High Transference Number of Li Ion in Highly Concentrated Lithium Bis(trifluoromethanesulfonyl)Amide/Dinitrile Liquid Electrolytes for Lithium Sulfur Batteries

摘要

Liquid electrolytes with high concentration of Li salts are receiving increasing attention owing to various unusual physicochemical and electrochemical properties. Recently, our group reported that Li~+ ion hopping conduction in highly concentrated solutions of lithium bis(fluorosulfonyl)amide (LiFSA) dissolved in dinitrile solvents, namely, succinonitrile (SN), glutaronitrile (GN), and adiponitrile (ADN), is involved in the ionic conduction. In the liquids with composition [LiFSA]/[dinitrile] > 1, the two cyano groups of the dinitrile coordinate to two different Li~+ ions and form solvent-bridged structures of Li~+-dinitrile-Li~+. In addition to the solvent-bridged structures, ionic aggregates (Li~+-FSA~--Li~+) are formed in these liquids. We report here the phase behaviors, solvate structures and transport properties of dinitrile-based highly concentrated electrolytes composed of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) with higher thermal and chemical stability. LiTFSA/SN and LiTFSA/ADN binary mixtures form stable solvates at the composition of LiTFSA-(Dinitrile)_(1.5) and keep solid state at room temperature. On the other hands, LiTFSA/GN mixtures become glass forming liquids and maintains liquid state in a wide range of composition. Raman spectra for the liquids of LiTFSA/GN revealed that the average Li~+ ion solvation number in the range of 1/8 ≤ [LiTFSA]/[GN] ≤ 1/3 is 3.1. In the highly concentrated solutions with compositions of [LiTFSA]/[GN] ≥ 1/3, the coordination of TFSA~- anion to Li~+ ion becomes remarkable due to the deficiency of GN and contact ion pairs and ionic aggregates are formed. The self-diffusion coefficients of Li~+, TFSA~-, and GN measured with pulsed field gradient (PFG) NMR suggested that Li~+ ion diffuses faster than the anion in the concentrated electrolytes. This suggests that Li~+ ion exchanges the anions in the ionic aggregates and moves forward leaving the anions. In other words, the ion-exchange conduction mode emerges in the high concentration region. We estimated the Li~+ transference number in an electrolyte of [LiTFSA]/[GN] = 1/1.5 using the Bruce-Vincent method~3 and found that the electrolyte possesses a high transference number of ca. 0.7. Finally, we applied the [LiTFSA]/[GN] = 1/1.5 electrolyte to a lithium sulfur cell. Thanks to the high transference number, the cell showed a relatively high rate capability, regardless of its low ionic conductivity (0.21 mS cm~(-1)) at room temperature.