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A Facile Methodology for the Development of a Printable and Flexible All-Solid-State Rechargeable Battery Bibekananda De,† Amit Yadav,‡ Salman Khan,‡ and Kamal K. Kar*,†,‡ †

Department of Mechanical Engineering and ‡Materials Science Programme, Advanced Nanoengineering Materials laboratory, Indian Institute of Technology Kanpur, Kanpur-208016, India S Supporting Information *

ABSTRACT: Development of printable and flexible energy storage devices is one of the most promising technologies for wearable electronics in textile industry. The present work involves the design of a printable and flexible all-solid-state rechargeable battery for wearable electronics in textile applications. Copper-coated carbon fiber is used to make a poly(ethylene oxide) (PEO)-based polymer nanocomposite for a flexible and conductive current collector layer. Lithium iron phosphate (LiFePO4) and titanium dioxide (TiO2) are utilized to prepare the cathode and anode layers, respectively, with PEO and carbon black composites. The PEO- and Li salt-based solid composite separator layer is utilized for the solidstate and safe electrolyte. Fabrication of all these layers and assembly of them through coating on fabrics are performed in the open atmosphere without using any complex processing, as PEO prevents the degradation of the materials in the open atmosphere. The performance of the battery is evaluated through charge− discharge and open-circuit voltage analyses. The battery shows an open-circuit voltage of ∼2.67 V and discharge time ∼2000 s. It shows similar performance at different repeated bending angles (0° to 180°) and continuous bending along with long cycle life. The application of the battery is also investigated for printable and wearable textile applications. Therefore, this printable, flexible, easily processable, and nontoxic battery with this performance has great potential to be used in portable and wearable textile electronics. KEYWORDS: flexible solid-state battery, printable energy storage device, copper-coated carbon fiber, polymeric composite, wearable electronics, lithium

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INTRODUCTION Having attributes of being small, thin, lightweight, inexpensive, flexible, or even rollup in order to meet the current demands of the modern market, portable electronics are the emerging and promising technologies for the next generation optoelectronic devices such as smart phones, computers, cameras, rollup displays, and other smart electronics and wearable devices.1−4 The increasing level of pollution due to the combustion of fossil fuels is driving us to generate energy from environmentally friendly sources such as solar, wind, geothermal, etc. However, such types of renewable energy sources mainly depend on weather for power supply, which is unpredictable and inconsistent. Therefore, efficient storage and delivery of these sustainable energies is extremely important. Flexible and rechargeable batteries are one of the most attractive and promising candidates for this purpose.5 Several types of flexible batteries such as flexible alkaline batteries, 6−8 plastics batteries,9,10 polymer lithium−metal batteries,11 and rechargeable lithium-ion batteries,12−15 have been invented. Li-ion batteries exhibit higher volumetric and gravimetric energy density, higher output voltage, and longer cycle life compared to the other secondary batteries.16−19 Many novel technologies and processes, as well as several new types of materials such as nanotubes, carbon fiber, graphene, 3-D tubular and 2-D © XXXX American Chemical Society

nanomembranes, nanostructured silicon, and nanostructured metal oxide, have been utilized to make flexible electrode material for Li-ion batteries.20−24 Different kinds of flexible substrates, composite electrodes, and gel electrolytes have also been reportedly used to fabricate solid-state flexible batteries.22−26 However, it is still a great challenge to design a highly flexible all-solid-state battery with high mechanical strength, excellent electrical stability, and conductivity. Current batteries are also unable to sustain stable power, energy supply, and cyclic stability under frequent bending and twisting.22 This may be due to the nonflexible materials that are mostly used in the electrodes, as well as to poor contact between components of the batteries, particularly between active electrodes and current collectors. The problem is associated with electrolyte leakage, which is the main cause of short circuit damage to the battery. Thus, development of a mechanically strong flexible battery consisting of flexible electrodes embedded in a highly flexible and electrically conducting current collector with the use of conducting additives and binders is necessary. Ionic conductivity and mechanical properties of the flexible solid Received: March 23, 2017 Accepted: May 23, 2017 Published: May 23, 2017 A

DOI: 10.1021/acsami.7b04112 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

prepared by the reported method.29 Triton X-100 and methanol were obtained from Loba Chemie. Preparation of Cu-Coated CFs (Cu-CFs). An electroless deposition method was used to prepare Cu-CFs without using any external electrical power. The method consists of two steps. In the first step, a bunch of CFs (∼1000 filaments, length ∼10 cm, and weight ∼100 mg) were immersed into an aqueous glucose (Glucon-D) solution (5 g in 100 mL of deionized water) for 2 h at room temperature to generate some aldehyde groups on the surface of the fibers, which will assist the binding of copper during coating. Then these CFs were transferred into an aqueous silver nitrate (AgNO3) solution (2 g in 100 mL of deionized water) and dipped for 30 min to activate the surface of the fibers. In the second step, copper was coated on the surface of the fiber by dipping the activated CFs into a bath containing an aqueous solution of cupric sulfate (CuSO4·5H2O, 2.60 g) as the copper source, formaldehyde (HCHO, 1.75 mL) as the reducing agent, sodium potassium tartrate (KNaC4H4O6·4H2O, 4.30 g) as the chelating agent, and sodium hydroxide (NaOH, 2.10 g) as the neutralizing agent for the reaction. Before the CFs were dipped, these reagents were mixed with deionized water and stirred for 30 min at room temperature. The dipping process was continued until all the CFs were covered with copper. The thickness of the copper coat was mainly dependent on the dipping time. Some amount of benzonitrozole was used to remove adhesion phenomena during this coating. Fabrication of the All-Solid-State Flexible Battery. An allsolid-state flexible battery was fabricated that consisted of polymer composite-based flexible electrodes separated by flexible solid electrolyte and embedded in a highly flexible and conducting current collector. First, a flexible and conducting current collector (CC) was prepared using a polymer composite of PEO, Cu-CFs, and CB. Different weight percentages of chopped Cu-CFs were mixed homogeneously with CB and PEO by stirring in methanol. The composition ratio of Cu-CFs, CB, and PEO was optimized through checking the electrical conductivity and dispersion level of the fibers. A flexible solid-state cathode and anode were prepared through mixing 50 wt % LiFePO4 and TiO2 separately with 50 wt % PEO by stirring in methanol to balance electronic conductivity and strength. A small amount of CB (0.5 wt %) was added to the solution in each electrode to provide a channel for electronic conductivity. In the same way, the flexible solid polymer electrolyte (SPE) was prepared by mixing PEO (60 wt %) with LiCF3SO3 (20 wt %) and chopped glass fibers (GFs) (20 wt %) in methanol. Then all respective materials in a dispersed state in methanol were used to fabricate the flexible solid-state battery. A small amount of Triton X-100 (4−5 drops) was added to increase the plasticity of the layers. All the respective solutions were stirred at 600 rpm at 80 °C for 30 min and followed by sonication overnight at room temperature to make homogeneous pastes. Then the materials (total amount of the active material, 6 mg) were drop-casted layer by layer on fabric substrates (area, 2 × 2 cm2) and dried in an oven at 50 °C. Aluminum foil was used as an electron collector on the anode side of the battery. Characterization. X-ray diffraction (XRD) was carried out at room temperature to analyze the structure of the materials on a Panalytical XPert diffractometer using Cu Kα radiation (0.154 nm) at a scan rate of 3° min−1. The elemental composition of the samples was analyzed using an EDS tungsten-electron microscope (W-SEM, Model JSM-6010LA, JEOL) at an accelerating voltage of 20 kV and magnification 2000×. The surface morphology of the layers of the battery was studied by scanning electron microscopy (SEM) using a QUANTA 200 FEI microscope at an operating voltage of 10 kV with different magnifications (500×, 1000×, etc.). The conductivities of the current collector and solid polymer electrolyte were measured by impedance spectroscopy and the four-probe method. The thickness of the samples was measured by a caliper, and the conductivity was obtained from the resistance. Electrochemical Analysis. The electrochemical analysis of the battery was carried out by a charge−discharge test. Constant currents of 0.010, 0.020, 0.030, and 0.040 mA were applied on a 2 × 2 cm2 battery sample during the charge−discharge test, and the maximum

polymer electrolyte also need to be developed. Further, the current collector provides the structural support for the battery component and ensures the proper electrical contact with active electrodes. However, the use of heavy weight metals in conventional current collectors reduces the gravimetric capacity of the battery. Active electrode material coated on the current collector may easily lose contact with the current collector when it is subjected to bending, and its original shape is also difficult to restore after deformation. When these metallic current collectors are replaced by lightweight flexible current collectors, the energy density and contact with the substrate can be improved.22 Therefore, in the present work an attempt has been made to develop a flexible battery by making all components solid-state flexible. In this battery, copper-coated carbon fibers (Cu-CFs) are used as current collector, lithium iron phosphate (LiFePO4) as cathode, and titanium dioxide (TiO2) as anode. Poly(ethylene oxide) (PEO) and carbon black are used in each layer to provide better conductivity and mechanical stability. Composites of Li salt, PEO, and chopped glass fibers (GFs) are utilized as a flexible separator membrane as well as a flexible solid-type electrolyte. Here the main advantage of the use of LiFePO4 as a cathode over other positive electrode materials is the superior thermal and chemical stability, which ensure better safety characteristics. The specific capacity of LiFePO4 is ∼170 mAh/g, which is higher than that of conventional LiCoO2. Ttitanium-based oxide has drawn attention in the field of Li-ion batteries as a negative electrode material because of its negligible safety issues. It also has other interesting features such as being less expensive and having low toxicity, excellent cycle life, and low volume change (2−3%) on both insertion and deinsertion of lithium. Furthermore, it has high electroactivity, good chemical stability, strong oxidation capability, high abundance, and structural diversity.27,28 It also shows excellent safety and good stability characteristics at the operating voltage of 1.5 V. Further, the use of solid polymer electrolyte has several specific advantages such as high safety, low cost, high energy density, easy fabrication, good electrochemical stability, and excellent compatibility with lithium salts. However, the ionic conductivity of PEO is high only above its melting point (∼60 °C), which narrows the practical application of the battery with PEO as the solid polymer electrolyte. Hence, to increase the ionic conductivity, glass fibers were added, which will also provide mechanical strength to the separator layer. Finally, the battery was fabricated in an open environment by coating the materials layer by layer on a fabric. So, the fabrication process is highly cost-effective and easy. The battery exhibited high electrochemical performance with long cycle life. It has sufficient conductivity and proper viscosity for coating and maintaining its function even when the battery is repeatedly bent. Therefore, this type of flexible, high performance battery with a nontoxic and environmentally friendly nature is highly attractive to the textile industry.

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EXPERIMENTAL SECTION

Materials. PEO (Mw = 500 000 g/mol) was obtained from Loba Chemie Pvt. Ltd. Carbon black (CB), LiCF3SO3, and TiO2 were purchased from Alfa Aesar, Aldrich, and E. Merck (India), respectively. Cupric sulfate, formaldehyde, sodium hydroxide, and sodium potassium tartrate were acquired from Aldrich. Polyacyronitrile (PAN)-based carbon fibers (CFs) (specific gravity ∼1.74 g cm−3, tensile strength ∼1900−2450 MPa, and diameter ∼8 μm) were purchased from Zoltek Corporation, Bridgeton, MO. LiFePO4 was B


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Figure 1. (a) Electroless deposition setup for Cu-CFs. Digital photographs of (b) Cu-CFs and (c) chopped Cu-CFs. (d) Schematic representation for the preparation of the electrodes, electrolyte, and current collector layers. (e) Schematic illustration and (f) digital photograph of the current collector layer. (g) Schematic diagram of the flexible fabric battery with its component layers. (h and i) Digital photograph of the battery coated on fabric. charge−discharge time was fixed for 5.5 h. The charge−discharge test was also performed in bending mode of the samples to check flexibility of the battery. The open circuit voltage (OCV) of the fabricated battery was measured using a multimeter.

CFs, as the standard redox potential for copper (E° = +0.337 V) is more electropositive than that of carbon (E° = −0.20 V) in the electrochemical series.30This cathodic reaction will continue until all the carbon fibers are covered with copper. There is a lack of adhesion between carbon fibers during coating, and thus a small amount of benzonitrozole solution is used to remove these phenomena. During deposition of copper, the potential of the medium must be lower than +0.337 V, as the copper ions are thermodynamically stable above this potential in acidic medium. In this reaction, oxidation of HCHO to formic acid or formate anion also occurs below this potential. Again, the local changes in pH value lead to precipitation of the metal in bulk solution. Therefore, a chelating agent, sodium potassium tartrate, is used to maintain the metal ions in solution by complexing the free metal ion that dissociates from the the metal complex. It also allows the solution to be used at higher pH value. The pH value of the copper solution keeps changing during the coating, which affects the rate of deposition and quality of the coating. Thus, NaOH base is added to maintain the pH of the solution. The

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RESULTS AND DISCUSSION Fabrication and Characterization of the Flexible Solid-State Battery. Preparation of the primary materials before design of the all-solid-state battery and fabrication of the battery are schematically illustrated in Figure 1. Preparation of the Cu-CFs-based polymer composite afforded a highly flexible and excellent conductive current collector. The Cu-CFs are produced through electroless copper deposition on CFs as shown in Figure 1a. The Cu2+ salt is reduced by HCHO (shown in eq 1) in the presence of NaOH and deposited on Cu 2 + + 2HCHO + 4OH− → Cu + 2HCOO− + 2H 2O + H 2

(1)

C


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Figure 2. SEM images (at 5000× magnification) of Cu-CFs at coating times of (a) 1 min, (b) 2 min, and (c) 5 min. (d) XRD pattern and (e) EDS spectrum of Cu-CFs.

Figure 3. XRD patterns of (a) Cu-CFs, PEO, and the current collector layer. (b) LiFePO4, PEO, and cathode layer. (c) LiCF3SO3, PEO, and separator layer. (d) TiO2, PEO, and anode layer.

thickness of the coating depends on the coating time and varies from 1 to 5 min. When time allowed is 1 min, the deposition is found to be much less, whereas when the deposition time is increased to 5 min, the coating is continuous and covers the whole surface of the CFs, as shown in Figure 2a−c. Figure 2

shows that with the increase in deposition time, the coating is more stable on the surface of the CFs. Therefore, a 5 min coating is the optimized time and the final thickness of the coating is ∼2.0 to 2.5 μm. The thickness of the coating was measured by comparing the thickness of the pure CF and CuD


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Figure 4. EDS spectra of (a) current collector layer, (b) cathode layer, (c) separator layer, and (d) anode layer. Here all the inset SEM images show the selected area for the analysis.

which correspond to the crystallographic planes of (101), (004), (200), (105), (211), and (204), respectively, according to JCPDS card no. 00-021-1272 of tetragonal anatase titanium oxide. In the XRD pattern of the anode layer, all the TiO2 peaks are present. However, the intensity of crystalline peaks for PEO is diminished at 2θ of 19.19° and 23.53°, which may be due to strong polar−polar interactions between PEO and TiO2. The very intense peaks of TiO2 also mask the peak intensity of PEO. Therefore, the XRD study shows that the chemical structure of all the layers is similar to the basic elements used to fabricate them. This is further supported by EDS study. The EDS analysis for the current collector, cathode, separator, and anode layers is shown in Figure 4. The current collector (Figure 4a) shows weight and atomic ratios of the elements carbon, oxygen, and copper to be 69.31:24.22:6.47 and 78.13:20.49:1.38, respectively. The elemental weight and atomic ratios of carbon, oxygen, phosphorus, and iron in the cathode layer (Figure 4b) are found to be 51.34:38.31:3.66:5.04 and 61.63:34.52:1.70:1.30, respectively. However, lithium is not detected in EDS analysis, as X-ray is used for this elemental analysis. The elements carbon, oxygen, fluorine, sulfur, and silicon are found to be 51.34:37.20:10.60:3.20:3.69 and 54.72:33.80:8.11:1.67:1.70 in weight and atomic ratios, respectively, in the EDS spectrum (Figure 4c) of the separator layer. Here carbon, oxygen, sulfur, and fluorine are the main elements of LiCF3SO3, and silicon comes from glass fiber used in the separator layer. Figure 4d shows the elemental ratios of carbon, oxygen, and TiO2 are 36.80:39.51:23.69 in weight and 50.83:40.97:8.21 in atomic, respectively, for the anode layer. Here carbon and oxygen come from PEO used in the anode layer.

CF. The SEM image of pure CF is shown in Figure S1. The same thickness of the coating was also found from the crosssectional SEM images of Cu-CF, as shown in Figure S2. XRD analysis of Cu-CFs (Figure 2d) shows three diffraction peaks at 2θ of 43.29°, 50.43°, and 74.13°, corresponding to the (111), (200), and (220) crystallographic planes of copper. This pattern perfectly matches the standard JCPDS card reference code no. PDF 00-004-0836 of the cubic copper system. The carbon (002) and (100) planes of CFs are found at 2θ of 25.80° and 38.80°, respectively. From the EDX spectrum (Figure 2e) of Cu-CFs, the atomic and weight fractions of Cu and CFs are found to be 98:2 and 99:1, respectively. The XRD patterns for different layers of the battery and their different components are shown in Figure 3. Figure 3a shows the XRD patterns of the current collector and its main components PEO and Cu-CFs. The carbon (002) and (100) peaks, and copper (111) and (200) planes, are found at 2θ of 16.73° and 23.34°, and 43.22° and 49.05° in the XRD pattern of the current collector. The PEO peak is located at 19.66°. Figure 3b shows XRD patterns of the cathode and its main constituents. The XRD pattern of LiFePO4 shows seven peaks at 2θ of 17.12°, 20.752°, 25.50°, 29.67°, 35.52°, 52.42°, and 61.58°, corresponding to the crystallographic planes of (020), (011), (111), (121), (131), (222), and (400), respectively, according to the JCPDS card no. 00-040-1499 of orthorhombic LiFePO4. All these peaks are found in the XRD pattern of the cathode layer with two peaks at 19.14° and 23.20° of the crystalline polymer PEO whereas the intense peak of PEO is decreased in the XRD pattern of the solid-state polymer electrolyte (Figure 3c) after the addition of Li salt.31 In the XRD pattern (Figure 3d) of pure TiO2, six peaks are observed at 2θ of 25.48°, 37.76°, 48.09°, 54.08°, 55.11°, and 62.76°, E


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Figure 5. SEM images of the current collector layer (a) at magnification 5000×. (b) Cross-section area at magnification 500× and (c) cross-section area at magnification 2000×. (d) SEM images of the cathode layer at magnification 2000× and (e) cross-section area at magnification 500×. (f) Cross-section SEM image of the cathode attached to the current collector at magnification 1000×. SEM image of the separator layer at magnifications (g) 250× and (h) 5000×. (i) Cross-section image of separator layer attached to the current collector and cathode layers at magnification 2000×. (j) SEM image of the anode layer at magnification 5000×. Cross-section SEM images of the full battery (k) at 1000× and (l) 500× magnifications.

Figure 6. V−I characteristic curves of the current collector layer (a) without addition of Cu-CFs and (b) with addition of Cu-CFs (with different bending conditions).

F


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the increase in the amount of Li salt as given in Table 2. Glass fiber also increases the ionic conductivity as well as mechanical strength of the layer by providing channels for the movement of ions. To balance the conductivity, mechanical property, and dispersion level of the solid polymer electrolyte, 20 wt % Li salt and 20 wt % glass fiber optimizes the fabrication of the battery. Electrochemical Analysis. Electrochemical analysis of the battery is conducted through open-circuit voltage (OCV) and charge−discharge cycling, as shown in Figure 8. Before OCV measurement, the batteries are fully charged at 0.040 mA of current. The OCV of the batteries is measured up to 100 min using aluminum foil as an electron collector on the anode side, and the value is found to be 2.67 V, as shown in Figure 8a. The OCV value is found to be only 0.67 V without using any electron collector on the anode side. The OCV value of the batteries starts to decrease after several hours of measurement. The charge−discharge cycles of the batteries are shown in Figure 8b. The charge−discharge cycle of the batteries with repeatable bending (fold and unfold) at different angles (30− 180°) is also measured to observe the flexibility of the battery. Figure 8b shows that the bending did not affect the charge− discharge characteristic of the battery. Charge and discharge curves of the battery at different currents are shown in Figure 9. The charging scheme generally consists of a constant current charging until the battery voltage reaches the charge voltage and then constant voltage charging, which allows the charge current to taper until it is very small. Battery voltage mainly consists of three contributions, the difference between the chemical potentials of the electrodes and two electrical potentials of the double layers at the surface of each electrode, which is called the overpotential. The double layer potential is due to the rate of transfer of ions between electrodes and electrolytes. Figure 9a shows the charge cycle at constant current of 0.010, 0.020, 0.030, and 0.040 mA for 1000 s, in which charging is started from ∼1.6−2.0 V. The charge curve at 0.010 mA of the current attains maximum voltage of 2.62 V. As we decrease the applied charge current, the obtained voltage is decreased at a constant charge time. The maximum obtained voltage for 0.040 mA of current is 2.66 V. This phenomenon also occurred in discharge cycles as shown in Figure 9b. The total discharge time is 1000 s for all discharge curves. In the case of the discharge curve at 0.010 mA of current, the battery started to decrease from 2.0 V and after some time it attained a plateau of 0.25 V. It discharges up to a low voltage value of 0.04 V at 0.040 mA of current. As we increase the discharge current, the discharge voltage also starts to decrease. The areal capacity of the battery is found to be 3.7 mAh/cm2 at the discharge current of 0.01 mA, which is excellent to be used in wearable electronics in the textile industry. Long cycle life is an essential key factor for a high performance rechargeable battery. Charge−discharge curves of the battery at different cycles are shown in Figure 9c and 9d. Figure 9c shows the maximum charging voltage obtained at 1st cycle (2.66 V) and does not drop much after the 10th (2.64 V) and 20th (2.56 V) cycles. The voltage drop between 1st and 10th cycle is only 0.02 V, and it increases up to only 0.08 V after the 10th to 20th cycles. For any type of high performance battery, discharge cycles are more important than charge cycles. Here Figure 9d shows the discharge curves for the 1st, 10th, and 20th cycles, where all the curves exhibit the same discharge time. This is due to the excellent retention structural integrity of the battery materials during electrochemical performance.

The scanning electron microscopic (SEM) analysis (Figure 5) represents the surface morphologies of different layers of the battery. Figure 5a−c shows the SEM images of the current collector layer at different magnifications. The homogeneous distribution of PEO, CB, and Cu-CFs can be found from the images. The homogeneous distribution of lithium iron phosphate particles into the PEO matrix in the cathode layer can also be seen in Figure 5d−f. The strong attachment of the cathode layer to the current collector is observed from Figure 5f. The distribution of chopped glass fibers in the separator layer is shown in Figure 5g. The homogeneous distribution and the polymer coating on the surface of the glass fibers (Figure 5h) provide good mechanical strength with flexibility to the layer. The excellent contact between separator, cathode, and current collector layers is shown in the cross-section image (Figure 5i). Figure 5j shows homogeneous distribution of TiO2 particles in the PEO matrix in the anode layer. The crosssectional view of the battery is shown in Figure 5k and 5j, where current collector, cathode, separator, and anode layers are strongly attached to each other. The average thickness of the current collector, cathode, separator, and anode layers is found to be 25, 19, 23, and 60 μm, respectively. Electrical and Ionic Conductivity. Favorable electrical conductivity of the current collector layer and ionic conductivity of electrolyte are required for an ideal battery. Therefore, chopped Cu-CFs are added to the PEO- and CBbased composites to make the current collector layer favor electrical conductivity. The V−I characteristic of the current collector layer without addition of Cu-CFs (50 wt % of PEO and 50 wt % of CB) is shown in Figure 6a. It shows a potential drop of 3.3 mV at 1 mA of current, and the value is increased sharply up to 19.46 mV only at 5 mA of current. The current collector made of 50 wt % PEO, 20 wt % chopped Cu-CFs, and 30 wt % CB shows only 0.19 mV potential drop at 1 mA of current, and it is increased up to only 1.8 mV at 10 mA of current as shown in Figure 6b. The measured conductivity of the current collector without addition and with addition of CuCFs is found to be 1 × 102 and 2433.09 (S/m), respectively, as given in Table 1. The conductivity also increases with the Table 1. Measured Electrical Conductivity Values of Current Collector Layers without Addition of Cu-CFs and with Addition of 20 wt % of Cu-CFs current collector

thickness (mm)

conductivity (S/m)

without addition of Cu-CFs with addition of Cu-CFs

0.60 0.50

1 × 102 2.43 × 103

increase in the amount of Cu-CFs; however, the dispersion level is decreased. Therefore, 20 wt % of Cu-CFs is the optimized amount to make the current collector layer. The conductivity of the layer remains the same at different angles of bending as shown in Figure 6b. It also shows negligible changes to electrical conductivity with continuous bending. On the other hand, the ionic conductivity of the separator layer or solid polymer electrolyte is measured with the variation of weight percentages of Li salt (10, 15, and 20) and chopped glass fiber (30, 25, and 20) from the impedance plot, as shown in Figure 7. The obtained complex impedance plot consists of a semicircular portion due to the parallel combination of a capacitor and a resistor and a linear spike region. The bulk resistance and ionic conductivity are measured from the fitting curve. The ionic conductivity of the electrolyte increases with G


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Figure 7. Complex impedance plots of the electrolyte consisting of (a) 10 wt % LiCF3SO3−30 wt % glass fiber; (b) 15 wt % LiCF3SO3−25 wt % glass fiber, and (c) 20 wt % LiCF3SO3−20 wt % glass fiber using same amount of PEO (60 wt %). (d) Ionic conductivity of solid electrolyte at different wt % of PEO, LiCF3SO3, and glass fiber.

The retention of structural morphology of the full battery after 20 cycles is shown in Figure S3. Application of the Battery. Continuous development of the market for wearable electronic devices such as smart clothes, smart watches, and electronic bracelets are creating new services and products, which transform our life into the next level.32−34 These portable flexible devices require highly efficient power sources. These products with a flexible power source would be directly worn on the human body and perform stably under different bending conditions. It is known that our

Table 2. Ionic Conductivity of the Solid Electrolyte Layer at Different Weight Percentages of Li Salt and Glass Fiber sample

bulk resistance (ohm)

pure PEO PEO−10% LiCF3SO3 PEO−10% LiCF3SO3−30% GFs PEO−15% LiCF3SO3−25% GFs PEO- 20% LiCF3SO3-20% GFs

− − 9700 460 425

ionic conductivity (S/cm) 1.51 2.00 3.09 6.52 7.05

× × × × ×

10−7 20 10−7 20 10−6 10−5 10−5

Figure 8. (a) Measurement of the open-circuit voltage of a battery, without the current collector and with the aluminum current collector at the anode side. (b) Charge−discharge cycles of the battery at different bending angles and continuous bending during testing. H


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Figure 9. Charge and discharge curves of the battery (a, b) at a constant current of 0.010, 0.020, 0.030, and 0.040 mA, and (c, d) at 1st, 10th, and 20th cycles.

clothes can bear severe deformations during use, and building a printable battery on them is key to their high performance. Compared with conventional batteries, these printable flexible batteries are very thin and light. The thickness of the battery is less than 100 μm. This type of flexible battery can be used in textile industries provided that the electrodes used in the batteries are nontoxic and environmentally friendly. All the batteries are made in an open environment and are still capable to produce up to ∼2.67 V, and the 50 wt % content of PEO prevents degradation in the open environment. An anode− anode current collector combination is used for the first time, and it increases the power of the battery compared to the copper current collector. Figure 10 presents an example of a textile battery (coated on fabric, 2 × 2 cm2) to power a ∼2.7 V light-emitting diode (LED). In Figure 10a, an LED bulb is lit in normal state, where as Figure 10b shows that the bulb is lit in bending state. The battery provides power to the LED light for several hours, and it can be recharged repeatedly. Figure 10c shows that the battery is capable to produce ∼2.67 V. For practical application, we have connected four batteries in a series to provide enough power to light a bulb for several hours, as shown in Figure 10d. Therefore, the battery shows that it is coated on fabric and it has high flexibility and mechanical strength with less wear and tear resistance to human sweat if it is properly packed.

Figure 10. Lighting a LED bulb in (a) normal state and (b) bending state. (c) Battery provides an open-circuit voltage of 2.67 V. (d) Lighting a LED bulb for several hours by connecting four batteries in a series.

fabrication method is facile and cost-effective, as the battery is fabricated in an open atmosphere without using any complex process. The use of polymer PEO in all the layers prevents the degradation of the battery in the open environment. Coppercoated carbon fibers and a polymer-based composite current collector exhibits high flexibility and conductivity and is easy to

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CONCLUSIONS The present study successfully developed a flexible all-solidstate high performance printable battery in four layers. The I


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(9) Sultana, I.; Rahman, M.; Wang, J.; Wang, C.; Wallace, G. G.; Liu, H. K. All-Polymer Battery System Based on Polypyrrole (PPy)/Para (Toluene Sulfonic Acid) (pTS) and Polypyrrole (PPy)/Indigo Carmine (IC) Free Standing Films. Electrochim. Acta 2012, 83, 209−215. (10) Nyholm, L.; Nyström, G.; Mihranyan, A.; Strømme, M. Toward Flexible Polymer and Paper-Based Energy Storage Devices. Adv. Mater. 2011, 23, 3751−3769. (11) Shin, J. H.; Henderson, M. A.; Passerini, S. PEO-Based Polymer Electrolytes with Ionic Liquids and Their Use in Lithium MetalPolymer Electrolyte Batteries. J. Electrochem. Soc. 2005, 152, A978− A983. (12) Liu, B.; Zhang, J.; Wang, X.; Chen, G.; Chen, D.; Zhou, C.; Shen, G. Hierarchical Three-Dimensional ZnCo2O4 Nanowire Arrays/ Carbon Cloth Anodes for a Novel Class of High-Performance Flexible Lithium-Ion Batteries. Nano Lett. 2012, 12, 3005−3011. (13) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nano-Sized Transition-Metal Oxides as Negative-Electrode Materials for Lithium-Ion Batteries. Nature 2000, 407, 496−499. (14) Tang, Y.; Zhang, Y.; Deng, J.; Wei, J.; Tam, H. L.; Chandran, B. K.; Dong, Z.; Chen, Z.; Chen, X. Mechanical Force-Driven Growth of Elongated Bending TiO2-Based Nanotubular Materials for Ultrafast Rechargeable Lithium Ion Batteries. Adv. Mater. 2014, 26, 6111−6118. (15) Balogun, M. S.; Yu, M.; Li, C.; Zhai, T.; Liu, Y.; Lu, X.; Tong, Y. Facile Synthesis of Titanium Nitride Nanowires on Carbon Fabric for Flexible and High-Rate Lithium Ion Batteries. J. Mater. Chem. A 2014, 2, 10825−10829. (16) Tan, G.; Wu, F.; Zhan, C.; Wang, J.; Mu, D.; Lu, J.; Amine, K. Solid-State Li-Ion Batteries Using Fast, Stable Glassy Nanocomposite Electrolytes for Good Safety and Long Cycle-Life. Nano Lett. 2016, 16, 1960−1968. (17) Li, W.; Yang, L.; Wang, J.; Xiang, B.; Yu, Y. ThreeDimensionally Interconnected TaS3 Nanowire Network as Anode for High-Performance Flexible Li-Ion Battery. ACS Appl. Mater. Interfaces 2015, 7, 5629−5633. (18) Balogun, M. S.; Yu, M.; Huang, Y.; Li, C.; Fang, P.; Liu, Y.; Lu, X.; Tong, Y. Binder-Free Fe2N Nanoparticles on Carbon Textile with High Power Density as Novel Anode for High-Performance Flexible Lithium Ion Batteries. Nano Energy 2015, 11, 348−355. (19) Balogun, M. S.; Qiu, W.; Lyu, F.; Luo, Y.; Meng, H.; Li, J.; Mai, W.; Mai, L.; Tong, Y. All-Flexible Lithium Ion Battery Based on Thermally-Etched Porous Carbon Cloth Anode and Cathode. Nano Energy 2016, 26, 446−455. (20) Li, M.; Wahyudi, W.; Kumar, P.; Wu, F.; Yang, X.; Li, H.; Li, L.J.; Ming, J. Scalable Approach To Construct Free-Standing and Flexible Carbon Networks for Lithium−Sulfur Battery. ACS Appl. Mater. Interfaces 2017, 9, 8047−8054. (21) Lee, S. W.; Gallant, B. M.; Lee, Y. M.; Yoshida, N.; Kim, D. Y.; Yamada, Y.; Noda, S.; Yamada, A.; Shao-Horn, Y. Self-Standing Positive Electrodes of Oxidized Few-Walled Carbon Nanotubes for Light-Weight and High-Power Lithium Batteries. Energy Environ. Sci. 2012, 5, 5437−5444. (22) Zhou, G.; Li, F.; Cheng, H. M. Progress in Flexible Lithium Batteries and Future Prospects. Energy Environ. Sci. 2014, 7, 1307− 1338. (23) Wang, X.; Chen, Y.; Schmidt, O. G.; Yan, C. Engineered Nanomembranes for Smart Energy Storage Devices. Chem. Soc. Rev. 2016, 45, 1308−1330. (24) Balogun, M. S.; Qiu, W.; Luo, Y.; Meng, H.; Mai, W.; Onasanya, A.; Olaniyi, T. K.; Tong, Y. A Review of the Development of Full Cell Lithium-Ion Batteries: The Impact of Nanostructured Anode Materials. Nano Res. 2016, 9, 2823−2851. (25) Balo, L.; Shalu; Gupta, H.; Singh, V. K.; Singh, R. K. Flexible Gel Polymer Electrolyte Based on Ionic Liquid EMIMTFSI for Rechargeable Battery Application. Electrochim. Acta 2017, 230, 123− 131. (26) Fu, J.; Zhang, J.; Song, X.; Zarrin, H.; Tian, X.; Qiao, J.; Rasen, L.; Li, K.; Chen, Z. A Flexible Solid-State Electrolyte for Wide-Scale

adhere to fabric substrates. Cathode and anode layers are prepared through PEO-based composites using Li salt and TiO2, respectively. A solid polymer composite electrolyte layer of PEO and chopped glass fibers is also developed for the battery, which exhibits excellent ionic conductivity and mechanical strength. The layers of the battery are successfully characterized using different spectroscopic, diffraction, and microscopic techniques. The battery possesses good electrochemical performance with long cycle life and flexibility. Therefore, this flexible, high performance, easily processable, safe, and nontoxic battery will be attractive as a small power source in textile applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04112. SEM image of CF, cross-sectional SEM images of CuCF, and cross-sectional SEM images of the full battery before and after cyclic performance (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91 512 2597687. Fax: +91 512 2597408. ORCID

Kamal K. Kar: 0000-0002-6449-3399 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B. De sincerely acknowledges the receipt of his National Post Doctoral Fellowship (N-PDF) (file no. PDF/2016/001338/ ES) from the Science and Engineering Research Board (SERB), Government of India.

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REFERENCES

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Research Article

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