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Flexible thin-film battery based on solid-like ionic liquid-polymer electrolyte
Journal of Power Sources 303 (2016) 17e21
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Flexible thin-film battery based on solid-like ionic liquid-polymer electrolyte Qin Li a, Haleh Ardebili a, b, * a b
Mechanical Engineering Department, University of Houston, Engineering Building 1, Houston, TX 77204-4006, USA Materials Science and Engineering Program, University of Houston, Engineering Building 1, Houston, TX 77204-4006, USA
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Solid-like gel polymer electrolyte was fabricated containing 60 wt% ionic liquid. The incorporation of nonvolatile ionic liquid offers enhanced safety and stability. A low-cost, scalable lamination method was employed to fabricate flexible LIBs. The flexible LIB could deliver a stable capacity in flat and bent configurations.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 July 2015 Received in revised form 20 October 2015 Accepted 27 October 2015 Available online xxx
The development of high-performance flexible batteries is imperative for several contemporary applications including flexible electronics, wearable sensors and implantable medical devices. However, traditional organic liquid-based electrolytes are not ideal for flexible batteries due to their inherent safety and stability issues. In this study, a non-volatile, non-flammable and safe ionic liquid (IL)-based polymer electrolyte film with solid-like feature is fabricated and incorporated in a flexible lithium ion battery. The ionic liquid is 1-Ethyl-3-methylimidazolium dicyanamide (EMIMDCA) and the polymer is composed of poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP). The electrolyte exhibits good thermal stability (i.e. no weight loss up to 300 C) and relatively high ionic conductivity (6 104 S cm1). The flexible thin-film lithium ion battery based on solid-like electrolyte film is encapsulated using a thermallamination process and demonstrates excellent electrochemical performance, in both flat and bent configurations. © 2015 Elsevier B.V. All rights reserved.
Keywords: Polymer electrolyte Ionic liquid Flexible lithium ion battery EMIMDCA PVDF
1. Introduction Flexible energy storage devices offer many advantages including compatibility with the flexible electronics and flexible applications [1]; for example, they can be attached to the biological organs or
* Corresponding author. Mechanical Engineering Department, University of Houston, Engineering Building 1, Houston, TX 77204-4006, USA. E-mail address: [email protected] (H. Ardebili). http://dx.doi.org/10.1016/j.jpowsour.2015.10.099 0378-7753/© 2015 Elsevier B.V. All rights reserved.
embedded in textiles/clothing. Among the various energy storage devices, lithium-ion batteries (LIBs) are ideal candidates to transform into flexible devices, due to their superior attributes including higher energy density and efficiency. Over the past several decades several structural and materials designs for the fabrication of flexible LIBs have been proposed [2e10]. Min Koo et al. developed an all-solid-state bendable lithium-ion battery based on ceramic electrolyte exhibiting a capacity of 106 mAh/cm2 [2]. Liangbing Hu et al. designed a paper-based lithium ion battery which employed
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Q. Li, H. Ardebili / Journal of Power Sources 303 (2016) 17e21
carbon nanotubes instead of the traditional metals as the current collector [3]. Sheng Xu and others proposed self-similar interconnects to pack a set of cells forming a highly stretchable battery [4]. Comprehensive surveys of this topic can be found in two recent reviews by Lee et al. [11] and Zhou et al. [12]. In designing flexible LIBs, electrolytes play a crucial role. The common liquid electrolytes e used in many pouch-type flexible batteries e are less than ideal due to the risk of leakage and lack of mechanical stability. On the other hand, solid electrolytes are superior in terms of stability and safety but their relatively low ionic conductivity results in insufficient electrochemical performance and power density. In order to develop high-performance LIBs with relatively safer electrolyte, compared to organic liquids, intense efforts have been directed toward the development of gel electrolytes. Gel polymer electrolytes are usually obtained by infusing a large amount of liquid electrolyte into a suitable polymer matrix [13,14]. The role of the polymer matrix is mainly to store the liquid electrolyte and provide mechanical support. The ionic conductivity of a gel electrolyte, governed mainly by the conductivity and amount of the liquid phase, is relatively high and can be competitive with respect to the traditional organic liquid electrolytes. Various polymers including poly(ethylene oxide) (PEO) [15,16], polyacrylonitrile (PAN) [17,18], poly(methyl methacrylate) (PMMA) [19,20], and poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP) [21,22], have been studied as potential matrices for gel electrolyte. Among these polymers, PVDF-HFP-based gel electrolytes exhibit excellent mechanical properties as well as high ionic conductivity (~103 S cm1) and have become the leading polymer matrix materials in this field. Unfortunately, because of the presence of reactive, volatile, and flammable organic solvents (e.g. ethylene carbonate (EC) and propylene carbonate (PC)), common gel electrolytes share some of the same safety concerns as associated with liquid electrolytes [23]. To further enhance the safety of the gel electrolytes, ionic liquids can be introduced in the polymer matrix to replace the organic liquid phase [24,25]. Ionic liquids are essentially low-melting temperature salts that remain in liquid phase at room temperature. They possess several favorable properties such as high ionic conductivity, non-volatility, non-flammability, and thermal, chemical stability [26]. Many attempts have been made to replace the traditional organic carbonates with ionic liquids in the solvent-salt and gel electrolytes [27e33]. However, as discussed in a critical review, batteries with these ionic liquid-based liquid or gel electrolytes usually exhibit poor cyclic and power performance compared to those made with conventional organic solvent-based electrolytes even though they exhibit similar ionic conductivities [34]. In order to achieve high ionic conductivity, both the cation and anion of the ionic liquid should be generally small and carry a welldelocalized charge [35]. Among many ionic liquids, 1-Ethyl-3methylimidazolium dicyanamide (EMIMDCA) has a relatively high ionic conductivity (27 mS/cm), however, the electrochemical window is relatively small (2.9 V). The high ion conductivity can be attributed to the small size of the dicyanamide anion [36,37]. Adding lithium salt can slightly increase the viscosity and thus decrease the ionic conductivity of ionic liquid [38]. In this study, EMIMDCA ionic liquid based polymer electrolyte film with solid-like feature is fabricated and incorporated into a flexible Li ion battery. The flexible battery demonstrates excellent electrochemical performance, in both flat and bent positions.
30% PVDF-HFP (polymer matrix), 60% EMIMDCA (ionic liquid), and 10% LiClO4 (salt). LiClO4 salt has been used mainly because of its stability, better tolerance to moisture, and lower cost. The gel electrolyte was prepared by the general solution-casting method: 1) 0.6 g PVDF-HFP (Mw ¼ 400,000, purchased from Sigma Aldrich) was added in 20 mL N,N-Dimethylformamide (purity 99.8%, Sigma Aldrich) and the mixture was stirred at about 80 C for 1 h; the weights of other materials were calculated based on the weight of PVDF-HFP; 2) after the PVDF-HFP was dissolved, the EMIMDCA (Sigma Aldrich) and LiClO4 (Sigma Aldrich) were added to the solution and stirred at 50 C overnight (about 12 h); 3) the viscous solution was poured into a glass petri-dish and placed in the vacuum oven at 50 C for 24 h to allow the solvent to evaporate. The obtained thin-film electrolyte with an averaged thickness of about 0.2 mm was stored in the dry glove box to prevent moisture absorption. 2.2. Characterizations of the electrolytes The electrochemical impedance spectroscopy of the PVDF-IL electrolyte was obtained using Metrohm Autolab with frequency response analysis (FRA) module. The sample was sandwiched between two stainless steel electrode discs and the complex impedance spectra were obtained in the frequency range of 10 Hze500 kHz. Thermogravimetric analysis (TGA) results were collected using Q50 TGA (TA Instruments) in the temperature region of 20 Ce600 C. The temperature scanning rate was 10 C min1. 2.3. Battery assembly and testing The anode was prepared by coating the slurry (Li4Ti5O12: Carbon black: PVDF ¼ 8:1:1 dissolved in N-Methyl-2-pyrrolidone; purchased from MTI Corporation) on the aluminum foil and dried in the vacuum oven at 80 C for 24 h. Since the Li4Ti5O12 anode has a higher potential for Li-intercalation (~1.5 v.s. Liþ/Li), it can be used with an aluminum foil current collector. The cathode (LiCoO2 on aluminum foil with mass loading of about 12 mg/cm2) was purchased from MTI Corporation. Both the cathode and anode were cut into 1.5 cm 1.5 cm squares and a copper strip was placed on the aluminum side of each electrode for electrical connection to the external devices. A piece of the PVDF-IL electrolyte was sandwiched between the cathode and anode and one drop (about 2 mg) of 1M LiClO4 in EMIMDCA was poured on the surface of each electrode to further reduce the interfacial resistance. This step is critical in reducing the interfacial resistance in the battery. Then, the battery was placed on a thermal-adhesive plastic laminating sheet and the laminating machine (Saturn SL-95 Laminating Machine, Amazon) was used to laminate the battery. After the lamination process, the extra parts of the sheet were cut and removed, and the copper strips were exposed for battery testing. The cyclic voltammograms of the battery were obtained using Metrohm Autolab at a scan rate of 0.05 mV/s from 1.5 V to 2.8 V. The cycle performance of the battery was tested by Arbin battery test equipment at a constant current density of 35 mA/mg (0.25 C of LiCoO2) in the voltage range of 1.6 Ve2.7 V. To evaluate the electrochemical performance of the laminated battery in bending condition, the battery was wrapped around a 2 cm radius cylinder (Fig. S4). 3. Results and discussion
2. Experimental 2.1. Preparation of the gel electrolyte The weight ratios of the gel electrolyte components consist of
Fig. 1a shows a freestanding gel electrolyte film, with solid-like appearance, composed of polyvinylidene fluoride-cohexafluoropropene (PVDF-HFP), 1-Ethyl-3-methylimidazolium dicyanamide (EMIMDCA) ionic liquid (Fig. S1), and LiClO4 lithium
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Fig. 1. (a) A photograph of the solid-like electrolyte sample, (b) the SEM image of the PVDF-IL electrolyte showing microporous structure, (c) Nyquist plot ( ) and the fitting curve (solid line) of the PVDF-IL electrolyte, (d) comparison of TGA results of PVDF-IL and PVDF-HFP/C/PC electrolytes.
salt. The inhomogeneous surface of the sample indicates the nonuniform distribution of PVDF-HFP during its recrystallization. The slight non-uniformity observed in the thickness of the electrolyte film (dried in the petri-dish) is attributed to the lack of complete
flatness of the bottom of the petri-dish. This variation may affect the conductivity of the electrolyte film. The microstructure, ionic conductivity, and thermal stability of the solid-like electrolyte (PVDF-IL hereinafter) were examined. Due to the micro-porous
Fig. 2. (a) All components of the lithium ion battery are stacked in appropriate order and fed to the laminating machine, (b) photo of the multi-layer LIB, (c) schematics of the flexible Li ion battery, and (d) SEM image of the battery cross section.
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structure of PVDF-HFP, the liquid phase d ionic liquid combined with dissolved lithium saltdare well trapped in the electrolyte film. The microporous structure of the film is clearly evident from the scanning electron microscope (SEM) image shown in Fig. 1b. The electrolyte film was punched into thin disk shaped samples for complex impedance spectroscopy based ion conductivity measurements. The frequency ranged from 500 kHz to 10 Hz. Fig. 1c shows the Nyquist plot of the electrolyte. Unlike the case of common PEO-based solid polymer electrolytes, the Nyquist plot for the fabricated PVDF-IL electrolyte does not exhibit a semi-circle in the high frequency region. The intrinsic resistance is estimated by fitting an equivalent circuit to the Nyquist plot. The ionic conductivity s was calculated by the following equation
s¼
d RA
(1)
where d is the thickness of the sample, R is the resistance obtained from the Nyquist plot and A is the surface area of the sample. The calculated ionic conductivity of the fabricated PVDF-IL electrolyte is about 6 104 S cm1. This value is similar with that of other PVDFIL gel electrolytes with different ionic liquids (Table S1, Supplementary information). For gel electrolytes made with general organic carbonates, thermal stability is a significant concern. Due to its intrinsic volatility, the liquid phase will be gradually released during the fabrication and storage. This leads to the degradation of the gel electrolyte since the ionic conductivity is predominantly determined by the amount of liquid in the gel (Fig. S2 Supplementary Information). In sharp contrast, ionic liquids are nonvolatile so that the PVDF-IL gel electrolyte can maintain stable composition. Fig. 1d compares the thermogravimetric analysis (TGA) results of our PVDF-IL electrolyte and gel electrolyte composed of PVDF-EC þ PC. The weight loss of the former is negligible even when the temperature rises to 300 C. Using a simple and economical approach, a flexible lithium ion battery was fabricated based on the solid-like PVDF-IL gel electrolyte. The fabrication processes of the electrolyte and the flexible battery are described in the experimental section. A common office-type laminating machine (Fig. 2)a) was used to laminate the flexible battery layers covered by thermal-adhesive plastic sheets with a total process time just under a few minutes. The obtained laminated lithium ion battery (Fig. 2b) has a multi-layer structure as illustrated in Fig. 2c. The thermal-adhesive laminating plastic sheet acts as the encapsulation to protect the battery from the outside environment and the lamination process produces the pressure to keep all the layers in good contact. Maintaining adequate contact between the layers is essential for developing a high performance flexible lithium ion battery. This aspect is especially critical for the traditional carbon-based anodes and organic carbonate-based electrolytes due to the swelling caused by the gas generated from the electrolyte/electrode reactions. Even after replacing carbon-based anode with Li4Ti5O12 this problem persists [39] and a degassing process is generally required. Because of the high stability of ionic liquids, the gassing problem appears to be suppressed in the flexible batteries fabricated in this study. From the scanning electro microscopy (SEM) image of the cross-section of the flexible battery (Fig. 2d), it can be observed that the boundary between the active material (LiCoO2 or Li4Ti5O12) and the PVDF-IL layers has disappeared, indicating that the two layers have merged after the hot laminating process. On the other hand, there appears to be a small gap between the active material and the aluminum current collector. This is most likely due to the process of cutting and removal of the sample from the flexible battery causing the release of the applied pressure from encapsulation.
Fig. 3. (a) Cyclic voltammograms (CVs) of the flexible battery with LiCoO2/Al cathode, Li4Ti5O12/Al anode, and PVDF-IL electrolyte, (b) chargeedischarge profiles of the battery for 1st (flat), 10th (flat), and 20th (bending) cycles.
The electrochemical performance of the flexible battery was also evaluated. The electrochemical stability of the battery was examined by cyclic voltammetry as shown in Fig. 3a. In the voltage range 1.5 Ve2.8 V, two obvious peaks are observed in the voltammograms corresponding to the intercalation/deintercalation of lithium ions. The heights of these two peaks are almost identical which indicates good reversibility of the lithium ion insertion and extraction. No other peaks are found in the voltage range thus confirming that no side reaction has occurred. Fig. 3b shows the galvanostatic chargeedischarge profiles of the battery for selected cycles. Instead of the flat plateaus typically observed for the batteries based on organic liquid electrolytes, the voltage for the fabricated flexible LIBs, exhibits an increasing/decreasing trend. This trend was also observed in the case of the battery based on polyethylene oxide (PEO) gel electrolyte [4] and it may be attributed to the high polarization of the polymer electrolyte. The battery is charged and discharged in both flat and bent positions (Fig. 4a) and it can be seen that except for the first cycle, the Coulombic efficiencies of all the cycles are about 98%, consistent with the cyclic voltammograms. The battery can deliver about 300 mAh/cm2 capacity for 20 cycles. When the battery is transformed from flat to bent position, a capacity jump occurs. This may be attributed to the increased pressure between the layers under bending and the reduction of the interfacial resistance. On the other hand, capacity fading can be observed after 10 cycles and is attributed to the degradation of the electrode structure in the
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21
Acknowledgments The authors acknowledge financial support from NSF CAREER (CMMI-1254477) and TcSUH.
Appendix A. Supplementary information Supplementary information associated with this article can be found at http://dx.doi.org/10.1016/j.jpowsour.2015.10.099.
References
Fig. 4. (a) Discharge capacities and Coulombic efficiencies of the battery for 20 cycles in flat and bent (2 cm radius) positions, (b) the battery is able to light up a red LED (~2.0 V) while in bending. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
flexible battery. Furthermore, moisture penetration, impurities, and the absence of assembly pressure (similar to that of coin-cells) may contribute to the performance degradation in flexible thin-film batteries. Capacity fading has also been observed in previous studies of flexible lithium-ion battery [4]. The working voltage of the battery is about 2.1 V (Fig. 4b) and the battery safety is ensured through the use of non-volatile, inflammable PVDF-IL electrolyte. Therefore, the flexible battery fabricated in this study, based on PVDF-ionic liquid electrolyte, can be a promising energy storage device for integration with flexible electronics and applications. 4. Conclusions In summary, a safer flexible lithium ion battery is fabricated based on a solid-like gel electrolyte consisting of EMIMDCA ionic liquid and PVDF-HFP polymer matrix. The ionic conductivity was found to be 6 104 S cm1 with a polymer to ionic liquid ratio of 1:2. This solid-like electrolyte is non-volatile with a stable composition up to 300 C. This electrolyte is an ideal candidate for a low-cost, simple lamination method to fabricate high performance flexible lithium ion batteries. The battery shows relatively stable energy delivery capability and can function in both flat and bent configurations.
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Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Flexible thin-film battery based on solid-like ionic liquid-polymer electrolyte Qin Li a, Haleh Ardebili a, b, * a b
Mechanical Engineering Department, University of Houston, Engineering Building 1, Houston, TX 77204-4006, USA Materials Science and Engineering Program, University of Houston, Engineering Building 1, Houston, TX 77204-4006, USA
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Solid-like gel polymer electrolyte was fabricated containing 60 wt% ionic liquid. The incorporation of nonvolatile ionic liquid offers enhanced safety and stability. A low-cost, scalable lamination method was employed to fabricate flexible LIBs. The flexible LIB could deliver a stable capacity in flat and bent configurations.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 July 2015 Received in revised form 20 October 2015 Accepted 27 October 2015 Available online xxx
The development of high-performance flexible batteries is imperative for several contemporary applications including flexible electronics, wearable sensors and implantable medical devices. However, traditional organic liquid-based electrolytes are not ideal for flexible batteries due to their inherent safety and stability issues. In this study, a non-volatile, non-flammable and safe ionic liquid (IL)-based polymer electrolyte film with solid-like feature is fabricated and incorporated in a flexible lithium ion battery. The ionic liquid is 1-Ethyl-3-methylimidazolium dicyanamide (EMIMDCA) and the polymer is composed of poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP). The electrolyte exhibits good thermal stability (i.e. no weight loss up to 300 C) and relatively high ionic conductivity (6 104 S cm1). The flexible thin-film lithium ion battery based on solid-like electrolyte film is encapsulated using a thermallamination process and demonstrates excellent electrochemical performance, in both flat and bent configurations. © 2015 Elsevier B.V. All rights reserved.
Keywords: Polymer electrolyte Ionic liquid Flexible lithium ion battery EMIMDCA PVDF
1. Introduction Flexible energy storage devices offer many advantages including compatibility with the flexible electronics and flexible applications [1]; for example, they can be attached to the biological organs or
* Corresponding author. Mechanical Engineering Department, University of Houston, Engineering Building 1, Houston, TX 77204-4006, USA. E-mail address: [email protected] (H. Ardebili). http://dx.doi.org/10.1016/j.jpowsour.2015.10.099 0378-7753/© 2015 Elsevier B.V. All rights reserved.
embedded in textiles/clothing. Among the various energy storage devices, lithium-ion batteries (LIBs) are ideal candidates to transform into flexible devices, due to their superior attributes including higher energy density and efficiency. Over the past several decades several structural and materials designs for the fabrication of flexible LIBs have been proposed [2e10]. Min Koo et al. developed an all-solid-state bendable lithium-ion battery based on ceramic electrolyte exhibiting a capacity of 106 mAh/cm2 [2]. Liangbing Hu et al. designed a paper-based lithium ion battery which employed
18
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carbon nanotubes instead of the traditional metals as the current collector [3]. Sheng Xu and others proposed self-similar interconnects to pack a set of cells forming a highly stretchable battery [4]. Comprehensive surveys of this topic can be found in two recent reviews by Lee et al. [11] and Zhou et al. [12]. In designing flexible LIBs, electrolytes play a crucial role. The common liquid electrolytes e used in many pouch-type flexible batteries e are less than ideal due to the risk of leakage and lack of mechanical stability. On the other hand, solid electrolytes are superior in terms of stability and safety but their relatively low ionic conductivity results in insufficient electrochemical performance and power density. In order to develop high-performance LIBs with relatively safer electrolyte, compared to organic liquids, intense efforts have been directed toward the development of gel electrolytes. Gel polymer electrolytes are usually obtained by infusing a large amount of liquid electrolyte into a suitable polymer matrix [13,14]. The role of the polymer matrix is mainly to store the liquid electrolyte and provide mechanical support. The ionic conductivity of a gel electrolyte, governed mainly by the conductivity and amount of the liquid phase, is relatively high and can be competitive with respect to the traditional organic liquid electrolytes. Various polymers including poly(ethylene oxide) (PEO) [15,16], polyacrylonitrile (PAN) [17,18], poly(methyl methacrylate) (PMMA) [19,20], and poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP) [21,22], have been studied as potential matrices for gel electrolyte. Among these polymers, PVDF-HFP-based gel electrolytes exhibit excellent mechanical properties as well as high ionic conductivity (~103 S cm1) and have become the leading polymer matrix materials in this field. Unfortunately, because of the presence of reactive, volatile, and flammable organic solvents (e.g. ethylene carbonate (EC) and propylene carbonate (PC)), common gel electrolytes share some of the same safety concerns as associated with liquid electrolytes [23]. To further enhance the safety of the gel electrolytes, ionic liquids can be introduced in the polymer matrix to replace the organic liquid phase [24,25]. Ionic liquids are essentially low-melting temperature salts that remain in liquid phase at room temperature. They possess several favorable properties such as high ionic conductivity, non-volatility, non-flammability, and thermal, chemical stability [26]. Many attempts have been made to replace the traditional organic carbonates with ionic liquids in the solvent-salt and gel electrolytes [27e33]. However, as discussed in a critical review, batteries with these ionic liquid-based liquid or gel electrolytes usually exhibit poor cyclic and power performance compared to those made with conventional organic solvent-based electrolytes even though they exhibit similar ionic conductivities [34]. In order to achieve high ionic conductivity, both the cation and anion of the ionic liquid should be generally small and carry a welldelocalized charge [35]. Among many ionic liquids, 1-Ethyl-3methylimidazolium dicyanamide (EMIMDCA) has a relatively high ionic conductivity (27 mS/cm), however, the electrochemical window is relatively small (2.9 V). The high ion conductivity can be attributed to the small size of the dicyanamide anion [36,37]. Adding lithium salt can slightly increase the viscosity and thus decrease the ionic conductivity of ionic liquid [38]. In this study, EMIMDCA ionic liquid based polymer electrolyte film with solid-like feature is fabricated and incorporated into a flexible Li ion battery. The flexible battery demonstrates excellent electrochemical performance, in both flat and bent positions.
30% PVDF-HFP (polymer matrix), 60% EMIMDCA (ionic liquid), and 10% LiClO4 (salt). LiClO4 salt has been used mainly because of its stability, better tolerance to moisture, and lower cost. The gel electrolyte was prepared by the general solution-casting method: 1) 0.6 g PVDF-HFP (Mw ¼ 400,000, purchased from Sigma Aldrich) was added in 20 mL N,N-Dimethylformamide (purity 99.8%, Sigma Aldrich) and the mixture was stirred at about 80 C for 1 h; the weights of other materials were calculated based on the weight of PVDF-HFP; 2) after the PVDF-HFP was dissolved, the EMIMDCA (Sigma Aldrich) and LiClO4 (Sigma Aldrich) were added to the solution and stirred at 50 C overnight (about 12 h); 3) the viscous solution was poured into a glass petri-dish and placed in the vacuum oven at 50 C for 24 h to allow the solvent to evaporate. The obtained thin-film electrolyte with an averaged thickness of about 0.2 mm was stored in the dry glove box to prevent moisture absorption. 2.2. Characterizations of the electrolytes The electrochemical impedance spectroscopy of the PVDF-IL electrolyte was obtained using Metrohm Autolab with frequency response analysis (FRA) module. The sample was sandwiched between two stainless steel electrode discs and the complex impedance spectra were obtained in the frequency range of 10 Hze500 kHz. Thermogravimetric analysis (TGA) results were collected using Q50 TGA (TA Instruments) in the temperature region of 20 Ce600 C. The temperature scanning rate was 10 C min1. 2.3. Battery assembly and testing The anode was prepared by coating the slurry (Li4Ti5O12: Carbon black: PVDF ¼ 8:1:1 dissolved in N-Methyl-2-pyrrolidone; purchased from MTI Corporation) on the aluminum foil and dried in the vacuum oven at 80 C for 24 h. Since the Li4Ti5O12 anode has a higher potential for Li-intercalation (~1.5 v.s. Liþ/Li), it can be used with an aluminum foil current collector. The cathode (LiCoO2 on aluminum foil with mass loading of about 12 mg/cm2) was purchased from MTI Corporation. Both the cathode and anode were cut into 1.5 cm 1.5 cm squares and a copper strip was placed on the aluminum side of each electrode for electrical connection to the external devices. A piece of the PVDF-IL electrolyte was sandwiched between the cathode and anode and one drop (about 2 mg) of 1M LiClO4 in EMIMDCA was poured on the surface of each electrode to further reduce the interfacial resistance. This step is critical in reducing the interfacial resistance in the battery. Then, the battery was placed on a thermal-adhesive plastic laminating sheet and the laminating machine (Saturn SL-95 Laminating Machine, Amazon) was used to laminate the battery. After the lamination process, the extra parts of the sheet were cut and removed, and the copper strips were exposed for battery testing. The cyclic voltammograms of the battery were obtained using Metrohm Autolab at a scan rate of 0.05 mV/s from 1.5 V to 2.8 V. The cycle performance of the battery was tested by Arbin battery test equipment at a constant current density of 35 mA/mg (0.25 C of LiCoO2) in the voltage range of 1.6 Ve2.7 V. To evaluate the electrochemical performance of the laminated battery in bending condition, the battery was wrapped around a 2 cm radius cylinder (Fig. S4). 3. Results and discussion
2. Experimental 2.1. Preparation of the gel electrolyte The weight ratios of the gel electrolyte components consist of
Fig. 1a shows a freestanding gel electrolyte film, with solid-like appearance, composed of polyvinylidene fluoride-cohexafluoropropene (PVDF-HFP), 1-Ethyl-3-methylimidazolium dicyanamide (EMIMDCA) ionic liquid (Fig. S1), and LiClO4 lithium
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Fig. 1. (a) A photograph of the solid-like electrolyte sample, (b) the SEM image of the PVDF-IL electrolyte showing microporous structure, (c) Nyquist plot ( ) and the fitting curve (solid line) of the PVDF-IL electrolyte, (d) comparison of TGA results of PVDF-IL and PVDF-HFP/C/PC electrolytes.
salt. The inhomogeneous surface of the sample indicates the nonuniform distribution of PVDF-HFP during its recrystallization. The slight non-uniformity observed in the thickness of the electrolyte film (dried in the petri-dish) is attributed to the lack of complete
flatness of the bottom of the petri-dish. This variation may affect the conductivity of the electrolyte film. The microstructure, ionic conductivity, and thermal stability of the solid-like electrolyte (PVDF-IL hereinafter) were examined. Due to the micro-porous
Fig. 2. (a) All components of the lithium ion battery are stacked in appropriate order and fed to the laminating machine, (b) photo of the multi-layer LIB, (c) schematics of the flexible Li ion battery, and (d) SEM image of the battery cross section.
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structure of PVDF-HFP, the liquid phase d ionic liquid combined with dissolved lithium saltdare well trapped in the electrolyte film. The microporous structure of the film is clearly evident from the scanning electron microscope (SEM) image shown in Fig. 1b. The electrolyte film was punched into thin disk shaped samples for complex impedance spectroscopy based ion conductivity measurements. The frequency ranged from 500 kHz to 10 Hz. Fig. 1c shows the Nyquist plot of the electrolyte. Unlike the case of common PEO-based solid polymer electrolytes, the Nyquist plot for the fabricated PVDF-IL electrolyte does not exhibit a semi-circle in the high frequency region. The intrinsic resistance is estimated by fitting an equivalent circuit to the Nyquist plot. The ionic conductivity s was calculated by the following equation
s¼
d RA
(1)
where d is the thickness of the sample, R is the resistance obtained from the Nyquist plot and A is the surface area of the sample. The calculated ionic conductivity of the fabricated PVDF-IL electrolyte is about 6 104 S cm1. This value is similar with that of other PVDFIL gel electrolytes with different ionic liquids (Table S1, Supplementary information). For gel electrolytes made with general organic carbonates, thermal stability is a significant concern. Due to its intrinsic volatility, the liquid phase will be gradually released during the fabrication and storage. This leads to the degradation of the gel electrolyte since the ionic conductivity is predominantly determined by the amount of liquid in the gel (Fig. S2 Supplementary Information). In sharp contrast, ionic liquids are nonvolatile so that the PVDF-IL gel electrolyte can maintain stable composition. Fig. 1d compares the thermogravimetric analysis (TGA) results of our PVDF-IL electrolyte and gel electrolyte composed of PVDF-EC þ PC. The weight loss of the former is negligible even when the temperature rises to 300 C. Using a simple and economical approach, a flexible lithium ion battery was fabricated based on the solid-like PVDF-IL gel electrolyte. The fabrication processes of the electrolyte and the flexible battery are described in the experimental section. A common office-type laminating machine (Fig. 2)a) was used to laminate the flexible battery layers covered by thermal-adhesive plastic sheets with a total process time just under a few minutes. The obtained laminated lithium ion battery (Fig. 2b) has a multi-layer structure as illustrated in Fig. 2c. The thermal-adhesive laminating plastic sheet acts as the encapsulation to protect the battery from the outside environment and the lamination process produces the pressure to keep all the layers in good contact. Maintaining adequate contact between the layers is essential for developing a high performance flexible lithium ion battery. This aspect is especially critical for the traditional carbon-based anodes and organic carbonate-based electrolytes due to the swelling caused by the gas generated from the electrolyte/electrode reactions. Even after replacing carbon-based anode with Li4Ti5O12 this problem persists [39] and a degassing process is generally required. Because of the high stability of ionic liquids, the gassing problem appears to be suppressed in the flexible batteries fabricated in this study. From the scanning electro microscopy (SEM) image of the cross-section of the flexible battery (Fig. 2d), it can be observed that the boundary between the active material (LiCoO2 or Li4Ti5O12) and the PVDF-IL layers has disappeared, indicating that the two layers have merged after the hot laminating process. On the other hand, there appears to be a small gap between the active material and the aluminum current collector. This is most likely due to the process of cutting and removal of the sample from the flexible battery causing the release of the applied pressure from encapsulation.
Fig. 3. (a) Cyclic voltammograms (CVs) of the flexible battery with LiCoO2/Al cathode, Li4Ti5O12/Al anode, and PVDF-IL electrolyte, (b) chargeedischarge profiles of the battery for 1st (flat), 10th (flat), and 20th (bending) cycles.
The electrochemical performance of the flexible battery was also evaluated. The electrochemical stability of the battery was examined by cyclic voltammetry as shown in Fig. 3a. In the voltage range 1.5 Ve2.8 V, two obvious peaks are observed in the voltammograms corresponding to the intercalation/deintercalation of lithium ions. The heights of these two peaks are almost identical which indicates good reversibility of the lithium ion insertion and extraction. No other peaks are found in the voltage range thus confirming that no side reaction has occurred. Fig. 3b shows the galvanostatic chargeedischarge profiles of the battery for selected cycles. Instead of the flat plateaus typically observed for the batteries based on organic liquid electrolytes, the voltage for the fabricated flexible LIBs, exhibits an increasing/decreasing trend. This trend was also observed in the case of the battery based on polyethylene oxide (PEO) gel electrolyte [4] and it may be attributed to the high polarization of the polymer electrolyte. The battery is charged and discharged in both flat and bent positions (Fig. 4a) and it can be seen that except for the first cycle, the Coulombic efficiencies of all the cycles are about 98%, consistent with the cyclic voltammograms. The battery can deliver about 300 mAh/cm2 capacity for 20 cycles. When the battery is transformed from flat to bent position, a capacity jump occurs. This may be attributed to the increased pressure between the layers under bending and the reduction of the interfacial resistance. On the other hand, capacity fading can be observed after 10 cycles and is attributed to the degradation of the electrode structure in the
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Acknowledgments The authors acknowledge financial support from NSF CAREER (CMMI-1254477) and TcSUH.
Appendix A. Supplementary information Supplementary information associated with this article can be found at http://dx.doi.org/10.1016/j.jpowsour.2015.10.099.
References
Fig. 4. (a) Discharge capacities and Coulombic efficiencies of the battery for 20 cycles in flat and bent (2 cm radius) positions, (b) the battery is able to light up a red LED (~2.0 V) while in bending. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
flexible battery. Furthermore, moisture penetration, impurities, and the absence of assembly pressure (similar to that of coin-cells) may contribute to the performance degradation in flexible thin-film batteries. Capacity fading has also been observed in previous studies of flexible lithium-ion battery [4]. The working voltage of the battery is about 2.1 V (Fig. 4b) and the battery safety is ensured through the use of non-volatile, inflammable PVDF-IL electrolyte. Therefore, the flexible battery fabricated in this study, based on PVDF-ionic liquid electrolyte, can be a promising energy storage device for integration with flexible electronics and applications. 4. Conclusions In summary, a safer flexible lithium ion battery is fabricated based on a solid-like gel electrolyte consisting of EMIMDCA ionic liquid and PVDF-HFP polymer matrix. The ionic conductivity was found to be 6 104 S cm1 with a polymer to ionic liquid ratio of 1:2. This solid-like electrolyte is non-volatile with a stable composition up to 300 C. This electrolyte is an ideal candidate for a low-cost, simple lamination method to fabricate high performance flexible lithium ion batteries. The battery shows relatively stable energy delivery capability and can function in both flat and bent configurations.
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