Quantifying Lithium Salt and Polymer Density Distributions in Ion-Conducting Block Polymers

Block polymer (BP) electrolytes offer an enhanced balance of performance and stability for next-generation lithium-ion batteries, but to develop these technologies further, it is essential to understand both the overall self-assembly behavior and the distinct salt and polymer distributions within the specific nanoscale domains. To this end, we have quantitatively examined the lithium salt and polymer distributions in salt-doped polystyrene-b-poly(oligo-oxyethylene methacrylate) [PS-POEM] BPs and explored how the presence of salt affects BP chain conformations and thermodynamics. With respect to the lithium salt distribution, neutron reflectometry was leveraged to infer the salt distributions in POEM domains for a series of lithium salts. All salt-doped BP specimens exhibited lithium salt distributions that were strongly correlated with the POEM density profiles. Furthermore, using X-ray reflectometry in combination with strong-segregation theory, we estimated effective Flory-Huggins interaction parameters (χ_eff) and the POEM statistical segment lengths (b_POEM) as a function of salt concentration and rationalized the noted trends in χ_eff and b_POEM in terms of lithium counterion basicity.