Design of Nanostructured Heterogeneous Solid Ionic Coatings through a Multiscale Defect Model

Understanding of the electrical conduction, that is, ionic and electronic conduction, through the solid electrolyte interphase (SEI) is critical to the design of durable lithium-ion batteries (LIBs) with high rate capability and long life. It is believed that an ideal SEI should not only be an ionic conductor, but also an electronic insulator. In this study, we present a theoretical design of an artificial SEI consisting of lithium fluoride (LiF) and lithium carbonate (Li₂CO₃) on a LIB anode based on a newly developed density functional theory (DFT) informed space charge model. We demonstrate that the migration of lattice Li ions from LiF phase to form Li interstitials in Li₂CO₃ is energetically favorable near the LiF/Li₂CO₃ interface. At equilibrium, this interfacial defect reaction establishes a space charge potential across the interface, which causes the accumulation of ionic carriers but the depletion of electronic carriers near the LiF/Li₂CO₃ interface. To utilize this space charge effect, we propose a computationally designed, nanostructured artificial SEI structure with high density of interfaces of LiF and Li₂CO₃ perpendicular to the electrode. On the basis of this structure, the influences of grain size and volume ratio of the two phases were studied. Our results reveal that reducing the grain size of Li₂CO₃ in the nanostructured composite can promote ionic carriers and increase the ionic conductivity through the composite SEI by orders of magnitude. At the same time, the electronic conductivity is reduced due to electron depletion near the LiF/Li₂CO₃ interface. Furthermore, an optimal volume fraction that ensures high ionic and low electronic conduction was predicted.