Single-ion electrolytes are of considerable interest as electrolytes for battery systems, as they hold the key for the realization of safe, long-lasting, high-energy batteries. Here, we study the relationship between the ion conductivity and mechanical modulus of single-ion-conducting polyanion miktoarm star copolymers composed of poly(styrene-4-sulfonyltrifluoromethylsulfonyl) imide lithium (PSTFSILi) arms that are a complement to longer ion-conducting poly(ethylene oxide) (PEO) arms, (PSTFSILi) n (PEO) n , where n ≈ 22, attached to a poly-(divinylbenzene) (PDVB) cross-linked core. The degree of polymerization of the PEO arms was kept fixed at 105, while that of PSTFSILi was systematically varied from 5 to 18 approximately, resulting in molar ratios, r = [Li + ]/[EO], from 0.048 to 0.171, respectively. We show that, due to the fact that these macromolecular systems may be envisioned as core/shell nanoparticles (with the core region composed of PSTFSILi and PEO segments while the shell is composed of the longer PEO arms), their ion conductivity may be simply rationalized in terms of their differences in volume fractions of PSTFSI and PEO and corresponding changes in segmental dynamics due to the degree and strength of the Li + /EO complexation. Notably, due to the macromolecular architecture, the rheological measurements reveal a solid-like behavior of the single-ion electrolytes with a shear modulus G′ ≈ 1 GPa for the (PSTFSILi) n (PEO) n with the largest volume fraction of PSTFSILi (0.42). Our data reveal that these systems possess a significantly better trade-off between mechanical response and ion conductivity compared to the linear polyanionic block copolymer analogues. Our macromolecular design approach offers a tremendous potential for the design of high-performance single-ion solid polymer electrolytes.

Nanoparticles are commonly added to polymer electrolytes to enhance both their mechanical and ion transport properties. Previous work reports significant increases in the ionic conductivity and Li-ion transference in nanocomposite electrolytes with inert, ceramic fillers. The mechanistic understanding of this property enhancement, however, assumes nanoparticle dispersion states�namely, well-dispersed or percolating aggregates�that are seldom quantified using small-angle scattering. In this work, we carefully control the inter-silica nanoparticle structure (where each NP has a diameter D = 14 nm) in a model polymer electrolyte system (PEO:LiTFSI). We find that hydrophobically modified silica NPs are stabilized against aggregation in an organic solvent by inter-NP electrostatic repulsion. Favorable NP surface chemistry and a strongly negative zeta potential promote compatibility with PEO and the resulting electrolyte. Upon prolonged thermal annealing, the nanocomposite electrolytes display structure factors with characteristic interparticle spacings determined by particle volume fraction. Thermal annealing and particle structuring yield significant increases in the storage modulus, G′, at 90 °C for the PEO/NP mixtures. We measure the dielectric spectra and blocking-electrode (κ b ) conductivities from −100 to 100 °C, and the Li + current fraction (ρ Li + ) in symmetric Li-metal cells at 90 °C. We find that nanoparticles monotonically decrease the bulk ionic conductivity of PEO:LiTFSI at a rate faster than Maxwell's prediction for transport in composite media, while ρ Li + does not significantly change as a function of particle loading. Thus, when nanoparticle dispersion is controlled in polymer electrolytes, Li + conductivity monotonically, i.e., (κ b ρ Li + ), decreases but favorable mechanical properties are realized. These results imply that percolating aggregates of ceramic surfaces, as opposed to physically separated particles, probably are required to achieve increases in bulk, ionic conductivity.

Over the last year, the synthesis of polyanion copolymers has attracted considerable attention for their utilization in single-ion electrolytes in lithium battery applications. Morphology has a significant impact on the...