Defect Thermodynamics and Ion Transport in Inorganic SEI Compounds

In rechargeable batteries, electrolytes are often unstable under operating potentials which results in the formation of a mixture of organic and inorganic compounds on the electrodes. This solid electrolyte interphase (SEI) has profound effects on battery performance. Thus, characterization of the SEI compounds in terms of ionic and electronic transport is essential for the understanding of the limitations in charge-discharge rates, overpotential, electrode passivation, metal plating etc. LiF and NaF are two of the common SEI compounds primarily formed when electrolytes contain fluorinated salts in Li- and Na-ion batteries. LiOH on the other hand, results from water content in the electrolyte. These materials have wide band gaps that favor the insulation of electrode surface by impeding further reduction of electrolyte. However, the ionic transport and the mediating defects in these compounds are still in question. In this study, we conducted density functional theory calculations to identify the defect formation energies and concentrations in a range of relevant Li and Na chemical potentials. We considered several possible defect sites at different charge states. The close packing of the LiF and NaF makes it energetically unfavorable for interstitials to be present in considerable quantities. However this is not the case for LiOH. Accordingly, vacancies of the constituent elements are found to be predominant defects in fluorides. Diffusion of Li and Na is also evaluated by employing nudged-elastic band (NEB) method in defected structures. The activated states and the diffusion barriers of possible transport routes will be presented.