How the PEG terminals affect the electrochemical properties of polymer electrolytes in lithium metal batteries

Poly(ethylene oxide) (PEO) or poly(ethylene glycol) (PEG) based electrolytes for lithium batteries have arouse great interest, however their application is still hindered by low ion conductivity (σ) at room temperature, narrow electrochemical stability window (ESW) and low lithium ion transference number (LTN). Herein, we report the synthesis and comparative investigation of four kinds of methoxy poly(ethylene glycol) (MPEG) based electrolytes with different terminals, including MPEG with a hydroxyl terminal (MPEG-OH), a propyl carbonate terminal (MPEG-PC), a methoxy terminal (MPEG–OCH3) and a trifluoroethoxy terminal (MPEG-3F). It is found that the terminals in MPEG electrolytes exert great influences on the physical and electrochemical properties, e.g., crystallization, viscosity, dissociation capacity of Li salts, chemical environment of fluorine, σ, ESW, LTN and the cycling performance of lithium metal batteries. The -OH terminal is reactive with less stability, while -PC and –OCH3 result in moderate ionic transport. In comparison, MPEG-3F containing a trifluoroethoxy terminal affords high σ because of the good dissociation of LiTFSI, high LTN because of encapsulating the TFSI anions, and wide ESW due to high-voltage-resistant fluorocarbon.