Effect of Ion Distribution on Conductivity of Block Copolymer Electrolytes

TLDR

The electrolytes examined are mixtures of bis(trifluoromethane)sulfonimide lithium salt and symmetric poly(styrene‑block‑ethylene oxide) copolymers (SEO). Energy‑filtered transmission electron microscopy mapped lithium ion distribution, and self‑consistent field theory calculations of local stress fields in the block copolymer microdomains explain how ion localization changes with copolymer molecular weight. SEO/salt mixtures exhibit higher conductivity at higher copolymer molecular weight, with lithium ions increasingly confined to the center of PEO lamellae and stress‑induced interference reducing chain coordination near channel walls.

Abstract

Energy-filtered transmission electron microscopy (EFTEM) was used to determine the distribution of lithium ions in solid polymer electrolytes for lithium batteries. The electrolytes of interest are mixtures of bis(trifluoromethane)sulfonimide lithium salt and symmetric poly(styrene-block-ethylene oxide) copolymers (SEO). In contrast to current solid and liquid electrolytes, the conductivity of SEO/salt mixtures increases with increasing molecular weight of the copolymers. EFTEM results show that the salt is increasingly localized in the middle of the poly(ethylene oxide) (PEO) lamellae as the molecular weight of the copolymers is increased. Calculations of the inhomogeneous local stress field in block copolymer microdomains, modeled using self-consistent field theory, provide a quantitative explanation for this observation. These stresses, which increase with increasing molecular weight, interfere with the ability of PEO chains to coordinate with lithium cations near the walls of the PEO channels where ion mobility is expected to be low.