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

By Chinnam, Parameswara Rao; Chatare, Vijay; Chereddy, Sumanth; Mantravadi, Ramya; Gau, Michael; Schwab, Joe; Wunder, Stephanie L.
Published in J. Mater. Chem. A The Royal Society of Chemistry 2016

Abstract

Solid ion-gel separators for lithium or lithium ion batteries have been prepared with high lithium ion transference numbers (tLi+ = 0.36), high room temperature ionic conductivities ([sigma] [rightward arrow] 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 tLi+ 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 tLi+ 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 (Tg), fibrillar structure of MC in the ion gels.

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