DOI: 10.1002/chem.201406380
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Ionic Conductivity of b-Cyclodextrin–Polyethylene-Oxide/AlkaliMetal-Salt Complex Ling-Yun Yang,[a] Xiao-Bin Fu,[a] Tai-Qiang Chen,[b] Li-Kun Pan,[b] Peng Ji,[c] Ye-Feng Yao,*[a] and Qun Chen[a] Abstract: Highly conductive, crystalline, polymer electrolytes, b-cyclodextrin (b-CD)–polyethylene oxide (PEO)/ LiAsF6 and b-CD–PEO/NaAsF6, were prepared through supramolecular self-assembly of PEO, b-CD, and LiAsF6/ NaAsF6. The assembled b-CDs form nanochannels in which the PEO/X + (X = Li, Na) complexes are confined. The nanochannels provide a pathway for directional motion of the alkali metal ions and, at the same time, separate the cations and the anions by size exclusion.
High ionic conductivity is of relevance in the practical applications of solid polymer electrolytes (SPEs) formed by dissolving alkali metal salts in solid, coordinating polymers, such as polyethylene oxide (PEO).[1–5] Efforts to improve the ionic conductivity of SPEs have been conducted during the past decades.[6–10] In the literature, the research interests are mainly focused on amorphous SPEs,[11–13] where the Brownian motions of polymer segments are considered to be the driving force for the transport of alkali metal ions. We now know that the conductivity of crystalline SPEs, such as the PEO/Li + complex crystals, might even exceed that of the amorphous counterparts.[14, 15] Unlike for amorphous SPEs, the transport of alkali metal ions in crystalline SPEs is considered to be initiated by the well-defined segmental motions in the crystal lattice.[16–18] The high conductivity can be attributed to the well-separated cations and anions and the directional motion of the cations in the crystal lattices.
Recently, a new type of crystalline polymer electrolyte, prepared by the supramolecular self-assembly of PEO, a-CD, and LiAsF6, was reported to have a high conductivity at room temperature.[19] The high ionic conductivity of the material was attributed to the nanochannel structure formed by self-assembly of a-CD and PEO. In this work, we report two, new, highly conductive, crystalline, polymer electrolytes: the b-CD-PEO/Li + complex and the b-CD–PEO/Na + complex. In these two materials, b-CDs are used instead of a-CDs to form nanochannels with a larger cavity diameter. Na + ions are used instead of Li + ions to demonstrate the potential of this type of complex structure on the design of sodium polyelectrolytes. Figure 1 a illustrates the synthesis and the structural model of the CD–PEO/X + (X = Li, Na) complex electrolytes. Only PEO and b-CD together cannot assemble into the inclusion complex because the cavity size of b-CD is too big to contain a single PEO chain, but too small to contain two PEO chains.[20, 21] This situation can be changed by addition of Li + /Na + ions into the b-CD–PEO self-assembled system. The assembled structure of PEO and b-CD in the complex samples is confirmed by solid state NMR, wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). Figure 1 b shows the 13C NMR cross-polarization/magic angle spinning (CP/MAS) spectra of b-CD–PEO/Li + and b-CD–PEO/Na + . For a comparison, Figure 1 b also shows the 13C CP/MAS spectrum of b-CD. Compared with b-CD, the signal splitting in the spec-
[a] Dr. L.-Y. Yang, Dr. X.-B. Fu, Prof. Y.-F. Yao, Prof. Q. Chen Physics Department and Shanghai Key Laboratory of Magnetic Resonance East China Normal University North Zhongshan Road 3663, 200062 Shanghai (P. R. China) E-mail: [email protected] [b] Dr. T.-Q. Chen, Prof. L.-K. Pan Engineering Research Center for Nanophotonics and Advanced Instrument East China Normal University North Zhongshan Road 3663, 200062 Shanghai (P.R. China) [c] Dr. P. Ji Shanghai Key Laboratory of Green Chemistry and Chemical Processes Department of Chemistry East China Normal University North Zhongshan Road 3663, 200062 Shanghai (P.R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201406380. Chem. Eur. J. 2015, 21, 6346 – 6349
Figure 1. a) Synthesis and structural model of the CD–PEO/X + (X = Li, Na) complexes. b) 13C CP/MAS spectra of b-CD, b-CD–PEO/Li + , and b-CD–PEO/ Na + . c) DSC curves of PEO, b-CD–PEO/Li + , and b-CD–PEO/Na + . d) WXRD pattern of b-CD, b-CD–PEO/Li + , b-CD–PEO/Na + , and neat PEO. The complex samples have the same feed ratio (b-CD–EO:X + , X = Li, Na) of 1.2:6:1.
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Communication tra of the complex samples almost disappears, indicating assimilation of the conformations of the glucose units of b-CD in the complex samples. According to Harada[22] and our previous work,[23] the assimilated conformations of the glucose units are indicative of assembled structures of PEO and b-CD. Furthermore, the PEO chains that are threaded into the b-CDs are considered to be in a complete amorphous state. This is confirmed by the DSC curves in Figure 1 c and the WAXD patterns in Figure 1 d, where the melting peaks and the diffraction peaks of the PEO crystal completely disappear. By analogy with traditional ceramic electrolytes, such as NabAl2O3[24, 25] and RbAg4I5,[26, 27] the structures of the b-CD– PEO/Li + and b-CD–PEO/Na + complex crystals are quite intriguing. The assembled structure of PEO and b-CD forms the frame structure, where the aligned b-CDs form the nanochannels providing directional vacancies/interstitials for the transport of the alkali metal ions. The alkali metal ions in the samples (i.e., Li + and Na + ) may coordinate with oxygen atoms from both bCD and PEO, resulting in different states and dynamics.[19] Figure 2 a shows the 1H–7Li CP/MAS spectrum of b-CD–[D]PEO/Li + . To facilitate the signal assignment, this complex sample was synthesized with fully deuterated PEO ([D]PEO). Only one peak (¢0.71 ppm) is observed in the spectrum. Attributed to the 1 H–7Li cross-polarization process, this signal can be assigned to the 7Li ions located in the 1H-enriched environment. For b-CD– [D]PEO/Li + , the 1H-enriched environment only lies on the assembled b-CDs. We thus assign the peak at ¢0.71 ppm to the Li + ions close to b-CDs (Li + -A). Figure 2 b is the single-pulse excitation spectrum of b-CD–[D]PEO/Li + . A second peak at ¢1.17 ppm is clearly observed. According to the chemical shift,[19] this peak is assigned to the Li + ions close to PEO (Li + -B). Signal decomposition in Figure 2 b yields the ratio of Li + -A:Li + -B = 70:30. Similarly, we analyzed the 23Na NMR spectra in Figure 2 c and 2 d and tentatively assigned the peak at ¢16.6 ppm to the Na + ions close to b-CD (Na + -A), and the peak at ¢24.2 ppm to the Na + ions close to PEO (Na + -B). Signal decomposition yields the ratio of Na + -A:Na + -B = 75:25. Figure 3 a shows the electronic impedance spectra (EIS) of bCD–PEO/Li + and b-CD–PEO/Na + at 313 K. A semicircle arc connected with a skew line is observed in the spectra, indicating
Figure 2. 7Li NMR of b-CD–[D]PEO/Li + : a) 1H–7Li CP/MAS spectrum; b) 7Li single-pulse excitation spectrum. 23Na NMR of b-CD–[D]PEO/Na + : c) 1H–23Na CP/MAS spectrum; d) 23Na single-pulse excitation spectrum. The experimental temperature was 235 K. High-power 1H decoupling was applied during signal acquisition. % refers to the integral percentage. Chem. Eur. J. 2015, 21, 6346 – 6349
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Figure 3. a) EIS of b-CD–PEO/Na + and b-CD–PEO/Li + at 313 K. b) Arrhenius plots of the conductivities of b-CD–PEO/Na + , b-CD–PEO/Li + , and a-CD–PEO/ Li + .
that the both complex samples are typical electrolytes. Analysis of the EIS data shows that b-CD–PEO/Na + has a conductivity of 5.5 Õ 10¢7 S cm¢1 at 293 K, which is almost two orders of magnitude higher than that of b-CD–PEO/Li + (8.3 Õ 10¢9 S cm¢1) at the same temperature. The difference in conductivities of b-CD–PEO/Na + and b-CD–PEO/Li + indicates that the type of alkali metal ion may have a profound effect on the conductivity of the materials; this is in line with the observation reported for the PEO/LiAsF6 and PEO/NaAsF6 complex crystals by Bruce et al.[28] We also compared the conductivities of b-CD–PEO/Na + and b-CD–PEO/Li + with a-CD–PEO/Li + .[19] Comparison shows that, in the temperature window of 293– 323 K, the conductivities of b-CD–PEO/Na + and b-CD–PEO/Li + are both much higher than that of a-CD–PEO/Li + , indicating that ion transport can be facilitated by the big cavity of b-CD. Figure 3 b shows the temperature-dependent conductivities of b-CD–PEO/Na + , b-CD–PEO/Li + , and a-CD–PEO/Li + . All of the samples exhibit clear linear Arrhenius plots with the activation energies of 58.2 kJ mol¢1 for b-CD–PEO/Na + , ¢1 + 123.2 kJ mol for b-CD–PEO/Li , and 75.1 kJ mol¢1 for a-CD– PEO/Li + . In the literature, similar linearity has been observed for the PEO/Li + crystalline polymer electrolytes.[16, 29, 30] The different activation energies of the complex samples are indicative of the different energetic barriers for ion transport. A detailed discussion is given below. The molecular dynamics in b-CD–PEO/Li + and b-CD–PEO/ Na + was probed by various solid-state NMR techniques. Firstly, 7 Li–7Li 2D exchange NMR was performed on b-CD–PEO/Li + to study the dynamics of Li + ions. Interestingly, no clear exchange process was observed between Li + -A and Li + -B (for the 7Li–7Li 2D exchange spectrum, see Figure S3 in the Supporting Information). This indicates the Li + -A ions are spatially separated from the Li + -B ions; this is different from the observation for a-CD–PEO/Li + , where the Li + ions close to the aCDs are exchangeable with the Li + ions close to the PEO.[19] For b-CD–PEO/Na + , 23Na–23Na 2D exchange NMR spectroscopy was run to probe the dynamics of the Na + ions. But this experiment failed because of the very short T1 relaxation time of 23 Na + ions (T1 = 5 ms, see Figure S4 in the Supporting Information). Instead, we performed temperature-dependent 23Na single-pulse excitation experiments on the sample (see Figure S5 in the Supporting Information). The temperature-dependent 23Na spectra show that the 23Na signals (i.e., Na + -A and
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Communication Na + -B), which are well resolved at low temperatures (e.g., 235 K), merge together into one signal at room temperature. This temperature-dependent change in line shapes indicates the presence of the exchange process between Na + -A and Na + -B. In consideration of the coordination between Li + /Na + ions and ether oxygen atoms of PEO segments, the segmental dynamics of PEO can thus reflect the dynamics of the coordinated Li + /Na + ions. To monitor the segmental dynamics of PEO, temperature-dependent 2H NMR was applied on the deuterated complex samples, b-CD–[D]PEO/Li + and b-CD–[D]PEO/Na + . Figure 4 shows the temperature-dependent 2H NMR spectra of
Figure 4. Left: Motion models for the trans-trans-gauche (ttg) sequences of PEO and the corresponding simulated 2H NMR spectra. Right: Experimental 2 H NMR spectra of: a) a-CD–[D]PEO/Li + ; b) b-CD–[D]PEO/Li + ; c) b-CD– [D]PEO/Na + , acquired at different temperatures.
b-CD–[D]PEO/Li + , b-CD–[D]PEO/Na + and a-CD–[D]PEO/Li + . The 2H NMR spectra of the complex samples are quite similar: In the low-temperature range, the typical Pake line shape with a quadrupolar splitting of 114 kHz is observed in the spectra of all of the samples. With increasing temperature, a singlet appears in the middle of the Pake pattern. The singlet becomes more and more dominant in the spectrum at higher temperatures. Following Beckham[31] and Tonelli,[32] the Pake patterns with a quadrupolar splitting of 114 kHz indicate that the PEO chains in the nanochannels adopt the ttg conformational sequence. With increasing temperature, the segments in the ttg conformational sequence perform the discrete four-site jump (see the motion model in Figure 4). The appearance of a singlet can be attributed to the modulation of the discrete four-site jump present in the ttg conformational sequences of the PEO chain. The coexistence of the singlet and the Pake pattern in the spectra indicates that while the jump motions occur, the other parts of the PEO chains in the nanochannel still remain static. A similar tendency of the line-shape change in the temperature-dependent 2H NMR spectra in Figure 4 indicates that the PEO chain segments in the three complexes have quite similar dynamic environments. The differences in segmental dynamics of the PEO chains of the three complex samples are confirmed by the different Chem. Eur. J. 2015, 21, 6346 – 6349
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starting temperatures of the four-site jump motion in the samples. As marked by the squares in Figure 4, the starting temperatures of the four-site jump motion are 290 K for a-CD– [D]PEO/Li + , 275 K for b-CD–[D]PEO/Li + and 268 K for b-CD– [D]PEO/Na + . a-CD–[D]PEO/Li + and b-CD–[D]PEO/Li + are formed from the same polymer and salt. The difference in the starting temperatures can thus be attributed to the different sizes of the nanochannels, likely originating from the different cavity sizes of a-CD and b-CD. b-CD–[D]PEO/Li + and b-CD– [D]PEO/Na + are formed from the same polymer and cyclodextrin. The difference in the starting temperatures can be attributed to the different salts, or strictly speaking, the different cations. The different starting temperatures of the four-site jump motion in b-CD–[D]PEO/Li + and b-CD–[D]PEO/Na + can thus be considered as a good reflection of the influence of the alkali metal ions on the dynamics of the PEO chain segments in the samples. In search for the origin of the different activation energies of the three complex samples shown in Figure 3 b, we realized that for b-CD–PEO/Li + , which has the highest activation energy in the three complex samples, no exchange process occurs between the Li + ions close to b-CD and the Li + ions close to PEO, whereas for a-CD–PEO/Li + and b-CD–PEO/Na + , which both have much lower activation energies than b-CD– PEO/Li + , a clear exchange process is observed between the Li + /Na + ions close to CDs and the Li + /Na + ions close to PEO. We thus propose that the exchange process between the “inner” ions (close to PEO) and the ions attached on the nanochannels (a/b-CDs) facilitates the ion transport by opening up the bottlenecks between the two neighboring coordination sites of the PEO/Li + (PEO/Na + ) complex structure inside the nanochannel,[16, 17] thus lowering the activation energy for conduction. We are currently endeavoring to investigate this issue. In summary, in this work we report two highly conductive SPEs, b-CD–PEO/Li + and b-CD–PEO/Na + . Structure analysis shows that these SPEs consist of nanochannels formed by the aligned b-CDs in which the PEO/X + (X = Li, Na) complexes are confined. Dynamic 2H NMR shows that the segmental motion of the PEO chains in the nanochannels is close to the discrete four-site jump motion. A positive correlation between the macroscopic conductivity and the segmental mobility of PEO chains in the nanochannels was found in the samples. Our results also show that the mobility of the PEO segments can be improved by increasing the cavity size of the nanochannel, or replacing Li + ions with Na + ions. This provides directions to further improve the conductivity of SPE.
Experimental Section Materials: The protonated PEO (Mw = 2700) and b-CD were purchased from Sigma–Aldrich, China. The deuterated PEO (Mn = 2400) was purchased from Polymer Source, Canada. Lithium hexafluoroarsenate (LiAsF6) and sodium hexafluoroarsenate (NaAsF6) were purchased from Alfa Aesar, China. Sample preparation: The mixture of PEO and LiAsF6 or NaAsF6 (with a specific feed ratio) was dissolved in an aqueous solution and added to the saturated solution of CD. After mixing, the solu-
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Communication tion was kept in an oven at 303 K for crystallization. After 2 weeks, the samples were dried in vacuum for a week at 313 K before use in other experiments. In the main text, all of the studied complex samples have the same feed ratio (b-CD–EO:Li + /Na + ) of 1.2:6:1. In this feed ratio, the complex samples have a dominant nanochannel structure and no crystal phases of b-CD or PEO (see Figure S1 and S2 in the Supporting Information). Solid-state NMR measurements: All 13C, 7Li and 23Na solid-state NMR experiments were performed on a Bruker AVANCE III 600 WB spectrometer operating at 150.91, 158.83 and 233.23 MHz for 13C, 23 Na and 7Li, respectively. A 4 mm double resonance MAS probe was used for the experiments. The spin rate was set to 8 kHz in all experiments. The 1H 908 pulse was 4 ms and the tppm15 decoupling sequence was used during signal acquisition. For 1H–23Na cross-polarization, the cross-polarization time was 2 ms. For 1H– 7 Li NMR measurements, the cross-polarization time was 200 ms. The 7 Li, 23Na and 13C chemical shifts were calibrated by using a LiCl aqueous solution (1 mol L¢1, d = 0 ppm), a NaCl solution (1 mol L¢1, d = 0 ppm), or Adamantane (d = 38.5 ppm), respectively. 2
H NMR experiments were performed on Bruker AVANCE III 300 WB spectrometer operating at 46.08 MHz by using a homemade static probe. The solid echo was used to record the spectra. The 2H 908 pulse was 2.3 ms. The echo delays were varied between 20 and 30 ms. XRD and EIS: XRD measurements were performed on Bruker D8 ADVANCE with Cu Ka (1.5406 æ) radiation (40 kV, 40 mA). All samples were mounted on the same sample holder and scanned from 2q = 5–458 at a speed of 108 min¢1 at room temperature. The EIS measurements were performed with an electrochemical workstation (AUTOLAB PGSTAT302N). The Frequency range was 1 MHz to 0.1 Hz with the amplitude voltage of 100 mV. The polymer electrolyte disks were sandwiched between two stainless steel plates in a two-electrode cell, which was located within an argon-filled stainless steel chamber.
Acknowledgements Y.Y. acknowledges financial support from NSFC grant no. 21174039. L.-Y.Y. acknowledges financial support from Outstanding Doctoral Dissertation Cultivation Plan of Action No. PY2014008. Keywords: inclusion complexes · ion transport · molecular dynamics · polymer electrolytes · solid-state NMR [1] M. Armand, Adv. Mater. 1990, 2, 278 – 286. [2] D. E. Fenton, J. M. Parker, P. V. Wright, Polymer 1973, 14, 589. [3] J. Hassoun, F. Croce, M. Armand, B. Scrosati, Angew. Chem. Int. Ed. 2011, 50, 2999 – 3002; Angew. Chem. 2011, 123, 3055 – 3058.
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[4] W. A. Henderson, N. R. Brooks, V. G. Young, J. Am. Chem. Soc. 2003, 125, 12098 – 12099. [5] E. Castillo-Martnez, J. Carretero-Gonzlez, M. Armand, Angew. Chem. Int. Ed. 2014, 53, 5341 – 5345; Angew. Chem. 2014, 126, 5445 – 5449. [6] V. Aravindan, J. Gnanaraj, S. Madhavi, H. Liu, Chem. Eur. J. 2011, 17, 14326 – 14346. [7] I. McKenzie, M. Harada, R. F. Kiefl, C. D. P. Levy, W. A. MacFarlane, G. D. Morris, S. Ogata, M. R. Pearson, J. Sugiyama, J. Am. Chem. Soc. 2014, 136, 7833 – 7836. [8] J. F. M. Oudenhoven, L. Baggetto, P. H. L. Notten, Adv. Energy Mater. 2011, 1, 10 – 33. [9] M. Winter, J. O. Besenhard, M. E. Spahr, P. Novk, Adv. Mater. 1998, 10, 725 – 763. [10] Y. Yin, S. Xin, Y. Guo, L. Wan, Angew. Chem. Int. Ed. 2013, 52, 13186 – 13200; Angew. Chem. 2013, 125, 13426 – 13441. [11] M. Echeverri, C. Hamad, T. Kyu, Solid State Ionics 2014, 254, 92 – 100. [12] W. Wieczorek, A. Zalewska, D. Raducha, Z. Florjan´czyk, J. R. Stevens, A. Ferry, P. Jacobsson, Macromolecules 1996, 29, 143 – 155. [13] L. Zhang, B. L. Chaloux, T. Saito, M. A. Hickner, J. L. Lutkenhaus, Macromolecules 2011, 44, 9723 – 9730. [14] Z. Gadjourova, Y. G. Andreev, D. P. Tunstall, P. G. Bruce, Nature 2001, 412, 520 – 523. [15] G. S. MacGlashan, Y. G. Andreev, P. G. Bruce, Nature 1999, 398, 792 – 794. [16] E. Staunton, Y. G. Andreev, P. G. Bruce, J. Am. Chem. Soc. 2005, 127, 12176 – 12177. [17] Z. Stoeva, I. Martin-Litas, E. Staunton, Y. G. Andreev, P. G. Bruce, J. Am. Chem. Soc. 2003, 125, 4619 – 4626. [18] C. Zhang, Y. G. Andreev, P. G. Bruce, Angew. Chem. Int. Ed. 2007, 46, 2848 – 2850; Angew. Chem. 2007, 119, 2906 – 2908. [19] L. Yang, D. Wei, M. Xu, Y. Yao, Q. Chen, Angew. Chem. Int. Ed. 2014, 53, 3631 – 3635; Angew. Chem. 2014, 126, 3705 – 3709. [20] A. Harada, J. Li, M. Kamachi, Nature 1992, 356, 325 – 327. [21] A. Harada, M. Okada, J. Li, M. Kamachi, Macromolecules 1995, 28, 8406 – 8411. [22] H. Okumura, Y. Kawaguchi, A. Harada, Macromolecules 2001, 34, 6338 – 6343. [23] L. Chen, X. Zhu, D. Yan, Y. Chen, Q. Chen, Y. Yao, Angew. Chem. Int. Ed. 2006, 45, 87 – 90; Angew. Chem. 2006, 118, 93 – 96. [24] G. Adachi, N. Imanaka, H. Aono, Adv. Mater. 1996, 8, 127 – 135. [25] C. Kun Kuo, A. Tan, P. Sarkar, P. S. Nicholson, Solid State Ionics 1990, 37, 303 – 306. [26] Y. Inaguma, C. Liquan, M. Itoh, T. Nakamura, T. Uchida, H. Ikuta, M. Wakihara, Solid State Commun. 1993, 86, 689 – 693. [27] D. Kurzeja, M. Nold, M. Ebert, G. Staikov, W. J. Lorenz, A. Froese, R. Speck, W. Wiesbeck, M. W. Breiter, Solid State Ionics 1993, 66, 113 – 129. [28] C. Zhang, S. Gamble, D. Ainsworth, A. M. Z. Slawin, Y. G. Andreev, P. G. Bruce, Nat. mater. 2009, 8, 580 – 584. [29] C. Zhang, E. Staunton, Y. G. Andreev, P. G. Bruce, J. Am. Chem. Soc. 2005, 127, 18305 – 18308. [30] D. Devaux, R. Bouchet, D. Gl¦, R. Denoyel, Solid State Ionics 2012, 227, 119 – 127. [31] T. E. Girardeau, J. Leisen, H. W. Beckham, Macromol. Chem. Phys. 2005, 206, 998 – 1005. [32] J. Lu, P. A. Mirau, A. E. Tonelli, Prog. Polym. Sci. 2002, 27, 357 – 401. Received: December 7, 2014 Published online on March 10, 2015
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Ionic Conductivity of b-Cyclodextrin–Polyethylene-Oxide/AlkaliMetal-Salt Complex Ling-Yun Yang,[a] Xiao-Bin Fu,[a] Tai-Qiang Chen,[b] Li-Kun Pan,[b] Peng Ji,[c] Ye-Feng Yao,*[a] and Qun Chen[a] Abstract: Highly conductive, crystalline, polymer electrolytes, b-cyclodextrin (b-CD)–polyethylene oxide (PEO)/ LiAsF6 and b-CD–PEO/NaAsF6, were prepared through supramolecular self-assembly of PEO, b-CD, and LiAsF6/ NaAsF6. The assembled b-CDs form nanochannels in which the PEO/X + (X = Li, Na) complexes are confined. The nanochannels provide a pathway for directional motion of the alkali metal ions and, at the same time, separate the cations and the anions by size exclusion.
High ionic conductivity is of relevance in the practical applications of solid polymer electrolytes (SPEs) formed by dissolving alkali metal salts in solid, coordinating polymers, such as polyethylene oxide (PEO).[1–5] Efforts to improve the ionic conductivity of SPEs have been conducted during the past decades.[6–10] In the literature, the research interests are mainly focused on amorphous SPEs,[11–13] where the Brownian motions of polymer segments are considered to be the driving force for the transport of alkali metal ions. We now know that the conductivity of crystalline SPEs, such as the PEO/Li + complex crystals, might even exceed that of the amorphous counterparts.[14, 15] Unlike for amorphous SPEs, the transport of alkali metal ions in crystalline SPEs is considered to be initiated by the well-defined segmental motions in the crystal lattice.[16–18] The high conductivity can be attributed to the well-separated cations and anions and the directional motion of the cations in the crystal lattices.
Recently, a new type of crystalline polymer electrolyte, prepared by the supramolecular self-assembly of PEO, a-CD, and LiAsF6, was reported to have a high conductivity at room temperature.[19] The high ionic conductivity of the material was attributed to the nanochannel structure formed by self-assembly of a-CD and PEO. In this work, we report two, new, highly conductive, crystalline, polymer electrolytes: the b-CD-PEO/Li + complex and the b-CD–PEO/Na + complex. In these two materials, b-CDs are used instead of a-CDs to form nanochannels with a larger cavity diameter. Na + ions are used instead of Li + ions to demonstrate the potential of this type of complex structure on the design of sodium polyelectrolytes. Figure 1 a illustrates the synthesis and the structural model of the CD–PEO/X + (X = Li, Na) complex electrolytes. Only PEO and b-CD together cannot assemble into the inclusion complex because the cavity size of b-CD is too big to contain a single PEO chain, but too small to contain two PEO chains.[20, 21] This situation can be changed by addition of Li + /Na + ions into the b-CD–PEO self-assembled system. The assembled structure of PEO and b-CD in the complex samples is confirmed by solid state NMR, wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). Figure 1 b shows the 13C NMR cross-polarization/magic angle spinning (CP/MAS) spectra of b-CD–PEO/Li + and b-CD–PEO/Na + . For a comparison, Figure 1 b also shows the 13C CP/MAS spectrum of b-CD. Compared with b-CD, the signal splitting in the spec-
[a] Dr. L.-Y. Yang, Dr. X.-B. Fu, Prof. Y.-F. Yao, Prof. Q. Chen Physics Department and Shanghai Key Laboratory of Magnetic Resonance East China Normal University North Zhongshan Road 3663, 200062 Shanghai (P. R. China) E-mail: [email protected] [b] Dr. T.-Q. Chen, Prof. L.-K. Pan Engineering Research Center for Nanophotonics and Advanced Instrument East China Normal University North Zhongshan Road 3663, 200062 Shanghai (P.R. China) [c] Dr. P. Ji Shanghai Key Laboratory of Green Chemistry and Chemical Processes Department of Chemistry East China Normal University North Zhongshan Road 3663, 200062 Shanghai (P.R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201406380. Chem. Eur. J. 2015, 21, 6346 – 6349
Figure 1. a) Synthesis and structural model of the CD–PEO/X + (X = Li, Na) complexes. b) 13C CP/MAS spectra of b-CD, b-CD–PEO/Li + , and b-CD–PEO/ Na + . c) DSC curves of PEO, b-CD–PEO/Li + , and b-CD–PEO/Na + . d) WXRD pattern of b-CD, b-CD–PEO/Li + , b-CD–PEO/Na + , and neat PEO. The complex samples have the same feed ratio (b-CD–EO:X + , X = Li, Na) of 1.2:6:1.
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Communication tra of the complex samples almost disappears, indicating assimilation of the conformations of the glucose units of b-CD in the complex samples. According to Harada[22] and our previous work,[23] the assimilated conformations of the glucose units are indicative of assembled structures of PEO and b-CD. Furthermore, the PEO chains that are threaded into the b-CDs are considered to be in a complete amorphous state. This is confirmed by the DSC curves in Figure 1 c and the WAXD patterns in Figure 1 d, where the melting peaks and the diffraction peaks of the PEO crystal completely disappear. By analogy with traditional ceramic electrolytes, such as NabAl2O3[24, 25] and RbAg4I5,[26, 27] the structures of the b-CD– PEO/Li + and b-CD–PEO/Na + complex crystals are quite intriguing. The assembled structure of PEO and b-CD forms the frame structure, where the aligned b-CDs form the nanochannels providing directional vacancies/interstitials for the transport of the alkali metal ions. The alkali metal ions in the samples (i.e., Li + and Na + ) may coordinate with oxygen atoms from both bCD and PEO, resulting in different states and dynamics.[19] Figure 2 a shows the 1H–7Li CP/MAS spectrum of b-CD–[D]PEO/Li + . To facilitate the signal assignment, this complex sample was synthesized with fully deuterated PEO ([D]PEO). Only one peak (¢0.71 ppm) is observed in the spectrum. Attributed to the 1 H–7Li cross-polarization process, this signal can be assigned to the 7Li ions located in the 1H-enriched environment. For b-CD– [D]PEO/Li + , the 1H-enriched environment only lies on the assembled b-CDs. We thus assign the peak at ¢0.71 ppm to the Li + ions close to b-CDs (Li + -A). Figure 2 b is the single-pulse excitation spectrum of b-CD–[D]PEO/Li + . A second peak at ¢1.17 ppm is clearly observed. According to the chemical shift,[19] this peak is assigned to the Li + ions close to PEO (Li + -B). Signal decomposition in Figure 2 b yields the ratio of Li + -A:Li + -B = 70:30. Similarly, we analyzed the 23Na NMR spectra in Figure 2 c and 2 d and tentatively assigned the peak at ¢16.6 ppm to the Na + ions close to b-CD (Na + -A), and the peak at ¢24.2 ppm to the Na + ions close to PEO (Na + -B). Signal decomposition yields the ratio of Na + -A:Na + -B = 75:25. Figure 3 a shows the electronic impedance spectra (EIS) of bCD–PEO/Li + and b-CD–PEO/Na + at 313 K. A semicircle arc connected with a skew line is observed in the spectra, indicating
Figure 2. 7Li NMR of b-CD–[D]PEO/Li + : a) 1H–7Li CP/MAS spectrum; b) 7Li single-pulse excitation spectrum. 23Na NMR of b-CD–[D]PEO/Na + : c) 1H–23Na CP/MAS spectrum; d) 23Na single-pulse excitation spectrum. The experimental temperature was 235 K. High-power 1H decoupling was applied during signal acquisition. % refers to the integral percentage. Chem. Eur. J. 2015, 21, 6346 – 6349
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Figure 3. a) EIS of b-CD–PEO/Na + and b-CD–PEO/Li + at 313 K. b) Arrhenius plots of the conductivities of b-CD–PEO/Na + , b-CD–PEO/Li + , and a-CD–PEO/ Li + .
that the both complex samples are typical electrolytes. Analysis of the EIS data shows that b-CD–PEO/Na + has a conductivity of 5.5 Õ 10¢7 S cm¢1 at 293 K, which is almost two orders of magnitude higher than that of b-CD–PEO/Li + (8.3 Õ 10¢9 S cm¢1) at the same temperature. The difference in conductivities of b-CD–PEO/Na + and b-CD–PEO/Li + indicates that the type of alkali metal ion may have a profound effect on the conductivity of the materials; this is in line with the observation reported for the PEO/LiAsF6 and PEO/NaAsF6 complex crystals by Bruce et al.[28] We also compared the conductivities of b-CD–PEO/Na + and b-CD–PEO/Li + with a-CD–PEO/Li + .[19] Comparison shows that, in the temperature window of 293– 323 K, the conductivities of b-CD–PEO/Na + and b-CD–PEO/Li + are both much higher than that of a-CD–PEO/Li + , indicating that ion transport can be facilitated by the big cavity of b-CD. Figure 3 b shows the temperature-dependent conductivities of b-CD–PEO/Na + , b-CD–PEO/Li + , and a-CD–PEO/Li + . All of the samples exhibit clear linear Arrhenius plots with the activation energies of 58.2 kJ mol¢1 for b-CD–PEO/Na + , ¢1 + 123.2 kJ mol for b-CD–PEO/Li , and 75.1 kJ mol¢1 for a-CD– PEO/Li + . In the literature, similar linearity has been observed for the PEO/Li + crystalline polymer electrolytes.[16, 29, 30] The different activation energies of the complex samples are indicative of the different energetic barriers for ion transport. A detailed discussion is given below. The molecular dynamics in b-CD–PEO/Li + and b-CD–PEO/ Na + was probed by various solid-state NMR techniques. Firstly, 7 Li–7Li 2D exchange NMR was performed on b-CD–PEO/Li + to study the dynamics of Li + ions. Interestingly, no clear exchange process was observed between Li + -A and Li + -B (for the 7Li–7Li 2D exchange spectrum, see Figure S3 in the Supporting Information). This indicates the Li + -A ions are spatially separated from the Li + -B ions; this is different from the observation for a-CD–PEO/Li + , where the Li + ions close to the aCDs are exchangeable with the Li + ions close to the PEO.[19] For b-CD–PEO/Na + , 23Na–23Na 2D exchange NMR spectroscopy was run to probe the dynamics of the Na + ions. But this experiment failed because of the very short T1 relaxation time of 23 Na + ions (T1 = 5 ms, see Figure S4 in the Supporting Information). Instead, we performed temperature-dependent 23Na single-pulse excitation experiments on the sample (see Figure S5 in the Supporting Information). The temperature-dependent 23Na spectra show that the 23Na signals (i.e., Na + -A and
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Communication Na + -B), which are well resolved at low temperatures (e.g., 235 K), merge together into one signal at room temperature. This temperature-dependent change in line shapes indicates the presence of the exchange process between Na + -A and Na + -B. In consideration of the coordination between Li + /Na + ions and ether oxygen atoms of PEO segments, the segmental dynamics of PEO can thus reflect the dynamics of the coordinated Li + /Na + ions. To monitor the segmental dynamics of PEO, temperature-dependent 2H NMR was applied on the deuterated complex samples, b-CD–[D]PEO/Li + and b-CD–[D]PEO/Na + . Figure 4 shows the temperature-dependent 2H NMR spectra of
Figure 4. Left: Motion models for the trans-trans-gauche (ttg) sequences of PEO and the corresponding simulated 2H NMR spectra. Right: Experimental 2 H NMR spectra of: a) a-CD–[D]PEO/Li + ; b) b-CD–[D]PEO/Li + ; c) b-CD– [D]PEO/Na + , acquired at different temperatures.
b-CD–[D]PEO/Li + , b-CD–[D]PEO/Na + and a-CD–[D]PEO/Li + . The 2H NMR spectra of the complex samples are quite similar: In the low-temperature range, the typical Pake line shape with a quadrupolar splitting of 114 kHz is observed in the spectra of all of the samples. With increasing temperature, a singlet appears in the middle of the Pake pattern. The singlet becomes more and more dominant in the spectrum at higher temperatures. Following Beckham[31] and Tonelli,[32] the Pake patterns with a quadrupolar splitting of 114 kHz indicate that the PEO chains in the nanochannels adopt the ttg conformational sequence. With increasing temperature, the segments in the ttg conformational sequence perform the discrete four-site jump (see the motion model in Figure 4). The appearance of a singlet can be attributed to the modulation of the discrete four-site jump present in the ttg conformational sequences of the PEO chain. The coexistence of the singlet and the Pake pattern in the spectra indicates that while the jump motions occur, the other parts of the PEO chains in the nanochannel still remain static. A similar tendency of the line-shape change in the temperature-dependent 2H NMR spectra in Figure 4 indicates that the PEO chain segments in the three complexes have quite similar dynamic environments. The differences in segmental dynamics of the PEO chains of the three complex samples are confirmed by the different Chem. Eur. J. 2015, 21, 6346 – 6349
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starting temperatures of the four-site jump motion in the samples. As marked by the squares in Figure 4, the starting temperatures of the four-site jump motion are 290 K for a-CD– [D]PEO/Li + , 275 K for b-CD–[D]PEO/Li + and 268 K for b-CD– [D]PEO/Na + . a-CD–[D]PEO/Li + and b-CD–[D]PEO/Li + are formed from the same polymer and salt. The difference in the starting temperatures can thus be attributed to the different sizes of the nanochannels, likely originating from the different cavity sizes of a-CD and b-CD. b-CD–[D]PEO/Li + and b-CD– [D]PEO/Na + are formed from the same polymer and cyclodextrin. The difference in the starting temperatures can be attributed to the different salts, or strictly speaking, the different cations. The different starting temperatures of the four-site jump motion in b-CD–[D]PEO/Li + and b-CD–[D]PEO/Na + can thus be considered as a good reflection of the influence of the alkali metal ions on the dynamics of the PEO chain segments in the samples. In search for the origin of the different activation energies of the three complex samples shown in Figure 3 b, we realized that for b-CD–PEO/Li + , which has the highest activation energy in the three complex samples, no exchange process occurs between the Li + ions close to b-CD and the Li + ions close to PEO, whereas for a-CD–PEO/Li + and b-CD–PEO/Na + , which both have much lower activation energies than b-CD– PEO/Li + , a clear exchange process is observed between the Li + /Na + ions close to CDs and the Li + /Na + ions close to PEO. We thus propose that the exchange process between the “inner” ions (close to PEO) and the ions attached on the nanochannels (a/b-CDs) facilitates the ion transport by opening up the bottlenecks between the two neighboring coordination sites of the PEO/Li + (PEO/Na + ) complex structure inside the nanochannel,[16, 17] thus lowering the activation energy for conduction. We are currently endeavoring to investigate this issue. In summary, in this work we report two highly conductive SPEs, b-CD–PEO/Li + and b-CD–PEO/Na + . Structure analysis shows that these SPEs consist of nanochannels formed by the aligned b-CDs in which the PEO/X + (X = Li, Na) complexes are confined. Dynamic 2H NMR shows that the segmental motion of the PEO chains in the nanochannels is close to the discrete four-site jump motion. A positive correlation between the macroscopic conductivity and the segmental mobility of PEO chains in the nanochannels was found in the samples. Our results also show that the mobility of the PEO segments can be improved by increasing the cavity size of the nanochannel, or replacing Li + ions with Na + ions. This provides directions to further improve the conductivity of SPE.
Experimental Section Materials: The protonated PEO (Mw = 2700) and b-CD were purchased from Sigma–Aldrich, China. The deuterated PEO (Mn = 2400) was purchased from Polymer Source, Canada. Lithium hexafluoroarsenate (LiAsF6) and sodium hexafluoroarsenate (NaAsF6) were purchased from Alfa Aesar, China. Sample preparation: The mixture of PEO and LiAsF6 or NaAsF6 (with a specific feed ratio) was dissolved in an aqueous solution and added to the saturated solution of CD. After mixing, the solu-
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Communication tion was kept in an oven at 303 K for crystallization. After 2 weeks, the samples were dried in vacuum for a week at 313 K before use in other experiments. In the main text, all of the studied complex samples have the same feed ratio (b-CD–EO:Li + /Na + ) of 1.2:6:1. In this feed ratio, the complex samples have a dominant nanochannel structure and no crystal phases of b-CD or PEO (see Figure S1 and S2 in the Supporting Information). Solid-state NMR measurements: All 13C, 7Li and 23Na solid-state NMR experiments were performed on a Bruker AVANCE III 600 WB spectrometer operating at 150.91, 158.83 and 233.23 MHz for 13C, 23 Na and 7Li, respectively. A 4 mm double resonance MAS probe was used for the experiments. The spin rate was set to 8 kHz in all experiments. The 1H 908 pulse was 4 ms and the tppm15 decoupling sequence was used during signal acquisition. For 1H–23Na cross-polarization, the cross-polarization time was 2 ms. For 1H– 7 Li NMR measurements, the cross-polarization time was 200 ms. The 7 Li, 23Na and 13C chemical shifts were calibrated by using a LiCl aqueous solution (1 mol L¢1, d = 0 ppm), a NaCl solution (1 mol L¢1, d = 0 ppm), or Adamantane (d = 38.5 ppm), respectively. 2
H NMR experiments were performed on Bruker AVANCE III 300 WB spectrometer operating at 46.08 MHz by using a homemade static probe. The solid echo was used to record the spectra. The 2H 908 pulse was 2.3 ms. The echo delays were varied between 20 and 30 ms. XRD and EIS: XRD measurements were performed on Bruker D8 ADVANCE with Cu Ka (1.5406 æ) radiation (40 kV, 40 mA). All samples were mounted on the same sample holder and scanned from 2q = 5–458 at a speed of 108 min¢1 at room temperature. The EIS measurements were performed with an electrochemical workstation (AUTOLAB PGSTAT302N). The Frequency range was 1 MHz to 0.1 Hz with the amplitude voltage of 100 mV. The polymer electrolyte disks were sandwiched between two stainless steel plates in a two-electrode cell, which was located within an argon-filled stainless steel chamber.
Acknowledgements Y.Y. acknowledges financial support from NSFC grant no. 21174039. L.-Y.Y. acknowledges financial support from Outstanding Doctoral Dissertation Cultivation Plan of Action No. PY2014008. Keywords: inclusion complexes · ion transport · molecular dynamics · polymer electrolytes · solid-state NMR [1] M. Armand, Adv. Mater. 1990, 2, 278 – 286. [2] D. E. Fenton, J. M. Parker, P. V. Wright, Polymer 1973, 14, 589. [3] J. Hassoun, F. Croce, M. Armand, B. Scrosati, Angew. Chem. Int. Ed. 2011, 50, 2999 – 3002; Angew. Chem. 2011, 123, 3055 – 3058.
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