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Swelling Mechanism of Polyoxazoline-Based Gel Polymer Electrolytes for Lithium-Ion Batteries
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  • Mayank Pathak
    Mayank Pathak
    Institute of Macromolecular Chemistry of the Czech Academy of Sciences, Heyrovského nám. 2, 162 06 Prague, Czechia
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  • Andrii Mahun
    Andrii Mahun
    Institute of Macromolecular Chemistry of the Czech Academy of Sciences, Heyrovského nám. 2, 162 06 Prague, Czechia
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    Orcidhttps://orcid.org/0000-0003-1057-2910
  • Peter Černoch
    Peter Černoch
    Institute of Macromolecular Chemistry of the Czech Academy of Sciences, Heyrovského nám. 2, 162 06 Prague, Czechia
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  • Zuzana Morávková*
    Zuzana Morávková
    Institute of Macromolecular Chemistry of the Czech Academy of Sciences, Heyrovského nám. 2, 162 06 Prague, Czechia
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0001-8128-1040
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ACS Applied Polymer Materials

Cite this: ACS Appl. Polym. Mater. 2025, 7, 4, 2371–2383
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https://doi.org/10.1021/acsapm.4c03473
Published February 13, 2025

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Abstract

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Gel polymer electrolytes for lithium-ion batteries are typically prepared by swelling a preprepared cross-linked polymer network with a solution of lithium salt. This step of the electrolyte preparation is assumed to be straightforward, and very little attention has been paid to the investigation of the swelling mechanism. We questioned the common assumption that the feed solution swells the polymer network at the same concentration and fed a polyoxazoline network with several concentrations of lithium bis(oxalato)borate and lithium bis(trifluoromethane)sulfonimide solutions in propylene carbonate and diglyme. We thoroughly analyzed the obtained gels using NMR and Raman spectroscopy and evaluated the content of the solvent and salt in the polymer matrix, their mutual interactions, and mobilities. We found that this assumption was invalid and created a model of swelling of a polyoxazoline network with a salt solution. Our model implies the importance of often neglected solvent–polymer interactions or lack thereof. In the context of the function of the polyoxazoline system, our results provide insights into the efficiency of polymer matrices for gel polymer electrolytes, which will help avoid inefficient polymer matrices in the future.

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Introduction

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Environmental consciousness, ease of accessibility, cost-effectiveness, and human health concerns are evolving factors due to which consumers are shifting toward electric automobiles and portable/flexible electronics. (1) Battery is the main constituent of such advanced equipment. Li-ion batteries are the most used energy devices in the current days due to their high energy density, long cycle life, and portability. (2)
Among all the components of a battery system, the electrolyte plays a crucial role in determining the overall performance of the device. By the principle of the Li-ion battery, i.e., oxidizing/reducing lithium on the electrodes and transferring Li cations between them, an ideal battery electrolyte must be a good conductor of Li-ions, but electronically and anionically insulating. This main property is closely connected to the dissociation of the lithium salt and the interaction of the ions with other components of the system. The electrolyte should also be chemically, electrochemically, and thermally stable, should have low interfacial resistance (good wetting of the electrodes and formation of a suitable solid─electrolyte interface (SEI)), good mechanical properties (flexible and durable to serve as a separator as well), safe (nontoxic, nonflammable, and environmentally friendly), and affordable. (3)
Owing to their excellent ionic conductivity, liquid organic electrolytes, including propylene carbonate and ethylene carbonate, are generally used in commercial batteries; however, they have the disadvantages of flammability, explosivity, and other safety concerns. (4,5) The leakage of such compounds is also hazardous to the environment. (6) Solid electrolytes, on the other hand, suffer from low conductivities. (7) As a solution, gel polymer electrolytes (GPEs) are in focus to replace liquid electrolytes in next-generation batteries due to their decreased safety risks, wide electrochemical potential window, and comparable ionic mobility to liquid electrolytes. (7) In addition, their mechanical stability and flexibility allow their use in advanced flexible energy storage systems. (8)
GPEs are generally considered superior in terms of ionic conductivity (more than 10–4 S cm–1) to (dry) solid polymer electrolytes due to the better dissociation of salt and higher ion mobility. (9) GPEs are typically prepared by swelling the host polymer matrix in an electrolytic solution composed of electrolytic salt and a solvent at a specific concentration. (10) For Li-ion batteries, lithium salts like lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethane)sulfonimide (LiTFSI), lithium perchlorate (LiClO4), lithium chloride (LiCl), etc., are dissolved in an organic solvent with a high dielectric constant, such as carbonates or ethylene glycols (glymes), to make an electrolytic solution. (11)
In the GPE, lithium ions can be transported by two distinct mechanisms: vehicular and matrix-assisted. (12−15) The vehicular transport mechanism connects GPEs to liquid electrolytes. It is based on the free motion of an ion together with its solvation shell formed solely by the low-molecular component of the system (the solvent). It is generally faster, with ion mobility mainly dependent on the local viscosity of the environment and the size of the solvated ion. The matrix-assisted conduction mechanism, on the other hand, is typical for solid-state electrolytes. (14) The lithium ions interact with one or more coordination sites on the matrix and travel by continuously breaking the coordination at one site and coordinating to the next one. The ion mobility is given mainly by the coordination strength between the ion and the coordination site and the flexibility of the matrix. (14) In addition to salt dissociation, the overall conductivity of GPEs depends on the interplay of these two conduction mechanisms, and the high contribution of “liquid-like” vehicular conductivity is the main reason why GPEs perform generally better than solid electrolytes. (10)
Highly swollen gels allow a more liquid-like local environment, thereby providing better conductivity; on the other hand, they tend to be mechanically more fragile. This dilemma is the only aspect of swelling discussed in the literature so far. It has been assumed that the lithium salt solution swells in the matrix at its original concentration; therefore, the salt concentration giving the highest conductivity in the pure solution was used for GPE preparation. However, in a GPE, some of the ions are typically coordinated to the polymer matrix; therefore, the ion-pairing dependence on the salt concentration will differ. Similarly, the ideal salt/solvent ratio should also differed.
Numerous polymer materials, including poly(ethylene oxide), poly(acrylonitrile), poly(vinylidene fluoride), poly(vinylidene fluoride-co-hexafluoropropylene), and poly(methyl methacrylate), have been explored as the polymer host matrix in GPEs in the past. (11,16−19) Although they perform well in GPEs, these polymers face the challenges of environmental concerns and waste generation due to their nonbiodegradability. In this regard, scientists are exploring polymers that are sustainable, relatively less toxic, and biodegradable in nature for the greener GPEs. (18,20,21)
Polyoxazolines (POx) have been continuously researched as biocompatible and green polymer materials since their first use as food additives in 1966. (22) POx has also been successfully utilized in drug delivery, gene therapy, tissue engineering, and hydrogels owing to its alterable physicochemical characteristics. (23,24) However, very few reports have suggested its use as GPEs in energy storage devices. (25−27) This group reported POx-based GPEs synthesized using cationic ring-opening copolymerization (CROP) (25) followed by swelling in Li-salt solution and investigated their ionic conductivity and thermal properties. (27) In another work, Yang et al. explored POx as an additive for lithium–sulfur batteries. (28)
POx is a polymer that can illustrate the complexity of the swelling of an electrolyte solution into a polymer matrix. It is insoluble in pure solvents used in batteries; however, it swells in lithium salt solutions. Therefore, we intend to study the effect of the feed solution concentration on the effective lithum:solvent ratio in the polymer matrix, ion coordination in the system, and ionic mobility. We aim to explain the swelling process and put it in the context of the practical usability of GPEs.

Materials and Methods

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Materials

Butyronitrile (BN), methyl p-toluene sulfonate (MTS), barium oxide, benzoyl chloride, propylene carbonate (PC), 2-methoxyethyl ether (Diglyme, DG), ethylene glycol bis-mercaptoacetate (3-mercaptopropionate) (EGBM), and 2-hydroxy-2-methylpropiophenone (PI) were purchased from Merck. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(oxalato)borate (LiBOB) salts were purchased from Sigma-Aldrich (battery grade). Chloroform was obtained from Lach-Ner (Czech Republic). 2-Nonyl-2-oxazoline (NonOx) and 2-[dec-9′-enyl]-2-oxazoline (DecOx) were prepared according to the literature. (29) BN was stored over a molecular sieve (3 Å). The 2-oxazoline monomers were vacuum distilled twice: first over barium oxide and subsequently with 1 vol % benzoyl chloride. Freshly distilled monomers were stored under an argon atmosphere at 4 °C. All other chemicals were used as received.

Copolymers Synthesis

The choice of the solvent and the polymerization process have been described in detail in our previous work. (30) Briefly, poly[(2-nonyl-2-oxazoline)-co-(2-dec-9′-enyl-2-oxazoline)] (poly(NonOx-coDecOx)) copolymers were synthesized by living CROP (Figure 1) of 90% NonOx and 10% DecOx using BN as a solvent and MTS as the initiator (with a 1:100 molar ratio of initiator to monomers). Total monomer concentration in the solvent was in all cases adjusted to 3 M. (30)

Figure 1

Figure 1. Simplified scheme of polymer electrolyte sample preparation, indicating cationic ring-opening (co)polymerization (CROP) of 2-oxazolines, cross-linking, and swelling of the samples in 1 M LiTFSI salt solution in propylene carbonate.

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Preparation of Cross-Linked Polymeric Films

Cross-linking of solvent-casted polymeric films was based on a thiol-ene click reaction. (31) A dithiol with ester groups, EGBM, was used as a cross-linker. Briefly, the polymer (500 mg) was weighed and thoroughly dissolved in 1 mL of chloroform. In this solution, a temporary (swelling enhancer) solvent (PC, 5% w/v to dry polymer weight), cross-linker (EGBM, 100 mol % relative to the molar fraction of incorporated double bonds in the polymer), and PI (20 mol % relative to the molar fraction of double bonds in the polymer) were added and the contents were mixed well. The final solution was cast into a Teflon (PTFE) Petri dish with an inner diameter of 45 mm, covered with a watch glass, and allowed to evaporate slowly for 48 h in a hood. The dry films were consequently irradiated for 60 min in a home-built UV-B source device equipped with two 16 W light tubes. The cross-linked films were removed from the supporting PTFE dish and denoted as dry POx.
The cross-linked POx plates were cut into small pieces and allowed to swell in excess solutions, as shown in Table 1. The process of swelling was followed in detail for 1 M solutions of LiTFSI in PC, LiBOB in PC, and LiBOB in DG. The pieces of material were weighed before swelling and after 3, 7, 11, 14, and 20 days of swelling. Swelling was calculated using the following equation m(swollen)−m(dry)m(dry)×100%. Dry samples, obtained by cross-linking the respective copolymers, contained 5% w/v of PC. The swelling experiment was conducted in an argon-filled glovebox.
Table 1. Sample Notation of Swollen Samples
components/concentration0.5 M1 M1.5 Mgroup notation
LiTFSI in PC0.5M-LiTFSI-PC-POx1M-LiTFSI-PC-POx1.5M-LiTFSI-PC-POxLiTFSI-PC-POx
LiBOB in PC0.5M-LiBOB-PC-POx1M-LiBOB-PC-POx1.5M-LiBOB-PC-POxLiBOB-PC-POx
LiBOB in DG0.5M-LiBOB-DG-POx1M-LiBOB-DG-POx1.5M-LiBOB-DG-POxLiBOB-DG-POx
To prepare a dried sample, 1M-LiBOB-DG-POx-dried, a piece of 1M-LiBOB-DG-POx was left in a vessel sealed with parafilm, which is water-tight but permeable to DG, for a week at room temperature.

Pulsed-Field Gradient (PFG) NMR

The samples for NMR and Raman measurements were prepared in an argon-filled glovebox. The swollen polymer films were cut into small pieces and placed in predried NMR tubes. Further, to prevent the permeation of air and moisture into the NMR tubes, the tubes were hermetically flame-sealed.
7Li, 19F, and 11B PFG NMR experiments were conducted on a Bruker Avance III 600 spectrometer (14.1 T, ν(1H) = 600.2 MHz) using a DiffBB diffusion probe head and 40 A gradient amplifiers to investigate the diffusion of Li+ cations, TFSI–, and BOB– anions inside the POx matrix. A double-stimulated echo (DSTE) pulse sequence (32) was used to measure the self-diffusion coefficient D. The attenuation of the peak intensity is described by the following equation (33)
I=I0e−Dγ2g2δ2(Δ−δ/3)
(1)
where I is the observed integral intensity, I0 is the reference integral intensity (unattenuated signal intensity), D is the self-diffusion coefficient, γ is the gyromagnetic ratio of the observed nucleus, g is the gradient strength, δ is the length of the gradient, and Δ is the diffusion time. Experiments were carried out with a diffusion time Δ of 12–20 ms and a gradient duration δ of 1 ms, while the gradient strength (with a maximum of 11.38 T/m) was varied in 8 steps, providing more than 80% signal attenuation. The diffusion measurements were conducted in the temperature range of 295–353 K; after the change in the temperature, the samples were allowed to stabilize for at least 10 min (to ensure thermal equilibrium) before collecting NMR data. The self-diffusion coefficients were obtained by (lest-squares) fitting eq 1 using TopSpin 3.2 pl5 (Dynamic center) software. (34)

Raman Spectroscopy

FT-Raman spectra of all samples placed in NMR tubes, as described above, were measured using a Thermo Nicolet 6700 FTIR spectrometer with an FT-Raman module NXR (Nd:YAG laser 1064 nm) in backscattering geometry, a resolution of 2 cm–1, and 1024 scans per spectrum. Relevant regions were deconvoluted into Voigt bands using Omnic software. (35)
A band area ratio of selected bands was calculated and considered proportional to the physical system parameters such as the salt:solvent molar ratio. The band areas were obtained by deconvolution of the selected regions. As Raman spectroscopy is not inherently quantitative and a calibration model is needed, LiTFSI-PC, LiBOB-PC, and LiBOB-DG solutions at different concentrations were measured first. Then, the same band area ratios were calculated for the swollen GPEs and compared to those of the solutions.

Results and Discussion

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Swelling of the Polymer Matrix

POx displays a peculiar swelling behavior (Table 2): it does not measurably swell in neat PC or DG but swells in solutions of lithium salts. The final degree of swelling in the relatively well-working LiTFSI-PC system (27) was 144%, which was still quite dense. With the change in the anion from linear and flexible TFSI to bulky and rigid BOB, the swelling degree decreased slightly to 102%, and with the change of the solvent to DG, it drastically dropped to 12% (Table 2). The swelling degree thus depends on both the nature of the solvent and lithium salt.
Table 2. Swelling Degree of NonOx90DecOx10 Polymer Matrices Cross-Linked with EGBM and Swollen in 1 M Solutions of LiBOB in PC and DG, and LiTFSI in PC
 swellinga, %
time of swelling (days)1M-LiBOB-PC1M-LiBOB-DG1M-LiTFSI-PC
0000
332243
751595
11848135
1410212144
2010212144
a

Swelling was calculated using the equation: . Dry samples were obtained by cross-linking the respective copolymers, which already contained 5% w/v PC according to the procedure.

As the polymer matrix does not measurably swell in either of the pure solvents, the swelling must be driven by the dissolved salt (salting-in effect). (36) However, a large difference is observed with the change of the solvent, implying that solvent–salt–polymer interactions play a crucial role in the mechanism of swelling. This led us to further study the swelling mechanism in POx-based systems using PFG NMR and Raman spectroscopy.

Ion Mobilities

As mentioned in the Introduction, a well-performing battery electrolyte should not only have high conductivity but this conductivity should mainly be realized by Li cations. This property is described by the cationic transference number t+ (the fraction of the current transferred by the cations). As the concentrations of dissolved anions and cations are expected to be equal, t+ is dependent only on the cationic and anionic mobilities: t+ = (D+/(D++D–)), (37) where D+ and D– are the self-diffusion coefficients of the cation and anion, respectively. (14) Therefore, the faster the diffusion of the cation and the slower the diffusion of the anion, the higher the observed cation transference number and the better the electrolyte. The diffusion coefficients of the individual components were obtained by using PFG NMR experiments.
An attempt has been made to evaluate the mobilities of individual ions in 1 M systems. For the 1M-LiBOB-DG-POx sample, it is not possible to obtain the diffusion coefficients due to its low swelling degree and, as a result, a low concentration of the observed nuclei. This led to a poor signal-to-noise ratio (SNR) during the NMR diffusion experiments.
For the 1M-LiTFSI–PC-POx and 1M-LiBOB–PC-POx systems, the diffusion coefficients of the anions were determined over a relatively wide range of investigated temperatures (20 to 80 °C), whereas the diffusion coefficients of the Li+ cation were detectable only at higher temperatures (Figure 2). The diffusivities of the cation were not detectable at lower temperatures for two main reasons: (i) low rate (at the edge of the instrumental capabilities) of cation diffusion at those temperatures, and (ii) short spin–spin (T2) 7Li relaxation times, which contributes to the broadening of the NMR signal and deterioration of its SNR.

Figure 2

Figure 2. Temperature dependence of self-diffusion coefficients of Li cations (empty points) and corresponding anions (solid points) obtained by PFG NMR for 1M-LiTFSI-PC-POx and 1MLiBOB-PC-POx. Solid lines are the guidelines.

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The obtained temperature dependencies of the self-diffusion coefficients (Figure 2) reveal an increase of the self-diffusion coefficients with increasing temperature caused by the faster thermal motion of the species and the decrease of propylene carbonate viscosity. (38) Additionally, it was revealed that the diffusion rate (at the same temperature) of the cations is much smaller than that of the anions. This can be explained by the fact that Li+ cations interact with both solvent and polymer molecules (segments) while the anions stay (to a high extent) uncoordinated. The interactions of the cation with the solvent molecules led to the formation of a solvation shell, resulting in an increase in the effective cation radius. This, in turn, results in a decrease in the Li+ diffusion rate. Moreover, the coordination of cations to electronegative atoms (e.g., oxygen) from the polymer matrix further contributes to the decrease in their migration rate. It was also observed that in the case of the LiTFSI salt, both anions and cations diffused faster compared to the LiBOB salt (Figure 2). This might be addressed to different degrees of salt dissociation and as a result of different numbers of “free” ions, ion pairs and clusters, and immobilized ions (see the next paragraph) and the fact that in a PFG NMR experiment, diffusivities of all mobile moieties of interest are averaged. In addition, the obtained diffusion coefficients at a fixed temperature correlate well with the swelling of the samples; the higher the swelling of a sample, the higher the diffusion coefficients observed. Thus, 1M-LiTFSI-PC-POx displayed a higher diffusion coefficient for both ions than 1M-LiBOB-PC-POx, while 1M-LiBOB-DG-POx was unmeasurable.
It is noteworthy that PFG NMR allows the detection of only the mobile species, and it is not possible to quantitatively evaluate how large fractions of the ions are highly immobilized and thus not reflected in the above-mentioned measurements. As cation immobilization is commonly mentioned as an issue in polymer-based electrolytes, (13,15,39) it must be noted that there may be two types of ions present in the system: mobile and highly immobilized. As no mobile cations were detected at lower temperatures, we can assume that most cations were immobilized on the polymer matrix and became mobile only at higher temperatures. In order to investigate the ion–solvent–polymer interactions in more detail and gain insights into the POx swelling mechanism and ion migration in the prepared GPEs, Raman spectroscopy was comprehensively employed (see the following sections).

Salt–Solvent Molar Ratio

Raman spectroscopy is a promising tool to detect the specific bonding states of ions. (40,41) To test the hypothesis that the POx swells by the salting-in mechanism and the molar ratio between the salt and solvent in the polymer matrix differs from that in the feed solution, we have analyzed the intensities of typical Raman bands of the free anion, free solvent, solvent coordinated to the cation, and ion pairs, where possible.

LiTFSI-PC Series

The LiTFSI-PC solutions are well described in the literature (42) and allow for a simple analysis of the coordination state of the anion and solvent using Raman spectroscopy (Figures 3a and S1a). The ring-stretching band of PC at 712 cm–1 is known to be sensitive to interactions, and upon coordination with the Li cation, it shifts to 725 cm–1. (43−45) A growing band at this position is indeed observed in the spectra of solutions with increasing concentrations (Figures 3a, S1a, S2i, and S6d,e,g,h). The CF3 deformation of the TFSI anion, which is sensitive to ion association, occurs in the same region: free anions produce a band at 741 cm–1 and contact ion pairs (CIP) at 748 cm–1. (46−48) Such ion pairing was not observed in the solutions; however, the center of the band shifted to 742 cm–1 with increasing concentration (Figures 3a, S1a, S2i, and S6d,e,g,h), which may be related to the presence of solvent-separated ion pairs (SSIP). (49) As POx does not have any significant Raman features in this region, it serves well for the evaluation of the PC–Li and TFSI–Li interactions in the swollen polymer (Figures 3a, S1a, S2i, and S6a).

Figure 3

Figure 3. Raman spectra of the LiTFSI-PC and LiTFSI–PC-POx sample series in the region 685─760 cm–1 (a). Example of deconvolution of this spectral region for 2.0M-LiTFSI-PC (b). Concentration dependencies of the (I742+I748)/(I712+I725) ratio in the LiTFSI-PC and LiTFSI–PC-POx series together with the I742/(I712+I725) and I748/(I712+I725) ratios in the LiTFSI–PC-POx series (c); I725/I712 ratio in the LiTFSI-PC and LiTFSI–PC-POx series (d).

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The concentration of the salt in PC was calculated as the ratio of free TFSI or ion pairs to the sum of the PC-related bands (I742+I748)/(I712+I725) of band areas obtained by deconvolution of the 680─770 cm–1 region to Voigt bands (Figures 3b and S6). The data based on the solutions lie on a single line (Figure 3c). After swelling in the polymer, CIP became visible (Figures 3a,c, S1a, and S2i). The molar ratio of TFSI and PC does not seem to depend on the feed solution concentration, reaching out of the scale settled by solutions with double to triple the molar ratio of the 2 M solution, i.e., equivalent to the 4─6 M solution (however, the calibration is not linear in this range). The majority of the TFSI anions present in the swollen polymer environment are free (the band position at 741 cm–1 suggests the SSIP do not singificantly contribute), although CIP is present (Figure 3c).
The degree of lithium coordination to PC was visualized using the ratio of the bound and free PC ring-stretching band areas, I725/I712 (Figure 3d). Again, all solutions lie roughly on one curve. In swollen polymers, similar to the TFSI content, the ratio of coordinated PC does not seem to be related to the feed solution concentration and corresponds roughly to a 2 M solution. Here, the salt content appears lower than that indicated by the TFSI/PC ratio because lithium is also coordinated to the polymer matrix.

LiBOB-PC Series

When the TFSI anion was switched for BOB, the situation became complicated because the molecular structures of the BOB anion and PC were very similar, and most of their Raman bands overlapped (Figures S1b, S6b,q, and S7a,j). This included the above-mentioned marker bands (Figure 4a). The O–B–O deformation coupled with ring breathing appears at 742 cm–1 for CIP and around 727 cm–1 for the free anion, (50) overlapping with the marker of coordinated PC at 725 cm–1. In addition, the O–B–O deformation band observed in solid LiBOB coincides with the ring-stretching band of free PC at 712 cm–1. A group of BOB– bands can be observed in the low-frequency region at 310 cm–1 (torsion around the boron atom, all BOB– forms), 347 cm–1 (O═C–C═O bending, position typical for free BOB–anion), and 374 cm–1 (breathing mode, all BOB– forms) (Figure 4b). (50) However, no suitable band connected to the PC, not overlapped with BOB features, which could be used for the evaluation of the Li-PC interactions, was found. Therefore, the area ratio of the bands at 725 and 713 cm–1 was selected to correlate with the molar ratio between LiBOB and PC (Figures 4c and S6), although it is not directly proportional to a reasonable physical system descriptor (Figure 4d). The solutions lie on a single line. The swollen polymer samples, however, display a much higher I725/I713 ratio out of scale of the calibration, roughly corresponding to a 15 M solution (however, the calibration curve is most likely not linear in this range).

Figure 4

Figure 4. Raman spectra of the LiBOB-PC and LiBOB-PC-POx sample series in the regions 685─760 cm–1 (a) and 285─395 cm–1 (b). Example of deconvolution of the PC-relevant spectral region of 2.0M-LiBOB-PC (c). Concentration dependence of the I725/I712 ratio in the LiBOB-PC and LiBOB-PC-POx series (d).

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LiBOB-DG Series

LiBOB solutions in DG are easier to analyze using Raman spectroscopy than those in PC (Figure S1c). The O–B–O deformation coupled with ring breathing of the free BOB anion appears at 724/725 cm–1 and does not coincide with any band of DG or POx (Figures 5c, S2i, and S6). In addition, a small peak of O–B–O deformation in the free BOB anion at 704 cm–1 is observed. The group of bands above 300 cm–1 is also well resolved, with the 374 cm–1 one being the least overlapped with the broad feature of DG around 330 cm–1 (Figures 5d, 6d, S2k, and S7).

Figure 5

Figure 5. Raman spectra of the LiBOB-DG and LiBOB-DG-POx sample series in the regions 2690─3100 cm–1 (a), 775─910 cm–1 (b), 685─760 cm–1 (c) and 285─395 cm–1 (d). The 2 M solution was separated into an opaque heavier phase and a lighter solution phase; Raman spectra were obtained near the solution level (top) and near the bottom of the cuvette (bottom).

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Figure 6

Figure 6. Concentration dependencies of the DG CH3 stretching band position and area normalized to the total area of the CH-stretching region for the LiBOB-DG series (a). Example of deconvolution of the CH-stretching spectral region of 2.0M-LiBOB-DG (b). I375/I2827, I348/I2827, and I880/I2827 ratios in the LiBOB-DG and LiBOB-DG-POx series (c). Example of deconvolution of the BOB anion-related spectral region of 2.0M-LiBOB-DG (d). The 2 M solution was separated into an opaque heavier phase and a lighter solution phase, and the data calculated from the spectrum measured near the solution level are displayed as 1.9 M and from the bottom of the cuvette as 2.1 M for clarity. The spectra near the solution level were used for the deconvolution examples.

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The DG CH2 rocking bands in the region 800─900 cm–1 are known to be sensitive to the conformation of the molecule; the maximum at 880 cm–1 is connected to the crown-ether-like conformation formed when coordinated to Li ions. (51) The growth of the 880 cm–1 maximum is indeed observed with increasing salt concentration (Figures 5b, S1c, S2h, and S5). However, this region, as well as other DG bands sensitive to Li-coordination, overlap with the Raman bands of POx (Figures S1c and S5). The only DG-related band distinguishable in the spectra of the LiBOB-DG-POx series that does not overlap with any other feature is CH3 stretching at around 2827 cm–1 (Figures 5a, S1c, S2a, and S3). This band shifts with increasing salt concentration of the solution from 2822 to 2834 cm–1 while its area normalized to the sum of the CH stretching region remains constant (Figures 6a,b and S3). In the spectra of the swollen samples, it appears only as a faint shoulder, and its exact position could not be evaluated very reliably. Therefore, we do not describe the Li–solvent interaction in either the LiBOB-PC-POx or the LiBOB-DG-POx series.
The content of LiBOB can be determined using the area of the band at 372 cm–1 (O═C–C═O deformation coupled with ring breathing) shared by both the free BOB anions and ion pairs, divided by the area of the DG-related band at around 2827 cm–1. A good linear dependence is observed (Figure 6c). The band at 348 cm–1 (O═C–C═O deformation), which is related to free anions, has a very similar concentration dependence; therefore, a strong contribution of CIP is not expected. The concentration dependence of the area of the band at 880 cm–1 related to the Li-coordinated DG skeletal vibration (52−54) is also practically linear as well.
After swelling to the POx matrix, the Li─DG marker band at 880 cm–1 was covered with the POx bands at 870 and 890 cm–1 (Figures 5d, S2h, and S5). The I375/I2827 and I345/I2827 band area ratios are, again, out of the scale obtained for pure solutions (Figure 6c), but this time, they depend on the feed solution concentration. These ratios (and thus the molar ratios) appear to increase by around 4× for 375 cm–1 and 7× for 345 cm–1 after swelling to POx. The obtained values are completely out of the scale obtainable for solutions and thus display only a trend.

Salt–Solvent Molar Ratio Summary

For all salt–solvent combinations, the salt–solvent molar ratio in the GPE is much higher than that in the feed solution, out of the scale achievable by solution calibration.
For solutions in PC, the salt–solvent molar ratio in the GPE is independent of the concentration of the feed solution, while there appears to be dependent in LiBOB-DG systems.

Coordination and Swelling

Li–POx Coordination

In GPEs, the Li ions are also coordinated to the polymer matrix besides the solvent (and anion). In the case of POx, the main coordination site is the amide carbonyl. Free amide carbonyl is observed at 1643 cm–1 (Figures 7e, S2b, and S4) and shifts to 1661 cm–1 upon Li-coordination. (27) The 1661 cm–1 shoulder is observed in the spectra of all swollen samples in this study (Figures 7a–c,f and S4). Li ions can coordinate with the oxygen and sulfur atoms of the cross-linker as well; however, these groups do not produce strong distinguishable bands in the Raman spectra (for example, the band of ester carbonyl should appear around 1730 cm–1, but is too weak to be distinguished (Figures 7 and S4)).

Figure 7

Figure 7. Raman spectra of the LiTFSI-PC-POx (a), LiBOB-PC-POx (b) and LiBOB-DG-POx (c) sample series in the region 1560─1900 cm–1. The I1661/I1643 band area ratio of the POx amide (d). Example of deconvolutions of the carbonyl stretching region of dry POx (e) and 1.5M-LiTFSI-PC-POx (f).

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Within the established theme, the percentage of bound amide carbonyl (I1661/(I1643+I1661) × 100%) does not depend on the concentration of the feed solution. It does not seem to depend much even on the type of salt and solvent used (1M-LiBOB-DG-POx is probably an outlier) and remains at around 18%.
The effect of swelling POx with the salt solution consists of direct interaction with the cation and the effect of increased volume on the chain conformation. To separate these two effects, we prepare a GPE with different salt:solvent molar ratios. As this ratio is not dependent on the feed solution and thus cannot be controlled by the swelling process, we used a trick to dry the GPE. The sample became visibly thin and whitened. As can be seen from the carbonyl region of the spectra (Figure 8a), water was not adsorbed on the sample.

Figure 8

Figure 8. Raman spectra of the selected LiBOB-DG and LiBOB-DG-POx samples in the regions 1850─1560 cm–1 (a), 200─100 cm–1 (b), 3100─2720 cm–1 (c), 915─805 cm–1 (d), 1340─1255 cm–1 (e), and 1100─950 cm–1 (f).

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The first point to observe in the spectrum of dried GPE (Figure 8a, S1c, S2b, and S4) is the high intensity of the BOB anion carbonyl bands at 1828 and 1777 cm–1. A similar increase is also observed for other BOB anion Raman bands at 950, 725, 373, 345, and 311 cm–1 (Figure S2i,k, S6, S7, Tables S1 and S2). The position of the 725 cm–1 band is between the positions observed in solutions and in the solid state, which suggests extensive ion pairing. (50) In addition, new bands were observed at 114 and 154 cm–1, roughly coinciding with the bands observed for solid LiBOB (Figures 8b and S7). These bands were not observed for the LiBOB solutions and were probably active only in higher ionic aggregates or in the absence of anion solvation. Their positions correspond to torsional vibrations of the anion molecule. (50)
A possible explanation for our observations can be connected, together with the lost transparency of the sample, with phase separation to a phase of dry polymer and a salt-rich system, where the salt is quasi-solid. Such material is more scattering, which leads to (1) an increased overall Raman signal and (2) higher white light scattering and white color of the sample.

Detailed Study of the DG State in the Dried System

As mentioned earlier, the CH3 stretching band at around 2827 cm–1 is the only DG band that is reasonably distinguished in the swollen samples. After drying, the shoulder on the low-frequency side of the CH-stretching region shifts to higher positions (Figures 8c and S3m), which is consistent with the behavior observed in the solutions (Figures 5a, 6a, S2a, and S 3f,i). However, its intensity increases ca. 10-fold. If we look at other bands where Li-coordinated DG contributes (3000 and 2934 cm–1), an increase is also observed after drying. On the other hand, the band related to free DG at 2983 cm–1, faintly reflected in the spectrum of fresh GPE, almost disappears upon drying (Figures 8a, S2a, and S3m).
In the DG CH2 rocking region (Figures 8c and S2h), where nothing could be observed in fresh GPEs, the typical band of Li-coordinated DG at 880 cm–1 was well distinguished after drying, even dominating the region (Figure S5f). Another maximum at 827 cm–1, which increased with increasing solution concentration, could be resolved as well. Two new bands appear at 900 and 850 cm–1, which are not been observed previously (Figure S5f).
The increase of all coordination-related bands is caused by two effects: (1) (almost) all DG molecules are now coordinated, and (2) the phase separation discussed above. The new bands may be connected to a superconcentrated phase, perhaps a cocrystal of DG and LiBOB, or an ordered structure around the amide coordination site. Cocrystals of LiBOB with other solvents have been reported before. (55)
Other bands of DG, which shift with the change in conformation connected with coordination, can be found at 1472, 1450, and 530 cm–1 for free DG and at 1475 and 572 cm–1 for Li-coordinated DG (Figure S2c,j).

Detailed Study of the Polymer Matrix State in the Dried System

The fraction of the bound amide carbonyl band increased to ∼30% after drying (Figure 8a). A similar effect is observed on the amide deformation band at 1300 cm–1, which shifts very slightly to a higher frequency upon swelling with lithium salt, and upon drying, it shifts even further to 1303 cm–1 (Figures 8e and S2d). These are the only direct interactions of the POx matrix, and both interact with lithium ions. No direct interaction with diglyme is observed, and its effect on the polymer matrix is manifested only through the change in the chain conformation on the volume change connected with swelling and drying. The conformation of the POx chains is reflected mainly in the region of straight-chain skeletal vibrations (Figures 8f, S2, Tables S1 and S2). The band at 1080 cm–1 increases both upon swelling and drying, which is thus probably related to the chain segments near the interaction site (Figure S2f). The band at 1028 cm–1 redshifts to 1026 cm–1 upon swelling, but blueshifts back upon drying (Figure S2f). This suggests its relation to sections of aliphatic chains that are simply stretched due to the volume change associated with swelling.

Summary of the Coordination and Swelling Analysis

As all bands related to the LiBOB salt (BOB anions, coordinated DG, and coordinated POx vibrations) increase upon drying, while the total content of these species compared to the noncoordinated POx (the rest of the sample) should not increase, we suggest that this intensity increase is related to the phase separation associated with the increased scattering efficiency from the salt-rich phase (both Raman and elastic scattering, and whitening of the sample).
This phase is solid-like rather than solution-like. The lithium cation remains coordinated to the amide site of POx, and the residual DG, together with the BOB counterion, shrinks around it to form a compact coordination shell.
The lithium cation with its coordination sphere, either full or dried, stretches the polymer segments around the coordination site occupied by the lithium cation.
The dried sample allowed us to identify the band at 1028/1026 cm–1 as related solely to swelling of the polymer matrix: stretching of the aliphatic side chains due to the volume transition. No direct interactions of the polymer matrix with the anion or the solvent were observed. It was also possible to identify other interaction-related features beyond the known interaction markers within the forest of the GPE bands (Tables S1 and S2).

Discussion of the Polymer Swelling Mechanism

The fact that the swelling of both LiTFSI and LiBOB solutions in PC displayed minimal dependence on the feed solution concentration and that the molar ratio of salt to solvent in the polymer matrix was drastically higher than that in the feed solution supports the salting-in swelling mechanism. The leading interaction here is the coordination of lithium to the polymer (amide groups of the polymer backbone, oxygen, and sulfur atoms of the cross-linker). This is also consistent with undetectable swelling in pure solvents. The lithium cations drag their solvation shells and counterions with them into the polymer matrix, leading to the preferred polymer–salt–solvent molar ratio in the polymer matrix. At room temperature, the lithium ion coordinates with POx, and its coordination sphere is formed by some solvent and the anion (probably in the form of SSIP), stretching the polymer matrix. The choice of solvent and anion dictates the volume of the Li-coordination sphere. As the DG molecule wraps around the cation to a crown-ether-like conformation and forms three coordination links to Li+, (50) fewer DG molecules are necessary to form a full solvation shell around Li+ than PC, which is rigid and coordinates singly via its oxygen-rich end. (44) The solvation shell with PC is thus larger in volume. Consequently, the swelling degree of the GPE is also higher with PC. If the swelling degree is higher, then there is more space for movement and the mobilities of all the components of the system are higher.
All swollen GPEs have similar rather low fractions of coordinated amide carbonyls. We suppose that, for some reason given by the POx matrix itself, this is the fraction of accessible amide groups, and the rest is sterically blocked. This is related to cross-link density (here constant for all samples). As in our previous work, we observed that the fraction of the bound carbonyl is dependent on the decenyloxazoline:nonyloxazoline ratio in the polymerization mixture. (27) The amide groups of POx are located at the polymer backbone, and inert aliphatic chains are attached to it in the comb architecture. This limits the steric accessibility of the amide sites. A higher cross-link density limits the mobility of the side chains, which probably blocks the amide sites even more.
This mechanism suggests that there are not many lithium ions that are coordinated only to solvent molecules and thus would be mobile. Indeed, at room temperature (the conditions of the swelling and Raman measurements), NMR spectroscopy could not detect lithium species with measurable mobility in any of the swollen polymer samples. If the polymer matrix is stretched enough by the salted-in solution (high degree of swelling), then there can theoretically be space for the free solution, but this was not observed.

Conclusions

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We confirmed that the swelling of POx GPEs is driven by a salting-in mechanism through lithium coordination to the polymer matrix. Thus, no mobile lithium ions were present at room temperature. The specific choice of anions and solvent dictates the volume of the lithium coordination sphere, which consequently influences the degree of swelling and mobilities of all components of the system.
Generally, the cation-driven swelling mechanism is not very beneficial for a battery GPE. It has been shown previously that polymers with anion-coordinating groups (where swelling is probably driven by anions rather than cations) are more efficient than traditional lithium-coordinating polymers. (56,57) Our work points to the fact that if the swelling of the polymer matrix is driven by cation–polymer coordination, the cations will be trapped, whereas if the polymer matrix is able to swell even in a pure solvent, free cations will also be present in the liquid phase of the polymer gel that can travel through the faster vehicular mechanism. Consequently, such materials must be more efficient. This conclusion gives our first recommendation: avoid polymer matrices that do not swell in pure solvents you intend to use.
We showed that only about 18% of the amide groups of POx are accessible for Li-coordination. This is most likely due to the steric hindrance caused by the comb-like architecture of the POx matrix. The effect of the polymer matrix architecture deserves further study; however, our observations suggest that the main chain of a polymer comb or brush is not the ideal location for a cation coordination site.

Data Availability

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The data underlying this study are openly available in the ASEP repository at [DOI 10.57680/asep.0605036, https://hdl.handle.net/11104/0362661].

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsapm.4c03473.

  • All full-range Raman spectra; all details of the relevant regions of the Raman spectra; examples of deconvoluted regions of Raman spectra; tables of Raman band positions, evolution, and attribution (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Mayank Pathak - Institute of Macromolecular Chemistry of the Czech Academy of Sciences, Heyrovského nám. 2, 162 06 Prague, Czechia
    • Andrii Mahun - Institute of Macromolecular Chemistry of the Czech Academy of Sciences, Heyrovského nám. 2, 162 06 Prague, Czechia;  Orcidhttps://orcid.org/0000-0003-1057-2910
    • Peter Černoch - Institute of Macromolecular Chemistry of the Czech Academy of Sciences, Heyrovského nám. 2, 162 06 Prague, Czechia
  • Author Contributions

    M.P.: Writing–original draft, investigation, formal analysis, validation, and visualization. A.M.: writing–original draft, methodology, investigation, data curation, validation, and visualization. P.Č.: methodology, investigation, and visualization. Z.M.: writing–original draft, writing–review and editing, methodology, data curation, validation, formal analysis, conceptualization, visualization, and supervision.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors gratefully acknowledge Dr. Sabina Abbrent for numerous discussions and the Institute of Macromolecular Chemistry of the Czech Academy of Sciences for the Institutional Support RVO: 61389013.

References

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    A review. Exploring highly foldable batteries with no safety hazard is a crucial task for the realization of portable, wearable, and implantable elec. devices. Given these concerns, developing solid-state batteries is one of the most promising routes to achieve this aspiration. Because of the excellent flexibility and processability, polyvinylidene fluoride (PVDF) based electrolytes possess great potential to pack high energy d. flexible batteries, however, suffers the various intrinsic shortcomings such as inferior ionic cond., a high degree of crystallinity, and lack of reactive groups. Clearing the progress of the present state and concluding the specific challenges faced by PVDF based electrolytes will help to develop PVDF based polymer batteries. In this review, we summarize the recent progress of gel polymer electrolytes and all solid polymer electrolytes based on PVDF. The ion transport mechanisms and prepn. methods of PVDF based electrolytes are briefly introduced. Meanwhile, the current design principle and properties of electrolytes are highlighted and systematically discussed. Some peculiar modified strategies performed in lithium-sulfur batteries and lithium-oxygen batteries are also included. Finally, this review describes the challenges and prospects of some solid-state electrolytes to provide strategies for manufg. high-performance PVDF electrolytes aimed at practical application with flexible requirements.
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    Poly(2-oxazoline)s: A comprehensive overview of polymer structures and their physical properties
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    A review. Poly(2-oxazoline)s (PAOx) are of increasing importance for a wide range of applications, mostly in the biomedical field. This review describes the synthesis of 2-oxazoline monomers and their cationic ring-opening polymn., and gives a comprehensive overview of all reported PAOx homopolymers. In the second part of the review, the polymer properties of these PAOx homopolymers with varying side-chain structures are discussed. Altogether, this review intends to serve as an encyclopedia for poly(2-oxazoline)s enabling the straightforward selection of a polymer structure with the desired properties for a certain application. © 2017 Society of Chem. Industry.
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    European Polymer Journal (2022), 178 (), 111484CODEN: EUPJAG; ISSN:0014-3057. (Elsevier Ltd.)
    A review. Poly(2-oxazoline) s are emerging innovative biomaterials that show analogous and even preferable properties compared to well-known counterparts, esp. poly(ethylene glycol). This polymer can be synthesized in a living and hence controlled manner using the cationic ring-opening polymn. method. In addn., various polyoxazolines are water-sol. or amphiphilic and relatively non-toxic, making them attractive as biomaterials. Polyoxazolines (10 and 20 kDa) are safe when used i.v. to rats and can be used for several biomedical applications. These results from various papers and patents conclude that POx is a favorable biopolymer. The present review aims to highlight the synthesis, structure, and properties of polyoxazolines, such as different derivs. of POx used in tissue engineering. Different polymn. methods and smart behaviors of POx are described as well. Finally, it aims to introduce the significance and potential of POx in biomedical applications like drug delivery, protein delivery, and gene delivery using a no. of leading refs. of recent years.
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    Drews, M.; Trötschler, T.; Bauer, M. Photocured Cationic Polyoxazoline Macromonomers as Gel Polymer Electrolytes for Lithium-Ion Batteries. ACS Appl. Polym. Mater. 2022, 4, 158– 168,  DOI: 10.1021/acsapm.1c01171
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    Photocured Cationic Polyoxazoline Macromonomers as Gel Polymer Electrolytes for Lithium-Ion Batteries
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    ACS Applied Polymer Materials (2022), 4 (1), 158-168CODEN: AAPMCD; ISSN:2637-6105. (American Chemical Society)
    In this work, we present a cationic vinylimidazolium-terminated poly(2-ethyl-2-oxazoline) (PEtOx) macromonomer as a key component of gel polymer electrolytes (GPE) for lithium-ion batteries. GPE prodn. followed a scalable process based on UV curing of the cationic PEtOx macromonomer with polyfunctional acrylic comonomers dissolved in an org. electrolyte (LP30), affording electrolyte-swollen polymeric ionic liq. (PIL) networks with PEtOx side chains. Thus, cathodes coated with a GPE layer of less than 200μm thickness were readily manufd. The PIL brush-type GPE is highly insol. but swellable in LP30 and exhibits pronounced electrolyte retaining ability against evapn. At 160°C, the wt. loss of the GPE amounted to around 5%. This is 12% less compared to an LP30-soaked com. Celgard separator. At room temp., the ionic cond. was 3.6 x 10-4 S/cm, surpassing that of a comparable Celgard/LP30 system. Contrary to LP30 in Celgard, cond. measurements for the PIL brush GPE did not indicate any crystn. of the liq. electrolyte at subambient temps. This was confirmed by differential scanning calorimetry, suggesting improved ionic mobility in the GPE over a wide temp. range. The electrochem. stability window of the PIL brush GPE is wide enough and fits all common lithium-ion cathode materials. In fact, the GPE exhibited exceptional oxidative stability of 5.2 V vs Li/Li+. Half-cell cycling expts. using a lithium iron phosphate cathode revealed high capacity values of 150 mAh/g at a current rate of C/10. When the current was increased to C/2, the capacity decreased to 120 mAh/g and the cell reached 80% of its initial capacity (referred to C/2) after 180 cycles. Thus, according to the first physicochem. and electrochem. investigations, the PEtOx-based PIL brush GPE represents a promising candidate with respect to lithium-ion battery operation.
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    A review. Following Sharpless' visionary characterization of several idealized reactions as click reactions, the materials science and synthetic chem. communities have pursued numerous routes toward the identification and implementation of these click reactions. Herein, the authors review the radical-mediated thiol-ene reaction as one such click reaction. This reaction has all the desirable features of a click reaction, being highly efficient, simple to execute with no side products and proceeding rapidly to high yield. Further, the thiol-ene reaction is most frequently photoinitiated, particularly for photopolymns. resulting in highly uniform polymer networks, promoting unique capabilities related to spatial and temporal control of the click reaction. The reaction mechanism and its implementation in various synthetic methodologies, biofunctionalization, surface and polymer modification, and polymn. are all reviewed.
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    A derivation is given of the effect of a time-dependent magnetic field gradient on the spin-echo expt., particularly in the presence of spin diffusion. There are several reasons for preferring certain kinds of time-dependent magnetic field gradients to the more usual steady gradient. If the gradient is reduced during the radio-frequency pulses, H1 need not be particularly large; if the gradient is small at the time of the echo, the echo will be broad and its amplitude easy to measure. Both of these relaxations of restrictions on the measurement of diffusion coeffs. by the spin-echo technique serve to extend its range of applicability. A pulsed gradient can be recommended when it is crit. to define the precise time period over which diffusion is being measured. The theoretical expression derived was verified exptl. for several choices of time-dependent magnetic field gradient. An app. is described suitable for the production of pulsed gradients with amplitudes as large as 100 gauss cm. The diffusion coeff. of dry glycerol at 26° ± 1° is (2.5 ± 0.2) × 10-3 cm.2/sec., a value smaller than can ordinarily be measured by the steady gradient method.
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    Concentration Polarization of a Polymer Electrolyte
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    Journal of the Electrochemical Society (2002), 149 (8), A1015-A1019CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    The salt concn. in a concd. binary polymer electrolyte was measured in situ by confocal Raman spectroscopy during galvanostatic polarization expts. The electrolyte studied was 0.8M lithium bis(trifluoromethanesulfonyl)imide in a copolymer of ethylene- and propylene oxide at 25°. Recent work with a transport model and characterization of the transport properties, for the same electrolyte, was verified with the spectroscopic results of this study. A good agreement between modeled and measured results was found. The spectroscopic method suited well for these studies. The possibilities of using a transport model are briefly demonstrated and discussed.
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    Hofmann, A.; Schulz, M.; Hanemann, T. Gel electrolytes based on ionic liquids for advanced lithium polymer batteries. Electrochim. Acta 2013, 89, 823– 831,  DOI: 10.1016/j.electacta.2012.10.144
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    Gel electrolytes based on ionic liquids for advanced lithium polymer batteries
    Hofmann, Andreas; Schulz, Michael; Hanemann, Thomas
    Electrochimica Acta (2013), 89 (), 823-831CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
    A new ionic liq.-based polymer gel electrolyte for Li ion batteries is prepd. by combining N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)azanide, org. carbonates, Li bis(trifluoromethylsulfonyl)azanide, and poly(vinylidene fluoride-co-hexafluoropropylene). The conductivities of the gel electrolytes at room temp. are ∼1-2 mS/cm. The polymer gel electrolytes work effectively with a graphite anode and a LiCo1/3Mn1/3Ni1/3O2 cathode. Further the addn. of vinylene carbonate and 4-vinylpyridine improves cell performance.
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    Journal of Physical Chemistry (1993), 97 (22), 5826-30CODEN: JPCHAX; ISSN:0022-3654.
    Raman and IR spectroscopic studies and cond. and viscosity measurements of propylene carbonate (PC) doped with various concns. of lithium perchlorate are reported. The assignment of the vibrational modes was supplemented by AM1 normal coordinate anal. Both Raman and IR spectra showed band splitting in the fundamental vibrational frequencies of PC and perchlorate anion. Spectral curve fitting within the totally sym. perchlorate band shape showed contributions of free ion, solvent-shared ion pairs, and contact ion pairs. Strong Li+-PC interaction was obsd. for the PC ring deformation band at 712 cm-1. Ion pairing as deduced by spectroscopic techniques provided a rationale to account for cond. and viscosity data.
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    Allen, J. L.; Borodin, O.; Seo, D. M.; Henderson, W. A. Combined quantum chemical/Raman spectroscopic analyses of Li+ cation solvation: Cyclic carbonate solvents - Ethylene carbonate and propylene carbonate. J. Power Sources 2014, 267, 821– 830,  DOI: 10.1016/j.jpowsour.2014.05.107
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    Combined quantum chemical/Raman spectroscopic analyses of Li+ cation solvation: Cyclic carbonate solvents-Ethylene carbonate and propylene carbonate
    Allen, Joshua L.; Borodin, Oleg; Seo, Daniel M.; Henderson, Wesley A.
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    Combined computational/Raman spectroscopic analyses of ethylene carbonate (EC) and propylene carbonate (PC) solvation interactions with lithium salts are reported. Previously reported Raman analyses of (EC)n-LiX mixts. have utilized faulty assumptions. In the present studies, d. functional theory (DFT) calcns. have provided corrections in terms of both the scaling factors for the solvent's Raman band intensity variations and information about band overlap. By accounting for these factors, the solvation nos. obtained from two different EC solvent bands are in excellent agreement with one another. The same anal. for PC, however, was quite challenging. Com. available PC is a racemic mixt. of (S)- and (R)-PC isomers. Based upon the quantum chem. calcns., each of these solvent isomers may exist as multiple conformers due to a low energy barrier for ring inversion, making deconvolution of the Raman bands daunting and inherently prone to significant error. Thus, Raman spectroscopy is able to accurately det. the extent of the EC. Li+ cation solvation interactions using the provided methodol., but a similar anal. of PC. Li+ cation solvation results in a significant underestimation of the actual solvation nos.
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    Gorobets, M. I.; Ataev, M. B.; Gafurov, M. M.; Kirillov, S. A. Raman study of solvation in solutions of lithium salts in dimethyl sulfoxide, propylene carbonate and dimethyl carbonate. J. Mol. Liq. 2015, 205, 98– 109,  DOI: 10.1016/j.molliq.2014.05.019
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    Raman study of solvation in solutions of lithium salts in dimethyl sulfoxide, propylene carbonate and dimethyl carbonate
    Gorobets, M. I.; Ataev, M. B.; Gafurov, M. M.; Kirillov, S. A.
    Journal of Molecular Liquids (2015), 205 (), 98-109CODEN: JMLIDT; ISSN:0167-7322. (Elsevier B.V.)
    Raman study of cation and anion solvation in DMSO, propylene carbonate and di-Me carbonate solns. of six lithium salts has been performed in the concn. range from 0.05 to 0.25 M fraction of a salt. The dependences of the amt. of the solvent particles involved in dimerization, hydrogen bonding, and solvation have been detd., and the mean solvation nos. have been found. It is concluded that in all solns. studied, notwithstanding the differences in the phys. properties of the solvent and in the structure of the anion, both the lithium cation and the anion solvation equil. are quant. similar. In all cases, solvation nos. of cations are close to two and do not vary with the growth of concn. In particular, in molten LiX·4S solvates, LiS+4 entities expected from the phase diagrams do not exist. It has been found that for the solvent mols. in dimers and in solvation spheres, non-coincidences between vibrational frequencies of isotropic and anisotropic lines, ΔνNCE = νaniso - νiso are of opposite signs signifying that the mutual orientation of mols. is different. In all systems studied, solvation nos. of anions decrease if the salt content is growing and are close to four in concd. solns. These striking similarities in the structure and concn. of solvated entities clearly signify that solvation phenomena have no decisive importance in detg. the properties of salt systems.
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    Ferry, A.; Marca, M.; Doeff, L. C. D. J. Transport Properly and Raman Spectroscopic Studies of the. J. Electrochem. Soc. 1998, 145, 1586– 1592,  DOI: 10.1149/1.1838522
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    Transport property and Raman spectroscopic studies of the polymer electrolyte system P(EO)n-NaTFSI
    Ferry, Anders; Doeff, Marca M.; De Jonghe, Lutgard C.
    Journal of the Electrochemical Society (1998), 145 (5), 1586-1592CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    Concn. dependences of a complete set of ionic transport properties are reported for the binary solid polymeric ion conductor poly(ethylene-oxide) [P(EO)], complexed with NaTFSI, [TFSI = (bis)trifluoromethanesulfonate imide, (CF3SO2)2N]. Measured transport properties include the ionic cond. (σ), the differential salt diffusion coeff. (Ds), and the cationic transference no. (t0+). To measure t0+, a recently developed technique based on concd. soln. theory is used. Under this framework no assumptions need to be made concerning ideality of the electrolytic soln. We find that for the P(EO)n-NaTFSI complexes the ionic conductivities, and differential salt diffusion coeffs. are much higher than those of the previously reported P(EO)n-NaCF3SO3 system. Ds also varies less with complex compn. in concd. solns. Interestingly, the cationic transference nos. are found to be neg. over a wide concn. range, i.e., t0- decreases with increasing salt concn. and reaches a low of -1.15 ± 0.25 at a compn. corresponding to an ether-oxygen to sodium-ion ratio of 7:1. This implies that the cationic current is mainly carried by complexed ions. Complementary Raman spectroscopic studies directly probing cation-anion and cation-polymer mol. interactions show that extensive interionic interactions occur in this system.
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    Umebayashi, Y.; Mitsugi, T.; Fukuda, S. Lithium ion solvation in room-temperature ionic liquids involving bis(trifluoromethanesulfonyl) imide anion studied by Raman spectroscopy and DFT calculations. J. Phys. Chem. B 2007, 111, 13028– 13032,  DOI: 10.1021/jp076869m
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    Lithium ion solvation in room-temperature ionic liquids involving bis(trifluoromethanesulfonyl) imide anion studied by Raman spectroscopy and DFT calculations
    Umebayashi, Yasuhiro; Mitsugi, Takushi; Fukuda, Shuhei; Fujimori, Takao; Fujii, Kenta; Kanzaki, Ryo; Takeuchi, Munetaka; Ishiguro, Shin-Ichi
    Journal of Physical Chemistry B (2007), 111 (45), 13028-13032CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    The solvation structure of the lithium ion in room-temp. ionic liqs. 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI+TFSI-) and N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP+TFSI-) has been studied by Raman spectroscopy and DFT calcns. Raman spectra of EMI+TFSI- and BMP+TFSI- contg. Li+TFSI- over the range 0.144-0.589 and 0.076-0.633 mol dm-3, resp., were measured at 298 K. A strong 744 cm-1 band of the free TFSI- ion in the bulk weakens with increasing concn. of the lithium ion, and it revealed by analyzing the intensity decrease that the two TFSI- ions bind to the metal ion. The lithium ion may be four-coordinated through the O atoms of two bidentate TFSI- ions. It has been established in our previous work that the TFSI- ion involves two conformers of C1 (cis) and C2 (trans) symmetries in equil., and the dipole moment of the C1 conformer is significantly larger than that of the C2 conformer. On the basis of these facts, the geometries and SCF energies of possible solvate ion clusters [Li(C1-TFSI-)2]-, [Li(C1-TFSI-)(C2-TFSI-)]-, and [Li(C2-TFSI-)2]- were examd. using the theor. DFT calcns. It is concluded that the C1 conformer is more preferred to the C2 conformer in the vicinity of the lithium ion.
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    Boschin, A.; Johansson, P. Characterization of NaX (X: TFSI, FSI) - PEO based solid polymer electrolytes for sodium batteries. Electrochim. Acta 2015, 175, 124– 133,  DOI: 10.1016/j.electacta.2015.03.228
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    Characterization of NaX (X: TFSI, FSI) - PEO based solid polymer electrolytes for sodium batteries
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    Solid polymers electrolytes (SPEs) based on Na bis(fluorosulfonyl)imide (NaFSI) and poly(ethylene oxide) (PEO) with different ether O to Na (O:Na) molar ratios (n), resulting in NaFSI(PEO)n materials are here presented for the 1st time. These SPEs are extensively compared with the corresponding NaTFSI(PEO)n system in terms of ionic conductivities, thermal properties, and charge carriers - to in detail outline both the role of the different anions used and the salt concns. employed. While for the most dil. systems (n = 20) the two SPE families show similar ionic conductivities in the entire temp. range studied (273-343 K), for n = 6 and n = 9 they differ significantly; at room temp., the NaFSI based SPEs show lower ionic conductivities than the NaTFSI based analogs. This difference is mainly ascribed to differences in the morphol.; while the NaTFSI salt, possibly by virtue of its large TFSI anion, acts to inhibit crystn., NaFSI rather seems to favor crystn. Also, careful Raman spectroscopy anal. of the charge carrier speciation reveal higher aggregates to be present in the most concd. SPE, NaFSI(PEO)6, and the NaFSI based SPEs in general to result in less free anions than the NaTFSI based SPEs. Also, as both NaTFSI(PEO)n and NaFSI(PEO)n for n = 20 and n = 9 exhibit very similar glass transition temps., the FSI ion seem to be equally plasticizing as the TFSI ion, but for n = 6 the different speciation in terms of charge carriers also affects the relative dynamics of the polymer chains.
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    Vibrational Spectroscopy and ab Initio Studies of Lithium Bis(oxalato)borate (LiBOB) in Different Solvents
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  • Abstract

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    Figure 1

    Figure 1. Simplified scheme of polymer electrolyte sample preparation, indicating cationic ring-opening (co)polymerization (CROP) of 2-oxazolines, cross-linking, and swelling of the samples in 1 M LiTFSI salt solution in propylene carbonate.

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    Figure 2

    Figure 2. Temperature dependence of self-diffusion coefficients of Li cations (empty points) and corresponding anions (solid points) obtained by PFG NMR for 1M-LiTFSI-PC-POx and 1MLiBOB-PC-POx. Solid lines are the guidelines.

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    Figure 3

    Figure 3. Raman spectra of the LiTFSI-PC and LiTFSI–PC-POx sample series in the region 685─760 cm–1 (a). Example of deconvolution of this spectral region for 2.0M-LiTFSI-PC (b). Concentration dependencies of the (I742+I748)/(I712+I725) ratio in the LiTFSI-PC and LiTFSI–PC-POx series together with the I742/(I712+I725) and I748/(I712+I725) ratios in the LiTFSI–PC-POx series (c); I725/I712 ratio in the LiTFSI-PC and LiTFSI–PC-POx series (d).

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    Figure 4

    Figure 4. Raman spectra of the LiBOB-PC and LiBOB-PC-POx sample series in the regions 685─760 cm–1 (a) and 285─395 cm–1 (b). Example of deconvolution of the PC-relevant spectral region of 2.0M-LiBOB-PC (c). Concentration dependence of the I725/I712 ratio in the LiBOB-PC and LiBOB-PC-POx series (d).

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    Figure 5

    Figure 5. Raman spectra of the LiBOB-DG and LiBOB-DG-POx sample series in the regions 2690─3100 cm–1 (a), 775─910 cm–1 (b), 685─760 cm–1 (c) and 285─395 cm–1 (d). The 2 M solution was separated into an opaque heavier phase and a lighter solution phase; Raman spectra were obtained near the solution level (top) and near the bottom of the cuvette (bottom).

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    Figure 6

    Figure 6. Concentration dependencies of the DG CH3 stretching band position and area normalized to the total area of the CH-stretching region for the LiBOB-DG series (a). Example of deconvolution of the CH-stretching spectral region of 2.0M-LiBOB-DG (b). I375/I2827, I348/I2827, and I880/I2827 ratios in the LiBOB-DG and LiBOB-DG-POx series (c). Example of deconvolution of the BOB anion-related spectral region of 2.0M-LiBOB-DG (d). The 2 M solution was separated into an opaque heavier phase and a lighter solution phase, and the data calculated from the spectrum measured near the solution level are displayed as 1.9 M and from the bottom of the cuvette as 2.1 M for clarity. The spectra near the solution level were used for the deconvolution examples.

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    Figure 7

    Figure 7. Raman spectra of the LiTFSI-PC-POx (a), LiBOB-PC-POx (b) and LiBOB-DG-POx (c) sample series in the region 1560─1900 cm–1. The I1661/I1643 band area ratio of the POx amide (d). Example of deconvolutions of the carbonyl stretching region of dry POx (e) and 1.5M-LiTFSI-PC-POx (f).

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    Figure 8

    Figure 8. Raman spectra of the selected LiBOB-DG and LiBOB-DG-POx samples in the regions 1850─1560 cm–1 (a), 200─100 cm–1 (b), 3100─2720 cm–1 (c), 915─805 cm–1 (d), 1340─1255 cm–1 (e), and 1100─950 cm–1 (f).

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      A review. Exploring highly foldable batteries with no safety hazard is a crucial task for the realization of portable, wearable, and implantable elec. devices. Given these concerns, developing solid-state batteries is one of the most promising routes to achieve this aspiration. Because of the excellent flexibility and processability, polyvinylidene fluoride (PVDF) based electrolytes possess great potential to pack high energy d. flexible batteries, however, suffers the various intrinsic shortcomings such as inferior ionic cond., a high degree of crystallinity, and lack of reactive groups. Clearing the progress of the present state and concluding the specific challenges faced by PVDF based electrolytes will help to develop PVDF based polymer batteries. In this review, we summarize the recent progress of gel polymer electrolytes and all solid polymer electrolytes based on PVDF. The ion transport mechanisms and prepn. methods of PVDF based electrolytes are briefly introduced. Meanwhile, the current design principle and properties of electrolytes are highlighted and systematically discussed. Some peculiar modified strategies performed in lithium-sulfur batteries and lithium-oxygen batteries are also included. Finally, this review describes the challenges and prospects of some solid-state electrolytes to provide strategies for manufg. high-performance PVDF electrolytes aimed at practical application with flexible requirements.
    20. 20
      Gou, J.; Liu, W.; Tang, A. A renewable and biodegradable nanocellulose-based gel polymer electrolyte for lithium-ion battery. J. Mater. Sci. 2020, 55, 10699– 10711,  DOI: 10.1007/s10853-020-04667-7
      There is no corresponding record for this reference.
    21. 21
      Abdulwahid, R. T.; Aziz, S. B.; Kadir, M. F. Z. Replacing synthetic polymer electrolytes in energy storage with flexible biodegradable alternatives: sustainable green biopolymer blend electrolyte for supercapacitor device. Mater. Today Sustain. 2023, 23, 100472  DOI: 10.1016/j.mtsust.2023.100472
      There is no corresponding record for this reference.
    22. 22
      Glassner, M.; Vergaelen, M.; Hoogenboom, R. Poly(2-oxazoline)s: A comprehensive overview of polymer structures and their physical properties. Polym. Int. 2018, 67, 32– 45,  DOI: 10.1002/pi.5457
      22
      Poly(2-oxazoline)s: A comprehensive overview of polymer structures and their physical properties
      Glassner, Mathias; Vergaelen, Maarten; Hoogenboom, Richard
      Polymer International (2018), 67 (1), 32-45CODEN: PLYIEI; ISSN:0959-8103. (John Wiley & Sons Ltd.)
      A review. Poly(2-oxazoline)s (PAOx) are of increasing importance for a wide range of applications, mostly in the biomedical field. This review describes the synthesis of 2-oxazoline monomers and their cationic ring-opening polymn., and gives a comprehensive overview of all reported PAOx homopolymers. In the second part of the review, the polymer properties of these PAOx homopolymers with varying side-chain structures are discussed. Altogether, this review intends to serve as an encyclopedia for poly(2-oxazoline)s enabling the straightforward selection of a polymer structure with the desired properties for a certain application. © 2017 Society of Chem. Industry.
    23. 23
      Estabrook, D. A. Functionalizable Poly(2-oxazoline) Stabilizers in A Perfluorocarbon-in-Water Nanoemulsion Drug Delivery System; University of California: Los Angeles, 2021.
      There is no corresponding record for this reference.
    24. 24
      Nemati Mahand, S.; Aliakbarzadeh, S.; Moghaddam, A. Polyoxazoline: A review article from polymerization to smart behaviors and biomedical applications. Eur. Polym. J. 2022, 178, 111484  DOI: 10.1016/j.eurpolymj.2022.111484
      24
      Polyoxazoline: A review article from polymerization to smart behaviors and biomedical applications
      Nemati Mahand, Saba; Aliakbarzadeh, Sanaz; Moghaddam, Armaghan; Salehi Moghaddam, Abolfazl; Kruppke, Benjamin; Nasrollahzadeh, Mahmoud; Khonakdar, Hossein Ali
      European Polymer Journal (2022), 178 (), 111484CODEN: EUPJAG; ISSN:0014-3057. (Elsevier Ltd.)
      A review. Poly(2-oxazoline) s are emerging innovative biomaterials that show analogous and even preferable properties compared to well-known counterparts, esp. poly(ethylene glycol). This polymer can be synthesized in a living and hence controlled manner using the cationic ring-opening polymn. method. In addn., various polyoxazolines are water-sol. or amphiphilic and relatively non-toxic, making them attractive as biomaterials. Polyoxazolines (10 and 20 kDa) are safe when used i.v. to rats and can be used for several biomedical applications. These results from various papers and patents conclude that POx is a favorable biopolymer. The present review aims to highlight the synthesis, structure, and properties of polyoxazolines, such as different derivs. of POx used in tissue engineering. Different polymn. methods and smart behaviors of POx are described as well. Finally, it aims to introduce the significance and potential of POx in biomedical applications like drug delivery, protein delivery, and gene delivery using a no. of leading refs. of recent years.
    25. 25
      Drews, M.; Trötschler, T.; Bauer, M. Photocured Cationic Polyoxazoline Macromonomers as Gel Polymer Electrolytes for Lithium-Ion Batteries. ACS Appl. Polym. Mater. 2022, 4, 158– 168,  DOI: 10.1021/acsapm.1c01171
      25
      Photocured Cationic Polyoxazoline Macromonomers as Gel Polymer Electrolytes for Lithium-Ion Batteries
      Drews, Mathias; Troetschler, Tobias; Bauer, Manuel; Guntupalli, Apurupa; Beichel, Witali; Gentischer, Harald; Muelhaupt, Rolf; Kerscher, Benjamin; Biro, Daniel
      ACS Applied Polymer Materials (2022), 4 (1), 158-168CODEN: AAPMCD; ISSN:2637-6105. (American Chemical Society)
      In this work, we present a cationic vinylimidazolium-terminated poly(2-ethyl-2-oxazoline) (PEtOx) macromonomer as a key component of gel polymer electrolytes (GPE) for lithium-ion batteries. GPE prodn. followed a scalable process based on UV curing of the cationic PEtOx macromonomer with polyfunctional acrylic comonomers dissolved in an org. electrolyte (LP30), affording electrolyte-swollen polymeric ionic liq. (PIL) networks with PEtOx side chains. Thus, cathodes coated with a GPE layer of less than 200μm thickness were readily manufd. The PIL brush-type GPE is highly insol. but swellable in LP30 and exhibits pronounced electrolyte retaining ability against evapn. At 160°C, the wt. loss of the GPE amounted to around 5%. This is 12% less compared to an LP30-soaked com. Celgard separator. At room temp., the ionic cond. was 3.6 x 10-4 S/cm, surpassing that of a comparable Celgard/LP30 system. Contrary to LP30 in Celgard, cond. measurements for the PIL brush GPE did not indicate any crystn. of the liq. electrolyte at subambient temps. This was confirmed by differential scanning calorimetry, suggesting improved ionic mobility in the GPE over a wide temp. range. The electrochem. stability window of the PIL brush GPE is wide enough and fits all common lithium-ion cathode materials. In fact, the GPE exhibited exceptional oxidative stability of 5.2 V vs Li/Li+. Half-cell cycling expts. using a lithium iron phosphate cathode revealed high capacity values of 150 mAh/g at a current rate of C/10. When the current was increased to C/2, the capacity decreased to 120 mAh/g and the cell reached 80% of its initial capacity (referred to C/2) after 180 cycles. Thus, according to the first physicochem. and electrochem. investigations, the PEtOx-based PIL brush GPE represents a promising candidate with respect to lithium-ion battery operation.
    26. 26
      Wylie, R. A. L.; Miller, B.; Connal, L. A.; Qiao, G. Oligomeric Poly(Oxazoline) as Potential Lithium Battery Electrolytes. J. Electrochem. Soc. 2022, 169, 060533  DOI: 10.1149/1945-7111/ac766d
      There is no corresponding record for this reference.
    27. 27
      Mahun, A.; Černoch, P.; Paruzel, B. Newly developed gel polymer electrolytes based on crosslinked poly(2-oxazolines). Solid State Ionics 2023, 389, 116096,  DOI: 10.1016/j.ssi.2022.116096
      There is no corresponding record for this reference.
    28. 28
      Yang, Q.; Yu, Y.; Huang, W. Poly(2-ethyl-2-oxazoline) as a Gel Additive to Improve the Performance of Sulfur Cathodes in Lithium-Sulfur Batteries. ChemElectroChem 2021, 8, 411– 417,  DOI: 10.1002/celc.202001594
      There is no corresponding record for this reference.
    29. 29
      Petit, C.; Luef, K. P.; Edler, M. Microwave-Assisted Syntheses in Recyclable Ionic Liquids: Photoresists Based on Renewable Resources. ChemSusChem 2015, 8, 3401– 3404,  DOI: 10.1002/cssc.201500847
      There is no corresponding record for this reference.
    30. 30
      Abbrent, S.; Mahun, A.; Smrčková, M. D. Copolymer chain formation of 2-oxazolines byin situ1H-NMR spectroscopy: dependence of sequential composition on substituent structure and monomer ratios. RSC Adv. 2021, 11, 10468– 10478,  DOI: 10.1039/D1RA01509E
      There is no corresponding record for this reference.
    31. 31
      Hoyle, C. E.; Bowman, C. N. Thiol-ene click chemistry. Angew. Chem. - Int. Ed. 2010, 49, 1540– 1573,  DOI: 10.1002/anie.200903924
      31
      Thiol-Ene Click Chemistry
      Hoyle, Charles E.; Bowman, Christopher N.
      Angewandte Chemie, International Edition (2010), 49 (9), 1540-1573CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Following Sharpless' visionary characterization of several idealized reactions as click reactions, the materials science and synthetic chem. communities have pursued numerous routes toward the identification and implementation of these click reactions. Herein, the authors review the radical-mediated thiol-ene reaction as one such click reaction. This reaction has all the desirable features of a click reaction, being highly efficient, simple to execute with no side products and proceeding rapidly to high yield. Further, the thiol-ene reaction is most frequently photoinitiated, particularly for photopolymns. resulting in highly uniform polymer networks, promoting unique capabilities related to spatial and temporal control of the click reaction. The reaction mechanism and its implementation in various synthetic methodologies, biofunctionalization, surface and polymer modification, and polymn. are all reviewed.
    32. 32
      Johnson, C. S. Diffusion ordered nuclear magnetic resonance spectroscopy: Principles and applications. Prog. Nucl. Magn. Reson. Spectrosc. 1999, 34, 203– 256,  DOI: 10.1016/S0079-6565(99)00003-5
      32
      Diffusion ordered nuclear magnetic resonance spectroscopy: principles and applications
      Johnson, C. S., Jr.
      Progress in Nuclear Magnetic Resonance Spectroscopy (1999), 34 (3,4), 203-256CODEN: PNMRAT; ISSN:0079-6565. (Elsevier Science B.V.)
      A review with 146 refs.
    33. 33
      Stejskal, E. O.; Tanner, J. E. Spin diffusion measurements: Spin echoes in the presence of a time-dependent field gradient. J. Chem. Phys. 1965, 42, 288– 292,  DOI: 10.1063/1.1695690
      33
      Spin diffusion measurements: spin echoes in the presence of a time-dependent field gradient
      Stejskal, E. O.; Tanner, J. E.
      Journal of Chemical Physics (1965), 42 (1), 288-92CODEN: JCPSA6; ISSN:0021-9606.
      A derivation is given of the effect of a time-dependent magnetic field gradient on the spin-echo expt., particularly in the presence of spin diffusion. There are several reasons for preferring certain kinds of time-dependent magnetic field gradients to the more usual steady gradient. If the gradient is reduced during the radio-frequency pulses, H1 need not be particularly large; if the gradient is small at the time of the echo, the echo will be broad and its amplitude easy to measure. Both of these relaxations of restrictions on the measurement of diffusion coeffs. by the spin-echo technique serve to extend its range of applicability. A pulsed gradient can be recommended when it is crit. to define the precise time period over which diffusion is being measured. The theoretical expression derived was verified exptl. for several choices of time-dependent magnetic field gradient. An app. is described suitable for the production of pulsed gradients with amplitudes as large as 100 gauss cm. The diffusion coeff. of dry glycerol at 26° ± 1° is (2.5 ± 0.2) × 10-3 cm.2/sec., a value smaller than can ordinarily be measured by the steady gradient method.
    34. 34
      Bruker. Biospin Topspin 3.2. 2020 https://www.bruker.com/en.html.
      There is no corresponding record for this reference.
    35. 35
      Thermo Fisher Scientific Inc. OMNIC, 2011, https://www.thermofisher.com/order/catalog/product/INQSOF018.
      There is no corresponding record for this reference.
    36. 36
      Shibata, M.; Sawayama, S.; Osugi, M.; Fujii, K. Structural aspect on “Salting-in” mechanism of PEG chains into a phosphonium-based ionic liquid using lithium salt. J. Mol. Liq. 2022, 366, 120255  DOI: 10.1016/j.molliq.2022.120255
      There is no corresponding record for this reference.
    37. 37
      Konefał, R.; Morávková, Z.; Paruzel, B. Effect of PAMAM dendrimers on interactions and transport of LiTFSI and NaTFSI in propylene carbonate-based electrolytes. Polymers 2020, 12, 1– 16,  DOI: 10.3390/polym12071595
      There is no corresponding record for this reference.
    38. 38
      Barthel, J.; Utz, M.; Groß, K.; Gores, H. J. Temperature and composition dependence of viscosity I. Propylene carbonate-dimethoxyethane mixtures and thermodynamics of fluid flow. J. Solution Chem. 1995, 24, 1109– 1123,  DOI: 10.1007/BF00972958
      There is no corresponding record for this reference.
    39. 39
      Bhatt, P. J.; Pathak, N.; Mishra, K.; Kanchan, D. K.; Kumar, D. Effect of Different Cations on Ion-Transport Behavior in Polymer Gel Electrolytes Intended for Application in Flexible Electrochemical Devices. J. Electron. Mater. 2022, 51, 1371– 1384,  DOI: 10.1007/s11664-021-09398-2
      There is no corresponding record for this reference.
    40. 40
      Adebahr, J.; Gavelin, P.; Ostrovskii, D.; Torell, L. M.; Wesslen, B. Raman study on intermolecular and ionic interactions in gel electrolytes. J. Mol. Struct. 1999, 482–483, 487– 490,  DOI: 10.1016/S0022-2860(98)00686-3
      There is no corresponding record for this reference.
    41. 41
      Georén, P.; Adebahr, J.; Jacobsson, P.; Lindbergh, G. Concentration Polarization of a Polymer Electrolyte. J. Electrochem. Soc. 2002, 149, A1015,  DOI: 10.1149/1.1487832
      41
      Concentration Polarization of a Polymer Electrolyte
      Georen, P.; Adebahr, J.; Jacobsson, P.; Lindbergh, G.
      Journal of the Electrochemical Society (2002), 149 (8), A1015-A1019CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      The salt concn. in a concd. binary polymer electrolyte was measured in situ by confocal Raman spectroscopy during galvanostatic polarization expts. The electrolyte studied was 0.8M lithium bis(trifluoromethanesulfonyl)imide in a copolymer of ethylene- and propylene oxide at 25°. Recent work with a transport model and characterization of the transport properties, for the same electrolyte, was verified with the spectroscopic results of this study. A good agreement between modeled and measured results was found. The spectroscopic method suited well for these studies. The possibilities of using a transport model are briefly demonstrated and discussed.
    42. 42
      Hofmann, A.; Schulz, M.; Hanemann, T. Gel electrolytes based on ionic liquids for advanced lithium polymer batteries. Electrochim. Acta 2013, 89, 823– 831,  DOI: 10.1016/j.electacta.2012.10.144
      42
      Gel electrolytes based on ionic liquids for advanced lithium polymer batteries
      Hofmann, Andreas; Schulz, Michael; Hanemann, Thomas
      Electrochimica Acta (2013), 89 (), 823-831CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      A new ionic liq.-based polymer gel electrolyte for Li ion batteries is prepd. by combining N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)azanide, org. carbonates, Li bis(trifluoromethylsulfonyl)azanide, and poly(vinylidene fluoride-co-hexafluoropropylene). The conductivities of the gel electrolytes at room temp. are ∼1-2 mS/cm. The polymer gel electrolytes work effectively with a graphite anode and a LiCo1/3Mn1/3Ni1/3O2 cathode. Further the addn. of vinylene carbonate and 4-vinylpyridine improves cell performance.
    43. 43
      Battisti, D.; Nazri, G. A.; Klassen, B.; Aroca, R. Vibrational studies of lithium perchlorate in propylene carbonate solutions. J. Phys. Chem. A 1993, 97, 5826– 5830,  DOI: 10.1021/j100124a007
      43
      Vibrational studies of lithium perchlorate in propylene carbonate solutions
      Battisti, D.; Nazri, G. A.; Klassen, B.; Aroca, R.
      Journal of Physical Chemistry (1993), 97 (22), 5826-30CODEN: JPCHAX; ISSN:0022-3654.
      Raman and IR spectroscopic studies and cond. and viscosity measurements of propylene carbonate (PC) doped with various concns. of lithium perchlorate are reported. The assignment of the vibrational modes was supplemented by AM1 normal coordinate anal. Both Raman and IR spectra showed band splitting in the fundamental vibrational frequencies of PC and perchlorate anion. Spectral curve fitting within the totally sym. perchlorate band shape showed contributions of free ion, solvent-shared ion pairs, and contact ion pairs. Strong Li+-PC interaction was obsd. for the PC ring deformation band at 712 cm-1. Ion pairing as deduced by spectroscopic techniques provided a rationale to account for cond. and viscosity data.
    44. 44
      Allen, J. L.; Borodin, O.; Seo, D. M.; Henderson, W. A. Combined quantum chemical/Raman spectroscopic analyses of Li+ cation solvation: Cyclic carbonate solvents - Ethylene carbonate and propylene carbonate. J. Power Sources 2014, 267, 821– 830,  DOI: 10.1016/j.jpowsour.2014.05.107
      44
      Combined quantum chemical/Raman spectroscopic analyses of Li+ cation solvation: Cyclic carbonate solvents-Ethylene carbonate and propylene carbonate
      Allen, Joshua L.; Borodin, Oleg; Seo, Daniel M.; Henderson, Wesley A.
      Journal of Power Sources (2014), 267 (), 821-830CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      Combined computational/Raman spectroscopic analyses of ethylene carbonate (EC) and propylene carbonate (PC) solvation interactions with lithium salts are reported. Previously reported Raman analyses of (EC)n-LiX mixts. have utilized faulty assumptions. In the present studies, d. functional theory (DFT) calcns. have provided corrections in terms of both the scaling factors for the solvent's Raman band intensity variations and information about band overlap. By accounting for these factors, the solvation nos. obtained from two different EC solvent bands are in excellent agreement with one another. The same anal. for PC, however, was quite challenging. Com. available PC is a racemic mixt. of (S)- and (R)-PC isomers. Based upon the quantum chem. calcns., each of these solvent isomers may exist as multiple conformers due to a low energy barrier for ring inversion, making deconvolution of the Raman bands daunting and inherently prone to significant error. Thus, Raman spectroscopy is able to accurately det. the extent of the EC. Li+ cation solvation interactions using the provided methodol., but a similar anal. of PC. Li+ cation solvation results in a significant underestimation of the actual solvation nos.
    45. 45
      Gorobets, M. I.; Ataev, M. B.; Gafurov, M. M.; Kirillov, S. A. Raman study of solvation in solutions of lithium salts in dimethyl sulfoxide, propylene carbonate and dimethyl carbonate. J. Mol. Liq. 2015, 205, 98– 109,  DOI: 10.1016/j.molliq.2014.05.019
      45
      Raman study of solvation in solutions of lithium salts in dimethyl sulfoxide, propylene carbonate and dimethyl carbonate
      Gorobets, M. I.; Ataev, M. B.; Gafurov, M. M.; Kirillov, S. A.
      Journal of Molecular Liquids (2015), 205 (), 98-109CODEN: JMLIDT; ISSN:0167-7322. (Elsevier B.V.)
      Raman study of cation and anion solvation in DMSO, propylene carbonate and di-Me carbonate solns. of six lithium salts has been performed in the concn. range from 0.05 to 0.25 M fraction of a salt. The dependences of the amt. of the solvent particles involved in dimerization, hydrogen bonding, and solvation have been detd., and the mean solvation nos. have been found. It is concluded that in all solns. studied, notwithstanding the differences in the phys. properties of the solvent and in the structure of the anion, both the lithium cation and the anion solvation equil. are quant. similar. In all cases, solvation nos. of cations are close to two and do not vary with the growth of concn. In particular, in molten LiX·4S solvates, LiS+4 entities expected from the phase diagrams do not exist. It has been found that for the solvent mols. in dimers and in solvation spheres, non-coincidences between vibrational frequencies of isotropic and anisotropic lines, ΔνNCE = νaniso - νiso are of opposite signs signifying that the mutual orientation of mols. is different. In all systems studied, solvation nos. of anions decrease if the salt content is growing and are close to four in concd. solns. These striking similarities in the structure and concn. of solvated entities clearly signify that solvation phenomena have no decisive importance in detg. the properties of salt systems.
    46. 46
      Ferry, A.; Marca, M.; Doeff, L. C. D. J. Transport Properly and Raman Spectroscopic Studies of the. J. Electrochem. Soc. 1998, 145, 1586– 1592,  DOI: 10.1149/1.1838522
      46
      Transport property and Raman spectroscopic studies of the polymer electrolyte system P(EO)n-NaTFSI
      Ferry, Anders; Doeff, Marca M.; De Jonghe, Lutgard C.
      Journal of the Electrochemical Society (1998), 145 (5), 1586-1592CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      Concn. dependences of a complete set of ionic transport properties are reported for the binary solid polymeric ion conductor poly(ethylene-oxide) [P(EO)], complexed with NaTFSI, [TFSI = (bis)trifluoromethanesulfonate imide, (CF3SO2)2N]. Measured transport properties include the ionic cond. (σ), the differential salt diffusion coeff. (Ds), and the cationic transference no. (t0+). To measure t0+, a recently developed technique based on concd. soln. theory is used. Under this framework no assumptions need to be made concerning ideality of the electrolytic soln. We find that for the P(EO)n-NaTFSI complexes the ionic conductivities, and differential salt diffusion coeffs. are much higher than those of the previously reported P(EO)n-NaCF3SO3 system. Ds also varies less with complex compn. in concd. solns. Interestingly, the cationic transference nos. are found to be neg. over a wide concn. range, i.e., t0- decreases with increasing salt concn. and reaches a low of -1.15 ± 0.25 at a compn. corresponding to an ether-oxygen to sodium-ion ratio of 7:1. This implies that the cationic current is mainly carried by complexed ions. Complementary Raman spectroscopic studies directly probing cation-anion and cation-polymer mol. interactions show that extensive interionic interactions occur in this system.
    47. 47
      Umebayashi, Y.; Mitsugi, T.; Fukuda, S. Lithium ion solvation in room-temperature ionic liquids involving bis(trifluoromethanesulfonyl) imide anion studied by Raman spectroscopy and DFT calculations. J. Phys. Chem. B 2007, 111, 13028– 13032,  DOI: 10.1021/jp076869m
      47
      Lithium ion solvation in room-temperature ionic liquids involving bis(trifluoromethanesulfonyl) imide anion studied by Raman spectroscopy and DFT calculations
      Umebayashi, Yasuhiro; Mitsugi, Takushi; Fukuda, Shuhei; Fujimori, Takao; Fujii, Kenta; Kanzaki, Ryo; Takeuchi, Munetaka; Ishiguro, Shin-Ichi
      Journal of Physical Chemistry B (2007), 111 (45), 13028-13032CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      The solvation structure of the lithium ion in room-temp. ionic liqs. 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI+TFSI-) and N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP+TFSI-) has been studied by Raman spectroscopy and DFT calcns. Raman spectra of EMI+TFSI- and BMP+TFSI- contg. Li+TFSI- over the range 0.144-0.589 and 0.076-0.633 mol dm-3, resp., were measured at 298 K. A strong 744 cm-1 band of the free TFSI- ion in the bulk weakens with increasing concn. of the lithium ion, and it revealed by analyzing the intensity decrease that the two TFSI- ions bind to the metal ion. The lithium ion may be four-coordinated through the O atoms of two bidentate TFSI- ions. It has been established in our previous work that the TFSI- ion involves two conformers of C1 (cis) and C2 (trans) symmetries in equil., and the dipole moment of the C1 conformer is significantly larger than that of the C2 conformer. On the basis of these facts, the geometries and SCF energies of possible solvate ion clusters [Li(C1-TFSI-)2]-, [Li(C1-TFSI-)(C2-TFSI-)]-, and [Li(C2-TFSI-)2]- were examd. using the theor. DFT calcns. It is concluded that the C1 conformer is more preferred to the C2 conformer in the vicinity of the lithium ion.
    48. 48
      Boschin, A.; Johansson, P. Characterization of NaX (X: TFSI, FSI) - PEO based solid polymer electrolytes for sodium batteries. Electrochim. Acta 2015, 175, 124– 133,  DOI: 10.1016/j.electacta.2015.03.228
      48
      Characterization of NaX (X: TFSI, FSI) - PEO based solid polymer electrolytes for sodium batteries
      Boschin, Andrea; Johansson, Patrik
      Electrochimica Acta (2015), 175 (), 124-133CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      Solid polymers electrolytes (SPEs) based on Na bis(fluorosulfonyl)imide (NaFSI) and poly(ethylene oxide) (PEO) with different ether O to Na (O:Na) molar ratios (n), resulting in NaFSI(PEO)n materials are here presented for the 1st time. These SPEs are extensively compared with the corresponding NaTFSI(PEO)n system in terms of ionic conductivities, thermal properties, and charge carriers - to in detail outline both the role of the different anions used and the salt concns. employed. While for the most dil. systems (n = 20) the two SPE families show similar ionic conductivities in the entire temp. range studied (273-343 K), for n = 6 and n = 9 they differ significantly; at room temp., the NaFSI based SPEs show lower ionic conductivities than the NaTFSI based analogs. This difference is mainly ascribed to differences in the morphol.; while the NaTFSI salt, possibly by virtue of its large TFSI anion, acts to inhibit crystn., NaFSI rather seems to favor crystn. Also, careful Raman spectroscopy anal. of the charge carrier speciation reveal higher aggregates to be present in the most concd. SPE, NaFSI(PEO)6, and the NaFSI based SPEs in general to result in less free anions than the NaTFSI based SPEs. Also, as both NaTFSI(PEO)n and NaFSI(PEO)n for n = 20 and n = 9 exhibit very similar glass transition temps., the FSI ion seem to be equally plasticizing as the TFSI ion, but for n = 6 the different speciation in terms of charge carriers also affects the relative dynamics of the polymer chains.
    49. 49
      Wang, Z.; Gao, W.; Huang, X.; Mo, Y.; Chen, L. Spectroscopic studies on interactions and microstructures in propylene carbonate - LiTFSI electrolytes. J. Raman Spectrosc. 2001, 32, 900– 905,  DOI: 10.1002/jrs.756
      There is no corresponding record for this reference.
    50. 50
      Holomb, R.; Xu, W.; Markusson, H.; Johansson, P.; Jacobsson, P. Vibrational spectroscopy and ab initio studies of lithium bis(oxalato)borate (LiBOB) in different solvents. J. Phys. Chem. A 2006, 110, 11467– 11472,  DOI: 10.1021/jp0626824
      50
      Vibrational Spectroscopy and ab Initio Studies of Lithium Bis(oxalato)borate (LiBOB) in Different Solvents
      Holomb, Roman; Xu, Wu; Markusson, Henrik; Johansson, Patrik; Jacobsson, Per
      Journal of Physical Chemistry A (2006), 110 (40), 11467-11472CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      The effect of Li ion coordination with the bis(oxalato)borate (BOB-) [B(C2O4)2]- anion in DMSO, PEG, PPG, and d-PPG was studied in detail by IR and Raman spectroscopy. Ab initio calcns. were performed to allow a consistent anal. of the exptl. data. The main features obsd. in the IR and Raman spectra correspond to the presence of free, un-coordinated, BOB- anions. Only using d-PPG as solvent a small amt. of Li+···BOB- ion pairs were detected. The Raman spectra and the calcns. together indicate that Li+ coordinates bidentately with 2 end-O atoms of the BOB- anion. The identification of ion pairs can be used to reveal limitations of LiBOB based electrolytes. The results for LiBOB are compared with literature on other Li salts.
    51. 51
      Yan, F.; Mukherjee, K.; Maroncelli, M.; Kim, H. J. Infrared Spectroscopy of Li+ Solvation in Diglyme: Ab Initio Molecular Dynamics and Experiment. J. Phys. Chem. B 2023, 127, 9191– 9203,  DOI: 10.1021/acs.jpcb.3c05612
      There is no corresponding record for this reference.
    52. 52
      Frech, R.; Huang, W. Polymer conformation and ionic association in complexes of lithium, sodium and potassium triflate with poly (ethylene oxide) oligomers. Solid State Ionics 1994, 72, 103– 107,  DOI: 10.1016/0167-2738(94)90132-5
      52
      Polymer conformation and ionic association in complexes of lithium, sodium and potassium triflate with poly(ethylene oxide) oligomers
      Frech, Roger; Huang, Weiwei
      Solid State Ionics (1994), 72 (Pt. 2), 103-7CODEN: SSIOD3; ISSN:0167-2738.
      Raman spectra of mono-, di-, tri-, and tetraglyme complexed with Li, Na, and K triflate salts have been measured as a function of chain length and salt concn. and temp. Spectral changes in the oligomer bands accompanying complexation are attributed to a conformational arrangement of the polyether backbone not obsd. in the pure liq. An anomalous chain length dependence is discussed in terms of cation coordination with the polyether oxygen atoms.
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      Matsuura, H.; Miyazawa, T. Vibrational analysis of molten poly(ethylene glycol). J. Polym. Sci. A-2 Polym. Phys. 1969, 7, 1735– 1744,  DOI: 10.1002/pol.1969.160071009
      53
      Vibrational analysis of molten poly(ethylene glycol)
      Matsuura, Hiroatsu; Miyazawa, Tatsuo
      Journal of Polymer Science, Polymer Physics Edition (1969), 7 (10), 1735-44CODEN: JPLPAY; ISSN:0098-1273.
      The ir absorption of polyethylene glycol was measured in the molten state. Characteristic bands of the molten state were identified. Normal vibrations and frequency distributions were treated for various con formation models with CH2CH2O repeat units. The ir absorption peaks of the molten state closely corresponded to the frequency distribution peaks of the trans-gauche-trans conformation with gauche OCH2CH2O groups, although ir bands due to trans OCH2CH2O groups were also observed. Vibrational assignments of the ir bands and Raman lines were made on the basis of potential energy distributions.
    54. 54
      Frech, R.; Huang, W. Conformational Changes in Diethylene Glycol Dimethyl Ether and Poly(ethylene oxide) Induced by Lithium Ion Complexation. Macromolecules 1995, 28, 1246– 1251,  DOI: 10.1021/ma00108a063
      54
      Conformational Changes in Diethylene Glycol Dimethyl Ether and Poly(ethylene oxide) Induced by Lithium Ion Complexation
      Frech, Roger; Huang, Weiwei
      Macromolecules (1995), 28 (4), 1246-51CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      Conformational changes in diethylene glycol di-Me ether (diglyme) and high-mol.-wt. poly(ethylene oxide) induced by complexation with lithium trifluoromethanesulfonate (LiCF3SO3) have been investigated using Raman scattering and IR-transmission spectroscopy. In both the diglyme and polymer complex, new bands were obsd. in spectral regions involving a significant amt. of CH2 bending motion. These bands are attributed to a conformation which is not energetically favored in the pure polymer or oligomer but which is stabilized through interactions of the cation with the polyether oxygen atoms. The structure of the new conformer is discussed.
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      Seo, D. M.; Boyle, P. D.; Sommer, R. D. Solvate structures and spectroscopic characterization of litfsi electrolytes. J. Phys. Chem. B 2014, 118, 13601– 13608,  DOI: 10.1021/jp505006x
      55
      Solvate Structures and Spectroscopic Characterization of LiTFSI Electrolytes
      Seo, Daniel M.; Boyle, Paul D.; Sommer, Roger D.; Daubert, James S.; Borodin, Oleg; Henderson, Wesley A.
      Journal of Physical Chemistry B (2014), 118 (47), 13601-13608CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
      A Raman spectroscopic evaluation of numerous cryst. solvates with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI or LiN(SO2CF3)2) has been conducted over a wide temp. range. Four new cryst. solvate structures - (PHEN)3:LiTFSI, (2,9-DMPHEN)2:LiTFSI, (G3)1:LiTFSI and (2,6-DMPy)1/2:LiTFSI with phenanthroline, 2,9-dimethyl[1,10]phenanthroline, triglyme, and 2,6-dimethylpyridine, resp. - have been detd. to aid in this study. The spectroscopic data have been correlated with varying modes of TFSI-···Li+ cation coordination within the solvate structures to create an electrolyte characterization tool to facilitate the Raman band deconvolution assignments for the detn. of ionic assocn. interactions within electrolytes contg. LiTFSI. It is found, however, that significant difficulties may be encountered when identifying the distributions of specific forms of TFSI- anion coordination present in liq. electrolyte mixts. due to the wide range of TFSI-···Li+ cation interactions possible and the overlap of the corresponding spectroscopic data signatures.
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      Sun, X. G.; Xu, W.; Zhang, S. S.; Angell, C. A. Polyanionic electrolytes with high alkali ion conductivity. J. Phys.: Condens. Matter 2001, 13, 8235– 8243,  DOI: 10.1088/0953-8984/13/36/301
      There is no corresponding record for this reference.
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      Sun, X.; Austen Angell, C. ‘Acid-in-chain’ versus ‘base-in-chain’ anionic polymer electrolytes for electrochemical devices. Electrochim. Acta 2001, 46, 1467– 1473,  DOI: 10.1016/S0013-4686(00)00741-6
      There is no corresponding record for this reference.

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