Quasi-Solid Semi-Interpenetrating Polymer Networks as Electrolytes

Article pubs.acs.org/JPCC

Quasi-Solid Semi-Interpenetrating 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 Nimai Bar,†,‡,§ Kota Ramanjaneyulu,†,‡,§ 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: A new class of quasi-solid polymer electrolytes where both the primary and secondary components of the synthesized semi-interpenetrating polymer networks (semiIPNs) are comprised of an ether backbone is investigated in detail. The study comprehensively discusses the dependence of physicochemical characteristics and effects on ion conduction behavior for the semi-IPN matrices with reference to the constituent composition, cross-link density, chain length between cross-links, extent of molecular entanglements, charge carrier concentration, and the role of anions. The choice of polyether to be the secondary component resulted in marked improvement in the matrix morphology, enhanced miscibility, better thermal properties, and significant increase in ionic conduction while retaining the quasi-solid nature and film-forming capability of these semi-IPNs. Ionic conductivity of 10−4−10−3 S cm−1 at ambient temperatures, notably without the use of additional plasticization, is achieved with optimization of the semi-IPN matrix as a function of various compositions, content of secondary component, different lithium salts, and electrolyte concentrations. A direct correlation of ionic conductivity to the anion size and ion dissociation such that −N(CF3SO2)2− > −ClO4− > −CF3SO3− > −I−/−I3− was observed. The nonlinear temperature dependence of ionic conductivity follows the Vogel−Tammann−Fulcher equation, indicating that the ionic hopping events are strongly coupled with the segmental motions. Evaluation of ion−polymer and polymer−polymer interactions, morphology studies, glass transition temperature, melting temperature, degree of crystallinity, thermal stability, and degradation onset all provide valuable insights into the overall behavior of these semi-IPN electrolytes. These encouraging results favorably indicate their potential applicability in next generation energy conversion and storage devices.

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INTRODUCTION An ideal solid polymer electrolyte (SPE) that can work for practical devices at ambient conditions has remained elusive for more three decades despite intensive research in the field. Nevertheless, it has not deterred researchers in their attempts to improve on the findings. With efforts toward developing materials to harness sustainable energy intensifying globally,1 the research area has been reinvigorated. A breakthrough can effectively improve the processing, performance, and lifetime of electrochemical devices such as excitonic solar cells and Li-ion rechargeable batteries while adding a degree of flexibility in device engineering.2−7 Since the early conceptualization8−10 and application of polymer electrolytes was demonstrated,11,12 several monomers and polymers and their combinations, yielding exotic architectures, have been attempted.13−17 Yet, none have so far quite bested the advantages offered by polyether matrices © 2013 American Chemical Society

with respect to their salt solvation capability, low glass transition temperature, and ionic conductivity.17 Understandably, it is also unlikely that any new monomers or polymers would be synthesized given the broad exploits of numerous researchers over the years. Low dimensional stability of polyethers coupled with the high propensity of crystallization for higher molecular weights necessitates their structural alterations to enable the promised advantages.16,17 However, any attempted modifications such as blending,18,19 networking,20−22 grafting,23,24 and/or copolymerization25−27 carried out on polyether systems significantly compromised the ion conductivity. Organic molecules with low molar mass and high dielectric constants, such as propylene carbonate (PC), Received: September 24, 2013 Revised: December 9, 2013 Published: December 10, 2013 159

dx.doi.org/10.1021/jp409533r | J. Phys. Chem. C 2014, 118, 159−174

The Journal of Physical Chemistry C

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possible applications in next generation energy conversion and storage devices.

ethylene carbonate (EC), vinyl carbonate (VC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), γ-butyrrolactone, and the like, have been frequently used and can effectively offset the loss of ionic conductivity with a liquid-like environment providing channels for ion transport within the polymer matrix.28−34 Nonetheless, the problems associated with leakage, evaporation, and drying out, and hence unquantifiable composition variation of these gel matrices, complicate and restrict their utility. Thus, achieving a suitable polymer matrix that can provide both structural integrity as well as appreciably high ionic conductivity (>10−3 S cm−1) at ambient temperatures without incorporating low molecular weight organic compounds remains a challenge for researchers. In our previous reports, we have demonstrated the advantages and feasibility of semi-interpenetrating polymer networks (semi-IPNs) as matrices that can possibly address both issues with relative ease of tailoring. The semi-IPN of polyethylene oxide−polyurethane/polyacrylonitrile (PEO− PU/PAN)35−38 showed considerably improved ion conductivity over that of full-IPNs (polyethylene oxide−polyurethane/ polyvinylpyridine, PEO−PU/PVP).39 Significant enhancement in conductivity behavior and physicochemical characteristics of these semi-IPN matrices was further demonstrated through a wide range of simple modifications and structural alterations. In successive attempts employing simple techniques such as variation of cross-link density, molecular weight between cross-links, oligomeric plasticization, control of the degree of crystallinity, and forming semi-IPN nanocomposites, our efforts have successfully pushed the ion conductivities in the range of 10−6−10−5 S cm−1 at ambient temperatures.40,41 The improved results achieved over the years for PEO−PU/PAN semi-IPNs, though appreciable, are far from practical suitability. Nevertheless, a thorough understanding of these complex systems has encouraged us to investigate other alternate polymer duos while retaining the semi-IPN architecture. The present study details the investigations of an alternate semi-IPN matrix wherein the second component of the polymer pair is also comprised of an ether backbone. Replacement of the rigid poly(acrylonitrile) and poly(vinylpyridine) components with the flexible ether backbone have yielded marked improvement in the matrix morpholology, thermal characteristics, and ion conductivity behavior while retaining the quasi-solid nature and film-forming capability of these semi-IPNs. Linear poly(ethylene glycol) dimethylether is selected as the alternate component of choice in the system, and the study comprehensively discusses the effects of (i) constituent composition (% of component II), (ii) cross-link density, (iii) macromolecular chain length between cross-links, (iv) molecular weight of the secondary polymeric component, (v) charge carrier concentration, and (vi) nature of anion on the matrix properties. The semi-IPN formation, morphology, improved miscibility of the system, thermal behavior, temperature dependence of ion conduction, and associated mechanisms all have been probed extensively employing FT-IR, SEM, EDAX, DSC, TG-DTA, and dc and ac evaluation techniques. Optimization of the matrix composition has led to a substantial improvement in ion conductivity (10−4−10−3 S cm−1) at ambient temperatures, notably without the usage of any additional plasticization involving small molecules. The results reaffirmed and reassured our original contention of using the semi-IPN architecture while indicating strong prospects of their

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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 ∼ 400, 1000, 2000, 4000, 8000, 10 000, 14 000) (Aldrich), poly(ethylene glycol) dimethylether (PEGDME, Mn ∼ 250, 500, 1000, 2000) (Aldrich), lithium perchlorate (LiClO 4 ) (Aldrich), bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) (Aldrich), lithium trifluoromethanesulfonate (LiTf) (Aldrich), lithium iodide (LiI) (Aldrich), granular iodine (I2) (Aldrich), N,N-dimethylaniline (DMA) (Rankem), tetrahydrofuran (THF) (Rankem), and acetonitrile (CH3CN) (S.D. Fine-Chem Ltd., India). Polyethylene glycols and the solvents (THF, CH3CN) were dried prior to synthesis. Synthesis of Semi-IPN Electrolyte Matrices. 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). Then the reaction vessel containing the isocyanateterminated 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 to the system to intimately entangle within the growing polymer network. The incorporation of electrolyte salt and/or redox couple of desired concentration, dissolved in a 1:1 solvent mixture of THF/CH3CN, is also achieved at this stage. 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, dried at room temperature for 24 h followed by curing at higher temperature and under inert atmosphere to ensure the completion of the isocyanate reaction (at 80 °C for 48 h) and to obtain the quasi-solid semi-IPN electrolyte matrix. The free-standing 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 parentheses as (60:40), (50:50), (40:60), and (30:70) indicating the respective weight percentage. Compositions beyond 70 wt % of PEGDME visibly lacked structural integrity and phase homogeneity and hence were not considered under this study. Synthesis of Semi-IPN Electrolyte Matrices with Variable Cross-Link Density. The variation in the crosslink density for the semi-IPN matrix was achieved by controlling the total −NCO/−OH ratio in the reaction mixture during stage I and II of synthesis as described above. The −NCO/−OH ratio was varied as 1.1, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, and 2.4 while keeping all other parameters constant to investigate the effect of increasing amount of hard segment and number of cross-links within the system. Synthesis of Semi-IPN Electrolyte Matrices with Variable Chain Length between Cross-Links and 160



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Entanglements. The semi-IPN architecture were tailored to study the effect of chain length between cross-links and degree of entanglements by either varying the average molecular weight of the macromonomer used in component I (PEG) or component II (PEGDME) during synthesis while keeping all other parameters and the electrolyte (LiClO4) used constant. The different molecular weights (Mn) of PEG used in the present study are 400, 1000, 2000, 4000, 8000, 10 000, and 14 000 in combination with PEGDME of Mn = 500. The semiIPNs of (30:70) weight compositions so formed are accordingly coded as P0.4K-PU/P2, P1K-PU/P2, P2K-PU/ P2, P4K-PU/P2, P8K-PU/P2, P10K-PU/P2, and P14K-PU/P2, respectively. Similarly, the different molecular weights of PEGDME (component II) used in the present study are 250, 500, 1000, and 2000 in combination with PEG of Mn = 4000 in component I. The sample compositions are accordingly designated as P4K-PU/P1, P4K-PU/P2, P4K-PU/P3 and P4K-PU/P4, respectively. Synthesis of Semi-IPN Electrolyte Matrices with Variable Electrolyte Composition. As described in the preceding section for the synthesis of semi-IPN matrices, the electrolyte composition was varied using four different lithium salts, namely, lithium perchlorate (LiClO4), lithium bis(trifluoromethane)sulfonimide (LiN(CF3SO2)2, LiTFSI), lithium trifluorosulfonate (LiCF3SO3, LiTf) and lithium iodide/iodine (LiI/I2; 1:0.1) as redox couple. The desired salts in requisite concentrations solvated in a 1:1 THF/CH3CN solvent system were incorporated within the semi-IPN matrix in stage II of the reactions. 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, 10, and so on. It should be noted that the concentration of the salt in the semiIPN increases significantly as the EO/Li ratio decreases. Characterization. Fourier transform infrared spectroscopy was used to follow the formation of the semi-IPN matrices and the salt solvation in the mid-FTIR absorption range of 4000− 400 cm−1 employing a Bruker ALPHA-T instrument. Typically, monomer and polymer samples (∼2−5 mg) were grinded with KBr (∼200 mg) and pressed into transparent pellets of approximate dimensions, Φ = 1.2 cm and t = 0.02 cm. The pellets were vacuum dried at 60 °C for 30 min prior to each run. The transmittance spectra collected for 256 scans with a resolution interval of 2 cm−1 were corrected for baseline, atmospheric interference and also were normalized when required before comparative evaluation. The morphology of the semi-IPN electrolytes were analyzed with scanning electron microscopy on a JEOL JSM-5600N instrument. The cross sections of the fractured semi-IPN matrices sputtered with gold and SEM images were acquired at different magnifications to ascertain the sample homogeneity, extent of phase separation, and porosity. Additional scanning electron microscopy studies and energy dispersive X-ray analysis (EDAX) were done on Zeiss equipment for the same composition and to ascertain sample homogeneity (See Supporting Information). The moisture content of the synthesized semi-IPN electrolytes were estimated using a Metler-Toledo Karl Fischer titrator. In a slightly modified procedure, dry nitrogen (IOLAR grade, purity 99.999%) was passed through a flow-meter over the samples of known weight heated in a closed chamber at 150 °C and delivered to the titrator solution using a needle. Pure nitrogen was used as a control prior to the estimation of the sample moisture content.

Differential scanning calorimetry was performed on a DSC Q200 differential scanning calorimeter (TA Instruments) under dry nitrogen atmosphere. The synthesized semi-IPN electrolyte samples were vacuum-dried overnight before carrying out the thermal studies. Typically a sample (5−10 mg) of the semi-IPN electrolyte was loaded in an aluminum pan and hermetically sealed, rapidly cooled down to −150 °C using liquid nitrogen, equilibrated for 5 min, and then heated to 150 °C at a scan rate of 10 °C min−1. The power and temperature scales were calibrated using pure indium, and an empty aluminum pan was used as the reference. The analysis of the thermograms was carried out using universal analysis software provided by TA Instruments. The glass transition temperature (Tg) was determined from the inflection point of the transitions. The estimated experimental error of the determination of the glass transition temperature (Tg) is ±2 °C. Melting and crystallization temperatures, when they occurred, were defined as the maxima of the melting endotherms (Tm) and crystallization exotherms (Tc), respectively. Heat of fusion (ΔHm) was estimated by the area integration of the endothermic melting peaks. Percentage crystallinity (% χ) was determined from the ratio of the experimentally measured enthalpy to the value of 205 J g−1 reported for the enthalpy of melting of 100% crystalline PEO and corrected for the weight fraction of actual PEO present in the semi-IPN matrix. The thermal stabilities of the synthesized semi-IPNs were assessed by a TA Q500 modulated thermo gravimetric analyzer. Semi-IPN samples, 10−20 mg, were carefully weighed in an aluminum pan and TG scans were recorded at a ramp rate of 10 °C min−1 under nitrogen atmosphere in the range of 35−600 °C. The direct current (dc) measurements were carried out on a Keithley 2612 System Source Meter R interfaced with an inhouse designed sample holder. The disc-shaped polymer films or quasi-solid samples were sandwiched between two springloaded stainless steel (SS) blocking electrodes with a Teflon spacer of appropriate thickness to ensure a surface area of ∼0.95 cm2. The sample holder was placed in a thermostatcontrolled heating chamber to carry out the variable temperature measurements over a range of ∼20−90 °C at an interval of ∼5−7 °C during heating. The samples were equilibrated at each temperature for 30 min prior to acquiring the voltage sweep data. The temperature was measured with an accuracy better than ±0.1 °C using a K-type thermocouple placed in close proximity with the sample. No corrections for thermal expansion of the cell were carried out. Analysis of the temperature dependence of the dc conductivity data was done by nonlinear least-squares fit (NLSF) using Microcal OriginPro 8.5 software. The maximum error associated with the simulated fits for the Arrhenius and Vogel−Tammann−Fulcher (VTF) equation is within ±2%. All the synthesized samples were vacuum-dried overnight at 80 °C before carrying out the characterizations. The alternating current (ac) electrochemical impedance measurements were carried out on a Zahner Zennium electrochemical workstation controlled by Thales Operational Software. The system was interfaced with a thermostatted oven equipped with parallel test channels independently connected to spring-loaded Swagelok cells to test the samples at identical conditions. The synthesized semi-IPN electrolyte samples were vacuum-dried overnight before carrying out the electrical measurements. Punched circular disc-shaped polymer films or quasi-solid samples of surface area ∼0.95 cm2 and thickness ∼0.06−0.08 cm were sandwiched between two 316 stainless 161



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Figure 1. Representative mid-FTIR spectra of the semi-IPN matrices (A) with LiClO4 as an electrolyte at various EO/Li mole ratios: (a) EO/Li = 10, (b) EO/Li = 15, (c) EO/Li = 20, (d) EO/Li = 30, (e) EO/Li = 60, and (f) EO/Li = 100. (B) Spectra of the semi-IPN matrices with salts LiN(CF3SO2)2 (a) EO/Li = 10, (b) EO/Li = 20, and (c) EO/Li = 30 and LiCF3SO3 (d) EO/Li = 10, (e) EO/Li = 20, (f) EO/Li = 30 as electrolyte.

ion conduction mechanism; and (iii) role of various anions, effect of phase homogeneity, cross-link density, segmental mobilities, porosity, and free volume on the overall charge transport dynamics that determine the ion conductivity in these complicated systems. The synthesis and kinetics of the reaction for the formation of polyethylene glycol−polyurethane networks and the semiIPNs of the same are now well established from the previous studies.35−41 Nevertheless, preliminary details of mid-FTIR analysis for this new class of semi-IPN formation is provided in Figures SI-1−SI-4 of the Supporting Information. Castor oil, primarily a trihydroxy fatty acid with an −OH value of ∼2.7, is used as a cross-linker for the polymer network formation. The pure castor oil spectrum shows a sharp carbonyl peak indicative of its ester linkage at ∼1746 cm−1 (see Supporting Information, Figure SI-1). Similarly, for the reactant N,N′-diphenylmethane diisocyanate, the characteristic −NCO stretching peak can be seen prominently at ca. 2277 cm−1 (Figure SI-2 of the Supporting Information). Also provided is a typical FTIR spectrum of pure polyethylene glycol (Mn = 4000) (Figure SI-3 of the Supporting Information), which is used as a macromonomer for chain extension and polymer network formation in the final polyurethane matrix. The distinctive vibrational band corresponding to the C−O−C stretching vibration at ∼1110 cm−1 is evidenced along with the strong −OH stretching and bending modes at 3600−3200 cm−1 and ca. 1630 cm−1, respectively. During the classic urethanation reaction employed for the polymer network formation, two significant changes can be followed from FTIR of aliquots withdrawn from the reaction mixture as a function of time. A gradual decrease of the isocyanate contribution with final disappearance of the signature peak (∼2277 cm−1) postcuring (for 48 h at 80 °C) indicates the progress and completion of the reaction (Figure SI-4 of the Supporting Information). Concurrently, an initial broadening and gradual appearance of a peak at ∼1725 cm−1 overlapped as a shoulder to the 1746 cm−1 peak from the ester carbonyl of castor oil are indicative of urethane formation, which finally almost merges into one broad band. The other major peaks in the spectra at ca. 3400 (b), 1520 (s), 2926, and 2859 cm−1 can be assigned to the free N− H stretching and bending vibrational modes of urethane linkages along with C−H stretching of the methylene and methyl groups, respectively.36−38

steel blocking electrodes with a Teflon spacer of appropriate dimension and loaded in the Swagelok assembly. The spring and Teflon spacer ensured the application of the same amount of spring pressure during the sample mounting and throughout the test. The sample holders were placed in the controlled heating chamber to carry out the variable temperature impedance measurements over a range of ∼20−90 °C at an interval of ∼5−7 °C during heating. The temperature was measured with an accuracy better than ±0.1 °C using a K-type thermocouple placed in close proximity with the sample. Samples were equilibrated at each temperature for 30 min prior to acquiring the frequency sweep impedance data. All data were collected following a frequency sweep through the 1 Hz to 4 MHz range at an alternating potential with a RMS-amplitude of 10 mV across the OCV of the assembled cells. No corrections for thermal expansion of the cells were carried out. The real part of the impedance was appropriately normalized for the cell dimensions and ionic conductivity (σ (S cm −1 )) was determined. All the data points plotted represent an average of at least three different sets of measurements under similar conditions with appropriate standard deviation provided as Yerror. Analysis of temperature dependence of the ionic conductivity data was done by nonlinear least-squares fits (NLSF) using Microcal OriginPro 8.5 software. The maximum error associated with the simulated fits for the Arrhenius and/or Vogel−Tammann−Fulcher (VTF) equation is within ±2%.

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RESULTS AND DISCUSSION Properties of polyethers as electrolytes have been extensively studied over the last three decades, making it an appropriate standard for validation of complex polymeric systems. The present investigation of this new class of quasi-solid semi-IPN matrices hence primarily involves polyethers as a major component for both the networks and secondary entanglements in the bulk. A detailed evaluation of the physicochemical and electrochemical properties of these quasi-solid polymer matrices was initiated with the aim of optimizing the electrolyte composition as a function of the primary and secondary components of the semi-IPN matrix, reactant ratios of the polymeric network, and concentration and type of electrolytes. The possible variables for the present study, though intentionally kept limited within the large parameter space available, helped to rationalize the key factors, such as, (i) ease of salt solvation and dissociation; (ii) charge carrier concentration and 162



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Figure 2. Scanning electron micrographs on the cross sections of the fractured semi-IPN electrolyte matrices for two typical compositions. (Panels a−d) P4K-PU/P2 (50:50)-LiClO4, −NCO/−OH = 1.2, EO/Li = 20. The inset boxes show the region of interest that is magnified in each successive micrograph. (Panels e and f) P4K-PU/P2 (30:70)-LiClO4, −NCO/−OH = 1.2, EO/Li = 20. The scale bar provided in the images are (a) 300 μm, (b) 100 μm, (c) 50 μm, (d) 10 μm, (e) 50 μm, and (f) 10 μm.

LiCF3SO3 system (Figure 1B). The strong bands at ∼1354 cm−1 and ∼1194 cm−1 (Figure 1B, spectra (a)−(c)) 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.47 The shoulder at 1058 cm−1 represents the contributions from free ions, contact ion pairs, and ion aggregates.47,48 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.45 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−1.49 Spectra (d)−(f) in Figure 1B for the LiCF3SO3 as electrolyte show a strong doublet at ca. 1290 and 1254 cm−1 that can be attributed to the νsym(CF3) and νasym(CF3), respectively.47 The 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 while the peak at ca. 640 cm−1 indicates the strong O−Li stretch.47,49

Figure 1A,B shows the representative FTIR stack plots for the three different electrolytes, LiClO4, LiN(CF3SO2)2, and LiCF3SO3, at different loading concentration in the semi-IPN matrix. The complexation of the salts, i.e., ion−ion and ion− polymer interactions, are evident in the spectral zone 1800− 400 cm−1, particularly the C−O−C stretching region of the polymer matrix. The strong band at ∼1112 cm−1, characteristic of C−O−C stretch, is observed with a shoulder apparent at ca. 1098 cm−1, which is attributed to the salt dissociation and formation of Li+-ion-mediated transient cross-links with ether oxygen.42 The ion−polymer interactions in the N−H and carbonyl stretching regions can also be evidenced.43,44 The band observed at ∼630 cm−1 corresponding to free ClO4− ions45,46 (highlighted in Figure 1A) appears stronger with increasing salt concentration (decreasing EO/Li ratio), suggesting excellent ion dissociation within the polymer matrix. The effect of electrolyte complexation with the polymer is significantly more pronounced for the LiN(CF3SO2)2 and 163



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Figure 3. (a) Representative illustration of a cocontinuous morphology and porous scaffold that can be achieved through formation of a 3D crosslinked interpenetrating polymer network. (b) A schematic representation of Li-ion solvation in the synthesized P4K-PU/P2 semi-IPN matrix.

Figure 4. (a) Representative complex plane Nyquist plots of the synthesized P4K-PU/P2 (50:50) semi-IPN polymer matrix as a function of different temperatures. (b) Log(σ) vs 1000/T plots depicting the dependence of ionic conductivity as a function of temperature and with variation of compositional weight ratio of component I/component II in the synthesized P4K-PU/P2 semi-IPN polymer matrix: (i) 60:40, (ii) 50:50, (iii) 40:60, and (iv) 30:70. LiClO4 salt is used as the electrolyte with EO/Li = 30 and the total −NCO/−OH ratio in the network (component I) = 1.2 is maintained. The error bars provided in (b) represent the standard deviation for at least three different samples.

ratios of the polymeric network maintained at −NCO/−OH = 1.2. The thickness of the polymer film can be estimated from Figure 2a to be ca. 0.64 mm, and the micrograph reveals a fairly homogeneous bulk. The successive images, Figure 2b−d, reveal an extraordinarily porous morphology of the semi-IPN matrix with an average pore size of 1−2 μm. The semi-IPN composition P4K-PU/P2 (30:70)-LiClO4, with −NCO/−OH = 1.2 and EO/Li = 20 (Figure 2e,f) exhibits a slightly different morphology with a considerable degree of phase separation probably between the castor oil−isocyanate polyurethane segments and the increased polyethylene glycol dimethylether phase. The magnified image of this (30:70) semiIPN matrix, however, displays an appreciably higher degree of porosity when compared to the (50:50) composition, albeit with smaller pore size distribution. Polymer electrolyte compositions beyond (30:70) visibly lacked structural integrity and phase homogeneity. The EDAX findings on the cross sections of the fractured semi-IPN electrolyte matrices, particularly for the apparently phase separated composition P4K-PU/P2 (30:70)-LiClO4, with −NCO/−OH = 1.2 and EO/Li = 20 confirmed appreciable constituent homogeneity irrespective of the scanned area (800 μm × 800 μm; 100 μm × 100 μm) (see Supporting Information, Figures SI-5 and SI-6). The overall features of these synthesized semi-IPN matrices as revealed from the detailed electron microscopy evaluation

Effect of Constituent Composition on Matrix Property and Ionic Conductivity. The selection of linear poly(ethylene glycol) dimethylether as an alternate component II in the present semi-IPN system is intended to enhance the miscibility of the constituent components while preserving the dimensional integrity within an acceptable degree. The increased entanglements in the matrix are expected to not only lower the degree of crystallinity but also provide considerable internal plasticization of the matrix that offers ion conduction channels and pathways. The semi-IPN morphology, matrix homogeneity, extent of phase separation, if any, and bulk porosity of the synthesized quasi-solid and solid polymer films postcuring were probed in detail using scanning electron microscopy. A representative series of electron micrographs depicting the cross-sectional views of the fractured polymer films for two typical compositions of P4K-PU/P2 semi-IPNs (50:50 and 30:70) are provided at different magnification in Figure 2a−f. The top and bottom layers of the films were essentially featureless with a predominantly smooth surface typical of solution-cast polymer films and revealed no significant information. The gold sputtered fractured films mounted transversely, however, revealed the bulk features of the semi-IPN matrix. Figure 2a− d depicts the morphology of a P4K-PU/P2 (50:50) semi-IPN doped with LiClO4 of EO/Li mole ratio of 20 and the reactant 164



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Table 1. Evaluation Summary of the Thermal Characteristics of the Various P4K-PU/P2 Semi-IPNs Compositions Synthesizeda sample name

EO/Li

Tg (°C)

P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2

(60:40) (50:50) (40:60) (30:70) (30:70) (30:70) (30:70) (30:70) (30:70) (30:70)

30 30 30 100 80 60 40 30 20 10

−64.6 −66.2 −67.0 −71.8 −70.1 −69.2 −68.2 −65.0 −63.6 −54.0

P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2

(30:70) (30:70) (30:70) (30:70)

10 15 20 30

−61.2 −63.5 −65.6 −71.2

P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2

(30:70) (30:70) (30:70) (30:70)

10 15 20 30

−60.8 −65.2 −69.1 −69.7

Tc (°C)

Tm1 (°C)

Tm2 (°C)

% χb

To (°C)

PEG Mn = 4000; PEGDME Mn = 500; Salt: LiClO4 − − 37 40.9 −36.8 − 32.2 38.2 −40.5 − 34.2 36.5 − 6.5 32.9 35.2 194 − 14.3 34.9 34.6 201 − 17.2 33.8 33.1 208 − 14.8 34.7 32.3 210 − 12.6 32.8 24.1 215 −38.6 − 28.7 14.7 213 − − − − 228 PEG Mn = 4000; PEGDME Mn = 500; Salt: LiTf − 9.5 27.8 17 231 − 13.0 31.5 13 212 − − 29.0 10 204 − − 33.5 8 206 PEG Mn = 4000; PEGDME Mn = 500; Salt: LiTFSI − − − − − 23.7 0.32 − −30.6 − −40.4 − 24.1 6.5 − − 9.1 31.6 12.2 −

Td1 (°C)

Td2 (°C)

Td3 (°C)

− − − 194−370 201−370 208−305 210−351 215 − 297 213−287 228−315

− − − 370−446 370−446 305−375 351−402 297 − 348 287−355 315−340

− − − 446− 446− 375− 402− 348− 355− 340−

231−343 212−325 204 − 334 206−332

343−448 325−372 334 − 442 332−423

448− 372− 442− 423−

− − − −

− − − −

− − − −

a The glass transition temperature (Tg), cold crystallization temperature (Tc), crystalline melting temperature (Tm), and degree of crystallinity (% χ) results were obtained by differential scanning calorimetry and analyzed using the software Universal Analysis provided by TA Instruments. The degradation onset temperature (To) and the successive stages of degradation (Td1, Td2, and Td3) were estimated from differential plots of the thermogravimetric scans. bNormalized values to 100% PEO. The degree of crystallinity (% χ) of the PEO fraction in the matrix is calculated from the equation χ = ΔHm(PEO)/ ΔHom(PEO), where ΔHom(PEO) = 205 J g−1 is the heat of melting per gram of 100% crystalline PEO and ΔHm(PEO) is the apparent enthalpy of melting per gram of the PEO.68,69

electrode−electrolyte impedance at low-frequency is manifested as a spike along with the partial contribution from the charge transfer resistance at the phase boundary, if any. In the mid-frequency range, the contribution from the distributed elements (resistance, capacitance, etc.) of the sample under examination contribute to the derivation of one or more depressed semicircular arcs while at very high frequency they tend toward the origin. Temperature-step electrochemical impedance spectroscopy hence can generate a wealth of information relevant to the microscopic processes involved in ionic conduction and mechanism of charge migration.37,50−54 To hold the focus of this investigation within the ambit of the present discussion, the analysis and results of the EIS are kept limited to the determination of bulk ionic conductivity (σb) of the synthesized semi-IPNs. The resistance of the sample bulk (Rb) is estimated from the intercept on the real axis (Z′) by either the spike manifested at the electrode−electrolyte interface or the depressed semicircular arc in the low- to mid-frequency region (Figure 4a). The temperature dependence of ion conductivity for four typical compositions of the synthesized P4K-PU/P2 semi-IPN polymer matrix at a fixed electrolyte concentration (EO/Li = 30) along with other reaction parameters maintained constant are presented in Figure 4b. It is evidenced that with increasing weight percent of component II, i.e., PEGDME (P2), the ion conductivity increases significantly throughout the temperature window of the study. The hypothesis that the predominant presence of ether linkages (C−O−C) in both the semi-IPN components, the P4K-PU network and P2 (PEGDME), would help to dissociate the electrolyte and solvate the Li+-ions quite effectively is reflected in the considerably high conductivity

demonstrate a porous cocontinuous matrix of the polymer with fairly homogeneous distribution of the components in the three-dimensional (3D) architecture of semi-interpenetrating polymer networks (Figure 3). The ionic conductivity and ion conduction behavior of the semi-IPN samples were assessed for the synthesized P4K-PU/ P2 compositions (60:40, 50:50, 40:60 ,and 30:70) using both dc and ac techniques. The dc measurement involved sensing the generated current through the sample bulk under a fast voltage sweep. The slope (= 1/R) obtained from the linear fit to the current−voltage data was normalized for sample area and thickness to determine the specific conductivity (σdc) of the bulk. Representative plots of specific conductivity determined as a function of temperature employing the direct current technique are provided in the Supporting Information (Figure SI-7). Nevertheless, the contribution of ion polarization associated with the dc technique and its effects cannot be prevented even with fast sweeps at low voltages, particularly when experiments are repeated on the same sample during temperature-step scanning. The ion conduction behavior was hence primarily assessed by evaluating the ac-response of the synthesized semi-IPNs using electrochemical impedance spectroscopy (EIS). A frequency sweep through the 1 Hz to 4 MHz range at an alternating potential with a RMS-amplitude of 10 mV across the OCV of the samples sandwiched between SS-0316 blocking electrodes generated the appropriate current response and was translated to complex impedance. The typical complex plane plots (Nyquist plots) derived from the real and imaginary parts of impedance for the P4K-PU/P2 (50:50) semi-IPN composition with LiClO4, EO/Li ratio =30 are depicted in Figure 4a. As can be observed from the representative plot, the 165



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Table 2. Conductivity Data and Fitting Parameters for the Various Compositions of Synthesized P4K-PU/P2 Semi-IPNsa sample name

−NCO/−OH

EO/Li

P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2

(60:40) (50:50) (40:60) (30:70)b (30:70) (30:70) (30:70) (30:70)b (30:70) (30:70) (30:70) (30:70) (30:70)

1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2

30 30 30 30 10 15 20 30 40 50 60 80 100

P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2

(30:70) (30:70) (30:70) (30:70) (30:70) (30:70) (30:70) (30:70)

1.1 1.2 1.4 1.6 1.8 2.0 2.2 2.4

30 30 30 30 30 30 30 30

σ33oC × 10−3 (S cm−1)

σ83oC × 10−3 (S cm−1)

PEG Mn = 4000; PEGDME Mn = 500; Salt: LiClO4 0.02 0.2 0.04 0.2 0.07 0.5 0.43 2.5 0.08 0.7 0.1 0.9 0.2 1.1 0.43 2.5 0.2 1.6 0.13 0.53 0.12 0.55 0.09 0.4 0.07 0.3 PEG Mn = 4000; PEGDME Mn = 500; Salt: LiClO4 0.5 3.3 0.43 2.5 0.25 1.5 0.23 1.6 0.20 1.4 0.12 1.5 0.1 0.78 0.08 0.69

Egc (eV)

log σ0d

Ead (eV)

T0d (°C)

0.55 0.41 0.57 − − − − − − − − − 0.34

−0.98 −0.34 −0.39 0.27 −0.54 0.56 −0.65 0.27 0.35 −0.65 −0.70 −1.05 −0.98

0.030 0.070 0.030 0.047 0.032 0.079 0.048 0.047 0.089 0.035 0.036 0.029 0.035

−83.3 −85.5 −37.1 −67.9 −35.9 −89.0 −71.1 −67.9 −125.0 −60.3 −62.5 −54.0 −62.9

0.46 − 0.48 0.43 − 0.42 − −

0.38 0.27 0.16 0.60 −0.04 −0.19 −0.05 0.36

0.035 0.047 0.063 0.047 0.043 0.106 0.053 0.064

−49.4 −67.9 −86.4 −73.2 −60.2 −119.5 −71.5 −84.4

The bulk ionic conductivity at 33 and 83 °C, Arrhenius activation energies (Eg), as well as the fitting parameters for the VTF equations. bThe data have been reproduced in duplicate for ease of comparison. cActivation energy from Arrhenius equation σ = σ0 exp(−Eg/kT). dParameters from VTF equation σ = σ0T−0.5 exp{−Ea/k(T − T0)}. a

(10−5−10−3 S cm−1) observed for the typical compositions studied. Initial DSC studies reveal that the macromonomer PEG4000 (P4K) used for the network synthesis and the component II (PEGDME) both possess a very high degree of crystallinity. A sharp endothermic melting peak (Tm) at ∼58 °C for P4K with an enthalpy of ca. 192 J g−1 corresponded to a predominantly crystalline bulk, with degree of crystallinity, % χ ∼ 94 (see Supporting Information, Figure SI-8a). The glass transition temperature (Tg) probably could not be ascertained owing to such high degree of crystallinity. For the oligomer PEGDME (Mn = 500), Figure SI-8b of the Supporting Information, Tg was observed at ∼−86 °C along with a noticeably broad Tm at ∼11 °C and % χ ∼ 52, resulting from a significant amount of crystalline phase present. The broad endothermic peak (Tm) witnessed with a prominent shoulder is understandably from the substantial polydispersity present in this low molecular weight oligomer. The inflection point (I) of the glass transition region for the P4K-PU network (Figure SI8c of the Supporting Information) is found to be −52 °C, with a clear indication of a completely amorphous polymer matrix in the absence of any melting region. The observation unambiguously indicates that the PEG macromonomer gets randomly entangled once incorporated in the cross-linked network, which in turn restricts formation of extensive Hbonding between the chains; hence, it cannot crystallize. With incorporation of PEGDME in the network (Figure SI-8d of the Supporting Information), the glass transition temperature does shift toward the lower temperature, ca. −71 °C, as expected for a sufficiently plasticized P4K-PU/P2 semi-IPN matrix. Nevertheless, clear evidence for the presence of crystallized domains is indicated with the appearance of two endothermic peaks indicated by Tm1 and Tm2. The broad lower melting

temperature peak, Tm1, observed at ∼ 3 °C, is due to the polydispersed oligomer PEGDME forming intramolecular Hbonds. This finding implies the existence of a small amount of exclusive crystalline PEGDME rich domains (microscopic phase separation) within the constrained confinements of the P4K-PU network. The appearance of the second endothermic peak, Tm2, at ∼35 °C is significantly shifted to lower temperature compared to the ∼58 °C for the pure PEG macromonomer. This is attributed to a mixed interface formed by the entangled PEG of the P4K-PU network and PEGDME chains that facilitates intermolecular H-bonding. Enhanced miscibility of the two components indicated by a steady decrease in glass transition temperatures (Tg) coupled with a noticeable decrease in the degree of crystallinity (% χ) when the weight percent of PEGDME increases (Table 1) supports the contention. It can be rationalized that the flexible, linear chain PEGDME oligomeric structures would preferentially solvate the Li+-ions (according to the hard−soft acid− base (HSAB) principle), while facilitating a progressively amorphous and plasticized ionic pathway within the P4K-PU network. The considerably low Tg ( LiClO4 > LiTf > LiI:I2 (see Table 3). A representative bar plot illustrating the comparative ion conductivity evaluation in P4K-PU/P2 semi-IPN polymer matrix observed for the four different salts at a typical EO/Li

ionic hopping events and ion percolation through the matrix. The EO/Li mole ratio of 10 shows a complete transition to Arrhenius behavior, implying that segmental motions are pinned because of excessive cross-linking. The possibility exists that at sufficiently high salt concentrations, formation of ion pairs, ion triplets, and higher ionic aggregates coexisting with free ions can also have detrimental effects on bulk ionic conductivity. Similar observations in temperature-dependent conductivity profiles for LiTf and LiI/I2 are provided in Supporting Information (Figures SI-11 and SI-12). The best ionic conductivity is observed to be achieved for P4K-PU/P2 (30:70) semi-IPN composition with an EO/Li ratio of 30 and LiTFSI as the electrolyte. The bulk conductivity (σ, S cm−1) values at two representative temperatures (33 and 83 °C) along with the activation energy and related parameters estimated from Arrhenius and VTF fits for ease of comparison are provided in Tables 2 and 3. The bulk ionic conductivity observed throughout the temperature range definitely shows 171



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porosity signifying appreciable miscibility of the two constituent polymeric phases and a large free volume conducive to ionic transport. The considerably low glass transition temperatures ( 190 °C) determined from thermogravimetry suggests the considerable thermal stability of these quasi-solid semi-IPNs. Bulk ionic conductivity of 10−4−10−3 S cm−1 at ambient temperatures, notably without the use of additional plasticization, is achieved with optimization of the semi-IPN matrix as a function of various compositions, content of secondary component, different lithium salts, and electrolyte concentrations. A direct correlation of ionic conductivity to the anion size and ion dissociation such that −N(CF3SO2)2− > −ClO4− > −CF3SO3− > −I−/−I3− was observed. The nonlinear temperature dependence of ionic conductivity follows the Vogel−Tammann−Fulcher (VTF) equation, indicating that the ionic hopping events are strongly coupled with the segmental motions. Tailoring the semi-IPN composition, evaluation of ion−polymer and polymer−polymer interactions, morphology studies, glass transition temperature (Tg), melting temperature (T m), degree of crystallinity (% χ), thermal stability, degradation onset (To) and its consequential effects on ionic conductivity all provide valuable insights into the overall behavior of these semi-IPN electrolytes. In summary, these reassuring and encouraging results favorably indicate their potential for applicability in next generation energy conversion and storage devices, such as dye-sensitized solar cells or lithiumion rechargeable batteries.

ratio of 30 is presented in Figure 9b. It is now understood that smaller cations, such as Li+, are highly electronegative and have low polarizability. Thus, they tend to behave as hard acids and have better interaction with the ether oxygen (hard base).16,17,66 In most cases, the formation of polymer−salt complexes are controlled by the solvation energy driven by cations. Nonetheless, the nature of anions plays an equally important role in charge dissociation and polarizability within the medium. Large anions with a delocalized charge cloud are weak bases and possess low ion−dipole stabilization energies. In addition, the lattice energies for salts of smaller cations and larger anions are relatively low. When in an aprotic medium, anions with lower charge density and basicity are easily prone to destabilization.16,67 This effect can be exemplified considering the role of the TFSI anion in a polymeric medium. The charge delocalization due to attachment of strong electronwithdrawing groups (CF3SO2)2− to a negative center (N-atom) coupled with the flexibility of the S−N−S bond provides an ideal combination. These characteristics do allow a high degree of dissociation of the electrolyte while the weak solvation of the TFSI anion in polyether-mediated dipoles minimizes contact ion pair formation. Additionally, owing to their flexible structures, a reasonable degree of plasticization in the parent matrix can be achieved. The findings from Watanabe et al.67 and later by Johansson et al.48 have shown that the electronwithdrawing order among the four anions is TFSI− > ClO4− > Tf− > I−/I3−, while the flexibility and plasticization are almost the same. Thus, the rationalization holds true for the present system, where the trend in ionic conductivity could be directly correlated to the anion size and ion dissociation as −N(CF3SO2)2− > −ClO4− > −CF3SO3− > −I−/−I3−.

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CONCLUSIONS A new class of quasi-solid semi-IPN polymer electrolytes where both the primary and secondary components are comprised of an ether backbone is extensively investigated. In this effort, a series of poly(ethylene glycol)-polyurethane/poly(ethylene glycol) dimethylether semi-interpenetrating polymer networks (P4K-PU/P2 semi-IPN) with diverse compositions and incorporated with four different lithium salts (LiClO4, LiTFSI, LiTf, (LiI:I2)) as electrolytes were synthesized at room temperature following a simple approach. The detailed study comprehensively discusses the dependence of physicochemical characteristics and effects on ion conduction behavior with reference to (i) the constituent composition, (ii) cross-link density, (iii) chain length between cross-link, (iv) extent of macromolecular entanglements, (v) charge carrier concentration, and (vi) the role of anions in the semi-IPN matrices. Choice of polyether to be the secondary component as well have resulted in marked improvement in the matrix morphology, enhanced miscibility, better thermal properties, and significant increase in ionic conduction while retaining the quasi-solid nature and film-forming capability of these semiIPNs. When compared to our earlier studies of PEO−PU/PVP full-IPNs, PEO−PU/PAN semi-IPNs, and PEO−PU/PAN semi-IPN titania nanocomposites, the overall physicochemical and electrochemical properties of this new class of P4K-PU/P2 semi-IPNs are significantly improved. The formation of semiIPNs, polymer−salt complexation, ion−ion and ion−polymer interactions are probed using mid-FTIR spectroscopy. The detailed evaluation of semi-IPN morphology using scanning electron microscopy coupled with EDAX revealed a 3D cocontinuous and fairly homogeneous bulk with extraordinary

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ASSOCIATED CONTENT

S Supporting Information *

Supplementary details on the formation of semi-IPNs probed by FTIR spectroscopy, additional scanning electron micrographs along with EDAX spectrum, initial evaluation by differential scanning calorimetry for the semi-IPN formation, data on the effects of chain lengths between cross-links and macromolecular entanglements, temperature dependence of ion conductivity behavior for LiCF3SO3 and (LiI:I2) as electrolytes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.B. and K.R. acknowledge 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 172



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Innovative Solutions for Solar Energy Storage (StoreSolar: NWP-0056) for the grants received. The authors sincerely appreciate the encouragements and considerable help received from Dr. S.V. Manorama, Dr. K.V.S.N. Raju, Dr. P.R. Bangal, Dr. R.K. Rana, Dr. M.S.L. Karuna, and Dr. R. Narayan during the course of this investigation.

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Quasi-Solid Semi-Interpenetrating Polymer Networks as Electrolytes

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