Quantifying Lithium Salt And Polymer Density Distributions In Nanostructured Ion-Conducting Block Polymers

Block polymer (BP) electrolytes offer significant advantages relative to existing liquid or polymer electrolytes due to their independently tunable ion transport and mechanical stability properties as a result of nanoscale self-assembly. Many of these nanostructured electrolytes are composed of a BP that is doped with a lithium salt to impart conductivity but which also alters the self-assembly (structure and thermodynamics) in comparison to the neat BP. By elucidating the effects of lithium salt concentration and counterion chemistry on the relevant salt and polymer density distributions, BP electrolytes with more efficient conductivity pathways can be developed. In this work, neutron and X-ray reflectometry (NR and XRR, respectively) were harnessed to determine the spatial distribution of salt and polymer in lamellae-forming polystyrene-block-poly(oligo-oxyethylene methacrylate) [PS-b POEM] films doped with various lithium salts. From the NR results, the distribution of lithium salts across domains appeared to match that of the POEM in the BP electrolyte for all salts tested. This finding of a salt distribution that was directly proportional to the POEM density profile facilitated quantitative analysis of polymer and salt XRR profiles using a strong-segregation theory framework. Through this approach, effective Flory Huggins interaction parameters (x(eff))s were deconvoluted from POEM statistical segment lengths (b(POEM))s. For all salts tested, chi(eff) increased at low salt concentrations and then plateaued at higher salt concentrations, while b(POEM), increased linearly across all salt concentrations. These findings can be leveraged to advance the next generation of salt-doped BP electrolyte materials that enhance the performance and mechanical stability of lithium-ion batteries.