Di-block copolymers as solid-state electrolytes for lithium ion batteries

Abstract

Novel and solvent-free routes for producing solid-state electrolyte membranes for lithium ion batteries containing Li[N(CF3SO2)2] (LiTFSI) and Arnitel® copolymers produced and provided by DSM are proposed and elaborated in this graduation project. Other lithium salts such as Li[B(C2O4)2] (LiBOB) and LiCl have also been tested, but more research has been focused on LiTFSI. Arnitel materials with variable proportions of poly(ethylene oxide) (PEO) and polybutylene terephthalate (PBT) have been used. The presented solvent-free methods of incorporating the lithium salt are intended as an alternative to the solvent route, that involves the use of hazardous HFiP, previously investigated in internal DSM research. The methods proposed in this work are based on two different concepts: one involves melting the polymer and mixing in the lithium salt (referred to as melt-mixing), while the other two techniques make use of water as a medium to carry and incorporate the salt in the polymer chains. This has been performed on Arnitel pellets (method named water soaking) and on thin films of the same material (referred to as thin film soaking). The water can then be removed by complete drying. Samples produced through melt-mixing and water soaking have then undergone hot pressing and thus thin (150-200µm) free-standing electrolytes have successfully been prepared. DSC and electrochemical analysis such as impedance measurements between room temperature and approximately 65°C, cyclic voltammetry and lithium plating experiments are presented. The samples prepared during this research proved to have room temperature conductivity as high as 10-5 S/cm. Electrochemical stability seems instead to be an issue in the lower voltage, but this is ascribed to the inherent behaviour between the lithium electrolyte and seems to be independent of the production method used. Such electrochemical results are aligned with the internal DSM data on samples produced via the solvent route: it is therefore possible to consider the proposed production routes as feasible alternatives for cheaper, safer and more environmentally friendly methods to produce Arnitel-based solid-state electrolytes for lithium ion batteries.