Continuation: Polymer electrolytes for reversible lithium cells – synthesis, electrical properties, modeling

Aim of project
The aim of the project was to obtain a polymer electrolyte which could be used in a rechargeable lithium cell and to study the relation between the microstructure of the polymer electrolyte, which can be a heterogeneous system because of its partial crystallization, introduction of unlimited fi ller or appearance of salt precipitation, and the ionic conduction and accompanying phenomena of electric polarization.

Polymer electrolytes, composed of modified poly(ethene oxide) of branched star structure and lithium imide salt LiN(CF3SO2)2, were synthesized and studied. The presence of a core of nanometer dimensions in star polymers gives an effect similar to PEO composite with a filler but without the agglomeration effect. The star structure of the polymer matrix influences the creation of stoichometric complexes, preventing the formation of crystalline structures and decrease of conductivity.

In terms of modeling composite polymer electrolytes, modifications of the resistor network model were created, facilitating a better presentation of the structure of the actual electrolyte. Impedance spectra registered at different temperatures for polymer electrolytes based on poly(ethene oxide) of various crystallization level, were analyzed in terms of modeling electrode processes which are of crucial importance for the use of polymer electrolytes in lithium cells. Impedance spectra measured with simultaneous registration of electrolyte image in a polarization microscope allowed to relate changes in impedance spectrum with electrolyte crystallization.

An advanced analysis was conducted of the measured impedance spectra and there was a correlation between the model of electrical polarization phenomena and dielectric relaxation, and the data on electrolyte structure on the basis of butyl acrylonitryle and acrylate copolymer.

Expected results
Observation of inhibition of crystallization processes in electrolytes composed of star modification of poly(ethene oxide) and the salt LiN(CF3SO2)2. Homogeneous structure of the electrolyte of molar ratio 1:1 EO:Li proves high solubility of the salt in the polymer matrix, which allows to use poly(ethene oxide) of star structure for creating electrolytes of ”polymer in salt” type. Modified technology of manufacture of composite polymer electrolytes will enable to obtain foils of homogeneous distribution of chemically pure unlimited filler of standardized grain size, which will allow to compare quantitatively experimental data with results of computer simulations of ionic conductivity.

Determination of parameters of the bilayer on the border: polymer electrolyte – metal electrode proved vital for the evaluation of contact quality between electrodes and the electrolyte. Different properties of the bilayer were observed in case of electrolytes of low and high content of lithium imide salt in poly(ethene oxide).

Electrical polarization of polymers of butyl acrylonitryle – acrylate and the salt LiN(CF3SO2)2 was analyzed on the basis of a model which takes into consideration beta relaxation (local mod) and alpha relaxation (segmental) of the polymer chain coupled with relaxation of charge carriers. The results point to the same mechanism of ionic conductivity in newly prepared and older foils of polymer electrolytes of not very high salt content.