(372e) Understanding Spatial Heterogeneities in Ion Transport in Composite Solid-Ion Conductors Based on Neutral Plastic Crystal–Polymer Hybrids

Heterogeneities intrinsic to composite solid-ion conductors (SICs) are thought to influence the solvation structure and dynamics for lithium ions (Li⁺) responsible for the ion current, as well as the diffusion paths available for ion transport. An understanding of the molecular origin of such heterogeneity and its impact on bulk Li⁺ transport is needed to intentionally design a SIC showcasing desirable properties for integration into a lithium metal battery. Here, we show that a lithium halide doped plastic crystal–polymer composite naturally forms a defect rich, high lithium mobility domain around the polymer backbone that is correlated with higher frequency of bond rotation within the plastic crystal. We show that a new class of SIC delivers high conductivity (1–5 mS cm^–1 at T = 20–25 ËšC), high transference number (t⁺_SS = 0.72) and naturally protects against overcharging in lithium metal battery. The key to our success is to engineer a defect rich domain through which the lithium ion diffusivity is order of magnitude higher than the bulk plastic crystal. Our choice of lithium halide also forms an electronically insulating interlayer in lithium metal battery allowing lithium to be reversibly plated at area specific resistance of 60 ohm cm² for >300 h. We anticipate that our work will reinvigorate the field of plastic crystal-based SICs as a new design principle for tailoring their transport and mechanical properties for use in solid-state batteries.