Transport of Li+ and O2 in cathodes of lithium-air batteries

G. HORWITZ; H. A. CORTÉS PAÉZ; M. FACTOROVICH; M. P. LONGINOTTI; H. R. CORTI

Buenos Aires

Congreso; 20th Topical Meeting of the International Society of Electrochemistry; 2017

International Society of Electrochemistry

It has been recognized that the level of dissociation of the lithium salt used in a lithium-air battery (LAB) plays a significant role in the oxygen reduction reaction (ORR) [1]. Moreover, the conductivity of the lithium electrolyte and the diffusivity of oxygen determine the overpotentials during charge/discharge of LABs [2]. Thus, it is of relevance to have tools for predicting the values of these transport coefficients, along with the speciation of lithium salts, in solvents used in LABs, particularly in glymes (glycol ethers). These solvents seem to be good candidates for non-aqueous LAB, in view of their good stability under the electrochemical conditions of the cathodic processes.The estimation of ionic conductivity and diffusion coefficients is commonly performed by resorting to simple hydrodynamic models, which lead to the well-known Stokes-Einstein relationship (SER) for the diffusion coefficients and the Walden rule (WR) for the ionic conductivity. In both cases the hydrodynamic model predict that the products lh and Dh/T (where l is the ionic molar conductivity, D is the diffusion coefficient, h is the solvent viscosity, and T is the temperature) are constants that only depend on the hydrodynamic radii of the species that are transported by concentration or potential gradients. A comprehensive analysis of the literature data indicates that SER underestimate the diffusion coefficient of oxygen in all the solvents of interest for LABs by a factor as large as 5. This is not unexpected in view that SER assumes that the size of the diffusing molecule is much higher that the size of the solvent molecules. We concluded that semiempirical equations that take into account the diffusion-viscosity decoupling, or free volume concepts applicable to the diffusion of small solutes in molecular solvents, describe much better the behavior of oxygen diffusivity with solvent fluidity. We also performed a detailed analysis of the electrical conductivity of lithium trifluoromethanesulfonate (LiTf: lithium triflate) and lithium bistrifluoromethanesulfonimidate (LiTFSI) in DME and diglyme, over a wide range of concentrations, including the concentrated region where the formation of triplets and quadruplets ions predominates. The maximum specific conductivities of these salts in the studied glymes are of the order of several mS.cm-1, which is appropriate for non-aqueous LABs. Precise measurements of the ionic conductivity of lithium salts in the dilute and moderate concentration regime allowed us to obtain reliable values of ion pairs and triplet ions formation constants, improving the speciation analysis of lithium salts in glymes, previously reported [3]. It is interesting to note that the infinite dilution conductivity of Li+ ions in aprotic solvents of different molecular sizes obeys WR. We have analyzed this fact in terms of ionic mobility models and we propose that dielectric friction, instead of viscous friction, determines the mobility of Li+ ions in these solvents.Finally, we report preliminary results for the partition and diffusion of Li+ ions in monolithic mesoporous carbon embedded with glymes, in order to reproduce conditions similar to those at the LAB´s cathodes. Tortuosity and confinement effects are discussed in relation to the pore sizes distribution of the carbon and the bulk transport properties of the electrolytes.