Entropic Penalty Governs Solvation Site Formation and Ion Transport Mechanisms in Mixed Polarity Copolymer Electrolytes

Recent solid polymer electrolyte (SPE) designs for Li⁺ conduction considers a synergistic polarity and mobility contrast to achieve improved conductivity. To assess such idea, we employ molecular dynamic (MD) simulations to compare Li⁺ solvation and transport behaviors in poly (oligoethylene methacrylate) (POEM) versus in its copolymers with poly(glycerol carbonate methacrylate) (PGCMA). Unexpectedly, Li⁺ is not solvated by the highly polar PGCMA, but by ether oxygens (EO) instead due to a lower entropic penalty. The presence of PGCMA promotes single-chain solvation and thereby suppresses inter-chain Li⁺ hopping. The conductivity difference between PGCMA-r-POEM and the less-mixed PGCMA-b-POEM is explained in terms of hybrid solvation site formation at varied degrees of local composition fluctuation. This work elaborates on how thermodynamic driving forces govern solvation site formation and Li⁺ transport in mixed SPEs containing microscopic interfaces.