Piperidinium-Based Ionic Liquids as an Electrolyte Solvent for Li-Ion Batteries: Effect of Number and Position of Oxygen Atom in Cation Side Chain on Electrolyte Property

Abstract

In the pursuit of developing reliable Li-ion batteries with high performance and safety, the use of ionic liquids is one of the most promising approaches due to its high thermal stability. There is, however, the critical issue concerning the significantly low Li-ion diffusion in an electrolyte solution, which is directly linked to the degradation of their rapid charge−discharge properties. Ionic liquids with ether-substituted cation have generated much interest as electrolyte solvents for Li-ion batteries because the ether group functions to reduce the coordination number and increase ionic conductivity. In the present study, we synthesized six types of ionic liquids consisting of bis(trifluoromethanesulfonyl)amide (TFSA) anions and piperidinium-based cations in which the attached side chains are 1-((2-methoxyethoxy)methyl)-1-methylpiperidinium (PP1MEM), 1-butoxymethyl-1-methylpiperidinium (PP1BM), 1-methyl-(2-propoxyethyl)piperidinium (PP1PE), 1-(3-ethoxypropyl)-1-methylpiperidinium (PP1EP), 1-(4-methoxybutyl)-1-methylpiperidinium (PP1MB), and 1-hexyl-1-methylpiperidinium (PP16). It was found that the ionic liquid with PP1MEM cations with two oxygen atoms in the side chain exhibited the lowest solvation number of TFSA anions (1.56), whereas PP1BM (1.78), PP1PE (1.86), PP1EP (2.08), and PP1MB-TFSA (2.07) resulted in a larger number, as the oxygen atoms in the alkyl side chain are located far from the positively charged nitrogen atoms. PP1MEM-TFSA with small solvation numbers enhanced the ionic conductivity and thereby achieved high electrode performance.