Ion-conducting block copolymers (BCPs) have attracted significant interest as conducting materials in solidstate lithium batteries. BCP self-assembly offers promise for designing ordered materials with nanoscale domains. Such nanostructures provide a facile method for introducing sufficient mechanical stability into polymer electrolyte membranes, while maintaining the ionic conductivity at levels similar to corresponding solvent-free homopolymer electrolytes. This ability to simultaneously control conductivity and mechanical integrity provides opportunities for the fabrication of sturdy, yet easily processable, solid-state lithium batteries. In this review, we first introduce several fundamental studies of ion conduction in homopolymers for the understanding of ion transport in the conducting domain of BCP systems. Then, we summarize recent experimental studies of BCP electrolytes with respect to the effects of salt-doping and morphology on ionic conductivity. Finally, we present some remaining challenges for BCP electrolytes and highlight several important areas for future research.

The phase behavior of lithium salt-doped poly(styrene-b-ethylene oxide) (PS−PEO) was studied as a function of salt concentration, lithium (Li) counterion (X), and annealing temperature. Three LiX salts were employed in this work: LiClO4, LiCF3SO3, and LiAsF6. Increasing the salt doping ratio ([EO]:[Li]) from 48:1 to 3:1 led to morphological changes from hexagonally packed cylinders to lamellar morphologies; however, comparison between the small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) results shows that the location of these morphological changes are altered by the different counterions. We interpret our SAXS, TEM, and differential scanning calorimetry data using strong segregation theory to estimate an effective interaction parameter (χeff) for our salt-doped copolymers. This χeff varies linearly with salt concentration for a single PS-PEO:LiX material; however, the slope for each χeff,PS−PEO:LiX is influenced by the nature of the counterion. We rationalize these effects by examining the Lewis acidity of each anion.

Nanoscale self-assembly of block copolymer thin films has garnered significant research interest for nanotemplate design and membrane applications. To fulfill these roles, control of thin film morphology and orientation is critical. Solvent vapor annealing (SVA) treatments can be used to kinetically trap morphologies in thin films not achievable by traditional thermal treatments, but many variables affect the outcome of SVA, including solvent choice, total solvent concentration/swollen film thickness, and solvent removal rate. In this work, we systematically examined the effect of solvent removal rate on the final thin film morphology of a cylinder-forming ABA triblock copolymer. By kinetically trapping the film morphologies at key points during the solvent removal process and then using successive ultraviolet ozone (UVO) etching steps followed by atomic force microscopy (AFM) imaging to examine the through-film morphologies of the films, we determined that the mechanism for cylinder reorientation from substrate-parallel to substrate-perpendicular involved the propagation of changes at the free surface through the film toward the substrate as a front. The degree of reorientation increased with successively slower solvent removal rates. Furthermore, the AFM/UVO etching scheme permitted facile real-space analysis of the thin film internal structure in comparison to cross-sectional transmission electron microscopy.

We explored the relationship between ionic conductivities and morphology in a lithium perchlorate-doped poly(styrene-b-ethylene oxide) (PS−PEO) system using ac impedance spectroscopy, in situ small-angle X-ray scattering, and transmission electron microscopy. To aid in morphological analysis, a flow alignment technique was used to orient nanoscale domains in order to facilitate characterization of nanostructures such as hexagonally perforated lamellae (HPL), hexagonally packed cylinders (HEX), and lamellae (LAM). Over the PEO volume fraction (f PEO ) from 0.70 to 0.75, the neat PS−PEO exhibits a morphological transition from HPL to HEX, while the salt-doped PS−PEO shows morphological transitions from LAM to HPL to HEX. Additionally, experiments on hotpressed specimens show that 3-D conducting pathways (HPL and HEX) exhibit much higher normalized conductivities than the corresponding 2-D conducting pathway (LAM) even after accounting for nonrandom domain orientations and slight PEO molecular weight changes across samples. Our results further suggest that using block copolymer electrolytes with 3-D conducting pathways can prevent a decrease in through-plane conductivities caused by the partial nanostructure alignment during sample preparation.

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