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Comparative study of aromatic hydrocarbons in bitumens and expelled oils generated by hydrous pyrolysis of coal
Carbon 147 (2019) 441e450
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Fabrication of a coaxial high performance fiber lithium-ion battery supported by a cotton yarn electrolyte reservoir Hyeonjun Song a, Seung-Yeol Jeon b, c, Youngjin Jeong a, d, * a
Department of Information Communication, Materials, and Chemistry Convergence Technology (BK-21 Plus), Soongsil University, Seoul, 06978, South Korea b Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD, 21218, USA c Hopkins Extreme Materials Institute, Johns Hopkins University, Baltimore, MD, 21218, USA d Department of Organic Materials and Fiber Engineering, Soongsil University, Seoul, 06978, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 October 2018 Received in revised form 22 February 2019 Accepted 25 February 2019 Available online 28 February 2019
Our report describes a coaxial fiber-type lithium-ion battery consisting of cotton core yarn wrapped with carbon nanotube (CNT) films and a nano-web separator. The CNT film was used as a current collector because of its high conductivity, flexibility, and network structure. The cotton yarn served as an electrolyte reservoir and a skeleton for the fiber shape. The cotton yarn even absorbed electrolyte as much as 9 times its own weight and its diameter swelled up to a 20%, which resulted in robust interfacial contact between components of the battery. Also, the swelling behavior of cotton yarn due to electrolyte up-take was simulated to clarify the function of cotton yarn. The nano-web separator was beneficial in accommodating the in-plane deformation occurred during the bending of the battery. This coaxial fiber battery exhibited stable performance even under bent or knotted states and delivered 144.82 mWhcm3 of volumetric energy density with high coloumbic efficiency of about 95%. Successful demonstration of the flexible fiber battery bespeaks a promising future for wearable electronic devices. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction There has been an explosive growth in demand for wearable IT devices, most of which are released in the form of wearable accessories. These products will occupy a more favorable position as they become lighter and more flexible; however, most of the devices, such as the smart watch, have a rigid structure that makes it difficult for them to conform to the shape of the human body [1e5]. As a power source for the wearable IT devices, lithium-ion batteries (LIBs) have been used because of their high energy density (150e200 Whkg1) and long-term stability [6,7]. However, they are not flexible enough to conform to the shape of the human body, a problem mainly ascribed to the way LIBs are fabricated. LIBs are generally prepared by mixing active materials with a polymer binder and a conducting agent, then coating the resulting slurries onto metal current collectors. The electrodes
* Corresponding author. Department of Information Communication, Materials, and Chemistry Convergence Technology (BK-21 plus), Soongsil University, Seoul, 06978, South Korea. E-mail address: [email protected] (Y. Jeong). https://doi.org/10.1016/j.carbon.2019.02.081 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
made in this way are vulnerable to bending deformation because of weak adhesion between the slurries and the metal current collectors [8]. Many efforts have been made to solve the problem, a typical attempt being replacement of the metallic current collectors with nanocarbon sheets made of carbon nanotubes (CNTs) or graphene [9e14]. However, most flexible batteries have been prepared in a film form which is not suitable for conforming to a curved surface. The film-type battery, which does not accommodate in-plan deformation, is buckled by the bending force, whereas a textile fabric does not buckle because it can accommodate in-plan deformation by moving fibers in the textile. If a fiber battery can be woven into a textile fabric, it will be possible to make a conformable wearable battery that integrates seamlessly into clothes. Recently there have been some attempts to create fiber-shaped batteries [15e26]. Among them is a cable-type battery with a hollow structure, consisting of a spring-like anode (Ni/Sn coated on Cu wire), a conventional cathode (LiCoO2, LCO), and a polyethylene nonwoven separator [15]. The distinguishing feature of the cable-type battery is that the electrolyte was stored in the hollow of the cable center. Therefore, the electrolyte could easily access into electrodes, which resulted in reduced internal
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resistance of the battery with a discharge capacity of 1 mAhcm1. However, the cable battery has a limited gravimetric/volumetric capacity due to the heavy metal and hollow structure. Ren, J. et al. also demonstrated a wire-shaped battery prepared by twisting CNT fibers together with a separator [20]. The CNT fibers acted as a current collector and a supporter for active materials. Compared with the cable-type battery, it was lighter and more flexible because no metallic wire was involved. The wire battery delivered a capacity of 0.0028 mAhcm1 (138 mAhg1), and the coloumbic efficiency was low at 85%. This result seems to be due to the structure being generated by twisting all the components together, making the facing area between anode and cathode small, and militating against the transport of Li ions. Also reported [21] was a coaxial-type fiber battery with a high volumetric energy density of 99.3 mWhcm3 that was fabricated by winding anode and cathode fibers spirally onto a cotton yarn. The cotton yarn, which acted as a substrate of the battery, imparted mechanical stability to the fiber battery but did not contribute to its electrochemical performance. A woven fabric inserted with the fiber battery to keep the shape of a straight line without a crimp was demonstrated. From these findings, one can see that there is still a long way to go toward the development of a highperformance, fiber-shaped battery that can be woven into a textile fabric. In this report we demonstrate a fiber battery with a coaxial structure made of the CNT film electrodes, a nano-web separator, and a cotton yarn (Fig. 1). The coaxial structure is constructed by winding the film electrodes and the separator onto a cotton yarn which acts as a framework of the fiber battery and confines electrolyte. The electrochemical performance of the battery is investigated in this study.
2. Experimental 2.1. Fabrication of electrodes To fabricate a CNT film, the precursor solution prepared by mixing acetone 98.0 wt% (99.7%, Samchun Chemical, Korea), ferrocene 0.2 wt% (98%, Sigma Aldrich), thiophene 0.8 wt% (99%, Sigma Aldrich), and polysorbate_20 1.0 wt% (Sigma Aldrich) was injected at a rate of 10 ml h1 into a vertical reactor heated to 1200 C with a carrier gas (H2) at the flow rate of 1000 sccm [27]. And then, the synthesized CNTs were continuously wounded onto cylindrical winder to form the CNT film at the bottom of reactor. The electrode slurry was prepared by mixing LCO (MTI Korea) or graphite (MTI Korea), carbon black (Super P, MTI Korea), and poly(vinylidone difluoride) (PVDF, MTI Korea) in N-methly-2-pyrrolidone (NMP, Samchun Chemical, Korea) solvent at a weight ratio of 7.5:1.5:1. The electrode slurry was coated on the CNT film with a thickness of 5 mm by doctor blade. The prepared electrode was dried at 120 C for 2 h in a vacuum oven. The thickness of the asprepared electrode was 20e30 mm. 2.2. Fabrication of batteries The electrochemical performance of the as-prepared electrode was evaluated by assembling a coin-type half-cell (CR 2032, MTI Korea). The CNT film coated with electrode slurries and Li metal were used as a working electrode and a counter electrode, respectively. Microporous Trilayer Membrane (PP/PE/PP; thickness, 25 mm; Celgard 2325) was used as the separator. A mixture of 1 M LiPF6 dissolved in ethylene carbonate (EC)/diethylene carbonate (DEC) (1:1 by volume) was used as the electrolyte. A full-cell was
Fig. 1. Schematic of the procedure for preparing electrodes and fabricating a coaxial fiber battery with a cotton yarn acting as a framework for fiber shape and electrolyte reservoir. The LCO-CNT film, nano-web, and graphite-CNT film are wound around the cotton yarn in sequence. The CNT films act as current collectors. (A colour version of this figure can be viewed online.)
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assembled by using the LCO-CNT film as a cathode and graphiteCNT film as an anode. The separator and electrolyte were same as those used in the half-cell. All the cells were assembled in an argonfilled glove box (Korea Kiyon). A coaxial fiber battery was prepared by winding LCO-CNT film, Polyvinylidene fluoride (PVDF) nano-web (thickness: ca. 10 mm, FTEnE, Korea), and graphite-CNT film around the cotton yarn sequentially as shown in Fig. 1. The electrode films are wound around 3 layers. Also, we used strapping tape, which is stable under organic electrolyte, to adhere metal wire (Al and Ni) onto CNT film electrode to make the external electrical contact. Then the assembled fiber battery was sealed with a heat-shrinkable tube with a diameter of 1 mm and heated with a heat gun at 110 C to shrink the tube so that all the components of electrodes closely adhered to each other. The fabricated fiber battery was dried 100 C under vacuum for overnight to remove residual moisture of the fiber battery. Finally, the electrolyte was injected into the fiber battery in an argon-filled glove box.
2.3. Characterization The morphology of the as-prepared electrode was imaged with a field emission scanning electron microscopy (Carl Zeiss, SIGMA). The ID/IG of CNT film was characterized by the Raman spectrometer (HORIBA, LabRAM HR UVevisible_NIR) using a 632.8 nm line of an argon laser. The cross section of the fiber battery was imaged with an optical microscopy (I-Megascope, SOMTECH). Galvanostactic charge/discharge tests were carried out over the voltage window of 0e2 V for anode half-cell (graphite-CNT film), 2.8e4.2 V for cathode half-cell (LCO-CNT film), and 2.8e4.3 V for full-cell using charge/discharge cyclers (Shin Corporation). The AC impedance measurement was carried out using an electrochemical impedance spectroscopy (Autolab, PGSTAT 302 N) in the frequency range of 100 kHz - 100 mHz with an AC voltage amplitude of 5 mV.
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3. Results & discussions 3.1. Morphological characterization of CNT film electrodes The CNT film is flexible enough to be used as a flexible current collector as shown in Fig. S1A where the film was wrapped around a thin rod. Its flexible nature is even maintained in liquid nitrogen [27], which comes from its network structure created by the entangled long CNT bundles (bundle diamter, about 20 nm; CNT diameter, about 4 nm) (See Fig. S1B). Also, it can be seen that the CNT film (surface area, 286.5 m2 g1 [28]) has a rough and porous surface that produces an increase in the interfacial contact area and the adhesion between the electrode materials and the CNT film [28], allowing the electrode materials to easily infiltrate into the spaces among the individual CNT bundles. Moreover, the CNT film has a low surface resistance (9.4 U sq1) and high crystal perfection (ID/IG ¼ 0.14, Fig. S1C), these features sufficiently replaces a metallic current collector [29]. The photograph in Fig. 2A shows that as-prepared LCO-CNT film electrode and graphite-CNT film electrode maintained their mechanical flexibility after the electrode materials were coated onto the CNT films. Even after the LCOCNT film electrode and the graphite-CNT film electrode passed through the needle eye and were wrapped around the needle, the LCO and the graphite were well-adhered on the CNT film, and no deformation was observed from the electrodes. The robust adhesion seems to be due to the rough surface of the CNT film. These outstanding properties are indispensable properties for flexible LIBs. Also, the SEM images in Fig. 2B and C shows that the electrode materials are uniformly coated on the CNT film. In addition, the thin CNT film of 5 mm is a remarkable feature when compared to a conventional metallic current collector (Al foil: 15 mm, Cu foil: 9 mm, MTI Korea), and it plays an important role in fabricating batteries with a high volumetric energy density. In this respect, the CNT film has proved to be a very suitable material for flexible electrode.
Fig. 2. (A) Photograph of the as-prepared graphite-CNT film electrode and the LCO-CNT film electrode being passed through a needle eye and wrapped around a needle, respectively, exhibiting the mechanical flexibility and stability of as-prepared electrodes. SEM images of (B) as-prepared graphite-CNT film electrode, and (C) LCO-CNT film electrode. (A colour version of this figure can be viewed online.)
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3.2. Electrochemical characterization of electrodes To use the CNT film as a current collector, it must be electrochemically stable. The stability of the CNT film was investigated with cyclic voltammetry (CV) analysis, and the results are shown in Fig. 3A and B. From these results it can be seen that any electrochemical reaction associated with the CNT film does not occur in the potential range of 2.8e4.2 V, which corresponds to the potential range of the LCO-CNT film electrode (cathode). The stability is also verified by the voltage profiles of the LCO-CNT film electrode in Fig. S2A where a typical graph of LCO electrode is observed. The LCO-CNT film electrode delivered a discharge capacity of 130 mAhg1 (based on the mass of the LCO) and a high coloumbic efficiency of 99% after 50 cycles at 0.5 C (1 C ¼ 145 mAg1), as shown in Fig. S2B. In contrast, some peaks related to redox reactions are observed in the CV curve of the CNT film in the potential range of 0e2 V (Fig. 3B), which corresponds to the potential range of the
graphite-CNT film electrode (anode). The cathodic peak at around 0.62 V appeared in the first cycle and disappeared in the subsequent cycles, which indicates the formation of the solid electrolyte interphase (SEI) layer on the CNT film [30,31]. The other cathodic peak and anodic peak at around 0 and 0.1 V are associated with lithiation and delithiation of lithium ions, respectively [32]. The similarity of peak intensity and area in subsequent reactions indicates that the electrochemical reaction occurs reversibly and that the CNT film also serves well as anode-active material. The graphite-CNT film electrode showed excellent cycling stability with a specific capacity of 353 mAhg1 (based on the mass of graphite) at 0.5 C and coloumbic efficiency of >99% after 50 cycles (see Figs. S2C and D). To know the contribution of CNT film to the capacity of graphite-CNT film electrode, a half-cell was assembled using only the CNT film without graphite. The discharge capacity of the CNT film maintained stability until the 50th cycle as shown in Fig. 3C. The contribution of CNT film to the capacity of graphite-CNT film
Fig. 3. Cyclic voltammetry of the CNT film in the ranges of (A) 2.8 Ve4.2 V (cathode) and (B) 0 Ve2 V (anode). (C) The absolute capacity of the graphite-CNT film electrode and the CNT film electrode. (D) The capacity contribution of the CNT film and the graphite. Comparison of the gravimetric specific capacity (based on the total mass of the electrode and the electrode materials) of (E) the LCO-Al foil and LCO-CNT film electrode, and (F) the graphite-Cu foil and the graphite-CNT film electrode. Much higher gravimetric specific capacity was achieved in the electrode with CNT film. (A colour version of this figure can be viewed online.)
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was about 5.7% (Fig. 3C and D), which is a small portion of the capacity obtained by the graphite-CNT film electrode. The use of CNT film as a current collector is highly advantageous in terms of gravimetric specific capacity because the CNT current collector (0.174 mgcm2) is so light that its density is only 1/45 of copper and 1/32 of aluminum current collector. The loading of electrode materials is about 2 mg cm2 and the weight ratio of electrode material to CNT film is 11. On the other hand, the weigt ratio of electrode materials to Al foil (MTI Korea, 5.61 mg cm2) and Cu foil (MTI Korea, 7.83 mg cm2) are 0.35 and 0.26. To compare the gravimetric specific capacity, coin cells were fabricated using metallic current collectors (Cu and Al foils). The gravimetric specific capacity based on the total mass of the electrode (Celectrode) and the mass of the LCO and the graphite (CLCO, Cgraphite) are shown in Fig. 3E and F. For the cathode part, no significant difference in the specific capacity (CLCO) was observed between the LCO-CNT film electrode and the LCO-Al foil electrode. However, the Celectrode of the LCO-CNT film electrode was 2 times higher than that of the LCO-Al foil electrode. Also, the graphite-CNT film electrode exhibited higher gravimetric specific capacity than the graphite-Cu foil electrode in both Celectrode and Cgraphite, as shown in Fig. 3F. The higher specific capacity is due to the contribution of CNT film as an active material. These findings reveal that the CNT film, which is stable in electrochemical reactions and has light weight, has a high potential as the current collector for flexible batteries with high capacity. After the electrochemical reactions in the half cell were verified, the graphite-CNT film and LCO-CNT film were assembled into a full cell. The full cell was charged/discharged at 0.5 C (1C ¼ 145 mAg1) after the formation process (1 cycle at 0.1 C) and the capacity ratio of graphite-CNT film anode to LCO-CNT film cathode was selected to be ~1.1:1 (N/P ratio ¼ 1.1) for the full cell capacity balance. The initial discharge capacity and the capacity retention of 141.84 mAhg1 and 84%, respectively, were obtained with high coloumbic efficiency (>98%) after 50 cycles, as shown in Figs. S3A and B. The reason for the low capacity retention compared to the half-cell is probably due to the absence of Li metal. The Li metal used as a counter electrode in the half cell can replenish Li ions lost during the charge/discharge process, while the full cell cannot replenish the lost Li ion because of an insufficient supply of lithium ion [33]. Therefore, additional work is required to improve the capacity retention with cycling of the full cell by adjusting the N/P ratio [34]. Additionally, a pouch-type full cell was fabricated to demonstrate its potential as a flexible battery as shown in Fig. S4. A series of photographs showed that a red LED worked well and the open circuit voltage (OCV) was kept stable even if the pouch cell was folded in four equal parts and unfolded back. This indicates that the prepared CNT film is very suitable as a flexible electrode. 3.3. Characterization of fiber battery The cotton yarn also has to be chemically stable in the electrolyte in order to conduct as electrolyte support. To verify the stability, the cotton fiber was immersed in the electrolyte for one day and the tensile property was measured. No obvious change in the tensile property was observed for the cotton (see Fig. S5), which indicates that the cotton yarn does not react with the electrolyte. Fig. 4A shows the fiber battery assembled with the as-prepared electrodes and cotton yarn. The fiber battery in the photograph has a length of 9 cm and a diameter of 0.91 mm. High volumetric energy density may be essential in order for the fiber battery to be used as a power source for wearable devices. Therefore, the amount of absorbed electrolyte and swelling are very important features considering the requirement of high volumetric energy density. The optical microscope (OM) image of the cross-section in Fig. 4B
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displays the coaxial structure of the fiber battery. The coaxial structure has large interfacial areas between the anode and cathode, which is a favorable condition for transfer and diffusion of lithium ions. A pair of the OM images (Fig. 4C and D) shows that the cotton yarn was swollen after absorbing the electrolyte. The degree of swelling and uptake of electrolyte were about 20% and 900%, respectively (calculated by equations (1) and (2)).
Degree of swelling ð%Þ ¼
Uptakeð%Þ ¼
D2 D1 100 D1
W2 W1 100 W1
(1)
(2)
where D1 and W1 are diameter and weight of the cotton yarn before absorbing the electrolyte, D2 and W2 are the diameter and weight of the cotton yarn after absorbing the electrolyte. High uptake of organic electrolyte (1 M LiPF6 in EC:DEC 1:1 v/v) by cotton yarn is due to internal empty space and the high polarity of cotton yarn [35]. Presence of the carbonate groups in solvents increases the polarity of electrolyte and leads to good affinity between the cotton yarn and the electrolyte. Unlike coin and pouch cells, the fiber battery has a very narrow internal space in which the electrolyte can be contained, which can make it difficult for the battery to operate normally. Therefore, the high electrolyte uptake of cotton yarn, which is 9 times its own weight, plays a very important role in achieving a high performance fiber battery. Moreover, the swelling of cotton yarn has an additional effect of making the contact between the components of the fiber battery more robust. It is not enough to tighten the elements of the fiber battery sufficiently by shrinkage of the thermal-shrinkage tube only. To clarify the function of cotton yarn, the stress induced by the swelling of cotton yarn was calculated using the modified stress generation model during the lithiation [36] with following equations.
s_ r ¼
! !# " E dVr _c _p Vr _c _p εr εr þ v εq εq ð1 þ nÞ ð1 þ nÞð1 2nÞ dr r " ! !# E dVr _c _p Vr _c _p εr εr εq εq s_ r s_ q ¼ (3) ð1 þ nÞ dr r
where r is fiber radius, ѵ is Poisson's ratio and Vr is radial velocity. As shown in Fig. 4E and (F), the electrolyte was completely absorbed into the cotton yarn in 2.5 s. Also, 86.8 kPa of stress in axial direction and 67.4 kPa of stress in circumferential direction of the cotton yarn were induced in the fiber battery by swelling (Simulation details in supporting information). The proof-of-concept experiments for the coaxial fiber type battery, the fiber battery was fabricated with cotton yarn (C-FB). Another fiber battery was constructed using polypropylene (PP) fiber (diameter: 0.24 mm, PP-FB) as a control, which does not absorb the electrolyte owing to the lack of affinity with the organic electrolyte and solid structure of the PP fiber. Compared to C-FB, the PP-FB will contain a smaller amount of electrolyte and less tightened interfacial contact between the electrodes. The electrochemical impedance spectroscopy (EIS) of fiber type symmetric battery (LCO/LCO) was conducted to verify role of cotton yarn and prove the simulation results. Since the effect of electrode can be excluded, the EIS of symmetric cell can provide direct insight into the role of cotton yarn in fiber shape battery. The high-frequency range (>100 kHz) and the mid-frequency range (100 kHze1 Hz) in the spectra are associated with the ohmic resistance and interfacial phenomena, respectively [37]. As shown in Figure G and H, PP-FB
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Fig. 4. (A) Photograph of the fiber battery with length of 9 cm. OM image of (B) cross section of fiber battery with diameter of 1.2 mm. OM images of side view of cotton fiber (C) before absorbing the electrolyte and (D) after absorbing the electrolyte. Simulation of (E) the expansion of cotton fiber and (F) the development of stress in the cotton fiber surface during expansion due to the absorption of electrolyte. Nyquist plots for fiber type symmetric cells using (G) cotton yarn and (H) PP fiber in the frequency range from 100 kHze100 mHz. (A colour version of this figure can be viewed online.)
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Fig. 5. (A) Voltage profiles of the C-FB battery at the 1st cycle. (B) Comparison of cycle performance of C-FB battery and PP-FB battery. (C) Nyquist plots of fiber batteries after 3 cycles in the frequency range from 100 kHze100 mHz (inset: Nyquist plots of the C-FB). (D) Comparison of volumetric energy density and specific capacity with other reported fiber shape batteries.
showed very high resistance compared to C-FB at all frequency. This results and previous simulation results strongly suggest that the cotton yarn conducted a critical role of performance of fiber battery by improving the contact between the components. This difference in core fiber affected the electrochemical performance as shown in Fig. 5. Fig. 5A shows that voltage profiles similar to those of the coin-type full cell were obtained from charge/discharge tests of the C-FB battery (N/P ratio ¼ 1.1), which rationalizes the principle of designing the C-FB battery. Fig. 5B shows the charge and discharge performance of the C-FB and the PP-FB batteries. The initial discharge capacity and the capacity retention of the C-FB battery were 0.24 mAhcm1 and 84%, respectively. It should be noted that the capacity retention of the CFB battery is similar to that of conventional coin cell (Fig. S3), which implies that the coaxial structure is well suited to fiber-type battery. In contrast, the PP-FB battery exhibited a discharge capacity of 0.09 mAhcm1, lower than that of the C-FB battery, which rapidly declined in subsequent cycles. The reason for the poor performance of the PP-FB battery can be deduced from the impedance spectra in Fig. 5C. As shown in Fig. 5C, the PP-FB battery has a higher resistance in both the high- and mid-frequency range than that of the CFB battery. These results are well agreed with Fig. 4G and H. Also, similar results were reported in the cable type battery with hollow structure [14]. The internal space by hollow structure helps electrolyte easily permeate and access cell components, which results in reducing cell resistance and making it operate normally. Likewise, the cotton yarn, which holds the electrolyte and forms a skeleton of fiber battery, increases accessibility of electrolyte to electrodes and enhances the electrochemical performance. The performance of C-FB battery with concentric structure was
compared with the wire battery fabricated by the combining of anode fiber and cathode fiber followed by twisting [20]. The C-FB battery delivered capacity of 0.24 mAhcm1, which is 86 times higher than that of the wire-shaped battery (0.0028 mAhcm1). Also, the volumetric energy density (based on the C-FB battery volume which includes cotton yarn) of the C-FB battery is 144.82 mWhcm3 (the areal energy density based on the cross-sectional area of the C-FB is 72.41 mWhcm2), which is 8 times higher than that of the wire battery (17.7 mWhcm3 [20]). Moreover, when the volumetric energy density of the C-FB battery is calculated only by the two electrode volumes, it reaches to 587.91 mWhcm3, which is 6 times higher than that of the coaxial fiber battery fabricated with a cotton yarn and silicon anode (99.3 mWhcm3, based on the electrode volume [21]). Comparison of volumetric energy density and specific capacity with other reported fiber shape batteries is presented in Fig. 5D and data about state-of-the-art fiber-shaped battery are summarized in Table S2. To best our knowledge, the volumetric energy density (587.91 mWhcm3) of the C-FB battery is the best value compared to others reported elsewhere. The difference arises from the fact that the cotton yarn was limited to its function as a substrate [21] while the cotton yarn in this study played the role of supporting electrolyte as well as improving the interfacial contact between the electrode elements. This indicates that the cotton yarn and the concentric structure play important roles in achieving good performance, which is also verified from the higher coloumbic efficiency of 95% than that of the wire battery (80%) [20]. The fiber battery used for energy storage in wearable devices needs to have adequate flexibility. In other words, the fiber battery should operate normally even under severe mechanical
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Fig. 6. (A) Photograph of a film-type separator buckled by bending deformation. (B) SEM image of the film-type separator. (C) Photograph of a PVDF nano-web separator under bending, exhibiting conformal deformation on the surface of electrode. (D) SEM image of the PVDF nano-web consisted of numerous nanofibers. (A colour version of this figure can be viewed online.)
Fig. 7. The photographs of LED lit by (A) an undeformed fiber battery and (B) a knotted fiber battery. The SEM image (inset of (B)) displays the surface of the bent part of electrode, exhibiting no falling off of active materials from the electrode. (C) Nyquist plots of undeformed and knotted fiber batteries after 3 cycles in the frequency range from 100 kHze100 mHz. (A colour version of this figure can be viewed online.)
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Fig. 8. The photographs of (A and B) examples of application of the fiber battery as a string of necklaces. (A colour version of this figure can be viewed online.)
deformation such as twisting and knotting. The flexibility of cathode and anode presented in Fig. 2 was due to the unique feature of CNT film as a three-dimensional nanoporous structure. Likewise, a separator should accommodate the deformation applied to the fiber battery. Generally, the separator is made of polyolefin film, which cannot accommodate in-plan deformation. As a result, the film-type separator buckles under bending deformation caused by the restricted movement in the plane as shown in Fig. 6A and B. The buckling may also impart undesirable deformations to the components of the fiber battery. As an alternative, the PVDF nano-web can be a proper candidate for the separator of the fiber battery. The PVDF nano-web can accommodate in-plane deformation because it consists of numerous nano fibers which can move in the plane as shown in Fig. 6C and D. In addition, the porous nanostructure of the PVDF nano-web is a favorable factor in achieving good electrochemical performances of the fiber battery because it shortens the lithium ion pathway and causes uniform redox reactions [38]. It has been verified that the fiber battery fabricated in this way works well under deformed conditions. Fig. 7A and B displays that a blue LED is well lit even when the fiber battery is knotted. The inset SEM image in Fig. 7B shows that the deformed surface of the electrode remained stable without a falling off of active materials even under a knotted condition, which demonstrates the outstanding mechanical stability of the as-prepared electrodes. To investigate the effect of the external force on the electrochemical performance of the fiber battery, the charge/discharge test was conducted for the knotted fiber battery and displayed in Fig. S6. Nevertheless, it is observed that there is a slight difference in the brightness of the LED turned on by the two fiber batteries, as shown in Fig. 7A and B. The reason for the brighter LED can be explained by the impedance results in Fig. 7C. It can be seen that the impedance decreased remarkably when the fiber battery was knotted. The reason for this can be explained as follows: 1) During the knotting, external force is applied to the components of the fiber battery, which increases interfacial force between the components of the fiber battery and decreases the distance between the electrodes. Consequently, it leads to a decrease of the lithium ion pathway and the charge transfer resistance. 2) It has been known that the impedance decreases as the amount of electrolyte increases [39]. The function of knotting is like squeezing the cotton fiber and letting the electrolyte come out. As a result, the amount of available electrolyte increases, which results in enhancing the ionic conductivity. In this study, a fiber battery has been fabricated and the battery has proven to work well under bending deformation such as knotting. Fig. 8A and B shows the high potential of the fiber battery as energy storage for wearable devices. These results suggest that the fiber battery can be put to practical use.
4. Conclusions In summary, a coaxial fiber battery was fabricated by winding CNT film electrodes onto a cotton yarn. Due to its unique structure, the fiber battery was flexible enough to make a knot. The cotton yarn, which functioned not only as a skeleton for the fiber shape, but also as an electrolyte reservoir, absorbing 900% of its own weight of electrolyte and swelling to about 20% in diameter after absorbing electrolyte. These characteristics of cotton yarn improved interfacial contact between components of the battery, which resulted in high performance of the coaxial fiber battery. Also, a nano-web was selected as a separator to avoid buckling by accommodating in-plane deformation. As a result, the coaxial fiber battery exhibited a high volumetric energy density of 144.82 mWhcm3 (based on the fiber battery's volume) and operated well even under severe mechanical deformation such as bending and knotting. From these results, it can be said that the novel coaxial fiber battery represents a significant step forward for wearable energy storage. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (NRF-2017R1A5A1015596). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.02.081. References [1] B. Liu, J. Zhang, X. Wang, G. Chen, D. Chen, C. Zhou, et al., Hierarchical threedimensional ZnCo2O4 nanowire arrays/carbon cloth anodes for a novel class of high-performance flexible lithium-ion batteries, Nano Lett. 12 (2012) 3005e3011. [2] H. Gwon, H.S. Kim, K.U. Lee, D.H. Seo, Y.C. Park, Y.S. Lee, et al., Flexible energy storage devices based on graphene paper, Energy Environ. Sci. 4 (2011) 1277e1283. [3] X. Duan, Assembled semiconductor nanowire thin films for high performance flexible macroelectronics, MRS Bull. 32 (2007) 134e141. [4] Z. Wang, H. Wang, B. Liu, W. Qiu, J. Zhang, S. Ran, et al., Transferable and flexible nanorod-assembled TiO2 cloths for dye-sensitized solar cells, photodetectors, and photocatalysts, ACS Nano. 5 (2011) 8412e8419. [5] K.A. Sierros, D.S. Hecht, D.A. Banerjee, N.J. Morris, L. Hu, G.C. Irvin, et al., Durable transparent carbon nanotube films for flexible device components, Thin Solid Films 518 (2010) 6977e6983. [6] H. Gwon, J. Hong, H. Kim, D.H. Seo, S. Jeon, K. Kang, Recent progress on flexible lithium rechargeable batteries, Energy Environ. Sci. 7 (2014) 538e551. [7] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359e367. [8] K. Wang, S. Luo, Y. Wu, X. He, F. Zhao, J. Wang, et al., Super-aligned carbon nanotube films as current collectors for lightweight and flexible lithium ion
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Fabrication of a coaxial high performance fiber lithium-ion battery supported by a cotton yarn electrolyte reservoir Hyeonjun Song a, Seung-Yeol Jeon b, c, Youngjin Jeong a, d, * a
Department of Information Communication, Materials, and Chemistry Convergence Technology (BK-21 Plus), Soongsil University, Seoul, 06978, South Korea b Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD, 21218, USA c Hopkins Extreme Materials Institute, Johns Hopkins University, Baltimore, MD, 21218, USA d Department of Organic Materials and Fiber Engineering, Soongsil University, Seoul, 06978, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 October 2018 Received in revised form 22 February 2019 Accepted 25 February 2019 Available online 28 February 2019
Our report describes a coaxial fiber-type lithium-ion battery consisting of cotton core yarn wrapped with carbon nanotube (CNT) films and a nano-web separator. The CNT film was used as a current collector because of its high conductivity, flexibility, and network structure. The cotton yarn served as an electrolyte reservoir and a skeleton for the fiber shape. The cotton yarn even absorbed electrolyte as much as 9 times its own weight and its diameter swelled up to a 20%, which resulted in robust interfacial contact between components of the battery. Also, the swelling behavior of cotton yarn due to electrolyte up-take was simulated to clarify the function of cotton yarn. The nano-web separator was beneficial in accommodating the in-plane deformation occurred during the bending of the battery. This coaxial fiber battery exhibited stable performance even under bent or knotted states and delivered 144.82 mWhcm3 of volumetric energy density with high coloumbic efficiency of about 95%. Successful demonstration of the flexible fiber battery bespeaks a promising future for wearable electronic devices. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction There has been an explosive growth in demand for wearable IT devices, most of which are released in the form of wearable accessories. These products will occupy a more favorable position as they become lighter and more flexible; however, most of the devices, such as the smart watch, have a rigid structure that makes it difficult for them to conform to the shape of the human body [1e5]. As a power source for the wearable IT devices, lithium-ion batteries (LIBs) have been used because of their high energy density (150e200 Whkg1) and long-term stability [6,7]. However, they are not flexible enough to conform to the shape of the human body, a problem mainly ascribed to the way LIBs are fabricated. LIBs are generally prepared by mixing active materials with a polymer binder and a conducting agent, then coating the resulting slurries onto metal current collectors. The electrodes
* Corresponding author. Department of Information Communication, Materials, and Chemistry Convergence Technology (BK-21 plus), Soongsil University, Seoul, 06978, South Korea. E-mail address: [email protected] (Y. Jeong). https://doi.org/10.1016/j.carbon.2019.02.081 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
made in this way are vulnerable to bending deformation because of weak adhesion between the slurries and the metal current collectors [8]. Many efforts have been made to solve the problem, a typical attempt being replacement of the metallic current collectors with nanocarbon sheets made of carbon nanotubes (CNTs) or graphene [9e14]. However, most flexible batteries have been prepared in a film form which is not suitable for conforming to a curved surface. The film-type battery, which does not accommodate in-plan deformation, is buckled by the bending force, whereas a textile fabric does not buckle because it can accommodate in-plan deformation by moving fibers in the textile. If a fiber battery can be woven into a textile fabric, it will be possible to make a conformable wearable battery that integrates seamlessly into clothes. Recently there have been some attempts to create fiber-shaped batteries [15e26]. Among them is a cable-type battery with a hollow structure, consisting of a spring-like anode (Ni/Sn coated on Cu wire), a conventional cathode (LiCoO2, LCO), and a polyethylene nonwoven separator [15]. The distinguishing feature of the cable-type battery is that the electrolyte was stored in the hollow of the cable center. Therefore, the electrolyte could easily access into electrodes, which resulted in reduced internal
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resistance of the battery with a discharge capacity of 1 mAhcm1. However, the cable battery has a limited gravimetric/volumetric capacity due to the heavy metal and hollow structure. Ren, J. et al. also demonstrated a wire-shaped battery prepared by twisting CNT fibers together with a separator [20]. The CNT fibers acted as a current collector and a supporter for active materials. Compared with the cable-type battery, it was lighter and more flexible because no metallic wire was involved. The wire battery delivered a capacity of 0.0028 mAhcm1 (138 mAhg1), and the coloumbic efficiency was low at 85%. This result seems to be due to the structure being generated by twisting all the components together, making the facing area between anode and cathode small, and militating against the transport of Li ions. Also reported [21] was a coaxial-type fiber battery with a high volumetric energy density of 99.3 mWhcm3 that was fabricated by winding anode and cathode fibers spirally onto a cotton yarn. The cotton yarn, which acted as a substrate of the battery, imparted mechanical stability to the fiber battery but did not contribute to its electrochemical performance. A woven fabric inserted with the fiber battery to keep the shape of a straight line without a crimp was demonstrated. From these findings, one can see that there is still a long way to go toward the development of a highperformance, fiber-shaped battery that can be woven into a textile fabric. In this report we demonstrate a fiber battery with a coaxial structure made of the CNT film electrodes, a nano-web separator, and a cotton yarn (Fig. 1). The coaxial structure is constructed by winding the film electrodes and the separator onto a cotton yarn which acts as a framework of the fiber battery and confines electrolyte. The electrochemical performance of the battery is investigated in this study.
2. Experimental 2.1. Fabrication of electrodes To fabricate a CNT film, the precursor solution prepared by mixing acetone 98.0 wt% (99.7%, Samchun Chemical, Korea), ferrocene 0.2 wt% (98%, Sigma Aldrich), thiophene 0.8 wt% (99%, Sigma Aldrich), and polysorbate_20 1.0 wt% (Sigma Aldrich) was injected at a rate of 10 ml h1 into a vertical reactor heated to 1200 C with a carrier gas (H2) at the flow rate of 1000 sccm [27]. And then, the synthesized CNTs were continuously wounded onto cylindrical winder to form the CNT film at the bottom of reactor. The electrode slurry was prepared by mixing LCO (MTI Korea) or graphite (MTI Korea), carbon black (Super P, MTI Korea), and poly(vinylidone difluoride) (PVDF, MTI Korea) in N-methly-2-pyrrolidone (NMP, Samchun Chemical, Korea) solvent at a weight ratio of 7.5:1.5:1. The electrode slurry was coated on the CNT film with a thickness of 5 mm by doctor blade. The prepared electrode was dried at 120 C for 2 h in a vacuum oven. The thickness of the asprepared electrode was 20e30 mm. 2.2. Fabrication of batteries The electrochemical performance of the as-prepared electrode was evaluated by assembling a coin-type half-cell (CR 2032, MTI Korea). The CNT film coated with electrode slurries and Li metal were used as a working electrode and a counter electrode, respectively. Microporous Trilayer Membrane (PP/PE/PP; thickness, 25 mm; Celgard 2325) was used as the separator. A mixture of 1 M LiPF6 dissolved in ethylene carbonate (EC)/diethylene carbonate (DEC) (1:1 by volume) was used as the electrolyte. A full-cell was
Fig. 1. Schematic of the procedure for preparing electrodes and fabricating a coaxial fiber battery with a cotton yarn acting as a framework for fiber shape and electrolyte reservoir. The LCO-CNT film, nano-web, and graphite-CNT film are wound around the cotton yarn in sequence. The CNT films act as current collectors. (A colour version of this figure can be viewed online.)
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assembled by using the LCO-CNT film as a cathode and graphiteCNT film as an anode. The separator and electrolyte were same as those used in the half-cell. All the cells were assembled in an argonfilled glove box (Korea Kiyon). A coaxial fiber battery was prepared by winding LCO-CNT film, Polyvinylidene fluoride (PVDF) nano-web (thickness: ca. 10 mm, FTEnE, Korea), and graphite-CNT film around the cotton yarn sequentially as shown in Fig. 1. The electrode films are wound around 3 layers. Also, we used strapping tape, which is stable under organic electrolyte, to adhere metal wire (Al and Ni) onto CNT film electrode to make the external electrical contact. Then the assembled fiber battery was sealed with a heat-shrinkable tube with a diameter of 1 mm and heated with a heat gun at 110 C to shrink the tube so that all the components of electrodes closely adhered to each other. The fabricated fiber battery was dried 100 C under vacuum for overnight to remove residual moisture of the fiber battery. Finally, the electrolyte was injected into the fiber battery in an argon-filled glove box.
2.3. Characterization The morphology of the as-prepared electrode was imaged with a field emission scanning electron microscopy (Carl Zeiss, SIGMA). The ID/IG of CNT film was characterized by the Raman spectrometer (HORIBA, LabRAM HR UVevisible_NIR) using a 632.8 nm line of an argon laser. The cross section of the fiber battery was imaged with an optical microscopy (I-Megascope, SOMTECH). Galvanostactic charge/discharge tests were carried out over the voltage window of 0e2 V for anode half-cell (graphite-CNT film), 2.8e4.2 V for cathode half-cell (LCO-CNT film), and 2.8e4.3 V for full-cell using charge/discharge cyclers (Shin Corporation). The AC impedance measurement was carried out using an electrochemical impedance spectroscopy (Autolab, PGSTAT 302 N) in the frequency range of 100 kHz - 100 mHz with an AC voltage amplitude of 5 mV.
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3. Results & discussions 3.1. Morphological characterization of CNT film electrodes The CNT film is flexible enough to be used as a flexible current collector as shown in Fig. S1A where the film was wrapped around a thin rod. Its flexible nature is even maintained in liquid nitrogen [27], which comes from its network structure created by the entangled long CNT bundles (bundle diamter, about 20 nm; CNT diameter, about 4 nm) (See Fig. S1B). Also, it can be seen that the CNT film (surface area, 286.5 m2 g1 [28]) has a rough and porous surface that produces an increase in the interfacial contact area and the adhesion between the electrode materials and the CNT film [28], allowing the electrode materials to easily infiltrate into the spaces among the individual CNT bundles. Moreover, the CNT film has a low surface resistance (9.4 U sq1) and high crystal perfection (ID/IG ¼ 0.14, Fig. S1C), these features sufficiently replaces a metallic current collector [29]. The photograph in Fig. 2A shows that as-prepared LCO-CNT film electrode and graphite-CNT film electrode maintained their mechanical flexibility after the electrode materials were coated onto the CNT films. Even after the LCOCNT film electrode and the graphite-CNT film electrode passed through the needle eye and were wrapped around the needle, the LCO and the graphite were well-adhered on the CNT film, and no deformation was observed from the electrodes. The robust adhesion seems to be due to the rough surface of the CNT film. These outstanding properties are indispensable properties for flexible LIBs. Also, the SEM images in Fig. 2B and C shows that the electrode materials are uniformly coated on the CNT film. In addition, the thin CNT film of 5 mm is a remarkable feature when compared to a conventional metallic current collector (Al foil: 15 mm, Cu foil: 9 mm, MTI Korea), and it plays an important role in fabricating batteries with a high volumetric energy density. In this respect, the CNT film has proved to be a very suitable material for flexible electrode.
Fig. 2. (A) Photograph of the as-prepared graphite-CNT film electrode and the LCO-CNT film electrode being passed through a needle eye and wrapped around a needle, respectively, exhibiting the mechanical flexibility and stability of as-prepared electrodes. SEM images of (B) as-prepared graphite-CNT film electrode, and (C) LCO-CNT film electrode. (A colour version of this figure can be viewed online.)
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3.2. Electrochemical characterization of electrodes To use the CNT film as a current collector, it must be electrochemically stable. The stability of the CNT film was investigated with cyclic voltammetry (CV) analysis, and the results are shown in Fig. 3A and B. From these results it can be seen that any electrochemical reaction associated with the CNT film does not occur in the potential range of 2.8e4.2 V, which corresponds to the potential range of the LCO-CNT film electrode (cathode). The stability is also verified by the voltage profiles of the LCO-CNT film electrode in Fig. S2A where a typical graph of LCO electrode is observed. The LCO-CNT film electrode delivered a discharge capacity of 130 mAhg1 (based on the mass of the LCO) and a high coloumbic efficiency of 99% after 50 cycles at 0.5 C (1 C ¼ 145 mAg1), as shown in Fig. S2B. In contrast, some peaks related to redox reactions are observed in the CV curve of the CNT film in the potential range of 0e2 V (Fig. 3B), which corresponds to the potential range of the
graphite-CNT film electrode (anode). The cathodic peak at around 0.62 V appeared in the first cycle and disappeared in the subsequent cycles, which indicates the formation of the solid electrolyte interphase (SEI) layer on the CNT film [30,31]. The other cathodic peak and anodic peak at around 0 and 0.1 V are associated with lithiation and delithiation of lithium ions, respectively [32]. The similarity of peak intensity and area in subsequent reactions indicates that the electrochemical reaction occurs reversibly and that the CNT film also serves well as anode-active material. The graphite-CNT film electrode showed excellent cycling stability with a specific capacity of 353 mAhg1 (based on the mass of graphite) at 0.5 C and coloumbic efficiency of >99% after 50 cycles (see Figs. S2C and D). To know the contribution of CNT film to the capacity of graphite-CNT film electrode, a half-cell was assembled using only the CNT film without graphite. The discharge capacity of the CNT film maintained stability until the 50th cycle as shown in Fig. 3C. The contribution of CNT film to the capacity of graphite-CNT film
Fig. 3. Cyclic voltammetry of the CNT film in the ranges of (A) 2.8 Ve4.2 V (cathode) and (B) 0 Ve2 V (anode). (C) The absolute capacity of the graphite-CNT film electrode and the CNT film electrode. (D) The capacity contribution of the CNT film and the graphite. Comparison of the gravimetric specific capacity (based on the total mass of the electrode and the electrode materials) of (E) the LCO-Al foil and LCO-CNT film electrode, and (F) the graphite-Cu foil and the graphite-CNT film electrode. Much higher gravimetric specific capacity was achieved in the electrode with CNT film. (A colour version of this figure can be viewed online.)
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was about 5.7% (Fig. 3C and D), which is a small portion of the capacity obtained by the graphite-CNT film electrode. The use of CNT film as a current collector is highly advantageous in terms of gravimetric specific capacity because the CNT current collector (0.174 mgcm2) is so light that its density is only 1/45 of copper and 1/32 of aluminum current collector. The loading of electrode materials is about 2 mg cm2 and the weight ratio of electrode material to CNT film is 11. On the other hand, the weigt ratio of electrode materials to Al foil (MTI Korea, 5.61 mg cm2) and Cu foil (MTI Korea, 7.83 mg cm2) are 0.35 and 0.26. To compare the gravimetric specific capacity, coin cells were fabricated using metallic current collectors (Cu and Al foils). The gravimetric specific capacity based on the total mass of the electrode (Celectrode) and the mass of the LCO and the graphite (CLCO, Cgraphite) are shown in Fig. 3E and F. For the cathode part, no significant difference in the specific capacity (CLCO) was observed between the LCO-CNT film electrode and the LCO-Al foil electrode. However, the Celectrode of the LCO-CNT film electrode was 2 times higher than that of the LCO-Al foil electrode. Also, the graphite-CNT film electrode exhibited higher gravimetric specific capacity than the graphite-Cu foil electrode in both Celectrode and Cgraphite, as shown in Fig. 3F. The higher specific capacity is due to the contribution of CNT film as an active material. These findings reveal that the CNT film, which is stable in electrochemical reactions and has light weight, has a high potential as the current collector for flexible batteries with high capacity. After the electrochemical reactions in the half cell were verified, the graphite-CNT film and LCO-CNT film were assembled into a full cell. The full cell was charged/discharged at 0.5 C (1C ¼ 145 mAg1) after the formation process (1 cycle at 0.1 C) and the capacity ratio of graphite-CNT film anode to LCO-CNT film cathode was selected to be ~1.1:1 (N/P ratio ¼ 1.1) for the full cell capacity balance. The initial discharge capacity and the capacity retention of 141.84 mAhg1 and 84%, respectively, were obtained with high coloumbic efficiency (>98%) after 50 cycles, as shown in Figs. S3A and B. The reason for the low capacity retention compared to the half-cell is probably due to the absence of Li metal. The Li metal used as a counter electrode in the half cell can replenish Li ions lost during the charge/discharge process, while the full cell cannot replenish the lost Li ion because of an insufficient supply of lithium ion [33]. Therefore, additional work is required to improve the capacity retention with cycling of the full cell by adjusting the N/P ratio [34]. Additionally, a pouch-type full cell was fabricated to demonstrate its potential as a flexible battery as shown in Fig. S4. A series of photographs showed that a red LED worked well and the open circuit voltage (OCV) was kept stable even if the pouch cell was folded in four equal parts and unfolded back. This indicates that the prepared CNT film is very suitable as a flexible electrode. 3.3. Characterization of fiber battery The cotton yarn also has to be chemically stable in the electrolyte in order to conduct as electrolyte support. To verify the stability, the cotton fiber was immersed in the electrolyte for one day and the tensile property was measured. No obvious change in the tensile property was observed for the cotton (see Fig. S5), which indicates that the cotton yarn does not react with the electrolyte. Fig. 4A shows the fiber battery assembled with the as-prepared electrodes and cotton yarn. The fiber battery in the photograph has a length of 9 cm and a diameter of 0.91 mm. High volumetric energy density may be essential in order for the fiber battery to be used as a power source for wearable devices. Therefore, the amount of absorbed electrolyte and swelling are very important features considering the requirement of high volumetric energy density. The optical microscope (OM) image of the cross-section in Fig. 4B
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displays the coaxial structure of the fiber battery. The coaxial structure has large interfacial areas between the anode and cathode, which is a favorable condition for transfer and diffusion of lithium ions. A pair of the OM images (Fig. 4C and D) shows that the cotton yarn was swollen after absorbing the electrolyte. The degree of swelling and uptake of electrolyte were about 20% and 900%, respectively (calculated by equations (1) and (2)).
Degree of swelling ð%Þ ¼
Uptakeð%Þ ¼
D2 D1 100 D1
W2 W1 100 W1
(1)
(2)
where D1 and W1 are diameter and weight of the cotton yarn before absorbing the electrolyte, D2 and W2 are the diameter and weight of the cotton yarn after absorbing the electrolyte. High uptake of organic electrolyte (1 M LiPF6 in EC:DEC 1:1 v/v) by cotton yarn is due to internal empty space and the high polarity of cotton yarn [35]. Presence of the carbonate groups in solvents increases the polarity of electrolyte and leads to good affinity between the cotton yarn and the electrolyte. Unlike coin and pouch cells, the fiber battery has a very narrow internal space in which the electrolyte can be contained, which can make it difficult for the battery to operate normally. Therefore, the high electrolyte uptake of cotton yarn, which is 9 times its own weight, plays a very important role in achieving a high performance fiber battery. Moreover, the swelling of cotton yarn has an additional effect of making the contact between the components of the fiber battery more robust. It is not enough to tighten the elements of the fiber battery sufficiently by shrinkage of the thermal-shrinkage tube only. To clarify the function of cotton yarn, the stress induced by the swelling of cotton yarn was calculated using the modified stress generation model during the lithiation [36] with following equations.
s_ r ¼
! !# " E dVr _c _p Vr _c _p εr εr þ v εq εq ð1 þ nÞ ð1 þ nÞð1 2nÞ dr r " ! !# E dVr _c _p Vr _c _p εr εr εq εq s_ r s_ q ¼ (3) ð1 þ nÞ dr r
where r is fiber radius, ѵ is Poisson's ratio and Vr is radial velocity. As shown in Fig. 4E and (F), the electrolyte was completely absorbed into the cotton yarn in 2.5 s. Also, 86.8 kPa of stress in axial direction and 67.4 kPa of stress in circumferential direction of the cotton yarn were induced in the fiber battery by swelling (Simulation details in supporting information). The proof-of-concept experiments for the coaxial fiber type battery, the fiber battery was fabricated with cotton yarn (C-FB). Another fiber battery was constructed using polypropylene (PP) fiber (diameter: 0.24 mm, PP-FB) as a control, which does not absorb the electrolyte owing to the lack of affinity with the organic electrolyte and solid structure of the PP fiber. Compared to C-FB, the PP-FB will contain a smaller amount of electrolyte and less tightened interfacial contact between the electrodes. The electrochemical impedance spectroscopy (EIS) of fiber type symmetric battery (LCO/LCO) was conducted to verify role of cotton yarn and prove the simulation results. Since the effect of electrode can be excluded, the EIS of symmetric cell can provide direct insight into the role of cotton yarn in fiber shape battery. The high-frequency range (>100 kHz) and the mid-frequency range (100 kHze1 Hz) in the spectra are associated with the ohmic resistance and interfacial phenomena, respectively [37]. As shown in Figure G and H, PP-FB
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Fig. 4. (A) Photograph of the fiber battery with length of 9 cm. OM image of (B) cross section of fiber battery with diameter of 1.2 mm. OM images of side view of cotton fiber (C) before absorbing the electrolyte and (D) after absorbing the electrolyte. Simulation of (E) the expansion of cotton fiber and (F) the development of stress in the cotton fiber surface during expansion due to the absorption of electrolyte. Nyquist plots for fiber type symmetric cells using (G) cotton yarn and (H) PP fiber in the frequency range from 100 kHze100 mHz. (A colour version of this figure can be viewed online.)
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Fig. 5. (A) Voltage profiles of the C-FB battery at the 1st cycle. (B) Comparison of cycle performance of C-FB battery and PP-FB battery. (C) Nyquist plots of fiber batteries after 3 cycles in the frequency range from 100 kHze100 mHz (inset: Nyquist plots of the C-FB). (D) Comparison of volumetric energy density and specific capacity with other reported fiber shape batteries.
showed very high resistance compared to C-FB at all frequency. This results and previous simulation results strongly suggest that the cotton yarn conducted a critical role of performance of fiber battery by improving the contact between the components. This difference in core fiber affected the electrochemical performance as shown in Fig. 5. Fig. 5A shows that voltage profiles similar to those of the coin-type full cell were obtained from charge/discharge tests of the C-FB battery (N/P ratio ¼ 1.1), which rationalizes the principle of designing the C-FB battery. Fig. 5B shows the charge and discharge performance of the C-FB and the PP-FB batteries. The initial discharge capacity and the capacity retention of the C-FB battery were 0.24 mAhcm1 and 84%, respectively. It should be noted that the capacity retention of the CFB battery is similar to that of conventional coin cell (Fig. S3), which implies that the coaxial structure is well suited to fiber-type battery. In contrast, the PP-FB battery exhibited a discharge capacity of 0.09 mAhcm1, lower than that of the C-FB battery, which rapidly declined in subsequent cycles. The reason for the poor performance of the PP-FB battery can be deduced from the impedance spectra in Fig. 5C. As shown in Fig. 5C, the PP-FB battery has a higher resistance in both the high- and mid-frequency range than that of the CFB battery. These results are well agreed with Fig. 4G and H. Also, similar results were reported in the cable type battery with hollow structure [14]. The internal space by hollow structure helps electrolyte easily permeate and access cell components, which results in reducing cell resistance and making it operate normally. Likewise, the cotton yarn, which holds the electrolyte and forms a skeleton of fiber battery, increases accessibility of electrolyte to electrodes and enhances the electrochemical performance. The performance of C-FB battery with concentric structure was
compared with the wire battery fabricated by the combining of anode fiber and cathode fiber followed by twisting [20]. The C-FB battery delivered capacity of 0.24 mAhcm1, which is 86 times higher than that of the wire-shaped battery (0.0028 mAhcm1). Also, the volumetric energy density (based on the C-FB battery volume which includes cotton yarn) of the C-FB battery is 144.82 mWhcm3 (the areal energy density based on the cross-sectional area of the C-FB is 72.41 mWhcm2), which is 8 times higher than that of the wire battery (17.7 mWhcm3 [20]). Moreover, when the volumetric energy density of the C-FB battery is calculated only by the two electrode volumes, it reaches to 587.91 mWhcm3, which is 6 times higher than that of the coaxial fiber battery fabricated with a cotton yarn and silicon anode (99.3 mWhcm3, based on the electrode volume [21]). Comparison of volumetric energy density and specific capacity with other reported fiber shape batteries is presented in Fig. 5D and data about state-of-the-art fiber-shaped battery are summarized in Table S2. To best our knowledge, the volumetric energy density (587.91 mWhcm3) of the C-FB battery is the best value compared to others reported elsewhere. The difference arises from the fact that the cotton yarn was limited to its function as a substrate [21] while the cotton yarn in this study played the role of supporting electrolyte as well as improving the interfacial contact between the electrode elements. This indicates that the cotton yarn and the concentric structure play important roles in achieving good performance, which is also verified from the higher coloumbic efficiency of 95% than that of the wire battery (80%) [20]. The fiber battery used for energy storage in wearable devices needs to have adequate flexibility. In other words, the fiber battery should operate normally even under severe mechanical
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Fig. 6. (A) Photograph of a film-type separator buckled by bending deformation. (B) SEM image of the film-type separator. (C) Photograph of a PVDF nano-web separator under bending, exhibiting conformal deformation on the surface of electrode. (D) SEM image of the PVDF nano-web consisted of numerous nanofibers. (A colour version of this figure can be viewed online.)
Fig. 7. The photographs of LED lit by (A) an undeformed fiber battery and (B) a knotted fiber battery. The SEM image (inset of (B)) displays the surface of the bent part of electrode, exhibiting no falling off of active materials from the electrode. (C) Nyquist plots of undeformed and knotted fiber batteries after 3 cycles in the frequency range from 100 kHze100 mHz. (A colour version of this figure can be viewed online.)
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Fig. 8. The photographs of (A and B) examples of application of the fiber battery as a string of necklaces. (A colour version of this figure can be viewed online.)
deformation such as twisting and knotting. The flexibility of cathode and anode presented in Fig. 2 was due to the unique feature of CNT film as a three-dimensional nanoporous structure. Likewise, a separator should accommodate the deformation applied to the fiber battery. Generally, the separator is made of polyolefin film, which cannot accommodate in-plan deformation. As a result, the film-type separator buckles under bending deformation caused by the restricted movement in the plane as shown in Fig. 6A and B. The buckling may also impart undesirable deformations to the components of the fiber battery. As an alternative, the PVDF nano-web can be a proper candidate for the separator of the fiber battery. The PVDF nano-web can accommodate in-plane deformation because it consists of numerous nano fibers which can move in the plane as shown in Fig. 6C and D. In addition, the porous nanostructure of the PVDF nano-web is a favorable factor in achieving good electrochemical performances of the fiber battery because it shortens the lithium ion pathway and causes uniform redox reactions [38]. It has been verified that the fiber battery fabricated in this way works well under deformed conditions. Fig. 7A and B displays that a blue LED is well lit even when the fiber battery is knotted. The inset SEM image in Fig. 7B shows that the deformed surface of the electrode remained stable without a falling off of active materials even under a knotted condition, which demonstrates the outstanding mechanical stability of the as-prepared electrodes. To investigate the effect of the external force on the electrochemical performance of the fiber battery, the charge/discharge test was conducted for the knotted fiber battery and displayed in Fig. S6. Nevertheless, it is observed that there is a slight difference in the brightness of the LED turned on by the two fiber batteries, as shown in Fig. 7A and B. The reason for the brighter LED can be explained by the impedance results in Fig. 7C. It can be seen that the impedance decreased remarkably when the fiber battery was knotted. The reason for this can be explained as follows: 1) During the knotting, external force is applied to the components of the fiber battery, which increases interfacial force between the components of the fiber battery and decreases the distance between the electrodes. Consequently, it leads to a decrease of the lithium ion pathway and the charge transfer resistance. 2) It has been known that the impedance decreases as the amount of electrolyte increases [39]. The function of knotting is like squeezing the cotton fiber and letting the electrolyte come out. As a result, the amount of available electrolyte increases, which results in enhancing the ionic conductivity. In this study, a fiber battery has been fabricated and the battery has proven to work well under bending deformation such as knotting. Fig. 8A and B shows the high potential of the fiber battery as energy storage for wearable devices. These results suggest that the fiber battery can be put to practical use.
4. Conclusions In summary, a coaxial fiber battery was fabricated by winding CNT film electrodes onto a cotton yarn. Due to its unique structure, the fiber battery was flexible enough to make a knot. The cotton yarn, which functioned not only as a skeleton for the fiber shape, but also as an electrolyte reservoir, absorbing 900% of its own weight of electrolyte and swelling to about 20% in diameter after absorbing electrolyte. These characteristics of cotton yarn improved interfacial contact between components of the battery, which resulted in high performance of the coaxial fiber battery. Also, a nano-web was selected as a separator to avoid buckling by accommodating in-plane deformation. As a result, the coaxial fiber battery exhibited a high volumetric energy density of 144.82 mWhcm3 (based on the fiber battery's volume) and operated well even under severe mechanical deformation such as bending and knotting. From these results, it can be said that the novel coaxial fiber battery represents a significant step forward for wearable energy storage. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (NRF-2017R1A5A1015596). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.02.081. References [1] B. Liu, J. Zhang, X. Wang, G. Chen, D. Chen, C. Zhou, et al., Hierarchical threedimensional ZnCo2O4 nanowire arrays/carbon cloth anodes for a novel class of high-performance flexible lithium-ion batteries, Nano Lett. 12 (2012) 3005e3011. [2] H. Gwon, H.S. Kim, K.U. Lee, D.H. Seo, Y.C. Park, Y.S. Lee, et al., Flexible energy storage devices based on graphene paper, Energy Environ. Sci. 4 (2011) 1277e1283. [3] X. Duan, Assembled semiconductor nanowire thin films for high performance flexible macroelectronics, MRS Bull. 32 (2007) 134e141. [4] Z. Wang, H. Wang, B. Liu, W. Qiu, J. Zhang, S. Ran, et al., Transferable and flexible nanorod-assembled TiO2 cloths for dye-sensitized solar cells, photodetectors, and photocatalysts, ACS Nano. 5 (2011) 8412e8419. [5] K.A. Sierros, D.S. Hecht, D.A. Banerjee, N.J. Morris, L. Hu, G.C. Irvin, et al., Durable transparent carbon nanotube films for flexible device components, Thin Solid Films 518 (2010) 6977e6983. [6] H. Gwon, J. Hong, H. Kim, D.H. Seo, S. Jeon, K. Kang, Recent progress on flexible lithium rechargeable batteries, Energy Environ. Sci. 7 (2014) 538e551. [7] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359e367. [8] K. Wang, S. Luo, Y. Wu, X. He, F. Zhao, J. Wang, et al., Super-aligned carbon nanotube films as current collectors for lightweight and flexible lithium ion
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