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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.

The increasing interest in developing safe and sustainable energy storage systems has led to the rapid rise in attention to superconcentrated electrolytes, commonly called water-in-salt (WiS). Several works indicate that the transport properties of these liquid electrolytes are related to the presence of nanodomains, but a detailed characterization of such structure is missing. Here, the structural nano-heterogeneity of lithium WiS electrolytes, comprising lithium trifluoromethanesulfonate (LiTf) and bis(trifluoromethanesulfonyl)imide (LiTFSI) solutions as a function of concentration and temperature, was assessed by resorting to the analysis of small-angle neutron scattering (SANS) patterns. Variations with the concentration of a correlation peak, rather temperature-independent, in a Q range around 3.5−5 nm −1 indicate that these electrolytes are composed of nanometric water-rich channels percolating a 3D dispersing anion-rich network, with differences between Tf and TFSI anions related to their distinct volumes and interactions. Furthermore, a common trend was found for both systems' morphology above a salt volume fraction of ∼0.5. These results imply that the determining factor in the formation of the nanostructure is the salt volume fraction (related to the anion size), rather than its molality. These findings may represent a paradigm shift for designing WiS electrolytes.

In situ subtractively normalized Fourier transform infrared spectroscopy (SNIFTIRS) experiments were performed simultaneously with electrochemical experiments relevant to Li-air battery operation on gold electrodes in two glyme-based electrolytes: diglyme (DG)...

The choice of optimal electrolytes is crucial for improving the electrochemical performance of a lithium air battery because it determines the morphology of the discharge products in the cathode and the conductivity of the electrolyte. We have critically analyzed an important aspect related to the behavior of highly associated electrolytes, as those used in lithium−air batteries: the prediction of the concentration of maximum conductivity. Lithium triflate and lithium bis(trifluoromethyl sulfonyl)imide in glymes of low dielectric constant (1,2-di-methoxyethane and bis(2-methoxy-ethyl)ether) were used as a model of electrolytes exhibiting strong ionic clustering. The viscosity and lithium transference number of all the electrolytes were measured, and it was found that the correlation between the concentration of maximum conductivity and coefficient that describes the high-order dependence of the electrolyte viscosity with the concentration is no longer valid in these electrolytes because of the failure of Walden's rule, although a qualitative correlation with the salts' association constant was observed.

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