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 T-g (-7 degrees 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(+)/FSI- 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.

Automated summary

In this study, styrene-based poly(ionic liquid) (PIL) diblock copolymers and their analogous PIL homopolymers were synthesized with different covalently attached cations and counteranions. Solid polymer electrolytes (SPEs) were prepared by mixing the polymer with corresponding salts under various salt concentrations, and the impacts of lithium salt concentration and cation/anion chemistry on electrochemical, morphological, transport, and physical properties were explored. The results indicate that SPEs with the MIm(+)/FSI- ion pair exhibit the highest conductivity and electrochemical stability compared to other SPEs.