Fabrication of Carbonaceous Nanotubes and Mesoporous Nanofibers

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Fabrication of Carbonaceous Nanotubes and Mesoporous Nanofibers as Stable Anode Materials for Lithium Ion Battery Jing Hu, Changzhen Shao, Baozong Li, Yonggang Yang, and Yi Li ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01060 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 17, 2018

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ACS Applied Nano Materials

Fabrication of Carbonaceous Nanotubes and Mesoporous Nanofibers as Stable Anode Materials for Lithium Ion Battery Jing Hu, Changzhen Shao, Baozong Li, Yonggang Yang and Yi Li*

Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P.R. China.

ABSTRACT The morphologies and pore architectures of carbon nanostructures could be precisely controlled via a self-assembly process. This work presented resorcinol-formaldehyde resin-silica composite nanofibers, which were synthesized through a sol-gel method under the help of co-structure directing agent (CSDA) (S)-β-citronellol and n-hexanol. The self-assembly process and the nanostructure of obtained composite nanofibers were changed by adding different CSDA. After carbonization and getting rid of silica, carbonaceous nanotubes and mesoporous nanofibers were obtained, respectively. Their electrochemical performance was tested as anode materials for lithium-ion batteries (LIBs). Stable discharge capacities of 574.3 mA h g-1 for carbonaceous nanotubes and 609.9 mA h g-1 for mesoporous carbonaceous nanofibers after 300 cycles were achieved at current density of 0.1 A g-1. Thus, a facile method of designing and fabricating excellent carbon anode candidates for LIBs was provided. 1

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KEYWORDS: sol-gel preparation; self-assembly; carbonaceous nanotubes; mesoporous nanofibers; lithium-ion batteries; anode material

1. INTRODUCTION

One-dimensional (1D) silica nanostructures with a high geometrical aspect ratio, such as nanotubes, nanobelts, nanorods and nanofibers, have been applied as novel units of nanodevices and feasible hard templates for 1D nanostructures1-3, therefore, they have attracted lots of researchers to investigate their special structures and applications. Generally, these 1D silica nanostructures can be synthesized via a sol-gel method4-12. The self-assembly of amphiphilic surfactants such as cetyl trimethylammonium chloride (CTAC) and cetyl trimethylammonium bromide (CTAB) are used as the templates, in the assistance of co-structure directing agents (CSDAs) such as ethyl acetate4, perfluorinated acids (PFOA) 5-8 and alkanols9, 10. The aggregation behavior between the surfactant and CSDA reduces the surface free energy and advances the formation of helical morphology. As reported, by changing the weight ratio of CSDA to surfactant, rather than the physical confinement at the nanoscale13, or chiral surfactants and chiral micelles at the molecular scale14, the morphology of silica nanostructures can be preciously tuned. For example, Yu et al. found that when the weight ratio of CTAB to PFOA was increased the CTAB-PFOA co-assembly transformed from helical hexagonal structure to vesicular structure11.

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In recent years, polymer-silica organic-inorganic micro/nano-architectures have received considerable attentions because they have the characteristics of both organic components and inorganic ones, which enable materials with a wealth of enhanced properties15-17. The typical methods to synthesize polymer-silica nanocomposites include blending18, in-situ polymerization19 and sol-gel process20-21. Especially, sol-gel process is implemented in the presence of polymeric inorganic precursors or organic precursors with functional groups. Due to the tremendous different reaction conditions between organic and inorganic precursors, it’s hard to create polymer-silica nanocomposites via one-step sol-gel way. However, in synthesizing colloidal silicas and resorcinol-formaldehyde (RF) resins, though they own very different reaction rates, the numerous similarities in their reaction mechanisms and reaction conditions suggest that a one-step synthesis of RF resin-silica nano-composites is feasible. For example, Sevilla et al.22 reported core@shell structured RF resin-silica nanospheres under typical Stöber conditions. Zhang et al.23 reported SiO2@SiO2/RF core−shell structured nanospheres using tetrapropyl orthosilicate as silica precursor. Noonan et al.24 demonstrated a feasible in-situ Stöber templating approach to prepare silica/polydopamine composite nanospheres. Up to date, the reports on 1D polymer-silica nanocomposites are still rare25, and the influence of CSDAs on the co-assembly in polymer-silica system needs further study.

The polymer-silica nanocomposites can be used as good carbon sources to fabricate carbons with special morphologies and pore structures, while the silica acts as bones or pore-makers26-29. As reported, mesoporous carbon nanospheres and 3



hollow carbon nanospheres are successfully produced by this “silica-assisted” method, using cationic surfactants (CTAB or CTAC) as the template, tetraethyl orthosilicate (TEOS) as the inorganic precursor, and polymer oligomer (such as phenolic resols, RF resin and 3-aminophenol- formaldehyde resin) as the organic precursor. For example, Qian et al.24 changed the molar ratio of TEOS/resorcinol in the reaction mixture and found that the cavity structure and particle size of mesoporous carbon nanospheres were varied. Chen et al.26 used 3-aminophenol-formaldehyde resin/silica composite as starting material to produce highly monodispersed N-containing hollow mesoporous carbon spheres. They tuned the particle sizes and shell thicknesses of the carbon spheres by adjusting the volume ratio of ethanol/water.

In this work, firstly, through a one-pot sol-gel approach, RF resin-silica composite nanofibers were prepared using resorcinol，formaldehyde and TEOS as precursors, CTAB as the template, (S)-β-citronellol or n-hexanol as the CSDA. The as-prepared composites were then carbonized at 600 °C and treated with HF to remove silica moiety, and carbonaceous nanotubes and mesoporous nanofibers were obtained finally. The co-assembly behavior during the reaction process was discussed, indicating the structure change of final carbonaceous products with the CSDA. The electrochemical performance of these carbons was also evaluated as anodes for lithium-ion batteries (LIBs). The results showed that they possessed high Li-storage capability and superior cycling stability, which might be ascribed to their special nanostructures. 4


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2. EXPERIMENTAL SECTION

2.1 Materials

HF aqueous solution (40 wt%), ammonium hydroxide (25-28 wt%), CTAB, TEOS, resorcinol and formaldehyde (37-40 wt%) were purchased from Sinopharm Chemical Reagent Co., Ltd., (S)-β-citronellol was purchased from Aladdin Industrial Corporation.

2.2 Preparation of RF resin-silica composite nanofibers, carbonaceous nanotubes and mesoporous nanofibers

The RF resin-silica composite nanofibers were synthesized according to the literature9. Typically, 182 mg of CTAB was added into a pre-mixture of 40 mL deionized water and 3.0 mL ammonia aqueous solution, and dissolved under stirring at 80 °C. After 15 min, 22.5 µL of (S)-β-citronellol was added into the mixture and 0.50 mL of TEOS was added 30 min later. Resorcinol-formaldehyde mixed alcohol solution, containing 100 mg of resorcinol, 140 µL of formaldehyde and 2.0 mL of ethanol, was then added dropwise into the solution. After standing at 80 ºC for 2.0 h, the mixture was filtrated and washed with water and ethanol, and yellow powder was obtained. The as-prepared product was extracted with the mixture of HCl and ethanol for 48 h, then dried at 50 ºC. The obtained RF resin-silica composite was then heated to 600 ºC under Ar atmosphere to produce carbon-silica composite, which was then immersed in 5.0 wt% HF solution for 24 h to throughly remove the silica. The final black product was washed with deionized water to neutral followed by drying in 5



vacuum at 40 ºC. The obtained RF resin-silica nnaofibers, carbon-silica nanofibers, carbonaceous nanotubes were donated as RSNFs-cn, CSNFs-cn, CNTs, respectively.

To prepare mesoporous carbonaceous nanofibers (denoted as CNFs), the starting RF resin-silica composite nanofibers (named as RSNFs-hex) were synthesized in the same way except that n-hexanol was used as CSDA instead of (S)-β-citronellol, and the molar ratio of n-hexanol to CTAB is 1/1. After the same post-treatment as above mentioned, carbon-silica nanofibers (named as CSNFs-hex) and CNFs were produced.

Herein, "cn" and "hex" were used as suffix to distinguish the resin-silica and carbon-silica nanofibers synthesized with (S)-β-citronellol (-cn) and n-hexanol (-hex) as the CSDA.

2.3 Characterization

Field emission scanning electron microscope (FE-SEM, Hitachi S-4800) and transmission electron microscope (TEM, FEI Tecnai G220) measurement were performed to inspect the morphology and structure. Raman spectra were measured on a RENISHAW Raman spectrometer (Ar+ laser source, λ= 532.5 nm). Wide angel X-ray diffraction (WAXRD, X’Pert-Pro MPD X-ray diffractometer) patterns were recorded using Cu Kα radiation (λ=0.154 nm). Thermal gravity analysis (TGA) was performed on a TG/TGA 6300 instrument. X-ray photoelectron energy spectrum (XPS) was measured with a VGESCALAB 220i-XL instrument. Nitrogen sorption isotherms were measured on a Micromeritics Tristar II 3020 instrument. The specific 6


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surface area was calculated using a Brunauer-Emmett-Teller (BET) method, while the pore size distribution was counted from the adsorption branch.by a Barrett-Joyner-Halon (BJH) method.

2.4 Electrochemical measurements

CR2016 coin-type cells were assembled in an Ar-filled glove box, using CNTs or CNFs electrode as working electrode, metallic lithium foil as counter electrode and Celgard 2325 porous film as separator. A solution of 1.0 M LiPF6, which is dissolved in an ethylene carbonate and diethyl carbonate mixed solvent (1:1 by colume), was used as the electrolyte. For the preparation, CNTs (or CNFs), acetylene black, and binder (polyvinylidene fluoride dissolved in N-methylpyrrolidone) were mixed evenly at the mass ratio of 8:1:1 to form a slurry, which was then coated on a Cu foil current collector uniformly followed by drying overnight under vacuum. To perform the electrochemical tests, after assembling, the qualified cells were charged and discharged between 0.01 and 3.0 V.

3. RESULTS AND DISCUSSIONS

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Scheme 1. Illustration of the synthesis process of CNTs and CNFs. As illustrated in Scheme 1, the aimed CNTs and CNFs are synthesized from the same starting materials under the help of different CSDAs. The morphologies and pore structures of as-obtained samples can be identified from Figure 1 and 2. RF resin-silica composites prepared using (S)-β-citronellol as CSDA (RSNFs-cn) are helical nanofibers (Figure 1a and d). The diameters and lengths of RSNFs-cn are 60-85 nm and 400-700 nm, respectively. We also changed the molar ratio of (S)-β-citronellol to CTAB in the reaction mixture, the morphologies of obtained RF resin-silica composites are changed, as shown in Figure S1 and S2. When no CSDA is added, the RF resin-silica composites are nanospheres. At the molar ratio of 0.1/1 (Figure S2a), the RF resin-silica composite are nanorods with diameters and lengths of 60-110 and 160-400 nm, respectively. When the molar ratio increases to 1/1, the homogeneity of RF resin-silica composite nanorods gets worse (Figure S2b). In our previous work10, helical mesoporous hybrid silica nanofibers were prepared using CTAB as the template under the assistance of CSDA, (S)-β-citronellol. We found that the (S)-β-citronellol could reduce the CTAB critical micelle concentration and enhance the formation of rod-like micelles. The main driven force was attributed to the surface free energy reduction as well as the entropy increase. Although it was reported that the tendency to form single-handed helical nano-structures could be enhanced by increasing the interactions between chiral dopant and CTAB30, however, since the interaction between CSDA and CTAB is too weak, the number of

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left-handed helical nanofibers is nearly the same to that of right-handed ones as calculated from Figure 1.

Figure 1. (a-c) FE-SEM and (d-f) TEM images of RSNFs-cn (a, d), CSNFs-cn (b, e) and CNTs (c, f). The molar ratio of (S)-β-citronellol to CTAB is 0.25/1. After removal of surfactant followed by carbonization at 600 ºC in Ar, carbonsilica composite nanofibers (CSNFs-cn, Figure 1b and e) were obtained. They maintain the fiber-like morphology of RSNFs-cn, and along the axial direction inside nanofibers, well-ordered mesoporous channels can be observed clearly (Figure 1e). CSNFs-cn sample was then treated with HF to get rid of silica moiety, and tubular cavities are left, as shown in Figure 1c and 1f. The obtained product (CNTs) are nanotubes. Their diameters and shell thickness are 50-75 and 5-10 nm, respectively. TGA, WAXRD and Raman analysis results (Figure S3 and S4) show that the walls of CNTs sample are mainly composed of amorphous carbon. 1.95 wt% of nitrogen is revealed by the elemental analysis, which is originated from the residue of CTAB. To

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further study the structure of CSNFs-cn, it was calcinated to get rid of carbon moiety, and helical mesoporous silica nanofibers were left (as shown in Figure S5a and S5c). Therefore, it can be deduced that the carbon-silica composite nanofibers are core-shell structure, comprising a helical mesoporous silica core and a carbon shell. That is, the original RF resin-silica composite nanofibers are core-shell structure. Thus, a possible formation process of RSNFs-cn is proposed. In the basic solution, the template CTAB molecules self-assemble into long rod-like micelles under the help of (S)-β-citronellol. The ammonium ions disperse uniformly on the outside surface of the micelles. The hydrolyzed silicate anions and phenolic anions adsorb onto the self-assemblies through electrostatic interactions. Herein, the hydrolyzation and polymerization rate of TEOS is much larger than that of resorcinol, therefore, a mesoporous silica nanofiber core is formed firstly. Thereafter, the RF resin oligomers deposit onto the surface of silica nanofibers. After further condensation, core-shell structure composite nanofibers are formed finally.

Figure 2. (a-c) FE-SEM and (d-f) TEM images of RSNFs-hex (a, d), CSNFs-hex (b, e) 10


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and CNFs (c, f). The molar ratio of n-hexanol to CTAB is 1/1. Helical RF resin-silica composite nanofibers prepared with n-hexanol as CSDA (RSNFs-hex) are displayed in Figure 2a and 2d. Their lengths and diameters are 350-800 nm and 45-75 nm, respectively. Similar to CNTs, helical carbonaceous nanofibers (CNFs) containing 1.72 % of nitrogen were produced after carbonization and removal of silica. Irregular mesopores with average size of 5-20 nm within the nanofibers can be identified in the TEM image (Figure 2f). These formation of the mesopores are ascribed to the removal of silica moiety. Unlike CNTs, mesopores in the CNFs didn’t fuse together to form a through-full inner cavity, indicating that the formation mechanism of RSNFs-hex is different from that of RSNFs-cn. During the preparation process of RSNFs-hex, under the help of n-hexanol, the hydrolyzation and polymerization rate of TEOS is almost equivalent to that of resorcinol/formaldehyde. Therefore, the hydrolyzed silicate oligomer and RF resin oligomers co-assemble with CTAB micelles simultaneously and polymerize together, and interpenetratedstructured RF resin-silica nanocomposites are gained. The woven structure of RSNFs-hex sample is also proved by its calcination product, as shown in Figure S5b and S5d. Unlike helical silica nanofibers with ordered mesoporous channels calcinated from RSNFs-cn (Figure S5a and S5c), the silicas calcinated from RSNFs-hex are helical nanofibers with disordered pores uniformly distributed on the walls. Obviously, these disordered pores are formed due to the removal of organic moiety.

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Figure 3. (a) Nitrogen sorption isotherms and (b) BJH pore distribution plots for CNTs and CNFs. Figure 3a shows the nitrogen sorption isotherms of CNTs and CNFs. CNTs display a type-IV curve with a distinct H2b hysteresis loop, indicating the existence of ink-bottle type mesopores. On the corresponding BJH pore distribution plot calculated from the adsorption branch (Figure 3b), a sharp peak appears at about 42 nm, which is corresponding to the inner radium of nanotubes. For CNFs, type-IV curves with distinct H3 hysteresis loop can be observed. Two capillary-condensation steps are identified at relative pressure (P/P0) of 0.45 to 0.70, and 0.90 to 1.00in Figure 3a. The former indicates the existence of mesopores, while the latter is originated from the voids among and within the CNFs. Most pore sizes are less than 10 nm for CNFs, as observed in its BJH pore distribution plot. The BET specific surface areas and pore volumes of CNTs and CNFs are 691.4 and 901.2 m2 g-1, and 1.869 and 0.816 cm3 g-1, respectively. In this work, since the cavities and mesopores within the carbons are formed due to the etching off silica, we can control the pore size and distribution by tuning the addition amount and time of TEOS in the reaction mixture. As reported, the porous nanoarchitecture can provide highly efficient ion-highways and benefit 12


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electrolyte diffusion and transport31, 32. Moreover, their large pore volumes can accommodate the strain relaxation during charge and discharge. We expect that the obtained carbons can show good electrochemical performance when applied as electrode materials.

Figure 4. (a, c) XPS survey spectra and (b, d) N 1s core level XPS spectra of CNTs (a, b) and CNFs (c, d). XPS spectra were measured and analyzed to investigate the elemental composition and bonding configuration of CNTs and CNFs. In XPS survey spectra (Figure 4a and 4c), typical peaks for C1s, N1s and O1s are shown. The N1s high-resolution XPS spectrum of CNTs (Figure 4b) is curve-fitted into three-type peaks, and three nitrogen electronic states: pyridinic-like (398.6 eV), pyrrolic-like (399.5 eV), and graphitic13



like (400.2 eV) are discerned. For CNFs, two-type peaks can also be decomposed and assigned to pyridinic-like nitrogen (398.6 eV) and pyrrolic-like nitrogen (399.9 eV), as shown in Figure 4d.

Figure 5. (a, b) Cycle performance of the CNTs and CNFs electrodes at a current density of 0.1 A g-1 in the voltage range of 0.01-3.0 V (versus Li/Li+); (c) Rate performance of the CNTs and CNFs electrodes at current densities varied from 0.5 A g-1 to 4 A g-1. Nano-carbons can be applied in LIBs as anode materials. Herein, to investigate the lithium storage performances of CNTs and CNFs, we carried out a series of 14


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electrochemical tests. The cyclic voltammograms (Figure S6) and galvanostatic discharge/charge profiles (Figure S7) for CNTs and CNFs electrodes were collected. The cycle performance (Figure 5a) was examined at a current density of 0.1 A g-1. The CNTs electrode exhibited high initial capacities of 1046.2 mAh g-1 (discharge) and 413.0 mAh g-1 (charge). The initial Coulombic efficiency is 39.5%. This low value is ascribed to the formation of solid electrolyte interphase (SEI) layer accompanying the electrolyte decomposition33, leading to the obvious discharge capacity loss in the next five cycles34. After a slight decrease, the capacity begins to increase slowly from the 61st cycle. The discharge capacity reaches 574.3 mAh g-1 at the 300th cycle, while the Coulombic efficiency keeps 98.1%. For the CNFs electrode, the first discharge capacity, 1357.3 mAh g-1, and charge capacity, 576.4 mAh g-1 are gained with a Coulombic efficiency of 42.5%. Apparent capacity dropping is observed in the next 10 cycles, then the capacity keeps at ~400 mAh g-1 till the 50th cycle. Thereafter, the capacity increases gradually and stabilizes at 609.9 mAh g-1 till the 300th cycle. Such capacity increasing phenomena, commonly being observed in metal-oxide anodes35 and reported in our previous work on nano-carbons36, are ascribed to electrode materials activation 37 and high-rate lithiation-induced reconstruction38.

To further verify the durability of CNTs and CNFs electrodes, they were examined at a higher rate of 0.3 A g-1 for longer cycles, as presented in Figure S8. After 500 cycles, CNTs electrode retains a discharge capacity of approximately 491.1 mA h g-1, while CNFs electrode shows a value of 456.6 mA h g-1. The obtained CNTs 15



and CNFs electrodes exhibit superior capacitance than commercial graphite anode, and comparable to many reported carbon anode materials such as ordered mesoporous carbon39, carbon nanotubes40, graphene sheets41 and heteroatoms doped graphitic carbon nanostructures42. The cycling stability of obtained CNTs and CNFs are comparable to or better than many reported high-performance N-graphene43, microporous carbon44 and hierarchical carbonaceous nanotubes45. Figure 5c shows the rating capability of CNTs form 0.1 and 4 A g-1. For CNTs, along with the increase of the current density, discharge capacities of 339.2 mA h g-1 (at 0.1 A g-1), 172.4 mA h g-1 (at 0.5 A g-1), 132.0 mA h g-1 (at 1 A g-1), and 76.8 mA h g-1 (at 4 A g-1) are achieved. The discharge capacity restores 177.1 mA h g-1 when the test current returns to 0.5 A g-1. The CNFs electrode displays inferior rate performance. It exhibits decent capacity retention: 142.6, 110.3 and 60.1 mA h g-1 as the current density increase from 0.5, 1 to 4 A g-1. The discharge capacity restores 168.5 mA h g-1 when the current density reverts to 0.5 A g-1, similar to that of the CNTs electrode.

Figure 6. Nyquist plots of CNTs and CNFs electrodes: (a) before and (b) after cycling. 16


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The Nyquist plots of CNTs and CNFs electrodes before cycling and after 300 cycles are shown in Figure 6. To our knowledge, the inclined line related to the diffusion coefficient appears at low frequency, which is connected to the Li+ diffusion process. The semicircle at the high frequency corresponds to Rct, which has a bearing on the electrochemical reaction at the solid/liquid interface. The starting impedance of CNTs is much lower than that of CNFs electrodes before cycling (191Ω vs. 351Ω), as shown in Figure 6a, possible reason is that CNTs have a higher N content than CNFs. As reported, nitrogen atom may efficiently enhance both the surface polarity and the electrical conductivity36, 38, therefore benefit the electrochemical performance. After 300 cycles, the impedances of CNTs and CNFs electrodes decrease to 32 and 62 Ω (Figure 6b). The reduced semicircles of both CNTs and CNFs after cycling suggest gradual improvement of the charge transport during cycling46. The observed improvement in capacity retention can also be ascribed to this reason.

4. CONCLUSIONS

This work shows a facile way to fabricate carbonaceous nanotubes and mesoporous carbon nanofibers from the same starting material, helical RF resin-silica composite nanofibers. By adding different CSDAs and tuning the molar ratio of template to CSDA, the self-assembly process changes, and the morphology and structure of the composites nanofibers vary synchronously. After carbonization and removal of silica, nano-carbons with different morphology and pore architecture are obtained. The

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carbonaceous nanotubes and mesoporous nanofibers electrodes are used in LIBs and exhibit superior Li-storage capability and cycling stability.

ASSOCIATED CONTENT

Supporting Information.

SEM images of RF resin-silica composite nanospheres and nanorods prepared using different precursors and adding different amount of CDSA; SEM and TEM images of silica nanofibers calcinated from CSNFs; TGA curves, WAXRD patterns, Raman spectra of CNTs and CNFs; Cyclic voltammograms for CNTs and CNFs electrodes; 1st, 2nd and 3th discharge and charge curves at 0.1 A g-1and cycle performance at 0.3 A g-1 for CNTs and CNFs electrodes.

AUTHOR INFORMATION

Corresponding Author Y. Li, Tel: +(86) 512 65882052; E-mail: [email protected] ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 21574095 and 51473106), and the Priority Academic Program Development of Jiangsu High Education Institutions (PAPD).

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Table of contents graphic

Carbonaceous nanotubes and mesoporous nanofibers were fabricated from the same RF resin-silica composites under the help of different co-structure directing agents.

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Fabrication of Carbonaceous Nanotubes and Mesoporous Nanofibers

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