Effects of Molecular Structure of Cross-Linked Solid Polymer Electrolytes on Ionic Conduction Behavior

Abstract

To improve the safety and energy density of Li batteries, all-solid-state batteries (ASSBs) are desired. Solid polymer electrolytes (SPEs) are expected to be promising candidates, owing to features such as their high flexibility and formability. To achieve high-performance ASSBs, we investigated molecular structure design by fabricating two types of SPEs. First, chemically cross-linked polyethylene oxide/polypropylene oxide (P(EO/PO))-based SPEs were formed with low-molecular-weight PEO as side chains. Second, physically cross-linked SPEs were fabricated with short PEO chains (G2) as soft-units and norbornene (NB) as hard-units with intramolecular phase separation. Evaluation of the physicochemical and electrochemical properties of these SPEs was carried out, and the degree of dissociation of the Li salt was investigated by Raman spectroscopy. The phenomenon of decoupling between the segmental motion of the SPEs and ionic conduction was confirmed by Vogel-Fulcher-Tamman equation analysis. P(NB/G2)-based SPEs also exhibited high thermal stability, which originated from the NB unit, according to TG-DTA and DSC measurements. Furthermore, ionic conductivity increased monotonically with the Li salt concentration, in contrast to conventional PEO-based SPEs. Physically cross-linked SPEs achieved the decoupling of ionic conduction from their segmental motion owing to their differences between the ionic and structural components of the SPEs.