Winding Aligned Carbon Nanotube Composite Yarns into Coaxial ...

Letter pubs.acs.org/NanoLett

Winding Aligned Carbon Nanotube Composite Yarns into Coaxial Fiber Full Batteries with High Performances Wei Weng, Qian Sun, Ye Zhang, Huijuan Lin, Jing Ren, Xin Lu, Min Wang, and Huisheng Peng* State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science and Laboratory of Advanced Materials, Fudan University, Shanghai 200438, China S Supporting Information *

ABSTRACT: Inspired by the fantastic and fast-growing wearable electronics such as Google Glass and Apple iWatch, matchable lightweight and weaveable energy storage systems are urgently demanded while remaining as a bottleneck in the whole technology. Fiber-shaped energy storage devices that can be woven into electronic textiles may represent a general and effective strategy to overcome the above difficulty. Here a coaxial fiber lithium-ion battery has been achieved by sequentially winding aligned carbon nanotube composite yarn cathode and anode onto a cotton fiber. Novel yarn structures are designed to enable a high performance with a linear energy density of 0.75 mWh cm−1. A wearable energy storage textile is also produced with an areal energy density of 4.5 mWh cm−2. KEYWORDS: Carbon nanotube, composite yarn, fiber shaped, textile, energy storage

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a huge volume change during the lithiation/delithiation process, resulting in a fast capacity decay.13 To this end, a second phase has been often incorporated to accommodate the Si volume change particularly by designing new interfacial structures at nanoscale, e.g., the formation of voids between two phases and production of scaffolds based on the second phase.14−17 Carbon nanotube (CNT) represents one of the most explored second phases in the Si accommodation because of the remarkable mechanical and electronic properties.18,19 For instance, Si had been deposited onto robust CNT scaffolds that were made of aligned CNT arrays or sheets, and high electrochemical stability was achieved during use.14,15 The aligned structure also favored a rapid charge transport with high electrochemical activity.20 Inspired by the above strategy in the planar LIBs, a coaxial, fiber-shaped full LIB is here developed by winding two aligned CNT composite yarns onto a cotton fiber. A hybrid layered structure is designed to make aligned CNT/Si composite yarn anode that can effectively accommodate the big volume change of Si. In the meantime, lithium manganate (LMO) due to its combined high safety, working voltage, structural stability, and low cost21 has been incorporated into aligned CNT yarn as the cathode. This coaxial fiber full LIB exhibits a linear capacity density of 0.22 mAh cm−1 or a linear energy density of 0.75 mWh cm−1. The fiber LIBs are further woven into a flexible energy storage textile with an areal energy density of 4.5 mWh cm−2.

ately, fancily integrated with clothes, glasses, and watches, the newborn wearable electronics represents a significant trend in the accelerated development of electronic technology. Having been or to be applied in healthcare, entertainment, communication, and military, the wearable electronics will potentially change our life and society far beyond the imagination we can conceive now.1 To this end, the textile technology that started from the dawn of civilization and has become a sophisticated and large-scale industry may provide a ubiquitous platform for the development in the wearable electronics. For instance, energy storage systems in a textile format can be directly assembled into the wearable electronic devices to satisfy the need for lightweight and high deformability.2,3 In the traditional textile technology, polymer fibers are the basic materials to be woven into clothes and other flexible structures. Therefore, it arouses a broad interest in the development of fiber-shaped energy storage devices that were also proposed to be woven into textiles.2,4−9 Some attempts were already made to fabricate fiber-shaped electrochemical capacitors (also named supercapacitors) typically by twisting two fiber electrodes.2,4−6 In contrast, it is rare to realize fibershaped lithium-ion batteries (LIBs) that are also highly desired. Compared with supercapacitors, LIBs possess higher energy densities and lower self-discharge losses that are necessary for mobile electronic facilities and various other electronic devices.10,11 However, it remains challenging to realize fibershaped LIBs possibly due to the difficulty in finding matching materials and designing appropriate structures. Silicon is widely studied as a promising electrochemically active material in the planar LIBs due to the highest theoretical specific capacity.12 However, as widely recognized, Si undergoes © 2014 American Chemical Society

Received: March 14, 2014 Revised: April 27, 2014 Published: May 15, 2014 3432

dx.doi.org/10.1021/nl5009647 | Nano Lett. 2014, 14, 3432−3438

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Figure 1. Schematic illustration of the synthesis of high-performance composite yarns: (a) CNT-LMO composite yarn; (b) CNT-Si/CNT composite yarn with a hybrid layered structure. Pink, blue, and yellow colors correspond to CNT, LMO, and Si, respectively. LMO is the abbreviation of lithium manganate.

Figure 2. (a) SEM image of bare CNT sheet. (b) SEM image of Si-coated CNT sheet. (c, d) SEM images of CNT-Si/CNT composite yarn with a hybrid layered structure at low and high magnifications, respectively. (e, f) SEM images of CNT-Si composite yarn without the hybrid layered structure at low and high magnifications, respectively. (g, h) SEM images of CNT-LMO composite yarn at low and high magnifications, respectively. 3433


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Figure 1 schematically shows the synthesis of cathode and anode composite yarns. Two different yarn structures were designed to incorporate electrochemically active materials for a high cyclic performance. Spinnable CNT arrays were synthesized by chemical vapor deposition,22 and the CNTs showed a multiwalled structure with a diameter of ∼12 nm (Figure S1). Aligned CNT sheets were continuously dry-drawn from the array with widths up to centimeters and lengths of meters. LMO undergoes less than 10% volume change during lithiation/delithiation cycling.23 Therefore, LMO particles were directly deposited onto the aligned CNT sheet, followed by scrolling into a CNT-LMO composite yarn (Figure 1a). On the contrary, a hybrid layered structure was developed to accommodate the Si that may undergo a dramatic volume change up to 300%.14 Si was first coated onto the CNT sheet by electron beam evaporation (Figure 1b). The Si-coated CNT sheets were then sandwiched between two bare CNT sheets and further scrolled into a composite yarn with the designed hybrid layered structure (denoted as CNT-Si/CNT composite yarn). A high alignment was demonstrated in the CNT sheets drawn from CNT arrays (Figure 2a). A lot of voids could be observed typically with sizes of hundreds of nanometers among the aligned CNTs. The voids were obviously decreased while the diameters of aligned CNTs had been increased after the deposition of Si (Figure 2b). However, the alignment was maintained. The loading amount of the Si was controlled by varying the deposition time. Si could be uniformly coated on the surfaces of aligned CNTs (Figures 2b and S2). Interestingly, after the Si coat, a coaxial structure was produced with the CNT as the core while Si as the sheath (Figure S2). As a comparison to CNT-Si/CNT composite yarn, the Si-coated CNT sheets could be also directly scrolled into a composite yarn without the hybrid layered structure when bare CNT sheets had not been used (denoted as CNT−Si composite yarn, Figure S3). For the convenience of the following comparing study, the CNT-Si/CNT and CNT-Si composite yarns were made to share an average diameter of ∼100 μm and Si weight percentage of 62% determined by thermogravimetric analysis (Figure S4). As expected, the CNTs and coaxial CNT/Si nanotubes remained highly aligned in the CNT-Si/CNT and CNT-Si composite yarns (Figures 2c−f). The maintained aligned structures were critically important for the composite yarns to form the desired hybrid layered structure. Figure 2g shows a typical SEM image of the resulting CNT-LMO yarn also with an average diameter of about 100 μm. Similar to the CNT-Si/ CNT composite yarn, the CNTs also remained aligned after deposition of LMO particles (Figure 2h). The LMO particles with an average diameter of 400 nm were mainly incorporated among the aligned CNTs. The LMO has a weight percentage of 75% in the CNT-LMO composite yarns (Figure S4). Figure 3a compares Raman spectra before and after coating of Si onto the CNTs. The characteristic peak centered at 513 cm−1 that corresponded to the Raman phonon vibration of crystalline Si24 was observed for the composite yarn. Interestingly, the intensity ratios of G to D bands in CNTs were increased after the coating of Si. Impurities such as amorphous carbon and defects or disordered structures of graphene sheets in CNTs will increase the intensity of D band. As the impurities are difficult to be removed during heating treatment,25 the increased G/D intensity ratios may be explained by the defect healing in CNTs during the coat of

Figure 3. (a) Raman spectra of bare CNT and CNT-Si composite yarns. (b) X-ray diffraction pattern of LMO.

Si at elevated temperatures, leading to a less disordered carbon in the composite yarn. For LMO, the X-ray diffraction pattern demonstrated a spinel structure of LiMn2O4 (Figure 3b).26 Half-cells were used to investigate the electrochemical property of the composite yarns that acted as an electrode with a lithium wire as the counter electrode. Lithiation/ delithiation was used to characterize the half-cell. In contrast, charge/discharge was used for the full cell. For the half-cell based on the CNT-Si and CNT-Si/CNT composite yarns, the current density and specific capacity were calculated on the basis of the total weight of active materials including both CNT and Si. The used specific capacities were 150 and 4200 mAh g−1 for the CNT (Figure S5) and Si, respectively. Therefore, the current density of 1 C for the CNT-Si and CNT-Si/CNT composite yarns (Si, 62 wt %) was calculated as 2661 mA g−1. For the half-cell based on the CNT-LMO composite yarn, the current density and specific capacity were calculated on the basis of the weight of active material of LMO alone, i.e., 148 mA g−1 at 1 C. The hybrid layered structure was found to be critical for a high electrochemical performance. The CNT-Si/CNT composite yarns were first studied in a voltage window of (0.005−3) V versus Li/Li+. Figure 4a represents typical cyclic voltammograms recorded at a rate of 0.1 mV s−1. The current peak between 1.0 and 0.5 V at the first lithiation curve was derived from the formation of solid electrolyte interface layers induced by Si and CNTs.27 The peaks at 0.17 and 0.04 V corresponded to the phase transition in amorphous LixSi and formation of crystalline Li15Si4, respectively,28,29 while the broad anodic peak at 0.6 V indicated a phase transition from amorphous LixSi to amorphous Si.29 Figure 4b shows the cyclic performance at 0.4 C. A high delithiation capacity of 2240 mAh g−1 (or 3521 mAh g−1 based on the Si) was obtained at the first cycle with 88% 3434


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Figure 4. Electrochemical performances of CNT-Si and CNT-Si/CNT composite yarns in a voltage window of (0.005−3.0) V versus Li/Li+. (a) Cyclic voltammograms of CNT-Si/CNT composite yarn at a rate of 0.1 mV s−1. (b) Comparison of cyclic performances between CNT-Si and CNTSi/CNT composite yarns at 0.4 C. (c) Long-life performance of the CNT-Si/CNT composite yarn at 2 C. (d) Rate capability of the CNT-Si/CNT composite yarn. (e, f) Structural evolutions of CNT-Si and CNT-Si/CNT composite yarns after 50 cycles at 0.4 C, respectively.

retention after 100 cycles for the CNT-Si/CNT composite yarn. In contrast, only 42% capacity was retained for the CNTSi composite yarn under the same condition. Figure 4c further shows the long-life performance of the half-cell based on the CNT-Si/CNT composite yarn at 2 C. A delithiation capacity of 1523 mAh g−1 was maintained after 400 cycles with capacity retention of 85%. Figure 4d exhibits the rate capability of the CNT-Si/CNT composite yarn at the increasing current densities from 0.2 to 5 C. A delithiation capacity of approximately 1000 mAh g−1 was achieved even at 5 C, indicating a good rate performance. To better understand the remarkable electrochemical properties derived from the hybrid layered structure in the CNT-Si/CNT composite yarn, the structural evolutions were also compared for the two kinds of composite yarns after 50 cycles at 0.4 C. Without the hybrid layered structure, a majority of Si layers were peeled off from the aligned CNTs (Figure 4e). In contrast, after the introduction of the hybrid layered structure, the Si layer had been stably maintained (Figure 4f), indicating that the designed structure can effectively accom-

modate the volume change of Si layers. Here the bare CNT sheet that is sandwiched between two Si-coated CNT sheets serves to buffer the volume change of Si and clamp the Si layer, so it is a key to control the thickness of the bare CNT sheet. Two layers of bare CNT sheets are used in this work. When only one layer is used, the electrochemical stability is much lower; when more than two layers are used, a similar high stability is achieved but with much lower specific capacities (Figure S6). Electrochemical properties of CNT-LMO composite yarns were investigated in a voltage window of 3.3−4.3 V versus Li/ Li+ in a half-cell. Figure 5a shows the voltage profiles of the CNT-LMO composite yarn at 0.5 C. Two distinct potential plateaus were clearly observed in both delithiation and lithiation curves. For a lithiation process, two characteristic plateaus at 4.1 and 3.9 V indicated a two-step process, i.e., from λ-MnO2 to Li0.5Mn2O4 and then to LiMn2O4.30 Figure 5b further shows a good rate capability with lithiation capacities of 100, 95, and 87 mAh g−1 at 0.5, 1, and 2 C, respectively. It also exhibited a good recovery. The long-life test showed a lithiation 3435


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as a demonstration. The coaxial fiber full LIB can be further sealed with an insulating layer. The mechanical properties of cotton fibers and CNT-LMO and CNT-Si/CNT composite yarns are compared in Figure S9. The tensile strengths of two composite yarns are both higher than that of the cotton fiber, which is ascribed to excellent mechanical properties of wound CNTs outside. The full cell was galvanostatically cycled at a voltage range of 2.0−4.3 V at a current density of 1 C (148 mA g−1) relative to the cathode. The length density of the coaxial fiber full LIB with a diameter of 2 mm was 27 mg cm−1 including 0.18 and 2.1 mg cm−1 for Si and LMO, respectively. Figure 6b shows the voltage profile of a full cell that was operated at an average voltage of 3.4 V, which corresponded to the potential difference between the CNT-LMO composite yarn electrode (4.0 V vs Li/Li+) and CNT-Si/CNT composite yarn electrode (0.6 V vs Li/Li+). A specific discharge capacity of 106.5 mAh g−1 was achieved at the first cycle based on the cathode. The long-life measurement showed that the capacity was retained by 87% after 100 cycles (Figure 6c), which agrees with the high stability of the CNTSi/CNT composite yarn anode. The full cell is designed on the basis of the capacity limited by the CNT-LMO cathode as the CNT-LMO composite exhibits a higher cyclic stability than CNT-Si/CNT, and excess Si-based electrode capacity can prevent lithium deposition during the charging process.31,32 Consequently, the capacities of CNT-Si/CNT anodes are selected to be approximately 2 times those of the CNT-LMO cathodes. To be mentioned, a full cell with excessive CNTLMO capacity is found to exhibit a lower cyclic stability than the one with excessive CNT-Si/CNT capacity (Figure S10). Based on the specific discharge capacity of 106.5 mAh g−1, a linear capacity density of 0.22 mAh cm−1 or a linear energy density of 0.75 mWh cm−1 and a gravimetric energy density of 27.7 mWh g−1 based on the total weight of the full cell were produced from the fiber LIB. Moreover, if based on all electrode materials, the volumetric and gravimetric energy densities were calculated as 99.3 mWh cm−3 and 242 mWh g−1, respectively. Though the linear capacity density (0.22 mAh cm−1) was smaller than that of the cable-type LIB using Cu wire as the skeleton (1 mAh cm−1),7 it can be enlarged by decreasing the pitch distance or increasing the diameter of composite yarn. As an example, the linear capacity density achieved 0.36 mAh cm−1 by decreasing the pitch distance from 300 to 200 μm for the same composite yarn (Figure S11). The fiber LIB had been further demonstrated to lighten up a light emission diode (Figure 6d). The coaxial fiber LIBs could be further woven into a flexible textile (Figure 6e), and an areal energy density of 4.5 mWh cm−2 could be achieved from the LIB text in an interlaced form with a separation distance of 3.3 mm (Figure S12). In conclusion, a family of aligned CNT composite yarns is developed to exhibit remarkable electrochemical properties, e.g., high specific capacities and capacity retentions. Two aligned CNT-LMO and CNT-Si/CNT composite yarns are sequentially wound onto a cotton fiber to make a coaxial fiber full battery. The fiber full LIB demonstrates a high linear energy density of 0.75 mWh cm−1, and it has been further woven into an energy storage textile with an areal energy density of 4.5 mWh cm−2. These novel fiber-shaped and textile energy storage devices show promising applications in a wide variety of fields, particularly, portable and wearable electronics. Experimental Section. Lithium manganate (LMO) nanoparticles were synthesized by a hydrothermal method. LiOH

Figure 5. Electrochemical performances of CNT-LMO composite yarns at 3.3−4.3 V versus Li/Li+: (a) voltage profiles at 0.5 C; (b) rate capability; (c) long-life performance at 1 C.

capacity of 101 mAh g−1 at the first cycle with a Coulombic efficiency of 92%, and the capacity was maintained by 94% after 100 cycles at 1 C (Figure 5c). The high electrochemical properties may be ascribed to the robust structures and efficient conducting pathways derived from the aligned CNTs. A coaxial fiber full LIB was finally fabricated based on the CNT-Si/CNT and CNT-LMO composite yarns as the anode and cathode, respectively (Figure 6a). Because of the light weight, widespread use, and commercial availability with low cost, a cotton fiber was used as the substrate that had been further covered with a shrinkable tube as a protecting layer. The resulting fiber substrate showed a diameter of 1 mm (Figure S7). The CNT-LMO and CNT-Si/CNT composite yarns were sequentially wound onto the cotton fiber while separated with a gel electrolyte. Figure S8 shows the morphology of a CNTLMO composite yarn that was wrapped on a cotton fiber. The linear capacity densities of fiber-shaped LIBs are increased with the decreasing pitch distance of the winded composite yarn. Herein, a pitch distance of ∼300 μm had been mainly studied 3436


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Figure 6. (a) Schematic illustration to the fabrication of the coaxial fiber full LIB. Red, blue, and yellow colors correspond to cotton fiber, CNTLMO composite yarn, and CNT-Si/CNT composite yarn, respectively. (b) Voltage profiles of the fiber-shaped LIB at 1 C between 2.0 and 4.3 V. (c) Long-life performance of the fiber-shaped LIB between 2.0 and 4.3 V at 1 C. (d) Photograph of a fiber-shaped LIB to lighten up a light emission diode. (e) Fiber-shaped full LIBs being woven into a textile.

(0.377 g) and MnO2 (1.37 g) were first dissolved in H2O (40 mL), followed by adding glucose (0.2 g) and H2O (40 mL), and reacted at 200 °C for 24 h. The resulting solid products were filtered and washed with distilled water and ethanol for at least three times. The weights of CNT-Si, CNT-Si/CNT, and CNT-LMO composite yarns were measured by a microbalance (Sartarious SE2, resolution of 0.1 μg). The weight percentages of Si and LMO in composite yarns were determined by thermogravimetric analysis. The structures were characterized by Raman microscopy (Renishaw, inVia) and X-ray diffraction (Bruker, D8 ADVANCE). The morphologies were characterized by scanning electron spectroscopy (SEM, Hitachi, 4800-1) and transmission electron microscopy (TEM, JEOL, JEM-2100F). Half-cells were used to investigate the electrochemical property of the composite yarn that directly acted as an electrode with a lithium wire as the counter electrode. LiPF6 in a mixture solvent of ethylene carbonate and diethyl carbonate (50/50, v/v) with a concentration of 1 M was used as the electrolyte. CNT-LMO and CNT-Si/CNT composite yarns served as cathode and anode in the full cell, respectively. The gel electrolyte was made as follows.33 1 M LiClO4 solution in ethylene carbonate/diethyl carbonate (50/50, v/v) was first prepared; 0.8 g of poly(vinylidene fluoride-co-hexafluoroprophlene) (PVDF-HFP) was dissolved in 12 mL of tetrahydrofuran; the LiClO4 solution was mixed with PVDF−HFP solution by a volume ratio of 1/6. After stirring for 12 h, a clear solution was obtained and further vacuum-dried to remove the solvent to form a gel electrolyte. Both half-cells and full cells were assembled in an argon-filled glovebox (Mikrouna, Super 1220/750) with both moisture and oxygen to be less than 1 ppm.

Electrochemical measurements were made in a BT2000 from Arbin. Galvanostatic measurements were carried out at a voltage range of 0.005−3.0 V versus Li/Li+ for CNT-Si and CNT-Si/CNT composite yarns, 3.3−4.3 V versus Li/Li+ for CNT-LMO composite yarns, and 2.0−4.3 V for full cells. For the full cell (limited by the cathode), the current density and specific capacity were based on the weight of the active cathode material. Prior to the assembly of the full cell, the CNT-Si/ CNT yarn anode was lithiated/delithiated for five cycles using lithium wire as the counter electrode to eliminate its irreversible capacity. To trace the structural evolution during cycling, the electrodes were soaked in acetonitrile overnight to wash the residual electrolyte and then cleaned by dipping them in an aqueous solution of acetic acid (1 mM) to remove the solid electrolyte interface layer and other impurities. Finally, they had been rinsed with deionized water prior to the measurement.34

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ASSOCIATED CONTENT

S Supporting Information *

Preparation of composite yarns, fabrication of full cells and characterization of CNT, coaxial CNT/Si nanotube, CNT-Si composite yarn, fiber battery, and textile battery. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (H.P.). Notes

The authors declare no competing financial interest. 3437


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(28) Du, C. Y.; Gao, C. H.; Yin, G. P.; Chen, M.; Wang, L. Energy Environ. Sci. 2011, 4, 1037−1042. (29) Nguyen, H. T.; Zamfir, M. R.; Duong, L. D.; Lee, Y. H.; Bondavalli, P.; Pribat, D. J. Mater. Chem. 2012, 22, 24618−24626. (30) Tang, H.; Chang, Z.; Zhao, H.; Yuan, X.-Z.; Wang, H.; Gao, S. J. Alloys Compd. 2013, 566, 16−21. (31) Huang, X.; Lin, M.; Tong, Q.; Li, X.; Ruan, Y.; Yang, Y. J. Power Sources 2012, 202, 352−356. (32) Xiang, H. F.; Zhang, X.; Jin, Q. Y.; Zhang, C. P.; Chen, C. H.; Ge, X. W. J. Power Sources 2008, 183, 355−360. (33) Yang, Y.; Jeong, S.; Hu, L. B.; Wu, H.; Lee, S. W.; Cui, Y. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 13013−13018. (34) Choi, J. W.; McDonough, J.; Jeong, S.; Yoo, J. S.; Chan, C. K.; Cui, Y. Nano Lett. 2010, 10, 1409−1413.

ACKNOWLEDGMENTS This work was supported by MOST (2011CB932503), NSFC (21225417), STCSM (12 nm0503200), the Fok Ying Tong Education Foundation, the Program for Special Appointments of Professors at Shanghai Institutions of Higher Learning, and the Program for Outstanding Young Scholars from the Organization Department of the CPC Central Committee.

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