in this work was the composition of 1 M NaClO4 (SigmaeAldrich) in a solvent mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (with a volume ratio of 1:1). Galvanostatic charge/discharge test were conducted with a battery test system. Cyclic voltammetry (CV) measurements were carried out at a scan rate of 0.2 mV s
1 between 0.01 and 3 V using 1470e electrochemical measurement system. Those electrochemical tests were carried out under a constant temperature of 25  C. 3. Results and discussion 3.1. Synthesis and characterization As presented in Fig. 1, the CNF@NPC synthetic procedures can be divided into five parts: agitation, dispersion, polymerization, collection and carbonization. The BC membrane was pulped to form BC nanofibers suspension. BC is a natural polymer hydrogel with three-dimensional polymeric networks and BC can also imbibe other monomeric, reactive and potentially polymerizable monomers into its networks for occupying its void volume and interacting with BC fiber chains [38]. The pyrrole monomer was adsorbed on the surface of BC and then the polypyrrole was coated onto the surface of BC nanofibers uniformly by the oxidation of pyrrole with ferric chloride as the oxidant in an acidic aqueous medium. In this work, two kinds of carbon sources have been used, BC served as both template and carbon precursor and pyrrole monomer also used as porous carbon precursor. Centrifugation and

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Fig. 1. Schematic of the CNF@NPC fabricate procedure. (A colour version of this figure can be viewed online.)

freeze-drying were carried out to collect the solid product. After heat treatment of the as-prepared BC@PPy at 700  C for 3 h, the CNF@NPC products were finally obtained. Through the coating and carbonization procedures, core-sheath structure and N-doped porous carbon composite was obtained. The morphology characterization of pure BC, BC@PPy precursor and CNF@NPC nanofibres were investigated using scanning electron microscope (SEM) and transmission electron microscope (TEM). The original BC pellicle (Fig. 2a) exhibits a water rich and transparent morphology. While pulping the BC membrane, the homogeneous BC suspension (Fig. 2b) was fabricated. When pyrrole monomer was added into BC suspension, the color of the BC nanofibres suspension was changed from ivory to black. By freeze-drying the resultant precipitate of polymerization reaction, the BC@PPy precursor (Fig. 2c) was formed. It is noted that the asprepared BC@PPy is lightweight, loosen and black. Fig. 2d presents a typical scanning electron microscopy (SEM) image of the pure BC. The BC nanofibres suspension was filtered off and sequentially freeze-dried, then it displays its nature nanofiber structure with 30e60 nm and BC nanofibers connect with each other to form interconnected network structure [37]. Fig. 2e presents the SEM image of the CNF@NPC composite, which was derived from BC@PPy precursor. In comparison to pure BC, the roughness and width of CNF@NPC nanofibres are increased to 80e110 nm after coating with NPC layer. The CNF@NPC nanofibers are contacted with each other to form the interconnected network (Fig. 2f) and the CNF derived from pure BC are encapsulated by NPC to fabricate core-sheath structure (Fig. 2g). Moreover, TEM image further reveals that PPy was deposited along the cellulose nanofibers (Fig. 2g), the result is consistent with SEM [39]. To further characterize the surface and the porous structure of the CNF@NPC and CNF, N2 absorption-desorption measurement

was also carried out. Fig. 3a shows a typical type IV isotherm with a hysteresis loop within a relative pressure P/P0 range of 0.4e1, suggesting the characteristic mesoporous structure. The adsorption-desorption hysteresis of CNF@NPC is larger than the same of CNF, suggesting that CNF@NPC possesses a larger volume of mesoporous [28]. The porosity data shows that the specific surface area (SSA) of CNF and CNF@NPC is 71.3 and 300.2 m2 g
1 respectively. It is indicated that surface area of CNF@NPC is increased sharply by coating NPC layer, which has porous structure with high SSA. As shown in Fig. 3b, compare to CNF, the pore size of the CNF@NPC is mainly distributed between 3 and 4 nm. The amount of mesopores is obviously increased, further verifying that NPC was successfully coated on the CNF and formed core-sheath structures, which is consistent with results of SEM and TEM. The porous structured CNF@NPC with large surface area will adequately contact with the electrolyte and facilitate good transport of electrons/sodium ion [23]. Fig. 4a presents the X-ray diffraction (XRD) pattern of CNF@NPC and CNF. In this XRD pattern of CNF@NPC and CNF, two broad peaks are observed at around 23 and 43 , which can be indexed to (002) and (100), respectively. It is demonstrated that they have semblable crystalline structure. The relatively weak peak at 43 can be ascribed to the low crystalline structure. This XRD pattern indicates an amorphous structure of CNF@NPC and CNF. Based on the XRD result, the interlayer spacing (d002) of CNF@NPC is 0.376 nm by calculation. The larger interlayer spacing than graphite (0.34 nm) makes CNF@NPC a suitable host material for SIBs to store sodium ion. Fig. 4b displays the Raman spectrum of the CNF@NPC, two peaks are observed at 1348 and 1588 cm
1, which are ascribed to D-band (for the sp3 configuration) and Gband (for graphitic configuration), respectively. The ID/IG used to characterize the graphitization of carbon materials value of CNF@NPC is calculated to be 0.87, suggesting a highly disordered

Z. Zhang et al. / Carbon 95 (2015) 552e559

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Fig. 2. Photograph of (a) the bacterial cellulose (BC) membrane, (b) BC nanofiber suspension, (c) BC@PPy precursor. SEM images of (d) pure BC, (e,f) CNF@NPC nanofibres, (f, g) HRTEM image of CNF@NPC nanofiber. (A colour version of this figure can be viewed online.)

structure of the CNF@NPC. The XPS spectra (Fig. 5) are used to confirm the N-doped of the CNF@NPC composite. In the XPS spectrum (inset of Fig. 4a), the peaks center at 284.5, 400.7, and 531.0 eV correspond to C 1 s of sp2 C, N 1 s of the doped N, and O 1 s of the absorbed O, respectively, and the atomic percentage of N in the sample is about 7.02 at%. The XPS spectrum of the C 1 s (Fig. 5a) can be divided into three peaks, which can be classified into the following bands: C in rings without N at 284.5 eV, C singly bound to O in phenol and ether (i.e. CeOH) at 285.5 eV and C doubly bound to O in ketone and quinine (i.e. NeC] O) at 288.0 eV. In the XPS spectrum of the N 1 s (Fig. 5b), four components are observed, suggesting that N atoms are in the four different bonding characters. The N 1 s spectrum is divided into three main peaks at 398.2, 399.6, and 400.7 eV, corresponding to pyridinic, pyrrolic/pyridonic, and graphitic nitrogen atoms,

respectively. The minor N 1 s peak at 404.0 eV originates from the N-oxide of pyridinic nirtrogen [28,40]. According to the XPS analysis, it is indicated that the CNF@NPC composite have successfully realized the N-doped. 3.2. Electrochemical performance of the CNF@NPC anode To analyze the electrochemical properties of the CNF@NPC anode, cyclic voltammetry (CV) tests and galvanostatic charge/ discharge measurements were performed. The CV curves for the first five cycles of the CNF@NPC electrode between 0.01 and 3.0 V are shown in Fig. 6a. For the cathodic process, three peaks locate at round 0.8, 0.25 and 0.01 V are observed during the first scan. The peak at 0.8 V and 0.25 V are due to the decomposition of the ethylene carbonate and diethyl carbonate electrolyte and

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Fig. 3. (a) N2 adsorptionedesorption isotherms and the pore size distribution of CNF and CNF@NPC. (A colour version of this figure can be viewed online.)

formation of a solid electrolyte interface (SEI) film [11,27], which disappear in the following cycles, indicating the reaction to be irreversible. The peak central at 0.01 V in the first scan is due to sodium ion insertion into porous carbon material [11]. In the subsequent cycles, this peak still exists, suggesting the insertion

Fig. 5. (a) the XPS C 1 s spectra of CNF@NPC, the inset picture is the XPS spectrum of CNF@NPC. (b) The XPS N 1 s spectra of the CNF@NPC. (A colour version of this figure can be viewed online.)

of the Naþ is reversible. After the first cycle, there is a peak at around 1.1 V, which is attributed to the reaction between the sodium ion and the surface functional group(s) [41]. The peak is

Fig. 4. (a) XRD patterns of the CNF@NPC and CNF. (b) Raman spectrum of the CNF@NPC. (A colour version of this figure can be viewed online.)

Z. Zhang et al. / Carbon 95 (2015) 552e559

Fig. 6. (a) Typical CV curves of the CNF@NPC electrode at a potential sweep rate of 0.2 mV s
1 and (b) The 1st, 5th and 200th chargeedischarge profiles of the CNF@NPC at 200 mA g
1. The inset picture shows the 5th, 10th, 100th and 200th chargeedischarge profiles of the CNF@NPC. (A colour version of this figure can be viewed online.)

still present in the following cycles, although it becomes less conspicuous, suggesting the reaction to be partially reversible. For the anodic process, Naþ extraction occurs over a broad potential range (0e1.5 V) and the reaction between sodium and the surface functional group(s) at the carbon surface take place at around 1.8 V [11]. Furthermore, after the first cycle, the subsequent CV curves almost coincide with each other, indicating outstanding reversibility of the CNF@NPC during sodium ion insertion and extraction. Fig. 6b exhibits the 1st, 5th and 200th chargeedischarge profiles of the CNF@NPC material within 0.01e3.0 V, at 200 mA g
1. During the first cycle, the CNF@NPC electrode delivers specific discharge and charge specific capacities of 756 and 222.5 mAh g
1, respectively, exhibiting a 29.4% initial coulombic efficiency. The large irreversible capacity loss can be ascribed to the electrolyte decomposition and SEI layer formation. Two main reaction models have been proposed to explain the Naþ insert into the disordered carbon reaction mechanisms [42]: (i) Naþ insertion occurs between the carbon layers corresponding to the slope region of the voltage profile; (ii) Naþ accommodates into the mesopores, which is ascribed to the plateau region of the voltage profile. In the following

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5th, 10th, 100th and 200th cycles, the reversible capacities are stablized and the discharge/charge curves become gentler, just as the CV result demonstrated. In order to gain further insight on the electrochemical performance of the cells, the CNF@NPC cathode was discharged and charged at different rates (Fig. 7a). The CNF@NPC electrode show outstanding capacity retention with high reversible capacities of 240, 183.1 and 153.7 mAh g
1 maintained at 100, 200 and 500 mA g
1 after 100 cycles. Discharge capacity become relatively stable after ~10 cycles at different rate. After several cycles, the coulombic efficiency approaches 100%. In comparison to the CNF anode electrode, The CNF@NPC electrode show much better rate performance (Fig. 7b). When tested at different current densities, the CNF@NPC present high reversible capacities, namely, 258.3, 228.4, 201.5, 171.8 and 146.5 mAh g
1 at 0.05, 0.1, 0.2, 0.5 and 1.0 A g
1, respectively. While cycling at a current density as high as 1000 mA g
1, a capacity of 146.5 mAh g
1 can still be reserved. When the rate is lowered back to 50 mA g
1, the capacity recovers to 239.3 mAh g
1. The better rate capability of the CNF@NPC electrode than the CNF electrode may benefit from abundant porous structure, high surface area, and the appropriate N-doped of the NPC shells [28]. Fig. 7c shows the cycling performances of the CNF and CNF@NPC anode electrode. Obviously, the cycling performance of the CNF@NPC electrode is impressive. When cycling at 500 mA g
1, the CNF@NPC shows an initial discharge capacity of 318.7 mAh g
1 and a stable capacity of 148.8 mAh g
1 preserved after 400 cycles with coulombic efficiencies of nearly 100% after several cycles. The decay rate is 0.13% of initial capacity/per cycle and the specific capacity loss become extremely slow after ~50 cycles, exerting outstanding cycling stability and high capacity with respect to the present work of SIBs [10,11,27,43]. By contrast, the retained specific capacity for the CNF anode is 60.5 mA h g
1 in the 400th cycle, showing a poor cycle performance. After coating with NPC, the properties of CNF are improved [28]. The good electrochemical performance may attribute to the synergies of the CNF cores and NPC outer layer, in other words, the particular coreesheath structure of CNF@NPC anode. As illustrated in Fig. 8, firstly, the inner CNF cores pyrolyzed from BC can ensure an efficient and uninterrupted electron transport and supply a firm 3D matrix. The NPC shells provide Naþ conductive pathways and adequately contact with the electrolyte. The O-containing and N-containing functional groups on the surface of CNF@NPC can facilitate the surface redox reactions [44]. Secondly, the large interlayer spacing between graphene layers of the coresheath structured CNF@NPC accelerates sodium ion transport and storage, which are extremely important for sodium ions since they are much larger than lithium ions [45]. Finally, N-doped can generate extrinsic defects on the graphene layer and form a disordered carbon structure, which further enhances sodium absorption properties [24]. 4. Conclusion The CNF@NPC core-sheath structured composite for SIBs is synthesized by a simple hydrothermal method. The inner CNF cores pyrolyzed from BC can ensure the high electronic conductivity and supply a firm 3D matrix. The outer NPC layers derived from PPy with high surface area and an appropriate N-doped improve the reversible capacity. Benefiting from the core-sheath structure and the large interlayer spacing, the CNF@NPC exhibit a high reversible capacity, including a high reversible capacity (240 mAh g
1 at 100 mA g
1 over 100 cycles), excellent rate capability (146.5 mAh g
1 at 1000 mA g
1), and superior cycling

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Fig. 7. (a) Cycle performance at current density 100 mA g
1, 200 mA g
1 and 500 mA g
1 of the CNF@NPC electrode. (b) Rate performance of the CNF and CNF@NPC electrode. (c) Long cycle performance at 500 mA g
1 of the CNF and CNF@NPC electrode. (A colour version of this figure can be viewed online.)

Fig. 8. Schematic illustration of sodium storage mechanism in CNF@NPC. (A colour version of this figure can be viewed online.)

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