Correlations between Salt-Induced Crystallization, Morphology, Segmental Dynamics, and Conductivity in Amorphous Block Copolymer Electrolytes
Published Web Locationhttps://doi.org/10.1021/acs.macromol.7b02415
Block copolymer electrolytes have been shown to increase the cycle life of rechargeable batteries that utilize high capacity lithium anodes. Most block copolymer electrolyte studies have been centered on polystyrene-b-poly(ethylene oxide) (SEO) mixed with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt and are limited to salt concentrations in the vicinity of r[Li]/[EO] = 0.1, where [Li] and [EO] are the concentration of lithium and ethylene oxide moieties, respectively, as the conductivity of poly(ethylene oxide) (PEO) homopolymer electrolytes is maximized at this concentration. In this work, we study the morphology and conductivity of electrolytes derived from a high molecular weight SEO block copolymer over the wide LiTFSI concentration range of 0 ≤ r ≤ 0.550. For electrolytes with r ≥ 0.125, the crystallization of PEO-LiTFSI complexes with stoichiometric ratios of 6:1, 3:1, or 2:1 (EO:Li) is shown to correlate with morphology as determined by small-angle X-ray scattering (SAXS) and scanning transmission electron microscopy (STEM): some samples that are semicrystalline at room temperature exhibit regions of periodic lamellar ordering, whereas those that do not crystallize have aperiodic packed-ellipsoid morphologies. SAXS profiles below and above the melting temperatures of the crystals are identical, indicating that the crystals are confined within the block copolymer domains. The conductivities of SEO/LiTFSI mixtures with r = 0.275, r = 0.300, and r = 0.350 are within experimental error of PEO/LiTFSI at the same salt concentrations, in spite of the presence of insulating polystyrene (PS) domains in the SEO/LiTFSI samples. We attribute this to enhanced segmental dynamics of the PEO chains in the SEO electrolytes, based on differential scanning calorimetry (DSC) measurements of glass transition temperatures. The dependence of ionic conductivity on salt concentration in the SEO/LiTFSI electrolytes at temperatures above the melting temperature of the crystalline complexes shows three local maxima that correlate with the formation of crystalline complexes when r ≥ 0.125. These maxima arise due to a combination of the enhanced segmental dynamics and changes in morphology observed by SAXS and STEM.