  Solid-ion conductors (SICs) comprising non-ionic plastic crystals and lithium
salts often require compositing with polymers to render them processable for
use in solid-state lithium-metal batteries. Here, we show that polymer-doped
plastic crystal SICs form a previously unrecognized plastic crystal-polymer
high entropy interphase, where ions selectively partition and exhibit a higher
fraction of matrix-separated ion pairs than in the bulk. Liithium ion
diffusivity in this interphase is an order of magnitude higher than in other
microenvironments due to an increase in the molar volume of the plastic crystal
in the vicinity of the polymer, which increases the frequency of bond rotation
in the plastic crystal required for ion conduction. Surprisingly, the polymer
does not directly participate in ion transport. These insights led us to
prepare SICs from specific polymers, plastic crystals, and lithium halide salts
that concomitantly deliver fast ion conduction at ambient and sub-ambient
temperatures and sustainable passivation of the lithium anode.
