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PEO-urea-LiTFSI ternary complex as solid polymer electrolytes
Polymer 99 (2016) 44e48
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Short communication
PEO-urea-LiTFSI ternary complex as solid polymer electrolytes Xiaojing Yan 1, Bo Peng 1, Bingwen Hu*, Qun Chen* State Key Laboratory of Precision Spectroscopy, Shanghai Key Laboratory of Magnetic Resonance, Institute of Functional Materials, School of Physics and Materials Science, East China Normal University, 3663 North Zhongshan Road, Shanghai 200062, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 March 2016 Received in revised form 14 June 2016 Accepted 23 June 2016 Available online 27 June 2016
A kind of highly-crystalline solid polymer electrolytes, based on the a-PEO-urea-LiTFSI ternary complex, is introduced here. The introduction of LiTFSI into a-PEO-urea inclusion compound has not changed the crystalline structure of the inclusion compound. Furthermore, the ionic conductivity of this a-PEO-ureaLiTFSI ternary complex has reached up to 6.03 105 S cm1 at 303 K, where the highest conductivity of the reported highly-crystalline polymer electrolyte is only 1.58 107 S cm1 for PEO6/LiAsF6. Our investigation might motivate further research on the different kinds of highly-crystalline polymer electrolytes based on the inclusion complexes. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Polymer electrolytes Inclusion complex PEO
1. Introduction Solid polymer electrolytes (SPEs) have attracted great research interests since 1970s, due to their potential application in all-solid rechargeable batteries [1e5]. In particular, PEO/LiX (here PEO is poly(ethylene oxide) [(CH2CH2O)n] and X refers to any anion) polymer electrolytes are regarded as promising materials for high energy density rechargeable batteries [6,7]. However, the poor conductivity of PEO-based SPEs at room temperature has restricted their applications. There are many efforts to improve the conductivity which focus on reducing the crystallinity and increasing the amount of amorphous state of the PEO-based complexes [8e11]. However, a series of highly-crystalline polymer electrolytes based on PEO of molecular weight of 1 103 g/mol and alkaline metal salts, which were considered to be insulators in the past, show high conductivity at room temperature [12,13]. The PEO6/ LiAsF6 system has highest conductivity (1.58 107 S cm1 at 303 K) among these highly-crystalline polymer electrolytes [12,13]. It is reported that lithium ions are located in the tunnels formed by pair of PEO chains along which they could migrate in those crystalline polymer electrolytes [12e17] and ion-conduction mechanisms was probed by different methods, such as molecular dynamics simulations and solid-state NMR [18e20]. The high ionic
* Corresponding authors. E-mail addresses: [email protected] (B. Hu), [email protected] (Q. Chen). 1 Equal contribution. http://dx.doi.org/10.1016/j.polymer.2016.06.056 0032-3861/© 2016 Elsevier Ltd. All rights reserved.
conductivity in this kind of material could be attributed to the directional motion of Liþ in the channels formed by the PEO chains. In our previous paper, we prepared a new type polymer electrolyte formed by PEO, a-cyclodextrin (aCD) and LiAsF6, in which the Liþ moves along the tunnel formed by a-cyclodextrin [21]. Furthermore, only one PEO chain threads along the tunnel in a complete amorphous state. This ternary complex aCD-PEO-LiAsF6 has higher conductivity than the PEO-LiAsF6, which has been ascribed to the directional motion of Liþ in the channels formed the a-cyclodextrin. The research on PEO-LiAsF6 and aCD-PEO-LiAsF6 motivated us to find new materials in which the Liþ could move along the channel in well-defined direction. Here we report a kind of highly-crystalline polymer electrolytes, a-PEO-urea-LiTFSI (here LiTFSI is lithium bis(trifluoromethane sulfonyl) imide) ternary complex, based on the inclusion complex a-PEO-urea. The ionic conductivity of this a-PEO-urea-LiTFSI ternary complex has reached up to 6.03 105 S cm1 at 303 K. It was also found out that the a-PEO-urea-LiAsF6 ternary complex also possesses high conductivity of 3.39 105 S cm1 at 303 K. The PEO-urea binary system has gained a lot of attention since a stable crystalline inclusion compound (IC) can be formed [22e24]. With a (EO)4:(urea)9 stoichiometry, six urea molecules form a hexagonal channel structure that includes the remaining three urea molecules and the PEO chain [22]. The introduction of LiTFSI into the channel of this PEO-urea compound only slightly changes the crystalline size, but does not change the crystalline structure of the inclusion compound. The Liþ ion could immigrate along the hexagonal channel and lead to high conductivity.
X. Yan et al. / Polymer 99 (2016) 44e48
2. Experimental 2.1. Sample pretreatment PEO with average molecular weights (Mw) of 100 000 g/mol and urea were purchased from Sigma-Aldrich and heated in vacuum drying oven at 70 C for 2 days before use. Acetonitrile (Jk chemical, 99.9% purity) and LiTFSI (Sigma-Aldrich, 99.95%) were used without further purification. 2.2. Preparation of a phase of PEO and urea complex The a phase of PEO and urea complex with the molar ratio of 4:9 (referred as a-PEO-urea) was prepared using a previously described co-crystallization method [24]. The mixture of PEO and urea was heated in an oven at 40 C. A white powdery sample was obtained after 5 days. Then the product was heated in the vacuum drying oven at room temperature for 1 month for further analysis. It should be mentioned here that quenching this a complex from its melting temperature and recrystallization will lead to a mixture of a complex and b complex [25]. 2.3. Preparation of PEO-urea-LiTFSI ternary complex PEO-urea-LiTFSI complex were prepared by dissolving LiTFSI of 0.1223 g (or 0.1957 g, 0.2447 g, 0.3262 g, 0.6525 g), PEO of 0.3 g, and urea of 0.9214 g in acetonitrile solution of 50 mL, and we can obtain the sample with EO:urea ¼ 4:9 and EO:LiTFSI ¼ 16:1, 10:1, 8:1, 6:1, 3:1, which will be referred as a-PEO16-urea-LiTFSI, a-PEO10-ureaLiTFSI, a-PEO8-urea-LiTFSI, a-PEO6-urea-LiTFSI and a-PEO3-ureaLiTFSI. The mixture was vigorously stirred overnight at room temperature. The mixture was then cast under the temperature of 40 C to evaporate the acetonitrile for one week to obtain the white powder. The powder was finally moved to a vacuum and dried at room temperature for 1 month for further analysis. It should be pointed out that when the EO:LiTFSI ratio reaches 2:1, the PEOurea-LiTFSI ternary complex becomes slightly liquefied, therefore the lowest EO:LiTFSI ratio is limited to 3:1.
45
thermometer inside to detect the temperature and the glass bottle was placed in an oil bath with a temperature controlling device to control the temperature of the coin cells. The equivalent electrical circuit used to fit the Nyquist plot could be found in Fig. S1a and one typical Nyquist plot is shown in Fig. S1b. The fitting result of R2 is the sample resistance, and then the ionic conductivity could be obtained by the equation s ¼ l=R2 S, where S is the pellet area and l is the pellet thickness. Wide angle X-ray diffractions of the disk-shaped samples with about 5 mm in diameter and 0.5 mm in thickness were recorded at ambient conditions on a BRUKER D8 Advance instrument using Ni filtered Cu Ka radiation. The supplied power was 3 kW. The scanning was carried out with 2q from 2 to 50 with a step of 0.02 , and the diffraction peak positions of the sample were calibrated from the pattern of aluminum. All SSNMR experiments were performed on a Bruker AVANCEIII spectrometer with a 1H frequency of 400 MHz and a 13C frequency of 100 MHz. Commercial Bruker triple-resonance 4 mm MAS probes and 4 mm zirconia rotors were used for all experiments. The 13 C chemical shifts were determined from the carbonyl carbon signal (diso ¼ 176.03 ppm) of glycine relative to tetramethylsilane (TMS). The spin rate of the 4 mm rotor was 8 kHz. The process of sealing the powder samples into the rotor was carried out in the glove box filled with N2 gas. All of the NMR experimental data were processed with Bruker Topspin 2.1 software. 3. Results and discussion Wide angle X-ray spectra in Fig. 1 are employed to probe the crystalline structure of the PEO-based complex. It’s obvious from Fig. 1aef that the peak positions only slightly shift to high angle with the increase of LiTFSI content, implying that a-PEO-ureaLiTFSI ternary complexes form the same crystalline structure as a-
2.4. Preparation of simple mixtures The mixture of PEO, urea and LiTFSI were prepared directly by simple mixing of PEO, urea and LiTFSI solid powders with the EO:Li ¼ 3:1. The mixture of a-PEO-urea and LiTFSI were prepared by simple mixing of a-PEO-urea and LiTFSI solid powders with the EO:Li ¼ 3:1. These two mixtures are not prepared by the solution casting method, but prepared by the mixture of different solid powders. 2.5. Characterization The powder sample was loaded into a polytetrafluoroethylene (PTFE) cylinder mold with the diameter of 16.24 mm and was pressed into a compact disc like pellet about 2.5 mm in thickness by two stainless steel rods with 50 bar pressure. The polymer electrolyte disks were sandwiched between 2 stainless steel electrodes and then sealed into a coin cell. All the sample handing was carried out in a N2-filled glove box. Impedance spectra were carried out with an Autolab PGSTAT 302N electrochemical workstation. An alternating voltage of 100 mV amplitude was employed and the data were collected over the frequency 0.1 Hz to 1 MHz. The dc conductivity at different temperatures was obtained from the complex impedance plots [2,3]. The coin cells were located in a glass bottle with a
Fig. 1. Wide angle X-ray diffraction patterns of a-PEO-urea (a) and PEO-urea-LiTFSI ternary complexes of (b) a-PEO16-urea-LiTFSI, (c) a-PEO10-urea-LiTFSI, (d) a-PEO8urea-LiTFSI, (e) a-PEO6-urea-LiTFSI, (f) a-PEO3-urea-LiTFSI.
46
X. Yan et al. / Polymer 99 (2016) 44e48
PEO-urea but with smaller unit cell lattice. The changes in relative intensity between peaks with respect to different LiTFSI content might be due to the inhomogeneous powder orientation or the growth of the samples in different orientation. This X-ray spectra without very broad humps for any amorphous background, confirm that these PEO-based complex are highly-crystalline. It should be pointed out that the diffraction peak at 2q around 22.3 belongs to urea, indicating that there is a small amount of free urea besides the complex. Chenite and Brisse showed that with the stoichiometry ratio of EO:urea ¼ 4:9 for a-PEO-urea, six urea molecules form the hexagonal channel structures that surround the other three urea molecules and PEO chains (Fig. 2) [23]. As in most urea inclusion complex, the guest PEO chains are packed inside hexagonal channels constructed from an essentially infinite network due to hydrogen bonding of the urea molecules [23]. The introduction of LiTFSI into the a-PEO-urea only slightly changes the cell lattice of the inclusion compound, but indeed it changes the melting point (See Fig. S2). The decreasing melting point with increasing LiTFSI should be ascribed to the plasticizing effect of LiTFSI. The bulky TFSI group of LiTFSI softens the polymer backbone and then weakens the interactive bonds within the polymer matrix (e.g. NH-O), which lowers down the melting point [26,27]. However, it is also possible that the inclusion of LiTFSI into the inclusion compounds has interrupted the channels so as to decrease the melting point. Adding the LiTFSI into the a-PEO-urea has enhanced the vibration peak of C]O for urea (Fig. S3) and has made the surface of a-PEOurea-LiTFSI smoother than that of a-PEO-urea in SEM spectra (Fig. S4). To probe the position of Liþ ion, we resort to solid state NMR with 2D CP (cross-polarization) correlation experiment. 2D 1H-7Li correlation spectrum of a-PEO3-urea-LiTFSI is shown in Fig. 3a. This spectrum showed that the Liþ correlates with both the NH2 group (~6 ppm) from urea and the CH2 group (~4 ppm) from the PEO
chain. This result suggested that the Liþ ion should reside in the hexagonal channel formed by six urea molecules, so that Liþ is close to both the urea and the PEO chain. Therefore this hexagonal channel structure could possibly facilitate the Liþ ion to move along the channel and lead to high conductivity. Moreover, in Fig. 3b, it could be seen that all the F signals from TFSI groups also correlate with both the NH2 group (~6 ppm) from urea and the CH2 group (~4 ppm), indicating that the TFSI groups also reside in the hexagonal channel (Fig. 2). The peak at 121.7 ppm could be ascribed to the CF3 group closed to the inner PEO chain and the inner urea ring, therefore this peak has the correlation with the PEO and the urea in Fig. 3b; the peak at 120 ppm could be ascribed to the CF3 group closed to the outer urea ring, therefore this peak mainly has the correlation with the urea in Fig. 3b. The correlation between the PEO and the CF3 has two peaks (top right peaks), which might be due to two different environments. The temperature-dependent conductivity for a-PEO-urea-LiTFSI ternary complexes is shown in Fig. 4. It could be observed that with the increase of the LiTFSI content, the conductivity increases. With EO:LiTFSI ¼ 3:1, the conductivity reaches ~6.03 105 S cm1 at 303 K. The ionic conductivity increases almost 2 orders of magnitude higher than the highest conductivity of the reported highlycrystalline polymer electrolyte (1.58 107 S cm1 for PEO6/ LiAsF6) [12]. Previous Raman spectroscopic studies reveal that urea breaks or weakens the bonding between the Liþ cations and TFSI anions in urea-LiTFSI system [28]. Therefore it is assumed here that in a-PEOurea-LiTFSI system, the urea also breaks or weakens the bonding between the Liþ cations and TFSI anions. Furthermore, the coordination of urea with the bulky TFSI group slightly decreases the channel size and immobilizes the bulky TFSI anions. This makes the small Liþ ion easier to move, leading to high conductivity. It should be pointed out that we could also employ LiAsF6 instead of LiTFSI, and the a-PEO3-urea-LiAsF6 ternary complex
Fig. 2. Ball and stick model of a-PEO-urea-LiTFSI crystal structure from Z axis. Red ball represents oxygen; blue ball represents nitrogen atom; green ball represents Liþ; big gray ball represents carbon atom; small gray ball represents hydrogen atom. Inner red framework is the PEO chain. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
X. Yan et al. / Polymer 99 (2016) 44e48
47
19F -2
ppm
1H
0
0
2
2
4
4
6
6
8
(a) 5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
(b) ppm
-110
-115
-120
-125
8
10 ppm
Fig. 3. 2D (1) 1H-7Li and (2) 1H-19F correlation spectra of a-PEO3-urea-LiTFSI under MAS rate vr ¼ 8 kHz. Cross-polarization contact time for 1H-7Li is 2 ms and that for 1H-19F is 1 ms.
changed the crystalline structure of the inclusion compound of aPEO-urea. Furthermore, the ionic conductivity of this PEO-ureaLiTFSI ternary complex has reached up to 6.03 105 S cm1 at 303 K. Meanwhile, the a-PEO3-urea-LiAsF6 ternary complex also reaches high conductivity of 3.39 105 S cm1 at 303 K. Although the urea might not be stable enough for practical use for the solid electrolytes [27], our investigation of this inclusion complex might offer alternative opportunities for improvement of the ionic conductivity of SPEs and motivate further research on the different kinds of highly-crystalline polymer electrolytes based on the inclusion complexes. Further work on the motion of PEO chain and Li ion together with the identification of the detail crystalline structure is in progress. Acknowledgements
Fig. 4. Temperature-dependent conductivity plots of PEO-urea-LiTFSI ternary complexes.
could reach high conductivity of 3.39 105 S cm1 at 303 K (See Fig. S5). Here we should mention that the urea-LiTFSI complex is a viscous liquid [28] with the conductivity of ~2.0 104 S cm1 at 303 K for urea3.3:LiTFSI [29]. At last the controlled experiments were done by (1) simple mixing of the a-PEO-urea complex with LiTFSI or (2) simple mixing of PEO, urea, and LiTFSI with the EO:TFSI ¼ 3:1. We found that the ionic conductivities for the two simple mixtures are ~6.31 106 S cm1 at 303 K, which is one order of magnitude lower than the a-PEO-urea-LiTFSI with 6.03 105 S cm1 at 303 K (See Fig. S6). The X-ray spectra demonstrate that the simple mixtures yield a large volume of urea and a small amount of PEO-ureaLiTFSI ternary complex (Fig. S7). 4. Conclusion A kind of highly-crystalline solid polymer electrolytes, based on the a-PEO-urea-LiTFSI ternary complex, is introduced here. The introduction of LiTFSI into this inclusion compound has not
Authors would like to acknowledge Large Instruments Open Foundation of East China Normal University, National Center for Magnetic Resonance in Wuhan (Wuhan Institute of Physics and Mathematics, CAS), National Natural Science Foundation of China (21373086), National Natural Science Foundation of China for Excellent Young Scholars (21522303), Basic Research Project of Shanghai Science and Technology Committee (No. 14JC1491000), National High Technology Research and Development Program of China (No. 2014AA123401) and National Key Basic Research Program of China (2013CB921800). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2016.06.056. References [1] M. Armand, J. Chabagno, M. Duclot, Extended abstracts, Second Int. Conf. Solid Electrolytes (1978) 20e22. [2] P.V. Wright, Br. Polym. J. 7 (5) (1975) 319e327. [3] K. Murata, S. Izuchi, Y. Yoshihisa, Electrochim. Acta 45 (8) (2000) 1501e1508. [4] B.B. Owens, J. Power Sources 90 (2000) 2e8. [5] Q. Li, R. He, J.O. Jensen, N.J. Bjerrum, Chem. Mater. 15 (26) (2003) 4896e4915. [6] W.H. Meyer, Adv. Mater. 10 (6) (1998) 439e448. [7] F.B. Dias, L. Plomp, J.B. Veldhuis, J. Power Sources 88 (2) (2000) 169e191.
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[8] M. Michael, M. Jacob, S. Prabaharan, S. Radhakrishna, Solid State Ion. 98 (3) (1997) 167e174. [9] S.T.C. Ng, M. Forsyth, D.R. MacFarlane, M. Garcia, M.E. Smith, J.H. Strange, Polymer 39 (25) (1998) 6261e6268. [10] J. Xi, X. Qiu, S. Zheng, X. Tang, Polymer 46 (15) (2005) 5702e5706. [11] A.M. Rocco, AdA. Carias, R.P. Pereira, Polymer 51 (22) (2010) 5151e5164. [12] Z. Stoeva, I. Martin-Litas, E. Staunton, Y. Andreev, P. Bruce, J. Am. Chem. Soc. 125 (15) (2003) 4619e4627. [13] C.-H. Zhang, S. Gamble, D. Ainsworth, A. Slawin, Y. Andreev, P. Bruce, Nat. Mater. 8 (2009) 580e584. [14] P. Lightfoot, M. Mehta, P. Bruce, Science 1993 (262) (1935) 883e885. [15] G. MacGlashan, Y. Andreev, P. Bruce, Nature 398 (1999) 192e194. [16] Z. Gadjourova, Y. Andreev, D. Tunstall, P. Bruce, Nature 412 (2001) 521e524. [17] Z. Gadjourova, D. Marero, K. Andersen, Y. Andreev, P. Bruce, Chem. Mater. 13 (4) (2001) 1282e1285. [18] D. Brandell, A. Liivat, A. Aabloo, J.O. Thomas, J. Mater. Chem. 15 (2005) 4338e4345.
[19] A. Liivat, D. Brandell, J.O. Thomas, J. Mater. Chem. 17 (2007) 3938e3946. [20] Q. Liu, B. Peng, M. Shen, B. Hu, Q. Chen, Solid State Ion. 255 (2014) 74e79. [21] L.-Y. Yang, D.-X. Wei, M. Xu, Y.-F. Yao, Q. Chen, Angew. Chem. Int. Ed. 53 (2014) 3631e3635. [22] Y. Liu, H. Antaya, C. Pellerin, J. Polym. Sci. Part B Polym. Phys. 46 (18) (2008) 1903e1913. [23] A. Chenite, F. Brisse, Macromolecules 24 (9) (1991) 2221e2225. [24] Y. Liu, C. Pellerin, Macromolecules 39 (26) (2006) 8886e8888. [25] Y. Liu, C. Pellerin, Polymer 50 (12) (2009) 2601e2607. [26] C.W. Liew, R. Durairaj, S. Ramesh, PLoS One 9 (7) (2014) e102815. [27] N.A. Stolwijk, C. Heddier, M. Reschke, M. Wiencierz, J. Bokeloh, G. Wilde, Macromolecules 46 (2013) 8580e8588. [28] H. Liang, H. Li, Z. Wang, F. Wu, L. Chen, X. Huang, J. Phys. Chem. B 105 (2001) 9966e9969. [29] R. Chen, F. Wu, H. Liang, L. Li, B. Xu, J. Electrochem. Soc. 152 (2005) A1979eA1984.
Contents lists available at ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Short communication
PEO-urea-LiTFSI ternary complex as solid polymer electrolytes Xiaojing Yan 1, Bo Peng 1, Bingwen Hu*, Qun Chen* State Key Laboratory of Precision Spectroscopy, Shanghai Key Laboratory of Magnetic Resonance, Institute of Functional Materials, School of Physics and Materials Science, East China Normal University, 3663 North Zhongshan Road, Shanghai 200062, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 March 2016 Received in revised form 14 June 2016 Accepted 23 June 2016 Available online 27 June 2016
A kind of highly-crystalline solid polymer electrolytes, based on the a-PEO-urea-LiTFSI ternary complex, is introduced here. The introduction of LiTFSI into a-PEO-urea inclusion compound has not changed the crystalline structure of the inclusion compound. Furthermore, the ionic conductivity of this a-PEO-ureaLiTFSI ternary complex has reached up to 6.03 105 S cm1 at 303 K, where the highest conductivity of the reported highly-crystalline polymer electrolyte is only 1.58 107 S cm1 for PEO6/LiAsF6. Our investigation might motivate further research on the different kinds of highly-crystalline polymer electrolytes based on the inclusion complexes. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Polymer electrolytes Inclusion complex PEO
1. Introduction Solid polymer electrolytes (SPEs) have attracted great research interests since 1970s, due to their potential application in all-solid rechargeable batteries [1e5]. In particular, PEO/LiX (here PEO is poly(ethylene oxide) [(CH2CH2O)n] and X refers to any anion) polymer electrolytes are regarded as promising materials for high energy density rechargeable batteries [6,7]. However, the poor conductivity of PEO-based SPEs at room temperature has restricted their applications. There are many efforts to improve the conductivity which focus on reducing the crystallinity and increasing the amount of amorphous state of the PEO-based complexes [8e11]. However, a series of highly-crystalline polymer electrolytes based on PEO of molecular weight of 1 103 g/mol and alkaline metal salts, which were considered to be insulators in the past, show high conductivity at room temperature [12,13]. The PEO6/ LiAsF6 system has highest conductivity (1.58 107 S cm1 at 303 K) among these highly-crystalline polymer electrolytes [12,13]. It is reported that lithium ions are located in the tunnels formed by pair of PEO chains along which they could migrate in those crystalline polymer electrolytes [12e17] and ion-conduction mechanisms was probed by different methods, such as molecular dynamics simulations and solid-state NMR [18e20]. The high ionic
* Corresponding authors. E-mail addresses: [email protected] (B. Hu), [email protected] (Q. Chen). 1 Equal contribution. http://dx.doi.org/10.1016/j.polymer.2016.06.056 0032-3861/© 2016 Elsevier Ltd. All rights reserved.
conductivity in this kind of material could be attributed to the directional motion of Liþ in the channels formed by the PEO chains. In our previous paper, we prepared a new type polymer electrolyte formed by PEO, a-cyclodextrin (aCD) and LiAsF6, in which the Liþ moves along the tunnel formed by a-cyclodextrin [21]. Furthermore, only one PEO chain threads along the tunnel in a complete amorphous state. This ternary complex aCD-PEO-LiAsF6 has higher conductivity than the PEO-LiAsF6, which has been ascribed to the directional motion of Liþ in the channels formed the a-cyclodextrin. The research on PEO-LiAsF6 and aCD-PEO-LiAsF6 motivated us to find new materials in which the Liþ could move along the channel in well-defined direction. Here we report a kind of highly-crystalline polymer electrolytes, a-PEO-urea-LiTFSI (here LiTFSI is lithium bis(trifluoromethane sulfonyl) imide) ternary complex, based on the inclusion complex a-PEO-urea. The ionic conductivity of this a-PEO-urea-LiTFSI ternary complex has reached up to 6.03 105 S cm1 at 303 K. It was also found out that the a-PEO-urea-LiAsF6 ternary complex also possesses high conductivity of 3.39 105 S cm1 at 303 K. The PEO-urea binary system has gained a lot of attention since a stable crystalline inclusion compound (IC) can be formed [22e24]. With a (EO)4:(urea)9 stoichiometry, six urea molecules form a hexagonal channel structure that includes the remaining three urea molecules and the PEO chain [22]. The introduction of LiTFSI into the channel of this PEO-urea compound only slightly changes the crystalline size, but does not change the crystalline structure of the inclusion compound. The Liþ ion could immigrate along the hexagonal channel and lead to high conductivity.
X. Yan et al. / Polymer 99 (2016) 44e48
2. Experimental 2.1. Sample pretreatment PEO with average molecular weights (Mw) of 100 000 g/mol and urea were purchased from Sigma-Aldrich and heated in vacuum drying oven at 70 C for 2 days before use. Acetonitrile (Jk chemical, 99.9% purity) and LiTFSI (Sigma-Aldrich, 99.95%) were used without further purification. 2.2. Preparation of a phase of PEO and urea complex The a phase of PEO and urea complex with the molar ratio of 4:9 (referred as a-PEO-urea) was prepared using a previously described co-crystallization method [24]. The mixture of PEO and urea was heated in an oven at 40 C. A white powdery sample was obtained after 5 days. Then the product was heated in the vacuum drying oven at room temperature for 1 month for further analysis. It should be mentioned here that quenching this a complex from its melting temperature and recrystallization will lead to a mixture of a complex and b complex [25]. 2.3. Preparation of PEO-urea-LiTFSI ternary complex PEO-urea-LiTFSI complex were prepared by dissolving LiTFSI of 0.1223 g (or 0.1957 g, 0.2447 g, 0.3262 g, 0.6525 g), PEO of 0.3 g, and urea of 0.9214 g in acetonitrile solution of 50 mL, and we can obtain the sample with EO:urea ¼ 4:9 and EO:LiTFSI ¼ 16:1, 10:1, 8:1, 6:1, 3:1, which will be referred as a-PEO16-urea-LiTFSI, a-PEO10-ureaLiTFSI, a-PEO8-urea-LiTFSI, a-PEO6-urea-LiTFSI and a-PEO3-ureaLiTFSI. The mixture was vigorously stirred overnight at room temperature. The mixture was then cast under the temperature of 40 C to evaporate the acetonitrile for one week to obtain the white powder. The powder was finally moved to a vacuum and dried at room temperature for 1 month for further analysis. It should be pointed out that when the EO:LiTFSI ratio reaches 2:1, the PEOurea-LiTFSI ternary complex becomes slightly liquefied, therefore the lowest EO:LiTFSI ratio is limited to 3:1.
45
thermometer inside to detect the temperature and the glass bottle was placed in an oil bath with a temperature controlling device to control the temperature of the coin cells. The equivalent electrical circuit used to fit the Nyquist plot could be found in Fig. S1a and one typical Nyquist plot is shown in Fig. S1b. The fitting result of R2 is the sample resistance, and then the ionic conductivity could be obtained by the equation s ¼ l=R2 S, where S is the pellet area and l is the pellet thickness. Wide angle X-ray diffractions of the disk-shaped samples with about 5 mm in diameter and 0.5 mm in thickness were recorded at ambient conditions on a BRUKER D8 Advance instrument using Ni filtered Cu Ka radiation. The supplied power was 3 kW. The scanning was carried out with 2q from 2 to 50 with a step of 0.02 , and the diffraction peak positions of the sample were calibrated from the pattern of aluminum. All SSNMR experiments were performed on a Bruker AVANCEIII spectrometer with a 1H frequency of 400 MHz and a 13C frequency of 100 MHz. Commercial Bruker triple-resonance 4 mm MAS probes and 4 mm zirconia rotors were used for all experiments. The 13 C chemical shifts were determined from the carbonyl carbon signal (diso ¼ 176.03 ppm) of glycine relative to tetramethylsilane (TMS). The spin rate of the 4 mm rotor was 8 kHz. The process of sealing the powder samples into the rotor was carried out in the glove box filled with N2 gas. All of the NMR experimental data were processed with Bruker Topspin 2.1 software. 3. Results and discussion Wide angle X-ray spectra in Fig. 1 are employed to probe the crystalline structure of the PEO-based complex. It’s obvious from Fig. 1aef that the peak positions only slightly shift to high angle with the increase of LiTFSI content, implying that a-PEO-ureaLiTFSI ternary complexes form the same crystalline structure as a-
2.4. Preparation of simple mixtures The mixture of PEO, urea and LiTFSI were prepared directly by simple mixing of PEO, urea and LiTFSI solid powders with the EO:Li ¼ 3:1. The mixture of a-PEO-urea and LiTFSI were prepared by simple mixing of a-PEO-urea and LiTFSI solid powders with the EO:Li ¼ 3:1. These two mixtures are not prepared by the solution casting method, but prepared by the mixture of different solid powders. 2.5. Characterization The powder sample was loaded into a polytetrafluoroethylene (PTFE) cylinder mold with the diameter of 16.24 mm and was pressed into a compact disc like pellet about 2.5 mm in thickness by two stainless steel rods with 50 bar pressure. The polymer electrolyte disks were sandwiched between 2 stainless steel electrodes and then sealed into a coin cell. All the sample handing was carried out in a N2-filled glove box. Impedance spectra were carried out with an Autolab PGSTAT 302N electrochemical workstation. An alternating voltage of 100 mV amplitude was employed and the data were collected over the frequency 0.1 Hz to 1 MHz. The dc conductivity at different temperatures was obtained from the complex impedance plots [2,3]. The coin cells were located in a glass bottle with a
Fig. 1. Wide angle X-ray diffraction patterns of a-PEO-urea (a) and PEO-urea-LiTFSI ternary complexes of (b) a-PEO16-urea-LiTFSI, (c) a-PEO10-urea-LiTFSI, (d) a-PEO8urea-LiTFSI, (e) a-PEO6-urea-LiTFSI, (f) a-PEO3-urea-LiTFSI.
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PEO-urea but with smaller unit cell lattice. The changes in relative intensity between peaks with respect to different LiTFSI content might be due to the inhomogeneous powder orientation or the growth of the samples in different orientation. This X-ray spectra without very broad humps for any amorphous background, confirm that these PEO-based complex are highly-crystalline. It should be pointed out that the diffraction peak at 2q around 22.3 belongs to urea, indicating that there is a small amount of free urea besides the complex. Chenite and Brisse showed that with the stoichiometry ratio of EO:urea ¼ 4:9 for a-PEO-urea, six urea molecules form the hexagonal channel structures that surround the other three urea molecules and PEO chains (Fig. 2) [23]. As in most urea inclusion complex, the guest PEO chains are packed inside hexagonal channels constructed from an essentially infinite network due to hydrogen bonding of the urea molecules [23]. The introduction of LiTFSI into the a-PEO-urea only slightly changes the cell lattice of the inclusion compound, but indeed it changes the melting point (See Fig. S2). The decreasing melting point with increasing LiTFSI should be ascribed to the plasticizing effect of LiTFSI. The bulky TFSI group of LiTFSI softens the polymer backbone and then weakens the interactive bonds within the polymer matrix (e.g. NH-O), which lowers down the melting point [26,27]. However, it is also possible that the inclusion of LiTFSI into the inclusion compounds has interrupted the channels so as to decrease the melting point. Adding the LiTFSI into the a-PEO-urea has enhanced the vibration peak of C]O for urea (Fig. S3) and has made the surface of a-PEOurea-LiTFSI smoother than that of a-PEO-urea in SEM spectra (Fig. S4). To probe the position of Liþ ion, we resort to solid state NMR with 2D CP (cross-polarization) correlation experiment. 2D 1H-7Li correlation spectrum of a-PEO3-urea-LiTFSI is shown in Fig. 3a. This spectrum showed that the Liþ correlates with both the NH2 group (~6 ppm) from urea and the CH2 group (~4 ppm) from the PEO
chain. This result suggested that the Liþ ion should reside in the hexagonal channel formed by six urea molecules, so that Liþ is close to both the urea and the PEO chain. Therefore this hexagonal channel structure could possibly facilitate the Liþ ion to move along the channel and lead to high conductivity. Moreover, in Fig. 3b, it could be seen that all the F signals from TFSI groups also correlate with both the NH2 group (~6 ppm) from urea and the CH2 group (~4 ppm), indicating that the TFSI groups also reside in the hexagonal channel (Fig. 2). The peak at 121.7 ppm could be ascribed to the CF3 group closed to the inner PEO chain and the inner urea ring, therefore this peak has the correlation with the PEO and the urea in Fig. 3b; the peak at 120 ppm could be ascribed to the CF3 group closed to the outer urea ring, therefore this peak mainly has the correlation with the urea in Fig. 3b. The correlation between the PEO and the CF3 has two peaks (top right peaks), which might be due to two different environments. The temperature-dependent conductivity for a-PEO-urea-LiTFSI ternary complexes is shown in Fig. 4. It could be observed that with the increase of the LiTFSI content, the conductivity increases. With EO:LiTFSI ¼ 3:1, the conductivity reaches ~6.03 105 S cm1 at 303 K. The ionic conductivity increases almost 2 orders of magnitude higher than the highest conductivity of the reported highlycrystalline polymer electrolyte (1.58 107 S cm1 for PEO6/ LiAsF6) [12]. Previous Raman spectroscopic studies reveal that urea breaks or weakens the bonding between the Liþ cations and TFSI anions in urea-LiTFSI system [28]. Therefore it is assumed here that in a-PEOurea-LiTFSI system, the urea also breaks or weakens the bonding between the Liþ cations and TFSI anions. Furthermore, the coordination of urea with the bulky TFSI group slightly decreases the channel size and immobilizes the bulky TFSI anions. This makes the small Liþ ion easier to move, leading to high conductivity. It should be pointed out that we could also employ LiAsF6 instead of LiTFSI, and the a-PEO3-urea-LiAsF6 ternary complex
Fig. 2. Ball and stick model of a-PEO-urea-LiTFSI crystal structure from Z axis. Red ball represents oxygen; blue ball represents nitrogen atom; green ball represents Liþ; big gray ball represents carbon atom; small gray ball represents hydrogen atom. Inner red framework is the PEO chain. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
X. Yan et al. / Polymer 99 (2016) 44e48
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19F -2
ppm
1H
0
0
2
2
4
4
6
6
8
(a) 5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
(b) ppm
-110
-115
-120
-125
8
10 ppm
Fig. 3. 2D (1) 1H-7Li and (2) 1H-19F correlation spectra of a-PEO3-urea-LiTFSI under MAS rate vr ¼ 8 kHz. Cross-polarization contact time for 1H-7Li is 2 ms and that for 1H-19F is 1 ms.
changed the crystalline structure of the inclusion compound of aPEO-urea. Furthermore, the ionic conductivity of this PEO-ureaLiTFSI ternary complex has reached up to 6.03 105 S cm1 at 303 K. Meanwhile, the a-PEO3-urea-LiAsF6 ternary complex also reaches high conductivity of 3.39 105 S cm1 at 303 K. Although the urea might not be stable enough for practical use for the solid electrolytes [27], our investigation of this inclusion complex might offer alternative opportunities for improvement of the ionic conductivity of SPEs and motivate further research on the different kinds of highly-crystalline polymer electrolytes based on the inclusion complexes. Further work on the motion of PEO chain and Li ion together with the identification of the detail crystalline structure is in progress. Acknowledgements
Fig. 4. Temperature-dependent conductivity plots of PEO-urea-LiTFSI ternary complexes.
could reach high conductivity of 3.39 105 S cm1 at 303 K (See Fig. S5). Here we should mention that the urea-LiTFSI complex is a viscous liquid [28] with the conductivity of ~2.0 104 S cm1 at 303 K for urea3.3:LiTFSI [29]. At last the controlled experiments were done by (1) simple mixing of the a-PEO-urea complex with LiTFSI or (2) simple mixing of PEO, urea, and LiTFSI with the EO:TFSI ¼ 3:1. We found that the ionic conductivities for the two simple mixtures are ~6.31 106 S cm1 at 303 K, which is one order of magnitude lower than the a-PEO-urea-LiTFSI with 6.03 105 S cm1 at 303 K (See Fig. S6). The X-ray spectra demonstrate that the simple mixtures yield a large volume of urea and a small amount of PEO-ureaLiTFSI ternary complex (Fig. S7). 4. Conclusion A kind of highly-crystalline solid polymer electrolytes, based on the a-PEO-urea-LiTFSI ternary complex, is introduced here. The introduction of LiTFSI into this inclusion compound has not
Authors would like to acknowledge Large Instruments Open Foundation of East China Normal University, National Center for Magnetic Resonance in Wuhan (Wuhan Institute of Physics and Mathematics, CAS), National Natural Science Foundation of China (21373086), National Natural Science Foundation of China for Excellent Young Scholars (21522303), Basic Research Project of Shanghai Science and Technology Committee (No. 14JC1491000), National High Technology Research and Development Program of China (No. 2014AA123401) and National Key Basic Research Program of China (2013CB921800). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2016.06.056. References [1] M. Armand, J. Chabagno, M. Duclot, Extended abstracts, Second Int. Conf. Solid Electrolytes (1978) 20e22. [2] P.V. Wright, Br. Polym. J. 7 (5) (1975) 319e327. [3] K. Murata, S. Izuchi, Y. Yoshihisa, Electrochim. Acta 45 (8) (2000) 1501e1508. [4] B.B. Owens, J. Power Sources 90 (2000) 2e8. [5] Q. Li, R. He, J.O. Jensen, N.J. Bjerrum, Chem. Mater. 15 (26) (2003) 4896e4915. [6] W.H. Meyer, Adv. Mater. 10 (6) (1998) 439e448. [7] F.B. Dias, L. Plomp, J.B. Veldhuis, J. Power Sources 88 (2) (2000) 169e191.
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