Ionic liquids (ILs) with bis(trifluoromethylsulfonyl)amide (TFSA) anion show high ionic conductivity over 10-2 S cm-1 at room temperature and thermal stability over 400 °C. We have been investigating additives to improve the Li-ion conductivity in ILs. One of candidates is β-cyclodextirn (β-CD), which is known to form a host-guest complex by the inclusion of TFSA anion. In this study, we investigated the effect of β-CD on Li-ion transport properties of IL electrolytes. A hydrophobic IL, N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([P13][TFSA]), was synthesized and was mixed with a given amount of LiTFSA (IL : LiTFSA = 18 : 1 (mole ratio)). Acetylated β-cyclodextrin (Acβ-CD) in which the hydroxyl groups are substituted with acetyl group was used since β-CD was insoluble in the IL. After the addition of Acβ-CD to the IL/LiTFSA composites in the range of mole ratio from LiTFSA : Acβ-CD = 1 : 0.5 to 1 : 1.5. 19F NMR measurements were carried out to investigate the interaction between Acβ-CD and TFSA anion. The chemical shifts of the CF3 group of IL/LiTFSA/Acβ-CD (18 : 1 : x) (x = 0, 0.5, 1.0, and 1.5) composites were -80.00, -79.88, -79.80, and -79.73 ppm, respectively. In all IL/LiTFSA/Acβ-CD composites, the chemical shift of the CF3 group shifted to a higher magnetic field as compared with that of IL/LiTFSA, suggesting that Acβ-CD formed a complex with TFSA anion. The Li transference number (tLi+) values of IL/LiTFSA/Acβ-CD (18 : 1 : x) (x = 0, 0.5, 1.0, and 1.5) composites were 0.27, 0.17, 0.09 and 0.08 at 60 °C, respectively. The tLi+ values decreased with increasing the mole ratio of Acβ-CD. The oxidation potential of the IL/LiTFSA composite was 3.5 V vs Li/Li+, whereas the oxidation stability of the IL/LiTFSA/Acβ-CD was improved when the mole ratio of Acβ-CD was above 1.0.

Addition effects of cyclodextrin in ionic liquid electrolytes (VI): Analysis of lithium-ion transport property

256th National Meeting (2018)

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