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-blockpoly(ethylene oxide) (PS-b-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 r = 0.03−0.55. The numbers x and y in SEO(x−y) are the molecular weights of the blocks in kg mol −1 , and the r value is the molar ratio of salt to ethylene oxide moieties. Smallangle 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 ≤ r ≤ 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 (r = 0.21) as in PEO (r = 0.11). We propose that these observations are due to the effect of salt concentration on the grain structure in SEO electrolytes.

A significant limitation of rechargeable lithiumion batteries arises because most of the ionic current is carried by the anion, the ion that does not participate in energyproducing reactions. Single-ion-conducting block copolymer electrolytes, wherein all of the current is carried by the lithium cations, have the potential to dramatically improve battery performance. The relationship between ionic conductivity and morphology of single-ion-conducting poly(ethylene oxide)-bpolystyrenesulfonyllithium(trifluoromethylsulfonyl)imide (PEO−PSLiTFSI) diblock copolymers was studied by smallangle X-ray scattering and ac impedance spectroscopy. At low temperatures, an ordered lamellar phase is obtained, and the "mobile" lithium ions are trapped in the form of ionic clusters in the glassy polystyrene-rich microphase. An increase in temperature results in a thermodynamic transition to a disordered phase. Above this transition temperature, the lithium ions are released from the clusters, and ionic conductivity increases by several orders of magnitude. This morphology−conductivity relationship is very different from all previously published data on published electrolytes. The ability to design electrolytes wherein most of the current is carried by the lithium ions, to sequester them in nonconducting domains and release them when necessary, has the potential to enable new strategies for controlling the charge−discharge characteristics of rechargeable lithium batteries.