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

Abstract

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 (σ → 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 SiO_1.5 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 gels.