Polyaniline hollow nanofibers prepared by controllable sacrifice-template route as high-performance cathode materials for sodium-ion batteries

Electrochimica Acta 301 (2019) 352e358

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Polyaniline hollow nanofibers prepared by controllable sacrificetemplate route as high-performance cathode materials for sodium-ion batteries Haixia Han a, Haiyan Lu a, Xiaoyu Jiang a, Faping Zhong b, **, Xinping Ai a, Hanxi Yang a, Yuliang Cao a, * a College of Chemistry and Molecular Sciences, Hubei International Scientific and Technological Cooperation Base of Sustainable Resource and Energy, Wuhan University, Wuhan, 430072, China b National Engineering Research Center of Advanced Energy Storage Materials, Changsha, 410205, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 October 2018 Received in revised form 1 February 2019 Accepted 2 February 2019 Available online 4 February 2019

Polyaniline hollow nanofibers (PANI-HNFs) are successfully synthesized by using poly (methyl methacrylate) nanofibers as a sacrifice-template via an in-situ polymerization with a subsequent dissolution process. The polyaniline hollow nanofibers as cathode material for sodium-ion batteries exhibit a high reversible capacity of 153 mA h g
1 at 0.3 C (1C ¼ 150 mA g
1), high cycling stability (73.3% capacity retention after 1000 cycles) and rate capability (70 mA h g
1 at 8 C). The high electrochemical performance of the polyaniline hollow nanofibers originates from the one dimension hollow nanostructures with unique morphologies and highly reversible doping/dedoping property of the conducting polymer, so as to ensure well structural stability and good electrical/ionic conducting connectivity. Furthermore, this strategy can controllably prepare various hollow one dimension structures with different tube diameters and wall sizes, which has potential application for energy storage materials, catalysts, drugdelivered materials and chemical microreactors. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Polyaniline Hollow nanofibers Sacrifice-template Cathode Sodium ion batteries

1. Introduction Sodium-ion batteries (SIBs) have attracted increasing attention recently due to the abundant reserves and low cost of Na resource on the earth compared with those of Li [1e3]. However, many electrode materials for lithium ion batteries (LIBs) display very limited activity toward SIBs because of the larger radius of sodium ion (0.095 nm) than that of lithium ion (0.072 nm), which is a barrier to reversible sodiation and desodiation of electrode materials. Thus, it is crucial to explore new electrode materials with high capacity and excellent cyclic stability for high performance in SIBs [4]. As of now, the research of cathode materials for SIBs is mainly focused on layered transition metal oxides [5,6], polyanion-type compounds [7,8], prussian blue analogues [9] and organic cathode materials [10e12]. Among these cathode materials, organic cathode materials that include radical compounds [13], carbonyl

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (F. Zhong), [email protected] (Y. Cao). https://doi.org/10.1016/j.electacta.2019.02.002 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

compounds [14] and conducting polymers [15] have received considerable attention due to their abundance, environmental friendliness, safety and high theoretical capacity [3,14,16]. Among the many organic cathode materials, PANI as one kind of conducting polymers has been playing a great role in the energy storage and conversion devices because of its high specific capacity (296 mA h g
1), chemical stability, low cost and high electrical conductivity [17e19]. However, repeated doping/de-doping of anion and over-oxidation of PANI at high potential make polymer structure cracking, leading to poor electrical conductivity, which causes inferior cycling stability and rate capability [20]. To address these concerns, one of the effective methods is to modify active group on PANI chains [21]. Zhou et al. [22] synthesized Na-rich PANI cathode by grafting the electron-withdrawing eSO3Na group on PANI chains, which provides reversible insertion/extraction sodium ions, but also as an electron-withdrawing group promotes the electrochemical activation of the polyaniline chains, leading to a high reversible capacity of 133 mA h g
1 at a current density of 50 mA g
1. Zhao et al. [23] reported an o-nitroaniline-grafted PANI, exhibiting a reversible capacity of 180 mA h g
1 at 50 mA g
1 via

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increasing redox potential and enhancing stability of PANI. In addition, PANI can combine with carbon materials to improve its electrochemical performance by strengthening electrical conductivity and flexibility, such as PANI-MWCNT nanocomposites [24], PANI-graphene nanocomposites [25]. The other available strategy is to design nanostructured PANI with well-controlled morphology, which can obtain good electrochemical performance via facilitating electrolyte penetration, enhancing ionic transfer and electrical conductivity of the PANI electrodes [20], such as PANI sponge-like structures [26], whisker-like [27], hollow polyaniline nanofibers [28], nanospheres, nanofibers and nanotubes have been examined [29]. 1D nanomaterials have been recognized as one desirable material for applications in energy storage systems because their unique structure offers shorter ion-pathways, enhanced surface-tovolume ratio and efficient 1D electrical conduction [30,31]. Various methods such as self-assembly [32], electrospinning [33,34] and template-assisted [29] have been employed to prepare PANI 1D nanomaterials. Nevertheless, it is difficult to effectively synthesize long and adjustable PANI hollow nanofibers. Hence, in this work, we report a facile and controllable sacrifice-template strategy by combined electrospinning method to fabricate PANI hollow nanofibers with different diameters and wall sizes. The prepared PANIHNFs as a cathode material for SIBs showed remarkably cycling and rate capability, which was ascribed to the unique hollow 1D nanostructural stability and good ionic/electrical conducting connectivity. 2. Experimental 2.1. Materials preparation 0.9 g PMMA, 9.1 g DMF were mixed to form the electrospinning precursor solution. The solution was stirred for 8 h at room temperature. The electrosinning system (ET-3556H, Ucalery, Beijing, China) was employed. The setup for electrospinning consists of a high voltage, a syringe and a needle and a driving pump. The fluid flow rate was 0.015 mL/min. The applied voltage was 12 kV, and the distance between the spinneret and collection plate was 15 cm. PANI-HNFs were synthesized as flows: firstly, 0.7 g aniline (distilled under reduced pressure before using) was added to 100 mL 1 M HCl solution and then stirred for 0.5 h. Then the electrospinning PMMA nanofibers were transformed to the above solution and stirred overnight until the fibers dispersed uniformly. Later the mixtures were stirred at 4  C in an ice-water bath for 10 min, subsequently ammonium persulfate solution (the mol ratio to aniline was 1.25) cooled in advance in an ice-water bath was gradually added into the mixture. The polymerization reaction was performed in a freezer (4  C) overnight. Finally, the resulting PMMA-PANI core-shell nanofibers were immersed in hydrazine 10 min, then filtered and washed with deionized water several times and dried in vacuum oven at 60  C overnight. The resultant fibers were immersed in toluene for 6 h with gentle stirring at 60  C, so that the PMMA core component was removed to form the PANI-HNFs. PANI nanoparticles (PANI-NPs) were synthesized by using the similar chemical oxidative polymerization method but PMMA nanofibers were not added. 2.2. Materials characterization The morphological feature of the PANI-HNFs was examined by using a scanning electron microscope (FE-SEM, ZEISS Merlin Compact VP, Germany; EDS, Oxford Instruments Link ISIS) and a transmission electron microscope (TEM, JEM-2100 HR). The crystalline structure of the PANI-HNFs was characterized by X-ray

353

diffraction (XRD, Shimadzu XRD-6000) with Cu-Ka radiation. Fourier transform infrared (FT-IR) spectra were recorded on a NICOLET 5700 FTIR spectrometer in the absorption. 2.3. Electrochemical measurements The electrodes consisted of 70 wt% active material (PANI-HNFs), 20 wt% super P and 10 wt% poly-tetrafluoroethylene (PTFE). The mass loading of the active material in the electrode was about 1.6 mg cm
2. The charge-discharge performances of the electrode were carried out by 2016 coin-type cells using the cathode as the working electrode and a Na disk as the counter electrode, the electrolyte was 1 M NaClO4 dissolved in ethylene carbonate (EC)e diethyl carbonate (DEC) (1: 1 by volume) with 5 vol% fluoroethylene carbonate (FEC). All the cells were assembled in a glove box with water/oxygen content lower than 0.1 ppm and tested at room temperature. The galvanostatic chargeedischarge tests were performed in the range between 2.5 and 4.2 V at different current densities on a Neware cycler (Shenzhen Neware Electronics Co., China). Cyclic voltammetric measurements were examined with the coin cells at different scan rates using an Autolab PGSTAT128 N (Eco Chemie, Netherlands). 3. Results and discussion The fabrication scheme of PANI-HNFs is showed in Fig. 1a. It can be seen that PANI was coated on the surface of PMMA nanofiber through polymerization reaction to form coaxial PMMA@PANI nanofiber. And then the internal PMMA was dissolved to obtain a PANI hollow nanofiber. The FESEM image (Fig. 1b) shows that bare PMMA nanofibers prepared by the electrospinning method have uniform and long-rang structure with about 0.8 mm in diameter. After surface polymerization of aniline, coaxial PMMA@PANI nanofibers were easily obtained (Fig. 1c), which do not have any changes in morphology and structure, except for an increase in diameter (the inset of Fig. 1c). Subsequently, the PANI-HNFs were synthesized by removing the internal PMMA through a dissolution process (Fig. 1d). As can be seen, the as-prepared PANI-HNFs exhibit a uniform tube-like structure with about 1.2 mm in diameter and several micrometers in length (Fig. 1d), suggesting that the fiber morphology is well maintained compared with the PMMA nanofibers (Fig. 1b, ~800 nm in diameter) and PMMA@PANI nanofibers (Fig. 1c, ~1.2 mm in diameter). Fig. 1e shows TEM image of PANIHNFs, which further confirmed the hollow structure of PANI with average wall-thickness of about 180 nm. The images in Fig. 1 indicate the successful preparation of PANI-HNFs by using PMMA nanofibers as sacrificial template. The XRD pattern and FT-IR of PANI-HNFs are shown in Fig. 2. The broad peak of PANI-HNFs centered at 20 is attributed to the scattering of PANI molecular chains, which is the characteristic Bragg diffraction peak (020) of PANI (Fig. 2a) [35,36]. The weak and broad characteristics of diffraction peak indicates a typical amorphous state of the PANI-HNFs. From FT-IR spectra (Fig. 2b), there are five apparent absorption peaks observed at 1600, 1500, 1290, 1160 and 820 cm
1. Among these peaks, the bands at 1160 and 820 cm
1 are associated with the bending of CeH in-plane and out-of-plane of aromatic ring [37]. The band at 1290 cm
1 is assigned to the CeN stretching of an aromatic amine, 1600 and 1500 cm
1 belong to the C]C stretching of quinonoid (Q) and benzenoid (B) rings [38]. The above mentioned peaks represent a typical PANI structure [37,39]. The intensity ratio of the 1500 cm
1 to the 1600 cm
1 peak (I1500/ I1600) is sufficiently high, indicating that the PANI polymer exists mainly in a reduced state [22]. To evaluate the electrochemical performance of PANI-HNFs as cathode material for SIBs, 2016 coin-type Na/PANI-HNFs half-cells

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Fig. 1. (a) Scheme for the fabrication of the PANI-HNFs. FESEM images of (b) bare PMMA nanofibers, (c) PMMA@PANI nanofibers and (d) PANI-HNFs. (e) TEM image of PANI-HNFs.

Fig. 2. Structural characterization of the PANI-HNFs. (a) XRD diffraction pattern and (b) FT-IR spectra of the PANI-HNFs.

were assembled. The cyclic voltammetry (CV) curves of PANI-HNFs in the first 3 cycles at a scan rate of 0.1 mV s
1 in a voltage range of 2.5e4.2 V were shown in Fig. 3a. The main CV features of PANI appeared as two pairs of broad redox peaks at 3.2/2.7 V (denoted as Ⅰ/Ⅳ) and 4.0/3.8 V (denoted as Ⅱ/Ⅲ), similar to the CV response of PANI electrodes in prior literature as shown in Table. S1, which are associated with the doping/dedoping reactions of ClO
4 anions into/ from the para-disubstituted benzene ring and quinone diimine structures of the polymer, respectively (Fig. 3b) [18]. The CV curves in the subsequent cycles remained almost unchanged (Fig. 3a), demonstrating high reversibility of the doping/dedoping reactions

for PANI-HNFs. Fig. 3c shows the charge-discharge curves of PANIHNFs at a current of 20 mA g
1 between 2.5 and 4.2 V. The PANIHNFs electrode delivers a high charge and discharge capacity of 230 and 180 mA h g
1 respectively in the initial cycle with a coulombic efficiency of 78.3%. In the subsequent cycles, the coincidence of the discharge curves is in good agreement with the CV results, indicating that the electrode has a good reversibility. To better understand the insufficient redox reaction of PANI-HNFs in the charge-discharge process, the PANI-HNFs electrodes at different potential depths were characterized by FT-IR analysis. The spectra have characteristic peaks from where the effect of doping

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Fig. 3. (a) Cyclic voltammetry curves for the first 3 cycles of the PANI-HNFs at 0.1 mV s
1. (b) The main reaction mechanism of the PANI-HNFs. (c) Charge and discharge curves of the PANI-HNFs at 0.13 C. (d) Rate performance of the PANI-HNFs. (e) Cycling performance of the PANI-HNFs electrodes at 5.3 C. FESEM images of PANI-HNFs electrode (f) before and (g) after cycles.

on PANI with ClO
4 can be evaluated. As Fig. S1 shows, on comparison with the as-prepared PANI-HNFs electrode (Fig. S1a), the characteristic peaks in the spectra of the electrode first charged to 4.2 V (Fig. S1b) are shifted to lower wave numbers. The bands at 1600 cm
1 (N-Quinoid ring (Q)eN stretching) and 1500 cm
1 (Nbenzenoid ring (B)eN stretching) are shifted to 1566 cm
1 and

1465 cm
1, and an obviously decreased intensity ratio of I1465/I1566 is observed, suggesting the conformational change from the benzenoid structure to the quinonoid structure of the polyaniline backbone [37,40]. A relatively small shift in the wavenumbers is also observed in the CeH region that 820 cm
1 (out of plane bending of CeH of aromatic ring) is shifted to 792 cm
1. Doping

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delocalization of the ring electrons takes place that decreases ring electron density so that absorption bands appear at the lower wave numbers. The significant lowering of the wave numbers is attributed to effective doping [40,41]. In addition, a new 1074 cm
1 band featuring the CleO bond vibration in ClO
4 anions also appeared noticeably. These results demonstrate that the doping reaction of ClO
4 anions occurs during charging process. When discharged to 3.4 V (Fig. S1c), the characteristic peaks of PANI are slightly shifted to higher wave numbers that compared with the electrode charged to 4.2 V, suggesting a partial of ClO
4 anions dedoped from the PANI chains in the discharge process. At the fully discharged state (2.5 V) (Fig. S1d), the PANI-HNFs electrode shows a very similar FT-IR pattern to its pristine state (Fig. S1a), except that the intensity ratio of I1500/I1600 is slightly smaller than the pristine value and the band at 1074 cm
1 (CleO) is observed, indicating an increase of the number of quinonoid structure in the discharged PANI due to the partial oxidation of materials and a part of the active sites on the PANI chains were irreversibly occupied by ClO
4 anions after the first cycle [40,41]. The result is in agreement with the charge and discharge curves (Fig. 3c, with a coulombic efficiency of 78.3% in the initial cycle), demonstrating that the incomplete doping/dedoping and partial oxidation of materials during the charge-discharge process with the decrease of the active sites cause lower reversible capacity of PANI than its theoretical capacity. Fig. 3d displays the rate capability of PANI-HNFs electrode. The PANI-HNFs electrode exhibits a reversible capacity of 153, 147, 135, 117, 90, 70 and 55 mA h g
1 at 0.3, 0.7, 1.3, 2.7, 5.3, 8 and 10.7 C (1C ¼ 150 mA g
1), respectively. When returning to the low current density of 100 mA g
1, the capacity can still be recovered to the initial capacity. For comparison, the PANI nanoparticles were also synthesized by using the similar chemical oxidative polymerization method but PMMA nanofibers were not added that show the granular morphology of agglomerated nanoparticles (70e100 nm) (Fig. S2). It can be found that the PANI-HNFs also exhibit more excellent rate capacity compared with the PANI nanoparticles (Fig. S3). The excellent rate performance of PANI-HNFs is mainly attributed to its unique hollow fiber structure, which is in favor of the formation of the good electronic and ionic conductive network. To further evaluate the long-cycle performance, the cycling behavior of the PANI-HNFs electrode at 5.3 C (800 mA g
1) in the voltage range of 2.5e4.2 V is shown in Fig. 3e. It is observed that the PANI-HNFs electrode exhibits great long-term cycling performance with a capacity retention of 73.3% over 1000 cycles. The above electrochemical tests showed that the PANI-HNFs cathode had excellent electrochemical performance, compared to most of the previous reports on PANI cathodes as shown in Table. S1. To prove the structural stability of this material during the cycles, the SEM images of PANI-HNFs electrode before and after cycles are shown in Fig. 3f and g, respectively. After 1000 cycles, the shape of PANI-

HNFs electrode can still be well maintained without any mechanical breaking, further demonstrating the stable electrochemical properties of this material (Fig. 3g). Overall, the PANI-HNFs cathode displays excellent cycling stability and good rate capability, due to uniform hollow structure to alleviate volume change during charging and discharging, which shows promising application in energy storage. In order to further investigate the reaction properties of PANIHNFs electrode, the CV curves of PANI-HNFs electrode at different scan rates are shown in Fig. 4a. It can be clearly seen that the curves at different scan rates are similar in shape, which indicating that PANI-HNFs electrode has a good electrochemical reversibility and fast kinetics. Usually the peak current (i) and sweep rate (v) obey the following relationship [42]:

iðVÞ ¼ avb

(1)

logðiÞ ¼ logðaÞ þ blogðvÞ

(2)

Both a and b are empirical parameters. There are two welldefined conditions: b ¼ 0.5, represents a diffusion-controlled behavior, whereas b ¼ 1 indicates an ideal pseudocapacitive effect. Fig. 4b shows the relationship of log (v) and log (i) at the anodic and cathodic peaks, which the b-values for the anodic and cathodic reactions are determined to be 0.89, 0.72, 0.76 and 0.77 at the Ⅰ, Ⅱ, Ⅲ and Ⅳ(denoted in Fig. 5a), respectively. It was further confirmed that the doping/dedoping reactions of anion occurred at the redox peaks, and capacity was contributed by the capacitance and diffusion. According to the following equation we can obtain the contribution of capacitance to the capacity [43]:

iðVÞ ¼ k1 v þ k2 v1=2


.  i v1=2 ¼ k1 v1=2 þ k2

(3)

In equation (3), k1v and k2v1/2 represent the pseudocapacitive effect and diffusion-controlled insertion processes, respectively [44]. It can be calculated that the pseudocapacitive contributions for PANI-HNFs electrode are 78.6, 81.4, 84.3, 87.4, and 90.3% at the scan rates of 0.1, 0.2, 0.3, 0.4, and 0.5 mV s
1, respectively (Fig. 4c). The capacitance contribution increases as the sweep speed increase because of the rapid charge/discharge characteristics associated with capacitive processes [42]. Besides, the PANI 1D hollow structure provides a large number of active sites in contact with electrolyte to increase interfacial adsorption of anions, which promotes the electrochemical reaction. According to above discussion, it is easy to find that hollow fiber structure can realize stable cycling and rate performance for PANI electrode, which originates from fluent liquid-phase ionic diffusion, short solid-state ionic diffusion pathway and long-range electrical conductivity. The hollow fiber structure can be easily prepared by

Fig. 4. (a) CV curves at different sweep rates increasing from 0.1 to 0.5 mV s
1. (b) Plots of log (i) vs. log (v) (peak current: i, sweep rate: v) of the PANI-HNFs. (c) The percentages of pseudocapacitive contributions at different sweep rates.

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Fig. 5. FESEM images of difference diameters for (a) ~0.4 mm and (b) ~2 mm of PANI-HNFs.

using PMMA nanofibers as the sacrifice template via an in-situ polymerization with a subsequent dissolution process. Moreover, the sacrifice template method can easily tune tube diameter and wall thickness. Fig. 5 shows the FESEM images of different diameters (0.4 and 2 mm) of PANI-HNFs with wall thickness about 140 nm, which obtained in the sacrifice template approach. Therefore, we can easily obtain a series of PANI-HNFs with different diameters and wall thicknesses by adjusting the diameter of PMMA fibers and the amount of aniline for surface polymerization. In consequence, the hollow fibers with different tube diameter and wall thickness can provide different reaction and transport space, which are widely applied in catalysis, biomaterial, drug delivery and energy storage fields.

[6]

[7]

[8]

[9]

[10] [11]

4. Conclusions [12]

In summary, PANI-HNFs with adjustable hollow structure were synthesized via a sacrifice-template method. The typical PANIHNFs as cathode for sodium-ion batteries exhibited a high reversible capacity of 153 mA h g
1 at 0.3 C (50 mA g
1), rate capability (70 mA h g
1 at 8 C) and excellent cycling stability (73.3% capacity retention after 1000 cycles). The excellent electrochemical performance of PANI-HNFs is mainly due to its unique hollow fiber structure, which increases the contact area between material and electrolyte, and forms good electronic and ionic conductive network. Moreover, the simple and effective method may also prepare a series of PANI-HNFs with different diameters and wall thicknesses to apply in biomaterial, microreactors and other energy fields. Acknowledgements This work was support by National Key Research Program of China (No. 2016YFB0100200), JCKY2016130B010 and the National Nature Science Foundation of China (Nos. 21333007 and 21673165).

[13]

[14]

[15] [16]

[17] [18] [19] [20]

[21]

[22]

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.02.002.

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