CONICET | Buscador de Institutos y Recursos Humanos

Glycol ethers, or glymes, have been recognized asgood candidates as solvents for lithium−air batteries because theyexhibit relatively good stability in the presence of superoxideradicals. Diglyme (bis(2-methoxy-ethyl)ether), in spite of its lowdonor number, has been found to promote the solutionmechanism for the formation of Li2O2 during the dischargereaction, leading to large deposits, that is, high capacities. It hasbeen suggested that lithium salt association in these types ofsolvents could be responsible for this behavior. Thus, theknowledge of the speciation and transport behavior of lithiumsalts in these types of solvents is relevant for the optimization ofthe lithium−air battery performance. In this work, a compre-hensive study of lithium trifluoromethanesulfonate (LiTf) andlithium 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 tripletions formation constants have been obtained by resorting to well-known equations that describe the concentration dependenceof the molar conductivities in highly associated electrolytes, and we found that the system LiTf/DME would be the best topromote bulky Li2O2 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 torationalize these differences in terms of the competing interactions of the O-sites of the ethers and the SOx groups of thecorresponding anions with Li+ ion. The limiting Li+ diffusivity derived from the fractional Walden rule agrees quite well withthose obtained from MD simulations, when solvent viscosity is conveniently rescaled.