Lithium Salt Distribution and Thermodynamics in Electrolytes Based on Short Perfluoropolyether-block-Poly(ethylene oxide) Copolymers

Abstract

Wide-angle X-ray scattering (WAXS) was used to study the distribution of salt in short-chain disordered block copolymer electrolytes with concentration fluctuations on the length scale of 1 nm. The electrolytes were binary mixtures of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt and a short block copolymer comprising perfluoroether (PFE) segments covalently bonded to ethylene oxide (EO) segments. We develop a method to analyze scattering data from mixtures of block copolymers and salt where salt distribution is not known beforehand, using a minimal number of adjustable parameters. The WAXS peak due to scattering from disordered concentration fluctuations is fit to a random phase approximation (RPA) model that includes a parameter to describe the partitioning of salt between the perfluoropolyether (PFPE)-rich and poly(ethylene oxide) (PEO)-rich concentration fluctuations. This method enables quantification of the salt distribution and the effective Flory–Huggins interaction parameter between polymer segments. We posit that attractive interactions between the TFSI– anion and PFPE (fluorous effect) drive some of the salt into the PFPE-rich fluctuations. On the other hand, the attractive interactions between Li+ and EO segments drive the remaining salt molecules into the PEO-rich fluctuations. We use WAXS to quantify LiTFSI partitioning between the PFE and EO segments in the block copolymer. The segregation between blocks, quantified by an effective Flory–Huggins interaction parameter between polymer segments, decreases with increasing salt concentration, behavior that is atypical for block copolymer electrolytes.