High Li⁺ Conduction and Selectivity in Polymeric Ionic Liquid Electrolyte Complexes

Polymer electrolyte complexes (PECs) formed by blending oppositely charged polymeric ionic liquids (PILs) have been predicted to form electrostatically stabilized structures on the nanometer scale. We demonstrate that since solvent-free PECs can dissolve very high quantities of salt, they possess high ionic conductivity.  In this work, imidazoliums-tethered polysiloxane (polycations) and bis(trisfluoromethanesulfonyl)imides (TFSI)-tethered polyacrylate (polyanions) with varying charge densities are blended to form PECs.  PECs with an ionic charge on every monomer (100% charge density) are homogeneous due to the strong interpolymeric electrostatic interactions, solvate up to 50 wt% salt, and demonstrate high ionic conductivity (2×10⁻³ S/cm) and Li⁺ transference number (t₊ = 0.6) at 90 °C.  PECs with 10% charge density demonstrate a local correlation length of ca. 5 nm, while the addition of LiTFSI salt alters the structure to increase the correlation length by screening ionic interactions. This work guides our understanding of the structure and internal interactions of solvent-free charge-containing polymer complexes and the experimental design of such polyelectrolyte systems.