Effect of Grain Size on the Ionic Conductivity of a Block Copolymer Electrolyte

A systematic study of the dependence of ionic conductivity on the grain size of a lamellar block copolymer electrolyte was performed. A freeze-dried mixture of poly­(styrene)-<i>block</i>-poly­(ethylene oxide) and lithium bis­(trifluoromethylsulfonyl)­imide salt was heated in steps from 29 to 116 °C and then cooled back to 29 °C with an annealing time ranging from 30 to 60 min at each temperature. Grain structure and ionic conductivity during these steps were quantified by <i>in situ</i> small-angle X-ray scattering and ac impedance spectroscopy, respectively. Conductivity depends both on grain structure and temperature. A normalization scheme to decouple the dependence of conductivity on temperature and grain structure is described. Ionic conductivity at a given temperature was found to decrease by a factor of 5.2 ± 0.9 as the SAXS measure of grain size increased from 13 to 88 nm. The fact that in the system studied, large, well-formed lamellar grains are less conducting than poorly defined, small grains suggests a new approach for optimizing the transport properties of block copolymer electrolytes. Further work is necessary to confirm the generality of this finding

Structure and Ionic Conductivity of Polystyrene-<i>block</i>-poly(ethylene oxide) Electrolytes in the High Salt Concentration Limit

We explore the relationship between the morphology and ionic conductivity of block copolymer electrolytes over a wide range of salt concentrations for the system polystyrene-<i>block</i>-poly­(ethylene oxide) (PS-<i>b</i>-PEO, SEO) mixed with lithium bis­(trifluoro­methane­sulfonyl)­imide salt (LiTFSI). Two SEO polymers were studied, SEO(16–16) and SEO(4.9–5.5), over the salt concentration range <i>r</i> = 0.03–0.55. The numbers <i>x</i> and <i>y</i> in SEO­(<i>x</i>–<i>y</i>) are the molecular weights of the blocks in kg mol^–1, and the <i>r</i> value is the molar ratio of salt to ethylene oxide moieties. Small-angle X-ray scattering was used to characterize morphology and grain size at 120 °C, differential scanning calorimetry was used to study the crystallinity and the glass transition temperature of the PEO-rich microphase, and ac impedance spectroscopy was used to measure ionic conductivity as a function of temperature. The most surprising observation of our study is that ionic conductivity in the concentration regime 0.11 ≤ <i>r</i> ≤ 0.21 increases in SEO electrolytes but decreases in PEO electrolytes. The maximum in ionic conductivity with salt concentration occurs at about twice the salt concentration in SEO (<i>r</i> = 0.21) as in PEO (<i>r</i> = 0.11). We propose that these observations are due to the effect of salt concentration on the grain structure in SEO electrolytes

Phase Behavior and Electrochemical Characterization of Blends of Perfluoropolyether, Poly(ethylene glycol), and a Lithium Salt

Electrolytes consisting of low molecular weight perfluoropolyether (PFPE), poly­(ethylene glycol) (PEG), and lithium bis­(trifluoromethanesulfonyl)­imide (LiTFSI) blends were prepared and systematically studied for salt concentration and stoichiometry effects on the materials’ thermal and electrochemical properties. Herein we report that the tunable ratios of PFPE and PEG allow for precise control of crystalline melting and glass transition temperature properties. These blended liquid polymer electrolytes are inherently nonflammable and remain stable in the amorphous phase from approximately 150 °C down to −85 °C. The ionic conductivity of the electrolytes are on the order of 10^–4 S/cm at 30 °C, which makes them suitable for rechargeable lithium batteries