Ionic Transport and Speciation of Lithium Salts in Glymes: Experimental and Theoretical Results for Electrolytes of Interest for Lithium–Air Batteries

Abstract

Glycol ethers, or glymes, have been recognized as good candidates as solvents for lithium–air batteries because they exhibit relatively good stability in the presence of superoxide radicals. Diglyme (bis(2-methoxy-ethyl)ether), in spite of its low donor number, has been found to promote the solution mechanism for the formation of Li 2 O 2 during the discharge reaction, leading to large deposits, that is, high capacities. It has been suggested that lithium salt association in these types of solvents could be responsible for this behavior. Thus, the knowledge of the speciation and transport behavior of lithium salts in these types of solvents is relevant for the optimization of the lithium–air battery performance. In this work, a comprehensive study of lithium trifluoromethanesulfonate (LiTf) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1,2-di-methoxyethane (DME) and diglyme, over a wide range of concentrations, have been performed. Consistent ion pairs and triplet ions formation constants have been obtained by resorting to well-known equations that describe the concentration dependence of the molar conductivities in highly associated electrolytes, and we found that the system LiTf/DME would be the best to promote bulky Li 2 O 2 deposits. Unexpected differences are observed for the association constants of LiTf and, to a lesser extent, for LiTFSI, in DME and diglyme, whose dielectric constants are similar. Molecular dynamics (MD) simulations allowed us to rationalize these differences in terms of the competing interactions of the O-sites of the ethers and the SO x groups of the corresponding anions with Li + ion. The limiting Li + diffusivity derived from the fractional Walden rule agrees quite well with those obtained from MD simulations, when solvent viscosity is conveniently rescaled.

Structure of Magnesium Chloride Complexes in Ethereal Systems: Computational Comparison of THF and Glymes as Solvents for Magnesium Battery Electrolytes

“…Also, longer glymes, up to tetraglyme (G4), have been used successfully, although some increase in overpotential is observed [12,22,23] . In general, an increase in glyme length gives a (desired) lower volatility of the electrolyte, but at the cost of higher viscosity and slower transport kinetics [22,24–26] . Undoubtedly, that also affects the induced structure of the complexes and the reached equilibrium, which is crucial for magnesium electrolytes, determining many parameters, including the battery lifetime.…”

Quantitative Mapping of Molecular Substituents to Macroscopic Properties Enables Predictive Design of Oligoethylene Glycol-Based Lithium Electrolytes

“… 52 , 53 In addition, molal conductivity and viscosity are inversely correlated (see Figure S3 for the correlations between conductivity and molar viscosity, also known as Walden analysis), in agreement with data collected for glyme-based electrolytes as well. 52 , 54 , 55 Finally, introducing substituents was generally observed to lower molal ionic conductivity and increase viscosity compared to the parent octaglyme-based electrolyte.…”

Quantitative Mapping of Molecular Substituents to Macroscopic Properties Enables Predictive Design of Oligoethyleneglycol-Based Lithium Electrolytes

“…50,51 In addition, molal conductivity and viscosity are inversely correlated (see Figure S3 for the correlations between conductivity and molar viscosity, also known as Walden analysis), in agreement with data collected for glyme-based electrolytes as well. 50,52,53 Finally, substituents were generally observed to lower molal ionic conductivity and increase viscosity compared to the parent octaglyme-based electrolyte.…”