High ionic conductivity of polyether-based network polymer electrolytes with hyperbranched side chains

To achieve solvent-free polymer electrolytes with high ionic conductivity, network polymer electrolytes with hyperbranched ether side chains were synthesized. A monosubstituted epoxide monomer, 2-(2-methoxyethoxy)ethyl glycidyl ether (MEEGE), was copolymerized with ethylene oxide (EO) in the presence of 2-(2-methoxyethoxy)ethanol by base-catalyzed anionic ring-opening polymerization to semiterechelic poly[ethylene oxide-co-2-(2-methoxyethoxy)ethyl glycidyl ether] [P(EO/MEEGE)] oligomers with a hydroxyl terminal functional group, which was esterified with acrylic acid to a polyether macromonomer. Network polymer electrolytes were obtained by photo-cross-linking mixtures of the macromonomer, an electrolyte salt, and a photoinitiator. These polymer electrolytes consist of polyether networks with hyperbranched side chains and an electrolyte salt. The ionic conductivity changed with the molecular weight of the macromonomers and was largest when the macromonomer with molecular weight of ca. 1000 was used, although the glass transition temperature of the polymer electrolytes was nearly constant. The highest conductivity of 1 x 10(-4) S cm(-1) at 30 degrees C, 1 x 10(-3) S cm(-1) at 80 degrees C, was obtained with lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) as the electrolyte salt. An electrochemically stable potential window of the network polymer electrolytes was obtained by the microelectrode technique.