Multi-ionic lithium salts increase lithium ion transference numbers in ionic liquid gel separators

Solid ion-gel separators for lithium or lithium ion batteries have been prepared with high lithium ion transference numbers (t(Li+) = 0.36), high room temperature ionic conductivities (sigma -> 10(-3) S cm(-1)), and moduli in the MPa range. These were formed from the room temperature ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide (PYR14TFSI), a polysilsesquioxane multi-ionic lithium salt that contains four phenyl groups and four pendent LiTFSI (4mer-(LiTFSI)(4)) on a SiO1.5 ring, and methyl cellulose (MC). After co-dissolution in dimethyl sulfoxide (DMSO) and evaporation of the DMSO, ion-gels were formed at different PYR14TFSI/MC ratios but at constant 0.20 M 4mer-(LiTFSI)(4). Conductivity decreased but t(Li+) increased with increased MC content. Differential scanning calorimetry, dynamical mechanical analysis and X-ray diffraction data indicated that the PYR14TFSI/MC/4mer-(LiTFSI)(4) ion gels were micro-phase separated into a conductive PYR14TFSI/4mer-(LiTFSI)(4) phase and one in which the MC was swollen with PYR14TFSI/4mer-(LiTFSI)(4). The high t(Li+) was attributed to the large size of the anion, its decreased ability as the result of its rigid structure to form ion aggregates, hydrophobic/ hydrogen bonding interactions of the 4mer-(TFSI-)(4) anion with MC, and participation of the MC hydroxyl groups in the solvation sphere of Li+, weakening its interaction with the 4mer-(TFSI-)(4) and TFSI- anions. The high moduli were the result of the preserved semi-crystalline, high glass transition (T-g), fibrillar structure of MC in the ion