A series of novel branched copolyethers, poly((1,3-dioxIolane)-co-((2-(2-methoxylethoxyl)methyl)oxirane)) (P(DXL-co-MEMO)) and poly( (1,3-dioxIolane)-co-( (2-(2-(2-methoxylethoxyl)ethoxyl)methyl)oxirane)) (P(DXL-co-ME2MO)), were synthesized via cationic polymerization, and the structures of these copolymers were characterized by H-1 NMR and GPC. Ionic conductivities of copolyether/lithium bis(trifluoromethanesulphonyl) imide (LiTFSI), polyethylene oxide (PEO)/LiTFSI and PEO/copolyether/LiTFSI were investigated via electrochemical impedance spectroscopy. The results showed that the ionic conductivity of the P(DXL-co-MEMO)/LiTFSI polymer electrolyte was higher than that of the P(DXL-co-ME2MO)/LiTFSI polymer electrolyte, and the room temperature ionic conductivity of the P(DXL-co-MEMO)/LiTFSI polymer electrolyte was up to 2.2 x 10(-4) S cm(-1). Compared with the PEO/LiTFSI polymer electrolyte, the ionic conductivities of the PEO/copolyether/LiTFSI composite polymer electrolytes had a great improvement, but their thermal stabilities did not obviously reduce. (C) 2014 Elsevier B.V. All rights reserved.