**4.3.1 Ac-impedance spectroscopy**

Impedance spectroscopy (IS) is a powerful analytical tool to characterize the electrical properties of materials and their interfaces with electronically conducting electrodes. It is also widely been used to envisage the dynamics of bound or mobile charge in the bulk or interfacial regions of any kind of solid or liquid material: ionic, semiconducting, mixed electronic–ionic and even insulator (dielectric) (Barsoukov & Macdonald, 2005). The principle of the impedance spectroscopy is based on the ability of a medium to pass an alternating electrical or frequency current. It is well functioned by conducting current and measuring the potential difference created by the circulation of this current. When an electric field is applied across the sample, the polar group might be activated as dipoles which always interact with the corresponding ions due to the Coulombic electric force. Thus, these dipole moments will rearrange themselves under the influence of the external electric field, depending on the mobility of backbone. So, lithium cations can travel faster along these activated or polarizing areas to reach opposite of the electrode and generate current (Selvasekarapandian et al., 2006).

The prepared samples were subjected to ac–impedance spectroscopy. The thickness of the samples was measured by using micrometer screw gauge. The ionic conductivities of the samples were determined, by using HIOKI 3532–50 LCR HiTESTER, over a frequency range between 50 Hz and 1 MHz. The ionic conductivity was measured from ambient temperature to 100 °C. Samples were mounted on the holder with stainless steel (SS) blocking electrodes under spring pressure with the configuration SS/SPE/SS.
