The Effect of Miscibility Lengthscale on Ion Transport Behavior in Polymer-Mixture Electrolytes*

 

To tackle the tradeoff between mechanical strength and Li⁺ conductivity of the well-studied PEO, a rigid component is often added, by blending or copolymerization, to provide mechanical support. But, unlike small molecules, the degree of polymer miscibility can be ambiguous, and its effects on Li⁺ transport are unclear. Here, we studied two types of polymer mixtures (50:50 wt%) of PEO and PMMA: a polymer blend and a symmetric block copolymer PEO-b-PMMA, each doped with LiTFSI salt (r=0.05). Their Li⁺ transport and structural properties are compared between each other and to those of an unmixed PEO-LiTFSI electrolyte. We find that for the two mixtures, the critical lengthscale for miscibility is 5-10 Å. While the local Li⁺ solvation structures are the same (<5 Å), the presence of non-conducting PMMA near PEO causes differences in Li⁺ solvation environment beyond 6 Å. PMMA disrupts the network of viable solvation sites and separates them into in clusters. For that, we propose a quantitative model that considers Li⁺ transport within and across clusters. This newly acquired understanding about molecular packing, ion solvation behavior, and ion transport model in the context of polymer electrolyte mixtures should inform future design of nonhomogeneous polymer electrolyte systems.