Australian Journal of Chemistry | 2019
Liquid Structures and Transport Properties of Lithium Bis(fluorosulfonyl)amide/Glyme Solvate Ionic Liquids for Lithium Batteries
Abstract
The liquid structures and transport properties of electrolytes composed of lithium bis(fluorosulfonyl)amide (Li[FSA]) and glyme (triglyme (G3) or tetraglyme (G4)) were investigated. Raman spectroscopy indicated that the 1\u2009:\u20091 mixtures of Li[FSA] and glyme (G3 or G4) are solvate ionic liquids (SILs) comprising a cationic [Li(glyme)]+ complex and the [FSA]− anion. In Li[FSA]-excess liquids with Li[FSA]/glyme molar ratios greater than 1, anionic Lix[FSA]y(y\u2009–\u2009x)– complexes were formed in addition to the cationic [Li(glyme)]+ complex. Pulsed field gradient NMR measurements revealed that the self-diffusion coefficients of Li+ (DLi) and glyme (Dglyme) are identical in the Li[FSA]/glyme\u2009=\u20091 liquid, suggesting that Li+ and glyme diffuse together and that a long-lived cationic [Li(glyme)]+ complex is formed in the SIL. The ratio of the self-diffusion coefficients of [FSA]− and Li+, DFSA/DLi, was essentially constant at ~1.1–1.3 in the Li[FSA]/glyme\u2009 \u20091 liquid, indicating that the ion transport mechanism in the electrolyte changed at the composition of Li[FSA]/glyme\u2009=\u20091. The oxidative stability of the electrolytes was enhanced as the Li[FSA] concentration increased. Furthermore, Al corrosion was suppressed in the electrolytes for which Li[FSA]/glyme\u2009>\u20091. A battery consisting of a Li metal anode, a LiNi1/3Mn1/3Co1/3O2 cathode, and Li[FSA]/G3\u2009=\u20092 electrolyte exhibited a discharge capacity of 105\u2009mA\u2009h\u2009g−1 at a current density of 1.3\u2009mA\u2009cm−2, regardless of its low ionic conductivity of 0.2\u2009mS\u2009cm−1.