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Investigation of the free volume characteristics of PEO based solid state polymer electrolyte by means of positron annihilation spectroscopy
Solid State Ionics 339 (2019) 114990
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Investigation of the free volume characteristics of PEO based solid state polymer electrolyte by means of positron annihilation spectroscopy ⁎
Pranav Utpallaa,b, S.K. Sharmaa,b, , K. Sudarshana,b, M. Sahuc, P.K. Pujaria,b,
T
⁎
a
Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India Homi Bhabha National Institute, Anushaktinagar, Mumbai 400094, India c Radioanalytical Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Poly (ethylene oxide) Solid state electrolyte Positron annihilation spectroscopy Doppler broadening spectroscopy Free volume
Ion conduction in poly (ethylene oxide), PEO, based electrolytes are explained considering the ion movement coupled with polymer segmental motions or free volume. Herein, we describe the application of positron annihilation spectroscopy to investigate the free volume structure of PEO-LiTFSI solid state electrolyte with varying electrolyte concentration. Addition of LiTFSI indicates plasticization effect through the increase in free volume size of polymer matrix. Depth dependent Doppler broadening measurements have shown a decrease in positron/ positronium diffusion length and S-parameter value on addition of electrolyte. The observed decrease in Sparameter and positronium intensity could not be ascribed to positronium reaction (inhibition) with dissociated ions or salt aggregates as the variation does not follow the established inhibition expression. The SeW correlation curves indicate that the chemical environment at positron/positronium annihilation sites in PEO-LiTFSI is similar to pristine PEO. It again confirms that reduction in positronium intensity and S-parameter cannot be attributed to positronium inhibition. The present study shows that resultant positron/positronium annihilation parameters can be directly attributed to the change in molecular packing or free volume structure of the solid state electrolytes.
1. Introduction Poly (ethylene oxide), PEO, based solid state polymer electrolytes prepared with Li salts such as LiClO4, LiPF6 and LiTFSI etc. show very high ionic conductivity [1–5]. The higher ionic conductivity of PEO based electrolytes is attributed to the formation of ether oxygen-Li complex which facilitates the dissociation of Li salts [6,7]. The ion conduction mechanism in these electrolytes has been investigated in detail using various techniques [8–10]. It has been shown that ion movement in high molecular weight PEO matrix occurs through its amorphous phase [3,8–11]. Studies have been focused on enhancement of amorphous phase of PEO by addition of passive and active fillers in PEO matrix [3,12]. Broadband dielectric spectroscopy has been primarily used to investigate the segmental dynamics of PEO based electrolytes [3,9]. These studies along with atomistic simulations have shown that ion movement through PEO amorphous phase is coupled with segmental dynamics [8,9]. The segmental motions of PEO chains lead to a free volume distribution in the matrix. A free volume model for the ions movement in polymers has been proposed. According to this model, a critical free volume is required for the movement of ions
⁎
in a polymer matrix [13,14]. Many studies have been performed to correlate the free volume of PEO based electrolytes with its conductivity [13–16]. Haldar et al. [16] have studied temperature dependent free volume and ionic conductivity of PEO-NH4ClO4. The study showed that both free volume and ionic conductivity increase drastically after glass transition temperature. Bamford et al. [13] have investigated the role of free volume on ionic conductivity of PEO-LiClO4 electrolytes. Again, it has been shown that free volume and ionic conductivity increase with the increase in temperature having a steep rise after glass transition temperature. The study showed that a critical free volume of ~ 1nm3 is required for the elementary jump of an ion in these electrolytes and this critical volume is independent of type of ions [13]. In these studies, positron annihilation lifetime spectroscopy (PALS) has been used to investigate the free volume of solid state polymer electrolytes [13–16]. PALS involves the measurement of positron lifetime in materials. In case of polymer based materials, a number of positrons annihilate through positronium (Ps) formation. Ps is a bound state of positron and electron. It exists in two energy states viz. para-Positronium (p-Ps, intrinsic lifetime 0.125 ns) and ortho-Positronium (o-Ps, intrinsic lifetime
Corresponding authors at: Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India. E-mail addresses: [email protected] (S.K. Sharma), [email protected] (P.K. Pujari).
https://doi.org/10.1016/j.ssi.2019.05.025 Received 1 April 2019; Received in revised form 16 May 2019; Accepted 30 May 2019 0167-2738/ © 2019 Elsevier B.V. All rights reserved.
Solid State Ionics 339 (2019) 114990
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2. Experimental
142 ns). In polymeric materials, o-Ps is trapped in free volume holes present in the matrix as a result of segmental motion. The lifetime of localized Ps is reduced from 142 ns depending on the size of free volume holes. The process is called pick-off annihilation wherein positron from a trapped o-Ps annihilates with the opposite spin electron from the surrounding via two photons mode. The o-Ps pick-off lifetime (τpo, ns) is correlated with the free volume hole's radius (rh, nm) through well established Tao-Eldrup equation [17,18].
2.1. Preparation of PEO-LiTFSI films and characterization Commercially available Poly (ethylene oxide) (PEO, molecular weight = 300,000 Da), LiTFSI (purity > 99.9%) and acetonitrile (purity > 99.0%) have been used for the preparation of solid state electrolyte films. PEO and LiTFSI were dried in vacuum oven at 50 °C before their use. 3 g of PEO was dissolved in acetonitrile at room temperature through magnetic stirring and ultrasonication. Calculated amount of LiTFSI (0, 0.5, 1.0, 2.0, 5.0, 10.0 wt% w.r.t. composite) was added to PEO solution under magnetic stirring that was continued for more than six hours. The PEO-LiTFSI solution was cast on glass petri dish and left for three days for evaporation of the solvent. The films were dried under vacuum (~10−3 mbar) at 50 °C for 3 h. The films could be taken off easily from the Petri-dish. The films were stored in a desiccator and taken out before their characterization. These films are referred as PEO-xLi, where x represent the loading of LiTFSI, for example PEO-10Li represents a PEO film loaded with 10 wt% LiTFSI salt. XRD measurements were carried out using bench top Proto AXRD system with 2θ varying from 100 to 700 (Cu Kα wavelength = 0.154 nm). Differential scanning calorimetry (DSC; model number DSC 823e/700 of M/s. Mettler Toledo GmbH, Switzerland) measurements were carried out in nitrogen atmosphere at heating rate of 5 K/min. FTIR spectra were recorded by Bruker alpha platinum ATRFTIR spectrometer in the frequency range of 500–4000 cm−1.
−1
τpo =
1⎡ rh 1 2πrh ⎞ ⎤ + 1− sin ⎛ 2⎢ rh + δr 2π ⎝ rh + δr ⎠ ⎥ ⎦ ⎣ ⎜
⎟
(1)
where (δr = 0.166 nm) is an empirical parameter [19]. The o-Ps pick-off intensity is correlated to number density of free volume holes in polymer materials [20,21]. In the presence of species reactive with Ps, o-Ps lifetime may be reduced due to spin-conversion and chemical quenching effects. In case of quenching, the experimentally measured oPs lifetime (τ3) is given by Eq. (2) where k (mol−1 s−1) and c (moles) are quenching constant and concentration of the species, respectively [22].
1 1 = + kc τ3 τpo
(2)
The o-Ps intensity (Ipo) which is related to number density of free volume holes is also reduced through inhibition process. In the presence of electron or positron scavenger groups, the experimental o-Ps intensity (I3) is given through Eq. (3) where k′ (mol−1) and c (moles) are the inhibition constant and concentration of the inhibitor [22].
I3 =
2.2. Positron annihilation spectroscopy PALS measurements have been carried out using a fast-fast coincidence spectrometer (time resolution = 0.280 ns) coupled to two plastic scintillation detectors placed opposite to each other. In order to measure PALS spectra (total counts > 106), a positron source (22Na, activity ~ 10 microcurie) sealed in a kapton envelope was sandwiched between two stacks (thickness > 1.5 mm) of polymer electrolyte films. The fraction of positrons annihilating in source material and kapton were corrected by acquiring a PALS spectrum of a reference material (silicon single crystal). PALS spectra have been analyzed using PALSfit to extract the positron/positronium lifetime components and corresponding intensities in different samples [23]. Depth dependent Doppler broadening measurements have been carried out using an HPGe detector (energy resolution = 2.0 keV at 1332 keV of 60Co) coupled with a slow positron beam. The details of slow positron beam can be obtained from ref. [24]. Positrons emitted from a radioactive source are moderated using a tungsten moderator and transported using magnetic field. The implantation energy of positrons at the sample is increased by floating the sample at a requisite voltage. Doppler broadening spectra (total counts > 2 × 105) are acquired at different positron implantation energies. The Doppler broadened annihilation peak is analyzed through line shape (S- and W-) parameters. S-parameter is calculated as fractional area under the central region (511 ± 0.77 keV) of the photo peak whereas the Wparameter is calculated as the fractional area under the wing region (2.30 keV ≤ │Eγ-511 keV│ ≤ 5.76 keV) of the photo peak, where Eγ is the energy of annihilation photon.
Ipo (1 + k′c)
(3)
As mentioned before, the ionic conductivity of polymer solid state electrolyte is correlated to free volume distribution of polymer matrix. In the studies focused on free volume correlations with ionic conductivity [13–16], free volume size determined from o-Ps lifetime has been primarily considered assuming that no quenching occurs in the presence of Li salts and dissociated ions. The variation observed in the o-Ps intensity has not been correlated with the number density of free volumes speculating the inhibition of Ps formation. However, in this regard, no experimental evidence has been provided in these studies [13–16]. In order to study ionic conductivity mechanism, PEO based electrolyte films of thickness ~ 100–200 μm are prepared using solvent casting method. It has been shown in literature that unsaturated forces and interfacial interaction with casting surface lead to depth dependent variation in segmental relaxation or free volume distribution in polymer films. In such a case, ion movements which are coupled to segmental relaxation will also have deterministic effect from the depth dependent variation in free volume of these films [9]. In the present study, we have prepared PEO based solid electrolyte films loaded with varying content of LiTFSI salt using solvent casting method. The polymer salt complexation and amorphous-crystalline structure of PEO based films have been investigated using X-ray diffraction, Differential Scanning Calorimetry and Fourier Transform Infrared (FTIR) spectroscopy. PALS measurements have been carried out to measure the o-Ps lifetime and intensity as a function of LiTFSI loading. Depth dependent Doppler broadening spectroscopy has been carried out using a slow positron beam. The study shows a uniform depth dependent free volume distribution from both the surfaces. The study through SeW correlation curves shows identical chemical environment at positron/positronium annihilation sites. It confirms that the variation in o-Ps lifetime and intensity represent the change in free volume size and number density as a result of polymer-salt complexation.
3. Results and discussion Fig. 1 shows the XRD patterns of pure PEO and PEO-xLi electrolyte samples. The diffraction peaks at 2θ ~ 19.0 and 23.2 degrees shown in case of pure PEO are consistent with literature. These diffraction peaks are attributed to the inter-chain and intra-chain fold distance of 4.67 and 3.83 Ǻ, respectively [25]. On loading of LiTFSI, formation of any new crystalline phase is ruled out by absence of any new diffraction peaks. The peak positions are not shifted as a result of loading of LiTFSI 2
Solid State Ionics 339 (2019) 114990
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annihilation rate) in their increasing order is attributed to p-Ps, free positron and o-Ps annihilation in the polymer matrix. The corresponding intensity is attributed to the fraction of positrons annihilating through these three states. Fig. 4 shows the o-Ps lifetime and corresponding intensity as a function of electrolyte (LiTFSI) concentration. With the addition of salt, an increase in o-Ps lifetime and a decrease in o-Ps intensity are observed. These observations are consistent with the variations observed in o-Ps annihilation parameters in PEO with addition of Li salts [13,16]. According to Eq. (2), o-Ps lifetime decreases as a result of quenching. The observed increase in o-Ps lifetime as a function of electrolyte concentration up to 10% indicate that there is no quenching of Ps in PEO matrix in the presence of Li salts. Fig. 4 shows a decrease in o-Ps intensity which has been attributed to inhibition of Ps formation in PEO based polymer matrix loaded with Li based salt without any experimental evidence [13]. Hirata et al. [32] have studied the inhibition of positronium in three different polymers by doping 2,2′Dinitrobiphenyl (DNB). It has been shown that the o-Ps intensity decreases with the loading of DNB in all the three polymers. The inhibition constants k′ (wt%−1) for these polymers were observed in the range of 0.68 to 1.2. Mohammed et al. [33] and Xia et al. [34] have studied effect of CuCl2 and PdCl2 doping on intensity of positronium in poly (vinyl alcohol) and chitosan, respectively. The observed decrease in o-Ps intensity in these studies was observed to follow a trend given by eq. 3. These studies have shown that the value of inhibition constant (~0.14 wt%−1 and 0.22 wt%−1 for CuCl2-PVA and PdCl2-chitosan, respectively) in polymer matrix is lower than the aqueous solution of these salts. The calculated values of o-Ps intensity using Eq. (3) with average value of inhibition constant (k’ = 0.18 wt%−1) from refs. [33,34] is also shown in Fig. 4 (triangle symbols). The observed trend in experimental o-Ps intensity does not strictly follow the curve represented by Eq. (3) which indicates that the observed decrease in o-Ps intensity cannot be strictly attributed to inhibition process. In such a case, the observed variation in o-Ps intensity can be attributed to variation in free volume hole density in PEO matrix as a result of PEO-salt complexation. However, to the best of our knowledge, no experimental evidence is available in literature supporting the fact that o-Ps intensity variation in PEO based electrolytes can be attributed to the variation in local structure at molecular level of the polymer matrix. The local structure at the molecular level of the polymer matrix determines the positron/positronium diffusion in the polymer matrix. In order to investigate the depth dependent free volume structure of PEO based electrolytes, Doppler broadening measurements have been carried out by implanting monoenergetic positrons using a slow positron beam. The Doppler broadening spectroscopy of annihilation radiation involves the measurement of broadening of annihilation peak using high purity germanium (HPGe) detectors. As mentioned before, positron either as free positron or through the formation of Ps in polymeric materials undergoes two-gamma annihilation. The energy of annihilation photons is shifted by ΔE = ± cp/2 as a result of momentum conservation during the annihilation event, where c and p are the velocity of light and momentum of the annihilating electron in the direction of annihilation photons, respectively. In case of positron annihilation in a material, these shifts result in the broadening of annihilation peak. In case of polymers, the central region of the annihilation peak is primarily populated by the annihilation of p-Ps and free positrons with low momentum electrons (valence electrons). Any variation in Ps formation and chemical environment at annihilation sites lead to variation in area under low momentum (central region) and high momentum (wing region) of annihilation peak. The fractional area under the central and wing region is defined as S- and W- parameters, respectively. S- and Wparameters have been calculated from depth dependent Doppler broadening spectra as described in the experimental section [34,35]. Fig. 5 shows the S-E profiles of top and bottom surfaces of pure PEO and PEO-10Li samples. The top axis shows the positron implantation depth (< z > , nm) in polymer films calculated using < z > = 40E1.6/ ρ, where ρ is the density of polymeric films. It is observed from the
θ
Fig. 1. XRD patterns of pure PEO and PEO-xLi samples.
salt in polymer matrix. It shows that the crystalline structure of PEO matrix remains intact even after 10 wt% of LiTFSI. However, the intensity of the diffraction peaks is observed to decrease with the loading of LiTFSI which indicates a decrease in the crystallinity of the samples. It has been reported in literature that addition of Li based salts act as plasticizer for PEO leading to a decrease in its crystallinity [26]. In order to determine the crystallinity of the PEO based electrolytes, DSC measurements have been carried out. Fig. 2 shows the DSC thermograms of the heating cycle of all the samples. An endothermic peak at ~65 °C is observed showing the melting of crystalline PEO in the samples. The crystallinity of the samples have been evaluated from the ratio of enthalpy of melting of PEO in the sample to enthalpy of 100% crystalline PEO (214.6 Jg−1) [27]. The evaluated crystallinity is given in Table 1. Addition of LiTFSI leads to plasticization of PEO, however its effect is drastic at higher loadings. FTIR spectroscopy is used to investigate the complexation or interaction of Li salt with polymer molecules. Fig. 3 shows the FTIR spectra of pure PEO, LiTFSI salt as well as PEO-xLi electrolyte samples. The bands at 850, 1104, 1460 and 2900 cm−1 are attributed to CH2 rocking, asymmetric COC stretching, asymmetric CH2 bending, and symmetric and asymmetric CeH stretching modes of PEO [28,29]. Multiple peaks in the range of 1050 to 1350 cm−1 are assigned to TFSI anion. In the case of LiTFSI doped polymer samples, new band at ~1190 cm−1 appear indicating the dissociation of LiTFSI in PEO [30]. However, small changes are observed in the intensity of the bands indicating the variations in the local environment at microscopic level [9,31]. The modification of the local environment at the molecular level has been investigated using positron annihilation spectroscopy. PALS spectra of all the samples were analyzed using PALSfit into three decaying exponentials [23]. The positron lifetime (inverse of
Fig. 2. DSC thermograms (heating cycle) of pure PEO and PEO-xLi samples. 3
Solid State Ionics 339 (2019) 114990
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Table 1 The value of fitted parameters (Sbulk, Wbulk and Diffusion length) using VEPFIT and the crystallinity determined from DSC measurements. Sample
Sbulk
PEO-0Li PEO-0.5Li PEO-1Li PEO-2Li PEO-5Li PEO-10Li
0.5555 0.5526 0.5497 0.5510 0.5449 0.5432
Wbulk ± ± ± ± ± ±
0.0003 0.0003 0.0003 0.0003 0.0003 0.0003
0.0309 0.0317 0.0326 0.0318 0.0335 0.0341
± ± ± ± ± ±
0.0005 0.0005 0.0005 0.0005 0.0005 0.0005
Diffusion length (nm)
Crystallinity (%)
66.3 ± 14.5 13.6 ± 1.6 12.5 ± 2.6 12.7 ± 1.7 6.5 ± 0.6 8.8 ± 1.9
58.4 56.6 62.8 48.2 41.5 41.1
± ± ± ± ± ±
0.2 0.2 0.2 0.3 0.1 0.1
μ
Fig. 3. FTIR spectra of PEO, LiTFSI and PEO-xLi samples. The marked arrows show the IR bands corresponding to dissociated ions and Li salt aggregates in PEO matrix.
Fig. 5. S-E profiles of top and bottom surfaces of PEO-0Li and PEO-10Li electrolyte samples. The top axis shows the positron implantation depth (μm).
attributed to formation of higher number of positronium at higher depth as well as reduction in Ps diffusion to vacuum where it annihilates through three gamma annihilation mode. W-E profiles are complementary to S-E profiles due to definition of these parameters and are shown in Fig. S1 (Supplementary information, SI). The positron implantation profiles follow Makhovian profile and become broader with the increase in positron energy [36,37]. The experimental S-E profiles are convoluted with the positron implantation profile in the matrix. In order to extract positron/positronium diffusion length in the polymer matrix as well as characteristic S-parameter corresponding to different region of the matrix, S-E or W-E profiles are fitted using variable energy positron fit (VEPFIT) [38]. The computer program VEPFIT takes into account the implantation of positrons, their diffusion and trapping at low density regions before annihilation in the material. According to VEPFIT, a polymeric sample can be considered as multilayered having different molecular packing at different layers. In such a case, the experimental S-parameter at a particular implantation energy E, S(E), can be written as weighted average of S-parameters (Eq. (4)) corresponding to different layers as well as surface of the sample. The fitting parameters fs and fi are the fractions of positrons annihilating at surface and in the ith layer, whereas Ss and Si are the Sparameter corresponding to surface and ith layer.
Fig. 4. o-Ps annihilation parameters (lifetime and corresponding intensities) in PEO-xLi electrolytes. The open triangle symbols represent the calculated o-Ps intensity considering inhibition process with inhibition constant (k′ = 0.18 wt %−1) [33,34].
figure that S-E profiles of top and bottom surfaces of pure PEO as well as PEO-10Li samples are identical. These identical S-E profiles indicate similar free volume structure at sub-surface regions. It also shows that interfacial interaction at polymer-glass surface in pure PEO as well as in PEO-xLi samples do not lead to any modification of molecular packing at the bottom surface compared to the top surface of the films. Fig. 6 (a–b) shows the S-E profiles of top surface of pure PEO and PEO-xLi samples. The S-parameter increases continuously with the increase in positron implantation depth and reaches a constant value which corresponds to the free volume structure of polymer matrix in the bulk. Such type of S-E profiles is typical for polymeric materials [36,37]. The increase in S-parameter with the increase in positron energy is
S (E) = fs Ss +
∑ fi Si i
(4)
The solid lines through the data points in Fig. 6 (a–b) shows the fitting of S-E profiles using VEPFIT. The extracted parameters from the fitting are shown in Table 1. The value of positron/Ps diffusion length in pure PEO is 66.3 ± 14.5 nm. The values of diffusion lengths reported in amorphous polymers are in the range of 10–20 nm which is shorter compared to semiconductors and metals [36,37,39]. The higher 4
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μ
the increase in loading of Li salt. This increase in relaxation time of the segmental motions has been attributed to the rigidity of PEO lattice which is increased by intra- and inter-chain cross-linking by Li ions [41]. Similar results have been reported about the segmental relaxation dynamics using broadband dielectric spectroscopy [3,9]. The reduction in segmental dynamics or larger relaxation times is also observed through the increase in o-Ps lifetime as shown in Fig. 4. The positronium diffusion in the polymer matrix occurs through free volume holes which are redistributed in the polymer matrix with time due to segmental relaxation of polymer chains [13]. In the case of a rigid framework, the free volume distribution with time slows down and the probability of diffusion of Ps among free volume hole is reduced. As a result, a decrease in diffusion length of positron/Ps is observed on loading of LiTFSI in PEO. However, the effect of presence of dissociated ions and salt aggregates on Ps diffusion cannot be ignored. It is interesting to note from Table 1 that Ps diffusion length does not decrease drastically with loading of Li salt from 0.5 to 10 wt%. It indicates that reduction in positron/Ps diffusion length in PEO based electrolytes is not associated with positron/Ps complexation with Li salt aggregates and dissociated ions. This fact is also supported from SeW correlation curves as discussed later. Fig. 7 shows the S-parameter corresponding to bulk of the matrix evaluated by fitting of S-E profiles through VEPFIT. It is observed from the figure that S-parameter decreases sharply with the loading of Li salt. However, the reduction in S-parameter is drastically high in case of 5 and 10 wt% samples. The S-parameter is defined as fractional area in the low momentum region of annihilation peak wherein annihilation of p-Ps and free positron with valence electron predominantly contribute. In such a case, the value of S-parameter depends on Ps intensity as well as p-Ps zero point energy which is related to free volume hole size (o-Ps lifetime). The variation in Ps intensity and lifetime are counter intuitive as shown in Fig. 4. The reduction in S-parameter is highly consistent with the reduction in Ps intensity as shown in Fig. 4. It shows that with the increase in Li salt loading, number density of free volume decreases leading to a decrease in S-parameter as well as o-Ps intensity. In order to support this fact, SeW correlation curves as a function of positron implantation depths or positron implantation energy are plotted in Fig. S2 (SI). Recently, Madzarevic et al. [43] have investigated the chemical environment at the annihilation sites (free volumes) in poly (ether imide) films using depth dependent Doppler broadening measurements. It has been shown through this study that SeW data points are grouped according to the chemical composition of the films which modifies the electronic environment at the annihilation sites (free volumes). Fig. S2 shows that the data cluster corresponding to bulk of pure PEO and PEO-Li are grouped at one location indicating that the chemical environment at free volumes are uniform throughout the investigated depth of these samples.
μ
Fig. 6. (a–b): S-E profiles of PEO-xLi samples (x = 0, 0.5, 1.0, 2.0, 5.0 and 10.0). The top axis shows the positron implantation depth in the samples. The solid lines through the data points show the fit of data using VEPFIT.
value of diffusion length observed in case of pure PEO can be attributed to its high crystallinity and contribution from diffusion of free positron. The value of positron/Ps diffusion length decreases drastically on loading of Li salt, however the values are within the range of reported diffusion lengths in various pure polymers [36,37,39]. It shows that with the loading of Li salt, positron/positronium diffusion through the matrix is reduced drastically. This reduction of positron/Ps diffusion length in the polymer matrix can be attributed to two factors viz. (i) modification in free volume structure or segmental relaxation of polymer matrix, and (ii) Ps quenching or inhibition due to presence of Li salt aggregates and dissociated ions in the polymer matrix. Dielectric broadband spectroscopy and neutron scattering measurements on PEO based solid state electrolytes have been carried out in literature to investigate the effect of Li salt loading on the segmental relaxations of PEO [3,9,10,40–42]. Mao et al. [41] have shown using neutron scattering that α-relaxation which is attributed to the translational motion of the polymer chains, is slowed down drastically with
Fig. 7. Variation in S-parameter corresponding to bulk of PEO-xLi films evaluated from VEPFIT of S-E profiles as a function of LiTFSI loading. 5
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4. Conclusions Positron/Positronium diffusion in PEO-LiTFSI has been investigated using depth dependent Doppler broadening measurements. The reduction in positron/positronium diffusion length on loading of Li salt has been attributed to the increased rigidity of PEO lattice. The SeW correlation curves confirmed the similar chemical environment at annihilation sites (free volumes) in pure PEO and PEO-LiTFSI samples. It indicates that the variation in o-Ps annihilation parameters are not a consequence of Ps reaction (inhibition or quenching) with dissociated ions and salt aggregates present in the samples. The o-Ps lifetime and intensity can be directly correlated to the free volume holes size and number density of free volumes in PEO based solid state electrolytes. The observed variations in relative free volume determined using o-Ps annihilation is consistent with the reported variations in the ionic conductivity.
Fig. 8. SeW correlation plots for PEO-xLi samples.
Acknowledgement The authors thank Dr. B. G. Vats from BARC, Mumbai for his help in FTIR measurements. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ssi.2019.05.025. References [1] A.M. Stephan, K.S. Mahm, Review on composite polymer electrolytes for lithium batteries, Polymer 47 (2006) 5952–5964. [2] Z. Xue, D. He, X. Xie, Poly (ethylene oxide) based electrolytes for lithium ion batteries, J. Mater. Chem. A 3 (2015) 19218–19253. [3] W. Wang, E. Yi, A.J. Fici, R.M. Laine, J. Kieffer, Lithium ions conducting poly (ethylene oxide)-based solid state electrolytes containing active or passive ceramic nanoparticles, J. Phys. Chem. C 121 (2017) 2563–2573. [4] J. Zhang, N. Zho, M. Zhang, Y. Li, P.K. Chu, X. Guo, Z. Di, X. Wang, H. Li, Flexible and ion conducting membrane electrolyte for solid state lithium batteries: dispersion of garnet nanoparticles into insulating poly (ethylene oxide), Nano Energy 28 (2016) 447–454. [5] J. Gurriddappa, W. Madhuri, R.P. Suvarna, K. Priyadasan, Studies on the morphology and conductivity of PEO/LiClO4, Mat. Today: Proceed 3 (2016) 1450–1459. [6] P.V. Wright, Electrical conductivity in ionic complexes of poly (ethylene oxide), British Poly. J. 7 (1975) 319–327. [7] M. Armand, Polymer solid electrolytes-an overview, Sol. Stat. Ion. 9 (1983) 745–754. [8] D.J. Brroks, B.V. Merinov, W.A. Goddard, B. Kozinsky, J. Mailoa, Atomistic description of ionic diffusion in PEO-LiTFSI: effect of temperature, molecular weight and ionic concentration, Macromolecules 51 (2018) 8987–8995. [9] T. Dam, S.S. Jena, D.K. Pradhan, Coupled ion conduction mechanism and dielectric relaxation phenomenon in PEO20–LiCF3SO3-based ion conducting polymer nanocomposite electrolytes, J. Phys. Chem. C 122 (2018) 4133–4143. [10] K.I.S. Mongcopa, M. Tyagi, J.P. Mailoa, G. Samsonidze, B. Kozinsky, S.A. Mullin, D.A. Gribble, H. Watanabe, N.P. Balsara, Relationship between segmental dynamics measured by quasi-elastic neutron scattering and conductivity in polymer electrolytes, ACS Macro Lett. 7 (2018) 504–508. [11] S.B. Aziz, T.J. Woo, M.F.Z. Kadir, H.A. Ahmed, A conceptual review on polymer electrolytes and ion transport models, J. Sci. Adv. Mater. Device 3 (2018) 1–7. [12] H. Papananou, Tuning polymer crystallinity via the appropriate selection of inorganic nanoadditives, Polymer 157 (2018) 111–121. [13] D. Bamford, A. Reiche, G. Dlubek, F. Alloin, J.Y. Sanchez, M.A. Alam, Ionic conductivity, glass transition, and local free volume in poly(ethylene oxide) electrolytes: single and mixed ion conductors, J. Chem. Phys. 118 (2003) 9420. [14] T. Miyamoto, K. Shibayama, Free-volume model for ionic conductivity in polymers, J. Appl. Phys. 44 (1973) 5372. [15] D. Bamford, G. Dlubek, A. Reiche, M.A. Alam, W. Meyer, P. Galvosas, F. Rittig, The local free volume, glass transition, and ionic conductivity in a polymer electrolyte: a positron lifetime study, J. Chem. Phys. 115 (2001) 7260. [16] B. Haldar, R.M. Singru, K.K. Maurya, S. Chandra, Temperature dependence of positron-annihilation lifetime, free volume, conductivity, ionic mobility, and number of charge carriers in a polymer electrolyte polyethylene oxide complexed with NH4ClO4, Phys. Rev. B 54 (1996) 7143. [17] S.J. Tao, Positronium annihilation in molecular substances, J. Chem. Phys. 56 (1972) 5499–5510. [18] M. Eldrup, D. Lightbody, J.N. Sherwood, The temperature dependence of positron lifetimes in solid pivalic acid, Chem. Phys. 63 (1981) 51–58.
Fig. 9. Relative free volume (%) of PEO-xLi samples determined using o-Ps annihilation parameters.
In order to investigate the effect of Li cross-linking on electronic momentum distribution at the annihilation sites, SeW correlation curves using S- and W- parameter values corresponding to bulk of the samples are plotted in Fig. 8. In order to evaluate the W-parameter corresponding to bulk of the samples, W-E profiles, akin to S-E profiles, have been fitted using VEPFIT. The solid line in Fig. S1 passing through the data points shows the fitting of W-E profiles using VEPFIT. The diffusion length values evaluated from the fitting of S-E profiles have been used as a fixed parameter to carry out the fitting of W-E profiles. The evaluated W-parameters corresponding to bulk of the samples are also given in Table 1. It is observed from Fig. 8 that SeW data points corresponding to bulk of the samples follow a straight line. It shows that the chemical environment at the free volume holes in all the samples is similar to pure PEO and the free volume intensity measured in these electrolytes can be directly related to number density of free volume holes. On this basis, the relative free volume (fv, %) of PEO-LiTFSI electrolytes can be calculated using following expression,
fv =
4 3 πrh I3 3
(5)
where, rh (nm) is the radius of free volume holes assumed to be spherical [20,21]. Fig. 9 shows the relative free volume of the PEO-xLi electrolytes calculated using Eq. (5). The figure shows that the relative free volume increases linearly up to 5 wt% loading of Li salt. At higher loading (10 wt%), a drastic increase in free volume is observed. This observation is consistent with the variation observed in ionic conductivity of PEO-LiTFSI based electrolytes wherein a drastic increase is observed in ionic conductivity at loadings > 10 wt% [44,45]. 6
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[32] K. Hirata, Y. Kobayashi, Y. Ujihira, Diffusion coefficient of Ps in amorphous polymers, J. Chem. Soc. Faraday Transactions 92 (1996) 985–998. [33] H.F.M. Mohamed, Y. Ito, A.M.A. El-Sayed, E.E. Abdel-Hady, Positron annihilation in poly (vinylalcohol) doped with CuCl2, Polymer 37 (1996) 1529–1533. [34] R. Xia, X. Cao, M. Gao, P. Zheng, M. Zeng, B. Wang, L. Wei, Probing sub-nanolevel molecular packing and correlated positron annihilation characteristics of ionic cross-linked chitosan membranes using positron annihilation spectroscopy, Phys. Chem. Chem. Phys. 19 (2017) 3616–3626. [35] G. Houdalakis, A.D. Gotsis, H. Schut, S.J. Picken, The free volume in acrylic resin/ laponite nanocomposite coatings, Eur. Polym. J. 47 (2011) 264–272. [36] E.S. Dehghani, S. Aghion, W. Anwand, G. Consolati, R. Ferragut, G. Panzarasa, Investigating the structure of crosslinked polymer brushes (brush-gels) by means of positron annihilation spectroscopy, Eur. Polym. J. 99 (2018) 415–421. [37] H. Cao, R. Zhang, J.P. Yuan, C.M. Huang, Y.C. Jean, R. Suzuki, T. Ohdaira, B. Nielsen, Free-volume hole model for positronium formation in polymers: surface studies, J. Phys. Condens. Matter 10 (1998) 10429–10442. [38] V.A. Veen, H. Schut, J.D. Vries, R.A. Hakvoort, M.R. Ijpma, Analysis of positron profiling data by means of “VEPFIT”, AIP Conf. Proc. 218 (1991) 171–198. [39] M. Debowska, Influence of polymer crystallinity and morphology on positron annihilation characteristics, Acta Phys. Pol. A 110 (2006) 559–568. [40] G.M. Mao, R.F. Perea, W.S. Howells, D.L. Price, M.L. Saboungi, Relaxation in polymer electrolytes on the nanosecond timescale, Nature 405 (2000) 163–165. [41] G. Mao, M.L. Saboungi, D.L. Price, α-Relaxation in PEO-LiTFSI polymer electrolyte, Macromolecules 35 (2002) 415–419. [42] G. Mao, M.L. Saboungi, D.L. Price, M.B. Armand, W.S. Howells, Structure of liquid PEO-LiTFSI electrolyte, Phys. Rev. Lett. 84 (2000) 5536. [43] Z.P. Madzarevic, H. Schut, J. Cizek, T.J. Dingemans, Free volume in poly(ether imide) membranes measured by positron annihilation lifetime spectroscopy and doppler broadening of annihilation radiation, Macromolecules 51 (2018) 9925–9932. [44] M.S. Suait, A. Ahmad, H. Hamzah, M.Y. Rahman, Preparation and characterization of PMMA-MG49-LiClO4 solid polymeric electrolyte, J. Phys. D. Appl. Phys. 42 (2009) 055410. [45] S. Klomgkar, J. Punchusak, Effect of addition of LiCF3SO3 salt on conductivity, thermal and mechanical properties of PEO-LiCF3SO3 solid polymer electrolyte, Int. J. Chem. Eng. Appl. 6 (2015) 165–168.
[19] P.E. Mallon, Application to polymers, in: Y.C. Jean, P.E. Mallon, D.M. Schrader (Eds.), Principles and Applications of Positron and Positronium Chemistry, World Scientific, London, 2003. [20] Y. Kobayashi, W. Zheng, E.F. Meyer, J.D. McGervey, A.M. Jamelson, R. Simha, Free volume and physical aging of poly(vinyl acetate) studied by positron annihilation, Macromolecules 22 (1989) 2302–2306. [21] Y.Y. Wang, H. Nakanishi, Y.C. Jean, T.C. Sandreczki, Positron annihilation in amine-cured epoxy polymers—pressure dependence, J. Polym. Sci.:Part B: Polym. Phys. 28 (1990) 1431–1441. [22] Duplatre, Positronium chemistry, in: Y.C. Jean, P.E. Mallon, D.M. Schrader (Eds.), Principles and Applications of Positron and Positronium Chemistry, World Scientific, London, 2003. [23] P. Kirkegaard, M. Eldrup, Positronfit: a versatile program for analyzing positron lifetime spectra, Comput. Phys. Commun. 35 (1984) 401–409. [24] P. Maheshwari, P.K. Pujari, S.K. Sharma, K. Sudarshan, D. Dutta, S. Samanta, A. Singh, D.K. Aswal, R.A. Kumar, I. Samajdar, Defect profiling in organic semiconductor multilayers, Org. Electron. 13 (2012) 1409–1419. [25] H. Tadokoro, Y. Chatani, T. Yoshihara, S. Tahara, S. Murahashi, Structural studies on polyethers, [-(CH2)m-O-]n. II. Molecular structure of polyethylene oxide, Macromol. Chem. 73 (1964) 109–127. [26] C.W. Walker, M. Solomon, Improvement of ionic conductivity in plasticized PEO based solid polymer electrolytes, J. Electrochem. Soc. 140 (1993) 3409–3412. [27] B. Laik, L. Legrand, A. Chausse, R. Messina, Ion-ion interactions and lithium stability in a crosslinked PEO containing lithium salts, Electrochim. Acta 44 (1998) 773–780. [28] R.H. Beaumont, B. Clegg, G. Gee, J.B.M. Herbert, D.J. Marks, R.C. Roberts, D. Sims, Heat capacities of propylene oxide and of some polymers of ethylene and propylene oxides, Polymer 7 (1966) 401–417. [29] N. Inoue, M. Xu, S. Petrucci, Temperature dependence of ionic association and of molecular relaxation dynamics of lithium hexafluoroarsenate in 2-methyltetrahydrofuran, J. Phys. Chem. 91 (1987) 4628–4635. [30] S.J. Wen, T.J. Richardson, D.I. Ghantous, K.A. Stribel, P.N. Ross, E.J. Cairns, J. Electroanal. Chem. 408 (1996) 113–118. [31] M.R. Johan, O.H. Shy, S. Ibrahim, S.M.M. Yassin, T.Y. Hui, Effects of Al2O3 filler and EC plasticizer on the ionic conductivity enhancement of solid PEO-LiCF3SO3 solid polymer electrolyte, Solid State Ionics 196 (2011) 41–47.
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Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Investigation of the free volume characteristics of PEO based solid state polymer electrolyte by means of positron annihilation spectroscopy ⁎
Pranav Utpallaa,b, S.K. Sharmaa,b, , K. Sudarshana,b, M. Sahuc, P.K. Pujaria,b,
T
⁎
a
Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India Homi Bhabha National Institute, Anushaktinagar, Mumbai 400094, India c Radioanalytical Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Poly (ethylene oxide) Solid state electrolyte Positron annihilation spectroscopy Doppler broadening spectroscopy Free volume
Ion conduction in poly (ethylene oxide), PEO, based electrolytes are explained considering the ion movement coupled with polymer segmental motions or free volume. Herein, we describe the application of positron annihilation spectroscopy to investigate the free volume structure of PEO-LiTFSI solid state electrolyte with varying electrolyte concentration. Addition of LiTFSI indicates plasticization effect through the increase in free volume size of polymer matrix. Depth dependent Doppler broadening measurements have shown a decrease in positron/ positronium diffusion length and S-parameter value on addition of electrolyte. The observed decrease in Sparameter and positronium intensity could not be ascribed to positronium reaction (inhibition) with dissociated ions or salt aggregates as the variation does not follow the established inhibition expression. The SeW correlation curves indicate that the chemical environment at positron/positronium annihilation sites in PEO-LiTFSI is similar to pristine PEO. It again confirms that reduction in positronium intensity and S-parameter cannot be attributed to positronium inhibition. The present study shows that resultant positron/positronium annihilation parameters can be directly attributed to the change in molecular packing or free volume structure of the solid state electrolytes.
1. Introduction Poly (ethylene oxide), PEO, based solid state polymer electrolytes prepared with Li salts such as LiClO4, LiPF6 and LiTFSI etc. show very high ionic conductivity [1–5]. The higher ionic conductivity of PEO based electrolytes is attributed to the formation of ether oxygen-Li complex which facilitates the dissociation of Li salts [6,7]. The ion conduction mechanism in these electrolytes has been investigated in detail using various techniques [8–10]. It has been shown that ion movement in high molecular weight PEO matrix occurs through its amorphous phase [3,8–11]. Studies have been focused on enhancement of amorphous phase of PEO by addition of passive and active fillers in PEO matrix [3,12]. Broadband dielectric spectroscopy has been primarily used to investigate the segmental dynamics of PEO based electrolytes [3,9]. These studies along with atomistic simulations have shown that ion movement through PEO amorphous phase is coupled with segmental dynamics [8,9]. The segmental motions of PEO chains lead to a free volume distribution in the matrix. A free volume model for the ions movement in polymers has been proposed. According to this model, a critical free volume is required for the movement of ions
⁎
in a polymer matrix [13,14]. Many studies have been performed to correlate the free volume of PEO based electrolytes with its conductivity [13–16]. Haldar et al. [16] have studied temperature dependent free volume and ionic conductivity of PEO-NH4ClO4. The study showed that both free volume and ionic conductivity increase drastically after glass transition temperature. Bamford et al. [13] have investigated the role of free volume on ionic conductivity of PEO-LiClO4 electrolytes. Again, it has been shown that free volume and ionic conductivity increase with the increase in temperature having a steep rise after glass transition temperature. The study showed that a critical free volume of ~ 1nm3 is required for the elementary jump of an ion in these electrolytes and this critical volume is independent of type of ions [13]. In these studies, positron annihilation lifetime spectroscopy (PALS) has been used to investigate the free volume of solid state polymer electrolytes [13–16]. PALS involves the measurement of positron lifetime in materials. In case of polymer based materials, a number of positrons annihilate through positronium (Ps) formation. Ps is a bound state of positron and electron. It exists in two energy states viz. para-Positronium (p-Ps, intrinsic lifetime 0.125 ns) and ortho-Positronium (o-Ps, intrinsic lifetime
Corresponding authors at: Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India. E-mail addresses: [email protected] (S.K. Sharma), [email protected] (P.K. Pujari).
https://doi.org/10.1016/j.ssi.2019.05.025 Received 1 April 2019; Received in revised form 16 May 2019; Accepted 30 May 2019 0167-2738/ © 2019 Elsevier B.V. All rights reserved.
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2. Experimental
142 ns). In polymeric materials, o-Ps is trapped in free volume holes present in the matrix as a result of segmental motion. The lifetime of localized Ps is reduced from 142 ns depending on the size of free volume holes. The process is called pick-off annihilation wherein positron from a trapped o-Ps annihilates with the opposite spin electron from the surrounding via two photons mode. The o-Ps pick-off lifetime (τpo, ns) is correlated with the free volume hole's radius (rh, nm) through well established Tao-Eldrup equation [17,18].
2.1. Preparation of PEO-LiTFSI films and characterization Commercially available Poly (ethylene oxide) (PEO, molecular weight = 300,000 Da), LiTFSI (purity > 99.9%) and acetonitrile (purity > 99.0%) have been used for the preparation of solid state electrolyte films. PEO and LiTFSI were dried in vacuum oven at 50 °C before their use. 3 g of PEO was dissolved in acetonitrile at room temperature through magnetic stirring and ultrasonication. Calculated amount of LiTFSI (0, 0.5, 1.0, 2.0, 5.0, 10.0 wt% w.r.t. composite) was added to PEO solution under magnetic stirring that was continued for more than six hours. The PEO-LiTFSI solution was cast on glass petri dish and left for three days for evaporation of the solvent. The films were dried under vacuum (~10−3 mbar) at 50 °C for 3 h. The films could be taken off easily from the Petri-dish. The films were stored in a desiccator and taken out before their characterization. These films are referred as PEO-xLi, where x represent the loading of LiTFSI, for example PEO-10Li represents a PEO film loaded with 10 wt% LiTFSI salt. XRD measurements were carried out using bench top Proto AXRD system with 2θ varying from 100 to 700 (Cu Kα wavelength = 0.154 nm). Differential scanning calorimetry (DSC; model number DSC 823e/700 of M/s. Mettler Toledo GmbH, Switzerland) measurements were carried out in nitrogen atmosphere at heating rate of 5 K/min. FTIR spectra were recorded by Bruker alpha platinum ATRFTIR spectrometer in the frequency range of 500–4000 cm−1.
−1
τpo =
1⎡ rh 1 2πrh ⎞ ⎤ + 1− sin ⎛ 2⎢ rh + δr 2π ⎝ rh + δr ⎠ ⎥ ⎦ ⎣ ⎜
⎟
(1)
where (δr = 0.166 nm) is an empirical parameter [19]. The o-Ps pick-off intensity is correlated to number density of free volume holes in polymer materials [20,21]. In the presence of species reactive with Ps, o-Ps lifetime may be reduced due to spin-conversion and chemical quenching effects. In case of quenching, the experimentally measured oPs lifetime (τ3) is given by Eq. (2) where k (mol−1 s−1) and c (moles) are quenching constant and concentration of the species, respectively [22].
1 1 = + kc τ3 τpo
(2)
The o-Ps intensity (Ipo) which is related to number density of free volume holes is also reduced through inhibition process. In the presence of electron or positron scavenger groups, the experimental o-Ps intensity (I3) is given through Eq. (3) where k′ (mol−1) and c (moles) are the inhibition constant and concentration of the inhibitor [22].
I3 =
2.2. Positron annihilation spectroscopy PALS measurements have been carried out using a fast-fast coincidence spectrometer (time resolution = 0.280 ns) coupled to two plastic scintillation detectors placed opposite to each other. In order to measure PALS spectra (total counts > 106), a positron source (22Na, activity ~ 10 microcurie) sealed in a kapton envelope was sandwiched between two stacks (thickness > 1.5 mm) of polymer electrolyte films. The fraction of positrons annihilating in source material and kapton were corrected by acquiring a PALS spectrum of a reference material (silicon single crystal). PALS spectra have been analyzed using PALSfit to extract the positron/positronium lifetime components and corresponding intensities in different samples [23]. Depth dependent Doppler broadening measurements have been carried out using an HPGe detector (energy resolution = 2.0 keV at 1332 keV of 60Co) coupled with a slow positron beam. The details of slow positron beam can be obtained from ref. [24]. Positrons emitted from a radioactive source are moderated using a tungsten moderator and transported using magnetic field. The implantation energy of positrons at the sample is increased by floating the sample at a requisite voltage. Doppler broadening spectra (total counts > 2 × 105) are acquired at different positron implantation energies. The Doppler broadened annihilation peak is analyzed through line shape (S- and W-) parameters. S-parameter is calculated as fractional area under the central region (511 ± 0.77 keV) of the photo peak whereas the Wparameter is calculated as the fractional area under the wing region (2.30 keV ≤ │Eγ-511 keV│ ≤ 5.76 keV) of the photo peak, where Eγ is the energy of annihilation photon.
Ipo (1 + k′c)
(3)
As mentioned before, the ionic conductivity of polymer solid state electrolyte is correlated to free volume distribution of polymer matrix. In the studies focused on free volume correlations with ionic conductivity [13–16], free volume size determined from o-Ps lifetime has been primarily considered assuming that no quenching occurs in the presence of Li salts and dissociated ions. The variation observed in the o-Ps intensity has not been correlated with the number density of free volumes speculating the inhibition of Ps formation. However, in this regard, no experimental evidence has been provided in these studies [13–16]. In order to study ionic conductivity mechanism, PEO based electrolyte films of thickness ~ 100–200 μm are prepared using solvent casting method. It has been shown in literature that unsaturated forces and interfacial interaction with casting surface lead to depth dependent variation in segmental relaxation or free volume distribution in polymer films. In such a case, ion movements which are coupled to segmental relaxation will also have deterministic effect from the depth dependent variation in free volume of these films [9]. In the present study, we have prepared PEO based solid electrolyte films loaded with varying content of LiTFSI salt using solvent casting method. The polymer salt complexation and amorphous-crystalline structure of PEO based films have been investigated using X-ray diffraction, Differential Scanning Calorimetry and Fourier Transform Infrared (FTIR) spectroscopy. PALS measurements have been carried out to measure the o-Ps lifetime and intensity as a function of LiTFSI loading. Depth dependent Doppler broadening spectroscopy has been carried out using a slow positron beam. The study shows a uniform depth dependent free volume distribution from both the surfaces. The study through SeW correlation curves shows identical chemical environment at positron/positronium annihilation sites. It confirms that the variation in o-Ps lifetime and intensity represent the change in free volume size and number density as a result of polymer-salt complexation.
3. Results and discussion Fig. 1 shows the XRD patterns of pure PEO and PEO-xLi electrolyte samples. The diffraction peaks at 2θ ~ 19.0 and 23.2 degrees shown in case of pure PEO are consistent with literature. These diffraction peaks are attributed to the inter-chain and intra-chain fold distance of 4.67 and 3.83 Ǻ, respectively [25]. On loading of LiTFSI, formation of any new crystalline phase is ruled out by absence of any new diffraction peaks. The peak positions are not shifted as a result of loading of LiTFSI 2
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annihilation rate) in their increasing order is attributed to p-Ps, free positron and o-Ps annihilation in the polymer matrix. The corresponding intensity is attributed to the fraction of positrons annihilating through these three states. Fig. 4 shows the o-Ps lifetime and corresponding intensity as a function of electrolyte (LiTFSI) concentration. With the addition of salt, an increase in o-Ps lifetime and a decrease in o-Ps intensity are observed. These observations are consistent with the variations observed in o-Ps annihilation parameters in PEO with addition of Li salts [13,16]. According to Eq. (2), o-Ps lifetime decreases as a result of quenching. The observed increase in o-Ps lifetime as a function of electrolyte concentration up to 10% indicate that there is no quenching of Ps in PEO matrix in the presence of Li salts. Fig. 4 shows a decrease in o-Ps intensity which has been attributed to inhibition of Ps formation in PEO based polymer matrix loaded with Li based salt without any experimental evidence [13]. Hirata et al. [32] have studied the inhibition of positronium in three different polymers by doping 2,2′Dinitrobiphenyl (DNB). It has been shown that the o-Ps intensity decreases with the loading of DNB in all the three polymers. The inhibition constants k′ (wt%−1) for these polymers were observed in the range of 0.68 to 1.2. Mohammed et al. [33] and Xia et al. [34] have studied effect of CuCl2 and PdCl2 doping on intensity of positronium in poly (vinyl alcohol) and chitosan, respectively. The observed decrease in o-Ps intensity in these studies was observed to follow a trend given by eq. 3. These studies have shown that the value of inhibition constant (~0.14 wt%−1 and 0.22 wt%−1 for CuCl2-PVA and PdCl2-chitosan, respectively) in polymer matrix is lower than the aqueous solution of these salts. The calculated values of o-Ps intensity using Eq. (3) with average value of inhibition constant (k’ = 0.18 wt%−1) from refs. [33,34] is also shown in Fig. 4 (triangle symbols). The observed trend in experimental o-Ps intensity does not strictly follow the curve represented by Eq. (3) which indicates that the observed decrease in o-Ps intensity cannot be strictly attributed to inhibition process. In such a case, the observed variation in o-Ps intensity can be attributed to variation in free volume hole density in PEO matrix as a result of PEO-salt complexation. However, to the best of our knowledge, no experimental evidence is available in literature supporting the fact that o-Ps intensity variation in PEO based electrolytes can be attributed to the variation in local structure at molecular level of the polymer matrix. The local structure at the molecular level of the polymer matrix determines the positron/positronium diffusion in the polymer matrix. In order to investigate the depth dependent free volume structure of PEO based electrolytes, Doppler broadening measurements have been carried out by implanting monoenergetic positrons using a slow positron beam. The Doppler broadening spectroscopy of annihilation radiation involves the measurement of broadening of annihilation peak using high purity germanium (HPGe) detectors. As mentioned before, positron either as free positron or through the formation of Ps in polymeric materials undergoes two-gamma annihilation. The energy of annihilation photons is shifted by ΔE = ± cp/2 as a result of momentum conservation during the annihilation event, where c and p are the velocity of light and momentum of the annihilating electron in the direction of annihilation photons, respectively. In case of positron annihilation in a material, these shifts result in the broadening of annihilation peak. In case of polymers, the central region of the annihilation peak is primarily populated by the annihilation of p-Ps and free positrons with low momentum electrons (valence electrons). Any variation in Ps formation and chemical environment at annihilation sites lead to variation in area under low momentum (central region) and high momentum (wing region) of annihilation peak. The fractional area under the central and wing region is defined as S- and W- parameters, respectively. S- and Wparameters have been calculated from depth dependent Doppler broadening spectra as described in the experimental section [34,35]. Fig. 5 shows the S-E profiles of top and bottom surfaces of pure PEO and PEO-10Li samples. The top axis shows the positron implantation depth (< z > , nm) in polymer films calculated using < z > = 40E1.6/ ρ, where ρ is the density of polymeric films. It is observed from the
θ
Fig. 1. XRD patterns of pure PEO and PEO-xLi samples.
salt in polymer matrix. It shows that the crystalline structure of PEO matrix remains intact even after 10 wt% of LiTFSI. However, the intensity of the diffraction peaks is observed to decrease with the loading of LiTFSI which indicates a decrease in the crystallinity of the samples. It has been reported in literature that addition of Li based salts act as plasticizer for PEO leading to a decrease in its crystallinity [26]. In order to determine the crystallinity of the PEO based electrolytes, DSC measurements have been carried out. Fig. 2 shows the DSC thermograms of the heating cycle of all the samples. An endothermic peak at ~65 °C is observed showing the melting of crystalline PEO in the samples. The crystallinity of the samples have been evaluated from the ratio of enthalpy of melting of PEO in the sample to enthalpy of 100% crystalline PEO (214.6 Jg−1) [27]. The evaluated crystallinity is given in Table 1. Addition of LiTFSI leads to plasticization of PEO, however its effect is drastic at higher loadings. FTIR spectroscopy is used to investigate the complexation or interaction of Li salt with polymer molecules. Fig. 3 shows the FTIR spectra of pure PEO, LiTFSI salt as well as PEO-xLi electrolyte samples. The bands at 850, 1104, 1460 and 2900 cm−1 are attributed to CH2 rocking, asymmetric COC stretching, asymmetric CH2 bending, and symmetric and asymmetric CeH stretching modes of PEO [28,29]. Multiple peaks in the range of 1050 to 1350 cm−1 are assigned to TFSI anion. In the case of LiTFSI doped polymer samples, new band at ~1190 cm−1 appear indicating the dissociation of LiTFSI in PEO [30]. However, small changes are observed in the intensity of the bands indicating the variations in the local environment at microscopic level [9,31]. The modification of the local environment at the molecular level has been investigated using positron annihilation spectroscopy. PALS spectra of all the samples were analyzed using PALSfit into three decaying exponentials [23]. The positron lifetime (inverse of
Fig. 2. DSC thermograms (heating cycle) of pure PEO and PEO-xLi samples. 3
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Table 1 The value of fitted parameters (Sbulk, Wbulk and Diffusion length) using VEPFIT and the crystallinity determined from DSC measurements. Sample
Sbulk
PEO-0Li PEO-0.5Li PEO-1Li PEO-2Li PEO-5Li PEO-10Li
0.5555 0.5526 0.5497 0.5510 0.5449 0.5432
Wbulk ± ± ± ± ± ±
0.0003 0.0003 0.0003 0.0003 0.0003 0.0003
0.0309 0.0317 0.0326 0.0318 0.0335 0.0341
± ± ± ± ± ±
0.0005 0.0005 0.0005 0.0005 0.0005 0.0005
Diffusion length (nm)
Crystallinity (%)
66.3 ± 14.5 13.6 ± 1.6 12.5 ± 2.6 12.7 ± 1.7 6.5 ± 0.6 8.8 ± 1.9
58.4 56.6 62.8 48.2 41.5 41.1
± ± ± ± ± ±
0.2 0.2 0.2 0.3 0.1 0.1
μ
Fig. 3. FTIR spectra of PEO, LiTFSI and PEO-xLi samples. The marked arrows show the IR bands corresponding to dissociated ions and Li salt aggregates in PEO matrix.
Fig. 5. S-E profiles of top and bottom surfaces of PEO-0Li and PEO-10Li electrolyte samples. The top axis shows the positron implantation depth (μm).
attributed to formation of higher number of positronium at higher depth as well as reduction in Ps diffusion to vacuum where it annihilates through three gamma annihilation mode. W-E profiles are complementary to S-E profiles due to definition of these parameters and are shown in Fig. S1 (Supplementary information, SI). The positron implantation profiles follow Makhovian profile and become broader with the increase in positron energy [36,37]. The experimental S-E profiles are convoluted with the positron implantation profile in the matrix. In order to extract positron/positronium diffusion length in the polymer matrix as well as characteristic S-parameter corresponding to different region of the matrix, S-E or W-E profiles are fitted using variable energy positron fit (VEPFIT) [38]. The computer program VEPFIT takes into account the implantation of positrons, their diffusion and trapping at low density regions before annihilation in the material. According to VEPFIT, a polymeric sample can be considered as multilayered having different molecular packing at different layers. In such a case, the experimental S-parameter at a particular implantation energy E, S(E), can be written as weighted average of S-parameters (Eq. (4)) corresponding to different layers as well as surface of the sample. The fitting parameters fs and fi are the fractions of positrons annihilating at surface and in the ith layer, whereas Ss and Si are the Sparameter corresponding to surface and ith layer.
Fig. 4. o-Ps annihilation parameters (lifetime and corresponding intensities) in PEO-xLi electrolytes. The open triangle symbols represent the calculated o-Ps intensity considering inhibition process with inhibition constant (k′ = 0.18 wt %−1) [33,34].
figure that S-E profiles of top and bottom surfaces of pure PEO as well as PEO-10Li samples are identical. These identical S-E profiles indicate similar free volume structure at sub-surface regions. It also shows that interfacial interaction at polymer-glass surface in pure PEO as well as in PEO-xLi samples do not lead to any modification of molecular packing at the bottom surface compared to the top surface of the films. Fig. 6 (a–b) shows the S-E profiles of top surface of pure PEO and PEO-xLi samples. The S-parameter increases continuously with the increase in positron implantation depth and reaches a constant value which corresponds to the free volume structure of polymer matrix in the bulk. Such type of S-E profiles is typical for polymeric materials [36,37]. The increase in S-parameter with the increase in positron energy is
S (E) = fs Ss +
∑ fi Si i
(4)
The solid lines through the data points in Fig. 6 (a–b) shows the fitting of S-E profiles using VEPFIT. The extracted parameters from the fitting are shown in Table 1. The value of positron/Ps diffusion length in pure PEO is 66.3 ± 14.5 nm. The values of diffusion lengths reported in amorphous polymers are in the range of 10–20 nm which is shorter compared to semiconductors and metals [36,37,39]. The higher 4
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μ
the increase in loading of Li salt. This increase in relaxation time of the segmental motions has been attributed to the rigidity of PEO lattice which is increased by intra- and inter-chain cross-linking by Li ions [41]. Similar results have been reported about the segmental relaxation dynamics using broadband dielectric spectroscopy [3,9]. The reduction in segmental dynamics or larger relaxation times is also observed through the increase in o-Ps lifetime as shown in Fig. 4. The positronium diffusion in the polymer matrix occurs through free volume holes which are redistributed in the polymer matrix with time due to segmental relaxation of polymer chains [13]. In the case of a rigid framework, the free volume distribution with time slows down and the probability of diffusion of Ps among free volume hole is reduced. As a result, a decrease in diffusion length of positron/Ps is observed on loading of LiTFSI in PEO. However, the effect of presence of dissociated ions and salt aggregates on Ps diffusion cannot be ignored. It is interesting to note from Table 1 that Ps diffusion length does not decrease drastically with loading of Li salt from 0.5 to 10 wt%. It indicates that reduction in positron/Ps diffusion length in PEO based electrolytes is not associated with positron/Ps complexation with Li salt aggregates and dissociated ions. This fact is also supported from SeW correlation curves as discussed later. Fig. 7 shows the S-parameter corresponding to bulk of the matrix evaluated by fitting of S-E profiles through VEPFIT. It is observed from the figure that S-parameter decreases sharply with the loading of Li salt. However, the reduction in S-parameter is drastically high in case of 5 and 10 wt% samples. The S-parameter is defined as fractional area in the low momentum region of annihilation peak wherein annihilation of p-Ps and free positron with valence electron predominantly contribute. In such a case, the value of S-parameter depends on Ps intensity as well as p-Ps zero point energy which is related to free volume hole size (o-Ps lifetime). The variation in Ps intensity and lifetime are counter intuitive as shown in Fig. 4. The reduction in S-parameter is highly consistent with the reduction in Ps intensity as shown in Fig. 4. It shows that with the increase in Li salt loading, number density of free volume decreases leading to a decrease in S-parameter as well as o-Ps intensity. In order to support this fact, SeW correlation curves as a function of positron implantation depths or positron implantation energy are plotted in Fig. S2 (SI). Recently, Madzarevic et al. [43] have investigated the chemical environment at the annihilation sites (free volumes) in poly (ether imide) films using depth dependent Doppler broadening measurements. It has been shown through this study that SeW data points are grouped according to the chemical composition of the films which modifies the electronic environment at the annihilation sites (free volumes). Fig. S2 shows that the data cluster corresponding to bulk of pure PEO and PEO-Li are grouped at one location indicating that the chemical environment at free volumes are uniform throughout the investigated depth of these samples.
μ
Fig. 6. (a–b): S-E profiles of PEO-xLi samples (x = 0, 0.5, 1.0, 2.0, 5.0 and 10.0). The top axis shows the positron implantation depth in the samples. The solid lines through the data points show the fit of data using VEPFIT.
value of diffusion length observed in case of pure PEO can be attributed to its high crystallinity and contribution from diffusion of free positron. The value of positron/Ps diffusion length decreases drastically on loading of Li salt, however the values are within the range of reported diffusion lengths in various pure polymers [36,37,39]. It shows that with the loading of Li salt, positron/positronium diffusion through the matrix is reduced drastically. This reduction of positron/Ps diffusion length in the polymer matrix can be attributed to two factors viz. (i) modification in free volume structure or segmental relaxation of polymer matrix, and (ii) Ps quenching or inhibition due to presence of Li salt aggregates and dissociated ions in the polymer matrix. Dielectric broadband spectroscopy and neutron scattering measurements on PEO based solid state electrolytes have been carried out in literature to investigate the effect of Li salt loading on the segmental relaxations of PEO [3,9,10,40–42]. Mao et al. [41] have shown using neutron scattering that α-relaxation which is attributed to the translational motion of the polymer chains, is slowed down drastically with
Fig. 7. Variation in S-parameter corresponding to bulk of PEO-xLi films evaluated from VEPFIT of S-E profiles as a function of LiTFSI loading. 5
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4. Conclusions Positron/Positronium diffusion in PEO-LiTFSI has been investigated using depth dependent Doppler broadening measurements. The reduction in positron/positronium diffusion length on loading of Li salt has been attributed to the increased rigidity of PEO lattice. The SeW correlation curves confirmed the similar chemical environment at annihilation sites (free volumes) in pure PEO and PEO-LiTFSI samples. It indicates that the variation in o-Ps annihilation parameters are not a consequence of Ps reaction (inhibition or quenching) with dissociated ions and salt aggregates present in the samples. The o-Ps lifetime and intensity can be directly correlated to the free volume holes size and number density of free volumes in PEO based solid state electrolytes. The observed variations in relative free volume determined using o-Ps annihilation is consistent with the reported variations in the ionic conductivity.
Fig. 8. SeW correlation plots for PEO-xLi samples.
Acknowledgement The authors thank Dr. B. G. Vats from BARC, Mumbai for his help in FTIR measurements. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ssi.2019.05.025. References [1] A.M. Stephan, K.S. Mahm, Review on composite polymer electrolytes for lithium batteries, Polymer 47 (2006) 5952–5964. [2] Z. Xue, D. He, X. Xie, Poly (ethylene oxide) based electrolytes for lithium ion batteries, J. Mater. Chem. A 3 (2015) 19218–19253. [3] W. Wang, E. Yi, A.J. Fici, R.M. Laine, J. Kieffer, Lithium ions conducting poly (ethylene oxide)-based solid state electrolytes containing active or passive ceramic nanoparticles, J. Phys. Chem. C 121 (2017) 2563–2573. [4] J. Zhang, N. Zho, M. Zhang, Y. Li, P.K. Chu, X. Guo, Z. Di, X. Wang, H. Li, Flexible and ion conducting membrane electrolyte for solid state lithium batteries: dispersion of garnet nanoparticles into insulating poly (ethylene oxide), Nano Energy 28 (2016) 447–454. [5] J. Gurriddappa, W. Madhuri, R.P. Suvarna, K. Priyadasan, Studies on the morphology and conductivity of PEO/LiClO4, Mat. Today: Proceed 3 (2016) 1450–1459. [6] P.V. Wright, Electrical conductivity in ionic complexes of poly (ethylene oxide), British Poly. J. 7 (1975) 319–327. [7] M. Armand, Polymer solid electrolytes-an overview, Sol. Stat. Ion. 9 (1983) 745–754. [8] D.J. Brroks, B.V. Merinov, W.A. Goddard, B. Kozinsky, J. Mailoa, Atomistic description of ionic diffusion in PEO-LiTFSI: effect of temperature, molecular weight and ionic concentration, Macromolecules 51 (2018) 8987–8995. [9] T. Dam, S.S. Jena, D.K. Pradhan, Coupled ion conduction mechanism and dielectric relaxation phenomenon in PEO20–LiCF3SO3-based ion conducting polymer nanocomposite electrolytes, J. Phys. Chem. C 122 (2018) 4133–4143. [10] K.I.S. Mongcopa, M. Tyagi, J.P. Mailoa, G. Samsonidze, B. Kozinsky, S.A. Mullin, D.A. Gribble, H. Watanabe, N.P. Balsara, Relationship between segmental dynamics measured by quasi-elastic neutron scattering and conductivity in polymer electrolytes, ACS Macro Lett. 7 (2018) 504–508. [11] S.B. Aziz, T.J. Woo, M.F.Z. Kadir, H.A. Ahmed, A conceptual review on polymer electrolytes and ion transport models, J. Sci. Adv. Mater. Device 3 (2018) 1–7. [12] H. Papananou, Tuning polymer crystallinity via the appropriate selection of inorganic nanoadditives, Polymer 157 (2018) 111–121. [13] D. Bamford, A. Reiche, G. Dlubek, F. Alloin, J.Y. Sanchez, M.A. Alam, Ionic conductivity, glass transition, and local free volume in poly(ethylene oxide) electrolytes: single and mixed ion conductors, J. Chem. Phys. 118 (2003) 9420. [14] T. Miyamoto, K. Shibayama, Free-volume model for ionic conductivity in polymers, J. Appl. Phys. 44 (1973) 5372. [15] D. Bamford, G. Dlubek, A. Reiche, M.A. Alam, W. Meyer, P. Galvosas, F. Rittig, The local free volume, glass transition, and ionic conductivity in a polymer electrolyte: a positron lifetime study, J. Chem. Phys. 115 (2001) 7260. [16] B. Haldar, R.M. Singru, K.K. Maurya, S. Chandra, Temperature dependence of positron-annihilation lifetime, free volume, conductivity, ionic mobility, and number of charge carriers in a polymer electrolyte polyethylene oxide complexed with NH4ClO4, Phys. Rev. B 54 (1996) 7143. [17] S.J. Tao, Positronium annihilation in molecular substances, J. Chem. Phys. 56 (1972) 5499–5510. [18] M. Eldrup, D. Lightbody, J.N. Sherwood, The temperature dependence of positron lifetimes in solid pivalic acid, Chem. Phys. 63 (1981) 51–58.
Fig. 9. Relative free volume (%) of PEO-xLi samples determined using o-Ps annihilation parameters.
In order to investigate the effect of Li cross-linking on electronic momentum distribution at the annihilation sites, SeW correlation curves using S- and W- parameter values corresponding to bulk of the samples are plotted in Fig. 8. In order to evaluate the W-parameter corresponding to bulk of the samples, W-E profiles, akin to S-E profiles, have been fitted using VEPFIT. The solid line in Fig. S1 passing through the data points shows the fitting of W-E profiles using VEPFIT. The diffusion length values evaluated from the fitting of S-E profiles have been used as a fixed parameter to carry out the fitting of W-E profiles. The evaluated W-parameters corresponding to bulk of the samples are also given in Table 1. It is observed from Fig. 8 that SeW data points corresponding to bulk of the samples follow a straight line. It shows that the chemical environment at the free volume holes in all the samples is similar to pure PEO and the free volume intensity measured in these electrolytes can be directly related to number density of free volume holes. On this basis, the relative free volume (fv, %) of PEO-LiTFSI electrolytes can be calculated using following expression,
fv =
4 3 πrh I3 3
(5)
where, rh (nm) is the radius of free volume holes assumed to be spherical [20,21]. Fig. 9 shows the relative free volume of the PEO-xLi electrolytes calculated using Eq. (5). The figure shows that the relative free volume increases linearly up to 5 wt% loading of Li salt. At higher loading (10 wt%), a drastic increase in free volume is observed. This observation is consistent with the variation observed in ionic conductivity of PEO-LiTFSI based electrolytes wherein a drastic increase is observed in ionic conductivity at loadings > 10 wt% [44,45]. 6
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