Lithium-Ion Transport in Poly(ionic liquid) Diblock Copolymer Electrolytes: Impact of Salt Concentration and Cation and Anion Chemistry

Styrene-based poly(ionic liquid) (PIL) diblock copolymers and their analogous PIL homopolymers were synthesized in this study with various covalently attached cations (methylimidazolium (MIm+) and methylpyrrolidinium (MPyr+)) and counteranions (bis(trifluoromethanesulfonyl)imide (TFSI–) and bis(fluorosulfonyl)imide (FSI–)). Solid polymer electrolytes (SPEs) were prepared by mixing the polymer with the corresponding salts (Li+TFSI– and Li+FSI–) under various salt concentrations r = [Li+]/[PIL] (mol/mol) = 0.1–0.8. The impacts of lithium salt concentration and cation/anion chemistry were explored in regards to electrochemical, morphological, transport, and physical properties. The results show that the SPE with the MIm+/FSI– ion pair has the lowest PIL Tg (−7 °C), ca. 1–3 orders of magnitude higher conductivity compared to other SPEs as well as high electrochemical stability (lithium–metal stripping-plating). SPEs with the FSI– anion exhibit an ion-hopping-dominated transport mechanism and similar ion conductivities compared to their analogous PIL homopolymer SPEs at the same salt concentrations. The negative transference number of the SPE with the MIm+/TFSI– ion pair at a high salt concentration indicates the formation of larger anion-rich clusters and results in lower conductivity. This work reveals the impact of cation/anion chemistries on salt-doped PIL block polymers, which may enable new highly stable SPEs for lithium batteries.