Research Article www.acsami.org
Facile and Reliable in Situ Polymerization of Poly(Ethyl Cyanoacrylate)-Based Polymer Electrolytes toward Flexible Lithium Batteries Yanyan Cui,†,‡ Jingchao Chai,‡,§ Huiping Du,‡,§ Yulong Duan,‡,§ Guangwen Xie,*,† Zhihong Liu,*,‡,§ and Guanglei Cui*,‡,§ †
Key Laboratory of Nanomaterials, Qingdao University of Science and Technology, No. 53 Zhengzhou Road, Qingdao 266042, People’s Republic of China ‡ Qingdao Industrial Energy Storage Research Institute, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, People’s Republic of China § University of Chinese Academy of Sciences, No. 19A Yuquan Road, 100049, Beijing, China S Supporting Information *
ABSTRACT: Polycyanoacrylate is a very promising matrix for polymer electrolyte, which possesses advantages of strong binding and high electrochemical stability owing to the functional nitrile groups. Herein, a facile and reliable in situ polymerization strategy of poly(ethyl cyanoacrylate) (PECA) based gel polymer electrolytes (GPE) via a high efficient anionic polymerization was introduced consisting of PECA and 4 M LiClO4 in carbonate solvents. The in situ polymerized PECA gel polymer electrolyte achieved an excellent ionic conductivity (2.7 × 10−3 S cm−1) at room temperature, and exhibited a considerable electrochemical stability window up to 4.8 V vs Li/Li+. The LiFePO4/PECA-GPE/Li and LiNi1.5Mn0.5O4/PECAGPE/Li batteries using this in-situ-polymerized GPE delivered stable charge/ discharge profiles, considerable rate capability, and excellent cycling performance. These results demonstrated this reliable in situ polymerization process is a very promising strategy to prepare high performance polymer electrolytes for flexible thin-film batteries, micropower lithium batteries, and deformable lithium batteries for special purpose. KEYWORDS: Li-ion battery, poly(ethyl cyanoacrylate), in situ polymerization, flexible, polymer electrolytes
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INTRODUCTION
oxide) (PEO) electrolyte membranes consisted a sequence of processes, that is, polymer dissolution, solution casting, and solvent evaporation.10,12,13 In addition, it consumed a large amount of solvents to dissolve the polymer, which made this process tedious and environmentally nonbenign. Currently, in situ polymerization of gel polymer electrolytes becomes a popular strategy with the monomer and initiator dissolved in a real liquid electrolyte and subsequently was injected into the electrodes and to form gel polymer by a thermal or UV-light initiation. Polyacrylonitrile (PAN) and poly(methyl methacrylate) (PMMA) based gel electrolytes could be achieved in this facile process.14,15 However, the free radical polymerization reaction between confined electrodes were sometimes not reliable and lack of reproducibility since the surface side effect and quenching effect always happened. Therefore, a facile and reliable in situ polymerization strategy independent of side effect is highly desirable.
Recently, there is a strong stimulus to develop lightweight and flexible lithium ion batteries (LIB) meeting the energy storage demand for wearable electronic devices.1−4 The conventional lithium ion batteries use nonaqueous liquid electrolytes and contain a large amount of flammable organic carbonate solvents, causing potential risk of electrolyte leakage, thermal runway, or fire hazards.5,6 All solid-state polymer batteries have achieved a great progress in recent years. However, there is still a huge challenge to commercialize them unless some problems, such as relatively lower ionic conductivity, high voltage (>4.5 V vs Li/ Li+) electrochemical instability, and lithium metal interfacial incompatibility could be well resolved.7 As a compromise, gel polymer electrolytes (GPE) combine both merits of solid-state electrolytes and the nonaqueous liquid electrolytes, that is, high ionic conductivity, superior electrochemical stability with functional additives, flexible processability, and no leakage. Although GPEs are good candidates to prepare flexible LIBs with improved safety,8−11 they usually suffer from a complicated and tedious preparation process. The fabrication of poly(vinylidene fluoride-hexafluoro propylene) (PVDF-HFP) and poly(ethylene © 2017 American Chemical Society
Received: December 18, 2016 Accepted: February 23, 2017 Published: February 23, 2017 8737
DOI: 10.1021/acsami.6b16218 ACS Appl. Mater. Interfaces 2017, 9, 8737−8741
Research Article
ACS Applied Materials & Interfaces Cyanoacrylate is a well-known family of powerful instant glues for various materials interfacial adhesion. In our lab, we recently found that polycyanoacrylate polymer electrolyte possessed advantages of high electrochemical stability above 4.8 V vs Li/Li+ owing to the functional nitrile groups.7,16,17 In addition, the nitrile and ester groups favorably interact with the Li+ ions of the lithium salt, which was beneficial for the salt dissolution and charge transport.18 However, these polycyanoacrylate membranes in our previous reports were made via an ex-situ polymerization which was very tedious and solvents-consuming. It is noted that, in ethyl cyanoacrylate’s (ECA) chemical structure, two electron-withdrawing groups, that is, cyan group and ester group, are covalently linked to the unsaturated carbon atom, which makes them highly activated for efficient polymerization.19,20 Traces of water, weak bases, and anions can initiate the anionic polymerization.21 Herein, a facile and reliable in situ polymerization strategy of poly(ethyl cyanoacrylate) (PECA) based gel polymer electrolytes was presented. Moreover, this reliable in situ polymerization process is a very promising strategy to prepare high performance polymer electrolytes for flexible thin-film batteries, micropower lithium batteries, and deformable shapes lithium batteries for special purpose.
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Scheme 1. Anionic Polymerization Mechanism Initiated by Metal Li
the rapid chain propagation. The anionic active species are stabilized by both strong electron-withdrawing side groups, namely, the nitrile and ester groups, featuring high reaction activity. Moreover, compared to the frequently used free radical polymerization mechanism, the anionic polymerization possesses some advantages, that is, tolerance of oxygen and moisture, lower polymerization activation energy, and long living active center.21−25 Tolerance of oxygen and moisture allows its polymerization occurred under air atmosphere without an argon-filled glovebox. Lower reaction activation energy and long living active center make polymerization happen at ambient temperature or even lower temperature without a heating process and result in a high molecular weigh polymer independence of interfacial side effects. That is why the PECA is extensively used as a powerful glue for interfacial adhesion for various materials. Therefore, this facile and reliable in situ polymerization strategy via an anionic mechanism is very promising to prepare high performance polymer electrolytes. Since commercial lithium salt LiFP6 always contains trace of HF and PF5 capable of terminating the active anionic center, LiClO4 was chosen as lithium salt in this paper due to its good compatibility with ECA polymerization. In the case of using lithium metal foil as counter electrode, the in situ polymerization of ECA could be directly initialized without addition of any other initiators. The FTIR spectra comparison of ECA and PECA−GPE were shown in Figure S1. There were four strong absorption bands at 2991 (−CH2, asymmetrical stretching vibration), 2260 (−C N, stretching vibration), 1750 (CO, stretching vibration), and 1254 cm−1 (C−O, stretching vibration) in both IR spectra of ECA and PECA composite, which were in good accordance with previous reports.20,21 The disappearance of peaks at 3127 ( CH2, stretching vibration) and 1665 cm−1 (-CC-, stretching vibration) in PECA confirmed that the polymerization reaction was completed. The dependence of ionic conductivity of the in situ polymerized GPE was shown in Figure 1. The ionic conductivity of GPE reached 2.7 × 10−3 S cm−1 at room temperature, which was comparable to that of liquid-phase electrolyte with a
EXPERIMENTAL SECTION
Ethyl-cyanoacrylate (ECA, Heowns, 98%), ethylene carbonate (EC, Macklin, battery grade), dimethyl carbonate (DMC, Macklin, battery grade), and lithium perchlorate (LiClO4, Aladdion, battery grade) were commercially obtained. All the chemicals were used as received without further purification and stored in a argon-filled glovebox. Ethyl-cyanoacrylate (1 mL), as a highly active monomer, was homogeneously mixed with liquid electrolyte (3 mL, 4 M LiClO4 in a nonaqueous solution of ethylene EC/DMC with a volume ratio of 1:1) and initiator (lithium powder, ∼100 ppm) in a glovebox (water content ≤ 1 ppm), the precursor solution could be completely gelated within 2 h. The morphology was observed by Hitachi S-4800 field emission scanning electron microscopy (FESEM). The polymerization were characterized using a PerkinElmer Frontier FTIR spectrometer in the range of 4000−400 cm−1. Coin cells SS/PECA-GPE/SS for ionic conductivity measurements were assembled by sandwiching two stainless-steel plate electrodes with in situ polymerized PECA on a stainless steel. The ionic conductivities of the GPEs were analyzed by an AC impedance method using a test cell with temperatures ranging between 20 and 80 °C and calculated according to σ = d × s−1 × Rb, where σ is the ionic conductivity of the electrolyte, d is the distance between the two electrode, s is the surface area of the electrolyte, and Rb is the intercept at the real axis in the impedance. The AC impedance measurements were carried out with a potential amplitude of 5 mV and a frequency range of 0.1 Hz to 1 MHz by Zahner Zennium electrochemical working station. The cyclic voltammetry (CV) curves were recorded from −1.0 to 6.0 V vs Li/Li+ at a scanning rate of 0.1 mV s−1 using a stainless-steel (SS) as the working electrode and lithium metal as the counter and reference electrode on a Shanghai Chenhua CHI660C electrochemical workstation. LiFePO4/PECA-GPE/Li and LiNi0.5Mn1.5O4/PECA-GPE/Li cells were assembled to evaluate the electrochemical performance of the GPE. The cathode consisted of 80 wt % active material (LiFePO4 or LiNi0.5Mn1.5O4, respectively), 10 wt % super P and 10 wt % binder (PVDF). These charge and discharge profiles were galvanostatically obtained by using a LAND battery test system at ambient temperature.
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RESULTS AND DISCUSSION The anionic polymerization mechanism of ECA initiated by alkaline metal Li was depicted in Scheme 1. One electron transfers from metal Li to the unsaturated CC double bonds and subsequently results in anionic active species, which initialize
Figure 1. Temperature dependent on ionic conductivity of as prepared PECA-GPE. 8738
DOI: 10.1021/acsami.6b16218 ACS Appl. Mater. Interfaces 2017, 9, 8737−8741
Research Article
ACS Applied Materials & Interfaces separator Celgard 2500 (2.4 × 10−3 S cm−1).26 It could be observed that the ionic conductivity increases with increasing temperature complying well with the Arrhenius equation, σ = exp(−Ea/RT), where A is the pre-exponential factor, Ea is the activation energy, and R is the ideal gas constant. The transference number of lithium ion is also very important for the polymer electrolyte.18 The major challenges for dual ion conduction in LIBs are the low Li+ ion transference number and severe polarizations caused by moving anions.27 In addition, the mobile anions also take part in undesirable side reactions at the electrodes, which can also adversely affect the performance of batteries. The transference number of Li+ ion estimated by chronoamperometry was demonstrated in Figure 2. In this
Figure 4. Time evolution of the interfacial resistance (Rs) within Li/ PECA-GPE/Li for in situ polymerized GPE and ex-situ polymerized GPE, respectively.
monomer of ECA can easily penetrate into the cathode composite, so after polymerization process PECA can be in situ formed within cathode composite and provide continuous ionic channels deep into the electrodes. The good interfacial contact between electrolyte and electrode materials can be confirmed from the cross section SEM images. As shown in Figure S3, there are two obvious interfaces in the cross section, the polymer electrolyte spread into the electrode very uniformly and penetrate into the electrode material. This can provide the electrodes with good ionic conductivity and excellent interfacial performance. These results guaranteed that the in situ polymerized GPE possessed the advantage of lower interfacial resistance than that of ex-situ GPE. Figure 5 showed the electrochemical performance of the coin cell LiFePO4/PECA-GPE/Li using GPE prepared by in situ polymerization. The peaks of CV at 3.3 and 3.5 V versus Li/Li+ were typical redox peaks of LiFePO4 corresponding to extraction and insertion of lithium ion, respectively as shown in Figure 5a. The distance between redox peaks was about 0.2 V at the scan rate of 0.1−0.2 mV/s, which means the smaller polarization of electrochemical reaction, indicating good feasibility of this in situ polymerization method. The charging/discharging behaviors, were shown in Figure 5b. The charge−discharge curves presented typical flat-shaped voltage profiles. Moreover, the potential difference between the charge and discharge plateaus was about 0.07 V at 0.2 C, which was as small as that of liquid electrolyte. The rate capability and cycling performance were shown in Figure 6. The reversible capacities of gel electrolyte were about 155, 145, 140, 120, and 110 mAh g−1 at the discharge rate of 0.2, 0.5, 1 (0.8 mA/cm2), 2, and 3 C, respectively. This was mainly due to the superior ionic conductivity of PECA-GPE and excellent contact between PECA-GPE and the electrode materials. Besides, the in situ polymerized GPE can offer good interfacial compatibility between the electrodes and the electrolytes during cycling by analyzing the AC impedance spectra after first and 100th cycle (Figure S4). The cycling performance (Figure 6b) of the gel electrolyte was evaluated at a constant charge/discharge current density (1 C/1 C). After 100 cycles, the capacity remained over 90%, indicating superior cycling performance using this in situ polymerized PECA-GPE. High-voltage lithium batteries of LiNi0.5Mn1.5O4/PECAGPE/Li was also assembled to evaluate the high voltage properties of the in situ polymerized PECA-GPC. As can be seen from Figure 7a, the charge/discharge profiles exhibited good plateaus with a small polarization, which was due to the high ionic conductivity of the GPE and well penetration of electrolyte into cathodes. The capacities of gel electrolyte remained 122 mAh g−1 (93% retention) at the rate of 1 C (0.6 mA/cm2) after 100th cycle (Figure 7b). These findings
Figure 2. Chronoamperometry profiles in Li/PECA-GPE/Li with a step potential of 10 mV.
system, the Li+ ion transference value of 0.45 was obtained, which was higher than that of conventional liquid electrolyte (0.32).28 The high value of the ionic conductivity and the high Li+ ion transference number of the GPE was mainly due to the strong electron withdrawing capability of ester and cyano groups resulting in interactions with ClO4−, ClO4−···(δ+) C−CN (δ−), or ClO4−···(δ+) C−CO (δ+), which were beneficial for the higher lithium ion transference number.29 An electrochemically stable window up to 4.8 V would be achieved, which was favorable for potential application in high energy batteries (shown in Figure 3). Figure 4 showed the comparison of time
Figure 3. Cyclic voltammetry curve of Li/PECA-GPE/SS cell at a scan rate of 0.1 mV s−1.
dependent interfacial resistance between the as prepared GPE and ex-situ polymerized GPE. It indicated that the in situ polymerized GPE had low interfacial resistance as desirable. The interfacial stability between lithium anode and gel electrolyte was shown in Figure S2. The interface impedance initially increased and reached a stable plateau, indicating that the lithium anode was passivated to form a stable SEI favorable for a long cycling life.30 The gel electrolyte prepared by in situ polymerization on the electrode was shown in Figure S3. The low-viscous liquid 8739
DOI: 10.1021/acsami.6b16218 ACS Appl. Mater. Interfaces 2017, 9, 8737−8741
Research Article
ACS Applied Materials & Interfaces
Figure 5. (a) Cyclic voltammetry curves of the LiFePO4/PECA-GPE/Li. (b) Charging and discharging curves at varied C rates.
Figure 6. (a) Charge capacities at 0.2, 0.5, 1, 2, and 3 C rate capability. (b) Cycling behavior of the LiFePO4/PECA-GPE/Li batteries.
Figure 7. (a) Charge/discharge curves of LiNi0.5Mn1.5O4/Li in the 5th, 30th, 60th, and 100th cycles. (b) Cycling behavior of the LiNi0.5Mn1.5O4/PECAGPE/Li batteries at the rate of 1 C.
flexibility. The battery could deliver the capacity of about 130 mAh g−1 after 20 cycles with high Coulombic efficiencies approaching to unit. These results demonstrated this simple and convenient method was a very promising strategy to prepare flexible thin-film batteries for flexible electronic device.
demonstrate the in situ PECA-GPE was a promising polymer electrolyte for matching well with the 5 V-class cathode. The electrochemical performance of the flexible battery was also evaluated. Figure 8 showed the photo and schematics of flexible monolithic LiFePO4/PECA-GPE/Li battery. The battery was able to light up a LED in bending states indicating of good
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CONCLUSIONS We have prepared a poly(ethyl cyanoacrylate) based GPE electrolyte by in situ generation technology via a facile and reliable anionic polymerization. The PECA-GPE electrolyte possessed excellent ionic conductivity and high transference number. Excellent cycling behavior and C-rate capability were also achieved thanks to the considerable interface performance. These results demonstrated that this reliable in situ polymerization process may be a very promising strategy to prepare flexible thin-film batteries or deformable shapes rechargeable batteries.
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Figure 8. (a) Photo of a LiFePO4/PECA-GPE/Li flexible Li ion battery. (b) Schematic illustration of the LiFePO4/PECA-GPE/Li flexible Li ion battery. (c) Charging/discharging capacities and Coulombic efficiencies of this battery at 0.5 C (0.2 mA/cm2) for 20 cycles in bending states.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b16218. 8740
DOI: 10.1021/acsami.6b16218 ACS Appl. Mater. Interfaces 2017, 9, 8737−8741
Research Article
ACS Applied Materials & Interfaces
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FTIR spectra of PECA and ECA, the interfacial resistance for in-suit polymerization GPE and solvent casting GPE film, the optical image of the surface of the gel electrolyte by in situ polymerization on the electrode, typical SEM images of the cross section of the electrode, and nyquist plots of the GPE measured before and after 100 cycles of operation (PDF)
AUTHOR INFORMATION
Corresponding Authors
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[email protected]. ORCID
Guanglei Cui: 0000-0002-8008-7673 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA09010105), Shandong Provincial Natural Science Foundation, China (Grant No. ZR2015QZ01), and “135” Projects Fund of CAS-QIBEBT Director Innovation Foundation.
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DOI: 10.1021/acsami.6b16218 ACS Appl. Mater. Interfaces 2017, 9, 8737−8741
