Lithium-Containing Zwitterionic Poly(Ionic Liquid)s as Polymer

Aug 2, 2017 - ACS Omega 2018 3 (9), 10564-10571 ... Single lithium-ion polymer electrolytes based on poly(ionic liquid)s for lithium-ion batteries. Ya...
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Lithium-Containing Zwitterionic Poly(Ionic Liquid)s as Polymer Electrolytes for Lithium-Ion Batteries Fei Lu,† Xinpei Gao,† Aoli Wu,† Na Sun,† Lijuan Shi,*,‡ and Liqiang Zheng*,† †

Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan, 250100, People’s Republic of China ‡ Key Laboratory of Coal Science and Technology, Taiyuan University of Technology, Ministry of Education, Taiyuan, 030024, People’s Republic of China S Supporting Information *

ABSTRACT: Polymer electrolytes are considered as the good candidates for the new-generation-safe lithium-ion battery. Herein, a free-standing and flexible polymer electrolyte film based on a lithium-containing zwitterionic poly(ionic liquid) (PIL) was constructed with and without propylene carbonate (PC) by in-situ photopolymerization. In this system, the lithium-containing IL synthesized by equimolecular neutralization of imidazolium-type zwitterion 3-(1-vinyl-3-imidazolio)propanesulfonate (VIPS) and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) can both serve as the polymeric matrix of the polymer electrolytes to maintain sufficient mechanical strength and form Li+-rich channels for lithium-ion transportation. The ion−dipole interaction between the lithium ion and the polar solvent PC can further improve the lithium-ion conduction, resulting in a comparable ionic conductivity of ∼10−3 S/cm at 30 °C. Charge−discharge cycling performance of Li/LiFePO4 half-cell equipped with the PIL-based polymer electrolyte indicates the possibility of practical application. Simultaneously, the lithium-containing zwitterionic PIL fabricated by this facile method provides a promising model system for understanding the molecular interactions in promoting the lithium-ion conduction.

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INTRODUCTION Safety concerns on volatilization and flammability of the conventional liquid electrolyte and hazards associated with possible cell explosion are the major issues preventing the largescale application of lithium-ion batteries (LIBs).1 Recently, research aimed at reducing safety hazards in LIBs has a particular focus on the polymer electrolytes to replace the flammable organic alkyl carbonate electrolytes.2−9 In technological application, polymer electrolytes are expected to overcome some concerns such as cell leakage, corrosion, and portability. Furthermore, the use of polymer electrolyte provides the possibility to produce flexible thin-film batteries without any additional separator. One of the most promising approaches to further optimize the polymer electrolyte is the introduction of ionic liquids (ILs) owing to their unique properties such as good thermal stability, negligible vapor pressure, and nonflammability, together with high ionic conductivity and wide electrochemical window.10−12 Combining ILs with polymer electrolyte gives advantages in terms of improved safety and a higher operating temperature range. Thus, poly(ionic liquid)s (PILs), as a combination of polymers and ILs, have gained increasing attention as potential polymer electrolytes in the electrochemistry field. PILs are generally prepared by conventional or controlled radical polymerization of ionic liquid monomers.13−15 The presence of an IL moiety in the repeating unit of PIL chains can integrate © 2017 American Chemical Society

some desirable characteristics of IL into the polymeric matrix.16−19 Until now, different kinds of PILs based on pyrrolidinium, guanidinium, tetraalkylammonium, and imidazolium have been reported to serve as polymer electrolytes for advanced LIBs.20−25 However, the intrinsic high ionic conductivity of ILs is always useless for battery performance since the transported ion components in ILs are not the target ions of electrolyte, such as lithium ions for LIBs and protons for fuel cells. Thus, extra lithium salts, LiX (X− = anion), should be added to satisfy the strict requirement of target ions in practical battery application. Instead, the migration of component ions in ILs would inhibit selective transportation of target ions. Setting Li+ as a part of ILs to construct lithium-containing ILs offers the prospect to reduce the transportable species in electrolyte. Metalcontaining ILs can be achieved by designing ILs that have a coordinating group covalently linked to the cations or that have a metal complex as anions.26−28 It is a big challenge to prepare lithium-containing ILs by coordination rather than by simple dissolution, and few studies have been performed.29,30 To address this issue, zwitterion, as a covalent combination of cations and anions, can be proposed as another candidate to Received: June 26, 2017 Revised: August 2, 2017 Published: August 2, 2017 17756

DOI: 10.1021/acs.jpcc.7b06242 J. Phys. Chem. C 2017, 121, 17756−17763

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a UV arc lamp with an intensity of 9 mW/cm2 for 5 min at room temperature from both top and bottom surfaces of the plates to obtain free-standing polymeric films. The obtained films were stored in an argon-filled glovebox. Instrumentation. 7Li NMR spectra were recorded using a Bruker Advance 400 spectrometer at 155 MHz. Chemical shifts of 7Li signals were quoted to internal standard LiCl (δ = 0.00) in D2O. Fourier transform infrared (FT-IR) spectra were obtained with a resolution of 2 cm−1 using a BIORADFTS-165 spectrometer. The morphologies of the polymeric films were investigated by scanning electronic microscopy (SEM, JEOL JSM-7600F) after gold sputter coating. Thermogravimetric analysis (TGA) was performed on a Rheometric Scientific thermoanalyzer (Piscataway, NJ) under a flow of nitrogen with a heating rate of 10 °C/min from 30 to 800 °C. Differential scanning calorimetry (DSC) measurement was carried out on a PerkinElmer 8500 calorimeter with a heating rate of 5 °C/min from −25 to 180 °C under a flow of nitrogen. Mechanics under a flow of nitrogen al tensile test was performed using a Universal Testing Machine (Yashima Works Ltd. Co., model RTM-IT) at room temperature. The crosshead displacement speed of testing was set at the rate of 10 mm/min. The polymeric film with thickness around 0.1 mm and size of 6 mm × 25 mm was used for testing. Temperature dependence of ionic conductivity was determined using AC impedance spectroscopic technique in the frequency of 100−106 Hz with 0.3 mV oscillating voltage. The films of blended electrolytes were sandwiched between a pair of stainless steel electrodes. The measurements were performed in a temperature range from 30 to 150 °C, and the cell was thermally equilibrated for 1 h at each temperature before measurement. The values of ionic conductivity (σ, in S/cm) were calculated from the measured resistance following the equation of σ = d/AR, where d (cm) is the thickness of the film, A (cm2) is the cross-sectional area of the film, and R (Ω) is the bulk resistance derived from Nyquist plot. Linear sweep voltammetry was performed on an electrochemical analyzer in the range of 0−6 V with a scan rate of 1 mV/s using a [stainless steel/polymer electrolyte/Li metal] CR2032-type coin cell. All the cells (and hereafter) were assembled in an argon-filled glovebox. The lithium-ion transference number (tLi+) of the polymer electrolytes was derived by combining AC impedance and DC polarization techniques using a [Li metal/polymer electrolyte/ Li metal] symmetric cell at room temperature. The cell was subjected to polarization by applying a DC voltage of 10 mV for 1000 s to determine the initial and final current. Simultaneously, the cell resistances were also measured before and after the DC polarization using AC impedance. Galvanostatic cycling tests were conducted on a LAND CT2001A system using a [LiFePO4/polymer electrolyte/Li metal] half-cell. The LiFePO4 electrode (LiFePO4:acetylene black:poly(vinylidene fluoride) = 8:1:1, in N-methylpyrrolidone to form a uniform slurry, loaded onto aluminum foil and then dried) was purchased from Liming Power Co. and was cut into discs (12 mm in diameter with a loading density of 4.5 ± 0.2 mg/cm2) before use. The charge−discharge cycling was preformed within a potential range of 2.5−4.3 V (vs Li+/Li) at a current density of 17 mA/g at 30 °C.

prepare lithium-containing ILs. The equimolar mixture of zwitterion and lithium salt can result in an IL-like ion pair with low melting point because of the hard and soft, acids and bases principle.31 This approach provides a new preparative direction to turn a lithium salt into a liquid. In the IL-like ion pair, Li+ is the only transportable cation since the covalently combined zwitterions have no ion migration induced by the potential gradient. In this contribution, a polymerizable room-temperature ionic liquid [VIPS][LiTFSI] was synthesized by equimolecular neutralization of imidazolium-type zwitterion 3-(1-vinyl-3imidazolio)propanesulfonate (VIPS) and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) on the basis of the intermolecular electrostatic interactions. Upon UV crosslinking, the lithium-containing IL can be polymerized into a flexible free-standing electrolyte film. To further enhance the transportation of Li+ in the polymer electrolyte, propylene carbonate (PC) was chosen as the solvent of the gel polymer electrolyte because the organic carbonate was practically used in LIBs because of the high dielectric constant for the dissociation of lithium ion and sufficient electrochemical stability.32,33 Especially, the polymerized [VIPS][LiTFSI] can work as the polymeric matrix to form both thin, flexible film and also Li+-rich channels for lithium ion transportation. We expect that the lithium ion mobility in the obtained polymer electrolytes can be improved by the interactions between imidazolium-type zwitterion and LiTFSI, as well as by incorporating PC as the polar agent. This facile construction of lithium-containing polymerizable ionic liquid will provide a promising direction of understanding the molecular interactions in promoting lithium-ion conduction.

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EXPERIMENTAL SECTION Materials. 1-Vinylimidazole (97%) was purchased from Sigma-Aldrich. 1,3-Propane sultone (99%), propylene carbonate (99.5%), lithiumbis(trifluoromethanesulfonyl)imide (98%), N,N′-methylenebis(acrylamide) (98%), and 2-hydroxy-2-methylpropiophenone (99%) were obtained from J&K Scientific, Ltd. All the materials were used without purification. Synthesis of [VIPS][LiTFSI]. First, 3-(1-vinyl-3-imidazolio)propanesulfonate (VIPS) was prepared according to our previous work.34 Then, the obtained VIPS (0.05 mol, 10.80 g) was dissolved in a minimal amount of methanol. An equimolar amount of LiTFSI (0.05 mol, 14.35 g) was also dissolved in methanol and was added dropwise to the solution. The mixture was stirred at room temperature for 24 h. After vacuum rotary evaporation of methanol, the obtained viscous liquid was dried under vacuum at room temperature for 48 h. The purity of the product was confirmed by 1H and 19F NMR. 1 H NMR (D2O, 400.13 MHz): 9.04 (s, 1H), 7.72 (s, 1H), 7.56 (s, 1H), 7.10 (dd, J = 15.6, 8.7 Hz, 1H), 5.76 (d, J = 15.6 Hz, 1H), 5.38 (d, J = 8.7 Hz, 1H), 4.35 (t, J = 7.2 Hz, 2H), 2.89 (t, J = 7.2 Hz, 2H), 2.30 (m, 2H). 19F NMR (D2O, 376.46 MHz): −79.20 (s, 6F). Preparation of Electrolyte Films. First, homogeneous solutions were prepared by the addition of different amounts (from 0 to 90 wt % to total weight) of PC into [VIPS][LiTFSI]. Then N,N′-methylenebis(acrylamide) (2 wt % to the monomer) and 2-hydroxy-2-methylpropiophenone (0.5 wt % to the monomer) were added to the solution. The resulting reactive mixture was sandwiched between two quartz plates (separated by a 200 μm thick tape) and was irradiated by

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RESULTS AND DISCUSSION The lithium-containing IL [VIPS][LiTFSI] (shown in Figure 1b) was prepared by evaporation of a methanol solution of 17757

DOI: 10.1021/acs.jpcc.7b06242 J. Phys. Chem. C 2017, 121, 17756−17763

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Figure 1. (a) Chemical structures and photographs of imidazolium-based zwitterion VIPS and lithium salt LiTFSI; (b) chemical structure and photograph of monomeric IL [VIPS][LiTFSI]; (c) a photograph of the obtained polymer electrolyte film after photo-cross-linking; schematic representation of (d) polymerized zwitterion VIPS, (e) PIL [VIPS][LiTFSI], and (f) PIL organogel in the presence of PC.

equimolar zwitterion VIPS and LiTFSI. Although VIPS is a solid-state salt with high melting point, a polymerizable roomtemperature IL can be obtained with the addition of LiTFSI. The formation of ionic liquid results in the disappearance of the melting peak of VIPS in the DSC curve (Figure S1). Electrostatic shielding effects are expected to reduce the strong intermolecular interaction between zwitterionic-type VIPS considerably and to result in the decrease of the melting poiont.31 The ion exchange between VIPS and LiTFSI promotes the formation of IL-like ion pairs because of the hard and soft, acids and bases principle. Then, the lithium-ion conducting polymer electrolytes based on poly([VIPS][LiTFSI]) were synthesized using an economical and ecofriendly UV-induced polymerization. In brief, a precursor solution was prepared first by mixing the monomeric IL [VIPS][LiTFSI] with N,N′-methylenebis(acrylamide) as a cross-linker, 2-hydroxy-2-methylpropiophenone as a photoinitiator, and different amounts of PC as solvent. After photocross-linking, a transparent free-standing polymer electrolyte film was obtained. Indeed, organic carbonates such as PC have been previously reported to be used as additives to improve the overall properties of lithium-ion conducting electrolytes, especially in terms of ionic conductivity owing to their high dielectric constant and relatively low viscosity.35,36 Therefore, the intramolecular interactions within poly([VIPS][LiTFSI]), as well as the interactions between poly([VIPS][LiTFSI]) and PC, are expected to have a favorable effect on the properties of the obtained lithium-ion conducting polymer electrolytes. The sample name and the composition of each PIL-based polymer electrolytes investigated in this work are listed in Table S1, and the inverted-bottle test for samples with different amounts of PC before and after UV irradiation is shown in Figure S2 of the Supporting Information. To examine the interactions between VIPS, lithium ions, and PC, 7Li NMR spectra of LiTFSI; the binary mixture of VIPS and LiTFSI; and the tertiary mixture of VIPS, LiTFSI, and PC were recorded in D2O solution with LiCl as an external chemical shift reference in the coaxial tube. As shown in Figure 2, the 7Li chemical shift of LiTFSI exhibits a downfield shift

Figure 2. 7Li NMR spectra of LiTFSI (0.1 M), VIPS/LiTFSI (0.1 M/ 0.1 M), and VIPS/LiTFSI/PC (0.1 M/0.1 M/1 M) in D2O solution at 25 °C. LiCl (5 M) in D2O solution was used as an external chemical shift reference in the coaxial tube.

from 0.19 to 0.21 ppm with the addition of VIPS. This result indicates the formation of a complex between VIPS and LiTFSI because of the interactions generated by lithium ion and sulfonate anion.37−40 A further downfield shift to 0.22 ppm is observed with the subsequent addition of PC into the binary mixture of VIPS and LiTFSI, which can be ascribed to the ion− dipole interactions between lithium ion and PC. All the 7Li signals were detected by the addition of monomeric zwitterion VIPS because of the poor solubility in D2O after polymerization. The interactions among zwitterions, lithium ions, and PC were also confirmed by IR spectra in the bulk state. The IR spectra of powder monomeric VIPS, polymeric film poly([VIPS][LiTFSI]) without PC, and gel electrolyte film containing 50 wt % PC (PIL/50PC) were recorded at room temperature and are shown in Figure 3. The original zwitterion VIPS shows two absorption bands at 1033 and 1179 cm−1, which can be assigned to the SO stretching vibrations of the sulfate group.41,42 After the addition of equimolecular LiTFSI, 17758

DOI: 10.1021/acs.jpcc.7b06242 J. Phys. Chem. C 2017, 121, 17756−17763

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Thermogravimetric analysis was further conducted to examine the thermal stability of the electrolyte films. Figure S3 illustrates the TGA curves of the pure PIL film and PIL/ 50PC gel electrolyte film. The pure PIL matrix is thermally stable up to 400 °C, well above the melting point of Li metal. The obvious evaporation of PC for PIL/50PC starts around 100 °C (the evaporation is slower when the samples are sandwiched in the conductivity measurement) and is sufficient for the lithium-ion battery practical application. It is also of paramount importance to investigate the mechanical property of PIL film in order to meet the requirement during LIBs packaging and cycling. As shown in Figure S4, typically, the neat PIL film has a tensile strength at break around 11 MPa and a tensile elongation at break over 70%. With the addition of PC, the mechanical strength of the gel electrolyte films dramatically decreases because of the dilution of the polymer chains by polar solvent. The ionic conductivities of the pure PIL film and the gel polymer electrolyte films with different contents of PC were measured as a function of temperature by the altering current impedance method. As shown in Figure 5a, all the electrolyte

Figure 3. IR spectra of VIPS (black curve), PIL poly([VIPS][LiTFSI]) (red curve), and PIL/50PC (blue curve).

these absorption bands of the obtained PIL exhibit prominent blue shifts to 1053 and 1183 cm−1, respectively. Futher addition of 50 wt % PC leads to higher wavenumber shifts to 1057 and 1185 cm−1, respectively. This result is a clear indication that the sulfonate group of VIPS interacts not only with lithium ion but also with PC. Such an interaction could facilitate the transportation of lithium ion as well as stabilization of the lithium salt.43 Scanning electron microscopy (SEM) characterization provides direct information on the morphology of the PILbased polymer electrolyte films. As shown in Figure 4a, the

Figure 4. SEM images of the cross sections for (a) pure PIL film, (b) gel electrolyte film PIL/10PC, (c) gel electrolyte film PIL/50PC, and (d) gel electrolyte film PIL/70PC.

Figure 5. (a) Ionic conductivities as a function of temperature for the PIL/χPC polymer electrolytes (χ is the weight fraction of PC); (b) time dependence of ionic conductivities at 30 °C for the PIL/χPC polymer electrolytes.

cross section of the pristine PIL film is uniform and dense. However, the gel electrolyte films PIL/χPC (χ is the weight fraction of PC) show a spongelike morphology, and distinctive round-shaped particles stacking onto each other can be observed after a complete evaporation of PC. The irregular nanoparticles are closely interacted and built up the backbones of the porous PIL complex. The morphology of these samples is similar to each other, and a larger amount of PC could result in a more remarkable foam structure as shown in Figure 4c and d.

films show a gradual increase in ionic conductivity with the increase of temperature. The pure PIL film without PC exhibits increasing ionic conductivities from 2.4 × 10−5 to 2.9 × 10−3 S/ cm in the temperature range of 30−150 °C. Previous work reported that the ionic conductivity of another similar zwitterionic-type salt 3-(1-ethyl-3-imidazolio)propanesulfonate without any lithium salts was below 10−9 S/cm at room temperature.31 Although the ion density in zwitterionic salts is 17759

DOI: 10.1021/acs.jpcc.7b06242 J. Phys. Chem. C 2017, 121, 17756−17763

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The Journal of Physical Chemistry C high, there is no ion migration induced by the potential gradient among zwitterions, resulting in a low ionic conductivity. With the addition of an equimolar amount of LiTFSI into 3-(1-ethyl-3-imidazolio)propanesulfonate, the ionic conductivity of this binary mixture increased to 10−5 S/ cm at 50 °C because of the formation of ionic liquid.31 Although the polymerization process can lead to a significant decrease in the ionic conductivity,44,45 the neat poly([VIPS][LiTFSI]) film in our system shows a comparable ionic conductivity around 1.1 × 10−4 at 50 °C. The segmental mobility generated by cross-linker N,N′-methylenebis(acrylamide) in the main chain may be attributed to the reservation of ionic conductivity for the PIL poly([VIPS][LiTFSI]) because of the formation of the flexible segment for solvating and desolvating lithium ions.46,47 It can be seen that the addition of PC into the PIL significantly increases the ionic conductivities of the gel polymer electrolyte films at 30 °C. With the addition of 90 wt % PC, the ionic conductivity of PIL/90PC film can reach up to 4.1 × 10−3 S/cm at 30 °C, which is 2 magnitudes higher than that of pure PIL film. This observation suggests that the presence of PC would enhance the ion dissociation and migration in the system. The ionic conductivity of liquid electrolyte containing LiTFSI and organic carbonate has been reported in the value of ∼5 × 10−3 S/cm at room temperature.48,49 The PIL polymer electrolytes prepared by this facile method can simultaneously raise the practical safety as well as maintain the ionic conducting properties. The possible transportation processes in PIL-based polymer electrolyte are illustrated in Figure 1. The polymerized zwitterion VIPS with covalent combination of cation and anion is expected to form ionic channels but without any transportable ions (Figure 1d). After the addition of equimolecular lithium salt LiTFSI into VIPS, the lithium ions can be dissociated by the intermolecular interactions between LiTFSI and VIPS to form a polymerizable room-temperature IL. As a result, the ionic channels formed by VIPS are also filled with migratable ions. Upon photopolymerization, the PIL matrix with lithium-ion-rich channels can be obtained (Figure 1e). At the same time, the lithium ion mobility can be further enhanced by incorporating PC as a polar solvent to form a PIL organogel. The ion−dipole interactions between lithium-ion and PC are also favorable to promote the lithium ion dissociation and to improve the ionic conductivity (Figure 1f). Reduction in ionic conductivity caused by liquid leakage is one of the major issues to hinder the application of gel electrolytes. Figure 5b shows the trend of the ionic conductivity values of pure PIL film and gel polymer electrolyte films after three months of storage. Clearly, the conductivities of pure PIL film and PIL/30PC gel polymer film are highly stable after such long-time storage. However, the ionic conductivities of gel polymer film containing higher content of PC decrease with time because of the solvent loss. Also, the gel polymer film with higher content of PC has more liquid leakage and drastic reduction in ionic conductivity. Thus, in the following electrochemical tests, PIL/30PC gel electrolyte was chosen as a representative system. The transference number of lithium ion (tLi+) can be used to evaluate the lithium-ion conductivity of the electrolyte and is measured by AC impedance and chronoamperometry shown in the Supporting Information.50,51 The typical impedance spectra before and after chronoamperometry of the PIL/30PC polymer electrolyte at room temperature are depicted in Figure 6a, and

Figure 6. (a) Impedance spectra before and after chronoamperometry for a Li symmetric cell with PIL/30PC polymer electrolyte; the inset is the time-dependence response of DC polarization; (b) linear sweep voltammetry plot of a Li asymmetric cell with stainless steel as working electrode and PIL/30PC polymer electrolyte.

the inset shows the time-dependence response of DC polarization. It can be found that the measured I0 and Is are 0.46 and 0.12 μA, and the measured R0 and Rs are 8159 and 16585 Ω, repectively. According to eq S1, the value of tLi+ for PIL/30PC was calculated as 0.20. The values of tLi+ for the PILbased polymer electrolytes with different amounts of PC were also determined and are listed in Table S2. The electrochemical stability is also a crucial parameter to evaluate the potential application of the electrolyte. Figure 6b shows the linear sweep voltammetry plot of PIL/30PC polymer electrolyte as a representative. No significant decomposition is observed in the high potential range over 4.5 V versus Li/Li+. The achievement with sufficient electrochemical stability makes the as-prepared PIL polymer electrolytes promising candidates for the practical lithium-ion batteries. As an example, Li/LiFePO4 half-cell with PIL/30PC film as electrolyte was assembled to explore the cycling performance at 30 °C. As shown in Figure 7, the cell shows good initial discharge capacity of 120.4 mAh g−1, and the corresponding Coulombic efficiency is 89.1%. However, subsequently, the discharge capacity of the half-cell decreases slowly with the cycling, and the capacity decreases to 99.2 mAh g−1 in the 30th cycle, indicating that some irreversible electrochemical reactions of the electrode occurred. Generally, the voltage gap between the charge and discharge plateaus can be used to estimate the electrode polarization to denote electrochemical 17760

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the possibility of practical application. The zwitterionic PILbased polymer electrolytes prepared by this one-step method can simultaneously raise the practical safety and maintain the ionic conducting properties. We expect that this work will provide a rather appealing and promising perspective for the advanced safer lithium-ion battery.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06242. The compositions of PIL-based polymer electrolytes. The DSC curves of VIPS and IL. The TGA curves of PIL and PIL/50PC films. The stress−strain curve of PIL film. Impedance spectra before and after chronoamperometry for a Li symmetric cell with PIL/50PC or PIL/70PC electrolyte film. The theories and equations for calculation of the lithium-ion transference number (tLi+). The cyclic voltammogram of LiFePO4 half-cell with PIL/30PC film as electrolyte (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Liqiang Zheng: 0000-0003-0422-9585 Notes

Figure 7. (a) Charge−discharge curves and (b) cycle performance of LiFePO4 half-cell with PIL/30PC film as electrolyte at the current density of 17 mA/g at 30 °C.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (No. 21573132, No. 21403151) and China Postdoctoral Science Foundation (No. 2016M592174).

irreversibility.52,53 The bigger value of the voltage gap means the higher degree of the electrode polarization, resulting in poor cycling performance. As shown in Figure 7a, the value of the voltage gap between charge and discharge plateaus is much bigger than the previous works, indicating the higher polarization degree of the electrode.54,55 The reasons for polarization in batteries are always complex, and the optimization of the gel polymer electrolyte is also ongoing. Anyway, the cycling performance proves that the lithiumcontaining PIL-based polymer electrolyte offers a prospect in the practical Li metal batteries.

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REFERENCES

(1) Kalhoff, J.; Eshetu, G. G.; Bresser, D.; Passerini, S. Safer Electrolytes for Lithium-Ion Batteries: State of the Art and Perspectives. ChemSusChem 2015, 8, 2154−2175. (2) Kammoun, M.; Berg, S.; Ardebili, H. Flexible Thin-Film Battery Based on Graphene-Oxide Embedded in Solid Polymer Electrolyte. Nanoscale 2015, 7, 17516−17522. (3) Zhang, P.; Li, M.; Yang, B.; Fang, Y.; Jiang, X.; Veith, G. M.; Sun, X. G.; Dai, S. Polymerized Ionic Networks with High Charge Density: Quasi-Solid Electrolytes in Lithium-Metal Batteries. Adv. Mater. 2015, 27, 8088−8094. (4) Zhou, R.; Liu, W.; Leong, Y. W.; Xu, J.; Lu, X. Sulfonic Acid- and Lithium Sulfonate-Grafted Poly(Vinylidene Fluoride) Electrospun Mats As Ionic Liquid Host for Electrochromic Device and LithiumIon Battery. ACS Appl. Mater. Interfaces 2015, 7, 16548−16557. (5) Li, X.; Zhang, Z.; Li, S.; Yang, L.; Hirano, S. Polymeric Ionic Liquid-Plastic Crystal Composite Electrolytes for Lithium Ion Batteries. J. Power Sources 2016, 307, 678−683. (6) Ma, Q.; Zhang, H.; Zhou, C.; Zheng, L.; Cheng, P.; Nie, J.; Feng, W.; Hu, Y. S.; Li, H.; Huang, X.; Chen, L.; Armand, M.; Zhou, Z. Single Lithium-Ion Conducting Polymer Electrolytes Based on a Super-Delocalized Polyanion. Angew. Chem., Int. Ed. 2016, 55, 2521− 2525. (7) Kuo, P. L.; Wu, C. A.; Lu, C. Y.; Tsao, C. H.; Hsu, C. H.; Hou, S. S. High Performance of Transferring Lithium Ion for PolyacrylonitrileInterpenetrating Crosslinked Polyoxyethylene Network as Gel Polymer Electrolyte. ACS Appl. Mater. Interfaces 2014, 6, 3156−3162.

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CONCLUSION In summary, a lithium-containing zwitterionic IL was successfully prepared by equimolar ion exchange between an imidazolium-based zwitterion and a lithium salt. The obtained polymerizable IL monomers with different amounts of PC were further applied as polymer electrolytes for lithium-ion battery and exhibited good thermal stability and high electrochemical stability. In particular, the pure PIL polymer without PC showed increasing ionic conductivities from 2.4 × 10−5 to 2.9 × 10−3 S/cm in the temperature range of 30−150 °C owing to the promotion of Li+ dissociation generated by the zwitterionic effect. With the addition of PC, the ionic conductivities significantly increased to ∼10−3 at room temperature because of the further ion−dipole interaction between Li+ and PC. The following charge−discharge cycling performance of Li/LiFePO4 half-cell equipped with PIL-based polymer electrolyte indicates 17761

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The Journal of Physical Chemistry C

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DOI: 10.1021/acs.jpcc.7b06242 J. Phys. Chem. C 2017, 121, 17756−17763

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DOI: 10.1021/acs.jpcc.7b06242 J. Phys. Chem. C 2017, 121, 17756−17763