Ionics DOI 10.1007/s11581-017-2036-7
ORIGINAL PAPER
Characterization of ion transport property in hot-press cast solid polymer electrolyte (SPE) films: [PEO: Zn(CF3SO3)2] Shrabani Karan 1 & Tripti Bala Sahu 1 & Manju Sahu 1 & Y. K. Mahipal 1 & R. C. Agrawal 1
Received: 29 July 2016 / Revised: 20 January 2017 / Accepted: 16 February 2017 # Springer-Verlag Berlin Heidelberg 2017
Abstract Characterization of ion transport property in dry solid polymer electrolyte (SPE) films: [PEO: Zn(CF3SO3)2] in different salt wt% ratio has been reported. SPE films have been prepared by a hot-press casting procedure. Salt concentration dependent conductivity study at room temperature identified SPE film: [90PEO: 10 Zn(CF3SO3)2] as optimum conducting composition (OCC) with σrt ~ 1.09 × 10−6 S/cm which is approximately three orders of magnitude higher than that of pure PEO host (σrt ~ 3.20 × 10−9 S/cm). The reason attributed for σrt enhancement has been the increase in degree of amorphous phase in polymeric host after salt complexation. This has been confirmed by X-ray diffraction (XRD), Fourier transform infrared (FTIR), differential scanning calorimetry (DSC), and polarized optical microscopy (POM) analysis. To evaluate the usefulness of SPE OCC film in all-solidstate-battery applications, ion transport property has been characterized in terms of basic ionic parameters viz. ionic conductivity (σ) and total ionic (tion)/cation (t+) transport numbers. Mechanism of ion transport has been explained by temperature dependent conductivity measurements and the activation energy (Ea) has been computed by least square linear fitting of Blog σ − 1/T^ Arrhenius plot.
Keywords Dry solid polymer electrolyte (SPE) . Hot-press casting procedure . Ionic/cationic transference number . All-solid-state polymer batteries
* R. C. Agrawal
[email protected] 1
Solid State Ionics Research Laboratory, School of Studies in Physics & Astrophysics, Pt. Ravishankar Shukla University, Raipur, CG 492010, India
Introduction Ion conducting electroactive polymers or dry polymer electrolytes viz. solid polymer electrolytes (SPEs)/composite polymer electrolytes (CPEs) are potential solid-state ionic materials for all-solid-state battery applications. Ion conduction in otherwise insulating polymers was reported for the first time by Fenton et al. [1]. Subsequently, the first practical battery, based on Li+-ion conducting poly (ethylene oxide) PEO-based polymer electrolyte, was demonstrated by Armand et al. [2]. These discoveries attracted widespread attention of researchers. As a result, a large number of polymer electrolytes involving different mobile ions viz. H+, Li+, Ag+, Cu+, Na+, K+, Mg2+, Zn2+, etc. have been investigated since then in the last nearly four decades and their cell performances have been tested by fabricating all-solid-state batteries in all possible shapes/sizes [3–15]. Majority of the polymer electrolyte materials reported so far commonly used high molar weight polar poly(ethylene oxide) PEO as polymeric host due to its inherent ability to dissolve wide variety of ionic salts in substantially larger proportion. Further, modern portable polymer electrolyte batteries available today in the market are based on PEO complexed with Li+− ion salts and lithium metal electrode couples. However, these batteries have reported to encounter some serious fire-hazards in the recent past which was mainly due to use of lithium chemicals [16, 17]. Hence, on account of safety of the batteries while in use, the need of some non-lithium chemicals based polymer electrolytes and electrodes has been strongly felt in the recent time. The work reported in this paper is an attempt in this direction. We report the synthesis of Zn2+-ion conducting SPE films: [(1−x) PEO: x Zn(CF3SO3)2] in different salt concentration (x). The film exhibiting optimum room temperature conductivity (σrt) was identified and has been referred to as optimum conducting composition (OCC) SPE film. The investigations on materials
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Fig. 1 σrt as a function of salt concentration (x) for hot-press SPE films: [(1−x) PEO: x Zn(CF3SO3)2)]
and ion transport properties of SPE OCC film have been done in order to evaluate its possible applicability in potential allsolid-state batteries.
Experimental SPE films: [(1−x) PEO: x Zn(CF3SO3)2] in varying salt concentrations viz. x = 1, 3, 5, 7, 10, 15, 12, 20, 25, 30 wt% have been synthesized by hot-press cast technique. This is a novel technique, originally proposed by Gray et al. [18] and adopted recently by many groups with slight modifications [19–21]. Hot-press casting has several advantages over the traditional solution cast method and has been recognized as the most rapid, least expensive, completely dry/solution free procedure for casting polymer electrolyte films. Films have been casted using AR grade precursor chemicals: poly(ethylene oxide) PEO (6 × 105 Mw, purity >99%, Aldrich, USA), Zn(CF3SO3)2 (98%, Aldrich, USA). Dry powder of constituent chemicals in appropriate wt% ratio was mixed physically for about ~30– 60 min. The homogeneously mixed powder was then heated Table 1 The values of σrt as a function of salt concentration (x) for hotpress SPE films: [(1−x) PEO: x Zn(CF3SO3)2)] along with σrt of pure PEO
x [Zn(CF3SO3)2] wt% 0 1 3 5 7 10 12 15 20
Fig. 2 BLog σ − 1/T^ plot for hot-press SPE films: [95 PEO: 5 Zn(CF3SO3)2] ( ); [90 PEO: 10 Zn(CF3SO3)2] (OCC) (■); [85 PEO:15 Zn(CF3SO3)2] ( )
close to melting/softening point of PEO, i.e., ~70 °C, with mixing continued for ~30–40 min. As a result, a soft lump/ slurry was obtained which was pressed (~1–2 ton/cm2) between two stainless steel (SS) cold blocks to form SPE film of uniform thickness ~100–150 μm. The salt concentration dependent conductivity study at room temperature (27 °C) revealed SPE OCC film. Characterization of ion transport property of SPE OCC film was done in terms of basic ionic parameters viz. conductivity (σ), total ionic (tion), and cationic (t+) transference numbers. σ− measurements were carried out by impedance spectroscopy (IS) using an LCR- meter (HIOKI IM 3533) in which the film sample was placed between two SS (blocking) electrodes and frequency was varied in the range (1 mHz– 200 KHz). Temperature dependent conductivity study was also done and activation energy (Ea) was also computed by linear list square fitting of log σ – 1/T Arrhenius plot. The total ionic transference number (tion) was determined using dc polarization transient ionic current (TIC) technique [22, 23] by placing SPE
σrt (S/cm) 3.2 × 10−09 3.86 × 10−07 4.92 × 10−07 7.23 × 10−07 8.55 × 10−07 1.09 × 10−06 7.78 × 10−07 5.79 × 10−07 5.34 × 10−07
90 PEO: 10 Zn(CF 3 SO 3 ) 2 is highest conducting composition
Fig. 3 TIC plot for SPE OCC film: [90 PEO: 10 Zn(CF3SO3)2]
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Results and discussion Ion transport studies
Fig. 4 I0/Is and R0/Rs values obtained from Bcurrent-time^ and complex impedance BZ′-Z″^ (inset) plots
OCC film between SS (blocking) electrodes. Cation (Zn2+) transport number (t+) measurement was done by a combined ac/dc technique in which the film/sample was placed between Zn (non-blocking) electrodes and evaluated with the help of following equation [24]: t þ ¼ I S ðΔV−I 0 R0 Þ=I 0 ðΔV−I S RS Þ
ð1Þ
where I0/IS and R0/RS are the initial/final current/resistance values respectively before/after polarization and ΔV is a fixed polarization voltage. The surface morphology of pure PEO and SPE OCC films was studied using POM (AXIOSKOP 40, ZEISS, Germany) and material properties were characterized by XRD (D2 phaser model: 08 discover, Bruker) and FTIR (IR Affinity–1 Shimadzu, Japan) techniques. DSC (STARe, SW 13.00, METTLER) analysis was done to study thermal response o the film materials and degree of crystallinity (Xc) was evaluated from DSC thermograms with the help of the following equation [25]: ΔH m 100% ð2Þ Xc ¼ ΔH 0m where ΔHm and ΔH 0m are heat enthalpy of complexed polymer and polymer in pure crystalline phase, respectively. Table 2 The values of σrt, tion, t+, Ea for SPE films along with σrt of pure PEO Film sample
σrt (S/cm)a
Pure PEO SPE:[95PEO: 5 Zn(C3SO3)2] SPE OCC:[90PEO:10 Zn(CF3SO3)2] SPE:85PEO: 15 Zn(C3SO3)2
– – 3.2 × 10−9 −7 7.23 × 10 0.96 0.15 1.09 × 10−6 0.97 0.17 5.79 × 10−7 0.96 0.16
a
Siemen/cm
tion
t+
Ea (eV) – 0.25 0.27 0.22
Salt concentration (x) dependent conductivity (σrt) variation for different hot-press cast SPE films: [(1−x) PEO: (x) Zn(CF3SO3)2] is shown in Fig. 1 as well as σrt values for different salt ratios are listed in Table 1. The conductivity increased abruptly on an initial addition of salt by x = 1 wt%, then remained almost same on further addition of salt up to x = 20 wt%. The films beyond x = 20 wt% looked mechanically unstable. A moderate size σpeak appeared at x = 10 wt% salt. SPE film: [90PEO: 10 Zn(CF3SO3)2] has been identified as SPE OCC film exhibiting σrt ~ 1.09 × 10−6 S/cm which is approximately three orders of magnitude higher than that of pure PEO host (σrt ~ 3.20 × 10−9 S/ cm). The reason attributed for σrt enhancement in SPE film is predominantly due to increase in the degree of amorphous phase in polymeric host as a consequence of complexation/dissociation of salt. Figure 2 shows log σ − 1/T plots for SPE OCC film: [90PEO: 10 Zn(CF3SO3)2] and two additional SPE films: [95PEO: 5 Zn(CF3SO3)2] and [85PEO: 15 Zn(CF3SO3)2]. The conductivity initially increased gradually/linearly as temperature increased, followed by an onset of upward change in slope around ~65–70 °C and then the conductivity increased relatively rapidly afterwards. In fact, the temperature region 65– 70 °C corresponds to the characteristic semicrystallineamorphous phase transition of PEO which usually occurs at ~69–70 °C. Log σ – 1/T plot below this transition temperature can be expressed by following Arrhenius straight line equation: logσ ¼ logσ0 −E a =kT
ð3Þ
where σ0 is the pre-exponential factor, k is Boltzmann constant, and Ea is the activation energy which was computed by linear least square fitting of log σ – 1/T plot below ~60 °C and found to be in the range 0.22–0.27 eV. The total ionic transference number (tion) was measured using d.c. polarization TIC technique, as mentioned in the BExperimental^ section. Figure 3 shows Bcurrent-time^ TIC plot when the film sample was subjected to fixed dc polarization potential V ≈ 1 V. tion ~0.97 was obtained from the ratio: Iion/IT when the film sample was polarized for ~3 h. This value is very close to unity; hence, indicative of the fact that the film material is predominantly ionic. It is well known that in polymer electrolytes both cations and anions move. Hence, the cationic (Zn2+) transference number (t+), which is more relevant for good battery performance, was evaluated separately by a combined ac/dc technique and with the help equation given in the BExperimental^ section. The values of I0/Is and R0/Rs were obtained from currenttime and BZ′−Z″^ plots shown in Fig. 4. t+ was found to be ~0.17 which is significantly low. However, the values of both σrt and t+ can be increased substantially by dispersing micro/nano particles
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Fig. 5 POM study for a pure PEO, b SPE OCC film: [90 PEO: 10Zn(CF3SO3)2], and c SPE film: [95 PEO: 5Zn(CF3SO3)2]
of an active/passive insulating filler material [14] as the second phase dispassed into SPE which acts as first-phase host, thereby to form micro/nano composite polymer electrolytes (CPEs). Alternatively, σrt can also be increased by introducing nanoionic effect on dry polymer electrolyte system through high energy ball milling of powder mixture of constituent chemicals prior to casting SPE/CPE films [26]. These works are currently being pursued in the present laboratory. Table 2 lists the values of σrt, tion, t+, and Ea for SPE OCC film along with σrt of pure PEO.
Fig. 6 XRD patterns for (a) pure PEO, (b) Zn(CF3SO3)2 salt, (c) SPE film: [95 PEO: 5 Zn(CF3SO3)2], (d) SPE OCC film: [90 PEO: 10 Zn(CF3SO3)2], and (e) SPE film: [85 PEO:15 Zn(CF3SO3)2]
Surface morphology study The surface morphology of the film samples was studied at room temperature by polarized optical microscope (POM). Figure 5a, b, c shows POM micrographs of pure PEO, SPE OCC: [90PEO: 10 Zn(CF 3 SO 3 ) 2 ] and SPE: [95PEO: 5Zn(CF3SO3)2] films. The existence of tightly interconnected large spherulites in POM micrograph of pure PEO (Fig. 5a) clearly indicated the semicrystalline-amorphous nature of polymer [27]. As a consequence of complexation of salt in polymer, it can be clearly noticed in POM micrographs of SPE films (Fig. 5b, c) that the size of spherulites decreased
Fig. 7 FTIR for (i) pure PEO, (ii) Zn(CF3SO3)2 salt, (iii) SPE film: [95 PEO: 5 Zn(CF3SO3)2], (iv) SPE OCC film: [90 PEO: 10 Zn(CF3SO3)2], and (v) SPE film: [85 PEO:15 Zn(CF3SO3)2]
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considerably along with the increase in amorphous region (dark region).
in polymeric host. All these changes in the spectral response confirmed the complexation of salt in the polymer.
Material property studies
Thermal property characterization study
Figure 6 illustrates XRD patterns for pure PEO, complexing salt Zn(CF 3 SO 3 ) 2 , and SPE OCC film: [90PEO: 10 Zn(CF3SO3)2]. In comparing these patterns, it can be clearly noticed that the intensity of two main peaks of PEO at 2θ ~19.3° and 23.3° has been suppressed substantially after complexation of salt in polymer. This is a clear indication of the complexation/dissolution of salt into polymeric host as well as decrease in degree of crystallinity and/or increase in degree of amorphosities in PEO. This has been further confirmed by FTIR studies. Figure 7 shows FTIR spectra of pure PEO, Zn(CF3SO3)2, and SPE OCC film. The characteristic vibrational bands of pure PEO at ~2238, ~2163, and ~1963 cm−1, peaks at ~525–530 cm−1 and ~1200 cm−1 related to C-O-C bending and stretching modes respectively and the bands at ~750–950, ~1820, ~2900–3000, ~1475, and ~845 cm −1 corresponding to symmetrical/asymmetrical stretching/vibration of CH2 group, CH2 bending, CH2 rocking, etc. and characteristic bands of Zn(CF3SO3)2, i.e., symmetric deformation mode CF3 at 769.60 cm−1, asymmetric stretching of CF3 at 1151.50 cm−1 of triflate ion, SO3 mode of free triflate ion at 663.51 cm−1 appeared suppressed considerably after complexation of salt
The sharp endothermic peak observed through DSC study at 71.09 °C corresponds to the crystalline melting temperature (Tm) of pure PEO (Fig. 8a). The endothermic peak for pure PEO shows the transition from 71.09 to 69.66 °C upon the addition of 10 wt% of Zn(CF3SO3)2 salt. The endothermic curves also indicate a reduction in PEO crystallinity. The relative percentage of crystallinity (Xc) of PEO as well as SPE film have been calculated using the following equation: Xc ¼
ΔH m ΔH 0m
100%
where ΔHm is the melting enthalpy estimated experimentally and ΔH 0m used as referenced is the melting enthalpy for 100% crystalline PEO (213.7 J g−1) used as reference [28]. The calculated values of Xc are summarized in Table 3. The degree of crystallinity of the electrolyte decreases with the addition of the salt, which causes an increase in the amorphous phase. The decrease in Tm and Xc due to the addition of salt completely confirms the increase in σrt.
Conclusion A non-lithium chemical-based solid polymer electrolyte (SPE) film: [90 PEO: 10 Zn(CF3SO3)2] has been synthesized using hot-press casting technique. Materials/ion transport properties have been characterized to evaluate its usefulness in the fabrication of all-solid-state batteries. This SPE film exhibited room temperature conductivity (σrt) ~1.09 × 10−6 S/cm, the total ionic transference number (tion) ~0.97 and t+ ~0.17. σrt and t+ values are quite low as regards to its use in a good performing battery. However, t+ as well as σrt can be improved significantly by way of dispersing micro/nano filler particles in SPE host and/or introducing nano-ionic effect through high energy ball milling of dry powder mixture of constituents prior to casting the polymer electrolytes film. Table 3
Fig. 8 DSC for (a) pure PEO, (b) SPE film: [95 PEO: 5Zn(CF3SO3)2], (c) SPE OCC film: [90 PEO: 5Zn(CF3SO3)2], and (d) SPE film: [85 PEO: 15Zn(CF3SO3)2]
The values of Tm, ΔHm, and Xc for SPE films and pure PEO
Film sample
Tm (°C)
ΔHm (J/g)
Xc (%)
Pure PEO SPE:[95PEO: 5 Zn(C3SO3)2] SPE OCC:[90PEO:10 Zn(CF3SO3)2] SPE:85PEO: 15 Zn(C3SO3)2
71.09 70.32 69.66 70.69
175.15 133.36 123.18 139.81
81.94 62.40 57.64 65.42
Ionics Acknowledgements The authors would like to thank Chhattisgarh Council of Science & Technology (CCOST) Raipur, India, for extending financial support through a Mini Research Project (MRP): No. 15039/ CCOST/MRP/13, date: 29/03/14.
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