Structure and interactions of novel ether-functionalised morpholinium and piperidinium ionic liquids with lithium salts

Abstract

Ionic liquids with ether-functionalised cations have generated much interest as electrolytes for Li-ion batteries because of their intrinsic safety properties such as non-flammability, in addition to the advantageous fluidity and lithium coordination environment afforded by the ether group. Here, four ionic liquids, N-methoxymethyl-N-piperidinium bis(fluorosulfonyl)imide and bis(trifluoromethanesulfonyl)imide ([C_1O1mpip][FSI] and [C_1O1mpip][TFSI]) and N-methoxymethyl-N-methylmorpholinium FSI and TFSI ([C_1O1mmor][FSI] and [C_1O1mmor][TFSI]) were synthesised and compared to explore the suitability of these ionic liquids as battery electrolytes. Properties fundamentally relevant to electrolyte applications, such as thermal stability, density, viscosity and ionic conductivity and electrochemical stability were analysed to evaluate the effects caused by the presence of ether-functionality in the side chain and/or in the organic cation ring. Electrolyte solutions were prepared with lithium bis(fluorosulfonyl)imide (LiFSI) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in a ratio of 1 : 9 salt : IL (mol/mol). The electrolyte solutions were found to exhibit glass transition temperatures between −64 and −94 °C. (LiFSI)_0.1([C_1O1mpip][FSI])_0.9 exhibited the highest ionic conductivity of 5.0 × 10⁻³ S cm⁻¹ and fastest Li⁺ diffusion of 1.6 × 10⁻¹¹ m² s⁻¹ at 30 °C. The TFSI-based ionic liquids and their electrolyte solutions were analysed with Raman spectroscopy and the [C_1O1mmor][TFSI] exhibited a lower average solvation number of Li⁺ (1.49) than [C_1O1mpip][TFSI] (1.65) demonstrating the benefit of an oxygen in the ring.