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PVDF–PZT nanocomposite film based self-charging power cell
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 105401 (http://iopscience.iop.org/0957-4484/25/10/105401) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 128.118.88.48 This content was downloaded on 30/05/2014 at 01:31
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Nanotechnology Nanotechnology 25 (2014) 105401 (7pp)
doi:10.1088/0957-4484/25/10/105401
PVDF–PZT nanocomposite film based self-charging power cell Yan Zhang1,3,5 , Yujing Zhang1 , Xinyu Xue2,5 , Chunxiao Cui2 , Bin He2 , Yuxin Nie2 , Ping Deng2 and Zhong Lin Wang3,4,5 1
Institute of Theoretical Physics, and Key Laboratory for Magnetism and Magnetic Materials of MOE, Lanzhou University, Lanzhou 730000, People’s Republic of China 2 College of Sciences, Northeastern University, Shenyang 110004, People’s Republic of China 3 Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, People’s Republic of China 4 School of Material Science and Engineering, Georgia Institute of Technology, GA 30332, USA E-mail: [email protected], [email protected] and [email protected] Received 25 September 2013, revised 9 December 2013 Accepted for publication 19 December 2013 Published 14 February 2014
Abstract
A novel PVDF–PZT nanocomposite film has been proposed and used as a piezoseparator in self-charging power cells (SCPCs). The structure, composed of poly(vinylidene fluoride) (PVDF) and lead zirconate titanate (PZT), provides a high piezoelectric output, because PZT in this nanocomposite film can improve the piezopotential compared to the pure PVDF film. The SCPC based on this nanocomposite film can be efficiently charged up by the mechanical deformation in the absence of an external power source. The charge capacity of the PVDF–PZT nanocomposite film based SCPC in 240 s is ∼0.010 µA h, higher than that of a pure PVDF film based SCPC (∼0.004 µA h). This is the first demonstration of using PVDF–PZT nanocomposite film as a piezoseparator for SCPC, and is an important step for the practical applications of SCPC for harvesting and storing mechanical energy. Keywords: nanocomposite, piezoseparator, piezopotential (Some figures may appear in colour only in the online journal)
1. Introduction
conversion/storage efficiency due to reducing energy waste on the rectifying circuit. In a typical piezo-electrochemical process, piezoelectric potential along the PVDF film created by mechanical deformation can drive lithium ions migrating from the cathode to the anode and reacting with the anode, so the SCPC can be charged up by a periodic mechanical deformation of the film. The film used as the piezoseparator is an important component. Therefore, exploring a new nanocomposite film with good piezoelectric property is essential for the SCPC. Recently, nanogenerator devices based on nanocomposite films have been reported, and the nanocomposite film provides a high piezoelectric output [11–13]. A new piezoseparator may be realized by hybridizing PZT and PVDF. Furthermore, the presence of PZT in the nanocomposite film may intensify the piezoelectric potential and change physical properties [14–16].
Energy storage and conversion are two important technologies in green energy. Usually, they are two distinct processes using two different devices, such as piezoelectric nanogenerators (NGs) and lithium-ion batteries [1–8]. Recently, we have demonstrated a piezo-electrochemical mechanism that can directly hybridize energy conversion and storage processes into one, through which the mechanical energy is directly converted and simultaneously stored as chemical energy [9, 10]. In those works, a well polarized poly(vinylidene fluoride) (PVDF) film is used as the piezoseparator of a self-charging power cell (SCPC) to replace the traditional separator of a lithium-ion battery [9, 10]. Compared with a conventional energy harvest/storage system, the SCPC has higher energy 5 Authors to whom any correspondence should be addressed.
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Figure 1. ((a), (b)) Schematic images of the fabrication process of the PVDF–PZT nanocomposite film. (c) Schematic structure image of a SCPC with a PVDF–PZT nanocomposite film as the piezoseparator for the charge storage component. (d) Photograph of the PVDF–PZT nanocomposite film. The scale is given in centimeters. (e) Sticking an SCPC underneath the button of a calculator, the mechanical energy generated by pressing the button can be converted and directly stored by the SCPC.
films are polarized for 30 min under an electric field of 20 kV mm−1 in silicone oil at 80 ◦ C [16, 18]. The SCPC has similar structure to that in our previous reports: cathode, anode and piezoseparator [9, 10]. In this work, well polarized PVDF–PZT nanocomposite film serves as the piezoseparator for the energy storage component. LiCoO2 composites on Al foils are used as the cathode while the anode is MWCNTs (on Cu foils). Due to its flexibility, smoothness and low cost, aluminum foil for the positive electrode and copper foil for the negative electrode have been adopted by major battery manufacturers in commercial battery fields such as mobile phones and notebook personal computers. LiCoO2 on aluminum foil and MWCNTs on copper foil have been confirmed as excellent cathode and anode for lithium-ion batteries due to their extremely high cycling performance [19,20]. Thus LiCoO2 and MWCNTs with high stability are very suitable for demonstrating this SCPC with the new piezoseparator. The poling direction of PVDF–PZT in figure 1 is from cathode to anode, that results in a positive piezoelectric field at the cathode (LiCoO2 ) and negative piezoelectric field at the anode (MWCNTs) under compressive strain. The SCPC device is fabricated by a sealed stainless-steel 2016-coin-type cell. The thickness of the LiCoO2 layer, PVDF–PZT film and MWCNT film is about 60, 90 and 50 µm, respectively. The overall thickness of the anode, cathode and separator is several hundred micrometers. The cathode, separator and anode are sealed in a stainless-steel 2016-coin-type cell and the diameter of the cell is 20 mm. The nanocomposite film can establish a high piezoelectric potential across its thickness due to the presence of piezoelectric polarization charges at the two opposite surfaces under externally applied stress, which not only converts mechanical energy into electricity, but also drives the migration of Li ions across the film. The presence of the PZT nanoparticles could intensify the piezopotential, which will be explained later. As shown in figure 1(e), SCPCs can be placed underneath the touch buttons of a calculator and the mechanical energy generated by pressing the button can be directly converted and simultaneously stored by the SCPC.
In this paper, we report a simple, effective and low-cost way to fabricate a PVDF–PZT nanocomposite film. The nanocomposite film has high conversion/storage efficiency as the piezoseparator of SCPCs. The nanocomposite structure, formed by hybridizing two piezoelectric materials (PVDF and PZT powders), shows good piezoelectric property. The stored capacity of the PVDF–PZT nanocomposite film based SCPC in 240 s is ∼0.010 µA h, while that of a pure PVDF film SCPC is ∼ 0.004 µA h. Thus, hybridizing two or more piezoelectric materials in nanocomposite films can be an effective approach for improving the performance of SCPCs. 2. Results and discussion
Previously, we have demonstrated the fabrication of a piezoelectric nanogenerator using a composite structure by embedding piezoelectric nanoparticles and nanotubes, such as PZT or BaTiO3 , in a polymer matrix to form a flexible and foldable film [13, 17]. The film was poled to exhibit a macroscopic piezoelectric polarization across its thickness direction. The piezoelectric field would drive the electrons to flow in external load periodically once a mechanical strain is applied. Such a structure has been used for converting mechanical energy into electricity. Alternatively, the piezopotential created in the composite film can be used for driving the diffusion of Li ions, so that it can be used for directly charging a Li battery. The fabrication of the PVDF–PZT nanocomposite film for an SCPC has been illustrated in figures 1(a)–(c). Firstly, PVDF powders and PZT nanoparticles were dissolved in DMF under stirring (figure 1(a)). After stirring for 15 min, the mixed solution was ultrasonically dispersed. Secondly, the ultimate mixed solution was spin-coated onto a substrate at a spinning rate of 900 rpm and dried at 50 ◦ C in an oven (figure 1(b)). The structure of the SCPC with PVDF–PZT nanocomposite film as piezoseparator is shown in figure 1(c). Figure 1(d) is a photograph of the PVDF–PZT nanocomposite film with yellow color. Before introducing them into SCPCs, PVDF–PZT nanocomposite 2
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Figure 2. (a) FESEM image of the PVDF–PZT nanocomposite film. (b) Cross sectional FESEM image of the PVDF–PZT nanocomposite film. (c) FESEM image of MWCNTs. (d) Cross sectional FESEM image of the SCPC device (LiCoO2 as cathode, PVDF–PZT nanocomposite film as piezoseparator and MWCNTs as anode).
to flow back and forth in the external circuit, resulting in an alternating output. figure 3(a) is the output voltage curve of positively polarized PVDF–PZT nanocomposite films under periodic compressive stress of 10 N. The peak value of the positive voltage is ∼1.3 V. When the nanocomposite film is negatively polarized, the output voltage is reversed, as shown in figure 3(b). The peak value is ∼−1 V. The results demonstrate that the PVDF–PZT nanocomposite film has a good piezoelectric property. The piezopotential distribution inside the PVDF–PZT nanocomposite film is illustrated in figure 4. Six PZT nanoparticles in PVDF are chosen to explain the piezopotential distribution. Figure 4(a) is in an equilibrium state without external compressive stress. The center of the PVDF-PZF nanocomposite layer should be expanded while the film is under compressive stress. For simplicity, the lateral expansion is neglected for easy understanding in figures 4(b) and 6(b)–(d). When external compressive stress is applied to the PVDF– PZT nanocomposite film, a piezoelectric field is established across its thickness (figure 4(b)). This is because, when a compressive strain is applied to the nanocomposite film along the poling direction, the dipole moments will be changed so that the equilibrium state is broken, leading to a voltage drop or a piezoelectric field across the surfaces. It should be emphasized that the piezopotential created by the PVDF– PZT nanocomposite film is quite different from that of pure PVDF film, as shown in figure 4(b). Firstly, PZT with a high piezopotential coefficient (500–600 pC N−1 ) could generate high piezopotential in the nanocomposite film. According to [21], the dipole moments of PZT in the PVDF matrix aligned
The microstructure of the samples and device is analyzed by field emission scanning electron microscopy (FESEM; Hitachi S4800). As shown in figure 2(a), PZT nanoparticles are well dispersed in PVDF matrix and the size of these nanoparticles is hundreds of nanometers. More pores are observed in the composite film, which may result in high piezopotential and more ionic conduction paths. The creation of the pores can be attributed to the contraction of PVDF during the solidification process. The dimension of the pores ranges from several tens of nanometers to micrometers. The pores are not uniformly distributed in the volume. Figure 2(b) is a cross-sectional FESEM image of the PVDF–PZT nanocomposite film. It can be seen that the thickness of the PVDF–PZT nanocomposite film is 60–110 µm. Figure 2(c) is a typical FESEM image of MWCNTs. It can be seen that the MWCNTs have lengths of 5–15 µm and diameters of 40–60 nm. The cross-sectional FESEM image of the SCPC device (figure 2(d)) demonstrates the sandwich-structured SCPC device, which is composed of three parts: anode (MWCNTs), cathode (LiCoO2 ) and piezoseparator (PVDF–PZT nanocomposite film). In figure 2(d), the delaminations between LiCoO2 , PVDF–PZT and MWCNT layers can be attributed to accidental delamination during cutting for the cross-sectional image. Figure 3 shows the piezoelectric properties of the PVDF– PZT nanocomposite films, which are sealed in 2016-type stainless-steel cells without injecting electrolyte. The positive poling direction of PVDF–PZT is from cathode to anode in figure 3. When the PVDF–PZT nanocomposite film is subjected to a compressive stress, a piezoelectric field will be established across its thickness and drives the electrons 3
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Figure 3. The piezoelectric characteristics of the (a) positively polarized and (b) negatively polarized PVDF–PZT nanocomposite film. Their
insets on the right are the detailed information on the piezoelectric output voltage for one cycle under the mechanical deformation.
Figure 4. Schematic images of piezopotential distribution on an SCPC device. (a) The cross-section image shows that six PZT nanoparticles
are distributed in the PVDF. (b) Schematic image of piezopotential distribution under external compressive stress. The color code shows the piezopotential distribution of the PVDF–PZT nanocomposite film.
was about 0.010 µA h. The self-charging process and the constant-current discharging process of the SCPC based on pure PVDF film under the same periodic force and frequency (10 N, 1.5 Hz) is shown in figure 5(b). The voltage of the SCPC increased from 210 to 252.3 mV in 240 s. After the self-charging process, the device was discharged back to its original voltage during 14 s under a constant discharge current of 1 µA. Thus, the storage capacity of the device was calculated as ∼0.004 µA h. This result confirms that SCPCs based on PVDF–PZT nanocomposite films can show better conversion efficiency than those fabricated with pure PVDF films. On one hand, the good conversion efficiency could be attributed to the nanocomposite film, which could create a high piezopotential. PZT nanoparticles in the nanocomposite film could intensify the piezoelectric potential, which can be ascribed to the geometrical strain confinement effect. Thus, a high piezoelectric field would result in more Li ions migrating from cathode to anode. On the other hand, more pores in the nanocomposite film can lead to more ionic conduction paths, which also enhances the transportation of Li ions. Keeping both the force and the frequency unchanged, as shown in figure 5(c), the SCPC based on the PVDF–PZT nanocomposite
in the same direction as those of PVDF. The piezoelectric constant of the PVDF–PZT film depends on PZT unless the volume fraction of PZT is very small. As a result, in our SCPC device, PZT in the PVDF will increase the piezoelectric constant of the PVDF–PZT piezoseparator above that of pure PVDF. Secondly, the geometrical strain confinement effect could intensify the piezoelectric potential [16]. In our SCPC devices, the PVDF–PZT film has a porous structure, which should increase strain under applied stress. Considering the above, the presence of PZT nanoparticles would increase the numbers of pores in the nanocomposite film, resulting in a smaller interpore distance and leading to a higher piezoelectric potential. Figure 5 is the self-charging process under periodic compressive straining and the corresponding discharging process of the SCPC. With a compressive force of 10 N applied to the SCPC based on the PVDF–PZT nanocomposite film at a frequency of 1.5 Hz, the voltage of the device increased from 210 to 297.6 mV in 240 s. After the self-charging process, the device was discharged back to its original voltage during 37 s under a constant discharge current of 1 µA, as shown in figure 5(a). In this case, the storage capacity of the SCPC 4
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Figure 5. Self-charging process under periodic compressive straining and corresponding discharging process of SCPC. (a) A typical self-charging process by applying cycled mechanical compressive strain to the device based on PVDF–PZT nanocomposite film as piezoseparator (blue shadowed region), during which the voltage keeps rising and the current flowing through the external circuit remains zero. In the discharging process (gray shadowed region), the stored power is released at a constant current of 1 µA. (b) Self-charging and discharging cycles of the SCPC based on pure PVDF film under a force of 10 N and frequency of 1.5 Hz. The discharge current of the pure PVDF SCPC is 1 µA. (c) Self-charging and discharging cycles of the same SCPC based on PVDF–PZT nanocomposite film at the same frequencies, revealing a good stability. (d) Self-charging and discharging cycles of the same SCPC with the PVDF–PZT nanocomposite film as piezoseparator at different frequencies.
film is charged to a similar level, indicating a high stability of the SCPC. When the frequencies were 1.5, 0.9, 0.6 and 0.4 Hz under an applied force of 10 N, the SCPC can be charged from 210 mV to 299.6, 291.6, 287.2 and 278.0 mV within 240 s (figure 5(d)), respectively. As the frequency increases, the self-charging effect will be enhanced. The working mechanism of the SCPC is an electrochemical process driven by piezoelectric potential, which is created by the deformation of the PVDF–PZT nanocomposite film. Different from our previous report, the PVDF–PZT nanocomposite film is used as the piezoseparator. At the beginning, the device is in a discharged state, with LiCoO2 as cathode and MWCNTs as anode, and the electrolyte (LiPF6 ) is distributed on the whole space, as shown in figure 6(a). When a compressive stress is applied to the device, the polarized PVDF–PZT nanocomposite film will generate a positive piezopotential on the cathode side and negative piezopotential on the anode side, as shown in figure 6(b). Under the driving of the piezopotential between cathode and anode, the Li ions in the electrolyte will migrate from cathode to anode through the ionic conduction paths present in the PVDF–PZT nanocomposite film (as shown in figure 6). It should be emphasized
that the piezopotential created by the nanocomposite film is higher than that for pure PVDF film. PZT nanoparticles in the nanocomposite film could intensify the piezoelectric potential, which can be ascribed to the geometrical strain confinement effect. Thus, a high piezoelectric field would result in more Li ions to migrate from cathode to anode. In addition, more pores in the nanocomposite film would lead to more ionic conduction paths, which also enhances the transport of Li ions. The piezopotential field and the increase of Li ions at the anode would drive the reaction at the cathode, (LiCoO2 ↔ Li1−x CoO2 + xLi+ + xe− ), to move to the right, resulting in the deintercalation of Li ions from LiCoO2 and the production of Li1−x CoO2 (figure 6(c)). At the same time, the reaction at the anode, (6C + Li+ + e− ↔ LiC6 ), would move in the forward direction, resulting in the intercalation of Li ions in MWCNTs and the production of LiC6 . In this process, Li+ would continuously migrate from the cathode to the anode and the device is charged up a little. Under compressive stress, the piezopotential could continuously drive the migration of Li ions until a point when the distribution of the Li+ can balance the piezoelectric field in the PVDF–PZT nanocomposite film (figure 6(d)). When the applied force is released, the piezopotential field in the PVDF–PZT nanocomposite film disappears 5
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Figure 6. The working mechanism of the SCPC with a PVDF–PZT nanocomposite film as the piezoseparator. (a) Schematic illustration of the SCPC in the discharged state with LiCoO2 as cathode and MWCNTs as anode. (b) When a compressive stress is applied to the device, the polarized PVDF–PZT nanocomposite film can create a piezopotential with positive polarity at the cathode and negative polarity at the anode. Compared with pure film, PVDF–PZT nanocomposite film would result in enhanced piezopotential. (c) Under the driving of the inner piezoelectric field, more Li ions from the cathode will migrate through the PVDF–PZT nanocomposite film in the electrolyte towards the anode, leading to the corresponding charging reactions at the two electrodes. The free electrons at the cathode and positive charges at the anode will dissipate inside the device system. More pores in the nanocomposite film may lead to more ionic conduction paths, which enhances the transport of Li ions. (d) The new chemical equilibrium of the two electrodes is re-established and the self-charging process ceases. (e) When the compressive stress is released, the piezoelectric field of the PVDF–PZT nanocomposite film disappears, breaking the electrochemical equilibrium, so that a portion of the Li ions will diffuse back to the cathode. (f) The system reaches a new electrochemical equilibrium, and a cycle of the self-charging process is completed.
and a portion of the Li ions diffuse back from the anode to the cathode (figure 6(e)) and reach an even distribution of Li ions again (figure 6(f)). By this time, a self-charging process is completed, resulting in oxidizing a small amount of LiCoO2 to Li1−x CoO2 and reducing some of the MWCNTs to LiC6 . Moreover, a small amount of mechanical energy is converted to electric energy and stored by chemical energy directly in this device. When the device is mechanically deformed again, the process is repeated, with the result of
another cycle of the charging process. In this self-charging mechanism, the role played by piezoelectricity is similar to the DC power supply used in the conventional charging of a Li-ion battery, and mechanical energy is converted and stored as chemical energy in one unit. During the progress of charging electrochemical reactions at the two electrodes, extra free electrons will transfer from the cathode to the anode, in order to maintain the charge neutrality and the continuity of the charging reaction. According to our previous works [9, 10], 6
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there are generally two ways for the electrons to transfer: either through the external circuitry or inside the battery system. The exact process is still to be further investigated. The relative efficiency of PVDF–PZT SCPC increases 2.5-fold compared with pure PVDF SCPC. For the PVDF–PZT nanocomposite film based SCPC, the high piezopotential and good energy conversion/storage efficiency can be explained as follows. Firstly, PZT nanoparticles in the nanocomposite film would intensify the piezoelectric potential, due to the geometrical strain confinement effect. In this case, the high piezopotential could result in more Li ions to migrate from cathode to anode, leading to good energy conversion/storage efficiency. Secondly, more pores in the nanocomposite film would increase the ionic conduction paths, leading to the enhanced transport of Li ions. Thus, a good conversion efficiency could also be obtained.
with liquid electrolyte (1 M LiPF6 in 1:1 ethylene carbonate:dimethyl carbonate). The voltage of the SCPC under applying compressive deformation is monitored by a BST-TC53 battery measurement system (MTI, China), which can also conduct the constantcurrent discharging process.
3. Conclusion
[1] Wang Z L and Song J 2006 Science 312 242–6 [2] Wang X D, Song J, Liu J and Wang Z L 2007 Science 316 102–5 [3] Qin Y, Wang X D and Wang Z L 2008 Nature 451 809–13 [4] Jang M, Ciobotaru M and Agelidis V G 2013 IEEE Trans. Indust. Informat. 9 1158–66 [5] Wang Z L and Wu W 2012 Angew Chem. Int. Edn. Engl. 51 11700–21 [6] Hsieh C-T, Lin J-Y and Mo C-Y 2011 Electrochim. Acta. 58 119–24 [7] Valvo M, Garcia-Tamayo E, Lafont U and Kelder E M 2011 J. Power Sources 196 10191–200 [8] Endo M, Nishimura Y, Takahashi T, Takeuchi K and Dresselhaus M S 1996 J. Phys. Chem. Solids 57 725–8 [9] Xue X, Wang S, Guo W, Zhang Y and Wang Z L 2012 Nano Lett. 12 5048–54 [10] Xue X, Deng P, Yuan S, Nie Y, He B, Xing L and Zhang Y 2013 Energy Environ. Sci. 6 2615–20 [11] Park K, Xu S, Liu Y, Hwang G, Kang S, Wang Z L and Lee K 2010 Nano Lett. 10 4939–43 [12] Chang C, Tran V H, Wang J, Fuh Y and Lin L 2010 Nano Lett. 10 726–31 [13] Park K et al 2012 Adv. Mater. 24 2999–3004 [14] Hilczer B, Kułek J, Markiewicz E, Kosec M and Maliˇc B 2002 J. Non-Cryst. Solids 305 67–173 [15] Komaba S, Yabuuchi N, Ozeki T, Han Z-J, Shimomura K, Yui H, Katayama Y and Miura T 2012 J. Phys. Chem. C 116 1380–9 [16] Cha S et al 2011 Nano Lett. 11 5142–7 [17] Lin Z, Yang Y, Wu J, Liu Y, Zhang F and Wang Z L 2012 J. Phys. Chem. Lett. 3 3599–604 [18] Lee M, Chen C-Y, Wang S, Cha S N, Park Y J, Kim J M, Chou L-J and Wang Z L 2012 Adv. Mater. 24 5283 [19] Koksbang R, Barker J, Shi H and Saidi M Y 1996 Solid State Ion. 84 1–21 [20] Che G L, Lakshmi B B, Fisher E R and Martin C R 1998 Nature 393 346–9 [21] Furukawa T, Ishida K and Fukada E 1979 J. Appl. Phys. 50 4904–12
Acknowledgments
This work was partly supported by the Fundamental Research Funds for the Central Universities (grant Nos lzujbky-2013-35, N120205001, and N120405010), the Knowledge Innovation Program of the Chinese Academy of Sciences (grant No KJCX2-YW-M13), and the National Natural Science Foundation of China (51102041 and 11104025). References
In summary, by using a PVDF–PZT composite thin film as the separator in a Li battery, the piezoelectric potential generated by an external mechanical stress inside the film can effectively drive the diffusion of Li ions. This is an SCPC that directly converts mechanical energy into electrochemical energy and stores it. The storage capacity of the PVDF–PZT nanocomposite film based SCPC is ∼0.010 µA h in 240 s, which is higher than that using a pure PVDF film based SCPC (∼0.004 µA h), showing that the PZT nanoparticles can be effective for improving the performance of the SCPC. Such a PVDF–PZT nanocomposite film based SCPC points in a direction for enhancing the performance of the SCPC toward practical applications. 4. Experimental section
Fabrication of the PVDF–PZT nanocomposite film: 1.176 g PVDF and 0.824 g PZT were added to 12 mL DMF and ultrasonically dispersed for 30 min. Then the mixture was spin-cast on the substrate for ∼60 s at a spinning rate of 900 rpm and dried at 50 ◦ C in an oven. PVDF powders (purchased from MTI, China) and PZT nanoparticles (purchased from Zibo Yuhai Electronic Ceramic Co.) were dissolved in DMF under stirring (figure 1(a)). Fabrication of the SCPC: firstly, PVDF–PZT nanocomposite film was poled at 80 ◦ C for 30 min using an electric field of 20 kV mm−1 . In the same way as in our previous report [9, 10], the SCPS is assembled in a vacuum glove box. The anode (MWCNT:conductive carbon:binder mixtures with a weight ratio of 8:1:1, pasted on copper foil), piezoseparator (well polarized PVDF–PZT nanocomposite film) and cathode (LiCoO2 :conductive carbon:binder mixtures with a weight ratio of 8:1:1, pasted on aluminum foil) are completely filled
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PVDF–PZT nanocomposite film based self-charging power cell
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 105401 (http://iopscience.iop.org/0957-4484/25/10/105401) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 128.118.88.48 This content was downloaded on 30/05/2014 at 01:31
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 105401 (7pp)
doi:10.1088/0957-4484/25/10/105401
PVDF–PZT nanocomposite film based self-charging power cell Yan Zhang1,3,5 , Yujing Zhang1 , Xinyu Xue2,5 , Chunxiao Cui2 , Bin He2 , Yuxin Nie2 , Ping Deng2 and Zhong Lin Wang3,4,5 1
Institute of Theoretical Physics, and Key Laboratory for Magnetism and Magnetic Materials of MOE, Lanzhou University, Lanzhou 730000, People’s Republic of China 2 College of Sciences, Northeastern University, Shenyang 110004, People’s Republic of China 3 Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, People’s Republic of China 4 School of Material Science and Engineering, Georgia Institute of Technology, GA 30332, USA E-mail: [email protected], [email protected] and [email protected] Received 25 September 2013, revised 9 December 2013 Accepted for publication 19 December 2013 Published 14 February 2014
Abstract
A novel PVDF–PZT nanocomposite film has been proposed and used as a piezoseparator in self-charging power cells (SCPCs). The structure, composed of poly(vinylidene fluoride) (PVDF) and lead zirconate titanate (PZT), provides a high piezoelectric output, because PZT in this nanocomposite film can improve the piezopotential compared to the pure PVDF film. The SCPC based on this nanocomposite film can be efficiently charged up by the mechanical deformation in the absence of an external power source. The charge capacity of the PVDF–PZT nanocomposite film based SCPC in 240 s is ∼0.010 µA h, higher than that of a pure PVDF film based SCPC (∼0.004 µA h). This is the first demonstration of using PVDF–PZT nanocomposite film as a piezoseparator for SCPC, and is an important step for the practical applications of SCPC for harvesting and storing mechanical energy. Keywords: nanocomposite, piezoseparator, piezopotential (Some figures may appear in colour only in the online journal)
1. Introduction
conversion/storage efficiency due to reducing energy waste on the rectifying circuit. In a typical piezo-electrochemical process, piezoelectric potential along the PVDF film created by mechanical deformation can drive lithium ions migrating from the cathode to the anode and reacting with the anode, so the SCPC can be charged up by a periodic mechanical deformation of the film. The film used as the piezoseparator is an important component. Therefore, exploring a new nanocomposite film with good piezoelectric property is essential for the SCPC. Recently, nanogenerator devices based on nanocomposite films have been reported, and the nanocomposite film provides a high piezoelectric output [11–13]. A new piezoseparator may be realized by hybridizing PZT and PVDF. Furthermore, the presence of PZT in the nanocomposite film may intensify the piezoelectric potential and change physical properties [14–16].
Energy storage and conversion are two important technologies in green energy. Usually, they are two distinct processes using two different devices, such as piezoelectric nanogenerators (NGs) and lithium-ion batteries [1–8]. Recently, we have demonstrated a piezo-electrochemical mechanism that can directly hybridize energy conversion and storage processes into one, through which the mechanical energy is directly converted and simultaneously stored as chemical energy [9, 10]. In those works, a well polarized poly(vinylidene fluoride) (PVDF) film is used as the piezoseparator of a self-charging power cell (SCPC) to replace the traditional separator of a lithium-ion battery [9, 10]. Compared with a conventional energy harvest/storage system, the SCPC has higher energy 5 Authors to whom any correspondence should be addressed.
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c 2014 IOP Publishing Ltd
Printed in the UK
Nanotechnology 25 (2014) 105401
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Figure 1. ((a), (b)) Schematic images of the fabrication process of the PVDF–PZT nanocomposite film. (c) Schematic structure image of a SCPC with a PVDF–PZT nanocomposite film as the piezoseparator for the charge storage component. (d) Photograph of the PVDF–PZT nanocomposite film. The scale is given in centimeters. (e) Sticking an SCPC underneath the button of a calculator, the mechanical energy generated by pressing the button can be converted and directly stored by the SCPC.
films are polarized for 30 min under an electric field of 20 kV mm−1 in silicone oil at 80 ◦ C [16, 18]. The SCPC has similar structure to that in our previous reports: cathode, anode and piezoseparator [9, 10]. In this work, well polarized PVDF–PZT nanocomposite film serves as the piezoseparator for the energy storage component. LiCoO2 composites on Al foils are used as the cathode while the anode is MWCNTs (on Cu foils). Due to its flexibility, smoothness and low cost, aluminum foil for the positive electrode and copper foil for the negative electrode have been adopted by major battery manufacturers in commercial battery fields such as mobile phones and notebook personal computers. LiCoO2 on aluminum foil and MWCNTs on copper foil have been confirmed as excellent cathode and anode for lithium-ion batteries due to their extremely high cycling performance [19,20]. Thus LiCoO2 and MWCNTs with high stability are very suitable for demonstrating this SCPC with the new piezoseparator. The poling direction of PVDF–PZT in figure 1 is from cathode to anode, that results in a positive piezoelectric field at the cathode (LiCoO2 ) and negative piezoelectric field at the anode (MWCNTs) under compressive strain. The SCPC device is fabricated by a sealed stainless-steel 2016-coin-type cell. The thickness of the LiCoO2 layer, PVDF–PZT film and MWCNT film is about 60, 90 and 50 µm, respectively. The overall thickness of the anode, cathode and separator is several hundred micrometers. The cathode, separator and anode are sealed in a stainless-steel 2016-coin-type cell and the diameter of the cell is 20 mm. The nanocomposite film can establish a high piezoelectric potential across its thickness due to the presence of piezoelectric polarization charges at the two opposite surfaces under externally applied stress, which not only converts mechanical energy into electricity, but also drives the migration of Li ions across the film. The presence of the PZT nanoparticles could intensify the piezopotential, which will be explained later. As shown in figure 1(e), SCPCs can be placed underneath the touch buttons of a calculator and the mechanical energy generated by pressing the button can be directly converted and simultaneously stored by the SCPC.
In this paper, we report a simple, effective and low-cost way to fabricate a PVDF–PZT nanocomposite film. The nanocomposite film has high conversion/storage efficiency as the piezoseparator of SCPCs. The nanocomposite structure, formed by hybridizing two piezoelectric materials (PVDF and PZT powders), shows good piezoelectric property. The stored capacity of the PVDF–PZT nanocomposite film based SCPC in 240 s is ∼0.010 µA h, while that of a pure PVDF film SCPC is ∼ 0.004 µA h. Thus, hybridizing two or more piezoelectric materials in nanocomposite films can be an effective approach for improving the performance of SCPCs. 2. Results and discussion
Previously, we have demonstrated the fabrication of a piezoelectric nanogenerator using a composite structure by embedding piezoelectric nanoparticles and nanotubes, such as PZT or BaTiO3 , in a polymer matrix to form a flexible and foldable film [13, 17]. The film was poled to exhibit a macroscopic piezoelectric polarization across its thickness direction. The piezoelectric field would drive the electrons to flow in external load periodically once a mechanical strain is applied. Such a structure has been used for converting mechanical energy into electricity. Alternatively, the piezopotential created in the composite film can be used for driving the diffusion of Li ions, so that it can be used for directly charging a Li battery. The fabrication of the PVDF–PZT nanocomposite film for an SCPC has been illustrated in figures 1(a)–(c). Firstly, PVDF powders and PZT nanoparticles were dissolved in DMF under stirring (figure 1(a)). After stirring for 15 min, the mixed solution was ultrasonically dispersed. Secondly, the ultimate mixed solution was spin-coated onto a substrate at a spinning rate of 900 rpm and dried at 50 ◦ C in an oven (figure 1(b)). The structure of the SCPC with PVDF–PZT nanocomposite film as piezoseparator is shown in figure 1(c). Figure 1(d) is a photograph of the PVDF–PZT nanocomposite film with yellow color. Before introducing them into SCPCs, PVDF–PZT nanocomposite 2
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Figure 2. (a) FESEM image of the PVDF–PZT nanocomposite film. (b) Cross sectional FESEM image of the PVDF–PZT nanocomposite film. (c) FESEM image of MWCNTs. (d) Cross sectional FESEM image of the SCPC device (LiCoO2 as cathode, PVDF–PZT nanocomposite film as piezoseparator and MWCNTs as anode).
to flow back and forth in the external circuit, resulting in an alternating output. figure 3(a) is the output voltage curve of positively polarized PVDF–PZT nanocomposite films under periodic compressive stress of 10 N. The peak value of the positive voltage is ∼1.3 V. When the nanocomposite film is negatively polarized, the output voltage is reversed, as shown in figure 3(b). The peak value is ∼−1 V. The results demonstrate that the PVDF–PZT nanocomposite film has a good piezoelectric property. The piezopotential distribution inside the PVDF–PZT nanocomposite film is illustrated in figure 4. Six PZT nanoparticles in PVDF are chosen to explain the piezopotential distribution. Figure 4(a) is in an equilibrium state without external compressive stress. The center of the PVDF-PZF nanocomposite layer should be expanded while the film is under compressive stress. For simplicity, the lateral expansion is neglected for easy understanding in figures 4(b) and 6(b)–(d). When external compressive stress is applied to the PVDF– PZT nanocomposite film, a piezoelectric field is established across its thickness (figure 4(b)). This is because, when a compressive strain is applied to the nanocomposite film along the poling direction, the dipole moments will be changed so that the equilibrium state is broken, leading to a voltage drop or a piezoelectric field across the surfaces. It should be emphasized that the piezopotential created by the PVDF– PZT nanocomposite film is quite different from that of pure PVDF film, as shown in figure 4(b). Firstly, PZT with a high piezopotential coefficient (500–600 pC N−1 ) could generate high piezopotential in the nanocomposite film. According to [21], the dipole moments of PZT in the PVDF matrix aligned
The microstructure of the samples and device is analyzed by field emission scanning electron microscopy (FESEM; Hitachi S4800). As shown in figure 2(a), PZT nanoparticles are well dispersed in PVDF matrix and the size of these nanoparticles is hundreds of nanometers. More pores are observed in the composite film, which may result in high piezopotential and more ionic conduction paths. The creation of the pores can be attributed to the contraction of PVDF during the solidification process. The dimension of the pores ranges from several tens of nanometers to micrometers. The pores are not uniformly distributed in the volume. Figure 2(b) is a cross-sectional FESEM image of the PVDF–PZT nanocomposite film. It can be seen that the thickness of the PVDF–PZT nanocomposite film is 60–110 µm. Figure 2(c) is a typical FESEM image of MWCNTs. It can be seen that the MWCNTs have lengths of 5–15 µm and diameters of 40–60 nm. The cross-sectional FESEM image of the SCPC device (figure 2(d)) demonstrates the sandwich-structured SCPC device, which is composed of three parts: anode (MWCNTs), cathode (LiCoO2 ) and piezoseparator (PVDF–PZT nanocomposite film). In figure 2(d), the delaminations between LiCoO2 , PVDF–PZT and MWCNT layers can be attributed to accidental delamination during cutting for the cross-sectional image. Figure 3 shows the piezoelectric properties of the PVDF– PZT nanocomposite films, which are sealed in 2016-type stainless-steel cells without injecting electrolyte. The positive poling direction of PVDF–PZT is from cathode to anode in figure 3. When the PVDF–PZT nanocomposite film is subjected to a compressive stress, a piezoelectric field will be established across its thickness and drives the electrons 3
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Figure 3. The piezoelectric characteristics of the (a) positively polarized and (b) negatively polarized PVDF–PZT nanocomposite film. Their
insets on the right are the detailed information on the piezoelectric output voltage for one cycle under the mechanical deformation.
Figure 4. Schematic images of piezopotential distribution on an SCPC device. (a) The cross-section image shows that six PZT nanoparticles
are distributed in the PVDF. (b) Schematic image of piezopotential distribution under external compressive stress. The color code shows the piezopotential distribution of the PVDF–PZT nanocomposite film.
was about 0.010 µA h. The self-charging process and the constant-current discharging process of the SCPC based on pure PVDF film under the same periodic force and frequency (10 N, 1.5 Hz) is shown in figure 5(b). The voltage of the SCPC increased from 210 to 252.3 mV in 240 s. After the self-charging process, the device was discharged back to its original voltage during 14 s under a constant discharge current of 1 µA. Thus, the storage capacity of the device was calculated as ∼0.004 µA h. This result confirms that SCPCs based on PVDF–PZT nanocomposite films can show better conversion efficiency than those fabricated with pure PVDF films. On one hand, the good conversion efficiency could be attributed to the nanocomposite film, which could create a high piezopotential. PZT nanoparticles in the nanocomposite film could intensify the piezoelectric potential, which can be ascribed to the geometrical strain confinement effect. Thus, a high piezoelectric field would result in more Li ions migrating from cathode to anode. On the other hand, more pores in the nanocomposite film can lead to more ionic conduction paths, which also enhances the transportation of Li ions. Keeping both the force and the frequency unchanged, as shown in figure 5(c), the SCPC based on the PVDF–PZT nanocomposite
in the same direction as those of PVDF. The piezoelectric constant of the PVDF–PZT film depends on PZT unless the volume fraction of PZT is very small. As a result, in our SCPC device, PZT in the PVDF will increase the piezoelectric constant of the PVDF–PZT piezoseparator above that of pure PVDF. Secondly, the geometrical strain confinement effect could intensify the piezoelectric potential [16]. In our SCPC devices, the PVDF–PZT film has a porous structure, which should increase strain under applied stress. Considering the above, the presence of PZT nanoparticles would increase the numbers of pores in the nanocomposite film, resulting in a smaller interpore distance and leading to a higher piezoelectric potential. Figure 5 is the self-charging process under periodic compressive straining and the corresponding discharging process of the SCPC. With a compressive force of 10 N applied to the SCPC based on the PVDF–PZT nanocomposite film at a frequency of 1.5 Hz, the voltage of the device increased from 210 to 297.6 mV in 240 s. After the self-charging process, the device was discharged back to its original voltage during 37 s under a constant discharge current of 1 µA, as shown in figure 5(a). In this case, the storage capacity of the SCPC 4
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Figure 5. Self-charging process under periodic compressive straining and corresponding discharging process of SCPC. (a) A typical self-charging process by applying cycled mechanical compressive strain to the device based on PVDF–PZT nanocomposite film as piezoseparator (blue shadowed region), during which the voltage keeps rising and the current flowing through the external circuit remains zero. In the discharging process (gray shadowed region), the stored power is released at a constant current of 1 µA. (b) Self-charging and discharging cycles of the SCPC based on pure PVDF film under a force of 10 N and frequency of 1.5 Hz. The discharge current of the pure PVDF SCPC is 1 µA. (c) Self-charging and discharging cycles of the same SCPC based on PVDF–PZT nanocomposite film at the same frequencies, revealing a good stability. (d) Self-charging and discharging cycles of the same SCPC with the PVDF–PZT nanocomposite film as piezoseparator at different frequencies.
film is charged to a similar level, indicating a high stability of the SCPC. When the frequencies were 1.5, 0.9, 0.6 and 0.4 Hz under an applied force of 10 N, the SCPC can be charged from 210 mV to 299.6, 291.6, 287.2 and 278.0 mV within 240 s (figure 5(d)), respectively. As the frequency increases, the self-charging effect will be enhanced. The working mechanism of the SCPC is an electrochemical process driven by piezoelectric potential, which is created by the deformation of the PVDF–PZT nanocomposite film. Different from our previous report, the PVDF–PZT nanocomposite film is used as the piezoseparator. At the beginning, the device is in a discharged state, with LiCoO2 as cathode and MWCNTs as anode, and the electrolyte (LiPF6 ) is distributed on the whole space, as shown in figure 6(a). When a compressive stress is applied to the device, the polarized PVDF–PZT nanocomposite film will generate a positive piezopotential on the cathode side and negative piezopotential on the anode side, as shown in figure 6(b). Under the driving of the piezopotential between cathode and anode, the Li ions in the electrolyte will migrate from cathode to anode through the ionic conduction paths present in the PVDF–PZT nanocomposite film (as shown in figure 6). It should be emphasized
that the piezopotential created by the nanocomposite film is higher than that for pure PVDF film. PZT nanoparticles in the nanocomposite film could intensify the piezoelectric potential, which can be ascribed to the geometrical strain confinement effect. Thus, a high piezoelectric field would result in more Li ions to migrate from cathode to anode. In addition, more pores in the nanocomposite film would lead to more ionic conduction paths, which also enhances the transport of Li ions. The piezopotential field and the increase of Li ions at the anode would drive the reaction at the cathode, (LiCoO2 ↔ Li1−x CoO2 + xLi+ + xe− ), to move to the right, resulting in the deintercalation of Li ions from LiCoO2 and the production of Li1−x CoO2 (figure 6(c)). At the same time, the reaction at the anode, (6C + Li+ + e− ↔ LiC6 ), would move in the forward direction, resulting in the intercalation of Li ions in MWCNTs and the production of LiC6 . In this process, Li+ would continuously migrate from the cathode to the anode and the device is charged up a little. Under compressive stress, the piezopotential could continuously drive the migration of Li ions until a point when the distribution of the Li+ can balance the piezoelectric field in the PVDF–PZT nanocomposite film (figure 6(d)). When the applied force is released, the piezopotential field in the PVDF–PZT nanocomposite film disappears 5
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Figure 6. The working mechanism of the SCPC with a PVDF–PZT nanocomposite film as the piezoseparator. (a) Schematic illustration of the SCPC in the discharged state with LiCoO2 as cathode and MWCNTs as anode. (b) When a compressive stress is applied to the device, the polarized PVDF–PZT nanocomposite film can create a piezopotential with positive polarity at the cathode and negative polarity at the anode. Compared with pure film, PVDF–PZT nanocomposite film would result in enhanced piezopotential. (c) Under the driving of the inner piezoelectric field, more Li ions from the cathode will migrate through the PVDF–PZT nanocomposite film in the electrolyte towards the anode, leading to the corresponding charging reactions at the two electrodes. The free electrons at the cathode and positive charges at the anode will dissipate inside the device system. More pores in the nanocomposite film may lead to more ionic conduction paths, which enhances the transport of Li ions. (d) The new chemical equilibrium of the two electrodes is re-established and the self-charging process ceases. (e) When the compressive stress is released, the piezoelectric field of the PVDF–PZT nanocomposite film disappears, breaking the electrochemical equilibrium, so that a portion of the Li ions will diffuse back to the cathode. (f) The system reaches a new electrochemical equilibrium, and a cycle of the self-charging process is completed.
and a portion of the Li ions diffuse back from the anode to the cathode (figure 6(e)) and reach an even distribution of Li ions again (figure 6(f)). By this time, a self-charging process is completed, resulting in oxidizing a small amount of LiCoO2 to Li1−x CoO2 and reducing some of the MWCNTs to LiC6 . Moreover, a small amount of mechanical energy is converted to electric energy and stored by chemical energy directly in this device. When the device is mechanically deformed again, the process is repeated, with the result of
another cycle of the charging process. In this self-charging mechanism, the role played by piezoelectricity is similar to the DC power supply used in the conventional charging of a Li-ion battery, and mechanical energy is converted and stored as chemical energy in one unit. During the progress of charging electrochemical reactions at the two electrodes, extra free electrons will transfer from the cathode to the anode, in order to maintain the charge neutrality and the continuity of the charging reaction. According to our previous works [9, 10], 6
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there are generally two ways for the electrons to transfer: either through the external circuitry or inside the battery system. The exact process is still to be further investigated. The relative efficiency of PVDF–PZT SCPC increases 2.5-fold compared with pure PVDF SCPC. For the PVDF–PZT nanocomposite film based SCPC, the high piezopotential and good energy conversion/storage efficiency can be explained as follows. Firstly, PZT nanoparticles in the nanocomposite film would intensify the piezoelectric potential, due to the geometrical strain confinement effect. In this case, the high piezopotential could result in more Li ions to migrate from cathode to anode, leading to good energy conversion/storage efficiency. Secondly, more pores in the nanocomposite film would increase the ionic conduction paths, leading to the enhanced transport of Li ions. Thus, a good conversion efficiency could also be obtained.
with liquid electrolyte (1 M LiPF6 in 1:1 ethylene carbonate:dimethyl carbonate). The voltage of the SCPC under applying compressive deformation is monitored by a BST-TC53 battery measurement system (MTI, China), which can also conduct the constantcurrent discharging process.
3. Conclusion
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Acknowledgments
This work was partly supported by the Fundamental Research Funds for the Central Universities (grant Nos lzujbky-2013-35, N120205001, and N120405010), the Knowledge Innovation Program of the Chinese Academy of Sciences (grant No KJCX2-YW-M13), and the National Natural Science Foundation of China (51102041 and 11104025). References
In summary, by using a PVDF–PZT composite thin film as the separator in a Li battery, the piezoelectric potential generated by an external mechanical stress inside the film can effectively drive the diffusion of Li ions. This is an SCPC that directly converts mechanical energy into electrochemical energy and stores it. The storage capacity of the PVDF–PZT nanocomposite film based SCPC is ∼0.010 µA h in 240 s, which is higher than that using a pure PVDF film based SCPC (∼0.004 µA h), showing that the PZT nanoparticles can be effective for improving the performance of the SCPC. Such a PVDF–PZT nanocomposite film based SCPC points in a direction for enhancing the performance of the SCPC toward practical applications. 4. Experimental section
Fabrication of the PVDF–PZT nanocomposite film: 1.176 g PVDF and 0.824 g PZT were added to 12 mL DMF and ultrasonically dispersed for 30 min. Then the mixture was spin-cast on the substrate for ∼60 s at a spinning rate of 900 rpm and dried at 50 ◦ C in an oven. PVDF powders (purchased from MTI, China) and PZT nanoparticles (purchased from Zibo Yuhai Electronic Ceramic Co.) were dissolved in DMF under stirring (figure 1(a)). Fabrication of the SCPC: firstly, PVDF–PZT nanocomposite film was poled at 80 ◦ C for 30 min using an electric field of 20 kV mm−1 . In the same way as in our previous report [9, 10], the SCPS is assembled in a vacuum glove box. The anode (MWCNT:conductive carbon:binder mixtures with a weight ratio of 8:1:1, pasted on copper foil), piezoseparator (well polarized PVDF–PZT nanocomposite film) and cathode (LiCoO2 :conductive carbon:binder mixtures with a weight ratio of 8:1:1, pasted on aluminum foil) are completely filled
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