On Replacing Poly(ethylene oxide) in Solid Block Copolymer Electrolytes by Poly(glycidyl methyl ether): Morphology and Ionic Conductivity

A new family of block copolymer electrolytes, where the "soft" block is synthesized via anionic ring opening copolymerization of ethylene oxide (EO) and glycidyl methyl ether (GME) and the "hard" block is glassy polystyrene (PS), overcomes many of the limitations of poly-(ethylene oxide) (PEO) for battery applications. Two block copolymer systems, PS-<i>b</i>-P-(EO-<i>co</i>-GME) with a GME content of 21% and PS-<i>b</i>-PGME containing a pure PGME block, were prepared, both with narrow dispersity (<i>Đ</i> = 1.03-1.15). All polyether blocks are structural isomers of PEO. Yet, in both structures, the polyether block is fully amorphous at all temperatures. When doped with LiN-(SO₂CF₃) (LiTFSI) at different ratios, the materials provide superior dc-conductivity values in comparison to the established dual ion conductors PS-<i>b</i>-PEO doped with LiTFSI or with LiCF₃SO₃ (LiTf). In addition, PS-<i>b</i>-PGME doped with (LiTFSI) has a higher conductivity (∼1 × 10^-5 S·cm^-1 at the PS glass temperature) than PS-<i>b</i>-P-(EO-<i>co</i>-GME) and a higher conductivity than the structurally similar single ion conductor polystyrene-<i>b</i>-poly-(ethylene oxide-<i>co</i>-(lithium trifluoromethane-sulfonamide)-ethyl glycidyl ether) (PS-<i>b</i>-P-(EO-<i>co</i>-LiTFSAEGE). PGME best combines favorable properties required for the design of the soft block in SPEs based on block copolymers: low liquid-to-glass temperature (<i>T</i> _g) nearly independent of molar mass, favorable molecular structure that can solubilize alkali metal salts, higher dielectric permittivity than PEO, and the absence of crystallization. These results suggest that PGME or PGME-containing polyether copolymers can replace PEO as the "soft" block in future SPEs.