Highly Branched Poly(ethylene oxide) for Solid Electrolytes

Abstract

Commercial Li ion batteries use liquid electrolytes between the electrodes to obtain good Li ion conductivity and thus good energy density. However, the liquids are highly flammable, which presents a great challenge in designing large scale batteries. Solid polymer electrolytes (SPEs) based on poly(ethylene oxide) (PEO) have been widely explored as an alternative of all-solid-state Li ion batteries. However, their wide adoption is prohibited due to their low conductivity at room temperatures (lower than 10 -5 S cm -1 ), presumably because PEO is semicrystalline and crystals may not be available for ion conductivity. The goal of this study is to design and prepare series of polymers containing amorphous PEO with high free volume, achieving higher Li ion conductivity than the semi-crystalline PEO. Specifically, polymers were prepared using UV photopolymerization from poly(ethylene glycol) diacrylate (PEGDA) and poly(ethylene glycol) methyl ether acrylate (PEGMEA). The addition of PEGMEA in PEGDA increases the free volume and polymer chain flexibility. Polymer electrolytes based on PEGDA-co-PEGMEA and lithium perchlorate (LiClO 4 ) have been prepared and characterized using DSC, WAXD and electrochemical impedance spectroscopy (EIS). At PEGMEA content of 90% in the polymer with an O:Li ratio of 14, the conductivity approaches the current industrial targeted value (10 -5 S/cm), which is two orders of magnitude higher than that of pure PEO. This presentation will also compare the results with those based on the PEO-containing materials using the Vogel-Fulcher-Tammann (VFT) equation. The implication of the VFT model on the potential of PEO-based SPEs in terms of ionic conductivity will be elucidated.