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Quasi-Solid Semi-Interpenetrating Polymer Networks as Electrolytes: Part II. Assessing the Modes of Ion−Ion and Ion−Polymer Interactions Employing Mid-Fourier Transform Infrared Vibrational Spectroscopy Nimai Bar†,‡,§ and Pratyay Basak*,†,‡,§ †

Nanomaterials Laboratory, Inorganic & Physical Chemistry Division, Council of Scientific & Industrial Research−Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad-500 007, Andhra Pradesh, India ‡ CSIR − Network Institutes for Solar Energy (CSIR-NISE), Hyderabad-500 007, Andhra Pradesh, India § Academy of Scientific and Innovative Research (AcSIR), Hyderabad-500 007, Andhra Pradesh, India S Supporting Information *

ABSTRACT: The present study is a detailed vibrational spectroscopy investigation on the ion−ion and ion−polymer interactions that exist post-solvation or complexation in a new class of quasi-solid polymer electrolytes. Fourier transformed infrared spectra of the synthesized semi-interpenetrating polymer networks (semi-IPNs) matrix of poly(ethyleneoxide)-polyurethane/poly(ethylene glycol) dimethyl ether (P4K-PU/P2 (30:70)) complexed with LiCF3SO3 and LiN(CF3SO2)2 is deconvoluted for three primary stretching zones (ether, carbonyl, and amine) to isolate and identify the ionic species and polymer segments involved. The analysis revealed crucial information pertaining to the localized ion-association behavior, vital clues regarding the competitive interactions present, and important insights into the mechanisms of charge transport. The spectroscopic signatures imply favorable presence of positively charged triple ions or higher aggregate species along with a significant amount of “free” ions and a preferential solubility of the added electrolytes in the amorphous domains of the polymer. Critical salt concentrations Cc of EO/Li = 20 and EO/Li = 30 estimated for the LiTf and LiTFSI systems, respectively, are in good agreement with experimentally observed conductivity results. The appreciably high ionic conductivity in these semi-IPN systems could be effectively rationalized considering the nature of ionic dissociation, reassociation, and competitive interaction mechanisms. The availability of a highly disordered matrix with an optimal number of free sites, excellent segmental mobility, appreciable free volume, and finally the existence of adequate labile ionic species, all aids in the high charge transport observed in this new class of quasi-solid electrolytes.

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10−4−10−3 S cm−1 is observed in a temperature window of 20− 80 °C for the optimized semi-IPN system.28 It is a significant milestone achieved for a quasi-solid polymer matrix, particularly in the absence of any external plasticization. Polymer chains consisting of heteroatoms (O, N, S) undergo ion complexation through Lewis acid−base interactions forming the basis of an electrolyte medium. Apart from theoretical predictions,29,30 these interactions can be probed using either infrared, Raman, or NMR spectroscopy techniques.31−34 Fourier transform vibrational spectroscopy analysis (mid-FTIR) has been used extensively and effectively to quantify the nature of ionic species (free-ions, ion pairs, ion triplets, or higher aggregates) present in these systems alongside the changes occurring in the polymer binding sites, crystalline−amorphous domain ratios, and extent of H-

INTRODUCTION

Polymer electrolytes (PEs) have remained the subject of intensive research since the late 1970s with multiple advantages envisaged that can possibly have huge impact on the stability, performance, design, and safety of next generation electrochemical devices.1−7 Meeting the targeted specifications for practical applications are understandably quite challenging primarily because of the sluggish dynamics of the macromolecular medium.3,8,9 Over the years, attempts by several research groups have considerably enriched the field and narrowed down the prerequisites of such systems.10−20 In continuation of our persistent efforts,21−27 we have recently reported on a new series of semi-interpenetrating polymer network (semi-IPN) compositions with comprehensive discussion on the effects of (i) constituent composition, (ii) crosslink density, (iii) macromolecular chain length between crosslinks, (iv) molecular weight of the secondary component, (v) charge carrier concentration, and (vi) nature of anion on the matrix properties.28 Encouragingly, bulk ionic conductivities of © 2014 American Chemical Society

Received: February 25, 2014 Revised: April 29, 2014 Published: April 29, 2014 10640

dx.doi.org/10.1021/jp501972u | J. Phys. Chem. C 2014, 118, 10640−10650

The Journal of Physical Chemistry C

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bonding.35−54 Major research groups, in particular, Frech et al.,39−44 Jacobsson et al.,45 and Stevans et al.45,46 have extensively contributed toward decoding the major frequencies associated with the various ionic species and ion−polymer associations albeit using relatively simpler model systems. In the present study, we attempt to consolidate our understanding on the rather complex semi-IPN system focusing on the key ion−ion and ion−polymer interactions that exist post-solvation or complexation in these matrices. A detailed investigation on the semi-IPN matrix of synthesized poly(ethylene-oxide)-polyurethane/poly(ethylene glycol) dimethyl ether (P4K-PU/P2 (30:70)) is undertaken using infrared vibrational spectroscopy. Lithium trifluoromethanesulfonate (LiTf, LiCF3SO3) and lithium bistrifluoromethane sulfonimide (LiTFSI, LiN(CF3SO2)2) are the two electrolytes of choice because of their strong spectroscopic signatures. The focus was primarily on the ether stretching region along with the urethane carbonyl and amine stretch zones, which are expected to contain vital information pertaining to the localized ionassociation behavior. Fourier transformed infrared spectra were deconvoluted for these three zones to isolate and identify the ionic species and polymer segments involved. The cation− polymer complexation, free anions, cation−anion associations, charge pairs, effect on H-bonding, and crystalline−amorphous contributions all are examined in considerable detail. An effort is made to quantitatively assess the changes occurring in the polymer bulk as a function of electrolyte concentration and correlate their effect on the physico-chemical properties observed.

significantly as the EO/Li ratio decreases. The reaction mixture is degassed, and vigorous mixing is continued for another 30 min, under inert atmosphere, to obtain a uniformly homogeneous viscous mix of an electrolyte composition. Finally, the viscous polymer solution is casted onto a Teflon Petri dish and dried at room temperature for 24 h followed by curing at higher temperature and inert atmosphere to ensure the completion of isocyanate reaction (at 80 °C for 48 h) and to obtain the quasi-solid semi-IPN electrolyte matrix. The freestanding films so obtained have an average thickness in the range of ∼0.06−0.08 cm. The synthesized semi-IPN samples are coded as P4K-PU/P2 in the text with the corresponding composition of component I and component II provided in brackets as (30:70) indicating the respective weight percentage. Characterization and Analysis. Fourier transform infrared spectroscopy employing a Bruker ALPHA-T instrument was used to follow salt solvation in the semi-IPNs. Typically, the polymer electrolyte samples (∼2−5 mg) were grinded with dried KBr (∼200 mg) and pressed into transparent pellets of approximate dimensions Φ = 1.2 cm and t = 0.02 cm; this was followed by vacuum drying at 60 °C for 30 min prior to each run. The transmittance spectra in mid-FTIR absorption range of 4000−400 cm−1 was collected for 256 scans with a resolution interval of 2 cm −1 and corrected for baseline. The deconvolution analysis of the experimental data was done by nonlinear least-squares fits (NLSF) using Microcal OriginPro 8.5 software. Generally, the procedure involves estimating the approximate peak positions of the component bands determined using second-order derivatives.55 The determination of the best estimate for other parameters of the component curves are then arrived at through repetitive iterations by the software wherein it attempts to minimize the sum of the squares of the difference between an experimental spectrum and a computed spectrum generated by summing the component curves. Quantitative values for band areas of heavily overlapped bands can be achieved by using curve-fitting procedures with appreciable accuracy. Though the class of band shape, i.e., Gaussian, Lorentzian, Voigt, or pseudo-Voigt replica depends on the sample and can be defined by the user.55 For most disordered systems, however, the statistical distribution of oscillators often dictates the change in band shapes from the natural Lorentzian distribution to a Gaussian form. As a consequence, typical band profiles observed in disordered materials, such as polymers and glasses, are Gaussian. Hence, the multiple peak fit in the present study is achieved using a Gaussian model where maximum error associated with the simulated fits is within ±1%.

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EXPERIMENTAL SECTION Materials. All the chemicals used were of reagent grade. The chemicals were castor oil (CO) (BSS grade), diphenylmethane-4,4′-diisocyanate (MDI) (Merck), poly(ethylene glycol) (PEG, Mn ∼ 4000) (Aldrich), poly(ethylene glycol) dimethyl ether (PEGDME, Mn ∼ 500) (Aldrich), lithium trifluoromethanesulfonate (LiTf) (Aldrich), bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) (Aldrich), N,Ndimethylaniline (DMA) (Rankem), tetrahydrofuran (THF) (Rankem), and acetonitrile (CH3CN) (S.D. Fine-Chem Ltd., India). Poly(ethylene glycol)s and the solvents (THF and CH3CN) were dried prior to synthesis. Synthesis of Semi-IPN Electrolyte Matrices.28 The process of preparing a typical semi-IPN electrolyte matrix involves forming a isocyanate terminated prepolymer by reacting castor oil (−OH value ∼2.7) with a diphenylmethane-4,4′-diisocyanate (MDI) in requisite amount for 1 h using THF as the solvent and nitrogen as inert atmosphere (stage I). Thereafter, the reaction vessel containing the isocyanate-terminated prepolymer is charged with the polyether macromonomer (PEG, Mn ∼ 4000) and room-temperature catalyst N,N-dimethylaniline (DMA) to initiate the formation of the polymer networks, component I (stage II). Concurrently, component II, i.e., PEGDME (Mn ∼ 500) having nonreactive end group in the preferred weight percent, is added within the system to intimately entangle within the growing polymer network. The incorporation of electrolyte salt (LiTf or LiTFSI) of desired concentration, dissolved in a 1:1 solvent mixture of THF/CH3CN, is also achieved at this stage. The salt concentrations for electrolyte matrices are denoted as the number of ethylene oxide (EO) units per Li+ ions and expressed as EO/Li ratios 30, 20, 15, and 10. It should be noted that the concentration of the salt in the semi-IPN increases

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RESULTS AND DISCUSSION Polyethers in various architectures have formed an extensively studied model of PEs, making it an appropriate reference for validation of complex polymeric systems. The quasi-solid semiIPN matrix (P4K-PU/P2) in the present investigation primarily involves polyethers as a major component both for the networks and secondary entanglements in the bulk. A detailed investigation is undertaken to study the spectroscopic signatures of (i) cation−polymer complexation, (ii) free anions, (iii) cation−anion associations, (iv) charge pairs or higher aggregates, (v) effect on H-bonding, and (vi) crystalline− amorphous contributions. The assessment is focused primarily on three spectral zones: the ether stretching region (C−O−C; ∼1200−1000 cm−1), the carbonyl region (CO; ∼1760− 1560 cm−1), and the amine stretching region (N−H; ∼ 3600 10641



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Figure 1. (A) Representative mid-FTIR spectra of the semi-IPN polymer matrices at four different concentrations of the electrolyte, LiCF3SO3: (a) EO/Li = 10, (b) EO/Li = 15, (c) EO/Li = 20, and (d) EO/Li = 30. (B) P4K-PU/P2 incorporated with LiN(CF3SO2)2 at different EO/Li ratios as follows: (a) EO/Li = 30, (b) EO/Li = 20, (c) EO/Li = 15, and (d) EO/Li = 10, respectively. Experimental data of EO/Li ratios 30, 20, 10, for LiTf and LiTFSI are reproduced from ref 28. The highlighted areas depicted in the plots corresponding to the ether (1200−1000 cm−1), carbonyl (1760− 1560 cm−1), and amine (3700−3050 cm−1) stretching regions are the primary zones of interest in the present investigation.

Figure 2. Schematic representation for the various possible coordination environment of the Li+-ion (labile transient ion associations or stronger complexation) and multiple possibilities of H-bonding (interchain/intrachain) in the ether, carbonyl, and amine stretching regions.

cm−1-3050 cm−1). These regions in vibrational spectroscopy are expected to contain vital information pertaining to the localized ion-association behavior and the dynamics of ion−polymer interactions. Deconvolution of the Fourier transformed infrared spectra for these three zones are carried out to isolate the various contributions, identify the ionic species and polymer segments involved, and quantitatively estimate the possible role they play on the overall ionic conductivity behavior of the semiIPN matrix. The formation of poly(ethylene glycol)−polyurethane networks and the semi-IPNs of the same are now well-established from the previous studies.21−28 Nevertheless, preliminary details of mid-FTIR analysis are provided in the Supporting Information (see Figure SI-1). Figure 1A,B presents the

representative mid-FTIR spectra for LiCF3SO3 (LiTf) and LiN(CF3SO2)2 (LiTFSI) as electrolytes at four different loading concentrations (EO/Li mole ratio = 30, 20, 15, 10) in the semiIPN matrix. The solvation of the salts, i.e., ion−polymer interactions due to complexation, are clearly evidenced. In particular, the strong band at ∼1108 cm−1 is characteristic of C−O−C stretch but is observed with a prominent shoulder apparent at ca. 1098 cm−1, which is attributed to the salt dissociation and formation of Li+-ion-mediated transient crosslinks with ether oxygen (see Figure 2).35 The effect of electrolyte complexation with the polymer is significantly more pronounced for the LiCF3SO3 system. Figure 1A for the LiCF3SO3 as electrolyte shows a strong doublet at ca. 1290 and 1254 cm−1 that can be attributed to the νsym(CF3) and 10642



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Figure 3. Representative mid-FTIR stack plots for the ether stretching region (νC−O−C, 1200−1000 cm−1) in semi-IPNs P4K-PU/P2 30:70 with (A) LiTf and (B) LiTFSI at different EO/Li ratios as electrolyte. Plots of d2y/dx2 are the corresponding second derivative of the spectral region followed by the deconvolution of individual compositions: (i) EO/Li = 30, (ii) EO/Li = 20, (iii) EO/Li = 15, and (iv) EO/Li = 10 for the respective salts. The deconvolution analysis with Gaussian multipeak fitting is done using OriginPro 8.5 Software; open circles represent the experimental data, and the red line depicts the cumulative fit for each spectra. The vertical dotted lines are a guide to the eyes.

νasym(CF3), respectively.36 A medium intensity band at ∼1035 cm−1 partially merged in the C−O−C region is the contribution from free ions, contact ion pairs, and higher aggregates, whereas the peak at ca. 640 cm−1 indicates the strong O−Li stretch.29,36 The strong bands at ∼1354 cm−1 and ∼1194 cm−1 for LiN(CF3SO2)2 (Figure 1B) loaded polymer matrix are indicative of the νasym(SO2) in-plane stretching vibrations and the νsym(SO2) from the imide anion along with the ν(C−O) of the −OCH3 group.36 The shoulder at ca. 1058 cm−1 can be associated with the contributions from SO2 stretching and νa(SNS) of the TFSI anion, respectively.29,30 The peaks at ca. 787 and 737 cm−1 correspond to ν(C−S) and the CF3 symmetric bend (δsym) of the imide ion, respectively.44 The asymmetric in-plane bending modes of SO2 in the imide anion gives a peak at ∼618 cm−1 along with two asymmetric bending vibrations (δasym) of CF3 at ca. 570 cm−1 and ca. 510 cm−1.29 The bands observed appear stronger with increasing salt concentration (decreasing EO/Li ratio) suggesting excellent

ion dissociation within the polymer matrix. The ion−polymer interactions and noticeable band broadening in the amine and carbonyl stretching regions are also evident.53,54 Apart from the various ion−polymer interactions, possibilities of multiple hydrogen bonding in these semi-IPNs between the proton donor (−N−H in urethane) and one of the three different proton acceptors (urethane carbonyl −CO, ether −C−O−C, and ester C(O)−O−C carbonyl) coexists (Figure 2). Apart from this, polyether urethanes are characterized by hard−hard segment hydrogen bonding (NH···OC) and hard−soft segment hydrogen bonding involving ether oxygen (NH···O) that represents the extent of mixing of the hard−soft phases. The degree of these interactions is estimated by rigorous deconvolution analysis of the spectrum in three major regions as discussed in the following sections. Ether (−C−O−C−) Stretching Region. Panels A and B of Figure 3 depict the stack plots for the mid-FTIR spectra in the ∼1200−1000 cm−1 region corresponding to the ether stretch ν(C−O−C) of the semi-IPNs P4K-PU/P2/LiCF3SO3 and 10643



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Table 1. Deconvolution Summary of Mid-FTIR Multi-Peak Fitting over the Ether Stretching Region (∼1200−1000 cm−1) for the Semi-IPNs P4K-PU/P2 (30:70) Matrix Incorporated with Li(CF3SO3) and LiN(CF3SO2)2 Salts peak position (wavenumber (cm−1)) and area percentage (% A) of the corresponding peaks

semi-IPN composition EO/Li 10 15 20 30

1015 (1) 1.93 1.60 0.99 0.54

1027 (2) 2.30 2.67 3.55 2.75

1034 (3) 9.18 7.53 6.13 7.60

EO/Li 10 15 20 30

(1) 1018 0.61 0.83 0.74 0.71

(2) 1029 3.45 1.90 3.58 2.57

(3) 1041 1.72 4.25 1.98 3.46

PEG Mn ∼ 4000, PEGDME Mn ∼ 500, LiCF3SO3 1051 1063 1075 1085 1093 1105 (4) (5) (6) (7) (8) (9) 4.13 2.28 10.43 5.35 7.51 18.96 2.27 3.53 10.74 4.49 10.75 16.25 2.90 3.34 9.60 6.05 8.51 17.75 3.69 2.34 12.11 4.52 9.53 14.98 PEG Mn ∼ 4000, PEGDME Mn 500, LiN(CF3SO2)2 (4) (5) (6) (7) (8) (9) 1053 1061 1073 1083 1095 1107 6.17 12.21 11.63 2.16 17.60 18.77 1.08 17.77 0.77 17.31 4.72 22.56 7.57 8.89 3.60 18.35 4.74 17.79 0.41 11.78 2.62 14.84 11.5 19.33

1118 (10) 7.97 10.56 10.32 10.05

1130 (11) 11.21 11.46 11.02 11.34

1143 (12) 5.81 7.39 10.70 10.74

1156 (13) 6.63 6.89 7.15 7.51

1168 (14) 6.30 3.85 1.99 2.28

(10) − − − − −

(11) 1127 13.83 11.83 11.38 10.48

(12) 1140 9.41 14.39 18.17 18.71

(13) 1149 2.43 2.60 3.20 3.60

(14) − − − − −

Figure 4. Relative FTIR band area percentage as determined for each EO/Li mole ratio in the ether stretching region for the quasi-solid polymer electrolytes (a) and (b) semi-IPN P4K-PU/P2 (30:70)/Li(CF3SO3); (c) and (d) semi-IPN P4K-PU/P2 (30:70)/LiN(CF3SO2)2.

be effectively resolved into 14 peaks, whereas for the P4K-PU/ P2/LiN(CF3SO2)2 the same region was composed of 12 bands. The quantitative approximations of the percent area obtained for each contributing species are summarized in Table 1 and Figure 4a−d. In the case of semi-IPN P4K-PU/P2/Li(CF3SO3), the bands centered around 1143 and 1105 cm−1 that dominates the spectrum are attributed to the stretching modes of free and Li+coordinated ether groups in amorphous domains, respectively. Band 1118 cm−1 arises because of the free (C−O−C) groups in the crystalline domains of the P4K-PU/P2 matrix. Both the 1143 and 1118 cm−1 contribution are seen to steadily decrease with increasing salt concentrations (Figure 4b). That the salt

P4K-PU/P2/LiN(CF3SO2)2, respectively, at four different electrolyte concentrations. The zoomed in region clearly indicates contribution from several species, as evident from the broad shoulders. Second-order derivative plots corresponding to the spectra confirms the presence of multiple hidden peaks and reveals a reasonable estimate of the number of peaks and the approximate peak position. As provided in the stacked plots for each of the four salt concentrations, deconvolutions were successfully achieved in each case. The cumulative best fits on the experimental data were obtained by iterative leastsquares fitting of multiple Gaussian models varying the peak position, width, and area using Microcal OriginPro 8.5 software. The ether region of semi-IPN P4K-PU/P2/Li(CF3SO3) could 10644



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The feature at ca. 1041 cm−1 can be assigned to the H-bonded C−O−C stretch of the PEG units.46,50 The bands centered at 1053 and 1061 cm−1 can be associated with the SO2 stretching and νa(SNS) of the TFSI anion, respectively.29,30 The 1149 cm−1 peak can be associated with the asymmetric stretching vibration of the CF3 for the TFSI anion.29,51,52 All the other significant contributions of the 12 peaks decoded for the LiTFSI, the peak positions at ca. 1127 and 1095 cm−1, can be assigned to the crystalline complex,38,39 whereas 1083 and 1073 cm−1 correspond to the vibrations of C(O)−O−C and the Li+ion-coordinated C(O)−O−C stretch of the urethane segment in semi-IPN P4K-PU/P2, respectively.46,50 From Table 1 and Figure 4, some of the key observations that can be inferred for the P4K-PU/P2 loaded with LiTf and LiTFSI systems are that several possibilities of ionic dissociation and association coexists within the complex semiIPN matrix. The spectroscopic signatures as observed for the LiTf system imply preferential presence of positively charged triple ions and higher aggregate species along with a significant amount of “free” ions. Interestingly, this observation carries particular significance at relatively higher salt concentration ranges, where most other studies indicate strong presence of contact ion pairs that probably impedes ionic conductivity severely. Barring the composition P4K-PU/P2 (30:70) with LiTFSI in EO/Li mole ratio =10, no substantial evidence of any detrimental change are observed in the assessment of ionic conductivities of these semi-IPN matrices (see Figures SI-4 and SI-5 of Supporting Information). The ion dissociation characteristics are hence indicative of a very robust system (semi-IPN P4K-PU/P2 (30:70)) and lends strong credence to our previous claims.28 The rather numerous bands associated with C−O−C give an apparent impression toward the lack of any uniform or appreciable change in the degree of complexation. Nonetheless, a closer observation reveals that this is probably true only when considering the overall degree of ion complexation (for both transient and stronger complexation). The most significant observation can be undoubtedly deduced from the change in the stretching band pertaining to the free C−O−C in the amorphous domains (1146 cm−1). The decrease in the band area along with the trend is noticeable with increasing salt concentration. No apparent trend could be similarly discerned for the free C−O−C in the crystalline domains (1118 cm−1) until a very high salt concentration (EO/Li = 10) was achieved. This signifies a preferential solubility of the added electrolytes in the amorphous domains of the polymer as also surmised in our previous report.22 Notably, any increase in the total complexation (C−O−C···Li+) within the matrix is not reflected in a concomitant increase in ion conductivity. It is now wellunderstood that the bulk ionic conductivity is dependent on not only the number of charge carriers coupled with optimal availability of vacant jump-sites but also equally on the degree of cross-links that restrict the segmental motion. This rationale can be extended in the present context to determine a critical salt concentration Cc where all the factors (cation−polymer complexation, free ions, cation−anion associations, charge pairs and higher aggregates, effect on H-bonding, crystalline− amorphous contributions) combine to provide the most favorable environment for a faster charge transport. The critical salt concentration Cc of EO/Li = 20 and EO/Li = 30 estimated for the LiTf and LiTFSI systems, respectively, are in good agreement with experimentally observed conductivity results (see Supporting Information).

complexation preferably occurs in the amorphous domains first is very evident from the trend where a faster decrease in the relative peak area is observed for the 1143 cm−1. Bands at ca. 1075, 1084, 1094, and 1130 cm−1 are assigned to the extent of bonding of Li+ ions with ether and urethane groups.37 The peak positions at 1132 and 1094 cm−1 are assigned for the crystalline complex38,39 The 1085 and 1075 cm−1 bands can be associated with the stretching vibration of C(O)−O−C and the Li+-ioncoordinated C(O)−O−C stretch of the urethane segment in P4K-PU/P2.46,50 The band centered at around 1060 cm−1 for all the polymer semi-IPN matrices is assigned to the combined asymmetric stretching for C−O−C plus the symmetric rocking modes of methylene groups (νas(C−O−C) + rs(CH2)).38 The νs(SO3) mode of “free” triflate anion, with its frequency reported as being cation-independent, can be unambiguously assigned to the peak positioned at 1034 cm−1.40,47 Any coordination with Li+ ion leads to a nondegeneracy in the νs(SO3) band, and the resulting signals are shifted to higher wavenumbers.38,41−43,46,48 The contribution at 1051 cm−1 in the spectra can be undoubtedly ascribed to the bridging aggregate of positively charged triplets, [Li2(CF3SO3)]+, and is in good agreement with earlier reports.46,56,57 Another component present at 1063 cm−1 in the νas(SO3) envelop is associated with a divalent positively charged counterpart, [Li3(CF3SO3)]2+, formed because of tridentate bridging.56 The bands found at 1027 and 1015 cm−1 can probably be associated with either weakly bonded triflate ions with a Li+-ion simultaneously interacting with carbonyl oxygen of the urethane groups or the CF3SO3− ions feebly H-bonded to the amine group of the urethane linkage.49 FTIR bands for the polymer samples P4K-PU/P2/Li(CF3SO3) at around 1156 and 1168 cm−1 are assigned to the asymmetric stretching vibration of the CF3 group (νas(CF3)) of the CF3SO3− anion in the crystalline and amorphous domains of the complex.38,44 Interestingly, absence of any contributions at ca. 1040 cm−1 provided clear evidence for the non-existence of any Li+· (CF3SO3)− contact ion pairs or negatively charged triplets, [Li(CF3SO3)2]−,46,56,57 indicating a very favorable dissociation of ionic charges within the polymeric matrix. Likewise in the case of semi-IPN P4K-PU/P2/LiN(CF3SO2)2, the bands centered around 1140 and 1107 cm−1 that dominate the spectrum are attributed to the stretching modes of free and Li+-coordinated ether groups in amorphous domains, respectively (Figure 3B). Unlike in the case of LiTfloaded semi-IPNs, the 1118 cm−1 band due to the free (C−O− C) groups in the crystalline domains of the P4K-PU/P2 matrix is evidently absent. This is consistent with our observation for the differential scanning calorimetry results obtained for the semi-IPN LiTFSI system where the presence of crystalline domains was found to be almost minimal or absent (see Supporting Information).28 This is probably because of the better ion dissociation and inherent plasticization of the semiIPN matrix by the relatively larger TFSI anion. Furthermore, it is also important to note that the relatively low symmetry of N(CF3SO2)2− makes the spectroscopic analysis less sensitive than that for the CF3SO3− ion30 and is manifested as noticeably subdued signals in the spectroscopic region 1040−1000 cm−1. The bands found at 1029 and 1018 cm−1, as in the case of LiTf, can probably be associated with either weakly bonded triflate ions with a Li+-ion simultaneously interacting with carbonyl oxygen of the urethane groups or the CF3SO3− ions feebly Hbonded to the amine group of the urethane linkage.49 The band at 1034 cm−1, as in the case of LiTf, is unmistakably absent. 10645



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Carbonyl Stretching (−CO) Region. Spectral deconvolution of the carbonyl group region (1760−1560 cm−1) of the quasi-solid polymer electrolytes P4K-PU/P2 semi-IPNs matrices loaded with LiTf and LiTFSI was similarly achieved using a multiple Gaussian peak fit function (Figure 5A,B).

Deconvolution for both the electrolyte compositions yielded 12 Gaussian bands of which most of the major peaks could be assigned without ambiguity (second-order derivative plots provided as Figure SI-6 of Supporting Information). The peak at 1746 cm−1 could be assigned to the ester carbonyl stretch, νs(C(O)−O−C), of the castor oil used as a crosslinker. The band at ca. 1729 cm−1 is unambiguously assigned for the stretching vibration of “free” urethane carbonyl or nonH-bonded −CO.46,50 The stretching frequency of ca. 1706 cm−1 corresponds to the disordered hydrogen-bonded urethane carbonyls, while ∼1689 cm−1 can be attributed to the ordered H-bonded urethane carbonyl groups slightly shifted toward lower frequency because of their interactions with Li+ions.46,50,58−60 A concomitant decrease of the peak area of 1729 cm−1 coupled with apparent increase in the sum of the 1706 and 1689 cm−1 peak areas with increasing salt concentration confirms salt complexation in the polyurethane segments. The band at ∼1666 cm−1 can be associated with the strong carbonyl stretch along with the aromatic −CC− stretch of the MDI moiety. The deconvoluted peaks at wavenumbers ca. 1650 and 1634 cm−1 as intermediate bands of modest intensity could possibly be ascribed to ordered or disordered interfacial domains in the polyurethane structure.59 The bands at ca. 1626 and 1618 cm−1 are indicative of strong interurethane or urea H-bonding,59,61,62 whereas the peak at 1605 cm−1 can be assigned to the aliphatic −CC of the castor oil. The peaks at ∼1595 and 1581 cm−1 correspond to the free and Li+-bonded amide II stretching regions and hence are apparently dependent on the salt concentration.59 The trends of the integral peak area distribution in the carbonyl region and associated changes in contributions with electrolyte concentrations are clearly evident from the data in Table 2 and Figure 6. Amine (N−H) Stretching Region. In P4K-PU/P2 semiIPNs, the polymeric architecture contains as a part of the urethane linkage a secondary amine which is directly attached to a carbonyl group. Deconvolution of amine stretching frequency carried out using Gaussian peakfit over the range of 3700−3050 cm−1 resulted in 11 peaks for both LiTf and LiTFSI electrolytes (Figure 7A,B). The major peaks isolated in this broad region could be ascertained with understandably a certain degree of uncertainty (second-order derivative plots provided as Figure SI-7 of Supporting Information). Nonethe-

Figure 5. Representative mid-FTIR stack plots for the carbonyl stretching region (νCO), 1780−1560 cm−1 in semi-IPNs P4K-PU/P2 30:70 with (A) LiTf and (B) LiTFSI at different EO/Li ratios as electrolyte. Deconvolution of individual compositions: (i) EO/Li = 30, (ii) EO/Li = 20, (iii) EO/Li = 15, and (iv) EO/Li = 10 for the respective salts are presented. The open circles represent the experimental data, and the red line depicts the cumulative fit for each spectra. The vertical dotted lines are a guide to the eyes.

Table 2. Deconvolution Summary of Mid-FTIR Multi-Peak Fitting over the Carbonyl Stretching Region (∼1760−1560 cm−1) for the Semi-IPNs P4K-PU/P2 (30:70) Matrix Incorporated with Li(CF3SO3) and LiN(CF3SO2)2 Salts peak position (wavenumber (cm−1)) and area percentage (% A) of the corresponding peaks

semi-IPN composition EO/Li 10 15 20 30

1581 (1) 2.08 1.86 0.82 0.24

1595 (2) 5.24 6.78 9.31 9.70

EO/Li 10 15 20 30

(1) 1581 1.10 1.20 1.53 2.22

(2) 1595 5.61 9.31 5.78 8.54

PEG Mn ∼ 4000, PEGDME Mn ∼ 500, LiCF3SO3 1605 1618 1626 1634 1650 (3) (4) (5) (6) (7) 8.06 1.38 24.58 7.28 16.49 3.40 16.38 0.80 16.73 17.17 6.25 8.23 4.53 9.47 17.72 7.14 6.27 7.56 6.46 15.43 PEG Mn ∼ 4000, PEGDME Mn 500, LiN(CF3SO2)2 (3) (4) (5) (6) (7) 1605 1618 1626 1634 1650 5.74 20.19 19.08 11.49 12.37 14.16 15.38 10.55 18.62 16.23 7.05 15.72 18.43 12.14 10.58 7.53 20.47 16.10 8.93 8.36 10646

1666 (8) 13.96 11.31 13.27 12.49

1689 (9) 6.92 7.18 5.23 7.47

1706 (10) 4.52 2.54 7.59 4.86

1729 (11) 5.73 8.98 10.95 13.43

1746 (12) 3.76 7.74 6.62 8.93

(8) 1666 5.96 0.35 3.73 8.18

(9) 1689 6.47 3.66 8.23 3.16

(10) 1706 5.25 5.23 6.68 8.30

(11) 1729 4.08 2.92 7.69 5.62

(12) 1746 2.66 2.39 2.43 2.61



Article

Figure 6. Relative FTIR band area percentage as determined for each EO/Li mole ratio in the carbonyl stretching region (1760 cm−1-1560 cm−1) for the quasi-solid polymer electrolytes (a) semi-IPN P4K-PU/P2 (30:70)/Li(CF3SO3) and (b) semi-IPN P4K-PU/P2 (30:70)/LiN(CF3SO2)2.

3452 cm−1 can be attributed to the “free” or nonbonded −N− H group for the hard and soft segments of the polyurethane matrix.46,50,58 As can be observed, their contribution does vary with the salt loading. When the non-H-bonded −N−H groups interacts with the Li+ cations to form transient cross-links, these labile bonds give rise to a band at slightly lower frequency, ca. 3410 cm−1.46 Unlike transient cross-links, possibilities of stronger interaction between the nitrogen lone-pair and Li+ions also coexist. The delocalization of lone-pair electrons because of strong metal-ion-mediated coordination effectively reduces the −N−H bond length. The vibrational energy of such a bond will be increased and manifest itself at a higher frequency. The observed 3533 cm−1 band can be attributed to such an environment.46 Two other major bands that could be ascertained are in the ∼3245−3251 cm−1 and ∼3312−3318 cm−1 regions for the −N−H hydrogen bonded to the ether oxygen of the poly(ethylene glycol) chain and the −N−H hydrogen bonded to carbonyl oxygen of the polyurethane segment.46,50,58 The 3363 cm−1 band is assigned to the Hbonded N−H with carbonyls where the carbonyls are likely interacting with Li+-ions.58 Under the circumstances of competitive interactions induced by the cations for the share of electron-rich oxygen, the strength of the hydrogen bond between the amine and carbonyl is weakened. A significantly stronger N−H bond yet in close interaction with the carbonyl hence shifts toward a slightly higher frequency. The possibility of a few unreacted −O−H groups of the used macrodiols (poly(ethylene glycol) and castor oil) definitely exists, and this is seen as a band at ca. 3633 cm−1. These −O−H groups interacting with Li+ shift to lower frequency, as observed with a contribution at 3587 cm−1, indicating a reduced participation in H-bonding. The band at ca. 3188 cm−1 can be attributed to the stretching frequency of sp2-hybridized carbon−hydrogen (−C− H) bond of MDI.63 The spectral assignments in both carbonyl (−CO) and amine (−N−H) regions imply significant coexistence of both inter- and intra-hydrogen bonded polyurethane segments as well as evidence of ion complexation. A strong presence of competitive interactions of extensive H-bonding and Li+-ionmediated complexation both creates a very conducive local environment in these domains for effective charge dissociation and provides labile/transient ionic species that can efficiently jump through the available free sites. The dynamic nature of ionic dissociation, reassociation, and migration through the

Figure 7. Representative mid-FTIR stack plots for the amine stretching region (νN−H) 3700−3050 cm−1 in semi-IPNs P4K-PU/ P2 30:70 with (A) LiTf and (B) LiTFSI at different EO/Li ratios as electrolyte. Deconvolution of individual compositions: (i) EO/Li = 30, (ii) EO/Li = 20, (iii) EO/Li = 15, and (iv) EO/Li = 10 for the respective salts are presented. The open circles represent the experimental data, and the red line depicts the cumulative fit for each spectra. The vertical dotted lines are a guide to the eyes.

less, the characteristic trends due to salt complexation and formation of transient cross-links were quite apparent (Figure 8, Table 3). The prominent bands at peak position 3498 and 10647



Article

Figure 8. Relative FTIR band area percentage as determined for each EO/Li mole ratio in the amine stretching region (3700−3100 cm−1) for the quasi-solid polymer electrolytes (a) semi-IPN P4K-PU/P2 (30:70)/Li(CF3SO3) and (b) semi-IPN P4K-PU/P2 (30:70)/LiN(CF3SO2)2.

Table 3. Deconvolution Summary of Mid-FTIR Multi-Peak Fitting over the Amine Stretching Region (∼3700−3050 cm−1) for the Semi-IPNs P4K-PU/P2 (30:70) Matrix Incorporated with Li(CF3SO3) and LiN(CF3SO2)2 Salts peak position (wavenumber (cm−1)) and area percentage (% A) of the corresponding peaks

semi-IPN composition (1) 3120 3.43 3.42 0.97 0.98

EO/Li 10 15 20 30

EO/Li 10 15 20 30

(1) 3120 3.47 3.86 3.03 3.95

(2) 3188 3.54 5.55 4.85 4.70

(2) 3188 6.13 4.27 6.40 3.22 (3) 3249 11.19 10.30 11.75 14.33

PEG Mn ∼ 4000, PEGDME Mn ∼ 500, LiCF3SO3 (3) (4) (5) (6) (7) 3249 3316 3363 3410 3452 8.74 10.42 11.33 12.46 9.75 7.72 15.60 9.41 11.96 15.39 7.42 13.58 5.40 21.08 10.17 9.30 13.74 10.47 4.47 24.00 PEG Mn ∼ 4000, PEGDME Mn 500, LiN(CF3SO2)2 (4) (5) (6) (7) 3316 3363 3410 3452 7.36 12.66 9.79 21.11 7.93 14.11 13.19 15.35 5.95 16.06 13.49 8.48 4.83 13.88 12.36 13.71

(8) 3498 14.02 11.88 17.46 9.96

(9) 3533 9.91 7.85 8.81 11.92 (9) 3533 3.92 6.66 8.11 11.14

(10) 3587 7.74 8.46 9.74 8.91 (10) 3587 11.42 9.16 9.31 9.51

(11) 3633 1.13 0.21 1.61 2.80 (11) 3633 1.53 2.01 1.51 1.61

positively charged triple ions and higher aggregate species along with a significant amount of “free” ions. Interestingly, there was no evidence of contact ion pairs (Li+·CF3SO3) or negatively charged triplets, [Li(CF3SO3)2]−, that usually tend to affect ion conductivity in such matrices adversely. The ion dissociation characteristics are indicative of a very robust system (semi-IPN P4K-PU/P2 (30:70)) and lends strong credence to our previous claims. The most significant observation can be undoubtedly deduced from the change in the stretching band pertaining to the free C−O−C in the amorphous domains (1146 cm−1). The decrease in the relative band area along with the trend is noticeable with increasing salt concentration. This signifies a preferential solubility of the added electrolytes in the amorphous domains of the polymer. A critical salt concentration Cc of EO/Li = 20 and EO/Li = 30 estimated for the LiTf and LiTFSI system, respectively, are in good agreement with experimentally observed conductivity results. Coexistence of extensive inter- and intra-hydrogen bonded polyurethane segments along with evidence of ion complexation indicates competitive interactions for the available heteroatoms. This dynamics creates a very conducive local environment in these domains for effective charge dissociation providing ample labile/transient ionic species that can efficiently jump through the available free sites. The appreciably high ionic conductivity

highly disordered matrix explains the appreciably high ionic conductivity (10−4−10−3 S cm−1) observed for these semi-IPN systems.

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(8) 3498 18.95 15.72 14.80 10.19

CONCLUSIONS

The main objective of this work was to attempt vibrational band assignments of the key ion−ion and ion−polymer interactions and to consolidate our understanding of a rather complex semi-IPN system. In this effort, a detailed investigation on the synthesized semi-IPN matrix of poly(ethylene-oxide)polyurethane/poly(ethylene glycol) dimethyl ether (P4K-PU/ P2 (30:70)) is undertaken, focusing on the key ion−ion and ion−polymer interactions that exist post-solvation or complexation. Fourier transformed infrared spectra were successfully deconvoluted for three primary stretching zones (ether, carbonyl, and amine) to isolate and identify the ionic species and polymer segments involved. As expected, the deconvolution studies revealed that these three zones contain crucial information pertaining to the localized ion association behavior (cation−polymer complexation, free anions, cation−anion associations, charge pairs, effect on H-bonding, and crystalline−amorphous contributions) that provided important insights and vital clues. The spectroscopic signatures as observed primarily for the LiTf system imply preferential presence of 10648



Article

(11) Wright, P. V. Electrical conductivity in ionic complexes of poly(ethylene oxide). Br. Polym. J. 1975, 7, 319−327. (12) Wright, P. V. An Anomalous Transition to a Lower Activation Energy for dc Electrical Conduction above the Glass-Transition Temperature. J. Polym. Sci., Polym. Phys. Ed. 1976, 14, 955−957. (13) Armand, M. B.; Chabagno, J. M.; Duclot, M. J. In Extended Abstracts, Second International Conference on Solid Electrolytes, St. Andrews, Scotland, 1978. (14) Armand, M. B.; Chabagano, J. M.; Duclot, M. J. In Fast Ion Transport in Solids: Electrodes Electrolytes, Proceedings of the International Conference on Fast Ion Transport in Solids, Electrodes, and Electrolytes, Lake Geneva, WI, May 21−25, 1979. (15) Fast Ion Transport in Solids; Vashishta, P., Mundy, J. N., Shenoy, G. K., Eds.; Elsevier/North-Holland: New York, 1979; p 131. (16) Ratner, M. A. In Polymer Electrolyte Reviews-1; MacCallum, J. R., Vincent, C. A., Eds. Elsevier Applied Science: New York, 1987; Vol. 1. (17) MaCallum, J. R;. Vincent, C. A. Polymer Electrolytes Reviews; Elsevier: London, 1987 (Vol 1), 1989 (Vol 2). (18) Ratner, M. A.; Shriver, D. F. Ion Transport in Solvent-Free Polymers. Chem. Rev. (Washington, DC, U.S.) 1988, 88, 109−124. (19) Gray, F. M. Polymer Electrolytes; The Royal Society of Chemistry: Cambridge, U.K., 1997. (20) Alamgir, M., Abraham, K. M. In Lithium Batteries-New Materials, Developments and Perspectives; Pistoia, G., Ed.; Elsevier: Amsterdam, 1994. (21) Basak, P.; Manorama, S. V. PEO-PU/PAN Semi-Interpenetrating Polymer Networks for SPEs: Influence of Physical Properties on the Electrical Characteristics. Solid State Ionics 2004, 167, 113−121. (22) Basak, P.; Manorama, S. V. Poly(ethylene oxide)−Polyurethane/ Poly(acrylonitrile) Semi-Interpenetrating Polymer Networks for Solid Polymer Electrolytes: Vibrational Spectroscopic Studies in Support of Electrical Behavior. Eur. Polym. J. 2004, 40, 1155−1162. (23) Basak, P.; Manorama, S. V.; Singh, R. K.; Prakash, O. Investigations on the Mechanisms of Ionic Conductivity in PEO− PU/PAN Semi-interpenetrating Polymer Network−Salt Complex Polymer Electrolytes: An Impedance Spectroscopy Study. J. Phys. Chem. B 2005, 109, 1174−1182. (24) Basak, P.; Manorama, S. V. Thermo-mechanical Properties of PEO-PU/PAN Semi- Interpenetrating Polymer Networks and their LiClO4 Salt-Complexes. J. Macromol. Sci., Part A: Pure Appl. Chem. 2006, A43, 369−382. (25) Basak, P.; Parkash, O.; Chatterji, P. R. Solid Polymer Electrolytes: Interpenetrating Polymer Networks doped with Lithium Perchlorate. J. Macromol. Sci., Part A: Pure Appl.Chem. 2001, A38 (4), 399−415. (26) Ramanjaneyulu, K.; Bar, N.; Shah, Md. S. A. S.; Manorama, S. V.; Basak, P. Semi-Interpenetrating Polymer Networks as Solid Polymer Electrolytes: Effects of Ion-Dissociation, Crosslink Density and Oligomeric Entanglements on the Conductivity Behavior in Poly(ethylene oxide)−Polyurethane/ Poly(acrylonitrile) Matrix. J. Power Sources 2012, 217, 29−36. (27) Shah, Md. S. A. S.; Basak, P.; Manorama, S. V. Polymer Nanocomposites as Solid Electrolytes: Evaluating Ion−Polymer and Polymer−Nanoparticle Interactions in PEG-PU/PAN Semi-IPNs and Titania Systems. J. Phys. Chem. C 2010, 114, 14281−14289. (28) Bar, N.; Ramanjaneyulu, K.; Basak, P. Quasi-Solid SemiInterpenetrating Polymer Networks as Electrolytes: Part I. Dependence of Physicochemical Characteristics and Ion Conduction Behavior on Matrix Composition, Cross-Link Density, Chain Length between Cross-Links, Molecular Entanglements, Charge Carrier Concentration, and Nature of Anion. J. Phys. Chem. C 2014, 118, 159−174. (29) Gejji, S. P.; Suresh, C. H.; Babu, K.; Gadre, S. R. Ab Initio Structure and Vibrational Frequencies of (CF3SO2)2N−Li+ Ion Pairs. J. Phys. Chem. A 1999, 103, 7474−7480. (30) Rey, I.; Johansson, P.; Lindgren, J.; Lassegues, J. C.; Grondin, J.; Servant, L. Spectroscopic and Theoretical Study of (CF3SO2)2N− (TFSI−) and (CF3SO2)2NH (HTFSI). J. Phys. Chem. A 1998, 102, 3249−3258.

in these semi-IPN systems (notably devoid of any external plasticization) could hence be effectively rationalized considering the nature of ionic dissociation, reassociation, and competitive interaction mechanisms. The availability of a highly disordered matrix with an optimal number of free sites, excellent segmental mobility, appreciable free volume, and finally existence of adequate labile ionic species all aid in the high charge transport observed in this new class of quasi-solid electrolytes.

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ASSOCIATED CONTENT

S Supporting Information *

Details on the formation of semi-IPNs probed by FTIR spectroscopy, differential scanning calorimetry thermograms for the semi-IPN formation, temperature dependence of ion conductivity behavior for LiCF3SO3 and LiN(CF3SO2)2 as electrolytes, and second-order derivative plots for the carbonyl and amine stretching regions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Phone: 040-27193225/27191386. Fax: +91-40-27160921. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.B. acknowledges Council of Scientific and Industrial Research (CSIR), India for financial assistance in the form of senior research fellowship (SRF). P.B. duly acknowledges the strong support of DST-Ramanujan Fellowship (GAP-0248), MNRE-CSIR TAPSUN Project on Dye Sensitized Solar Cells (DyeCell: GAP-0366), and CSIR TAPSUN Project on Innovative Solutions for Solar Energy Storage (StoreSolar: NWP-0056) for the grants received. The authors sincerely appreciate the encouragement and considerable help received from Dr. S.V. Manorama, Dr. K.V.S.N. Raju, Dr. R.K. Rana, and Dr. R. Narayan during the course of this investigation.

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REFERENCES

(1) Thomas, K. E.; Darling, R. M.; Newman, J. In Advances in Lithium Ion Batteries;van Schalkwijk, W. A., Scrosati, B., Eds.; Kluwer Academic/Plenum Publishers: New York, 2002; p 345. (2) Scrosati, B.; Garche, J. Lithium Batteries, Prospects and Future. J. Power Sources 2010, 195, 2419−2430. (3) Quartarone, E.; Mustarelli, P. Electrolytes for Solid-State Lithium Rechargeable Batteries: Recent Advances and Perspectives. Chem. Soc. Rev. 2011, 40, 2525−2540. (4) Gray, F. M. In Solid Polymer Electrolytes-Fundamentals and Technological Applications; VCH: Weinhem, Germany, 1991. (5) Kleitz, M.; Sapoval, B.; Ravaine, D., Eds. Solid State Ionics; North Holland: Amsterdam, 1983. (6) Lewis, N. S. Towards Cost-Effective Solar Energy Use. Science 2007, 315, 798−801. (7) Grätzel, M. Photoelectrochemical Cells. Nature (London, U.K.) 2001, 414, 338−344. (8) Kang, X. Nonaqueous Liquid Electrolytes for Lithium Based Rechargeable Batteries. Chem. Rev. (Washington, DC, U.S.) 2004, 104, 4303−4417. (9) Scrosati, B.; Croce, F.; Panero, S. Progress in Lithium Polymer Battery R&D. J. Power Sources 2001, 100, 93−100. (10) Fenton, D. E.; Parker, J. M.; Wright, P. V. Complexes of Alkali Metal Ions with Poly(ethylene oxide). Polymer 1973, 14, 589−589. 10649



Article

(31) Chaurasia, S. K.; Singh, R. K.; Chandra, S. Ion-Polymer Complexation and Ion-pair Formation in a Polymer Electrolyte PEO:LiPF6 Containing an Ionic Liquid Having Same Anion: A Raman Study. Vib. Spectrosc. 2013, 18, 190−195. (32) Ferry, A.; Jacobsson, P.; Torrel, L. M. The Molar Conductivity Behavior in Polymer Electrolytes at Low Salt Concentration: Raman Study of Poly(ethylene glycol) Complexed with LiCF3SO3. Electrochim. Acta 1995, 40, 2369−2373. (33) Thomas, K.-J.; Köster, L. v. W. Phase Separation and Local Cation Coordination in PEOnLiNTf2 Polymer Electrolytes: Lessons from Solid State NMR. Solid State Ionics 2008, 178, 1879−1889. (34) Abbrent, S.; Greenbaum, S. Recent Progress in NMR Spectroscopy of Polymer Electrolytes for Lithium batteries. Curr. Op. Colloid Interface Sci. 2013, 18, 228−244. (35) Edman, L. Ion Association and Ion Solvation Effects at the Crystalline−Amorphous Phase Transition in PEO−LiTFSI. J. Phys.Chem. B 2000, 104, 7254−7258. (36) Bishop, A. G.; MacFarlane, D. R.; McNaughton, D.; Forsyth, M. FT-IR Investigation of Ion Association in Plasticized Solid Polymer Electrolytes. J. Phys. Chem. 1996, 100, 2237−2243. (37) Hong, L.; Cui, Y.; Wang, X.; Tang, X. Conductivities and Spectroscopic Studies on Hyperbranched Polyurethane Electrolytes. J. Polym. Sci., Part B: Polym. Phys. 2003, 4, 1120. (38) Dissanayake, M. A. K. L.; Frech, R. Infrared Spectroscopic Study of the Phases and Phase Transitions in Poly(ethylene oxide) and Poly(ethylene oxide)−Lithium Trifluoromethanesulfonate Complexes. Macromolecules 1995, 28 (15), 5312. (39) Frech, R.; Papke, B. L.; Ratner, M. A.; Shriver, D. F. Vibrational Spectroscopy and Structure of Polymer Electrolytes, Poly(ethylene oxide) Complexes of Alkali Metal Salts. J. Phys. Chem. Solids 1981, 42, 493. (40) Chintapalli, S.; Quinton, C.; Frech, R.; Vincent, C. A. Compound Formation and Ionic Association in the Poly(ethylene oxide)−Potassium Triflate System. Macromolecules 1997, 30, 7472− 7477. (41) Bernsen, A.; Lindgren, J.; Huang, W.; Frech, R. Coordination and Conformation in PEO, PEGM and PEG Systems Containing Lithium or Lanthanum Triflate. Polymer 1995, 36 (23), 4471. (42) Frech, R.; Chintapalli, S.; Bruce, P. G.; Vincent, C. A. Structure of an Amorphous Polymer Electrolyte, Poly(ethylene oxide): LiCF3SO3. Chem. Commun. (Cambridge, U.K.) 1997, 157−158. (43) Frech, R.; Chintapalli, S.; Bruce, P. G.; Vincent, C. A. Crystalline and Amorphous Phases in the Poly(ethylene oxide) LiCF3SO3 System. Macromolecules 1999, 32, 808−813. (44) Chintapalli, S.; Zea, C.; Frech, R. Characterization studies on high molecular weight PEO-ammonium triflate complexes. Solid State Ionics 1996, 92, 205−212. (45) Jacobsson, P.; Albinsson, I.; Mellander, B.-E.; Stevens, J. R. Ion Association Effects and Phase Separation in Poly(propylene oxide) Modified Poly(dimethylsiloxane) Complexed with Triflate Salts. Polymer 1992, 33 (13), 2778. (46) van Heumen, J. D.; Stevens, J. R. The Role of Lithium Salts in the Conductivity and Phase Morphology of a Thermoplastic Polyurethane. Macromolecules 1995, 28, 4268−4277. (47) Wendsjo, A.; Lindgren, J.; Thomas, J. O.; Farrington, G. C. The Effect of Temperature and Concentration on the Local Environment in the System M(CF3SO3)2PEOn for M=Ni, Zn and Pb. Solid State Ionics 1992, 53−56, 1077. (48) Bernsen, A.; Lindgren, J. Free ions and ion pairing/clustering in the system LiCF3SO3:PPOn. Solid State Ionics 1993, 60, 37−41. (49) de Zea Bermudez, V.; Ostrovskii, D.; Carlos, L. D. Urethane Cross-linked Poly(oxyethylene)/Siliceous Nanohybrids Doped with Eu3+ Ions. Part 2. Ionic Association. Phys. Chem. Chem. Phys. 2004, 6 (3), 649. (50) Digar, M.; Humg, S. L.; Wang, H. L.; Wen, T. C.; Gopalan, A. Study of Ionic Conductivity and Microstructure of a Crosslinked Polyurethane Acrylate Electrolyte. Polymer 2002, 43, 681−691.

(51) Wen, S. J.; Richardson, T. J.; Ghantous, D. I.; Stribel, K. A.; Ross, P. N.; Crains, E. J. FTIR Characterization of PEO + LiN(CF3SO2)2 Electrolytes. J. Electroanal. Chem. 1996, 408, 113. (52) Wen, S. J.; Richardson, T. J.; Ghantous, D. I.; Stribel, K. A.; Ross, P. N.; Crains, E. J. Corrigendum to FTIR Characterization of PEO + LiN(CF3SO2)2 Electrolytes. J. Electroanal. Chem. 1996, 415, 197. (53) Wang, F. C.; Feve, M.; Lam, T. M.; Pascault, J. P. FTIR Analysis of Hydrogen Bonding in Amorphous Linear Aromatic Polyurethanes. I. Influence of Temperature. J. Polym. Sci., Part B: Polym. Phys. 1994, 32, 1305−1313. (54) Wang, F. C.; Feve, M.; Lam, T. M.; Pascault, J. P. FTIR Analysis of Hydrogen Bonding in Amorphous Linear Aromatic Polyurethanes. II. Influence of Styrene Solvent. J. Polym. Sci., Part B: Polym. Phys. 1994, 32, 1315−1320. (55) Stuart, B. Infrared Spectroscopy: Fundamentals and Applications; Analytical Techniques in Science Series, John Wiley & Sons: Chicester, U.K., 2004; p 45. (56) Huang, W.; Frech, R.; Wheeler, R. A. Molecular Structures and Normal Vibrations of Trifluoromethane sulfonate (CF3SO3−) and its Lithium Ion Pairs and Aggregates. J. Phys. Chem. 1994, 98, 100−110. (57) Stevens, J. R.; Jacobsson, P. A Comparison of Acetone and Poly(propylene glycol) as Solvents for Lithium Triflate and Lithium Perchlorate. Can. J. Chem. 1991, 69, 1980−1984. (58) Liang, Y.-H.; Wang, C.-C.; Chen, C.-Y. Synthesis and Characterization of a New Network Polymer Electrolyte Containing Polyether in the Main Chains and Side Chains. Eur. Polym. J. 2008, 44, 2376−2384. (59) Filip, D.; Macocinschi, D.; Ioanid, G. E.; Vlad, S. Multiblock Segmented Polyurethanes and Polyurethane-ureas based on Poly(ethylene oxide)/Poly(propylene oxide)/Poly(ethylene oxide) Macrodiols. Synthesis and Characterization. Polimery 2010, 55 (5), 358−365. (60) Watanabe, M.; Oohashi, S.-I.; Sanui, K.; Ogata, N.; Kobayashi, T.; Ohtaki, Z. Morphology and Ionic Conductivity of Polymer Complexes Formed by segmented Polyether Poly(urethane urea) and Lithium Perchlorate. Macromolecules 1985, 18, 1945−1950. (61) Garrett, J. T.; Xu, R.; Cho, J.; Runt, J. . Phase Separation of Diamine Chain Extended Poly(urethane) Copolymers: FTIR Spectroscopy and Phase Transitions. Polymer 2003, 44, 2711. (62) Yilgor, E.; Yurtsever, E.; Yilgor, I. Hydrogen Bonding and Polyurethane Morphology. II. Spectroscopic, Thermal and Crystallization Behavior of Polyether Blends with 1,3-dimethylurea and Model Urethane Compound. Polymer 2002, 43, 6561. (63) Chu, P. K.; Li, L. Characterization of Amorphous and Nanocrystalline Carbon Films. Mater. Chem. Phys. 2006, 96, 253−277.

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