B: Soft Matter, Fluid Interfaces, Colloids, Polymers, and Glassy MaterialsJanuary 18, 2026

The Role of Li⁺ Ions in Polyzwitterionic Ionogels: Gelator or Mobile Charge Carrier?
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Sajal Arwish
Sajal Arwish
Institute of Physical Chemistry, University of Münster, Corrensstr. 2830, 48149 Münster, Germany
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Mossab K. Alsaedi
Mossab K. Alsaedi
Tufts University, Department of Chemical & Biological Engineering, 4 Colby St., Medford, Massachusetts 02155, United States
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Ryan P. O’Hara
Ryan P. O’Hara
Tufts University, Department of Chemical & Biological Engineering, 4 Colby St., Medford, Massachusetts 02155, United States
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Ayse Asatekin
Ayse Asatekin
Tufts University, Department of Chemical & Biological Engineering, 4 Colby St., Medford, Massachusetts 02155, United States
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https://orcid.org/0000-0002-4704-1542
Matthew J. Panzer
Matthew J. Panzer
Tufts University, Department of Chemical & Biological Engineering, 4 Colby St., Medford, Massachusetts 02155, United States
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https://orcid.org/0000-0002-1741-8548
Monika Schönhoff*
Monika Schönhoff
Institute of Physical Chemistry, University of Münster, Corrensstr. 2830, 48149 Münster, Germany
*Email: [email protected]
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https://orcid.org/0000-0002-5299-783X

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The Journal of Physical Chemistry B

Cite this: J. Phys. Chem. B 2026, 130, 4, 1384–1394

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https://doi.org/10.1021/acs.jpcb.5c06389

Published January 18, 2026

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Abstract

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Introducing zwitterionic (ZI) polymers to lithium-containing, ionic liquid-based electrolytes can change the dynamics and local environment of Li⁺ ions. While Li⁺ ions enable the formation of noncovalently cross-linked ionogels, they may also act as free charge carriers. Aiming at an understanding of these roles, we investigate Li⁺ ion coordination and transport in ionogels consisting of 1-butyl-1-methyl pyrrolidinium bis(trifluoromethyl sulfonamide) (BMP TFSI), LiTFSI, and poly(2-(methacryloyloxyethyl phosphorylcholine)) p(MPC), obtained via in situ free radical polymerization. Ionic conductivity as well as self-diffusion of both IL ions benefit from increasing p(MPC) content, while Li⁺ diffusion is reduced. Simultaneously, ⁷Li spin relaxation documents Li⁺ ion immobilization, attributed to a strong affinity of Li⁺ to the negatively charged phosphate group of p(MPC). Raman spectroscopy confirms decreasing TFSI-Li⁺ coordination with increasing p(MPC) content. Mechanical analysis reveals a drastic increase in elastic modulus, suggesting noncovalent cross-link formation. Finally, the distribution of Li⁺ on different sites, such as mobile and anion-coordinated, single chain-coordinated, or dual-chain cross-linking Li⁺ is analyzed. The results allow for an in-depth discussion of the role of Li⁺, partly acting as a cross-linker and partly as a mobile charge carrier, providing guidelines for optimizing the balance between ionic conductivity and mechanical strength.

This publication is licensed for personal use by The American Chemical Society.

Subjects

Introduction

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In modern society, the development and increasing use of electric vehicles and advanced portable electronics calls for the advancement of energy storage technologies. Supercapacitors and Li⁺ ion-based batteries are widely used energy storage devices. (1) However, due to the nature of their applications (airplanes, vehicles, wearable devices, etc.), it is crucial to ensure that they operate without posing any safety threats on their users. In currently employed batteries, a critical component concerning safety is the electrolyte. (2,3) Its purpose is to provide high ionic conductivity, but no electronic conductivity. To this end, current electrolytes consist of a lithium salt in a carbonate mixture, with their flammability causing a severe risk. (4,5) Therefore, in ongoing research, ionic liquids (ILs), which are salts that are liquid at room temperature, are being investigated as an alternative and promising class of thermally stable and nonvolatile solvents that could replace conventional liquid electrolytes. (6−11) Moreover, they also show broad electrochemical stability windows, making them suitable components for high voltage battery systems. (12−14) A further major advantage is their structural variety, which results in a plethora of combinations of different cation and anion motifs. This allows for significant tunability of physical and chemical properties, (13) and makes ILs highly versatile, allowing for their usage in hybrid electrolytes (15) as well as for beyond-Li battery technologies. (16)

However, ILs often show strong Li⁺ ion correlations and cluster formation, resulting in high viscosity and low Li⁺ transference numbers. (17−21) Detailed studies based on MD simulations or Raman spectroscopy of the anion TFSI^– could clarify the structural arrangement of Li⁺-anion clusters, involving the determination of coordination numbers and the role of monodentate or bidentate coordinations. (22−26) Additionally, direct evidence of the role of such coordinated clusters in ion transport was given by electrophoretic NMR, when a vehicular transport of Li⁺ ions in net negatively charged Li⁺-anion clusters was identified in lithium salt-in-IL systems. (27,28) This leads to an inverted Li⁺ migration direction in an electric field and ultimately causes negative Li⁺ transference numbers. (27,29) Attempts to overcome the strong Li⁺-anion coordination involved the use of asymmetric anions, (30,31) or mixed anion compositions. (32) However, more successful strategies were based on the addition of electrolyte constituents competing with the anions for Li⁺ coordination. Following this concept, electrolyte compositions with either added cosolvents, polymer chains or coordinating cations, which provide Li⁺ coordination, could finally revert the Li⁺ ion drift direction. (33−35)

As an alternative approach to reduce Li⁺-anion coordination, the introduction of zwitterionic (ZI) materials has proven to be beneficial. (36) ZI polymers have gained interest due to their ability to form three-dimensional solid scaffolds within a lithium salt/IL phase while maintaining ionic conductivity that is comparable to, and in some cases, higher than their fully liquid counterparts. (12,37,38) The presence of ZI moieties on the polymer chain can significantly affect Li⁺ transport properties, with certain zwitterion chemistries promoting stronger interactions with Li⁺, thereby reducing its mobility. Among various polyzwitterions, poly(2-methacryloyloxyethyl phosphorylcholine) (p(MPC)) in particular demonstrated excellent compatibility with a hydrophobic IL/lithium salt system.^39, In these materials, Li⁺ ions can assume different roles, on one hand behaving as a mobile species to facilitate Li⁺ ion transport, and on the other hand acting as a gelator that can bridge p(MPC) chains, creating a nonflowing, IL phase-rich gel (ionogel). It is thus interesting to analyze the role of Li⁺ in detail in order to identify the interplay of both roles, enabling further optimization of these electrolytes. A recent MD study has analyzed Li coordination numbers and transport parameters in pMPC-based ionogels with different types of anions. (40)

Here, we investigate varying amounts of the ZI polymer p(MPC) prepared within a lithium salt-in-IL electrolyte, i.e. LiTFSI dissolved at a concentration of 1 mol/L in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP TFSI) mixed with the monomer MPC (see structures in Scheme 1). ZI p(MPC) (and consequently, above a certain minimum polymer content, ionogels) are produced via photoinitiated in situ free radical polymerization. Ionic conductivity and pulsed field gradient NMR diffusion experiments elucidate overall charge transport as well as individual ionic constituent transport, and are complemented by Raman studies of anion coordination, dynamic mechanical analysis of the network, and spin relaxation of ⁷Li. From these results, we analyze the coordination and mobility of Li⁺ ions, identifying three different proposed Li⁺ populations (see Scheme 1) that help to clarify the various roles that Li⁺ ions may play (e.g., mobile charge carrier versus immobilized cross-linker) within these materials for different ZI polymer contents.

Scheme 1. Structures of the Main Electrolyte Components (Top) and Illustration of Different Li⁺ Species within a p(MPC)-Supported Ionogel^a

Materials and Methods

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Materials

BMP TFSI (99%, Iolitec Ionic Liquids Technologies GmbH, or 99.9%, Solvionic), LiTFSI (99.95% trace metals basis, Sigma-Aldrich), and 2-hydroxy-2-methylpropiophenone (HOMPP, 97%, Sigma-Aldrich) were stored in a nitrogen-filled glovebox (Mbraun Inc., MBRAUN Unilab) or argon-filled glovebox (GS Glovebox Systemtechnik) until used. 2- methacryloyloxyethyl phosphorylcholine (MPC, 97%, Millipore Sigma, and Sigma-Aldrich) was stored in a refrigerator until used. Stainless-steel CR2032 coin-cell parts (SS304, MTI Corporation) and lithium discs (LIB-LIC60, MTI Corp.) were stored in an argon-filled glovebox (H₂O and O₂ < 0.5 ppm) until used. The LiTFSI salt was dried for 24 h at 100 °C and 10^–7 bar. All materials were used as received.

Ionogel Preparation

As the basis for the ionogels, a stock lithium salt-in-IL solution was prepared by dissolving LiTFSI into the ionic liquid BMP TFSI at a concentration of 1 mol/L. All sample preparation and handling was performed in a glovebox due to the hygroscopic nature of LiTFSI and the MPC monomer. MPC monomer was first added to a glass vial, then the required amount of LiTFSI/IL solution was added to realize 2–14 wt % MPC “ionogel precursor” solutions. The resulting precursor solutions were stirred at 80 rpm at room temperature until MPC was completely dissolved and the mixture was transparent. The photoinitiator (HOMPP) was then added to the solutions (6 mol % with respect to the amount of MPC). The HOMPP-containing precursor solutions were then stirred at 80 rpm for 15 s. Molds or tubes relevant to the respective characterization method were filled with precursor solution. They were then exposed to UV radiation (365 nm) from a hand-held lamp (Spectronic Corp., 8 W, or Winger Electronics., 3 W) in the containers for 10 min to initiate the in situ photopolymerization of the MPC. Ionogel samples cured overnight inside the glovebox prior to carrying out any characterization. Three replicate gels were prepared for each sample composition.

Samples for Pulsed Field Gradient NMR and Raman spectroscopy were directly prepared in 5 mm outer diameter NMR tubes. Ionogels for room temperature ionic conductivity measurements were prepared in cylindrical polytetrafluoroethylene (PTFE) 125 μL wells with gold-coated electrodes. For dynamic mechanical analysis (DMA), ionogel precursor solutions were injected with a needle and syringe into cylindrical PTFE washers (6.35 mm inner diameter and 3.15 mm thickness). PTFE washers were sandwiched between two glass slides and two thin films of polyethylene separating the washer mold cavity from the glass on each side to prevent the adherence of p(MPC) to the glass slides. Prior to DMA characterization, ionogels were removed from their PTFE washer molds.

Dynamic Light Scattering (DLS)

p(MPC) chains synthesized in situ in precursor solution were analyzed by dynamic light scattering (Nano Brook 90Plus PALS particle sizer, Brookhaven Instruments, Holtsville, NY) to determine their Stokes’ diameters in solution as an indicator of chain length. The instrument light source was a helium–neon laser with a fixed wavelength of 659 nm and an entrance aperture of 1 mm. Ionogels were mixed with ultrapure deionized water inside a 15 mL centrifuge tube at a concentration according to the monomer content of each ionogel (2 mg monomer/1 mL of solvent). BMP TFSI is insoluble in water, causing it to readily form a separate phase. However, p(MPC) is soluble in water, allowing us to perform a simple liquid–liquid extraction. The Stokes diameter of the polymer contained in the water phase of the dissolved ionogel was measured at 25 °C using a scattering angle of 90°. Three gels were measured through three consecutive runs to obtain average Stokes diameter and polydispersity index values.

Ionic Conductivity

Electrochemical impedance spectroscopy (EIS) was performed to determine ionogel ionic conductivities using a potentiostat with a built-in frequency response analyzer (VersaSTAT 3, Princeton Applied Research). EIS spectra were collected over a frequency range from 1 kHz to 100 kHz with a sinusoidal voltage amplitude of 10 mV. The values of ionic conductivity for each ionogel were calculated from the impedance values obtained at phase angle closest to zero (real impedance).

NMR Spectroscopy and Pulsed-Field Gradient NMR (PFG NMR)

NMR spectra and diffusion experiments were performed on a Bruker AVANCE Neo 400 MHz or AVANCE III HD 400 MHz NMR spectrometer equipped with gradient amplifiers, using either a gradient probe head with selective radiofrequency inserts for ¹H or ¹⁹F (Bruker Diff50 with maximum gradient strength of 28 T/m), or a broad band diffusion probe head (Bruker, DIFF BBO, with maximum gradient strength of 17 T/m). Measurements were performed at room temperature (25 °C), or in the range up to 65 °C, respectively. Temperature control was provided by an air stream and an internal temperature sensor, which was calibrated prior to experiments using a Pt100 thermocouple (Greisinger electronics) inserted in an oil-filled NMR tube. The self-diffusion coefficients for the nuclei ¹H, ⁷Li and ¹⁹F were measured by PFG-NMR using a stimulated echo pulse sequence and recording a series of spectra varying the gradient strength, g. The diffusion coefficients for distinct resonances were obtained from the decay of the signal intensity I with g employing the Stejskal-Tanner equation (eq 1). (41)

I = I_{0} \exp (- γ^{2} δ^{2} g^{2} D (Δ - \frac{δ}{3}))

(1)

Here, γ represents the gyromagnetic ratio of the observed nucleus, and I₀ indicates the signal intensity without gradient influence. The gradient pulse length duration δ was set to a fixed value between 1 and 2 ms, while the gradient strength g was incremented in a series of spectra up to a maximum value of 16 T/m, depending on the nature of the observed nucleus and sample. Spin relaxation rates R₁ and R₂ were determined by inversion recovery and a CPMG (Carr, Purcell, Meiboom, Gill) (42,43) pulse sequence, respectively, and fitted with corresponding exponential functions.

Raman Spectroscopy

The same samples sealed in NMR tubes were inserted into a Raman spectrometer (MultiRAM, Bruker, Rheinstetten) equipped with a laser providing 263 mW at 1064 nm. The Raman spectra were recorded at room temperature in a range of (30 – 3600) cm^–1 with a minimum resolution of 0.5 cm^–1. The spectra were baseline corrected and normalized to the integral of the Raman band region from 700 cm^–1 to 800 cm^–1. Spectral decomposition was carried out using Voigt profiles fitting in OPUS software (Bruker, Rheinstetten). For the region a fixed contribution of 60% Lorentzian and 40% Gaussian is used. The peak position, intensity, and width were treated as free parameters during the fitting process.

Dynamic Mechanical Analysis

Elastic modulus values for ionogels were obtained by compressing the gels using a texture analyzer (TA.XT Plus 100, Texture Technologies Corp.) at room temperature. Compression was performed at 0.01 mm/sec to achieve 15% strain with respect to the ionogel thickness. Elastic moduli values were calculated as the slopes of the linear regions on the stress–strain curves between 4% and 10% strain values.

Results and Discussion

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In Situ Polymerization and Molecular Weight

In order to characterize the molecular weight of p(MPC) produced by in situ polymerization and examine any potential effects of monomer concentration on average polymer size, as-prepared samples were immersed in ultrapure deionized water to extract the ZI polymer. As a selective solvent, water dissolves p(MPC). The ionic liquid, BMP TFSI, has minimal solubility in water and thus creates a separate phase. The resulting aqueous solution likely also contains some LiTFSI (not quantified), though studies have shown that the Stokes diameter of p(MPC) is not significantly affected by salt concentration. (44,45) This aqueous phase containing p(MPC) chains was analyzed with DLS to estimate the Stokes diameter, which is correlated with the molar mass of the polymer. Table 1 shows that as the p(MPC) wt % in the ionogel increases, the extracted p(MPC) Stokes diameter in aqueous solution was observed to decrease, which indicates a decrease in polymer molecular weight. The large Stokes diameter found in the 2 wt % samples, ∼42 nm, implies a very high molar mass. In comparison, past studies have reported the Stokes diameter for a p(MPC) sample with an M_n of 234,000 g/mol to be ∼22 nm in aqueous solution. (44,45) The high molar mass could be explained by the Trommsdorf effect, where the high viscosity of the reaction mixture in the ionic liquid decreases the ability of chain ends to meet each other and terminate the polymerization reaction, resulting in longer chains. (46) In other reaction media, increasing monomer concentrations tend to lead to higher molar mass due to increasing propagation rates compared with chain initiation and termination. The trend of decreasing Stokes diameter, and thus molar mass, with increasing monomer concentration appears to be linked with this specific polymerization environment. At higher MPC concentrations, the noncovalent cross-link density of p(MPC) is higher (presented below, see DMA section). These noncovalent interactions are also likely present between monomers in solution. As a result, the mobilities of both polymer chains and monomer molecules are much lower at higher MPC contents. This may have led to a decrease in the probability of active chain ends meeting and reacting with monomer molecules, decreasing polymerization rate compared with initiation rate and resulting in shorter chains. It should be noted that the 2 wt % p(MPC) sample was a highly viscous solution (insufficient polymer content to form a gel), while the 4 wt % p(MPC) and higher polymer content samples were all free-standing ionogels. The in situ synthesized polymers all displayed full conversion according to the ¹H NMR spectra of the dissolved material (see Supporting Information (SI), Figure S1), most likely due to the high initiator concentration used. This indicates that in situ synthesized p(MPC) ionogels are able to achieve full conversion for a wide range of compositions, and that the molecular weight of the polymer is inversely correlated with the p(MPC) content.

Table 1. Stokes Diameter and Polydispersity Index Measured by Dynamic Light Scattering for Dissolved p(MPC) Extracted from Polymer/LiTFSI-in-IL Samples of Varying Composition

p(MPC) content (wt %)	Stokes diameter (nm)	Polydispersity index
2	42 ± 2	0.15 ± 0.04
4	39 ± 3	0.16 ± 0.02
6	38 ± 1	0.14 ± 0.01
8	35 ± 3	0.15 ± 0.04
10	29 ± 2	0.15 ± 0.03
12	25 ± 0.2	0.14 ± 0.02
14	21 ± 0.8	0.16 ± 0.02

Ionic Conductivity

Room temperature (25 °C) ionic conductivities for neat 1 M LiTFSI/BMP TFSI solution (0 wt % p(MPC)) and samples containing 2–14 wt % p(MPC) are displayed in Figure 1. Comparing the ionic conductivity values of the p(MPC) ionogels (4 wt % p(MPC) and above) to that of the 1 M LiTFSI/BMP TFSI solution, all ionogels exhibited a higher ionic conductivity. The introduction of p(MPC) through in situ photopolymerization therefore results in a boost in total ionic conductivity (up to 50% increase in the case of the 8 wt % p(MPC) ionogel). This is attributed to the dissociative effect that the charges on the zwitterion have on the surrounding ion clusters reported previously, (37,38) i.e. TFSI^– is liberated from both Li⁺ and BMP⁺, which results in less anion–cation coordination and creates more mobile ions. The presence of the ZI polymer at a weight percent as low as 2 wt % resulted in an increase in ionic conductivity by ∼25%. In the higher range of 10–14 wt % p(MPC), the ionic conductivity starts to decrease, which can be attributed to the decrease in the liquid fraction of the ionogels and an increase in their solid (polymer) fraction.

Diffusion Coefficients of the Constituents

The room temperature self-diffusion coefficients of each nucleus (¹H, ¹⁹F, and ⁷Li) for neat LiTFSI/BMP TFSI solution and 2–14 wt % p(MPC) samples are displayed in Figure 2a, representing the diffusion of the BMP⁺ cation, TFSI^– anion, and Li⁺ ion, respectively. In the neat LiTFSI/BMP TFSI solution, the order of the diffusion coefficients is IL cation > IL anion > Li⁺, which is typical for Li salt-containing ILs, since Li strongly complexes with the anions. With increasing p(MPC) content, the samples exhibited an increase in self-diffusion coefficients for the ¹H and ¹⁹F nuclei. In contrast, the trend for ⁷Li is different, as there is a monotonic decrease of self-diffusivity observed with increasing p(MPC) content. The decreasing trend can be attributed to the binding of Li⁺ to the p(MPC) units, hence decreasing the number of free Li⁺ ions. On the other hand, the increase in diffusion of the other ionic constituents, both the IL cation and anion, is reminiscent of a dilution of Li⁺ ions in the IL, since an increase of Li⁺ concentration typically enhances viscosity and lowers the diffusion coefficients of all ionic constituents. (18,20,21,47) The increase in the BMP⁺ cation and TFSI^– anion self-diffusivities likely stems from enhanced IL ion pair/cluster dissociation by the ZI group of p(MPC), while a portion of the increased anion diffusion is also due to liberation of TFSI^– resulting from the strong p(MPC)-Li⁺ coordination.

Figure 2. (a) Self-diffusion coefficients of the nuclei (¹H, ¹⁹F, ⁷Li) obtained from PFG-NMR at room temperature (25 °C) as function of p(MPC) content. (b) Activation energies of diffusion as a function of p(MPC) content measured over a temperature range of 25–65 °C. The dotted lines are merely guides to the eye.

We note that ⁷Li diffusion measurements at p(MPC) contents above 8 wt % were not feasible due to T₂ relaxation times that were too rapid.

Temperature dependent diffusion coefficient data are shown in Figure S2 for the ¹H and ¹⁹F nuclei over the range of 25 to 65 °C. To estimate the activation energy of diffusion, the data is fitted by an Arrhenius equation (details in SI). The resulting activation energies are displayed in Figure 2b. The decrease in activation energy observed for both the BMP⁺ and TFSI^– ions with increasing p(MPC) content (up to 10 wt %) indicates a higher mobility within the polymer matrix and an improved ion transport mechanism. It can be concluded that the facilitated transport of these two constituents contributes to the improved ionic conductivity of the ionogel as compared to the neat LiTFSI/BMP TFSI solution (see Figure 1). The findings corroborate the idea that p(MPC) can enhance the dissociation of the LiTFSI and BMP TFSI anion–cation pairs/clusters, hence BMP⁺ and TFSI^– are becoming more freely available in an uncoordinated form, enhancing their contribution to the total ionic conductivity. On the other hand, a beneficial influence of MPC groups is not visible for Li⁺, as its diffusivity is decreasing with greater p(MPC) content. In line with arguments from previous work, (38,39) we can conclude that Li⁺ ions are strongly interacting with the zwitterionic MPC groups. While this has a beneficial effect in terms of dissolving Li⁺-anion clusters, thus enhancing overall conductivity, the effect on transport of the Li⁺ ion species is rather unfavorable. In the following sections, we will therefore focus on the Li⁺ ions and further elucidate their dynamics.

Li⁺ Ion Apparent Transference Number

In concentrated electrolytes, ion correlations generally cause invalidity of the Nernst–Einstein equation. Therefore, a transference number obtained from diffusion coefficients is a mere estimate under the assumption that correlations are negligible. (48) In this case, the Li⁺ apparent transference number (t_Li^*) is calculated using eq 2, where c_i and D_i represent the molar concentration and self-diffusion coefficients of each ionic constituent, i.

t_{L i}^{*} = \frac{c_{L i} D_{L i}}{c_{B M P} D_{B M P} + c_{L i} D_{L i} + c_{T F S I} D_{T F S I}}

(2)

Figure 3 shows that the contribution of Li⁺ to overall charge transport is decreasing with increasing p(MPC) content up to 8 wt %, which is again an indication of a strong Li⁺ interaction with the zwitterionic units. This behavior favors the liberation of anions from Li⁺ cations. The decrease of the Li⁺ apparent transference number is therefore due to two trends, namely an increase of the partial conductivity of the anions, and a decrease of the partial conductivity of Li⁺ ions. Thus, the increase in total ionic conductivity is dominated by the more mobile species (BMP⁺, TFSI^–), which was also evident in the diffusion data (Figure 2a).

Figure 3. Li⁺ ion apparent transference number as function of p(MPC) content.

⁷Li NMR Spectra and Spin–Spin Relaxation Rates

A directly evident influence of the p(MPC) content in the ionogels on the Li⁺ ions is seen in 1D ⁷Li spectra of the samples. Figure 4a shows ⁷Li spectra for compositions from 2 to 8 wt % p(MPC), where the decrease of the ⁷Li peak intensity, accompanied by broadening of the peak, indicates a decrease in the Li⁺ ion mobility at higher compositions, while above 8 wt % p(MPC) no ⁷Li signal was detectable.

Figure 4. (a) ⁷Li NMR spectra, (b) peak integral, (c) line width, and (d) ⁷Li spin–spin relaxation rate R₂ determined by a stimulated echo sequence (black) and R₂* determined by line width analysis (red star) for 1 M LiTFSI/BMP TFSI solution (0 wt % p(MPC)), 2 wt % p(MPC) solution, and 4–8 wt % p(MPC) free-standing ionogels.

Generally, line broadening may arise due to an increased heterogeneity of the local environment of nuclei (“heterogeneous broadening”), or due to motional constraints, which lead to a reduced lifetime of the excited state and thus to line broadening (“homogeneous broadening”). For an analysis of the contributions of either broadening mechanism, we analyzed the ⁷Li peak integral (Figure 4b) and the line width (full width at half-maximum, FWHM) (Figure 4c). In case of homogeneous broadening, the line width is connected to the spin–spin relaxation rate R₂. Figure 4d presents R₂ values of ⁷Li in the samples, as measured by a CPMG experiment, in comparison to R₂*, which is extracted from the line width as R₂* = π·FWHM. These values are in good agreement with each other, indicating that the broadening can be fully attributed to homogeneous broadening, which is induced by an immobilization of Li⁺ that enhances the spin–spin relaxation rate R₂.

The increase in relaxation rates with increasing p(MPC) content is strongly pronounced and suggests an immobilization of Li⁺ ions within the ionogels, which most probably is due to a stronger affinity of Li⁺ ions to the phosphate group of the MPC units. Since the echo decays of the CPMG experiment are exponential, a single relaxation rate describes the data. From this we conclude that bound and free Li⁺ ions are in fast exchange, such that the experiment yields an averaged relaxation rate over both sites. Higher polymer concentrations lead to more MPC repeat units around each Li⁺ ion, thus immobilizing a larger number of Li⁺ ions, contributing to the increase of R₂. Additional evidence for the restriction of local Li⁺ dynamics with increasing MPC content is provided by spin–lattice relaxation rates, R₁, as these show a strong decrease with increasing polymer content (see Figure S3a). Moreover, the ratio of R₂/R₁, which serves as an indicator for motional restriction, strongly increases with p(MPC) content, achieving values up to 500 (Figure S3b), which is far from the value of 1 expected for motional narrowing. Similar to the spin–spin relaxation enhancement, the loss of signal intensity can be attributed to Li⁺ ion immobilization. At 2 wt % p(MPC), for example, the molar ratio of Li⁺ to MPC is 8.5, thus the ⁷Li signal is dominated by free Li⁺ ions (including Li⁺ ions in Li⁺-anion clusters). In contrast, at 8 wt % p(MPC), the Li⁺ to MPC molar ratio is approximately 2, and the ⁷Li signal vanishes above this p(MPC) concentration. We thus conclude that an average coordination number of 0.5 MPC units per Li⁺ ion immobilizes the latter sufficiently to impede the observation of any liquid state NMR signal. Thus, from the ⁷Li NMR data, the presence of two different localizations of the Li⁺ ions can be concluded. As the ⁷Li spectra in Figure 4b are showing, these Li sites do not occur as a superposition, but as an averaged single resonance, which is shifting and broadening with increasing fraction of MPC-coordinated Li. In conclusion, the exchange time τ between the Li sites is fast compared to the inverse spectral spacing, such that an upper limit can be given as τ < 80 ms. In relation to this time scale, the Li coordinations are transient. The relevance of these mobile and immobile Li⁺ species for network formation and charge transport will be discussed further below in context of the results of the other characterization methods.

Raman Spectroscopy

Since with greater p(MPC) content the Li⁺ coordination environment appears to shift from coordination with TFSI^– anions to coordination with p(MPC), it is worthwhile to study the coordination state of the anions, which is feasible by Raman spectroscopy. The Raman peaks observed for neat 1 M LiTFSI/BMP TFSI solution occur at 742 cm^–1 for free TFSI^– ions and at 748 cm^–1 for TFSI^– ions coordinated to Li⁺. (25,49) Figure 5a shows the Raman spectra for the IL itself (see black curve), the LiTFSI/IL solution (red curve), and for samples with increasing amounts of p(MPC). Upon addition of LiTFSI to the IL, a peak of Li⁺-coordinated anions appears (see green dotted line), which then becomes weakened with increasing p(MPC) content. In order to quantify the fractions of free and Li⁺-coordinated anions, Raman spectra were deconvoluted by Voigt profiles (Figure S4). From the integrals of the “free” TFSI^– species (A_f) and the coordinated TFSI^– species (A_c) the integral ratio A_c/(A_c + A_f) is determined. According to a procedure introduced by Lassegues et al. (50) the Li⁺ coordination number, i.e. the number of anions coordinated per Li⁺ ion (n) is evaluated, taking the known LiTFSI salt fraction for each composition into account. This method requires equal Raman scattering cross sections of either TFSI^– species, however, the validity of this assumption has been shown in literature. (25,51) Details of the procedure and the relevant equations are given in the SI. The resulting Li⁺ coordination number is plotted against the p(MPC) composition and shown in Figure 5b. A decrease in coordination number of TFSI^– to Li⁺ is observed with increasing p(MPC) content, which is in line with the high binding affinity of Li⁺ ions to MPC units as compared to TFSI^–, as discussed above. Interestingly, however, the coordination number of anions to Li⁺ does not reduce to zero, even in the composition range where the molar ratio of MPC to Li⁺ is 0.5 (at 8 wt % p(MPC), see Table S1) or higher. At 14 wt % p(MPC) the molar ratio of MPC to Li⁺ is close to 1, and the average coordination environment of Li⁺ consists of 0.72 anions (Table S2), and one MPC group, if all MPC is coordinating. This shows that Li⁺ is still coordinated with some fraction of anions while coordinating to an MPC group. While in the 1 M LiTFSI/BMP TFSI solution a coordination of 2 TFSI^– anions per Li⁺ is found, consistent with a previous study, (27) the lower contribution of anions to Li⁺ coordination in the samples with larger p(MPC) contents may suggest a replacement of more than one coordinating anion by MPC groups in the Li⁺ ion coordination environment, such that Li⁺ acts as a cross-linker between polymer segments. However, the stoichiometry of the molar ratio of MPC to Li⁺ being close to 1 (at 14 wt % p(MPC)) would then equally require several Li⁺ ions coordinated to the same MPC group. Such a stoichiometry can occur if Li⁺-MPC clusters are formed. This scenario is in line with theoretical predictions of the same system, where MD simulations at high MPC content of 13% showed that 35% of the Li ions are coordinated to two MPC monomers, and 25% even coordinate to three MPC units. (40)

Figure 5. (a) Raman spectra and (b) Li⁺ coordination number (coordinated anions per Li⁺) as extracted from the fraction of coordinated anions in dependence on p(MPC) content. The green dotted line in a) marks the position of the band of coordinated TFSI^– and the black arrow represents the increase in p(MPC) content from 2 wt % (green spectrum) to 14 wt % (dark blue).

Dynamic Mechanical Analysis (DMA)

In order to elucidate the role of Li⁺ as a cross-linker in a p(MPC) network, dynamic mechanical properties of the samples were probed. Using stress–strain curves from compression tests, elastic modulus values were obtained for the ionogel compositions with 4, 6, 8, 10, 12, and 14 wt % p(MPC). As shown in Figure 6a, the elastic modulus values increased drastically (by 3 orders of magnitude) upon a 6 wt % increment in polymer content (3.80 ± 0.35 kPa and 3000 ± 330 kPa for 4 and 10 wt % p(MPC) ionogels, respectively). This is an indication of a similarly drastic increase in the cross-link density of the gels, which is directly proportional to elastic modulus. This finding corroborates the decrease in Li⁺ diffusivity observed with increasing p(MPC) content. More Li⁺ cations are noncovalently coordinating with increasing p(MPC), which reduces the number of mobile Li⁺ cations and increases the Li⁺ ion-mediated cross-link density of the gels.

Figure 6. (a) Elastic modulus values for ionogels containing 4 to 14 wt % p(MPC). (b) Calculated cross-link density within the ionogels, representing the number of Li⁺ ion-mediated noncovalent cross-links per nm³, and (c) the corresponding fraction of Li⁺ that is not participating in noncovalent cross-links.

Since the increasing modulus might include contributions from direct MPC-MPC-cross-links in addition to Li-mediated cross-links, it would be instructive to analyze a reference with MPC in IL without any Li salt. Unfortunately, MPC is not soluble in BMP TFSI, (38) however, previous preparation of an MPC-containing copolymer was performed in a similar IL, EMI TFSI. This yielded a transparent viscous polymer solution instead of a gel, apparently lacking cross-links. (52) Thus, direct MPC-MPC cross-links are unlikely to occur in BMP TFSI.

To gain more information from the elastic modulus values of the p(MPC) ionogels, the cross-link density and the concentration of non-cross-linked Li⁺ were calculated (details in SI). Figures 6b and 6c show the cross-link density and the fraction of non-cross-linked Li⁺ in the ionogels, respectively. As the p(MPC) content increases from 4 to 10 wt %, the cross-link density increases from 4.8 × 10^–5 nm^–3 to 0.06 nm^–3, which is around a three-order-of-magnitude increase, with only a slight decrease in the fraction of non-cross-linked Li⁺ (0.99 and 0.90 fraction of non-cross-linked Li⁺ for 4 and 10 wt % p(MPC) ionogels, respectively). This demonstrates the importance of Li⁺ in the formation of the noncovalent cross-links that bridge p(MPC) chains and hold these gels together, as well as the ability to obtain relatively stiff gels (3000 ± 330 kPa for 10 wt % p(MPC) ionogels) by using only ∼10% of the available Li⁺ ions in the system, leaving the remaining ∼90% of Li⁺ ions not in cross-links.

Discussion of Li⁺ Sites

The above results show that the introduction of polymerized ZI units within the 1 M LiTFSI/BMP TFSI solution has two different effects: On the one hand, the Li⁺ ion local mobility is severely hindered, on the other hand, mechanical stability is introduced by Li⁺ acting as a cross-linker for the polymeric network. In the following discussion, we draw conclusions from the combination of the above findings, in order to describe the nature of Li⁺ sites and the respective Li⁺ dynamics in these sites for different regimes of p(MPC) content. It is important to note that with the different characterization methods we monitor different properties of Li⁺ ions. First, the ⁷Li NMR intensity provides the fraction of Li⁺ ions detectable in liquid state spectra, which we designate as the “mobile” Li⁺ fraction, m. The fraction m is given in Table 2, and it decreases drastically with greater p(MPC) content, see Figure 7a. Any Li⁺ ions with liquid-like dynamics are included in this site, irrespective of their coordination environment. We define r to be the molar ratio of MPC to Li⁺ present in each sample. Assuming that all immobilized Li⁺ ions are coordinated to MPC groups, the immobilized fraction of Li⁺, (1 – m), can be related to the MPC amount by calculating b = (1 – m)/r as the number of Li⁺ ions immobilized per MPC group, see Table 2. Furthermore, the Raman spectra of the TFSI^– anions provide information about the anion-related average coordination number of Li⁺. Finally, the DMA analysis provides the fraction of non-cross-linked Li⁺ ions, f_Li, which are not involved in network formation, see Figure 7a. This allows a further distinction of the MPC-coordinated Li⁺ into cross-linking or non-cross-linking species.

Table 2. Collection of Resulting Parameters, Detailing the Distribution of Li⁺ on Different Sites as a Function of p(MPC) Content

	r	m	b	n	f_Li
Composition wt % p(MPC) (target value)	MPC:Li⁺ molar ratio (actual)	mobile fraction of Li⁺ (⁷Li NMR)	Immobilized Li⁺ per MPC	Coord. no. TFSI^– per Li⁺ (Raman)	noncross-linked Li⁺ (DMA)
0	0	1	0	1.81	1
2	0.13	0.96	0.31	1.65	1
4	0.24	0.73	1.13	1.51	1
6	0.34	0.47	1.55	1.25	1
8	0.50	0.24	1.52	0.98	0.98
10	0.62	0	1.60	0.88	0.90
12	0.69	0	1.40	0.81	0.75
14	0.92	0	1.10	0.72	0.44

Figure 7. (a) Distribution of Li⁺ in different sites; (b) – (d) Li⁺ coordination scenarios at 0, 8, and 14 wt % p(MPC), showing free, immobilized, and cross-linking Li⁺ ions. For a legend of the symbols, see Scheme 1. Note that the MPC:Li⁺ molar ratio and number of coordinating anions are shown according to scale, while the total number of IL cations and anions is not to scale to enhance clarity.

With these data, the different Li⁺ sites and the role of Li⁺ in cross-linking and charge transport can be discussed for the different concentration regimes of polymer in the ionogel. From Figure 7a it is clear that at no or low MPC content there are no Li⁺ species involved in network formation, i.e. f_Li = 1, and all Li⁺ species are mobile, m ∼ 1. In the neat LiTFSI/BMP TFSI solution, the coordination number n is almost two, suggesting a predominant coordination of Li⁺ by two TFSI^– anions, in accordance with literature. (27,51) This scenario is schematically depicted in Figure 7b, where Li⁺ is coordinated by anions and freely mobile.

With increasing MPC content the coordination number of anions per Li⁺ ion (n) decreases, see Table 2. This is accompanied by a decrease of m, and both trends mark the shift of Li⁺ coordination from anions to MPC, where apparently polymer-coordinated Li⁺ does not yield a narrow liquid state NMR signal. Interestingly, in the low polymer concentration range, the number of immobilized Li⁺ ions per MPC group, b, increases with polymer content, in spite of an enhanced availability of MPC groups. The increase could be a signature of a cooperative effect, where ionic clusters are formed by Li⁺ and MPC, which increase with MPC content, attracting an increasing number of Li⁺ ions per MPC group. At the same time, the fraction of non-cross-linking Li⁺, as extracted from the DMA analysis, remains equal to one, showing that virtually no cross-links are formed in the low concentration regime. Therefore, Li⁺-MPC coordinations remain local without providing interchain links.

At intermediate polymer content (r = 0.5 at 8 wt % p(MPC)), the coordination number is reduced to one anion per Li⁺, while ∼1.5 Li⁺ ions are immobilized per MPC group. Interestingly, while the Li⁺ coordination is further shifting from anions to MPC, the TFSI^– ions are still playing a role in Li⁺ coordination, forming a heterogeneous environment, as illustrated in Figure 7c. Here, Li⁺ is shared by MPC and anions in a mixed coordination. This regime also marks the onset of cross-linking, i.e. the decrease of f_Li. It is also evident from NMR that the immobilized Li⁺ fraction is increasing, reaching a plateau of 1 – m = 1 at higher MPC content. Here, we can conclude that all Li⁺ is coordinated by MPC and consequently immobilized. While anions still contribute to the average coordination environment, there is virtually no Li⁺ which is exclusively coordinated by anions, as this species would appear as a liquid state signal in ⁷Li NMR.

Finally, at the highest polymer content of 14% wt p(MPC), more than 50% of the Li⁺ ions are forming cross-links. This agrees well with predictions from MD simulations at high polymer content. (40) The remaining Li⁺ ions are not contributing to the network, but since they are immobile, they are likely strongly bound to the phosphate group of zwitterionic MPC, as evident from the mobile fraction of Li⁺ ions being zero. Figure 7d shows this scenario, where anions are still playing a major role in a mixed coordination environment of Li⁺, with a coordination number of n = 0.72 (at 14 wt % p(MPC)). In this concentration regime, Li⁺ is predominantly acting as a cross-linker in providing a stiff ionogel, and as even the non-cross-linking Li⁺ ions are coordinated to the network, it is questionable whether Li⁺ contributes at all to the total ionic conductivity.

We note here that while we discuss the distribution of Li⁺ on different sites as a static picture, there is continuous exchange between these sites. This is evident from observing only a single spectral line and single exponential in all diffusion as well as spin relaxation data, instead of two or more superimposed contributions. Since the time scale of exchange is rapid as compared to the experimental time scale, the resulting diffusion coefficients and spin relaxation rates are weighted averages over Li⁺ in different sites.

The role of the anion is interesting, as it provides some seemingly opposing trends with increasing MPC content: On the one hand the anion diffusion coefficient is increasing (see Figure 2a), while on the other hand the anion is contributing to the heterogeneous coordination environment of Li⁺, implying its immobilization close to MPC groups. Here, the Raman data shed some light on different anion fractions: The anion coordination number n per Li ion is decreasing from 1.81 to 0.72 (see Table 2). Thus, 60% of the anions previously contained in Li-anion clusters become liberated by Li-MPC interaction and contribute to anion diffusion enhancement. In contrast, 40% of anions remain coordinated to Li and become indirectly immobilized by MPC groups. These anions limit the diffusion enhancement, as the measured diffusion coefficient is a fast exchange average over both sites. Interestingly, the diffusion enhancement is rather similar for cation and anion (see Figure S5), demonstrating their mutual correlations. However, a slightly stronger enhancement of anion diffusion is observed, which is a signature of the partial cluster dissociation.

Implications for Transport

In summary, three distinct fractions of Li⁺ with different dynamic properties could be identified and their contributions estimated from different methods. The present system provides some interesting beneficial features, as the interactions of the p(MPC) polymer with Li⁺ can on the one hand enhance overall conductivity and can on the other hand provide mechanical stability by generating noncovalent cross-links. However, the interaction of Li⁺ with MPC units is so strong that it leads to immobilization and ultimately to a reduction of Li⁺ transport, as evidenced by the reduced Li⁺ diffusion coefficients and the resulting apparent transference numbers. While we cannot quantify the lifetime of the Li⁺-MPC coordination or assess potential contributions of Li⁺ transport along p(MPC) chain pathways, the enhancement of total ionic conductivity in the low p(MPC) content range is mostly due to the anions, which are liberated from their coordination to Li⁺ ions.

For a more beneficial ionogel system, it is desirable that the onset of ionic cross-linking would occur at a lower polymer content, where Li⁺ immobilization by MPC is not yet as detrimental. A strategy for improvement may lie in the architecture of the polymer: the present p(MPC) homopolymer bears a very high density of ZI units, and, according to the above discussion, this may favor local ionic clusters. Improved polymer architectures might involve copolymers with a reduced p(MPC) density along the chain. (52) This would reduce the fraction of Li⁺ being immobilized, while longer spacers between the MPC groups may enhance the accessibility of MPC groups for cross-linking, due to a more even distribution in space, which could shift the curve of f_Li to lower MPC content. Work along these lines is currently in progress.

Conclusions

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Our findings reveal that the incorporation of p(MPC) into the ionogel matrix significantly reduces the diffusivity of Li⁺ ions. This decrease suggests that Li⁺ ions are being partially immobilized due to complexation or physical cross-linking with the negatively charged phosphate groups of p(MPC). This Li⁺-MPC coordination is strong, and its relevance increases with p(MPC) content, at the cost of Li⁺-anion coordination, which is reduced. This liberates the anions, and probably reduces local viscosity within the IL/salt liquid phase, such that for both the IL cation and anion, an enhanced mobility contributes to a boost in total ionic conductivity. Interestingly, up to a medium p(MPC) content of 6 wt %, the number of immobilized Li⁺ ions per MPC group increases, indicating a cooperative effect. At this p(MPC) content, the fraction of Li⁺ ions acting as cross-linkers remains low, and a far larger MPC content is required for effective cross-linking. Considering the large number of immobilized Li⁺ per MPC at a still rather low fraction of cross-linking Li⁺ ions, it appears likely that a large fraction of Li⁺ undergoes a local coordination, possibly between neighboring MPC groups, which causes immobilization, while efficient cross-linking requires a larger MPC density with a beneficial spatial distribution. Thus, a guideline for promoting the role of Li⁺ ions as structural gelators is to dilute the ZI units on the chains in order to avoid local cross-linking and clustering. With this strategy, the strong immobilization of Li⁺ at the high charge density of the chain might be overcome, paving the way toward developing systems with a defined fraction of Li⁺ ions acting as cross-linkers, while still a sufficient fraction remains to ensure effective ion transport by mobile Li⁺ ions. This study highlights the significance of optimizing the composition and architecture of polyzwitterion-supported ionogels to achieve a good balance between mechanical integrity, Li⁺ ion transport, and ionic conductivity.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcb.5c06389.

Supporting data such as ¹H NMR spectra of dissolved ionogels, temperature-dependent diffusion coefficients, spin relaxation rates, deconvolution of Raman spectra and calculation of coordination number, estimation of cross-linking density from DMA data (PDF)

jp5c06389_si_001.pdf (751.43 kb)

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Author Information

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Corresponding Author
- Monika Schönhoff - Institute of Physical Chemistry, University of Münster, Corrensstr. 2830, 48149 Münster, Germany; https://orcid.org/0000-0002-5299-783X; Email: [email protected]
Authors
- Sajal Arwish - Institute of Physical Chemistry, University of Münster, Corrensstr. 2830, 48149 Münster, Germany
- Mossab K. Alsaedi - Tufts University, Department of Chemical & Biological Engineering, 4 Colby St., Medford, Massachusetts 02155, United States
- Ryan P. O’Hara - Tufts University, Department of Chemical & Biological Engineering, 4 Colby St., Medford, Massachusetts 02155, United States
- Ayse Asatekin - Tufts University, Department of Chemical & Biological Engineering, 4 Colby St., Medford, Massachusetts 02155, United States; https://orcid.org/0000-0002-4704-1542
- Matthew J. Panzer - Tufts University, Department of Chemical & Biological Engineering, 4 Colby St., Medford, Massachusetts 02155, United States; https://orcid.org/0000-0002-1741-8548
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.

Acknowledgments

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We would like to thank Professor David Kaplan in the Department of Biomedical Engineering at Tufts University for providing us with access to the texture analyzer used to obtain dynamic mechanical analysis (DMA) results. This project is funded by the DFG-NSF (German Science Foundation-US National Science Foundation) joint program CONFINE, project no. 509154483 (NSF award 2234243). The NMR spectrometer is funded by the DFG via proposal INST 211/999-1 FUGG, project ID 452849940.

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Abstract
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Scheme 1
Scheme 1. Structures of the Main Electrolyte Components (Top) and Illustration of Different Li⁺ Species within a p(MPC)-Supported Ionogel^a
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^aBridging Li⁺ that acts as noncovalent cross-linker between ZI units, non-bridging Li⁺ coordinated to a single ZI unit, and mobile Li⁺ in solution (bottom).
Figure 1
Figure 1. Total ionic conductivity at room temperature (25 °C) as a function of p(MPC) wt %.
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Figure 2
Figure 2. (a) Self-diffusion coefficients of the nuclei (¹H, ¹⁹F, ⁷Li) obtained from PFG-NMR at room temperature (25 °C) as function of p(MPC) content. (b) Activation energies of diffusion as a function of p(MPC) content measured over a temperature range of 25–65 °C. The dotted lines are merely guides to the eye.
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Figure 3
Figure 3. Li⁺ ion apparent transference number as function of p(MPC) content.
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Figure 4
Figure 4. (a) ⁷Li NMR spectra, (b) peak integral, (c) line width, and (d) ⁷Li spin–spin relaxation rate R₂ determined by a stimulated echo sequence (black) and R₂* determined by line width analysis (red star) for 1 M LiTFSI/BMP TFSI solution (0 wt % p(MPC)), 2 wt % p(MPC) solution, and 4–8 wt % p(MPC) free-standing ionogels.
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Figure 5
Figure 5. (a) Raman spectra and (b) Li⁺ coordination number (coordinated anions per Li⁺) as extracted from the fraction of coordinated anions in dependence on p(MPC) content. The green dotted line in a) marks the position of the band of coordinated TFSI^– and the black arrow represents the increase in p(MPC) content from 2 wt % (green spectrum) to 14 wt % (dark blue).
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Figure 6
Figure 6. (a) Elastic modulus values for ionogels containing 4 to 14 wt % p(MPC). (b) Calculated cross-link density within the ionogels, representing the number of Li⁺ ion-mediated noncovalent cross-links per nm³, and (c) the corresponding fraction of Li⁺ that is not participating in noncovalent cross-links.
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Figure 7
Figure 7. (a) Distribution of Li⁺ in different sites; (b) – (d) Li⁺ coordination scenarios at 0, 8, and 14 wt % p(MPC), showing free, immobilized, and cross-linking Li⁺ ions. For a legend of the symbols, see Scheme 1. Note that the MPC:Li⁺ molar ratio and number of coordinating anions are shown according to scale, while the total number of IL cations and anions is not to scale to enhance clarity.
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Supporting Information
Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcb.5c06389.
- Supporting data such as ¹H NMR spectra of dissolved ionogels, temperature-dependent diffusion coefficients, spin relaxation rates, deconvolution of Raman spectra and calculation of coordination number, estimation of cross-linking density from DMA data (PDF)
- jp5c06389_si_001.pdf (751.43 kb)
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The Role of Li+ Ions in Polyzwitterionic Ionogels: Gelator or Mobile Charge Carrier?Click to copy article linkArticle link copied!

The Journal of Physical Chemistry B

Abstract

Introduction

Scheme 1

Materials and Methods

Materials

Ionogel Preparation

Dynamic Light Scattering (DLS)

Ionic Conductivity

NMR Spectroscopy and Pulsed-Field Gradient NMR (PFG NMR)

Raman Spectroscopy

Dynamic Mechanical Analysis

Results and Discussion

In Situ Polymerization and Molecular Weight

Ionic Conductivity

Figure 1

Diffusion Coefficients of the Constituents

Figure 2

Li+ Ion Apparent Transference Number

Figure 3

7Li NMR Spectra and Spin–Spin Relaxation Rates

Figure 4

Raman Spectroscopy

Figure 5

Dynamic Mechanical Analysis (DMA)

Figure 6

Discussion of Li+ Sites

Figure 7

Implications for Transport

Conclusions

Supporting Information

Terms & Conditions

Author Information

Acknowledgments

References

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Abstract

Scheme 1

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

References

Supporting Information

Supporting Information

Terms & Conditions

The Role of Li⁺ Ions in Polyzwitterionic Ionogels: Gelator or Mobile Charge Carrier?
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Li⁺ Ion Apparent Transference Number

⁷Li NMR Spectra and Spin–Spin Relaxation Rates

Discussion of Li⁺ Sites