Influence of Al2O3 Nanowires on Ion Transport in Nanocomposite

Dec 6, 2018 - Choi, U. H.; Middleton, L. R.; Soccio, M.; Buitrago, C. F.; Aitken, B. S.; Masser, H.; Wagener, K. B.; Winey, K. I.; Runt, J. Dynamics o...
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Influence of Al2O3 Nanowires on Ion Transport in Nanocomposite Solid Polymer Electrolytes Suk Jin Kwon,† Byung Mun Jung,† Taehoon Kim,† Jinho Byun,‡ Jaekwang Lee,‡ Sang Bok Lee,*,† and U Hyeok Choi*,§ †

Functional Composites Department, Korea Institute of Materials Science, Changwon 51508, Korea Department of Physics, Pusan National University, Busan 46241, Korea § Department of Polymer Engineering, Pukyong National University, Busan 48547, Korea Downloaded via TULANE UNIV on December 8, 2018 at 02:25:36 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Understanding the ion transport mechanism in nanocomposite solid polymer electrolytes is necessary to develop next-generation electrochemical devices. We investigate the role of inorganic nanoparticle on ion conduction and segmental dynamics in cross-linked epoxy-based nanocomposite solid polymer electrolytes, complexed with Li+F3CSO2NSO2CF3− (LiTFSI) salt and Al2O3 nanowire, using dielectric relaxation spectroscopy. The addition of Al2O3 not only increases the ionic conductivity σDC by up to ∼10 times but also accelerates the segmental α motion compared to the host electrolyte. Increasing Al2O3 content leads to a reduction in segmental α relaxation temperature Tα (fast dynamics), resulting in increased ion mobility as well as an enhancement in segmental α relaxation strength Δεα, lowering ion dissociation energy, as revealed by density functional theory calculations, thereby providing more mobile ions for conduction. This ion transport investigation provides insights into the design of high-conductivity nanocomposite solid polymer electrolytes for energy applications.

1. INTRODUCTION For the development of next-generation safe secondary lithium-ion batteries (LIBs) with high energy/power density, solid polymer electrolytes (SPEs)1−5 having good electrochemical stability are widely considered as promising substitutes for carbonate-based organic liquid electrolytes having flammability and explosion concerns.6,7 The SPEs could also inhibit the penetration of lithium dendrites from an electrode to an electrolyte, thus enabling the utilization of lithium metal anode with a high theoretical specific capacity.8,9 Poly(ethylene oxide) (PEO) containing a Li salt is one of the examples for a SPE and has been widely studied since 1970.10−12 However, the SPEs based on PEO usually suffer from low room-temperature ionic conductivity, σDC, because of PEO crystallization, slowing down polymer segmental motion, and show high electrolyte−electrode interfacial resistance,13 limiting recharging rates and power density and hence resulting in decreased capacity utilization. To improve σDC for commercialization, one of the effective strategies is the incorporation of liquid plasticizers that increases polymer chain motion, yet resulting in a significant decline in mechanical properties.14−16 To find a solution to the issue, of considerable interest are nanocomposite solid polymer electrolytes (NSPEs), wherein inorganic nanoparticles such as Al2O3, SiO2, or TiO2 are added © XXXX American Chemical Society

to polymer hosts, allowing for the combination of the advantages of both inorganic materials and organic polymers.17−21 This approach has been successfully employed for enhancing the interfacial22,23 and mechanical24 properties of the PEO/Li salt electrolytes. Furthermore, the addition of inorganic particles enhanced ionic conductivity of a PEO− LiClO4 mixture containing TiO2 or Al2O3 nanoparticles, and the improvement is primarily due to the absence of the polymer host crystallinity, which is consistent with no crystallization peaks from the DSC measurement.17 Extensive experimental investigations focused on the nanoparticle effect on the conductivity and reported that the surface functionalities of the inorganic nanoparticles promoted local structural modifications by Lewis acid−base interactions between these surface groups and the ionic species of the salt.25−29 Consequently, the interaction allows the ion pairs to be favorably separated, resulting in the enhancement of the number density of mobile Li ion that can transport fast throughout the nanoparticle surface as the conductive pathways.20 Molecular dynamics simulations30,31 have also been used to investigate the effect of inorganic nanoparticles Received: July 26, 2018 Revised: October 23, 2018

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received, but LiTFSI was dried under vacuum at 120 °C for 24 h prior to use. 2.2. Sample Preparation. The SN and LiTFSI mixture (SN/ LiTFSI) was first prepared by adding 1 M LiTFSI into SN melted at 60 °C, and then the material was constantly mixed under magnetic stirring until LiTFSI was completely dissolved. The epoxy resin system, consisting of YD-128 and KBH-1089 in the ratio of 10:9 by weight, was then added into the SN/LiTFSI solution, and the mixture of 50 wt % epoxy resin and 50 wt % SN/LiTFSI (referred to SN/ LiTFSI50) was stirred at 30 °C.32 The required amount of Al2O3 nanowires was finally added to the epoxy resin-containing electrolyte solution (SN/LiTFSI50), and after vigorous magnetic stirring, the homogeneously dispersed nanoparticle epoxy-based electrolyte solution was achieved. Different ratios of SN/LiTFSI50 and Al2O3 [Y_Al2O3, where Y is the Al2O3 content (Y = 0, 5, 9, 17, and 20 wt %)] were prepared. The Al2O3 nanowire distribution within the epoxy matrix was characterized with energy-dispersive X-ray spectroscopy (EDS) mapping over the cross section of the Al2O3-containing nanocomposite solid polymer electrolyte (Figure S1). 2.3. Dielectric Spectroscopy. The measurements of ion and polymer dynamics for the SPE and NSPEs were performed using broadband dielectric relaxation spectrometer (DRS) consisting of a Novocontrol GmbH Concept 40 and a Quatro Cryosystem highquality temperature control system. For these dielectric measurements, uncured samples were initially placed on a freshly polished brass electrode with a 30 mm diameter. After the samples flowed to cover the bottom electrode in vacuo, silica spacers were placed on top of the sample to set the sample thickness at 100 μm. To build a parallel plate capacitor cell, another freshly polished brass electrode as a top electrode with a 15 mm diameter was located on top of the sample. After being in a vacuum oven at 60 °C for at least 30 min (allowing to be squeezed to a gap of 100 μm), the sample was positioned in the DRS (after the measurements were complete, the precise thickness was verified). Each sample was then cured at 80 °C for 2 h, 120 °C for 1 h, and 150 °C for 2 h in the Novocontrol sample chamber having vacuum-isolated cryostat with a heated stream of nitrogen, allowing to avoid a moisture effect on ion and polymer dynamics (Figures S2 and S3). With 0.1 V amplitude, dielectric spectra were then collected in isothermal frequency (10−1−107 Hz range) sweeps every 10 K, from 150 to −100 °C (the Supporting Information provides the complete dielectric spectra of the complex dielectric permittivity [ε′ and ε″] and conductivity [σ′ and σ″], with Figures S4−S8). 2.4. Ab Initio DFT Calculations. All the calculations were performed with the density functional theory (DFT) using the generalized gradient approximation (GGA) and the projectoraugmented wave method33 with a plane-wave basis, as implemented in the Vienna ab initio simulation package (VASP) code.34 We used a kinetic energy cutoff of 500 eV and Γ-centered 4 × 4 × 2 k-point meshes for the Brillouin zone integration. The calculations converged in energy to 10−6 eV/cell, and all compounds were allowed to relax until the forces were less than 1 × 10−2 eV Å−1.

on the ion transport in NSPEs, but their results revealed that the nanoparticles restricted the polymer segmental motions, thereby reducing the overall ion mobility and σDC of NSPEs. Although improvement of σDC by inorganic nanoparticle plasticization has been experimentally observed in the PEO/ salt complex, the impact of the inorganic nanoparticle on the ion transport mechanism is far from being fully understood in alternative NSPEs. In this study, Al2O3 nanowire, which could offer better conducting ion pathways because of the generation of effective percolation network rather than isolated spherical nanoparticles,19,20 is added to a chemically cross-linking epoxybased SPE. The SPE was previously prepared using a commercially available cross-linkable diglycidyl ether of bisphenol A epoxy resin (YD-128), an anhydride curing agent (KBH-1089), and a mixture of succinonitrile (SN) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), with the structures shown in Figure 1, by a thermal curing process.32

Figure 1. Chemical structures of a diglycidyl ether of bisphenol A (YD-128) epoxy resin, a methyl tetrahydrophthalic anhydride (KBH1089) curing agent, a succinonitrile (SN) plastic crystal, and a lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt.

The curing process can lead to a polymerization-induced phase separation in the materials comprising solvating SN/Li salt conducting domains with a mechanically robust epoxy framework, which is one of the strategies to simultaneously achieve high modulus and high ionic conductivity.32 Herein, results of a systematic investigation of the role of Al2O3 nanowire content (0. 5, 9, 17, and 20 wt % Al2O3) on the ion conduction and dielectric response of the epoxy-based NSPE are reported using experimental dielectric relaxation spectroscopy measurements and theoretical density functional theory (DFT) calculations. Understanding both experimental observations and theoretical predictions in the electrolytes provides useful insights for the design of advanced materials for high-performing and safe energy storage applications.

3. RESULTS AND DISCUSSION 3.1. Ionic Conductivity. Figure 2 displays the temperature dependence of DC conductivity, σDC, evaluated from the real part of the conductivity, σ′(ω) (Figure S9), for the SPE and NSPEs containing Al2O3 nanowires. Their σDC is well described by the Vogel−Fulcher−Tammann (VFT) equation:

2. EXPERIMENTAL SECTION 2.1. Materials. A diglycidyl ether of bisphenol A epoxy resin (YD128, Kukdo Chemical, Korea) and a methyl tetrahydrophthalic anhydride curing agent (KBH-1089, Kukdo Chemical, Korea) were used to construct a mechanical framework of these SPE and NSPEs. To incorporate a conducting phase into the epoxy matrix, a mixture of a succinonitrile (SN, Sigma-Aldrich, Korea) and a lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, Sigma-Aldrich, Korea) salt was used. An inorganic Al2O3 nanowire with 2−6 nm diameter and 200−400 nm length (Sigma-Aldrich, Korea) was used as an effective nanofiller for the epoxy-based solid polymer nanocomposites. These raw materials (YD-128, KBH-1089, SN, and Al2O3) were used as

ij DT0 yzz σDC = σ∞ expjjj− z j T − T0 zz k {

(1)

wherein σ∞ is the high-temperature limit of the conductivity, D is the so-called strength parameter and reciprocally related to fragility, and T0 is the Vogel temperature. Equation 1 with the three fitting parameters, listed in Table 1, is shown as the solid lines in Figure 2 (the Supporting B

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additional α2 relaxation, which has been observed in polymer− salt complexes39−41 and ionomers14,42,43 at lower frequencies, is likely related to ionic orientational polarization (such as ion exchanging states between isolated ion pairs and aggregates of pairs),43 in addition to the segmental α relaxation involving collective reorientational motions of the complexed SN− LiTFSI−Al2O3 segments, at higher frequencies. The peak relaxation frequency maxima ωmax and the relaxation strength Δε of both the α2 and α processes were obtained by fitting the derivative spectra εder(ω) with one power law for electrode polarization (EP, see dashed line in Figure 3) plus two Havriliak−Negami (HN) functions for the two dielectric relaxations (α2 and α, see dashed curves in Figure 3 and Supporting Information contains the validity of εder analysis, with Figure S11 and Tables S2−S7)

Figure 2. σDC vs 1000/T for SPE and NSPEs and its fit to the VFT equation (eq 1, solid curves). The inset shows composition variation in σDC at 298 K, where ΦAl2O3 is the weight fraction of Al2O3.

ÄÅ É iÄ ′ (ω) ÉÑÑ ÅÅ ∂εHN ′ (ω) ÑÑÑÑ yzzz π jjjÅÅÅÅ ∂εHN ÑÑ Å ÑÑ + ÅÅ Ñ z εder = Aω − jjjÅÅ ÅÅÅ ∂ ln ω ÑÑÑÑ zzz 2 jÅÅÅÇ ∂ ln ω ÑÑÑÖα Ç Öα { 2 k | l o o Δε o ′ (ω) = Realo with εHN m } o a b o [1 + (iω/ωHN) ] o o n ~

Information provides the methodology used in the nonlinear VFT fit, with Table S1). The Al2O3 addition results in a decrease in both D from 16.1 to 12.9 (more fragile) and T0 from 139 to 133 K (faster dynamics). The neat SPE (0% _Al2O3) exhibits σDC ∼ 1.9 × 10−6 S cm−1 at 298 K. Gradual enhancement in σDC is observed on addition of Al2O3 in the inset of Figure 2, showing Al2O3 composition variation in σDC at 298 K. Increasing σDC eventually saturates, with maximum enhancement in conductivity of 1 order of magnitude, found for the NSPE with 17%_Al2O3 (σDC ∼ 1.5 × 10−5 S cm−1), but adding further Al2O3 slightly reduces σDC of 20%_Al2O3. The enhanced conductivity from the nanowire incorporation is partly from lowering glass transition temperature by acting as a plasticizer, thereby improving ion mobility, as well as partly from promoting ion dissociation, as the affinity of inorganic nanowires toward Li salt (i.e., ion−nanowire complex formation), hence boosting mobile ion concentration.35 To demonstrate these two effects, dipolar relaxation processes are next analyzed to assess the Al2O3 effect on segmental dynamics. 3.2. Dielectric Relaxation. Figure 3 displays the derivative dielectric loss curves36 εder = −π/2{∂ε′(ω)/∂[ln ω]} of Al2O3free SPE (0%_Al2O3) vs Al2O3-containing NSPE (5, 9, 17, and 20%_Al2O3). It has been shown previously37,38 that SN, forming a plastic crystal phase that behaves an orientationally disordered phase, exhibits collective reorientational motions of the molecular dipoles under the applied AC field [the so-called segmental (α) motion that is correlated to the typical characteristics of the glass transition dynamics]. In contrast to SN, an additional dipolar relaxation (α2) is observed at lower frequencies, upon incorporating LiTFSI into SN (see Figure S10, where the three-component mixture of LiTFSI, SN, and Al2O3 also displays two relaxations such as α and α2 processes). These SPE (0%_Al2O3) and NSPE (5, 9, 17, and 20% _Al2O3) also show two dielectric relaxations designated as α and α2 in order of decreasing frequency (see Figure 3). The

−s

(2)

wherein A and s are constants, a and b are shape parameters (listed in Table S2), and ωHN is a characteristic frequency related to the frequency of maximal loss ωmax by43 i aπ yz zz ωmax = ωHNjjjsin k 2 + 2b {

1/ a

ij abπ yz jjsin zz k 2 + 2b {

−1/ a

(3)

The α2 relaxation frequency maxima ωα2 (Figure 4a) and α relaxation frequency maxima ωα (Figure 4b) follow a VFT temperature dependence with the same Vogel temperature T0 as was found for σDC (Figure 2): ij DT0 yzz ωmax = ω∞ expjjj− z j T − T0 zz k {

(4)

The solid curves in Figure 4a,b are fit to eq 4 using the T0 from the conductivity VFT fits in Figure 2 with the hightemperature limiting frequency ω∞ and the strength parameter D, as two fitting parameters listed in Table 1.44 Adding Al2O3 into the neat SPE steadily accelerates the ion rearrangement (α2) as well as the segmental motion (α) on addition of up to 17 wt % of Al2O3, which shows the highest σDC among these SPE and NSPEs. However, further increasing Al2O3 content (20%_Al2O3) slightly slows down both the α2 and α relaxation processes, indicating that incorporating more than 20 wt % Al2O3 into the SPE may hinder motion of the dipoles, thereby leading to a slight decrease in σDC. This is consistent with the relaxation temperatures Tωmax s, obtained by extrapolating the VFT fits of the α2 and α relaxations to ωmax = 0.01 rad s−1 (Tα2

Table 1. Fitting Parameters (Eq 1) of the VFT Temperature Dependence of σDC and the α2 and α Relaxation Frequencies σDC

α2 process

α process

sample

T0 (K)

σ∞ (S/cm)

D

log(ω∞) (rad/s)

D

log(ω∞) (rad/s)

D

0%_Al2O3 5%_Al2O3 9%_Al2O3 17%_Al2O3 20%_Al2O3

139 135 134 133 134

2.5 0.9 0.7 0.4 0.5

16.1 14.0 13.4 12.9 13.0

11.7 10.9 11.0 11.0 10.5

17.4 15.1 14.5 14.2 13.4

12.0 12.0 11.7 11.7 11.6

15.6 15.6 14.2 13.9 13.9

C

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Figure 3. Dielectric derivative spectra εder at 263 K for (a) 0% and 17%_Al2O3, (b) 5%_Al2O3, (c) 9%_Al2O3, and (d) 20%_Al2O3. The solid curves are fits of the sum of a power law for electrode polarization and two derivative forms of the Havriliak−Negami function for α2 and α processes to the εder data (dashed lines indicating individual contributions).

Figure 4. Temperature dependence of (a, b) ωmax and (c) Δε of the α2 (open symbols) and α (filled symbols) processes: the inset shows a magnified view of Δεα. (d) Compositional variation in Tα (open symbols, left axis) and Δεα (filled symbols, right axis) at 298 K (σDC at 298 K vs 100Δεα/Tα in the inset).

moment, than either Li+TFSI− contact ion pairs or separated ion pairs, of TFSI− with a SN solvated Li+. As Al2O3 content increases, Δεα2 decreases due to dilution of LiTFSI ions,

and Tα, listed in Table 2), which decrease with increasing Al2O3 content. In the neat SPE and Al2O3-containing NSPEs, the relaxation strength for the α2 process (Δεα2 , open symbols in Figure 4c) is consistently higher than that for the α process (Δεα, filled symbols in Figure 4c). The increase in Δε arises from the enhanced dipole moment imparted by ionic hopping rearrangement of the ionic groups composed of Li+ cations and TFSI− anions,43 which has much larger anion−cation equilibrium separation distance, providing a larger dipole

whereas Δεα increases with Al2O3 content. However, the logical trend was not maintained at the highest Al2O3 content (20%_Al2O3). For a further understanding of ion-pair states in the system, an average value of effective pair dipole moment was estimated by utilizing the Onsager relationship46,47 D

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ab initio DFT calculations. The configurations had been optimized for the 0 K binding energies in vacuum for the ion pairs as shown in Figure 5.

Table 2. Fitting Parameters of the VFT (Eq 4), Onsager (Eq 5), and BNN (Eq 6) Equations VFT fit

Onsager fit −3

BNN fit

sample

Tα2 (K)

Tα (K)

ν (nm )

m (D)

B

0%_Al2O3 5%_Al2O3 9%_Al2O3 17%_Al2O3 20%_Al2O3

216 204 199 196 196

206 200 195 192 193

1.39 1.32 1.25 1.12 1.05

5.5 6.2 7.3 7.9 7.6

1.7 3.2 2.2 1.6 2.1

a

Number density of dipoles ν was determined from a group contribution method based on molecular structure.45

a

Δε(2Δε + 3ε∞) (Δε + ε∞)(ε∞ + 2)2

=

νm2 9ε0kT

(5) Figure 5. Simulation results for an ion pair of LiTFSI (a) without and (b) with Al2O3 equilibrated at 0 K in vacuum, using ab initio DFT calculations [Li (green), N (light red), S (yellow), O (red), F (light gray), C (wine), and Al (gray)].

wherein ν is the number density of dipoles, m is their dipole moment, k is the Boltzmann constant, and ε∞ is the highfrequency limit of the dielectric constant (here taken to as ε∞ = n2, where n = 1.38 is the refractive index of LiTFSI). The dashed and solid lines in Figure 4c are the fits to eq 5 with the νm2 term as the sole fitting parameter, displaying that Δεαs of these materials are well described by the Onsager equation (Δεα ∼ 1/T, see the inset of Figure 4c). However, Δεα2 does not perfectly follow Δεα2 ∼ 1/T for the Al2O3-containing NSPEs, but for the Al 2 O 3 -free SPE (0%_Al2 O 3 ), its Δεα2 ∼ 1/T holds at all temperatures studied. This is not fully understood yet, probably indicating that the NSPEs have an additional source of polarization, like Maxwell−Wagner− Sillars interfacial polarization between Al2O3 nanowires and Li salts, which cannot be predicted by the Onsager equation (Figures S12 and S13). Using Δεα and ν, estimated from group contributions45 and listed in Table 2, the average dipole moments m from eq 5 are then obtained (see Table 2), and upon addition of up to 17 wt % Al2O3, m increases from 5.5 D (0%_Al2O3) to 7.9 D (17%_Al2O3). This presumably suggests that the stable complexes between Al2O3 and LiTFSI due to the Lewis acid/base interaction25,27 would increase an average distance between Li+ and TFSI− of ion pairs. The larger distance ultimately leads to an enhanced dipole moment of the average ion pair compared to the case of contact ion pairs (TFSI−Li+).16 In the simplest case, the Coulomb energy of ionic interaction is Ea = e2/4πε0εsr, telling us that the activation energy Ea of ionic dissociation is inversely proportional to the dielectric constant εs and ion distance r governing the ionic association.42,48 Therefore, we would expect that the increase in the ion distance upon the addition of Al2O3 would facilitate ion dissociation by lowering Ea. The Al2O3 composition dependence of Tα (open symbols) and Δεα at 298 K (filled symbols) is explicitly displayed in Figure 4d, where adding Al2O3 to the epoxy-based SPE not only lowers Tα (fast segmental motion) but also boosts Δεα (promotion of Li+TFSI− dissociation), up to 17 wt % Al2O3. The inset of Figure 4d also shows that σDC for these NSPEs are correlated to the product of Δεα and Tα−1, demonstrating that the enhanced conductivity is owing to a combination of fast segmental motion, resulting in increased ion mobility, and large α relaxation strength, lowering ion dissociation energy and providing more mobile ions for conduction. 3.3. Ab Initio DFT Calculations. To more quantitatively investigate the Li+ cation−TFSI− anion interaction with and without Al2O3, the LiTFSI binding energies were estimated by

The LiTFSI binding energy, listed in Table 3, is 5.4 eV and is similar to what was found by Johansson et al.49,50 and Borodin et al.51 Incorporating Al2O3 into LiTFSI leads to a significant decrease in the binding energy (1.7 eV) due to the structural rearrangement around lithium cation. As a result, the interatomic distance dLi−N and dLi−S between Li and either the nitrogen or sulfur of the anion increases (see Table 3), while the intraatomic distance dS−N between S and N of TFSI decreases from 1.63 to 1.61 Å, consistent with the FTIR observation (see Figure S14). The optimized structures at dLi−N = 1.95 and 2.59 Å result in a dipole moment of the contact and separated pair of m = 9.4 and 12.4 D for LiTFSI without and with Al2O3, respectively, showing that the DFT results are larger than the effective average dipole moment from Δε using eq 5 (Table 2). This is presumably because all these ions are not in the contact ion pair state but in the associated pairs such as quadrupoles or larger aggregates, which do not contribute to Δε. Upon the Al2O3 addition, the 30% increase in the dipole moments from both experiment and prediction is consistent with the observation in the enhanced Δεα and σDC. 3.4. Correlation between Conduction and Dielectric Relaxation. The role of Al2O3 on σDC can be further investigated using the Barton, Nakajima, and Namikawa (BNN) relation.53−55 This empirical scaling correlation suggests that σDC is proportional to the product of Δε and ωmax. The connection between the σDC and the α2 relaxation has been observed for many ionomers,16,43,56−59 and herein we demonstrate that for these electrolytes their σDCs are correlated to the α relaxation: σDC = Bε0Δεαωα

(6)

wherein B is a dimensionless empirical constant, showing B = 1.7 for the Al2O3 free SPE and 1.6 ≤ B ≤ 3.2 for the NSPE (listed in Table 2), and ε0 is the vacuum permittivity. Figure 6 shows the strong correlation between σDC/ε0 and Δεαωα, indicating that the ion diffusion is controlled by the segmental motion over wide ranges of temperature. With the B and Δεα, the reduced ionic conductivity [σDC/(BΔεα)] does quite condense all data to a common curve plotted against Tα/ T (see the inset of Figure 6). This further accounts for the enhanced σDC because of Al2O3 plasticization (allowing for fast E

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Table 3. Ab Initio DFT Calculations of Binding Energy, Interatomic and Intraatomic Spacing, and Dipole Moment at 0 K in a Vacuum for LiTFSI without and with Al2O3 interatomic spacing

intraatomic spacing

sample

binding energy (eV)

dLi−N (Å)

dLi−S (Å)

dS−N (Å)

dipole moment from DFT (D)

LiTFSI LiTFSI + Al2O3

5.4 1.7

1.95 2.59

2.86/2.95 3.07/3.26

1.61/1.63 1.61/1.61

9.4 12.4

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parameter of EP power law exponent and shape parameters, R-square values of the derivative spectra fits, frequency dependence of dielectric permittivity, derivative spectra at 263 K, FTIR spectra and stress− strain curves (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Figure 6. BNN relation of σDC/ε0 vs Δεαωα for the SPE and NSPEs, with the lines indicating the BNN relation with 1.6 ≤ B ≤ 3.2 (solid lines) and with B = 1 (dashed line). The inset shows σDC divided by the product of B and Δεα with respect to inverse temperature normalized by Tα.

Taehoon Kim: 0000-0003-1045-616X U Hyeok Choi: 0000-0002-0048-9550 Notes

The authors declare no competing financial interest.

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segmental motion) as well as the Al2O3−ion complex (promoting dissociation of ions).

ACKNOWLEDGMENTS This study was financially supported by the Fundamental Research Program (PNK5830) of the Korean Institute of Materials Science (KIMS) and by the Korea Institute of Energy Technology Evaluation and Planning (KETEP), and the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (Grant 20174010201460).

4. CONCLUSIONS The ion conduction and dielectric response of epoxy-based NSPEs containing a weak-binding LiTFSI and a solid plasticizer SN, complexed with Al2O3 are reported from dielectric relaxation spectroscopy and ab initio DFT calculations. The Al2O3-containing NSPEs showing fast segmental dynamics (lower Tα) have higher σDC than the Al2O3-free SPE showing slow segmental dynamics (higher Tα). Moreover, the Al2O3 enhances Δεα as well as its effective dipole moment, obtained from the Onsager equation: i.e., adding Al2O3 allows for the formation of stable complexes between Al2O3 and LiTFSI, leading to an increase in an average distance between Li+ and TFSI− of ion pairs and favoring ion dissociation, suggesting a lower energy barrier for ion conduction, consistent with our ab initio DFT calculations. Therefore, the combination of plasticization and solvation plays a vital role in an enhancement in the conductivity of the NSPE containing Al2O3 having σDC ∼ 1.5 × 10−5 S cm−1 and Young’s modulus ∼290 MPa shown in Figure S15.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01603. Energy-dispersive X-ray spectroscopy (EDS) elemental mapping, DC conductivity as a function of time, along with temperature, TGA thermogram, complex dielectric permittivity and conductivity, dielectric response of an epoxy-based NSPE containing 17 wt % Al2O3 nanowires, R-square values of the VFT fits, dielectric derivative spectra of the two components mixture (LiTFSI and SN) and three-component mixture (LiTFSI, SN, and Al2O3), derivative spectra and dielectric loss, fitting F

DOI: 10.1021/acs.macromol.8b01603 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b01603 Macromolecules XXXX, XXX, XXX−XXX