Effect of mixed anions on the transport properties and performance of an ionic liquid-based electrolyte for lithium-ion batteries

Abstract

In this work, the physical, transport and electrochemical properties of various electrolytic solutions containing the 1-propyl-1-methylpyrrolidinium bis[fluorosulfonyl]imide ([C3C1pyr][FSI]) mixed with the lithium bis[(trifluoromethyl)sulfonyl]imide (Li[TFSI]) over a wide range of compositions are reported as a function of temperature at atmospheric pressure. First, the ionicity, lithium transference number, and transport properties (viscosity and conductivity) as well as the volumetric properties (density and molar volume) were determined as a function of lithium salt concentration from 293 to 343 K. Second, the self-diffusion coefficient of each ion in solution was measured by nuclear magnetic resonance (NMR) spectroscopy with pulsed field gradients (PFG). Moreover, an analysis of the collected nuclear Overhauser effect (NOE) data along with ab initio and COSMO-RS calculations was conducted to depict intra and intermolecular neighbouring within the electrolytic mixtures. Based on this analysis, and as expected, all activation energies increase with the Li[TFSI] concentration in solution, and all activation energies were determined from the self-diffusion data for all ions. Interestingly, regardless of the composition in solution, these activation energies were similar, except for those determined for the [FSI]− anion. The activation energy of [FSI]− self-diffusion relatively decreases compared to the other ions as the lithium salt concentration increases. Furthermore, the lithium transference was strongly affected by the lithium salt concentration, reaching an optimal value and an ionicity of approximately 50 % at a molality close to 0.75 mol · kg−1. Finally, these electrolytes were used in lithium-ion batteries (i.e. Li/NMC and LTO/NMC), demonstrating a clear relationship between the electrolyte formulation, its transport parameters and battery performance.

Safe, Stable Cycling of Lithium Metal Batteries with Low‐Viscosity, Fire‐Retardant Locally Concentrated Ionic Liquid Electrolytes

“…Further, the activation energy for ion conduction ( E a ic , Figure 2c) can be deduced from Vogel–Tammann–Fulcher (VTF) analysis (Table S1, Supporting Information), indicating that LCIL exhibited a lower activation energy (5.3 kJ mol −1 ) than DIL and CIL (5.9 and 6.7 kJ mol −1 , respectively). These findings may be attributed to alterations in the conduction mechanism [ 6b,17 ] or nanoscale heterogeneity in the solution structure. [ 18 ]…”

“…Such t + enhancement at higher Li concentration agrees well with previous reports. [ 6c,21 ] Although neither t + NMR nor t + PP fully represents the true transference number ( t + 0 ), in both cases, the increased t + value at high concentration suggests that Li‐ion transport in CIL and LCIL is favored over anion transport.…”

“…The Li||LCO cell with CIL (CIL cell) exhibits a higher capacity than the DIL cell up to 1C, despite the high viscosity and low ionic conductivity. This counterintuitive behavior may be ascribed to the increased Li transport numbers, [ 21a,26 ] conductive SEI formation, [ 21a ] or an unusual transport mechanism [ 6b,c ] in IL electrolytes with a high Li‐salt concentration. Notable, under moderate cell conditions, such as much lower mass loading (<0.5 mAh cm −2 ), decent rate performance (>1C) could be attained, which is governed less by mass transport limitations at even higher C‐rates.…”

“…For instance, even at moderate Li salt concentration (≈1 m ), ILs significantly increase the electrolyte viscosity, thereby decreasing ionic conductivity tenfold. [ 6 ]…”

“…P13FSI has been widely studied as a promising IL for LMBs due to its favorable physicochemical and electrochemical properties. [ 6a,c,8b,12 ] Adding TTE enables a concentrated IL electrolyte without compromising the viscosity and improves the separator wettability, thereby remarkably enhancing the ionic conductivity and cell performance. Local ionic clusters composed of Li + , TFSI − , and FSI − at the microscopic level are spatially distributed by poorly solvating diluents but maintain their solvation structure.…”