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Ultrafast flame growth of carbon nanotubes for high-rate sodium storage
Journal of Power Sources 439 (2019) 227072
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Short communication
Ultrafast flame growth of carbon nanotubes for high-rate sodium storage Weiwei Han a, b, Dong Chen b, Qifei Li b, Weiling Liu c, Huaqiang Chu a, *, Xianhong Rui b, d, ** a
School of Energy and Environment, Anhui University of Technology, Ma’anshan, 243002, China Guangzhou Key Laboratory of Low-Dimensional Materials and Energy Storage Devices, Collaborative Innovation Center of Advanced Energy Materials, School of Materials and Energy, Guangdong University of Technology, Guangzhou, 510006, China c School of Materials Science and Engineering, Nanyang Technological University, 639798, Singapore d State Key Laboratory of Vanadium and Titanium Resources Comprehensive Utilization, Panzhihua, 617000, Sichuan, China b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� A binder-free anode is developed sodium-ion batteries (SIBs). � The anode is carbon nanotubes directly grown on nickel foam (CNTs-NF). � CNTs-NF is prepared by an ultrafast flame approach. � The CNTs-NF anode has a superior rate capability (95 mA h g 1 at 20 A g 1). � Post-mortem analysis of the sodiated CNTs-NF is carried out.
A R T I C L E I N F O
A B S T R A C T
Keywords: Sodium storage Carbon nanotubes Binder-free High-rate
Sodium-ion batteries (SIBs) are one of the most promising alternatives to lithium-ion batteries as large-scale energy storage systems (ESSs), owing to the natural abundance and low cost of sodium. Because of its capa bility to accommodate large-sized Naþ, amorphous carbon stands out as potential anode for SIBs among the reported carbonaceous materials. However, the practical application of amorphous carbon is still limited by the poor cycling stability and unsatisfactory rate capability. Herein, we report a binder-free SIB anode by directly growing carbon nanotubes on nickel foam (CNTs-NF) via a simple and fast flame approach. Owing to highly disordered turbostratic structures and presence of abundant defects, the CNTs-NF anode delivers excellent electrochemical performance. Electrochemical impedance spectroscopy and cyclic voltammetry demonstrate low charge transfer impedance and surface-induced capacitive behavior, which favor fast transport/diffusion of electrons/ions. Particularly, such anode exhibits a high reversible capacity of 258 mA h g 1 at 0.1 A g 1 with an initial Coulombic efficiency of as high as 82.2% and superior rate capability of 95 mA h g 1 at an ultrahigh current density of 20 A g 1. The high-rate CNTs-NF anode holds great potential in application for large-scale ESSs.
* Corresponding author. ** Corresponding author. School of Energy and Environment, Anhui University of Technology, Ma’anshan, 243002, China E-mail addresses: [email protected] (H. Chu), [email protected] (X. Rui). https://doi.org/10.1016/j.jpowsour.2019.227072 Received 10 July 2019; Received in revised form 16 August 2019; Accepted 26 August 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Journal of Power Sources 439 (2019) 227072
Fig. 1. (a) Schematic illustration of the fabrication process of the CNTs-NF. (b, c) SEM, (d, e) TEM and (f) HRTEM images of the CNTs-NF (inset in (f): SAED pattern).
g., 95 mA h g 1 even at an ultrahigh current density of 20 A g 1). It is believed that such CNTs-NF anode would hold great potential for application in SIBs.
1. Introduction Lithium-ion batteries (LIBs) have now been widely used in portable electronic devices, electric vehicles and large-scale energy storage sys tems (ESSs) [1–3]. However, future application of LIBs may be limited by the insufficient reserve (0.0017 wt%) and uneven distribution of lithium. Recently, alternative battery systems based on a diversity of alkaline metal ions (e.g., Naþ, Kþ, Zn2þ and etc.) have been developed. Among them, sodium ion batteries (SIBs), which use abundant (2.36 wt %) and low-cost sodium, have demonstrated great application potential for ESSs [4–6]. Inspired by LIBs, carbonaceous materials are the first to be consid ered as promising anodes for SIBs. However, due to the large radius of Naþ (1.02 Å) and high standard electrochemical potential ( 2.71 V vs. SHE), the theoretical capacity of graphite for SIBs is only 35 mA h g 1 (NaC60) [7], which is much lower than 372 mA h g 1 (LiC6) in LIBs. Therefore, extensive efforts have been devoted to develop facile and economical methods to prepare advanced carbon-based SIB anodes. To date, amorphous carbon materials, such as hard carbon [8,9] and soft carbon [10], have demonstrated superior capability for the insertio n/extraction of Naþ [11,12]. However, the further application of amorphous carbon in SIBs is still hindered by some challenging issues such as low reversible capacity, poor rate capability and inferior cycling performance. Constructing amorphous carbon materials with appro priate disorder or defects may be critical to improve the electrochemical performance. On the other hand, during the preparation process of conventional electrodes, the active materials are initially mixed with the conductive agents and inactive binders to form a uniform slurry and then pasted on the current collectors. The introduction of insulating binders would decrease the electrical conductivity of the whole electrode to a certain extent. Developing binder-free electrodes with excellent performance would be highly favorable for the practical application of SIBs. At pre sent, most of the binder-free electrodes are constructed by electro spinning [13,14], vacuum filtration [15,16] hydrothermal [17] and sol-gel [18,19] methods, most of which involve complicated processes and require further treatments of electrodes. Herein, we present a facile strategy to fabricate binder-free carbon nanotubes directly grown on nickel foam (CNTs-NF) as an anode ma terial for SIBs. The as-prepared binder-free CNTs-NF anode features a three-dimensional (3D) interconnected CNTs network structure with abundant pores and high degree of disorder, which can effectively facilitate the (de)intercalation of Naþ. As expected, the CNTs-NF ex hibits a reversible specific capacity of 258 mA h g 1 at 0.1 A g 1 (initial Coulombic efficiency is as high as 82.2%) and superior rate capacity (e.
2. Experimental 2.1. Fabrication of CNTs-NF anodes The CNTs-NF anodes were synthesized via a facile one-step flame approach, which is schematically illustrated in Fig. 1a. NFs with di ameters of 12 mm were initially washed with deionized water and ethanol for several times, and then put directly in the interior of the ethanol (purity: 95%) flame for 30 s to fabricate CNTs-NF anodes. 2.2. Materials characterization The phase structure was investigated by X-ray powder diffraction (XRD) on a Bruker AXS D8 advance X-ray diffractometer using Cu Kα radiation. The morphology was investigated by using a scanning elec tron microscopy (SEM) system (JEOL, Model JSM-7600F), and the nanostructure was characterized by using a transmission electron mi croscopy (TEM) system (JEOL, Model JEM-2100). Raman spectra were measured using a HORIBA HR EVOLUTION Raman spectrometer with a laser of 532 nm. The X-ray photoelectron spectroscopy (XPS) analysis was carried out with ESCALAB 250Xi spectrometer. FT-IR spectrum was recorded on a Fourier transform infrared spectrometer (PerkinElmer) with a DGTS detector. Nitrogen adsorption/desorption isotherms were conducted at 77 K (ASAP 2020). 2.3. Electrochemical measurements The CR2032 coin-type cells were assembled in a high-purity argonfilled glove box. The as-prepared CNTs-NF was directly used as the anode. For comparasion, the electrochemical performance of commer cial carbon nanotubes (c-CNTs, the electrical conductivity: ~0.9 � 103 S m 1, and the corresponding XRD and SEM characteriza tion are shown in Fig. S1) was also investigated. The c-CNTs anode was fabricated by mixing 90 wt% c-CNTs with poly(vinylidenefluoride) (PVDF) (10 wt%) in N-methylpyrrolidone (NMP) solvent and then pasted onto the copper foils. It is noted here that the mass loading of active materials was around 1.0 mg cm 2. Sodium foils were used as counter electrodes and Whatman glass fibers were used as separators. The electrolyte solution was made of 1 mol L 1 NaPF6 in 1,2-dimethoxy ethane (DME) solvent. The cells were tested on a NEWARE multichannel battery system. Cyclic voltammetry (CV) measurements were 2
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Journal of Power Sources 439 (2019) 227072
Fig. 2. (a) XRD pattern, (b) Raman spectrum, (c) XPS survey spectrum and (d) high-resolution XPS C1s spectrum of the CNTs-NF.
of CNTs is estimated to be ~259 m2 g 1 with an average pore size of ~4 nm (Fig. S5). Moreover, the four-point-probe measurement depicts high electrical conductivity of these CNTs (~0.4 � 103 S m 1). In addi tion, with a careful observation, as confirmed by TEM, HRTEM images and SAED pattern (Figs. 1e and S6), nickel nanoparticals located at one end of the CNTs are founded, which play a catalytic role for the growth of CNTs. Generally, the growth process involves two steps: (1) hydro carbon is initially cracked to carbon atoms (the main reaction pathways of ethanol oxidation are illustrated in Fig. S7) [20], which then adsor bed, dissolved and/or diffused on the catalyst of nickel nanoparticals; (2) a carbon layer is formed on the catalyst surface to form CNTs (the detailed growth mechanism is shown in Fig. S8) [21,22]. Here, it is worth mentioning that the interior of the ethanol flame is a reductive atmosphere, making NFs to show great catalytic active in the growth of CNTs without oxidation [23]. The phase structure of the CNTs-NF was analyzed by XRD (Fig. 2a). Strong peaks at 2θ of 44.5� , 51.8� and 76.4� are indexed to the cubic nickel (JCPDS No. 65–2865). No obvious carbon peaks are found, which may indicate that the carbon is amorphous. The Raman spectrum in Fig. 2b exhibits two bands centered at 1345 cm 1 and 1599 cm 1, cor responding to the D-band (disordered portion) and the G-band (ordered graphitic structure) of the carbon materials, respectively [24]. The in tensity ratio of the D-band to G-band (ID/IG) is determined to be 1.326, which is much higher than that of N-doped CNTs (ID/IG ¼ 0.85) [25], S, N-codoped carbon nanosheets (ID/IG ¼ 0.97) [26] and comparable to amorphous carbon (ID/IG ¼ 1.11) [12], indicating the presence of abundant disordered carbon, as well as defects and vacancies (being
conducted on an AUTOLAB electrochemical workstation in the voltage range of 0.01–2.5 V at scanning rates of 0.1–2.0 mV s 1. The electro chemical impedance spectra (EIS, frequency range: 0.01–1.0 � 105 Hz) of the as-assembled CR2032 coin-type cells were also evaluated on an AUTOLAB electrochemical workstation with AC voltage amplitude of ~5 mV at fully charged state during the fifth cycle (~2.5 V). 3. Results and discussion The microstructures and morphologies of the CNTs-NF were initially checked by SEM and TEM observations. As depicted in Fig. S2, a 3D skeleton of open-pore interconnected network structure is presented for NFs, and there are many Ni nanoparticles (20–50 nm) on the surface. After burning in ethanol flame for a short time of 30 s, one-dimensional (1D) carbon nanomaterials are observed to grow on the surface of NFs (Figs. 1b and S3). High-magnification SEM images (Figs. 1c and S3) reveal that the 1D carbon nanomaterials exhibit a crowded outgrowth mode with the length approximately ranging from 0.5 to 1.0 μm and the average diameter of 70 nm (Fig. S4). Noted that the 1D nanocarbons directly grown on NFs will be beneficial for the electron conductivity. TEM images in Fig. 1d and e clearly reveal that the 1D nanocarbons have a hollow tubular structure with the inner diameter of about 6 nm, sug gesting the formation of carbon nanotubes (CNTs). The high-resolution TEM (HRTEM) image in Fig. 1f unveils the amorphous feature of CNTs, which is further confirmed by the selected area electron diffraction (SAED) pattern with dispersed diffraction rings (insert of Fig. 1f). Based on the Brunauer-Emmett-Teller (BET) method, the specific surface area 3
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Journal of Power Sources 439 (2019) 227072
Fig. 3. Electrochemical performance of the CNTs-NF and c-CNTs anodes. (a) CV curves (scan rate: 0.1 mV s 1) and (b) voltage profiles (current density: 0.1 A g 1) of the CNTs-NF. (c) Cycling performance at 0.1 A g 1. (d) Rate capability and long cycling performance at 10 A g 1. (e) EIS and corresponding fitted equivalent circuit model (inset). CV curves of (f) CNTs-NF and (g) c-CNTs anodes at various scanning rates, and (h) the corresponding linear relationship between log(i) and log(ν) for cathodic peak at around 0.4 V.
consistent with the HRTEM results). For SIB applicaton, the disordered turbostratic structure and defects/vacancies are favorable for Naþ adsorption and intercalation [27]. FT-IR spectrum (Fig. S9) reveals that – O and there are abundant oxygen-containing functional groups (e.g., C– C–O) on the CNT surfaces. The XPS survey spectrum of the CNTs-NF shows a pronounced C 1s peak at 284.9 eV, and weak O 1s (533.1 eV) and Ni 2p (856.7 eV) peaks (Fig. 2c). Fig. 2d displays the high-resolution XPS C 1s spectrum of the – O and O– – C–O species CNTs-NF, which is fitted to C–C, C–O, C– centered at 284.8, 286.1, 287.3 and 290.8 eV, respectively [13,28]. The high-resolution XPS O 1s spectrum in Fig. S10 is divided into two – O and C–O species at 531.2 and sub-peaks, corresponding to C– 533.4 eV, respectively. The sodium storage properties of the CNTs-NF and c-CNTs anodes were investigated by CV, EIS and galvanostatic charge/discharge mea surements. Fig. 3a presents the CV curves of the CNTs-NF for the first three cycles at a scanning rate of 0.1 mV s 1. In the first cathodic scan, small peak at ~0.7 V is mainly attributed to the formation of a solid electrolyte interphase (SEI) film [29,30]. The broad cathodic peak at about 0.3 V can be assigned to the insertion of Naþ into CNTs [13]. In the anodic scan, two peaks at around 0.3 and 1.3 V are ascribed to the Naþ de-insertion from CNTs, and the electrochemical reaction between the Na ions and the oxygen-containing functional groups on the CNT
surfaces [31,32], respectively. In the following cycles, the CV curves exhibit good repeatability, indicating the high reversibility of the re actions. Fig. 3b illustrates the discharge-charge profiles of the CNTs-NF anode for the first three cycles at a current density of 0.1 A g 1. It de livers initial discharge and charge capacity of 314 and 258 mA h g 1 respectively, corresponding to an initial Coulombic efficiency (ICE) of as high as 82.2%. The high reversibility is mainly ascribed to the usage of ether electrolyte, which can inhibit the decomposition of electrolyte, resulting in the formation of relatively thin SEI film (by comparison, serious irreversible reactions are occurred in ester electrolyte, Fig. S11) [33]. Furthermore, the ICE value achieved here is higher than that of other nanostructured carbon materials reported, such as amorphous carbon (ICE ¼ 79%) [12], carbon nanofibers (ICE ¼ 58.2%) [27], carbon nanotubes (ICE ¼ 39%) [34] and carbon nanosheets (ICE ¼ 44%) [26]. Fig. 3c shows the cycling performance of CNTs-NF anode at a current density of 0.1 A g 1. It maintains a capacity of 234 mA h g 1 after 50 cycles, showing an excellent cycle stability (i.e., capacity retention: 91%). Meanwhile, the ICE increases rapidly to 95% in the second cycle and goes up to 100% within six cycles, demonstrating the high revers ibility of the insertion/de-insertion of Naþ in the CNTs-NF anode. In contrast, as shown in Figs. 3c and S12, the c-CNTs anode exhibits lower ICE (48%) and reversible capacity (e.g., only 134 mA h g 1 after 50 cycles at 0.1 A g 1). 4
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Journal of Power Sources 439 (2019) 227072
Fig. 4. (a) SEM, (b) TEM, (c) HRTEM images, and (d) Raman spectra of the sodiated CNTs-NF.
Fig. 3d presents the sodium storage performance of the CNTs-NF and c-CNTs anodes at different current densities from 0.1 to 20 A g 1. The CNTs-NF anode delivers reversible capacities of 257, 239, 226, 217, 205, 181 and 142 mA h g 1 at 0.1, 0.2, 0.5, 1, 2, 5 and 10 A g 1, respectively. Even at an ultrahigh current density of 20 A g 1, our CNTs-NF anode can still deliver a reversible capacity of 95 mA h g 1. In contrast, the c-CNTs anode exhibits lower capacities of 175, 159, 137, 118, 101, 81, 67 and 50 mA h g 1 at 0.1, 0.2, 0.5, 1, 2, 5, 10 and 20 A g 1, respectively. Furthermore, the high-rate long-term cycling performance of CNTs-NF anode was evaluated (Fig. 3d). At a high rate of 10 A g 1, it shows a reversible capacity of 134 mA h g 1 after 500 cycles, while the c-CNTs anode can only achieve a low capacity of 63 mA h g 1. The above results demonstrate that our CNTs-NF anode has excellent high-rate sodium storage performance, which is also significantly superior to the state-ofthe-art carbon-based nanomaterials (a comparison of the rate capability of our CNTs-NF anode and other carbon-based anodes for SIBs reported recently is summarized in Table S1). To gain more insights into the electrochemical performance, EIS measurements were carried out at the fifth fully charged state. As shown in Fig. 3e, the Nyquist plot contains a semicircle in the high frequency region and a sloping straight line in the low frequency region, repre senting interfacial resistance (R) involving the surface film resistance and internal charge transfer resistance [35] and the Warburg resistance of Naþ diffusion in the electrode [36,37], respectively. The corre sponding fitted equivalent circuit model was shown in the inset of Fig. 3e. The R for the CNTs-NF is only 8.5 Ω, while for the c-CNTs the value is as high as 27.0 Ω, implying faster charge-transfer kinetics for the CNTs-NF, which can be ascribed to the characteristics of binder-free that can reduce internal resistance and increase effective reaction interface, achieving higher utilization and capacity. For better understanding of the superior rate capability and the Naþ storage mechanism, CV measurements were performed at different sweep rates from 0.8 to 2.0 mV s 1 (Fig. 3f and g). The electrochemical process can be qualitatively determined by the following equation [38, 39]:
i ¼ aνb where a and b are adjustable parameters. The b value is determined by the slope of log(i) vs. log(ν). The b value is 0.5, suggesting the diffusioncontrolled process; when b value is 1, indicating a surface-induced capacitive behavior. As shown in Fig. 3h, the b values are calculated to be 0.98 for the CNTs-NF and 0.67 for the c-CNTs, indicating almost entirely surface-induced capacitive behavior of CNTs-NF anodes. It is generally believed that the surface capacitance behavior originates from the Naþ adsorbed on the CNT surface, which is more mobile upon charging and discharging of electrode, especially at high rates. The above results well explain the considerable rate performance of our CNTs-NF anodes even at ultrahigh current density. Additionally, post-mortem SEM and HRTEM of the sodiated CNTs-NF (Fig. 4a–c) indicate the negligible structural change after sodiation for CNTs-NF, which explains its good cycling performance. Raman spec troscopy was also carried out to examine the structural evolution of the CNTs-NF anode after sodiation (Fig. 4d). The slightly decreased ID/IG value can be ascribed to the adsorption of Naþ at surface sites, limiting the breathing motion of sp2 atoms in the rings at edge planes [40,41]. In general, the remarkable rate capability and cycling stability of our CNTs-NF anodes can be attributed to following aspects. Firstly, the CNTs supported by NF provide 3D connection networks, increasing the con tact area between electrolyte and active materials. Secondly, due to the highly disordered turbostratic structure, presence of abundant defects and surface functional groups, the binder-free CNTs-NF anodes show smaller charge transfer impedance and surface-induced capacitive pro cesses. Finally, the network formed from interlacing of nanoscale CNTs not only shortens the Naþ diffusion pathway, but also improves elec tronic conductivity of the electrode. 4. Conclusions In summary, we have developed a facile and novel method to directly 5
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Journal of Power Sources 439 (2019) 227072
grow carbon nanotubes on nickel foam (CNTs-NF). The binder-free CNTs-NF anodes exhibited a high reversible specific capacity (258 mA h g 1 at 0.1 A g 1 with an initial Coulombic efficiency of as high as 82.2%) and superior rate capability (95 mA h g 1 at 20 A g 1). The outstanding electrochemical performances of the CNTs-NF can be ascribed to the unique 3D intertwined networks, presence of abundant defects and disordered turbostratic structure. Considering the facile synthesis, low cost and high-rate performance, our CNTs-NF will be a promising anode for large-scale ESSs, which also provides a new insight toward the preparation of binder-free anodes.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
Ultrafast flame growth of carbon nanotubes for high-rate sodium storage Weiwei Han a, b, Dong Chen b, Qifei Li b, Weiling Liu c, Huaqiang Chu a, *, Xianhong Rui b, d, ** a
School of Energy and Environment, Anhui University of Technology, Ma’anshan, 243002, China Guangzhou Key Laboratory of Low-Dimensional Materials and Energy Storage Devices, Collaborative Innovation Center of Advanced Energy Materials, School of Materials and Energy, Guangdong University of Technology, Guangzhou, 510006, China c School of Materials Science and Engineering, Nanyang Technological University, 639798, Singapore d State Key Laboratory of Vanadium and Titanium Resources Comprehensive Utilization, Panzhihua, 617000, Sichuan, China b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� A binder-free anode is developed sodium-ion batteries (SIBs). � The anode is carbon nanotubes directly grown on nickel foam (CNTs-NF). � CNTs-NF is prepared by an ultrafast flame approach. � The CNTs-NF anode has a superior rate capability (95 mA h g 1 at 20 A g 1). � Post-mortem analysis of the sodiated CNTs-NF is carried out.
A R T I C L E I N F O
A B S T R A C T
Keywords: Sodium storage Carbon nanotubes Binder-free High-rate
Sodium-ion batteries (SIBs) are one of the most promising alternatives to lithium-ion batteries as large-scale energy storage systems (ESSs), owing to the natural abundance and low cost of sodium. Because of its capa bility to accommodate large-sized Naþ, amorphous carbon stands out as potential anode for SIBs among the reported carbonaceous materials. However, the practical application of amorphous carbon is still limited by the poor cycling stability and unsatisfactory rate capability. Herein, we report a binder-free SIB anode by directly growing carbon nanotubes on nickel foam (CNTs-NF) via a simple and fast flame approach. Owing to highly disordered turbostratic structures and presence of abundant defects, the CNTs-NF anode delivers excellent electrochemical performance. Electrochemical impedance spectroscopy and cyclic voltammetry demonstrate low charge transfer impedance and surface-induced capacitive behavior, which favor fast transport/diffusion of electrons/ions. Particularly, such anode exhibits a high reversible capacity of 258 mA h g 1 at 0.1 A g 1 with an initial Coulombic efficiency of as high as 82.2% and superior rate capability of 95 mA h g 1 at an ultrahigh current density of 20 A g 1. The high-rate CNTs-NF anode holds great potential in application for large-scale ESSs.
* Corresponding author. ** Corresponding author. School of Energy and Environment, Anhui University of Technology, Ma’anshan, 243002, China E-mail addresses: [email protected] (H. Chu), [email protected] (X. Rui). https://doi.org/10.1016/j.jpowsour.2019.227072 Received 10 July 2019; Received in revised form 16 August 2019; Accepted 26 August 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Journal of Power Sources 439 (2019) 227072
Fig. 1. (a) Schematic illustration of the fabrication process of the CNTs-NF. (b, c) SEM, (d, e) TEM and (f) HRTEM images of the CNTs-NF (inset in (f): SAED pattern).
g., 95 mA h g 1 even at an ultrahigh current density of 20 A g 1). It is believed that such CNTs-NF anode would hold great potential for application in SIBs.
1. Introduction Lithium-ion batteries (LIBs) have now been widely used in portable electronic devices, electric vehicles and large-scale energy storage sys tems (ESSs) [1–3]. However, future application of LIBs may be limited by the insufficient reserve (0.0017 wt%) and uneven distribution of lithium. Recently, alternative battery systems based on a diversity of alkaline metal ions (e.g., Naþ, Kþ, Zn2þ and etc.) have been developed. Among them, sodium ion batteries (SIBs), which use abundant (2.36 wt %) and low-cost sodium, have demonstrated great application potential for ESSs [4–6]. Inspired by LIBs, carbonaceous materials are the first to be consid ered as promising anodes for SIBs. However, due to the large radius of Naþ (1.02 Å) and high standard electrochemical potential ( 2.71 V vs. SHE), the theoretical capacity of graphite for SIBs is only 35 mA h g 1 (NaC60) [7], which is much lower than 372 mA h g 1 (LiC6) in LIBs. Therefore, extensive efforts have been devoted to develop facile and economical methods to prepare advanced carbon-based SIB anodes. To date, amorphous carbon materials, such as hard carbon [8,9] and soft carbon [10], have demonstrated superior capability for the insertio n/extraction of Naþ [11,12]. However, the further application of amorphous carbon in SIBs is still hindered by some challenging issues such as low reversible capacity, poor rate capability and inferior cycling performance. Constructing amorphous carbon materials with appro priate disorder or defects may be critical to improve the electrochemical performance. On the other hand, during the preparation process of conventional electrodes, the active materials are initially mixed with the conductive agents and inactive binders to form a uniform slurry and then pasted on the current collectors. The introduction of insulating binders would decrease the electrical conductivity of the whole electrode to a certain extent. Developing binder-free electrodes with excellent performance would be highly favorable for the practical application of SIBs. At pre sent, most of the binder-free electrodes are constructed by electro spinning [13,14], vacuum filtration [15,16] hydrothermal [17] and sol-gel [18,19] methods, most of which involve complicated processes and require further treatments of electrodes. Herein, we present a facile strategy to fabricate binder-free carbon nanotubes directly grown on nickel foam (CNTs-NF) as an anode ma terial for SIBs. The as-prepared binder-free CNTs-NF anode features a three-dimensional (3D) interconnected CNTs network structure with abundant pores and high degree of disorder, which can effectively facilitate the (de)intercalation of Naþ. As expected, the CNTs-NF ex hibits a reversible specific capacity of 258 mA h g 1 at 0.1 A g 1 (initial Coulombic efficiency is as high as 82.2%) and superior rate capacity (e.
2. Experimental 2.1. Fabrication of CNTs-NF anodes The CNTs-NF anodes were synthesized via a facile one-step flame approach, which is schematically illustrated in Fig. 1a. NFs with di ameters of 12 mm were initially washed with deionized water and ethanol for several times, and then put directly in the interior of the ethanol (purity: 95%) flame for 30 s to fabricate CNTs-NF anodes. 2.2. Materials characterization The phase structure was investigated by X-ray powder diffraction (XRD) on a Bruker AXS D8 advance X-ray diffractometer using Cu Kα radiation. The morphology was investigated by using a scanning elec tron microscopy (SEM) system (JEOL, Model JSM-7600F), and the nanostructure was characterized by using a transmission electron mi croscopy (TEM) system (JEOL, Model JEM-2100). Raman spectra were measured using a HORIBA HR EVOLUTION Raman spectrometer with a laser of 532 nm. The X-ray photoelectron spectroscopy (XPS) analysis was carried out with ESCALAB 250Xi spectrometer. FT-IR spectrum was recorded on a Fourier transform infrared spectrometer (PerkinElmer) with a DGTS detector. Nitrogen adsorption/desorption isotherms were conducted at 77 K (ASAP 2020). 2.3. Electrochemical measurements The CR2032 coin-type cells were assembled in a high-purity argonfilled glove box. The as-prepared CNTs-NF was directly used as the anode. For comparasion, the electrochemical performance of commer cial carbon nanotubes (c-CNTs, the electrical conductivity: ~0.9 � 103 S m 1, and the corresponding XRD and SEM characteriza tion are shown in Fig. S1) was also investigated. The c-CNTs anode was fabricated by mixing 90 wt% c-CNTs with poly(vinylidenefluoride) (PVDF) (10 wt%) in N-methylpyrrolidone (NMP) solvent and then pasted onto the copper foils. It is noted here that the mass loading of active materials was around 1.0 mg cm 2. Sodium foils were used as counter electrodes and Whatman glass fibers were used as separators. The electrolyte solution was made of 1 mol L 1 NaPF6 in 1,2-dimethoxy ethane (DME) solvent. The cells were tested on a NEWARE multichannel battery system. Cyclic voltammetry (CV) measurements were 2
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Journal of Power Sources 439 (2019) 227072
Fig. 2. (a) XRD pattern, (b) Raman spectrum, (c) XPS survey spectrum and (d) high-resolution XPS C1s spectrum of the CNTs-NF.
of CNTs is estimated to be ~259 m2 g 1 with an average pore size of ~4 nm (Fig. S5). Moreover, the four-point-probe measurement depicts high electrical conductivity of these CNTs (~0.4 � 103 S m 1). In addi tion, with a careful observation, as confirmed by TEM, HRTEM images and SAED pattern (Figs. 1e and S6), nickel nanoparticals located at one end of the CNTs are founded, which play a catalytic role for the growth of CNTs. Generally, the growth process involves two steps: (1) hydro carbon is initially cracked to carbon atoms (the main reaction pathways of ethanol oxidation are illustrated in Fig. S7) [20], which then adsor bed, dissolved and/or diffused on the catalyst of nickel nanoparticals; (2) a carbon layer is formed on the catalyst surface to form CNTs (the detailed growth mechanism is shown in Fig. S8) [21,22]. Here, it is worth mentioning that the interior of the ethanol flame is a reductive atmosphere, making NFs to show great catalytic active in the growth of CNTs without oxidation [23]. The phase structure of the CNTs-NF was analyzed by XRD (Fig. 2a). Strong peaks at 2θ of 44.5� , 51.8� and 76.4� are indexed to the cubic nickel (JCPDS No. 65–2865). No obvious carbon peaks are found, which may indicate that the carbon is amorphous. The Raman spectrum in Fig. 2b exhibits two bands centered at 1345 cm 1 and 1599 cm 1, cor responding to the D-band (disordered portion) and the G-band (ordered graphitic structure) of the carbon materials, respectively [24]. The in tensity ratio of the D-band to G-band (ID/IG) is determined to be 1.326, which is much higher than that of N-doped CNTs (ID/IG ¼ 0.85) [25], S, N-codoped carbon nanosheets (ID/IG ¼ 0.97) [26] and comparable to amorphous carbon (ID/IG ¼ 1.11) [12], indicating the presence of abundant disordered carbon, as well as defects and vacancies (being
conducted on an AUTOLAB electrochemical workstation in the voltage range of 0.01–2.5 V at scanning rates of 0.1–2.0 mV s 1. The electro chemical impedance spectra (EIS, frequency range: 0.01–1.0 � 105 Hz) of the as-assembled CR2032 coin-type cells were also evaluated on an AUTOLAB electrochemical workstation with AC voltage amplitude of ~5 mV at fully charged state during the fifth cycle (~2.5 V). 3. Results and discussion The microstructures and morphologies of the CNTs-NF were initially checked by SEM and TEM observations. As depicted in Fig. S2, a 3D skeleton of open-pore interconnected network structure is presented for NFs, and there are many Ni nanoparticles (20–50 nm) on the surface. After burning in ethanol flame for a short time of 30 s, one-dimensional (1D) carbon nanomaterials are observed to grow on the surface of NFs (Figs. 1b and S3). High-magnification SEM images (Figs. 1c and S3) reveal that the 1D carbon nanomaterials exhibit a crowded outgrowth mode with the length approximately ranging from 0.5 to 1.0 μm and the average diameter of 70 nm (Fig. S4). Noted that the 1D nanocarbons directly grown on NFs will be beneficial for the electron conductivity. TEM images in Fig. 1d and e clearly reveal that the 1D nanocarbons have a hollow tubular structure with the inner diameter of about 6 nm, sug gesting the formation of carbon nanotubes (CNTs). The high-resolution TEM (HRTEM) image in Fig. 1f unveils the amorphous feature of CNTs, which is further confirmed by the selected area electron diffraction (SAED) pattern with dispersed diffraction rings (insert of Fig. 1f). Based on the Brunauer-Emmett-Teller (BET) method, the specific surface area 3
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Fig. 3. Electrochemical performance of the CNTs-NF and c-CNTs anodes. (a) CV curves (scan rate: 0.1 mV s 1) and (b) voltage profiles (current density: 0.1 A g 1) of the CNTs-NF. (c) Cycling performance at 0.1 A g 1. (d) Rate capability and long cycling performance at 10 A g 1. (e) EIS and corresponding fitted equivalent circuit model (inset). CV curves of (f) CNTs-NF and (g) c-CNTs anodes at various scanning rates, and (h) the corresponding linear relationship between log(i) and log(ν) for cathodic peak at around 0.4 V.
consistent with the HRTEM results). For SIB applicaton, the disordered turbostratic structure and defects/vacancies are favorable for Naþ adsorption and intercalation [27]. FT-IR spectrum (Fig. S9) reveals that – O and there are abundant oxygen-containing functional groups (e.g., C– C–O) on the CNT surfaces. The XPS survey spectrum of the CNTs-NF shows a pronounced C 1s peak at 284.9 eV, and weak O 1s (533.1 eV) and Ni 2p (856.7 eV) peaks (Fig. 2c). Fig. 2d displays the high-resolution XPS C 1s spectrum of the – O and O– – C–O species CNTs-NF, which is fitted to C–C, C–O, C– centered at 284.8, 286.1, 287.3 and 290.8 eV, respectively [13,28]. The high-resolution XPS O 1s spectrum in Fig. S10 is divided into two – O and C–O species at 531.2 and sub-peaks, corresponding to C– 533.4 eV, respectively. The sodium storage properties of the CNTs-NF and c-CNTs anodes were investigated by CV, EIS and galvanostatic charge/discharge mea surements. Fig. 3a presents the CV curves of the CNTs-NF for the first three cycles at a scanning rate of 0.1 mV s 1. In the first cathodic scan, small peak at ~0.7 V is mainly attributed to the formation of a solid electrolyte interphase (SEI) film [29,30]. The broad cathodic peak at about 0.3 V can be assigned to the insertion of Naþ into CNTs [13]. In the anodic scan, two peaks at around 0.3 and 1.3 V are ascribed to the Naþ de-insertion from CNTs, and the electrochemical reaction between the Na ions and the oxygen-containing functional groups on the CNT
surfaces [31,32], respectively. In the following cycles, the CV curves exhibit good repeatability, indicating the high reversibility of the re actions. Fig. 3b illustrates the discharge-charge profiles of the CNTs-NF anode for the first three cycles at a current density of 0.1 A g 1. It de livers initial discharge and charge capacity of 314 and 258 mA h g 1 respectively, corresponding to an initial Coulombic efficiency (ICE) of as high as 82.2%. The high reversibility is mainly ascribed to the usage of ether electrolyte, which can inhibit the decomposition of electrolyte, resulting in the formation of relatively thin SEI film (by comparison, serious irreversible reactions are occurred in ester electrolyte, Fig. S11) [33]. Furthermore, the ICE value achieved here is higher than that of other nanostructured carbon materials reported, such as amorphous carbon (ICE ¼ 79%) [12], carbon nanofibers (ICE ¼ 58.2%) [27], carbon nanotubes (ICE ¼ 39%) [34] and carbon nanosheets (ICE ¼ 44%) [26]. Fig. 3c shows the cycling performance of CNTs-NF anode at a current density of 0.1 A g 1. It maintains a capacity of 234 mA h g 1 after 50 cycles, showing an excellent cycle stability (i.e., capacity retention: 91%). Meanwhile, the ICE increases rapidly to 95% in the second cycle and goes up to 100% within six cycles, demonstrating the high revers ibility of the insertion/de-insertion of Naþ in the CNTs-NF anode. In contrast, as shown in Figs. 3c and S12, the c-CNTs anode exhibits lower ICE (48%) and reversible capacity (e.g., only 134 mA h g 1 after 50 cycles at 0.1 A g 1). 4
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Fig. 4. (a) SEM, (b) TEM, (c) HRTEM images, and (d) Raman spectra of the sodiated CNTs-NF.
Fig. 3d presents the sodium storage performance of the CNTs-NF and c-CNTs anodes at different current densities from 0.1 to 20 A g 1. The CNTs-NF anode delivers reversible capacities of 257, 239, 226, 217, 205, 181 and 142 mA h g 1 at 0.1, 0.2, 0.5, 1, 2, 5 and 10 A g 1, respectively. Even at an ultrahigh current density of 20 A g 1, our CNTs-NF anode can still deliver a reversible capacity of 95 mA h g 1. In contrast, the c-CNTs anode exhibits lower capacities of 175, 159, 137, 118, 101, 81, 67 and 50 mA h g 1 at 0.1, 0.2, 0.5, 1, 2, 5, 10 and 20 A g 1, respectively. Furthermore, the high-rate long-term cycling performance of CNTs-NF anode was evaluated (Fig. 3d). At a high rate of 10 A g 1, it shows a reversible capacity of 134 mA h g 1 after 500 cycles, while the c-CNTs anode can only achieve a low capacity of 63 mA h g 1. The above results demonstrate that our CNTs-NF anode has excellent high-rate sodium storage performance, which is also significantly superior to the state-ofthe-art carbon-based nanomaterials (a comparison of the rate capability of our CNTs-NF anode and other carbon-based anodes for SIBs reported recently is summarized in Table S1). To gain more insights into the electrochemical performance, EIS measurements were carried out at the fifth fully charged state. As shown in Fig. 3e, the Nyquist plot contains a semicircle in the high frequency region and a sloping straight line in the low frequency region, repre senting interfacial resistance (R) involving the surface film resistance and internal charge transfer resistance [35] and the Warburg resistance of Naþ diffusion in the electrode [36,37], respectively. The corre sponding fitted equivalent circuit model was shown in the inset of Fig. 3e. The R for the CNTs-NF is only 8.5 Ω, while for the c-CNTs the value is as high as 27.0 Ω, implying faster charge-transfer kinetics for the CNTs-NF, which can be ascribed to the characteristics of binder-free that can reduce internal resistance and increase effective reaction interface, achieving higher utilization and capacity. For better understanding of the superior rate capability and the Naþ storage mechanism, CV measurements were performed at different sweep rates from 0.8 to 2.0 mV s 1 (Fig. 3f and g). The electrochemical process can be qualitatively determined by the following equation [38, 39]:
i ¼ aνb where a and b are adjustable parameters. The b value is determined by the slope of log(i) vs. log(ν). The b value is 0.5, suggesting the diffusioncontrolled process; when b value is 1, indicating a surface-induced capacitive behavior. As shown in Fig. 3h, the b values are calculated to be 0.98 for the CNTs-NF and 0.67 for the c-CNTs, indicating almost entirely surface-induced capacitive behavior of CNTs-NF anodes. It is generally believed that the surface capacitance behavior originates from the Naþ adsorbed on the CNT surface, which is more mobile upon charging and discharging of electrode, especially at high rates. The above results well explain the considerable rate performance of our CNTs-NF anodes even at ultrahigh current density. Additionally, post-mortem SEM and HRTEM of the sodiated CNTs-NF (Fig. 4a–c) indicate the negligible structural change after sodiation for CNTs-NF, which explains its good cycling performance. Raman spec troscopy was also carried out to examine the structural evolution of the CNTs-NF anode after sodiation (Fig. 4d). The slightly decreased ID/IG value can be ascribed to the adsorption of Naþ at surface sites, limiting the breathing motion of sp2 atoms in the rings at edge planes [40,41]. In general, the remarkable rate capability and cycling stability of our CNTs-NF anodes can be attributed to following aspects. Firstly, the CNTs supported by NF provide 3D connection networks, increasing the con tact area between electrolyte and active materials. Secondly, due to the highly disordered turbostratic structure, presence of abundant defects and surface functional groups, the binder-free CNTs-NF anodes show smaller charge transfer impedance and surface-induced capacitive pro cesses. Finally, the network formed from interlacing of nanoscale CNTs not only shortens the Naþ diffusion pathway, but also improves elec tronic conductivity of the electrode. 4. Conclusions In summary, we have developed a facile and novel method to directly 5
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grow carbon nanotubes on nickel foam (CNTs-NF). The binder-free CNTs-NF anodes exhibited a high reversible specific capacity (258 mA h g 1 at 0.1 A g 1 with an initial Coulombic efficiency of as high as 82.2%) and superior rate capability (95 mA h g 1 at 20 A g 1). The outstanding electrochemical performances of the CNTs-NF can be ascribed to the unique 3D intertwined networks, presence of abundant defects and disordered turbostratic structure. Considering the facile synthesis, low cost and high-rate performance, our CNTs-NF will be a promising anode for large-scale ESSs, which also provides a new insight toward the preparation of binder-free anodes.
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