Introduction

Ion conducting polymer electrolytes have drawn much attention because of their high energy density, good flexibility, mechanical strength, good cyclability, etc., for use as electrolytes in solid state lithium ion rechargeable batteries (Scrosati et al. 2001; Tarascon and Armand 2001; Dias et al. 2000). Mainly poly ethylene oxide (PEO)–lithium salt-based polymer electrolytes (Croce et al. 1998; Meyer 1998; Xi and Tang 2004; Gadjourova et al. 2001; Stoeva et al. 2003) have been studied extensively and the ionic conductivity is the key parameter of their performance. The dissolution of lithium salt in PEO electrolyte is due to the fact that PEO chains coil around the Li+ ions and thus separates the cations from the counter anions (Dias et al. 2000). It is well established (Mao et al. 2000) that at room temperature the nature of PEO is biphasic containing both crystalline and amorphous phases and the transport of Li+ cations in the polymer matrix is related to the local relaxation and segmental motion of PEO chains. These two mechanisms are achieved when the amorphous phase of PEO is dominant. Generally PEO tends to crystallize below its melting temperature (mp 60 °C) and thus shows much higher crystalline phases near room temperature and the conductivity of these electrolytes is much lower than 10−5 Ω−1 cm−1, which restricts its application in rechargeable batteries.

Over the last two decades, many efforts have been made to increase the conductivity of these polymer electrolytes. The addition of various plasticizers, such as tetraethylene glycol (TEG), ethylene carbonate (EC), propylene carbonate (PC) etc., to the polymer matrix increases their conductivity considerably (Frech and Chintapalli 1996; Kim and Smotkin 2002). However, they suffer from several drawbacks, such as decomposition, volatilization, reaction toward lithium metal electrode and also the deterioration of the mechanical properties. The addition of ceramic nanoparticles like Al2O3, TiO2 in PEO–LiClO4 electrolyte (Croce et al. 1998, 1999, 2001) and inclusion of inorganic nanoparticles as fillers in the PEO–lithium salt-based polymer electrolytes (Wang et al. 2009; Bhattacharya and Ghosh 2008; Fan et al. 2003; Ribeiro et al. 2001; Scrosati et al. 2000; Nan et al. 2003) improves the conductivity considerably but below 10−4 Ω−1 cm−1.

In this study synthesis of PEO–LiI polymer nancomposite electrolyte embedded with different concentrations of CdO nanoparticles of average size 2.5 nm has been reported. It has been observed that the increase of amorphous phase of PEO–LiI electrolytes and hence the electrical conductivity is maximum for 0.10 wt% CdO doping concentration.

Experimental

First, CdO nanoparticle was synthesized by dissolving cadmium chloride (CdCl2) in distilled water and then adding NaOH bead into the solution under stirring condition. A white precipitation of Cd(OH)2 was produced. Then it was filtered and washed for several times to remove the insoluble residue to find Cd(OH)2 colloid. It was then heat treated at 400 °C for 24 h to obtain the CdO nanoparticles powder.

To prepare PEO–LiI electrolyte, PEO (MW = 400000, Aldrich) and appropriate amount of vacuum dried anhydrous LiI (Aldrich) were dissolved together in acetonitrile. The molar ratio of the ethylene oxide segments to the lithium ions was maintained at [EO]/[Li] = 12. After stirring for 24 h at room temperature, the solutions became very thick after solvent evaporation. Then it was cast in PTFE containers and dried in a vacuum oven at 55 °C for 36 h to form the free standing homogeneous films of about 0.01 cm thick.

Finally, the PEO–LiI-x wt% CdO nanocomposite electrolytes were prepared by dissolving appropriate amount of PEO and vacuum dried anhydrous LiI together in acetonitrile. The molar ratio of the ethylene oxide segments to the lithium ions was kept same as stated above. Then different concentrations of CdO nanoparticles (x = 0.05–0.20 wt%) were dispersed in acetonitrile separately and added into the mixture under stirring condition. After stirring for 24 h at room temperature, the solutions became very thick and free standing homogeneous films were produced following the same procedure as for PEO–LiI electrolytes as stated above.

X-ray diffraction (XRD) pattern of the CdO powder, as-prepared PEO–LiI films and PEO–LiI–CdO nanocomposite films were recorded in an X-ray diffractometer (D8 ADVANCE, BRUKER AXS) using CuKα radiation (0.154 nm wavelength) at a scan rate of 1 degree min−1. The differential scanning calorimetry (DSC) experiments were performed at a heating scan rate of 10 °C min−1 in Perkin-Elmer DSC-7 in N2 atmosphere. The samples were mounted on a specimen stub and coated with platinum and a JSM-6700F field emission scanning electron microscope (FE-SEM) (JEOL Co.), equipped with a link elemental analyzer and a silicon detector, was used for studying the surface morphology of the films. For transmission electron microscopic (TEM) studies a few drops of gel samples just before casting were sonicated in acetonitrile for 15 min in an ultrasonic bath (EYELA) and were dropped in a 300 mesh copper grid. The TEM images were taken using a high resolution TEM (HR-TEM, JEOL, model JEM 2010). Ionic conductivity of the PEO–LiI and PEO–LiI–CdO films were determined at various temperatures by ac impedance spectroscopy in the anhydrous environment in the frequency range 10 Hz–2 MHz using a RLC meter (QuadTech, model 7600). A conductivity cell containing two stainless steel blocking electrodes was used for the ac impedance measurement.

Results and discussion

Figure 1a shows the XRD pattern of CdO powder. A perfect match of the diffraction peaks with those from JCPDS datasheet (JCPDS Datasheet No. 05-0640) indicates the absence of any impurity. The XRD patterns of the as-prepared PEO–LiI and PEO–LiI-x wt% CdO nanocomposite electrolytes are shown in Fig. 1b. No characteristic peaks for LiI have been observed in Fig. 1b, indicating that LiI forms complex with the ether oxygen of PEO (Wang et al. 2006). Two diffraction peaks (at 2θ = 19° and 23.5°) are observed for PEO–LiI electrolyte which are the characteristic peaks of PEO due to its crystalline phase. Figure 1b clearly indicates that the PEO–LiI electrolyte shows highest peak intensity, suggesting highest crystalline phase of PEO. With the addition of CdO in the PEO–LiI matrix, the peak intensity of crystalline PEO decreases and only trace of crystalline PEO has been detected for 0.10 wt% CdO nanoparticles in the PEO–LiI matrix, indicating that significant reduction in the crystalline phase of PEO and a homogeneous electrolyte with a high degree of dispersion of the CdO occurs for this composition. When the concentration of CdO is increased beyond the optimum value of 0.10 wt% the intensity of the two peaks tends to increase again, showing the increase of crystalline phase of PEO.

Fig. 1
figure 1

a XRD pattern for CdO powder. b XRD patterns for PEO–LiI-x wt% CdO polymer electrolytes: (i) for x = 0; (j) for x = 0.05; (k) for x = 0.10; (l) for x = 0.15, and (m) for x = 0.20

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The DSC traces of PEO–LiI and PEO–LiI-x wt% CdO nanocomposite electrolytes are displayed in Fig. 2. The thermodynamic parameters such as glass transition temperature (T g), melting temperature (T m), and melting enthalpy (ΔH m) were obtained from the DSC traces and are summarized in Table 1. The percentage of crystalline phase of PEO (X c) has been calculated using the equation

$$ X_{\text{c}} = \, \Updelta H_{\text{m}} /\Updelta H_{\text{PEO}} ,$$
(1)

where ΔH m is the melting enthalpy of the sample and ΔH PEO is the melting enthalpy (namely 213.7 J g−1) of completely crystallized PEO (Li and Hsu 1984). The inset of Fig. 2 shows the variation of T g with CdO doping concentration. It can be observed that when CdO nanoparticles is doped into PEO–LiI polymer electrolyte the glass transition temperature T g decreases from −36.4 °C to around −49 °C and does not change considerably with doping concentration. This result suggests improvement in the flexibility of the polymer host backbone. It is noted in Table 1 that the values of T m and ΔH m for all concentrations of CdO are lower than that for PEO–LiI electrolytes with lowest values for 0.10 wt%. A lower ΔH m value shows a reduced degree of crystalline phase. This decrease in crystallinity of PEO is due to the fact that the introduction of CdO nanoparticles lowers the reorganization tendency of polymer chain via the interaction with Li+ and PEO, promoting amorphous regions and leading to the formation of an amorphous interface area surrounding the nanoparticles (Croce et al. 1998; Nan et al. 2003). The lowest value of X c (11.8%) achieved for 0.10 wt% CdO suggests the formation of good complexation with a large ratio of amorphous phase to crystalline phase.

Fig. 2
figure 2

DSC thermograms for PEO–LiI-x wt% CdO polymer electrolytes: (a) for x = 0; (b) for x = 0.05; (c) for x = 0.10; (d) for x = 0.15, and (e) for x = 0.20. Inset shows the variation of glass transition temperature (T g) with CdO concentration

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Table 1 Comparison of thermal parameters of PEO–LiI–CdO nanocomposite electrolytes obtained from DSC
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Figure 3 shows the FE-SEM micrographs of PEO–LiI and PEO–LiI–CdO nanocomposite electrolytes for varying concentration of CdO nanoparticle. The electrolytes exhibit a spherulite like surface structure, which is a typical morphology of a crystalline PEO with flexible chains. The amorphous phase of PEO is also observed in the gap between spherulites. The small pores are caused by the fast evaporation of the acetonitrile solvent during the preparation process.

Fig. 3
figure 3

FE-SEM images of polymer electrolyte films of PEO–LiI-x wt% CdO: a for x = 0; b for x = 0.05; c for x = 0.10; d for x = 0.15, and e for x = 0.20

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The average size (diameter) of spherulites for PEO–LiI electrolytes (Fig. 3a) is found to be 120 μm. Also the spaces in between the spherulites i.e., the amorphous regions of PEO in this electrolyte are very narrow. It is interesting to note that when CdO is inserted into PEO–LiI electrolyte the number of PEO spherulites increases and the average size of spherulites decreases to about 50 μm (Fig. 3b–e), which are both beneficial for the decreasing of the crystalline phase of PEO (Xi et al. 2005, 2006). CdO acts as a nucleation site of spherulite and provides an obstacle for the spherulite growth (Jeong et al. 2007). It is observed that the size of the spherulites does not change appreciably with the variation of CdO concentration, but the change of amorphous phase between spherulites is significant. A dramatic improvement of the amorphous phase of PEO is observed when 0.10 wt% CdO is doped into the matrix (Fig. 3c). However, further increase of CdO content decrease the amorphous region between the spherulites. The increased amount of amorphous phase in between the spherulites is very much important for conduction of Li+ ions.

The transmission electron micrographs for the two samples of PEO–LiI–CdO nanocomposite electrolytes with 0.05 and 0.10 wt% CdO are shown in Fig. 4a and b, respectively. The TEM images show that CdO nanoparticles are distributed in the polymer matrix. The histograms for the distribution of the particle size, calculated from TEM images, are shown in Fig. 4c and d, respectively, for these samples. The average size of the particles has been obtained from the fits of the data to the log normal distribution (Soderlund et al. 1998) given by the following relation

$$ f\left( r \right) = {\frac{1}{{\sqrt {2\pi } \ln \sigma }}}\,{\exp }\left\{ { - {\frac{{( \ln r - \ln \rho )}}{{2\left( { \ln \sigma } \right)^{2} }}}} \right\}, $$
(2)

where r is particle diameter, ρ is the geometric mean diameter, and σ is the dimensionless geometric standard deviation. The average size of the particles obtained from the fits is 2.5 nm in both the cases. Similar distribution of particles has been observed with the same average particle size for other CdO contents.

Fig. 4
figure 4

TEM images of a PEO–LiI-0.05 wt% CdO polymer electrolytes and b PEO–LiI-0.15 wt% CdO polymer electrolytes. c and d are histograms for distribution of CdO nanoparticles shown for PEO–LiI-0.05 wt% CdO polymer electrolytes and PEO–LiI-0.15 wt% CdO polymer electrolytes, respectively. The solid curves are the fits to lognormal function (Eq. 2)

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Figure 5 shows the variation of the dc conductivity at T = 303 K, obtained from complex impedance plot, as a function of CdO concentration. The dc conductivity first increases sharply with the increase of CdO concentration and attains a maximum value for 0.10 wt% CdO and then decreases beyond this concentration. It may be noted that the dc conductivity (3 × 10−4 S cm−1) for nanocomposite electrolyte with 0.10 wt% CdO is about three orders in magnitude higher than that of the PEO–LiI electrolyte. Basically, the ion transport in PEO–Li salt-based electrolyte occurs due to the movement of Li+ ions through the amorphous phase of PEO (Croce et al. 1998, 1999; Xi and Tang 2004). For the present PEO–LiI–CdO, XRD, SEM, and DSC studies show that when 0.10 wt% CdO nanoparticles is doped into the PEO–LiI matrix the amorphous phase of PEO attains its maximum value. For the present compositions the incremental amorphous phase increases the movement of Li+ ions through the matrix which in turn increases the dc conductivity and the highest conductivity of the composition with 0.10 wt% of CdO is due to the maximum amorphous phase for this composition. The decrease in the conductivity for higher CdO concentration is attributed to the fact that at higher concentration of CdO, the aggregation of the nanoparticles takes place due to inter particle interaction which in turn provides a blockage on the transport of charge carriers (Wang et al. 2009; Fan et al. 2003; Nan et al. 2003; Kumar et al. 2001).

Fig. 5
figure 5

Variation of dc conductivity for PEO–LiI-x wt% CdO polymer electrolytes with CdO concentration at T = 303 K

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The variation of logarithmic electrical conductivity for all compositions with reciprocal temperature is shown in Fig. 6. An Arrhenius type of behavior is obtained for PEO–LiI electrolyte with activation energy of 1.98 eV. Other authors (Itho et al. 2003; Ramesh et al. 2002) also observed such type of behavior in polymeric system below melting point. But the conductivity variation of CdO-doped electrolytes has been well described by the Vogel–Tamman–Fulcher (VTF) relation (Vogel 1921; Tamman and Hesse 1926; Fulcher 1925) expressed as

$$ \sigma = \sigma_{0} T^{ - 1/2} { \exp }\left[ {{\frac{{ - E_{\text{a}} }}{{k_{\text{B}} \left( {T - T_{0} } \right)}}}} \right], $$
(3)

where σ 0 is the conductivity pre-exponential factor, k B is the Boltzman constant, E a is the Vogel activation energy, T 0 is the Vogel scaling temperature, and T is the absolute temperature. The conductivity data for the CdO-doped electrolytes were fitted to Eq. 3 using non linear least square fits. The fitting parameters σ0, E a, and T 0 are shown in Table 2. Generally, the value of (T g−T 0) is of the order of 50 K for polymeric system (Angell 1983; Natesan et al. 2006). The values of T 0 obtained from the fit are about 40 K lower than the values of T g. The VTF model proposes that the ionic conductivity is favored as a result of the segmental motion of the polymer chain in the matrix. A reasonably good fit of the conductivity to VTF relation over a wide range of temperature clearly indicates that a strong coupling exists between the ionic and the polymer chain segmental motions of the investigated CdO-doped electrolytes (Ribeiro et al. 2001; Fragiadakis et al. 2008).

Fig. 6
figure 6

Variation of dc conductivity with reciprocal temperature of PEO–LiI-x wt% CdO polymer electrolytes: square for x = 0; circle for x = 0.05; triangle for x = 0.10; inverted triangle for x = 0.15, and times symbol for x = 0.20. The solid lines are the VTF fits (Eq. 3)

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Table 2 The VTF fitting parameters obtained for of PEO–LiI–CdO nanocomposite electrolytes
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Conclusions

In this study, novel PEO–LiI polymer nanocomposite electrolyte embedded with CdO nanoparticles with improved electrical conductivity has been synthesized. CdO nanoparticles of average size 2.5 nm have been prepared by thermal route. For a doping concentration of 0.10 wt% CdO into the PEO–LiI matrix, the increase of amorphous phase of PEO is maximum. At this doping level the nanocomposite electrolyte forms a good complexion and a significant enhancement of the dc conductivity has been observed. However, beyond this concentration the conductivity decreases due to blocking effect on the transport of ions resulting from aggregation of nanoparticles. The temperature dependence of the dc conductivity of the CdO-doped electrolytes is well explained by the VTF relation.