Entropic Penalty Switches Li Solvation Site Formation and Transport Mechanisms in Mixed Polarity Copolymer Electrolytes.

Emerging solid polymer electrolyte (SPE) designs for efficient Li-ion (Li) conduction have relied on polarity and mobility contrast to improve conductivity. To further develop this concept, we employ simulations to examine Li solvation and transport in poly(oligo ethylene methacrylate) (POEM) and its copolymers with poly(glycerol carbonate methacrylate) (PGCMA). We find that Li is solvated by ether oxygens instead of the highly polar PGCMA, due to lower entropic penalties. The presence of PGCMA promotes single-chain solvation, thereby suppressing interchain Li hopping. The conductivity difference between random copolymer PGCMA--POEM and block copolymer PGCMA--POEM is explained in terms of a hybrid solvation site mechanism. With diffuse microscopic interfaces between domains, PGCMA near the POEM contributes to Li transport by forming hybrid solvation sites. The formation of such sites is hindered when PGCMA is locally concentrated. These findings help explain how thermodynamic driving forces govern Li solvation and transport in mixed SPEs.