Abstract
To unravel mechanistic details of the ion transport in liquid electrolytes, blends of the ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyr14TFSI), ethylene carbonate (EC) and dimethyl carbonate (DMC) with the conducting salts lithium hexafluorophosphate (LiPF6) and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) were investigated as a function of the IL concentration. Electrochemical impedance, Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR) and Raman spectroscopy supported by Molecular Dynamics (MD) simulations allowed the structural and dynamic correlations of the ion motions to be probed. Remarkably, we identified that though the individual correlations among different ion types exhibit a clear concentration dependence, their net effect is nearly constant throughout the entire concentration range, resulting in approximately equal transport and transference numbers, despite a monitored cross-over from carbonate-based lithium coordination to a TFSI-based ion coordination. In addition, though dynamical ion correlation could be found, the absolute values of the ionic conductivity are essentially determined by the overall viscosity of the electrolyte. The IL/carbonate blends with a Pyr14TFSI fraction of ∼10 wt% are found to be promising electrolyte solvents, with ionic conductivities and lithium ion transference numbers comparable to those of standard carbonate-based electrolytes while the thermal and electrochemical stabilities are considerably improved. In contrast, the choice of the conducting salt only marginally affects the transport properties.
Highlights
PaperAmong the available alternatives, room temperature ionic liquids (RTILs) offer superior thermal and electrochemical stabilities[3,19] but unfavorably high viscosity, which according to the Stokes–Einstein equation has negative impact on ion transport properties.[19,20] several efforts were made to improve ion mobility in ionic liquids thereby making them more attractive for lithium-ion battery applications
In contrast to proton conduction in aqueous phases, which is often based on vehicular diffusion or Grotthuss-like ‘‘hopping’’ of hydrated hydronium ions,[23,26,61] the presence of various solvents and additives in addition to lithium salts and/or ILs renders an explicit description of probable mechanisms and phenomena responsible for an observable charge transport in lithium ion batteries difficult
The lithium salt may fully dissociate into solvated lithium ions and anions while in case of more concentrated solutions, anions could possibly attach to the lithium ions yielding aggregates including ion pairs, triple ions or even so-called quadrupoles, respectively.[63]
Summary
Room temperature ionic liquids (RTILs) offer superior thermal and electrochemical stabilities[3,19] but unfavorably high viscosity, which according to the Stokes–Einstein equation has negative impact on ion transport properties.[19,20] several efforts were made to improve ion mobility in ionic liquids thereby making them more attractive for lithium-ion battery applications. Among the large variety of possible ILs, Pyr14TFSI is attractive due to its high decomposition temperature of 360 1C and its ability to improve both operational safety and ionic conductivity of carbonate-based electrolytes Macroscopic properties such as thermal properties of Pyr14TFSI/EC/DMC blends[38] are commonly considered, while often neglecting the molecular details or ion speciation that govern the achievable ion transport properties of such blends, except for recent work.[39] when aiming at a comprehensive understanding of probable ion transport processes in Pyr14TFSI/EC/DMC blends in the presence of lithium salts, it is important to consider macroscopic properties of electrolytes including viscosities, self-diffusion coefficients, molar conductivities, apparent activation energies or transport numbers and the degree of ion dissociation, respectively. The induced dipoles were determined iteratively where the corresponding dipole–dipole interactions were scaled to zero by a tapering function between 14.5 and 15 Å
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