In-situ study of the influence of crystallization on the ionic conductivity of polymer electrolytes

Simultaneous impedance measurements and optical observations of polymer electrolytes were conducted in an automated experimental setup, combining an impedance analyser, polarizing microscope with a heating stage and a digital camera. The polymer film was placed between glasses with indium tin oxide conductive layers, forming a transparent cell mounted in a custom-designed holder, which preserved an argon atmosphere. Results of in-situ studies for various compositions of poly( ethylene oxide) (PEO) with LiN(CF3SO2)(2) salt (LiTFSI), as well as pure PEO, are presented. In the investigated systems, crystallization had a strong impact on ionic conductivity. It was found that the initial growth of crystalline structures caused only a small fraction of the total decrease of conductivity. A large decrease in conductivity was observed during the second stage of crystallization, when no significant changes in microscope picture were observed. In pure PEO and the PEO: LiTFSI 6: 1 system, a dense crystalline structure developed, resulting in a decrease in conductivity of over two orders of magnitude. In dilute PEO: LiTFSI systems, a "loose" structure was formed, with amorphous areas preserved between crystallites, and conductivity decreased by only a factor of about 6.