Article pubs.acs.org/Macromolecules
Ionic Conductivity in Relation to Ternary Phase Diagram of Poly(ethylene oxide), Succinonitrile, and Lithium Bis(trifluoromethane)sulfonimide Blends Mauricio Echeverri, Namil Kim, and Thein Kyu* Department of Polymer Engineering, University of Akron, Akron, Ohio 44325, United States ABSTRACT: In an effort to develop free-standing lithium battery membrane, binary and ternary phase diagrams of poly(ethylene oxide) (PEO), bis(trifluoromethane)sulfonimide (LiTFSI), and succinonitrile (SCN) (i.e., solid plasticizer) mixtures have been established by means of differential scanning calorimetry and polarized optical microscopy. The occurrence of hydrogen bonds and/or coordination bonds in each binary pair (PEO/SCN, SCN/LiTFSI, and PEO/LiTFSI) was examined using Fourier transform infrared spectroscopy. The binary PEO/LiTFSI mixture exhibits a eutectic phase diagram with the liquid + crystal coexistence region having various crystal forms of the lithium salt, whereas the SCN/LiTFSI blend shows a wide noncrystalline region, which is highly desirable for organic solvent-free battery applications. The PEO/SCN blend shows a typical eutectic behavior, which is explicable in the framework of the Flory−Huggins theory in conjunction with the phase field theory of crystal solidification. Various coexistence regions of the PEO/SCN/LiTFSI mixtures have been mapped out using polarized optical microscopy and wide-angle X-ray diffraction. The ionic conductivity was determined at various coexistence regions such as isotropic noncrystalline liquid, crystal + liquid, liquid + plastic crystal regions using ac impedance spectroscopy. Of particular interest is that the conductivity in the isotropic liquid region is higher than those of the crystal (or plastic crystal) + liquid coexistence regions.
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INTRODUCTION With the growing concern of fossil energy shortage, organic solvent-free lithium battery membrane has been in great demand for electric automobiles. A typical polymer lithium battery is made up of blends of poly(ethylene oxide) (PEO) and a lithium salt, e.g., lithium bis(trifluoromethane)sulfonimide (LiTFSI).1,2 To increase the bulk conductivity of the membrane, it is a common practice to employ organic solvents such as propylene carbonate or dimethyl sulfoxide to solubilize PEO and concurrently dissociate lithium cations from the LiTFSI salt. PEO is a linear polyether chain that serves as a host for transporting dissociated Li ions and concurrently improving the solubility in the polymeric matrix.3 However, both PEO and LiTFSI are crystalline; the dendritic growth of lithium salt crystals combined with low flash point of the organic solvent often resulted in catastrophic failure, e.g., electrical short circuiting leading to battery combustion. Hence, the choice of appropriate functional materials with an optimal operating temperature range of −20 to 60 °C is of paramount importance.4 An alternative approach is to utilize a solid plasticizer as a means of suppressing the crystallinity of PEO and concurrently promoting the ionic dissociation of the lithium salt to render greater mobility to the carriers. Succinonitrile (SCN) is one of the solid plasticizers attempted for this purpose. SCN, being a neutral organic molecule, has low ionic conductivity by itself. However, it exhibited considerable improvement of ion © 2012 American Chemical Society
conductivity when compounded into the polymer/lithium salt mixture.5,6 Electrochemical stability of SCN has been examined by Abouimrane et al., who demonstrated that SCN is stable up to 4.5 V upon adding 4 mol % of LiTFSI.7 Below −40 °C, the crystalline structure of SCN is monoclinic having predominantly gauche conformation, but above that temperature the material transforms to a plastic crystal with a body centered cubic (bcc) structure.8 The majority of the chain conformation of SCN is gauche, but 20% are of trans-type. That is to say the positional order of SCN is sustained in this temperature range, above which there is no orientational order, characteristics of a plastic crystal.8 Therefore, it is crucial to understand the phase behavior of the binary and ternary blends of PEO/LiTFSI/ SCN in order to map out various coexistence regions as a function of concentration and temperature. The binary phase diagrams of the PEO/LiTFSI1,9 and SCN/LiTFSI10 blends may be found in the literature, but that of the binary PEO/SCN mixture as well as of their ternary mixtures is new. In this article, we selected LiTFSI because of its highest ion conductivity among all Li salts tested. PEO is a standard polymer, which has been widely used in polymer electrolyte solution. SCN is a solid plasticizer, which is capable of ionizing the Li salt quite effectively. Binary phase diagrams of PEO/ Received: April 26, 2012 Revised: July 10, 2012 Published: July 20, 2012 6068
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the DSC thermograms of the second heating cycle were used in the analysis to ensure all samples received the same thermal history. In order to examine the blend morphology at various temperatures, polarized optical microscopic (POM, Olympus BX60) pictures were taken using a 35 mm digital camera (EOS 300D, Canon) and a sample hot stage (TMS93, Linkam). After drying, the blend samples were covered with a glass slide and quenched to −50 °C and held isothermally for 10 min. Then, samples are heated to desired experimental temperatures at 2 °C/min. Infrared spectra for blends of LiTFSI/PEO, PEO/SCN and SCN/ LiTFSI were collected using a FTIR (Nicolet 380 Thermo Scientific); the acquired spectra represented the average of 32 scans with a resolution of 4 cm−1. The spectra of the LiTFSI/PEO mixture were acquired using KBr disks in the transmission mode in a hot stage at 100 °C, but for the blends containing SCN the spectra were collected at 80 °C. Ionic conductivities were measured for various PEO/SCN/LiTFSI ternary mixtures using an impedance analyzer (HP4192A LF). Samples were sealed between two parallel stainless steel polished plates with an area of 1 cm2 using an 8 mm Teflon spacer. Sample loading was carried out in a glovebox to prevent potential water intake. The power of 1 V in amplitude was employed in a frequency range from 13 MHz to 5 Hz. All samples were measured after stabilizing the instrument for 20 min. A temperature ramping was made to various PEO/SCN/Li salt concentration to determine the change of conductivity upon PEO crystal melting. Various samples with different compositions were further analyzed by means of wide-angle X-ray diffraction (WAXD) using a sealed tube X-ray generator (Bruker Instrument) with the monochromatized Cu Kα (Kα1) radiation of wavelength of 1.5417 Å. The pinhole collimator of 0.5 mm in diameter was utilized.
LiTFSI, PEO/SCN, and SCN/LiTFSI pairs have been established on the basis of differential scanning calorimetry (DSC) and polarized optical microscopy (POM) along with determination of specific interactions of SCN with its counterparts using Fourier transform infrared spectroscopy (FTIR). Subsequently, a ternary phase diagram was constructed to determine various coexistence regions including crystal + liquid, crystal + plastic crystal, etc., and the emerged domain morphologies. The role of solid plasticizer SCN on the eutectic melting of the constituent crystals and ion conductivity is elucidated, and subsequently the stable isotropic noncrystalline region is identified for possible development of an organic solvent-free polymer lithium battery.
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EXPERIMENTAL SECTION
Materials. Poly(ethylene oxide) (PEO), purchased from Sigma, exhibited a number-averaged molecular weight (Mn) of 44 000 g/mol with a polydispersity index (PDI) of 1.13 as determined by means of high performance liquid chromatography (HPLC, Model 1515, Waters) using polystyrene standards. Lithium bis(trifluoromethane)sulfonimide (LiTFSI) having a purity of 99.9% and succinotrile (SCN) (>99% purity) were purchased from Aldrich. The solvents, methylene chloride (99.9% purity) and acetone (99.9% purity), were bought from Fisher Scientific and Mallinckrodt, respectively. Sample Preparation and Characterization. Prior to blending, the as-received pure LiTFSI was dried at 150 °C under vacuum for 24 h and then kept in a desiccator at room temperature. All blends were prepared in a glovebox under dry nitrogen gas circulation because of their considerable moisture sensitivity. In the case of the PEO/LiTFSI blends, these mixtures were simultaneously dissolved in a methylene chloride/acetone solution (40:1 w/w). Solution casting was carried out at 80 °C under continuous nitrogen flow. Subsequently, these PEO/LiTFSI blends were further dried at 150 °C in a vacuum oven for 1 h and then kept at room temperature in a glovebox under nitrogen circulation until use. In the case of blends containing SCN, we employed drying protocols differently than PEO/LiTFSI blends because of potential degradation of SCN during prolonged thermal treatment at elevated temperatures. Several compositions of the PEO/SCN mixtures were dissolved in methylene chloride/acetone mixture (40:1 w/w), and then the PEO/SCN blend films were solvent cast at 80 °C for 1 h and kept in a glovebox under a nitrogen environment at room temperature. The above mixing scheme and solvent casting procedure were extended to the other binary SCN/LiTFSI blend as well as to the ternary SCN/LiTFSI/PEO systems. Thermal properties of the neat PEO, SCN, and LiTFSI samples were examined using a thermogravimetric analyzer (TGA; Model Q50, TA Instruments) at a heating rate of 5 °C/min from room temperature to 600 °C under a nitrogen atmosphere. In addition, isothermal TGA was performed by annealing the samples at various temperatures from 80 to 130 at 10 °C intervals for 30 min. The determination of melting points was carried out using differential scanning calorimetry (DSC) (TA Instruments, Model Q200). Blend samples weighing 8−10 mg were placed in aluminum pans and hermetically sealed with aluminum lids in a glovebox under nitrogen to avoid moisture absorption. All DSC scans were acquired at a ramp rate of 10 °C/min, unless indicated otherwise, under the dry nitrogen gas circulation at a flow rate of 50 mL/min. Both PEO/SCN and SCN/LiTFSI blends were first heated to 80 °C and subsequently quenching to −70 °C using the refrigerating unit and kept there for 10 min and then heated again for a second time to 80 °C at 10 °C/min. However, in the case of PEO/LiTFSI blends, DSC scans were carried out by first heating to 250 °C and then quenching to −70 °C and kept there for 10 min and then ramped again to 250 at 10 °C/min. The ternary blends were handled and characterized in the same conditions of the SCN/LiTFSI mixtures. Because of the considerable water sensitivity of LiTFSI and PEO, only
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THEORETICAL DESCRIPTION FOR EUTECTIC PHASE DIAGRAM To establish the phase diagram, the total free energy of a binary crystalline pair can be described in terms of the sum of the phase field free energy density of crystal solidification of each constituent crystal and the free energy density of liquid−liquid demixing.11,12 The free energy density of crystallization pertaining to the crystal phase order parameter of i th component (ψi) may be expressed in the Landau-type asymmetric double well form,13 i.e. ⎡ ζ(T )ζ0(Tm) 2 ζ(T )ζ0(Tm) 3 1 4 ⎤ f (ψi ) = W ⎢ ψi − ψi + ψi ⎥ ⎣ 2 3 4 ⎦ (1)
where ζ(T) is related to the energy barrier for the crystal nucleation and W is a coefficient representing energy penalty for the nucleation to overcome. ζ0(Tm) is the crystal phase order parameter at the potential well of crystal solidification; for small molecule systems, ζ0(Tm) may be taken as unity. The liquid−liquid demixing may be expressed in the framework of Flory−Huggins equation by taking into consideration the amorphous−amorphous interaction parameter,11,12 i.e., χFH = χaa. fFH (ϕ1) =
1−ϕ ϕ ln ϕ + ln(1 − ϕ) + χaa ϕ(1 − ϕ) r1 r2 (2)
where ϕ1 = ϕ and ϕ2 = 1 − ϕ are the volume fractions of components 1 and 2 that satisfy the incompressible condition. The parameters, r1 and r2 are the numbers of statistical segments of PEO and SCN, respectively. The total free energy of binary crystalline blends may be expressed by adding both contributions and taking into account 6069
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the interactions between the crystal and amorphous phases using the interaction parameters χc1a and χac2 for each crystal present in the mixture and the interaction between both crystalline portions, χc1c2,14 f (ψ1 , ψ2 , ϕ) = ϕf (ψ1) + (1 − ϕ)f (ψ2) + +
ϕ ln ϕ r1
1−ϕ ln(1 − ϕ) r2
+ {χaa + χc a ψ12 − 2χc c ψ1ψ2 + χac ψ2 2}ϕ 1
(1 − ϕ)
1 2
2
(3)
The interaction parameters of the crystalline−amorphous phases can be related to the enthalpy of crystallization of constituent crystals, i.e., χc1a ≈ ΔHc1/RT, χac2 ≈ ΔHc2/RT, and the crystal−crystal interaction can be treated in accordance with the geometric mean of the crystal−amorphous interactions, χc1c2 = cw(χc1aχac2)1/2. The parameter cw accounts for any departure from the ideality of the anisotropic interactions. The present theoretical model is strictly focused only on the elucidation of a simple binary system such as PEO/SCN pair, exhibiting a eutectic melting behavior. In the present case, intermolecular interactions under consideration are basically van der Waals forces. However, other binary systems having strong specific interactions such as hydrogen bonding or ionic interaction need modification of the enthalpic contributions. Physically, the crystal order parameter ψ1 represents onedimensional (or linear) crystallinity of the constituent 1; the product with its volume fraction (ψ1ϕ1) corresponds to the bulk crystallinity (i.e., crystalline fraction). On the other hand, the product of ϕ2 and ψ1 signifies the amount of amorphous materials of component 2 interacting with the crystalline phase of component 1. Hence, the term χcaϕ1ψ1ϕ2ψ1 denotes the crystalline−amorphous interaction. The same argument is valid for the second crystalline component, χacψ2ϕ2ψ2ϕ1. The crossinteraction term, χccψ1ϕ1ψ2ϕ2, can be interpreted as the crystalline−crystalline interaction that occurs when the component 1 crystals (ϕ1ψ1) and the component 2 crystals (ϕ2ψ2) form cocrystals.
Figure 1. (a) Nonisothermal TGA thermograms of pure PEO, SCN, and LiTFSI and the corresponding chemical structures. (b) Isothermal TGA theromograms for pure SCN at various temperatures showing the sample weight loss at a function isothermal annealing time.
Hence, we restricted the drying procedures to 80 °C for all blends containing SCN. Binary Phase Diagrams and Molecular Interactions. LiTFSI/PEO Blends. Figure 2a exhibits the DSC thermograms of the second heating cycles of LiTFSI/PEO blends showing the depressed trend of crystal melting transitions as a function of composition. PEO reveals a crystal melting transition at 64 °C, whereas LiTFSI shows a pronounced single melting peak at 230 °C. The melting temperature (i.e., the midpoint of the transition peak) of PEO decreases with increasing Li salt concentrations up to the 70/30 PEO/LiTFSI composition, and thereafter it abruptly disappears. A similar melting point depression occurs in the melting of LiTFSI crystal, but the behavior is more complex that shows one or more peaks in the enlarged DSC traces of the intermediate compositions from 55/45 to 5/95 PEO/LiTFSI (Figure 2b), implying possible multiple crystal forms of the Li salt upon addition of PEO. The phase diagram for the mixture LiTFSI/PEO was constructed by plotting crystal melting temperatures against PEO weight fraction (ϕPEO ) in conjunction with the morphological observations by POM at various coexistence regions (Figure 3). CrLiTFSI denotes the crystalline phase of LiTFSI, and Cr with superscripts α, β, and γ represents different crystal forms of incongruent crystals of LiTFSI formed at different blend compositions; CrPEO is the crystalline phase of PEO. Note that the PEO crystal is monoclinic; the unit cell structure of LiTFSI was neither determined nor reported in the literature due to the presence of various crystal modifications. This type of incongruent crystal melting behavior was studied by Lascaud et al.,9 who attributed the coordination bonds between the lithium cation and the oxygen atom of the ether linkage of PEO to be responsible for the incongruent lithium salt crystal forms. The overall phase behavior can be viewed as the melting point depression of LiTFSI incongruent crystals
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RESULTS AND DISCUSSION Thermal stabilities of the neat PEO, SCN, and LiTFSI samples were analyzed by means of TGA at a heating rate of 5 °C/min from room temperature to 600 °C under a nitrogen atmosphere. As shown in Figure 1a, the nonisothermal TGA thermograms of PEO and LiTFSI exhibit excellent thermal stability up to 380 °C, whereas the neat SCN appears stable up to 130 °C. However, SCN is known to be susceptible to degradation upon prolonged thermal treatment, and thus isothermal TGA experiments were carried out at various temperatures from 80 to 130 at 10 °C intervals. As can be noticed in Figure 1b, the TGA thermogram of neat SCN shows no change in weight at 80 °C for duration of 30 min tested. At a higher temperature of 90 °C, there is a slight weight loss of ∼8% after 30 min annealing. This weight loss becomes more pronounced in the isothermal TGA runs at higher temperatures of 100−130 °C. This finding implies the potential evaporation of SCN during prolonged thermal treatment at such elevated temperatures, although the nonisothermal TGA thermogram obtained at 5 °C/min showed thermal stability up to 130 °C. 6070
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crystal + liquid regions corresponding to various incongruent crystal forms occurring within the Li salt (pictures A−D) and liquid + PEO crystal (picture E). In Figure 4 is shown the asymmetric stretching (νa) at 1100 cm−1 corresponding to the C−O−C of PEO broadens as a
Figure 4. Infrared spectra of LiTFSI/PEO mixtures vertically shifted for clarity, showing the movement of the SO2 band with increasing PEO and broadening of the C−H stretching band suggestive of coordination bonds. Figure 2. DSC thermograms of LiTFSI/PEO mixtures vertically shifted for clarity (a) as a function of blend compositions. (b) Magnified DSC scans of the intermediate compositions of LiTFSI/ PEO from 55/45 to 5/95.
function of the lithium salt concentration (10−80 wt %), which may be ascribed to the coordination bonds of LiTFSI anion with the ether group of PEO (i.e., ion-dipole interaction).15,16 Also, the C−H symmetric stretching band of PEO at 2867 cm−1 shifts to a higher wavenumber for about 30 cm−1. A similar phenomenon was noticed in an analogous system by Rey et al.,17 who found that the C−H symmetric stretching peak associated with the crown ether-like local conformation
with the addition of PEO and vice versa, showing an isotropic region in the vicinity of 60 and 70 wt % of PEO. At higher concentrations of PEO, the PEO crystals dominate the phase morphology. The POM pictures reveal several coexistence
Figure 3. Eutectic phase diagram of LiTFSI/PEO mixture along with the POM micrographs of PEO and incongruent LiTFSI crystals corresponding to various coexistence regions. Dashed lines were drawn by hand to guide the reader’s eyes. 6071
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around the Li cation becoming stronger with increasing LiTFSI concentration. In the present case, the symmetric vibration peak of SO2 at 1327 cm−1 moves to a higher wavenumber with increasing PEO. The movement of SO2 symmetric stretching band may be a consequence of the SO2 is being freed up as the ether oxygen of PEO carries away the Li cations, thereby dissociating the Li salt. Concurrently, the SO2 symmetric stretching band broadens especially at intermediate compositions. According to Rey et al.,17 this kind of peak broadening of SO2 band is due to the overlapping peaks arising from different isomeric species of the dissociated anions. PEO/SCN Blends. Figure 5 exhibits the DSC thermograms for the PEO/SCN blends. Evidently, the melting transition
Figure 6. Binary phase diagram of the PEO/SCN mixture, exhibiting various coexistence regions and corresponding crystal morphologies. Solid symbols are the experimental points, and the liquidus and solidus lines are obtained via self-consistent solution.
of the majority component; for example, at a high concentration of SCN, the whole optical microscopic view is covered with the aligned dendrites having secondary and/or ternary side branches grown epitaxially from the main stems (picture B). Note that a structure with a pronounced orientational order is called dendrite, and without apparent orientational order, it is called seaweed.18 The lack of orientational order of plastic crystals of SCN at that temperature made the POM observation difficult, showing completely dark appearance under the crossed polarizing condition, and thus only the pictures under the parallel polarizations are depicted in pictures A and B. On the other hand, the PEO-rich region shows the sheaf-like structures or dense-branching lamellar morphology embedded in the isotropic phase (picture C). Picture D exhibits the impinged spherulites of PEO with distinct grain boundaries. The solidus and liquidus lines represented by the solid lines in Figure 6 were obtained by self-consistently solving the free energy equation (eq 3) using the material parameters and the experimental conditions: r1 = 1, r2 = 6 and ΔHc1 = 4000 J/mol, ΔHc2 = 8500 J/mol,19,20 where 1 and 2 represent SCN and PEO, respectively, along with their melting points Tm1 = 58.4 °C and Tm2 = 65.0 °C and cw = 0.01. Of particular importance is that our theoretical model captures the eutectic trend of the experimental phase diagram of the present PEO/SCN system. Figure 7a exhibits a set of infrared spectra in the isotropic phase of the PEO/SCN blends between 2270 and 2230 cm−1 where a broad cyanide peak of SCN is located at 2253 cm−1. As the PEO concentration increases to 70 wt %, this band shifts to a lower wavenumber for ∼4 cm−1. According to Mosier-Boss, who investigated an analogous system of crown ether and acetonitrile, there was a red-shift of the cyanide peak as a consequence of a loss of electrons during hydrogen bonding between the CH2 of acetonitrile and the oxygen of the crown ether.21 In fact, as can be noticed in Figure 7b, the peaks corresponding to the C−H stretching (symmetric and asymmetric) of SCN move to lower wavenumbers for >10 cm−1, thereby supporting the notion of the loss of electrons in
Figure 5. DSC thermograms of PEO/SCN mixtures vertically shifted for clarity in conjunction with the magnified thermograms in the temperature range of 10−30 °C.
temperatures of SCN at 58 °C and PEO at 64 °C decrease upon blending. The transition peaks between 15 and 25 °C, as manifested in the inset of Figure 5, correspond to the melting transitions of the constituents in the vicinity of the eutectic point. The endothermic peak around −38 °C is a typical phase transition from the solid crystal to the plastic crystal of SCN. Note that a structure with both positional and orientational order is called a “solid crystal”, and without the orientational order it is called a “plastic crystal”.8,18 The small exothermic bump around −15 °C may be attributed to the cold crystallization of PEO, which is pronounced especially at 20− 40 wt % of PEO concentrations. It should be noted that the cold crystallization is a nonequilibrium phenomenon resulting from the incomplete (or slow) crystallization during cooling, and thus it will not be dealted in the phase diagram analysis. The melting transition temperatures (i.e., the endothermic peaks) were plotted against concentration in the phase diagram as depicted in Figure 6. The solid circles denote the melting points obtained from the DSC experiments in comparison with the liquidus and solidus lines (represented by the solid lines) obtained by solving eq 3 self-consistently. The phase diagram is of a typical eutectic type, exhibiting various coexistence regions such as isotropic phase (L), plastic crystal of SCN (PCrSCN), crystalline phase of PEO (CrPEO), and CrSCN + CrPEO regions. In Figure 6 are shown the POM micrographs illustrating the morphologies corresponding to various coexistence regions labeled with the capital letters (A−D) in the phase diagram. Evidently, the domain morphology is dominated by the crystals 6072
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In these mixtures, the melting point depression of SCN occurs with the addition of LiTFSI up to 30 wt %, beyond which the crystal melting transition is no longer discernible in the DSC thermograms. The phase diagram based on DSC in combination with POM observations is shown in Figure 9.
Figure 9. Binary phase diagram of SCN/LiTFSI mixture showing various coexistence regions and corresponding crystal morphologies. Dashed lines were drawn by hand to guide the reader’s eyes.
Of particular interest is that there is a wide noncrystalline region where no crystal melting is detectable in the DSC measurements from 30 up to 90 wt % of LiTFSI. The existence of LiTFSI crystals can only be seen under POM above 90 wt % LiTFSI, but at
