Star Poly(ethylene oxide) as a Low Temperature Electrolyte and Crystallization Inhibitor

Abstract

A significant problem with electrolytes for lithium ion and lithium metal batteries is poor low temperature performance due to electrolyte crystallization. Highly branched polyethers are intrinsically resistant to crystallization because of their inability to pack efficiently, and, thus, they should have low melting temperatures and improved low temperature conductivities. In this report, we describe the characterization of tetrakis(2-methoxy(2-ethoxy(2-ethoxy)))pentaerythritol, a four-armed polyether star, which has a glass transition of −93 °C and no evidence of a melting transition. Electrolytes prepared with LiClO4 have conductivities of 6 × 10-5 S/cm at 0 °C and 3.7 × 10-7 S/cm at −40 °C, establishing the star architecture as an interesting electrolyte for low temperature applications. In contrast, electrolytes based on poly(ethylene glycol) dimethyl ether 500, a linear polymer of approximately the same molecular weight, crystallize at ∼14 °C, resulting in a drastic drop in conductivity to ∼10-7 S/cm at 0 °C. From a limited investigation of the use of star-shaped molecules to inhibit crystallization of poly(ethylene oxide), we conclude that crystallization of linear poly(ethylene oxide) drives phase separation of the linear and branched polyethers, limiting application of branched molecules as crystallization inhibitors.