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Property-Relaxation Correlations in 3DSiloxane/Polyether Hybrid Polymer Electrolytes Nicola Boaretto, Christine Joost, Henning Lorrmann, Keti Vezzù, Giuseppe Pace, and Vito Di Noto J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b01631 • Publication Date (Web): 27 Apr 2016 Downloaded from http://pubs.acs.org on May 11, 2016

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The Journal of Physical Chemistry

Property-Relaxation Correlations in 3DSiloxane/Polyether Hybrid Polymer Electrolytes Nicola Boaretto,†,‡ Christine Joost,‡ Henning Lorrmann,‡ Keti Vezzù,†,|| Giuseppe Pace,†,§ and Vito Di Noto§,||,* † Department of Chemical Sciences, University of Padua, via Marzolo 1, I-35131, Padova, Italy ‡ Fraunhofer-Institut für Silicatforschung ISC, Neunerplatz 2, D-97082, Würzburg, Germany § CNR-IENI, Corso Stati Uniti 4, I-35127, Padova, Italy || Section of Chemistry for Technology, Department of Industrial Engineering, University of Padua, via Marzolo 1, in Department of Chemical Sciences, I-35131, Padova, Italy.

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ABSTRACT

The thermomechanical and transport properties of a series of hybrid polymer electrolytes are examined, by means of differential scanning calorimetry, rheological analysis and broadband electric spectroscopy. The electrolytes are composed of 3D-oligosiloxane defect clusters, grafted with polyether chains and doped with LiClO4, with concentration ranging from 0 to 1.4 mol⋅kg-1. The thermomechanical properties are mainly modulated by the balance of interactions taking place within the polyether domains. The materials show low Tg and no crystallization in a wide salt concentration range, while the mechanical modulus, comprised between 104 Pa and 105 Pa, is stable up to, at least, 100 °C. A detailed electric characterization, combined with the results from vibrational spectroscopy analysis, elucidates the factors influencing the transport properties. The conductivity reaches 8⋅10-5 S⋅cm-1 at 30 °C for intermediate salt concentrations. The sluggishness of the host matrix appears to be the limiting factor depressing the conductivity at higher salt concentrations. Conversely, the appearance of ion aggregates plays a negligible role, at least in the concentration range examined.


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INTRODUCTION Solid polymer electrolytes (SPEs) have been proposed as possible substitutes for organic liquid electrolytes in lithium secondary batteries, mostly due to their lower flammability and higher resistance to dendrite growth.1 These materials, based mainly on complexes of polyether with lithium salts, are of great interest because of their low cost and toxicity, but the conductivity at room temperature is restricted to 10-6 S⋅cm-1 to 10-5 S⋅cm-1, which is too low for practical purposes. In order to increase the conductivity of SPEs, several strategies have been deployed.2 In particular, excellent results have been achieved through the development of gel-polymer electrolytes,3 but at the cost of reintroducing liquid flammable components in the system. In the field of pure-polymer electrolytes, considerable effort has been directed towards the development of composite and hybrid polymer electrolytes.4 The latter in particular consist of inorganic and organic nano-domains mixed on a molecular level. These materials are generally characterized by low crystallinity, high thermo-mechanical stability, and improved conductivity compared to standard PEO-LiX complexes.5 In a previous report,6 we described the synthesis of a new hybrid inorganic-organic electrolyte, consisting of complexes of LiClO4 with polyethylene chains, covalently bound to 3D-poly(siloxane) networks and further cross-linked through terminal epoxy moieties. The material was prepared by sol-gel reaction between poly(ethylene glycol)(8)-αmethyl,ω-propyltrimethoxysilane

ether

(Me-(PEG)8-PTMS),

and

3-

(glycidoxypropyl)trimethoxysilane (GPTMS). Lithium hydroxide was used during the sol-gel reaction, which resulted in the formation of lithium-rich polysiloxane domains. The sol-gel reaction was followed by lithium perchlorate dissolution and polymerization reaction of the epoxy moieties. In total, seven hybrid electrolytes with molar ratio n(LiClO4)/n(EO) (x) ranging from 0 to 0.1 were prepared (Table 1). In the previous report,6 the resulting structure was investigated


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by vibrational and NMR spectroscopy, and can be described as consisting of loosely cross-linked polyether domains covalently bound to partially condensed siloxane domains, with the noncondensed bonds being in form of lithium silicate. The electrolytes show thermal stabilities up to 250 °C, electrochemical stability window up to 4.5 V versus Li+/Li, and good contact stability versus lithium metal. The conductivity was found to increase with salt concentration, reaching a maximum of 8·10-5 S·cm-1 at 30 °C, for a molar ratio x = 0.05. The lithium transference numbers, determined by potentiostatic polarization measurements as described in Supporting Information, decrease with increasing salt concentration, reaching a plateau value of 0.2 for x > 0.05. In this report, results from differential scanning calorimetry, broadband electric spectroscopy, and rheological analysis are combined to study the thermal, transport, and viscoelastic properties of the materials reported previously. The correlation between thermal behavior, ion, and matrix dynamics, combined with the structural details given by vibrational spectroscopy, provides a powerful basis of information for understanding the material’s dynamic properties, and for designing and optimizing new high performing polymer electrolytes.


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EXPERIMENTAL Materials. Me-(PEG)8-PTMS (Gelest, 98 %) and GPTMS (Sigma-Aldrich 98 %) were used as received, LiOH (Sigma-Aldrich, 98 %) and LiClO4 (Sigma-Aldrich, 99.99 %) were dried for 24 h at 150 °C and 120 °C, respectively. All other reagents and solvents were used after purification by standard methods. All reagents were stored under Argon on molecular sieves to prevent contamination by moisture. All transfer and handling operations were performed in Argon atmosphere.

Electrolytes preparation. The hybrid precursor (HP) is prepared according to a procedure described in a previous report.6 The precursor is mixed with different amounts of LiClO4 and hybrid-polymer electrolytes are obtained by epoxy-ring opening reaction, catalyzed by BF3·EtNH2. The electrolytes are indicated as [HP/(LiClO4)x]. In total seven sets of electrolytes have been prepared, with x equal to 0, 0.005, 0.01, 0.02, 0.05, 0.076, 0.1 (Table 1). Table 1: Electrolytes composition, LiClO4 and Li+ concentration, and dissociation coefficient α for LiClO4 (from vibrational spectroscopy data) and lithium transference numbers.6 n(LiClO4) / n(EO)

c(LiClO4) / mol·kg-1

c(Li+) / mol·kg-1

α

t+

0

0

0.43

/

/

0.005

0.084

0.51

1

0.5

0.01

0.167

0.59

1

0.4

0.02

0.328

0.74

1

0.3

0.05

0.778

1.17

0.94

0.3

0.076

1.136

1.52

0.90

0.2

0.1

1.439

1.81

0.86

0.2


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To obtain the gel-polymer electrolytes, 2 % by weight of BF3·EtNH2 is added to a solution of LiClO4 in HP. The resulting viscous liquid is poured onto aluminum pans and the samples are then heated at 100 °C for 70 h on a heating plate, in Argon atmosphere. After cooling, the pellets are eventually peeled and characterized.

Instruments and methods. Differential scanning calorimetry (DSC) measurements were carried out with a DSC Q 20 (TA Instruments). The measurements are performed on ca. 15 mg of sample hermetically sealed in an aluminum pan with a heating rate of 2 K⋅min-1, in the temperature range comprised between −100 °C and 150 °C. Rheological analyses were performed with an Anton Paar MCR 502, equipped with a PP-25 parallel plate measuring system. The electrolyte membranes were subjected to a sinusoidal angular deformation of amplitude 0.18 mrad, with frequency varying from 0.1 Hz to 10 Hz, and with a sampling frequency of 10 points⋅dec-1. The normal applied force was set to 5 N. The experiments were performed in the temperature range between -20 °C and 100 °C, at intervals of 10 °C. The small oscillation amplitude allows ignoring the inertial effects, so that the system response can be considered in the linear viscoelastic regime. Electrical spectra were collected by broadband electric spectroscopy in the frequency range from 10 mHz to 1 MHz using a Novocontrol Alpha-A Analyzer over the temperature range from -100 °C to 100 °C. The temperature was controlled by using a customized cryostat operating with an N2 gas jet heating and cooling system. Each sample was sandwiched between two circular gold electrodes of diameter 12 mm and sealed in a cylindrical PTFE cell with external stainless steel case. The cell assembly was performed in glove box.


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RESULTS AND DISCUSSION DSC analysis. The DSC profiles of the seven samples were collected from -100 to 150 °C (Figure 1). For x = 0, four thermal transitions are detected: a glass transition temperature at 70 °C (Tg1), followed by a second weak secondary transition (Tg2) at -36 °C (Tg2) (Figure 2), and by a broad endothermic first order transition centered at 7 °C (Tm).

Figure 1: DSC profiles of the seven [HP/(LiClO4)x] electrolytes.

The latter large endothermic peak was assigned to the disruption of locally ordered polyether chains domains.7 The occurring of a double Tg reveals the presence of two types of amorphous polyether domains, characterized by polyether chains with different segmental motion. This is explained by assuming that a distribution of domains, consisting of disordered (amorphous)


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regions and locally ordered (crystalline) domains, is occurring in these materials. In this case, Tg1 was assigned to the glass transition event of the bulk amorphous domains, and Tg2 to the glass transition of amorphous interphase regions sandwiched between small-sized ordered domains.

Figure 2: Double Tg in the DSC profile of [HP/(LiClO4)0].

This assignment is supported by the broadness of the melting peak, which suggests the presence of crystalline regions with a large distribution of sizes. For 0.005 ≤ x ≤ 0.02, only two thermal transitions were detected: a second order transition between -70 °C and –60 °C, and a first order transition between 10 °C and 20 °C. For higher salt concentrations, no first order transition was revealed. The transition temperatures (Tg1, Tg2, Tm) and the first order transition enthalpies are reported in Table 2. The glass transition temperature Tg1 increases with salt concentration, from 70 °C (x = 0) to about -50 °C (x = 0.1). The dependence of the Tg1 from the salt concentration is explained with the increasing density of the dynamic crosslinks between polyether chains and lithium cations which, in turns, increases the viscosity of the system. The first order transition is observed only for x < 0.05, thus indicating that micro-crystalline domains form only at lower salt doping concentrations. No clear correlation between the specific melting enthalpies and the salt concentration is observed. On the contrary, the melting temperature Tm increases from 7 °C to


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16 °C between x = 0 and x = 0.005 and remains constant around 15 °C for higher salt concentrations.

Table 2: Transition temperatures, melting enthalpies, and crystalline fractions of the hybridpolymer/salt complexes [HP/(LiCLO4)x], as determined by DSC. The melting enthalpies ∆Hm and the crystalline fractions χ are normalized on the PEG mass fraction. Tg1

Tg2

Tm

∆Hm / J·g-1

x

χ(PEG) (PEG)

/ °C 0

-69

0.005

-36

7

56

0.51

-68

16

80

0.73

0.01

-59

17

61

0.55

0.02

-53

13

77

0.7

0.05

-56

0.076

-46

0.1

-49

The addition of even small quantities of salt results in the reorganization of the material's micro-structure: the higher Tm and the sharper shape of the related peak indicate larger crystalline domain sizes with a more defined segregation between crystalline and amorphous regions. Furthermore, the trend in the melting enthalpy, which is not depending on the salt concentration for x ≤ 0.02, suggests that the salt is coordinated predominantly in the amorphous domain, as observed in PEO/LiClO4 electrolytes at low salt concentrations (for x < 0.2).8 The melting enthalpy, normalized on the PEG fraction, is also considerably inferior to the one reported for


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poly(ethylene glycol) dimethyl ether of comparable molar mass, which is of 109 J⋅g-1 (Table 2).9 Where detected, the crystalline fraction is estimated to be comprised between 50 % and 70 % of the total polyether content. For higher salt concentrations (x > 0.02), no melting transition is revealed, and the electrolytes show the typical DSC profiles of an amorphous material in the whole temperature range examined.

Rheological analysis. The rheological properties of [HP(LiClO4)x] samples were investigated by oscillatory rheological analysis, in the temperature and frequency ranges comprised between 20 and 100 °C and between 0.1 and 10 Hz, respectively. Some results regarding [HP(LiClO4)0] were already anticipated in the first part of this study.6 To summarize, [HP(LiClO4)0] shows a strong mechanical transition between 0 and 20 °C, which is responsible for the elastic shear modulus drop from over 100 MPa in the low temperature region down to 50 kPa at higher temperatures. In the high temperature regime, the modulus decreases smoothly, reaching a value of 20 kPa at 100 °C (Figure S1). The observed mechanical transition corresponds to the first order transition observed by DSC, and it is consequently attributed to the disruption of microcrystalline domains formed by groups of ordered stacked polyether chains. The loss shear modulus G’’ represents the dispersive component of the complex mechanical response and its value is related to the dynamic viscosity η’ by the relationship G’’ = η’·ω, where ω is the radial frequency. In the case of [HP/(LiClO4)0], G’’ is about 10 % of the storage modulus G’, indicating that the mechanical properties resemble those of an elastic solid (Figure S2). Both G’ and G’’ are frequency dependent. G’’ shows a minimum at medium frequencies, at about 1 Hz, which corresponds to a plateau in the G’ spectra. The


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minimum in the G’’ spectra was used to evaluate the plateau value GN0 from the G’ spectra (Figure 3).10 This parameter is related to the concentration of cross-links or entanglements (ne), which are responsible for the mechanical properties typical of cross-linked or entangled polymeric systems: =

(Eq. 1)

Where ρ is the density of the material, T the absolute temperature, and is R the universal gas constant. In [HP/(LiClO4)0], ne decreases with temperature (Figure 4). A residual dependency of ne on temperature indicates that the plateau modulus GN0 does not stem from the secondary cross-link network formed by the polymerized epoxy moieties. Indeed, the concentration of chemical cross-links, cCL, is fixed at 0.67 meq⋅g-1, a value which is considerably higher respect to the one calculated from Eq. 1. For [HP/(LiClO4)0], ne at room temperature is equal to 0.0185 meq⋅g-1. Correspondently, a chain fragment comprised between two cross-links has a molar mass of 5.4⋅104 g⋅mol-1, approximately the size of 100 polysiloxane units. To explain this result, the structure of the hybrid matrix has to be considered. This can be described as a collection of networked macromolecular fragments, composed by hybrid subunits, and interconnected through the epoxy-originated dioxane bridges. The occurring of a temperature dependent plateau is ascribed to the formation of dynamic cross-links between these fragments, mediated by interactions within the chains of the polyether domains. By addition of LiClO4, the rheological behavior changes significantly. The most striking effect is the absence of the order-disorder transition for x > 0.02 (Figure 3). Over this threshold concentration, the high temperature regime is extended to the whole temperature range examined. This confirms the correspondence between the transitions observed by DSC and


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rheological analysis, and indicates that high salt concentrations hinder the 3D-reorganization of polymer ether chains.

Figure 3: The rubber plateau modulus, GN0, as a function of temperature.

In the high temperature regime, GN0 varies between 104 Pa and 105 Pa, depending on the salt concentration. At 20 °C and for x = 0.005, the plateau modulus drops to 7•103 Pa, it increases up to 6•104 Pa for x = 0.02, and it decreases again for higher salt concentrations. Since GN0 is proportional to ne, these data indicate that the density of crosslinks is maximum for x = 0.02. The dependency of ne on temperature varies with the salt concentration (Figure 4): as noted before, in [HP/(LiClO4)0] ne decreases with increasing temperature. At x = 0.005, ne increases with temperature up to 80 °C. At T > 80°C, the dependency of ne vs T shows again a negative slope, like in [HP/(LiClO4)0]. In the range 0.01 ≤ x ≤ 0.05, ne increases with temperature with an Arrhenius type behavior, with the steepest slope observed for x = 0.02.


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Figure 4: Molar concentration of mechanically active crosslinks (ne) in the high temperature regime. The dotted lines were introduced as a guide for the eye.

At x > 0.02, the slope decreases progressively, reaching negative values for x = 0.1. This complex behavior suggests the coexistence of at least two types of cohesive forces, which are responsible for the 3D cross-links and for the mechanical properties of the materials. The first one, which is predominating at very low and at high salt concentrations (x ≤ 0.005, x ≥ 0.05), is assigned to the short range dipole-dipole interaction between neighbor polyether chains, and diminishes in intensity with increasing temperature. The second interaction is prevailing in the intermediate concentration range and it is due to long range coulombic forces. Since these are mediated by the lithium-coordinating polyether chains, the result is the buildup of an effectively cross-linked network. The fading of this contribution at high salt concentrations is due to the progressive saturation of the polyether coordination sites and to the consequent formation of ion pairs and multiple ionic aggregates, which provide a plasticizing effect, and whose occurrence is confirmed by Raman spectroscopy for x ≥ 0.05.6 As noted before, the loss modulus spectra are characterized by a minimum in the intermediate frequency region (Figure S2). This corresponds to the border region between two dissipative processes which occur at different time scales. On the high frequency side, the onset of the


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Rouse process is observed, corresponding to the mechanical relaxation due to the cooperative polyether segmental motion. The process takes place at lower frequencies or at higher temperatures respect to the glassy region, and is strictly related to the glass transition process. The dissipative event on the low frequency side evidences the occurring of a weak terminal behavior, which is caused by the looseness of the crosslinking network. This is enhanced at low salt concentrations (x ≤ 0.005), where the physical crosslinking network, due to the electrostatic interactions, has a low impact. Indeed, for x ≤ 0.005, this relaxation corresponds to a decrease of G’. With increasing salt concentrations, this low frequency relaxation broadens and shifts to lower frequencies. The same is true also for the segmental relaxation, for x ≥ 0.05. This is indicative of an increased sluggishness and of diminished mobility of the polyether chains. Indeed, the decrease of the relaxation frequencies corresponds to increasing values of G'', which, as noted before, is proportional to the dynamic viscosity. To summarize, the following features emerged from the rheological analysis: i) The materials behave like loosely cross-linked elastic polymers, with G’’ < G’, elastic shear modulus comprised between 104 Pa and 105 Pa at room temperature, and partial terminal behavior. ii) A thermo-mechanic transition occurs at T ≈ 10 °C which is: a) comparable to the first order transition revealed in the DSC profiles, b) coinciding with a considerable drop of G’. iii) The rubber-plateau modulus, GN0, is influenced by the temperature and salt concentration, and its value depends on the balance between dipole-dipole and coulombic interactions taking place in the polyether domains. iv) A dissipative event is detected at low frequencies, and it is attributed to the occurring of a partial terminal behavior.


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v) G’’ and dynamic viscosity increase with increasing salt concentration.

Figure 5: 3D electric spectra of [HP/(LiClO4)0] between -100 and 100 °C, 10-2 and 107 Hz: a) real permittivity (ε'); b) imaginary permittivity (ε''); c) tanδ (ε''/ε'); real conductivity (σ').

Broadband electric spectroscopy measurements. The electrical response of the seven hybrid electrolytes was studied through broadband electric spectroscopy (BES), in the temperature range comprised between -100 °C and 100 °C and in the frequency range between 10-2 Hz and 107 Hz. Figure 5 shows the spectra of [HP/(LiClO4)0] in terms of: the real, ε’ (Figure 5a), and imaginary, ε” (Figure 5b), components of the complex permittivity, phase angle tanδ = ε''/ε'


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(Figure 5c), and the real component of the complex conductivity σ' = ωε''(Figure 5d). The analysis of the spectra allows the determination of characteristic parameters, characterizing the electric response of each sample. In detail, the spectra show several features, which are related to two groups of dissipative events, namely polarizations (pk) and dielectric relaxations (rj). The polarizations are associated to charge accumulation at the interfaces sample/electrode (pEL) and between nano-domains with different permittivity (pIP). The dielectric relaxations are attributed to dipole reorientation events of the polymer matrix. The analysis of these events allows the investigation of the relationship between the dynamics of the host polymer matrix and the macroscopic electrical properties of the whole material. Above the glass transition temperature, the electric response is dominated by the relaxations associated to free charges. In these profiles, the ionic conductivity, σDC, appears as: 1) a plateau in the real conductivity spectrum (Figure 5d), 2) as an increase of ε'' with decreasing frequency (Figure 5b), and 3) as a strong peak in tanδ (Figure 5c). As the frequency decreases, a polarization event (pEL) is observed, which corresponds to a decrease of σ' (Figure 5b) and to a steep increase in the real permittivity spectrum (Figure 5a). At even lower frequencies, a second conductivity plateau (σIP) is detected (Figure 5d), corresponding to the pIP polarization event. At high frequencies, several dielectric relaxations are also visible, which are indicated in the spectra as rj with 1 ≤ j ≤ 3. The dielectric relaxations can be discerned from the conductivity features as they: 1) are not associated to plateaus in σ'; 2) appear as peaks in ε'' with lower intensity than the polarization events; 3) cause an increase of σ' vs log(ω) with increasing frequency. In the spectra of ε” and tanδ of [HP/(LiClO4)0], three relaxations are detected (Figure 5b and 5c). The slowest relaxation, r3, is partially overlapped to the pEL event, although with respect to the other two dielectric phenomena it is much more intense (which is clearly visible as a shoulder in ε').


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The spectra of the polymer electrolytes [HP/(LiClO4)x], with x > 0 show features similar to that of [HP/(LiClO4)0] (Figure S3 – S8). Nevertheless, in [HP/(LiClO4)x] with x > 0, only two dielectric relaxations are detected. A quantitative evaluation of the parameters responsible for the electric response in samples is carried out by fitting the complex σ*(ω) and ε*(ω) (σ*(ω)=iωε*(ω)) spectra with Eq. 2:11

∗ !")

% ! ) !') %* !#"+* ),= #$ & +) +) " #".1 + !#"+* ),- 0 *

2

∆2

67 97

31 + 4#"+2 5 8

+ :

(Eq. 2)

where ε∞ is the permittivity extrapolated in the high frequency limit from ε’(ω) profiles. The second term represents the electrode (EP) and inter-domain (IP) polarization phenomena. In this case, σk is the conductivity associated with k = EP or IP polarization, which corresponds to the plateau values in the σ' spectra. τk = (2πfk)-1 is the polarization time and γk is the shape parameter expressing the distribution of polarization times. In agreement with other results,12 the overall conductivity σT is the superposition of all the conductivity pathways characterizing the long range charge migration phenomena in materials. The third term shows the dielectric j-th relaxations, with ∆εj the relaxation strength, τj = (2πfj)-1 the relaxation time, αj and βj the shape parameters, which take into account the distribution of relaxation times and the interactions of each dielectric mode with the environment. The fitting process is carried out simultaneously in the real and imaginary components of the complex permittivity and conductivity spectra (ε', ε'', σ', σ''), as described elsewhere.13 In [HP/(LiClO4)0], two polarizations and three dielectric phenomena are observed, whereas in [HP/(LiClO4)x], only two polarizations and two dielectric relaxations are detected. An example of such a fitting is shown in Supplementary materials (Figure S9).


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Figure 6: Conductivities σEP and σIP determined from the fitting of the electric spectra. The vertical lines indicate the transition temperatures, as determined by DSC. The roman numbers I, II, and III indicate the temperature regions delimited by the transition temperatures: I) glassy region; II) semi-crystalline region; III) amorphous region.

Bulk and inter-domain conductivities. Two conductivity pathways, σEP and σIP, are revealed in each sample. These contribute to the overall conductivity of the sample σT = σEP + σIP. Since the intensities of σEP and σIP differ by at least one order of magnitude (Figure 6), it can be stated that σT ≈ σEP. σEP is correlated to the charges migration process taking place in the polyether domains (bulk conductivity), whereas σIP to the migration processes occurring at the interfaces between silicate and polyether domains. The bulk conductivity (σEP) is associated to the charge


18

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accumulation at the interface between blocking electrodes and the sample. Since the polyether mass fraction in the hybrid matrix is about 0.77, the polysiloxane nano-clusters are not expected to hinder significantly the long-range charge migration process. The dependency of σEP on 1/T (Figure 6) shows the typical Vogel-Tamman-Fulcher (VTF) profile, thus indicating that the long range charge migration processes in this conductivity pathways is affected by the polymer host dynamics and morphology (Eq. 3):14 %? = @ ∙ B

?D C

∙ EFG $

−IJ & ! − )

(Eq. 3)

In Eq. 3, A is related to the density of carrier ions, Ea is the VTF pseudo-activation energy, and T0 is the ideal glass transition temperature, which is related to the Tg of the polyether domains by the relationship: T0 ≈ Tg – 40 °C. T0 is the temperature where the excess configurational entropy or the free volume of the polyether domain vanishes. For x < 0.05, the conductivity profiles show a change of slope in correspondence of Tm. As a consequence, the conductivity profiles show three temperature regions (I, II and III). I is the glassy region below Tg1, II is the semi-crystalline region between Tg1 and Tm, and III is the amorphous region beyond Tm. Figure 7 shows the activation energies (Ea,i) of the bulk and inter-domain conductivities, and of the dielectric relaxations (see next section). In the semi-crystalline region (II), the conductivity profile shows a low curvature, compatible with a VTF behavior with T0 = 100 K. The resulting activation energies are comprised between 30 kJ·mol-1 and 50 kJ·mol-1, one order of magnitude higher than in the amorphous region (III). As expected, the long range charge migration events are hindered in the semi-crystalline region (II), where the segmental motion of polyether chains is inhibited, thus confirming that the charge migration is predominantly confined into the amorphous domains. Beyond Tm, and for x ≤ 0.02, Ea,i values are ranging from 4 and 6 kJ·mol-1, while at x > 0.02 they are progressively increasing, reaching a final value of ca. 20 kJ·mol-1 for x = 0.1.


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This is in agreement with the decreasing values of σEP. The inter-domain conductivity (σIP) is characterized by Ea value slightly higher than σEP, indicating that σIP is associated with a less favorable migration pathway.

Figure 7: Activation energies of the charge migration processes and of the dielectric relaxations. The roman numbers on the horizontal axes indicate the temperature regions: I) glassy region; II) semi-crystalline region; III) amorphous region.

The detection of a conductivity profile in the [HP/(LiClO4)0] spectrum is unexpected, since no LiClO4 was added in the sample. This suggests that traces of mobile lithium cations, which are then able to move in the polyether domains, are formed concurrently by a fraction of dissociated lithium silicate of the hybrid matrix and by the presence of residual ion traces resulting from the synthesis process of the precursors. These findings are in accordance with other results elsewhere described for similar hybrid polymer electrolytes.15 However, as expected, the charge carrier concentration in this sample is extremely low. Indeed, σEP is more than one order or magnitude lower than that of [HP/(LiClO4)0.05], which, by assuming the same ionic mobility as for [HP/(LiClO4)0.005], corresponds to a charge carrier concentration of 0.008 mol·kg-1 and to a molar ratio Li:O = 5⋅10-4. If we consider that the residual conductivity is due only to the dissociated


20

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structural lithium silicate groups, this means that only 2 % of the whole Li+ present in the system is mobile.

Figure 8: Values of Log(σEP) between -30 and 100 °C, as a function of the LiClO4 concentration.

Figure 8 shows the dependency of σEP on the salt concentration. The σEP increases with raising salt concentration, reaching a maximum of 8⋅10-5 S⋅cm-1 at 30 °C for a LiClO4 concentration of about 0.8 mol·kg-1 (x = 0.05). The conductivity drop observed at x > 0.05 is only partially explained with the formation of contact ion pairs and other ionic aggregates. Raman spectroscopy data detected the presence of ion aggregates in the same concentration range,6 but the calculated dissociation coefficient is always higher than 0.8, whereas an almost tenfold drop in the conductivity at room temperature is observed. This phenomenon can be ascribed to other causes, such as the reduced dynamics of the polyether chains. With respect to σEP, σIP is less dependent on the salt concentration (data not shown). A direct comparison can be performed by analyzing the relative conductivities ρEP = σEP,x/σEP,0 and ρIP = σIP,x/σIP,0, with σEP,x and σIP,x the bulk and inter-domain conductivities of samples [HP/(LiClO4)x], respectively, and σIP,0 the corresponding values of the reference [HP/(LiClO4)0] material (Figure S10). ρEP shows a


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maximum for x = 0.05, and its values are comprised between 20 and 600. For x ≤ 0.05, ρIP shows significantly lower values and no clear dependency on the salt concentration, thus confirming that ρIP is related to the hopping process of the structural lithium ions generated by silicate groups of the matrix. These structural lithium ions are likely to be located at the interface between the polysiloxane and the polyether domains. Nonetheless, an increase of ρIP is observed for x > 0.05. This suggests that high salt concentrations may prompt the inter-domain ion motion, possibly by enhancing the ion exchange between polyether domains and silicate nanoclusters.

Figure 9: Dielectric relaxation frequencies of the seven polymer electrolytes [HP/(LiClO4)x]. The vertical lines indicate the transition temperatures, as determined by DSC. The roman numbers on the horizontal axes indicate the temperature regions: I) glassy region; II) semicrystalline region; III) amorphous region.


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Dielectric relaxations. The interplay between conduction mechanism and polymer host matrix dynamics play a crucial role in the conductivity mechanism of polymer electrolytes.14 In [HP/(LiClO4)0], three relaxations , αs, αm, and αf, were detected (Figure 5a). αf is pinpointed in the glass transition region by the low intensity peak in tanδ and ε'', αm by a peak in tanδ at slightly higher temperatures, and αs is evidenced in tanδ by the high frequency side shoulder of pEL,, to which corresponds the low intensity step decrease in ε'. The relaxation parameters (frequency, dielectric strengths and shape parameters α and β) of these dielectric events are estimated by the simultaneous fitting of σ*(ω), ε*(ω), and tanδ spectra of Figure 5. The dependence on temperature of the frequency of the dielectric relaxation modes is shown in Figure 9. Results are in agreement with previous studies on hybrid PEO-based electrolytes.12 The fastest relaxation, αf, was assigned to the diffusion of conformational states in free polyether chain fragments, which corresponds to the segmental motion of polyether chains non coordinating the ions. The intermediate frequency range event, αm, is observed only in [HP/(LiClO4)0] and it merges with αf at T > Tg2. αm is attributed to the segmental mode of “free” polyether chains interacting at the interphase with different domains in [HP/(LiClO4)0]. This assignment is confirmed by the presence of a double Tg in the DSC profiles. The slowest relaxation, αs, was assigned to the segmental mode of the polyether chains coordinating the lithium cations. The presence of an αs in [HP/(LiClO4)0] confirms that a fraction of the polyether chains of the host matrix is involved in coordination process of lithium cations. This observation is in accordance with the detection of a low intensity σEP event, and confirms that a fraction of the lithium cations neutralizing the silicate nano-domains are dissociated owing to the coordination process promoted by the presence of the polyether chain ligands.


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Figure 10: Dielectric relaxation strengths of the seven polymer electrolytes [HP/(LiClO4)x]. The roman numbers on the horizontal axes indicate the temperature regions: I) glassy region; II) semi-crystalline region; III) amorphous region.

αf and αm show dielectric strengths, ∆ε, comprised between 1 and 10 (Figure 10), whereas αs has dielectric strengths at least ten times higher. This is easily explained if we consider that αs corresponds to the relaxation of polyether segments coordinated with the lithium cations. These polyether-Li+ complexes form larger dipole moments which are able to reorient efficiently with the fluctuation of the electric field. The dielectric strength of αs decreases on temperature (log(∆ε(αs)) ∝ 1/T), whereas ∆ε(αm) of and ∆ε(αf) are only slightly increasing on temperature. This confirms that the interactions of the dipole moments associated to αs with the external electric field are stronger. Therefore, the effect of the thermal stimulation on this mode is expected to be more effective.


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The activation energies of the relaxation frequencies fi,j (Figure 7) were obtained by fitting the fi,j versus 1/T profiles with a VTF equation, and by assuming that a) T0 = Tg – 40 K in region III, and b) T0 = 100 K in regions I and II. Results show an almost exact correspondence between the activation energies of σEP and those of the relaxation frequency fα,s, both in the semi-crystalline and in the amorphous regions, thus indicating that αs plays a crucial role in modulating the long range charge migration processes. In the semi-crystalline region, the activation energy of fα,s is about 50 kJ⋅mol-1, decreasing to ca. 10 kJ⋅mol-1 in region III. The higher values of Ea in the semicrystalline region are caused by the interaction with the ordered stack domains, whose polyether chains are characterized by a negligible mobility. Correspondently, the same trend is also observed in the activation energy of σEP. On the contrary, the activation energy of σIP does not correspond to that of fα,s, thus suggesting that this secondary charge migration process is only partially modulated by the polyether segmental motion. Finally, the activation energy of fα,f, determined in regions I and II, are comprised between 5 and 20 kJ⋅mol-1. Since the values do not match with the ones characteristic of the conductivity in the same temperature regions, we can suggest that fα,f does not influence significantly the charge migration processes.

Figure 11: Segmental relaxation frequency as a function of the salt concentration.


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Another peculiarity is that fα,s is strongly dependent on the salt concentration. Indeed, fα,s increases at low salt concentrations and reaches a maximum between x = 0.02 and x = 0.05 (Figure 11), with the maximum shifting to lower concentrations by increasing temperatures. This dependency is similar to that of σEP, further confirming the relationship existing between fα,s and σEP.

Diffusion coefficients and average migration distances. Following the Dynamic Bond Percolation model, the ionic conductivity is related to the dielectric relaxation frequency through the Nernst-Einstein-Smoluchowski formalism.16 The average salt diffusion coefficients, Davg, were calculated by the Nernst-Einstein equation: MJNO =

%PQ R C 2TU

(Eq. 4)

where σEP is the bulk conductivity, F the Faraday constant, c the molal salt concentration, ρ the density of the sample, and α is the salt dissociation coefficient at room temperature. The values of α (Table 1) are calculated from previously published vibrational data,6 by using: U=

TYZ[\] @!VW ) = @!VW ) + @!VXW ) T^_YZ[\

(Eq. 5)

where A(ν1) and A(ν’1) are the areas of the vibrational bands attributed to the stretching of free and ion-paired LiClO4 respectively, cClO4- is the concentration of free perchlorate, and cLiClO4 is the total concentration of lithium perchlorate in the sample. At room temperatures, and for x ≤ 0.05, the Nernst-Einstein diffusion coefficients are comprised between 2 and 5·10-9 cm2·s-1 (Figure S11). For higher salt concentrations (x = 0.076 and x = 0.1), the diffusion coefficients decrease of one order of magnitude. These values are considerably lower than the ones obtained by NMR for analogous materials with a similar


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conductivity.17 In order to understand such a discrepancy, which is commonly observed in polymer electrolytes,18 it should be considered that: 1) Nernst-Einstein diffusion coefficients are directly correlated to the long range charge transfer processes, which takes place when cation exchange phenomena occur between different domain of bulk materials; 2) diffusion coefficients obtained by NMR are correlated to the local intra- and inter-chain elementary cation exchange processes, which typically show higher rates and higher diffusion coefficients. On this basis, the discrepancy between NMR and conductivity diffusion coefficients is expected and easily justified, indicating that, in bulk electrolytes, the local motions are not rate determining phenomena in modulating their long range charge migration events. For these reasons, in the material here proposed, the inconsistency between the NMR results and the Nernst-Einstein diffusion coefficients demonstrates that the conductivity in proposed electrolytes occurs owing to cation exchanges events between different domains of materials mediated by the polymer matrix reorganization processes (segmental motions).

Figure 12: Average salt diffusion coefficients plotted versus the frequency of the αs mode. The full line indicates an ideal linear correlation between Davg and fα,s.


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The log-log plot of the Nernst-Einstein diffusion coefficients versus the relaxation frequency of αs is plotted in Figure 12. At low salt concentrations, the diffusion coefficients show a good linear correlation on fα. Large deviations are observed only for the two highest salt concentrations, and especially at low temperature. For x = 0.1, the slope of the log-log plot is about 1.3, whereas for all the other samples the slope is about 1.0. The deviation from linearity may be caused by an increase of the ion association with decreasing temperature. At the same time, the formation of higher order ionic aggregates cannot be excluded. The presence of these species also contributes to lower the effective charge carrier concentration. At lower salt concentrations, ion-ion interactions are negligible, and the observed linear correlation indicates that the diffusion coefficients follow the Einstein-Smoluchowski equation:16 MJNO =

` !bJNO )C c6,e 3

(Eq. 6)

where λavg corresponds to the average migration distance covered by an ion in the timescale of a segmental relaxation event.

Figure 13: Migration distances as a function of salt concentration.

The results (Figure 13) show that λavg is comprised between 0.7 and 1.5 nm, it decreases with increasing salt concentration and reaches a minimum for x = 0.076. These distances are


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comparable to the characteristic dimensions within a single polyether chain, which confirms that λavg is related to inter-chain ion migrations taking place in the polyether domains. The migration distances of the anion (λ-) and of the cations (λ+) are evaluated by using the transference numbers t+ (Table 1). These were determined by the potentiostatic polarization method elsewhere reported,6 and briefly described in the Supporting Information: bf = 2g f bJNO

(Eq. 7a)

bB = 2!1 − g f )bJNO

(Eq. 7b)

In this analysis, the effect of temperature on the transference numbers is neglected. This is acceptable, since the transference numbers are almost independent on temperature.19 The results show that the migration distance of anions and cations is different and depend on the salt concentration. λ+ decreases fast and monotonically, reaching a steady value at about 0.4 nm. Conversely, λ- shows a minimum at x = 0.05, and it increases at higher salt concentrations. In both cases, a decrease of the hopping distance can be interpreted in terms of increasing electrostatic interactions. In the case of λ+, the limiting value corresponds approximately to the distance between two neighbor coordination sites within a single polyether chain (intra-chain migration). To sum up, the conductivity data were analyzed by applying the Nernst-Einstein formalism, and by correlating the resulting salt diffusion coefficients with the segmental relaxation frequency. The analysis showed that the two parameters are in linear correlation, if the formation of ionic aggregates can be neglected. As a final step, we estimated the average distances covered by cations and anions within the timescale of a segmental relaxation, by applying the previously determined transference numbers to the average characteristic distance λavg. The results showed


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that λavg is comparable to the size of a single polyether chain, and that increasing ion-ion interactions hinder both anion and cation mobility.


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CONCLUSIONS This work describes the thermo-mechanical and transport properties of a series of hybrid polymer electrolytes, consisting on partially cross-linked polyether chains covalently bound to lithium-rich siloxane clusters, and further doped with LiClO4. The materials are studied by differential scanning calorimetry (DSC), rheological analysis, and broadband electric spectroscopy (BES). The combination of these three techniques allowed us to study the interplay between the material’s structure, the polyether dynamics, and the ion conduction mechanism. In particular, multiple thermal transitions are detected by DSC, demonstrating a very complex thermal behavior and morphological structure. As a general remark, increasing salt concentrations result in the shift of the glass transition towards higher temperatures, decreasing the crystallinity of the polyether chains, which progressively raises the homogenization of the polyether domains. Indeed, [HP/(LiClO4)0] shows segregation between microcrystalline and amorphous domains, and double Tg behavior, indicating micro-phase separation in the amorphous domains. In the range 0.005 ≤ x ≤ 0.02, a single Tg and a first-order transition are observed. For higher salt concentrations, only one Tg is observed. The mechanical properties depend mostly on non-covalent interactions taking place in the polyether domains, which in turn are influenced by temperature and salt concentration. Increasing temperature and salt concentration destabilize the polyether micro-crystalline domains, prompting a thermo-mechanical transition to a disordered phase, which is characterized by a much lower elastic shear modulus. In the disordered phase, the mechanical properties are modulated by the balance between dipole-dipole interactions between polyether chains and long range ionic coulombic interactions.


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Broadband electric spectroscopy analysis is used to study the conductivity and the influence of polymer dynamics on conduction mechanism. Two conductivity pathways are observed. The first consists of the ion migration in the polyether domains, and the second of the motion of the lithium cations at the interface between polyether domains and siloxane clusters. The first conduction mechanism gives the main contribution to the overall conductivity and it is modulated by the segmental motions of the polyether chains. The synthesized hybrid electrolytes are characterized by a highly disordered 3D-structure, due to the presence of polysiloxane clusters, and to the partial crosslinking of the polyether chains. A tendency to crystallization is still observed, but the stability window of the ordered phase is limited to a small temperature and salt concentration range. Correspondently, a high conductivity in a wider temperature and concentration range is obtained. At high salt concentrations, the increasing ion-ion interactions, the formation of ionic aggregates, and the general increase of the medium viscosity are responsible of the drop in the ionic conductivity. The polyether segmental relaxations are themselves strongly influenced by the salt concentration. The relaxations frequency reaches a maximum for 0.02 ≤ x ≤ 0.05, similarly to what is observed for the bulk conductivity. In general, a linear correspondence between the segmental relaxations frequency, fαs, and the diffusion coefficients obtained from conductivity data, is observed. This dependence is used to evaluate the migration distances covered by the charges on the timescale of the segmental relaxations. These migration distances are compatible with the length of a single polyether chain. The results show that the ionic mobility is maximized if two conditions are satisfied: a) negligible ion-ion interactions and b) absence of locally ordered polyether domains. If both conditions are satisfied, the charge motion is effectively modulated only by the segmental


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relaxation frequency of the polyether domains. In the particular case, the optimal conditions are achieved at low salt concentrations (0.02 ≤ x ≤ 0.05). In these conditions, the conductivity at 30 °C

is

about

8⋅10-5 S⋅cm-1,

compatible

with

the

one

observed

for

networked

polysiloxane/polyether-based polymer electrolytes.20,21 By setting as threshold a lithium conductivity of 10-4 S⋅cm-1,22 these materials may be used as electrolytes in lithium batteries with operating temperature higher than 50 °C.


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ASSOCIATED CONTENT Supporting Information. Additional information includes the description of the method used for the determination of the lithium transference numbers, the complete rheological spectra (loss and storage moduli), an example of fitting of the electric spectra, the 3D-electric spectra and the relative conductivity of the seven electrolytes, the diffusion coefficients plotted against the inverse temperature.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: 00390498275229 Author Contributions All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This research was partly funded by the Project ZfAE „Zentrum für Angewandte Elektrochemie“ of the Free State of Bavaria, and by the Strategic Project of the University of Padova “MAESTRA - From Materials for Membrane-Electrode Assemblies to Electric Energy Conversion and Storage Devices”. We would also like to thank Birke-Elisabeth Olsowski for the support in the synthetic part.


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REFERENCES (1)

Fergus, J. W. Ceramic and Polymeric Solid Electrolytes for Lithium-Ion Batteries. J. Power Sources 2010, 195, 4554–4569.

(2)

Muldoon, J.; Bucur, C. B.; Boaretto, N.; Gregory, T.; Di Noto, V. Polymers: Opening Doors to Future Batteries. Polym. Rev. 2015, 55, 208–246.

(3)

Scrosati, B.; Garche, J. Lithium Batteries: Status, Prospects and Future. J. Power Sources 2010, 195, 2419–2430.

(4)

Di Noto, V.; Lavina, S.; Giffin, G. A.; Negro, E.; Scrosati, B. Polymer Electrolytes: Present, Past and Future. Electrochim. Acta 2011, 57, 4–13.

(5)

Skaarup, S.; West, K.; Zachau-Christiansen, B.; Popall, M.; Kappel, J.; Kron, J.; Eichinger, G.; Semrau, G. Towards Solid State Lithium Batteries Based on ORMOCER Electrolytes. Electrochim. Acta 1998, 43, 1589–1592.

(6)

Boaretto, N.; Bittner, A.; Brinkmann, C.; Olsowski, B.-E.; Schulz, J.; Seyfried, M.; Vezzù, K.; Popall, M.; Di Noto, V. Highly Conducting 3D-Hybrid Polymer Electrolytes for Lithium Batteries Based on Siloxane Networks and Cross-Linked Organic Polar Interphases. Chem. Mater. 2014, 26, 6339–6350.

(7)

Gorecki, W.; Jeannin, M.; Belorizky E.; Roux, C.; Armand, M. Physical Properties of PEO-LiTFSI Complexes. J. Phys. 1995, 7, 6823–6832.

(8)

Vallée, A.; Besner, S.; Prud'Homme, J. Comparative Study of Poly(Ethylene Oxide) Electrolytes Made with LiN(CF3SO2)2, LiCF3SO3 and LiClO4: Thermal Properties and Conductivity Behaviour. Electrochim. Acta 1992, 37, 1579–1583.


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 38

(9)

CARBOWAX(TM) Methoxypolyethylene Glycol (MPEG) 350: Technical Data Sheet.

(10)

Raju, V. R.; Menezes, E. V.; Marin, G.; Graessley, W. W.; Fetters, L. J. Concentration and molecular Weight Dependence of Viscoelastic Properties in Line and Star Polymers. Macromolecules 1981, 14, 1668–1676.

(11)

Kitajima, S.; Bertasi, F.; Vezzù, K.; Negro, E.; Tominaga, Y.; Di Noto, V. Dielectric Relaxations and Conduction Mechanisms in Polyether–Clay Composite Polymer Electrolytes under High Carbon Dioxide Pressure. Phys. Chem. Chem. Phys. 2013, 15, 16626–16633.

(12)

Bertasi, F.; Vezzù, K.; Giffin, G. A.; Nosach, T.; Sideris, P.; Greenbaum, S.; Vittadello, M.; Di Noto, V. Single-Ion-Conducting Nanocomposite Polymer Electrolytes Based on PEG400 and Anionic Nanoparticles: Part 2. Electrical Characterization. Int. J. Hydrogen Energy 2014, 39, 2884–2895.

(13)

Di Noto, V.; Giffin, G. A.; Vezzù, K.; Piga, M.; Lavina, S. Broadband Dielectric Spectroscopy: A Powerful Tool for the Determination of Charge Transfer Mechanisms in Ion Conductors. In: Knauth, P., Di Vona, M. L., Eds. Solid State Proton Conductors; John Wiley & Sons, Ltd: Chichester, UK, 2012; pp 109–183.

(14)

Di Noto, V. Electrical Spectroscopy Studies of Lithium and Magnesium Polymer Electrolytes Based on PEG400. J. Phys. Chem. B 2002, 106, 11139–11154.

(15)

Di Noto, V.; Furlani, M.; Lavina, S. Synthesis, Characterization and Ionic Conductivity of

Poly[(Oligoethylene

Oxide)

Ethoxysilane]

and

Poly[(Oligoethylene

Oxide)

Ethoxysilane] /(EuCl3)0.67. Polym. Adv. Technol. 1996, 7, 759–767.


36

Page 37 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(16)


Di Noto, V.; Vittadello, M.; Yoshida, K.; Lavina, S.; Negro, E.; Furukawa, T. Broadband Dielectric and Conductivity Spectroscopy of Li-Ion Conducting Three-Dimensional Hybrid Inorganic–Organic Networks as Polymer Electrolytes Based on Poly(Ethylene Glycol) 400, Zr and Al Nodes. Electrochim. Acta 2011, 57, 192–200.

(17)

Cheung, I.W.; Chin, K.B.; Greene, E.R.; Smart, M.C.; Abbrent, S.; Greenbaum, S.G.; Prakash, G.K.S.; Surampudi, S. Electrochemical and Solid State NMR Characterization of Composite PEO-Based Polymer Electrolytes. Electrochim. Acta 2003, 48, 2149-2156.

(18)

Bruce, P.G.; Vincent, C.A. Effect of Ion Association on Transport in Polymer Electrolytes. Faraday Discuss. Chem. Soc. 1989, 88, 43-54.

(19)

Dai, H.; Zawodzinski, T. A. The Dependence of Lithium Transference Numbers on Temperature, Salt Concentration and Anion Type in Poly(Vinylidene Fluoride)– Hexafluoropropylene Copolymer-Based Gel Electrolytes. J. Electroanal. Chem. 1998, 459, 111–119.

(20)

Popall, M.; Andrei, M.; Kappel, J.; Kron, J.; Olma, K.; Olsowski, B. ORMOCERs as Inorganic–Organic

Electrolytes

for

New

Solid

State

Lithium

Batteries

and

Supercapacitors. Electrochim. Acta 1998, 43, 1155–1161. (21)

Oh, B.; Vissers, D.; Zhang, Z.; West, R.; Tsukamoto, H.; Amine, K. New Interpenetrating Network Type Poly(Siloxane-g-Ethylene Oxide) Polymer Electrolyte for Lithium Battery. J. Power Sources 2003, 119-121, 442–447.

(22)

Goodenough, J. B.; Kim, Y. Challenges for Rechargeable Li Batteries. Chem. Mater. 2010, 22, 587–603.


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Property-Relaxation Correlations in 3D-Siloxane ... - ACS Publications

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