TiO2 Nanotubes/Ionic Liquid Electrolyte System Enhances Li-Ion Battery Performance

High capacity advanced lithium ion batteries (LIBs) utilizes non-volatile ionic liquid (IL) electrolyte is considered a  promising candidate to address the safety and durability issues of traditional LIBs. Compared to organic-based electrolyte, however, IL has lower ionic conductivity at almost temperature range and shows less effective contact with the electrodes in absence of organic solvent, thus created barrier of lithium transfer at the electrolyte-electrode interface, which in turn reduce the cell performance ¹. Current study is focused on the design of composite ionic liquid electrolyte which gives higher ionic conductivity coupled with the improved of interface stability. Highly conductive electrolyte could be achieved through optimizing synergistic function between ionic liquids, salt, and the PVdF substrate. By suitable composition of lithium-bis-(trifluoromethyl sulfonyl)imide (Li-TFSI) and 1-butyl-3-methyl imidazolium bis-(trifluoromethyl sulfonyl)imide (BMI-TFSI) ionic liquids in poly-(vyniliden fluoride-hexafluoropropylene) (PVDF-HFP) co-polymeric host material,  optimized membrane morphology is achieved through phase inversion technique during membrane casting, where an unique pore structure is formed.  High ionic conductivity in the order of 10^-3 Scm^-2 is observed in PVDF-HFP: BMI-TFSI: Li-TFSI (10:10:2, w/w) electrolyte system, which is found to be higher than that of commercial membrane with liquid electrolyte. . Furthermore, the addition of small amount of TiO₂ nanotubes (TiNT) incorporated onto the ionic liquid electrolyte system further raises the ionic conductivity up to ~1.5 x 10^-3 Scm^-2 (25 ^oC) and ~8.0 x 10^-3 Scm^-2 (80 ^oC), which is attributed both to higher lithium salt dissociation in presence of TiO₂ and the establishment of more direct ion conducting pathway ². Further addition of TiNT, however, leads to reduction of ionic conductivity, which drops significantly after 5 wt% (relative to the solvent) of TiNT addition, which is probably the result of inorganic self-aggregation. Wider thermal decomposition around ~400 ^oC from TGA and DSC shows a remarkable improvement in thermal stability of membrane containing TiNT. Battery performance in a Li/electrolyte-TiNT/LiFePO₄ coin cells indicates relatively high discharge specific capacity around ~136 mAh/g at 0.05 C moderate electrochemical cycling and shows better cycling performance with almost constant ~ 99% coulombic efficiency as a result of improved electrolyte interface stability to the electrode.

References:

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