Two phosphonium ionic liquids with high Li⁺ transport number

Abstract

This work presents the physicochemical characterization of two ionic liquids (ILs) with small phosphonium cations, triethylpenthylphosphonium bis(trifluoromethanesulfonyl)imide ([P2225][Tf2N]) and (2-methoxyethyl)trimethylphosphonium bis(trifluoromethanesulfonyl)imide ([P222(201)][Tf2N]), and their mixtures with Li(+). Properties such as the electrochemical window, density, viscosity and ionic conductivity are presented. The diffusion coefficient was obtained using two different techniques, PGSE-NMR and Li electrodeposition with microelectrodes. In addition, the Li(+) transport number was calculated using the PGSE-NMR technique and an electrochemical approach. The use of these three techniques showed that the PGSE-NMR technique underestimates the diffusion coefficient for charged species. The Li(+) transport number was found to be as high as 0.54. Raman spectroscopy and molecular dynamics simulations were used to evaluate the short-range structure of the liquids. These experiments suggested that the interaction between the Li(+) and the Tf2N(-) anion is similar to that seen with other ILs containing the same anion. However, the MD simulations also showed that the Li(+) ions interact differently with the cation containing an alkyl ether chain. The results found in this work suggest that these Li(+) mixtures have promising potential to be applied as electrolytes in batteries.

Cation effect on small phosphonium based ionic liquid electrolytes with high concentrations of lithium salt

“…Quaternary phosphonium-based ionic liquids (ILs) and their solid state analogs, organic ionic plastic crystals (OIPCs), have been developing rapidly in recent years as a result of many excellent properties being discovered and applied in a number of fields, such as lubricants, phase transfer catalysts, electrochemical media, optical materials, and CO 2 capture, to name a few. [1][2][3][4][5][6][7][8] As a relatively new class of ILs, their application in electrochemical devices, including rechargeable lithium and sodium batteries, has received particular attention of late.…”

Ionic Liquids Containing Sulfonium Cations as Electrolytes for Electrochemical Double Layer Capacitors

“…15,16 The structure of these components and their degree of coordination have substantial influence over the physical properties of ILs, with the nature and relative sizes of the ions being known to strongly influence viscosity. 8,17,18 This in turn affects ionic conductivity, which is a crucial parameter in the design of EDLCs as it has a profound influence on the equivalent series resistance (ESR) of the cells. For example, it is known that ILs containing the bis(trifluoromethanesulfonyl) imide ([Tf 2 N]) anion exhibit relatively low viscosities and wide electrochemical stability windows (ESWs).…”

Ion dynamics in halogen-free phosphonium bis(salicylato)borate ionic liquid electrolytes for lithium-ion batteries

“…Martins et al have calculated the lithium transference number in phosphonium-based ionic liquids using both NMR and electrochemical methods and have found that the values calculated from the electrochemical method are 2-3 fold larger than the values calculated from the NMR method. 50 These authors have explained that NMR method underestimates the diffusion coefficient of charged species because it considers all the species and aggregates between lithium cation and the anion that are larger than the small charged Li + cation. Thus, their low diffusion coefficients decrease the overall diffusion coefficient and transference number of Li + cation.…”