Synthesis and ion conduction mechanism on hot-pressed sodium ion conducting nano composite polymer electrolytes

1 Introduction

Ion conducting materials comparable to liquid/aqueous electrolytes, show great technological promises as potential electrolyte systems for the development of all solid-state electrochemical devices viz. batteries, fuel cells, super-capacitors, memories, electrochromic displays etc. (Van Gool, 1973; Mahan and Roth, 1976; Chandra, 1981; Laskar and Chandra, 1989; Maier, 2000; Maier et al., 2006). A large number of solid state ionic materials in different phases such as crystalline/polycrystalline, glassy/amorphous, composite, ceramic, polymeric etc. and involving a variety of mobile ionic species viz. H⁺, Li⁺, Ag⁺, Cu⁺, Na⁺, F⁻, O²⁻ etc. have been discovered in the last 3–4 decades. Among the known superionic materials, polymeric electrolytes, the conventional as well as micro/nano composites, attracted widespread attention in recent times. The polymer electrolyte membranes are the most appropriate candidates to fabricate/flexible/compact/laminated all-solid-state thin film batteries (Gray, 1991; Appetecchi et al., 2000, 2001; Bhide and Kariharan 2006; Mohapatra et al., 2008; Agrawal and Chandra, 2007; Kumar et al., 2007; Chandra et al., 2009; Chandra and Chandra, 2010; Chandra, 2010). Majorities of solid polymer electrolytes (SPEs) reported so far are mostly PEO-based electrolytes. Polymer electrolytes exhibiting room temperature conductivity ⩾10⁻⁴ Scm⁻¹ are considered as good candidates for practically useful devices. The high-ionic conduction in these systems is supported by the presence of a high degree of the amorphous phase in the polymeric host materials. However, the conventional polymer–salt complex films are usually mechanically less stable and have poor conductivity behavior. Very recently, it has been reported that the electrical and mechanical property enhancements in polymer electrolytes have been achieved by different techniques. The addition of nano-fillers such as SiO₂, TiO₂, Al₂O₃ etc. in the host polymer electrolytes is one of the widely used techniques (Croce et al., 1998; Qian et al., 2001; Dissanayak, 2004; Chandra et al., 2012; Hu et al., 2007; Shukla and Thakur, 2009; Dey et al., 2008, 2009; Lakshmi and Chandra, 2001, 2002; Choudhary and Sengwa, 2011a,b; Appetecchi et al., 2003). In general, the polymer electrolytes prepared via conventional solution–cast/sol–gel techniques in a variety of ionic salts are complexed/dissolved into different kinds of polymeric hosts. Recently, a hot-press (extrusion) method which is relatively a least-expensive as well as much more-rapid procedure is widely being employed to prepare solution free/dry SPEs and NCPEs (Agrawal and Chandra, 2007; Kumar et al., 2007; Chandra et al., 2009, 2011, 2010; Choudhary and Sengwa, 2011a, 2011b; Appetecchi et al., 2003; Agrawal et al., 2008). Among the known polymers, Poly (ethylene oxide) PEO is widely used as a host polymer because of its ability to dissolve a wide variety of metal salts, its good electrochemical stability and good mechanical properties (Baril et al., 1997; Mohan et al., 2006). Na⁺ ion salts have several advantageous merits over other alkali ion salts viz. K⁺, Li⁺ etc. They are relatively less expensive, abundantly available and less moisture prone.

To understand the ion conduction mechanisms and device applications of Na⁺ ion conducting hot-pressed nano-composite polymer electrolytes, the present paper reports, synthesis and ion conduction studies on SiO₂ dispersed hot-pressed sodium ion conducting nano-composite polymer electrolytes (100 − x)[70PEO:30NaHCO₃] + xSiO₂, where x is in wt.%. The material characterization and ion transport parameters are studied using different experimental as well as theoretical models. The solid-state polymeric battery fabrication is also reported by using SPE host and NCPE OCC for direct comparison.

2 Experimental

The AR grade precursor chemicals: poly (ethylene oxide) PEO (10⁵ MW, Aldrich, USA), NaHCO₃ (purity >98%, Merck, India), SiO₂ (>99.8%, size ∼8 nm, Sigma, USA) were used for the synthesis of NCPEs: (100 − x)[70PEO:30NaHCO₃] + xSiO₂, where x is in wt.%. Firstly the host polymer electrolyte: (PEO:NaHCO₃), was synthesized using the hot-press technique. Details related to the hot-press casting of solid polymer electrolytes/nano-composite polymer electrolytes were explained in our earlier communications (Agrawal and Chandra, 2007; Kumar et al., 2007; Chandra et al., 2009, 2010; Chandra et al., 2011, 2012). On the basis of the salt-concentration dependent-conductivity measurements, the composition: (70PEO:30NaHCO₃) was identified as the highest conducting polymer electrolyte, to be used as the first phase host for the dispersal of nano-sized SiO₂ particles as second phase dispersoid. For the synthesis of nano-composite polymer electrolytes (NCPEs), we followed the same hot-press method. The dry powders of host polymer: (70PEO:30NaHCO₃), were homogeneously mixed with different wt.% ratios of SiO₂, heated at ∼70 °C to form slurry, then hot-pressed between SS blocks resulting finally in a mechanically stable membrane. The conductivity (σ) measurements were carried out at room temperature as a function of SiO₂ concentration and the optimum conducting composition (OCC) of the nano-composite polymer electrolyte was identified. The ionic mobility (μ) and ionic transference number (t_ion) of SPE host and NCPE OCC membranes at room temperature were carried out employing the Transient Ionic Current (TIC) technique (Chandra et al., 1988; Hashmi and Chandra, 1995). The conductivity (σ) – measurements were carried out on different samples using an LCR-bridge (model: HIOKI 3520-01, Japan). Subsequently, mobile ion concentration (n) was evaluated from ‘σ’ and ‘μ’ data. Material characterization and polymer–salt/SiO₂ complexation were done with the help of XRD (model: Shimadzu), FTIR (model: Shimadzu-8400), DSC (model: Perkin Elmer), TGA (model: SDT Universal) techniques. The activation energy (E_a) values were also determined by temperature dependent conductivity studies on different samples of NCPEs.

A solid-state polymeric battery was fabricated using NCPE OCC and SPE host as electrolytes in the following cell configuration:

The cathode in the film form has been prepared by hot-pressing the physical mixture of elemental iodine (I₂), the conducting graphite (C) and NCPE in 1:1:1 weight ratios at ∼50 ^oC. The cell performances of both the cells were studied under different load conditions at room temperature. The cell potential discharge profiles were drawn as a function of time and the important cell parameters were calculated from the plateau region of the profiles.

3 Results and discussion

Fig. 1 shows the ‘log conductivity (σ) – y’ plot of hot-pressed solid polymeric electrolyte (SPE) films: (100 − y)PEO: y NaHCO₃, where y in wt.% at room temperature. Three orders of conductivity increased as the salt NaHCO₃ concentration increased. A moderate-sized σ – maxima σ (∼6.92 × 10⁻⁷ Scm⁻¹) was observed at 30 wt.% of NaHCO₃ [i.e. for the composition: (70PEO:30NaHCO₃)]. The addition of salt in PEO resulted in an increase in the degree of amorphicity and number of mobile ions which in turn gave rise to an abrupt increase in the conductivity of SPEs. The ionic conductivity of SPE films decreases on further addition of salts (beyond 30 wt.%) and this is due to the ion-ion association effect.

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Figure 1

‘Log σ – y’ plot of SPE films: (100 − y)PEO: y NaHCO₃, where y in wt.%.

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Fig. 2 shows the ‘log conductivity (σ) – x’ plot of nano-composite polymer electrolyte (NCPE) films: (100 − x)[70PEO:30NaHCO₃] + xSiO₂, where x is in wt.%. The ionic conductivity (σ) of NCPE increases with increasing the addition of nano-filler SiO₂ and two orders of σ – enhancement were observed from that of the pure SPE host: (70PEO:30NaHCO₃). The two maximum conductivity values of 2.04 × 10⁻⁵ and 1.1 × 10⁻⁵ Scm⁻¹ were observed at 5 and 12 wt.% of dispersal SiO₂. However, the maximum conductivity of 2.04 × 10⁻⁵ Scm⁻¹ has been observed at 5 wt.% of SiO₂ and this has been referred to as optimum conducting composition (OCC). The existence of two peaks for the NCPEs has already been reported by Lakshmi and Chandra (2001, 2002). The existence of two σ-maxima in the present NCPEs can be attributed to two separate percolation thresholds involving two different kinds of mobile species: cation (Na⁺) and anion ( ${\text{HCO}}_3^{-}$ ). The ionic conductivity enhancement in NCPE is also due to the Lewis acid–base reaction and it is also well accepted by various workers (Croce et al., 1998; Qian et al., 2001; Dissanayak, 2004; Chandra et al., 2012; Hu et al., 2007; Shukla and Thakur, 2009). The room temperature values of some of the ionic parameters: σ, μ, n, t_ion are listed in Table 1. It is obvious from the table that the increase in room temperature conductivity of NCPE OCC is due to the increase in mobility and number of mobile Na⁺ ions in the system.

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Figure 2

‘Log σ – x, plot of NCPE films: (100 − x)[70PEO:30NaHCO₃] + xSiO₂, where x is in wt.%.

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Fig. 3 shows the two types of possible cross linking: (a) transient cross-linking of polymer segments via cation-cation interaction and (b) transient cross-linking of polymer segments via cation–anion interaction. The formation of inter/intra – molecular transient cross linking of PEO chains with salt and filler SiO₂ is responsible for the conductivity enhancement in present NCPEs. This type of behavior in PEO-based systems was also reported by various workers (Wieczorek et al., 1998; Albinson and Mellander, 1993; Thakur, 2011).

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Figure 3

Schematic presentation of two kinds of cross-linking in NCPEs: (a) polymer segments via cation-cation interaction and (b) polymer segments via cation–anion interaction.

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Fig. 4 shows the XRD patterns of pure PEO, pure NaHCO₃, SPE host: (70PEO:30NaHCO₃), pure SiO₂, and NCPE OCC: 95[70PEO:30NaHCO₃] + 5SiO₂. The characteristic peaks of pure PEO are observed at 2θ = ∼17^o, 23^o and 39^o. The intensity of the peaks of pure PEO drastically decreases after the polymer–salt complexation, as shown in Fig. 4c. The decrease in the intensity of the peaks is indicative of an increase in amorphous nature of the polymer–salt complex. A new peak at 2θ = ∼20^o has been observed in the diffraction pattern by the addition of nano-filler SiO₂ (as shown in Fig. 4e) and it is indicative of the formation of nano-composite polymer electrolyte. Finally, it can be concluded from the figure that some of the peaks of pure PEO became relatively broader as well as less-prominent/ feeble after salt complexation and SiO₂ dispersal. This is usually attributed to the increase in the degree of amorphicity and confirmation of polymer–salt complexation as well as to some extent, dispersal of filler SiO₂ in the pure polymeric host (Dissanayak, 2004; Agrawal et al., 2008; Chandra et al., 2013).

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Figure 4

XRD patterns: (a) NaHCO₃, (b) pure PEO, (c) SPE host: (70PEO:30NaHCO₃), (d) pure nano-sized SiO₂ and (e) NCPE OCC: [95 (70PEO:30NaHCO₃) + 5SiO₂].

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Fig. 5 shows the FTIR spectra for the pure PEO, NaHCO₃, SPE host: (70PEO:30NaHCO₃) and NCPE OCC: 95[70PEO:30NaHCO₃] + 5SiO₂. The most important absorption features present in PEO are CH₂ bending at 1466 cm⁻¹, C–C stretching at 1147 cm⁻¹, C–O–C asymmetric stretching at 962 cm⁻¹, CH₂ asymmetric rocks at 843 cm⁻¹. It is clearly indicated from the spectra that the width of the C–O–C and C–H stretching modes decreases after addition of salts at ∼1100 and ∼2900 cm⁻¹ respectively and broadening of peaks at ∼820 cm⁻¹. The small changes in the spectra at ∼980 and ∼1600 cm⁻¹ were also observed after the addition of nano-filler SiO₂. Some additional bands appeared in the SPE host and NCPE OCC other than the pure PEO and salt NaHCO₃ such as between 4000–3200, 2700, 2400 cm⁻¹. The band in the spectral region 700–400 cm⁻¹ has been affected with dispersion of nano filler SiO₂ in the host polymer matrix. This may be due to changes in the chemical environment and ion–ion interactions associated with ${\text{HCO}}_3^{-}$ . Some other changes have also been observed in the ion spectral region of 1000–700 cm⁻¹, due to filler dispersion. The appearance of these extra bands and all the above results indicated that the complexation of PEO-salts/filler and such type of interesting behavior were also suggested in the literature (Kumar et al., 2007; Albinson and Mellander, 1993; Thakur, 2011).

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Figure 5

FTIR spectra: (a) pure PEO, (b) pure NaHCO₃, (c) SPE host: (70PEO:30NaHCO₃) and (d) NCPE OCC: 95 (70PEO:30NaHCO₃) + 5SiO₂.

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Fig. 6 shows the Scanning Electron Micrograph (SEM) images of SPE host: (70PEO:30NaHCO₃) and NCPE OCC: 95[70PEO:30NaHCO₃] + 5SiO₂. It can be clearly seen from the figures the smooth surface morphology observed in SPE host and NCPE OCC while the pure PEO shows the rough morphology structure (Mohapatra et al., 2008; Chu et al., 2003). The smooth surface morphology in SPE host and NCPE OCC is closely related to the reduction of PEO crystallinity.

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Figure 6

SEM images: (a) SPE host: (70PEO:30NaHCO₃) and (b) NCPE OCC: [95 (70PEO:30NaHCO₃) + 5SiO₂].

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Fig. 7 shows the DSC thermograms for the pure PEO, SPE host: (70PEO:30NaHCO₃) and NCPE OCC: 95[70PEO:30NaHCO₃] + 5SiO₂. The broad endothermic peak was observed in polymeric films at ∼54–70 ^oC and it is corresponding to the melting point temperature of pure PEO (∼70 ^oC), SPE host and NCPE OCC films. It is clearly shown from the figure that the melting temperature (T_m) of pure host decreased from 70 to 66 ^oC and this is usually observed in various PEO-based systems (Qian et al., 2001; Bhide and Kariharan, 2006; Wen et al., 1996; Stephan et al., 2009). In case of NCPE OCC the T_m decreased up to 54 ^oC. It is due to the interaction between the PEO backbone and the filler SiO₂ that has affected the main chain dynamics of the SPE host. A possible mechanism for this behavior could be understood with the creation of additional hopping sites and favorable conducting pathways for ionic migration. DSC analysis clearly indicated that NCPE OCC shows a low degree of crystallinity and high amorphous nature, which is responsible for increase in ionic conductivity. The degree of crystallinity (χ) has been calculated by assuming pure PEO being 100% with the equation $\chi =(\mathrm{\Delta }{\text{H}}_{\text{f}}/\mathrm{\Delta }{\text{H}}_{{\text{f}}^{\text{o}}})$ (where $\mathrm{\Delta }{\text{H}}_{{\text{f}}^{\text{o}}}$ is the heat of fusion of pure PEO and ΔH_f is the heat enthalpy related to salt in PEO). Table 2 lists the obtained degree of crystallinity (χ) and melting temperature (T_m) of polymeric films. The decrease in the degree of crystallinity and melting temperature in NCPE OCC is indicative of an increase in amorphousness.

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Figure 7

DSC thermograms: (a) pure PEO, (b) SPE host: (70PEO:30NaHCO₃) and (c) NCPE OCC: [95 (70PEO:30NaHCO₃) + 5SiO₂].

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Fig. 8 shows the Thermo Gravimetric Analysis (TGA) curves for pure PEO, SPE host: (70PEO:30NaHCO₃) and NCPE OCC: 95[70PEO:30NaHCO₃] + 5SiO₂. It can be clearly observed from the figure that the total weight loss for pure PEO is larger as compared to both SPE host and NCPE OCC. The total weight loss for NCPE OCC is ∼60% and SPE host is ∼76% whereas for pure PEO is ∼95%. The thermal stability of polymer is improved when the nano-filler SiO₂ is doped and it is indicative of the formation of hot-pressed NCPEs, similar type of behavior was also reported by Dey et al. (2009).

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Figure 8

TGA curves: (a) NCPE OCC: [95 (70PEO:30NaHCO₃) + 5SiO₂], (b) SPE host: (70PEO:30NaHCO₃) and (c) pure PEO.

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The ionic transference number (t_ion) of SPE host and NCPE OCC has been evaluated using the dc polarization technique. The variation of current with time is shown in Fig. 9. t_ion was calculated using the following equation:

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Figure 9

‘Current vs. time’ plot for the cell: SS // NCPE OCC // SS.

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Fig. 10 shows the ‘log σ – 1/T’ plots for hot-pressed NCPEs: (100 − x)[70PEO:30NaHCO₃] + xSiO₂, where x varies from 0 to 14. ‘Log σ – 1/T’ plots exhibited straight line behavior below at 60 ^oC and then upward changes have been observed in the slope after ∼60 ^oC, which is due to the well-known semi-crystalline to amorphous phase change/melting point temperature of PEO-based polymers, as reported in our earlier communications (Chandra et al., 2010, Chandra et al., 2011, 2012). The straight line portions of SPE host and NCPE OCC below the transition temperature (∼60 ^oC) governed the following Arrhenius type equations:

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Figure 10

‘Log σ − 1/T’ plots for hot-pressed NCPEs: (100 − x)[70PEO:30NaHCO₃] + xSiO₂, where x = 0 ( ), 2 ( ), 5 ( ), 10 ( ), 14 ( ) is in wt.%.

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Fig. 11 shows ‘log μ – 1/T’ and ‘log n – 1/T’ plots for the hot-pressed NCPE OCC: 95[70PEO:30NaHCO₃] + 5SiO₂. The linear portion of this Fig. 11 can be expressed by the following Arrhenius type equations:

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Figure 11

‘Log μ – 1/T’ and ‘Log n – 1/T’ plots for NCPE OCC: [95 (70PEO:30NaHCO₃) + 5SiO₂].

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Fig. 12 shows the cell potential discharge profiles for the cells I and II under different loads viz. 100 and 50 kΩ at room temperature. The Open Circuit Voltage (OCV) ∼2.7 V was obtained for both the cells. It can be clearly noticed that except for the initial potential drop, OCV value remained practically stable for ∼65 and ∼27 h. when discharged through 100 and 50 kΩ respectively (i.e. during a low current drain state). However, the cell potential decreased relatively faster when discharged through 50 kΩ load (i.e. during higher current drain states). Direct comparison of cell parameters, is listed in Table 3. On the basis of these studies, it can be observed that the cell I, based on newly synthesized NCPE, performed fairly satisfactorily especially during low current drains as compared to cell II.

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Figure 12

Cell potential discharge profiles for the solid state polymeric cells: (a) cell I: Na // [95(70PEO:30NaHCO₃) + 5SiO₂] // (C + I₂ + Electrolye), (b) cell II: Na // (70PEO:30NaHCO₃) // (C + I₂ + Electrolye), under 100 kΩ (●) and 50 kΩ (▴) load resistances at room temperature.

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4 Conclusions

A new Na⁺ ion conducting nano-composite polymer electrolyte (NCPE) membrane: 95 (70PEO:30NaHCO₃) + 5SiO₂ (wt.%) has been synthesized employing a hot-press/solvent free dry technique. Dispersal of nano filler SiO₂ particles into the polymer electrolyte host: (70PEO:30NaHCO₃) resulted in an enhancement in the electrical as well as mechanical properties of the film. The ionic conductivity enhancements were explained with the help of ionic mobility (μ), mobile ion concentration (n) studies and different models. The polymer–salt/SiO₂ complexation has been successfully explained with the help of XRD, FTIR, DSC and TGA studies. The ionic transference number measurements indicated the fact that approximately 92% of the total Na⁺ ions take part in conduction process in the hot-press synthesized NCPE system. Solid state polymeric batteries were fabricated using NCPE OCC and SPE host as electrolyte and the cell potential discharge characteristics have been studied under a different load condition at room temperature.