Highly-branched cross-linked poly(ethylene oxide) with enhanced ionic conductivity

Polymers containing poly(ethylene oxide) (PEO) have been investigated for solid electrolytes for lithium-ion batteries, which are preferred to be amorphous and have flexible polymer chains to achieve high Li ion conductivity. This paper reports ionic conductivity in a series of cross-linked PEOs prepared from poly(ethylene glycol) methyl ether acrylate (PEGMEA, monomer) and poly(ethylene glycol) diacrylate (PEGDA, cross-linker) via photopolymerization. The resulting copolymers contain short PEO side chains and thus high content of amorphous PEO. While the cross-linking enhances mechanical strength for solid electrolytes, the introduction of PEGMEA with methoxyl chain end groups decreases T_g and increases ionic conductivity. The use of acrylate groups for polymerization, instead of methacrylate groups, also yields low T_g and thus high conductivity. The effect of PEGMEA content, salt content, and temperature on the ionic conductivity was thoroughly investigated, and it was satisfactorily described using the Vogel-Fulcher-Tammann (VFT) equation. The effectiveness of PEO-based polymers used for solid electrolytes in advanced lithium batteries was also elucidated based on the VFT equation.