Supramolecular Self-Assembly of Methylated Rotaxanes for Solid

Letter Cite This: ACS Macro Lett. 2018, 7, 881−885

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Supramolecular Self-Assembly of Methylated Rotaxanes for Solid Polymer Electrolyte Application Laura Imholt,† Dengpan Dong,§ Dmitry Bedrov,§ Isidora Cekic-Laskovic,†,‡ Martin Winter,*,†,‡ and Gunther Brunklaus*,†,‡ †

Helmholtz-Institute Münster, IEK-12, Forschungszentrum Jülich GmbH, Corrensstraße 46, 48149 Münster, Germany MEET Battery Research Center/Institute of Physical Chemistry, University of Münster, Corrensstraße 46, 48149 Münster, Germany § Department of Materials Science & Engineering, University of Utah, Salt Lake City, Utah 84112, United States Downloaded via DURHAM UNIV on July 5, 2018 at 16:01:35 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Li+-conducting solid polymer electrolytes (SPEs) obtained from supramolecular self-assembly of trimethylated cyclodextrin (TMCD), poly(ethylene oxide) (PEO), and lithium salt are investigated for application in lithium-metal batteries (LMBs) and lithium-ion batteries (LIBs). The considered electrolytes comprise nanochannels for fast lithium-ion transport formed by CD threaded on PEO chains. It is demonstrated that tailored modification of CD beneficially influences the structure and transport properties of solid polymer electrolytes, thereby enabling their application in LMBs. Molecular dynamics (MD) simulation and experimental data reveal that modification of CDs shifts the steady state between lithium ions inside and outside the channels, in this way improving the achievable ionic conductivity. Notably, the designed SPEs facilitated galvanostatic cycling in LMBs at fast charging and discharging rates for more than 200 cycles and high Coulombic efficiency.

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upramolecular self-assembled structures as building blocks of functional aggregates have had a large impact on many research fields in chemistry and attracted even more interest when concepts for self-assembly of polymers were introduced that exploit (noncovalent) intermolecular interactions to achieve three-dimensional control of complex architectures.1−4 In particular, self-assembly of polyrotaxanes (PRs) and polypseudorotaxanes (PPRs) where molecular rings (host molecule) are threaded onto a macromolecular chain (guest molecule) via noncovalent host−guest interactions in aqueous solution yields systems with great potential as stimulusresponsive materials, molecular machines, or switches in the fields of nanotechnology and drug delivery.5−10 Cyclodextrins (CDs) with lipophilic inner cavities and hydrophilic outer surfaces have been extensively studied as host molecules mainly due to their bioavailability, low cytotoxicity, as well as capability of readily forming inclusion complexes with a variety of guest molecules,11 including poly(ethylene oxide) (PEO) and its derivatives.7,12−15 The head−tail/tail−head sequence of CD shown in Figure 1a represents the ideal structure with lowest energy.16 The major driving force for successful selfassembly comprises hydrophobic and van der Waals interactions between the polymer and CDs as well as hydrogen bonds17 between adjacent CD molecules. Since CD contains many hydroxyl groups, it may be readily modified, and substitution of hydroxyl groups with non-hydrogen-bearing © XXXX American Chemical Society

Figure 1. (a) Simplified structure of formed inclusion complexes of CDs (green) as host and PEO (yellow) as guest molecule and the location of lithium ions and anions and (b) 13C CP/MAS spectra of γCD, γ-CD/PEO, and γ-TMCD/PEO.

functional groups controls the formation of inter- and intramolecular hydrogen bonds. NMR spectroscopy combined with quantum chemical computation is able to provide detailed information on Received: May 25, 2018 Accepted: July 2, 2018

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DOI: 10.1021/acsmacrolett.8b00406 ACS Macro Lett. 2018, 7, 881−885

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ACS Macro Letters supramolecular assemblies or host−guest interactions,18,19 including insight into (nano)channel formation.20,21 Complex formation of CD with PEO changes glucopyranosyl conformations of CD, as reflected by the disappearance of multiplet NMR signals in 13C cross-polarization/magic angle spinning (CP/MAS) spectra (Figure 1b). So far, merely a few reports of complex formation of modified CDs and polymers are known in the literature, though methylation is the easiest and most prominent modification.22−24 After methylation, disappearance of the signal splitting was observed, thus documenting successful complexation. In addition, signal distribution for 13C peaks of C1 and C4 associated with linkage of neighboring glucose units is found, which clearly shows that in the case of methylated CDs a distribution of linkage angles between adjacent glucose molecules exists. This likely reflects the absence of interglucose hydrogen bonds; also note that due to steric hindrance of methyl groups at O(2)and O(3)-sides, the glucose units at C1 and C4 are more inclined to reduce ring strain.25 In addition, the MD data are in agreement with an angular distribution for TMCD (Figure S13). The obtained distorted ring structure renders the system more flexible due to the absence of interglucose hydrogen bonds, affording characteristic mobility of the PRs where CD can slide and rotate on polymer chains.26,27 Indeed, partially or fully methylated CDs afforded complexes with PEO in channel-type structure22,23,28 where the hydrophobicity of CD is increased due to methylation and hydrophobic interactions rather than hydrogen bond formation are the main factor for complexation. Notably, 1H NMR analysis of the obtained complexes showed that the CD:EO ratio of 1:2.8 is identical for both hydroxylated and methylated systems. Solvent-free SPEs gained much attention as potential materials for electrochemical storage devices, including polymer-based LMBs and LIBs, due to their low flammability, excellent processability, as well as temperature stability and in the case of LMBs better compatibility with Li metal anodes than organic solvent based liquid electrolytes.29,30 Prominent examples are PEO-LiTFSI systems where lithium ions are coordinated to ether oxygens of the polymer backbone. Currently, there is a controversial debate in the literature whether major contributions of ionic conductivity can be attributed to the amorphous or crystalline parts of the polymer,31,32 though state-of-the-art SPEs exhibit insufficient lithium-ion conductivity and poor compatibility with electrodes upon constant current cycling, particularly at higher Crates (fast charge and discharge), rendering them unsuitable for room-temperature LMBs and LIBs. Notably, Yang et al.13 and Fu et al.15 attempted to prepare SPEs based on CD/PEO complex formation, where nanochannels should provide directional pathways for fast lithium-ion transport, whereas the present anions should be excluded from entering the channels based on their size (Figure 1a). Their data revealed that the achievable ionic conductivity of the resulting complexes is quite poor, hence impairing their application as SPEs in electrochemical storage devices. Since an effective lithium-ion transport is often related to the flexibility of coordinated polymer chains, modification of CDs (such as methylation) could possibly result in less rigid structures while maintaining the crucial channel-type assembly. Within this work we compare ion transport properties of nonmodified CD/PEO and methylated CD/PEO complexes and their applicability as SPEs in LMBs. Based on combined

computational and experimental efforts, we show that the Liion transport in the considered complexes is quite different from anticipated mechanisms,12,13 thereby revealing that major contributions to ionic conductivity originate from lithium ions moving outside the present nanochannels rather than inside along the polymer chain(s), thus limiting the impact of polymer segmental dynamics.33 Also, methylation of CDs leads to almost 2 orders of magnitude higher ionic conductivity and three times higher transport number compared to nonmodified complexes, thus highlighting rational design strategies for future SPEs with superior properties, aiding other transport mechanisms than the traditional.34,35 PEO-based SPEs have been intensively studied for application in electrochemical storage devices, despite that the low ionic conductivities at temperatures below 60 °C due to crystalline to amorphous phase transition hinder their application.36 Upon threading of CDs onto PEO chains, phase transitions of PEO are not observed over a broad temperature range (Figure S3), and PEO chains within the channels are in a fully amorphous state. Though we utilized low molecular weight PEO (2000 g/mol) for complex formation, our electrolytes are in a solid state over a broad temperature range. The inclusion complexes of CD and PEO were obtained as precipitated solid powders and subjected to X-ray diffraction analysis. The resulting XRD patterns for CD, γ-CD/PEOLiTFSI, γ-TMCD, and γ-TMCD/PEO-LiTFSI are shown in Figure S4 (Supporting Information). A sharp reflection at 2θ = 19.8° is characteristic for a channel-type PPR structure of γCD/PEO-LiTFSI,37,38 whereas γ-TMCD appears amorphous. Upon mixing of α-TMCD with PEO, sharp reflexes appear at similar 2θ values as for PPR with nonmethylated CD, evidencing that even amorphous-like methylated CDs yield channel-type structures with PEO. For effective Li-ion transport, it is highly preferable to have amorphous-like structures where the Li ions can be transported along the channels rather than in highly crystalline rigid structures. The ionic conductivity of CD/PEO complexes increases with the ring size of CDs (α-, β-, and γ-CD) and temperature, but TMCD/PEO complexes exhibit increased ionic conductivities compared to PEO complexes with nonmethylated CDs (Figure 2). An ionic conductivity of ≈10−4 S cm−1 was determined for optimized γ-TMCD/PEO5LiTFSI complexes (see SI) at 100 °C, 1.5 magnitude higher than for nonmethylated γ-CD/PEO complexes. The Arrhenius-type activation energy of ionic conductivity lies in the same range for all systems (≈80 kJ/mol) reflecting that Li-transport is based on similar microscopic mechanisms but appears more effective for methylated CD/PEO. Despite promising overall ionic conductivity, the actual fraction of ionic conductivity based on Li-ion transport is more important, at least in view of application of SPEs in LMBs and LIBs. For this reason, we determined the lithium transport number of all prepared samples,31 and a data analysis (Figure S7) yields the transport number (t+ ≈ 0.34) for γ-TMCD/PEO5LiTFSI (EO:Li ratio = 5:1) at 333 K, a value three times higher than for typical PEO/ LiTFSI systems. Note that reported transport numbers range from 0.1 to 0.2 for PEO-LiTFSI-based systems at temperatures of 323 K up to 363 K,39,40 but since many different methods for the determination of such numbers are known, we measured both systems at highly comparable conditions. In the considered temperature range, negligible variation with temperature was observed, thus indicating that the selfdiffusion coefficients of the active species show comparable 882

DOI: 10.1021/acsmacrolett.8b00406 ACS Macro Lett. 2018, 7, 881−885

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environments is summarized in Figure 4 and shows that the total number of O atoms varies between 4 and 5 around Li,

Figure 4. Snapshot from MD simulation of the CD/PEO-LiTFSI system with one CD channel highlighted (top). Coordination numbers of oxygen atoms in the first coordination shell of Li ions inside and outside CD channels.

Figure 2. Ionic conductivity as a function of inverse temperature in the temperature range of 30−100 °C.

trends with temperature. Methylated PPRs exhibit superior cation transport properties, while nonmethylated PPRs due to prohibitively high resistances did not yield reliable data. We have conducted atomistic molecular dynamics (MD) simulations using polarizable force fields to derive a more detailed understanding of local Li-ion environments in these SPEs. Specifically, we investigated ordered arrays of CD/PEO complexes with LiTFSI salt initially placed outside of CD/PEO “channels” (see Supporting Information for additional details). The radial distribution function (RDF) between Li+ and various oxygen atoms has been used to analyze Li-ion local environments.41 If Li+ is within the first hydration shell of PEO oxygen, then it is defined as ion “inside” the CD/PEO tube/ channel. Within 10 ns of simulations at 400 K, the fraction of Li ions “inside” and “outside” CD channels reached a steady state value. For systems with nonmethylated CD, the ratio of Li ions interacting with PEO chains inside CD channels amounts to 35% (independent of the α or γ form of CD), while in systems with methylated CDs the ratio is in the range of 15−20% (Figure 3). Note that the achievable spectral

which is typical for organic and polymer electrolytes.43 As anticipated, Li ions inside CD channels have about three PEO oxygen atoms in the first coordination shell. Note that this number is almost independent whether we have one PEO chain (in α-CD) or two PEO chains (in γ-CD) within the channels contributing to the actual Li coordination. The remaining coordination is provided by the oxygen atoms from CDs. In the case of Li ions outside CD channels, the oxygen atoms from both TFSI and CDs compete for Li coordination. Due to selective modification of the CDs we could clearly increase the achievable ionic conductivity and transport number indicating that a significantly larger fraction of the overall charge transport is based on lithium ions. Lithium plating−stripping experiments demonstrated an excellent interfacial stability and absence of lithium dendrite growth (Figure 5b). Furthermore, the oxidative stability could be increased from 3.8 V up to 4.1 V vs Li/Li+ (Figure S9) enabling application of methylated PPR-based SPEs in Li/LFP batteries. Figure 5a displays the cycling results using γ-TMCD/ PEO5LiTFSI SPE at different C-rates at T = 60 °C with thin LFP cathodes (1.2 mg/cm2). Indeed, the SPE facilitates fast cycling (C-rate of 1C) for more than 200 charge and discharge cycles with remarkable capacity retention of 95% and an averaged Coulombic efficiency (CE) of 99.4% after 200 cycles, where the impressive first cycle CE of 96% reflects excellent stability and compatibility of the investigated SPE against lithium metal and LFP during galvanostatic cycling. In comparison, PEO-based SPE as state-of-the-art polymer electrolyte shows inferior long-term cycling behavior at 1C (48% capacity retention) with the first cycle CE of only 93.4%. In conclusion, we employed functionalized CDs to yield PPRs applicable as solid-state SPEs in LMBs, thereby shifting the equilibrium of lithium ions residing inside and outside the present nanochannels to a higher fraction of lithium ions moving outside the channels in the case of methylated complexes. Since the novel materials have improved ionic conductivity compared to nonmodified PPRs, Li-ion movement outside the channels appears highly preferable. In addition, our solvent-free polymer electrolytes provide three times higher transport numbers than common PEO−LiTFSI

Figure 3. (a) Li+−O radial distribution functions and apparent coordination numbers obtained from simulation of the α-CD/PEOLiTFSI system at 400 K. (b) Evolution of Li+ inside the CD/PEO tube obtained from simulations of different SPEs at 400 K.

resolution of IR data is not sufficient to unambiguously resolve overlapping contributions from Li−O (CD) and Li−O (PEO) species, rendering peak deconvolution difficult, but previous work confirmed the plausibility of the MD protocol.42 The analysis of present coordination numbers (CNs) of oxygen atoms around Li ions in different systems and local 883

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complexes with trimethylated CD, but instead of water, chloroform was used as solvent since complex formation is solvent selective and depends on the hydrophobicity. LFP Electrodes. The LiFePO4-based cathodes were obtained using an electrode paste of 80 wt % LiFePO4, 10 wt % Super C65 carbon black, and 10 wt % PVDF binder in NMP. Thin electrodes (30 μm, 1.2 mg/cm2) were used.

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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/acsmacrolett.8b00406. Further experimental details and data from electrochemical analysis, MD simulation, as well as spectroscopic characterization of the polymer electrolytes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Fax: (+49) 251 83-36032. E-mail: [email protected]. *E-mail: [email protected]; martin.winter@uni-muenster. de. ORCID

Dengpan Dong: 0000-0002-3381-3425 Dmitry Bedrov: 0000-0002-3884-3308 Gunther Brunklaus: 0000-0003-0030-1383 Notes

The authors declare the following competing financial interest(s): Patent application at German Patent and Trade Mark Office, reference 10 2017 010 000.4, www.dpma.de (note added in the manuscript). A patent application including contents of this manuscript has been submitted to the German Patent and Trade Mark Office (www.dpma.de) and is in progress.

Figure 5. (a) Specific discharge capacities and Coulombic efficiencies of a Li/γ-TMCD-PEO 5 LiTFSI/LFP cell (green) and Li/ PEO20LiTFSI/LFP (black) cycled at 60 °C. The operational potential window ranged from 2.5 to 3.8 V vs Li/Li+. (b) Voltage profile of a lithium plating/stripping experiment of Li|γ-TMCD/PEO-LiTFSI|Li at constant current of 0.1 mA cm−2 and T = 60 °C. Note that the applied lithium metal foil has a thickness of 500 μm.45

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ACKNOWLEDGMENTS D.D. and D.B. acknowledge the support from the project sponsored by the Army Research Laboratory under Cooperative Agreement Number W911NF-12-2-0023 and the Center of High Performance Computing at the University of Utah for computing resources.

systems resulting in enhanced Li-ion conductivities. Upon cycling, solid-state thin-film LMBs with PPR-based SPE featured fast charging/discharging capabilities at excellent capacity retention, demonstrating applicability of the materials. Since modification with nonflexible methoxy groups was adjuvant, it can be anticipated that utilization of short flexible polymer chains may lead to even better ion transport properties. Also, the concept of single ion conductors where selected anions are chemically fixed to the polymer backbone could be a promising perspective for PPRs.44 Therefore, our work may pave the way for the rational design of superior future solid-state polymer electrolytes for advanced LMBs.

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REFERENCES

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EXPERIMENTAL SECTION

Anhydrous α-, β-, and γ-CD were each dissolved in DMF. The mixture was cooled to 0 °C, and sodium hydride was added slowly. The mixture was stirred at room temperature for 30 min and then cooled down again to 0 °C. An excess of methyl iodide was added slowly, and the suspension was stirred for 48 h (RT). An excess of sodium hydride was decomposed by dry methanol at 0 °C. The mixture was poured onto crushed ice; the aqueous layer was extracted with chloroform; and the organic layer was washed with water, dried over Na2SO4, and dried under reduced pressure. Preparation of the CD/PEO Complex. A mixture of PEO and LiTFSI with a specific EO:Li feed ratio was dissolved in Millipore water. The solution was added to a saturated solution of α-, β-, or γCD, respectively. The same procedure was used in the case of 884

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