A Model of Concurrent Lithium Dendrite Growth, SEI Growth, SEI Penetration and Regrowth

We present a model that enables simulations of concurrent coupled lithium dendrite growth, SEI formation and growth on the dendrite surface, dendrite penetration of the SEI layer, and SEI regrowth. The concept of SEI coverage density is introduced, which is conceptually similar to SEI thickness but broader. SEI and dendrite growth interacts with each other by evolving dendrite morphology, stretching of SEI by the dendrite surface, and the effect of evolving SEI thickness (coverage density) on lithium deposition reaction. The model shows that even under low current density, small protrusions on lithium surface can still grow into dendrites by penetrating SEI and causing inhomogeneous electrical resistance on dendrite surface. We find that dendrite growth shows two distinct stages: fast acceleration during SEI penetration, and stable growth after reaching an equilibrium SEI thickness at the dendrite tip. Under low current density, increasing current scales up the growth rate of dendrite without much shape change. High current density causes dendrite morphology to change significantly, forming a needle shape. We show that local SEI defects/inhomogeneity can induce dendrite formation and growth. Our study suggests that flat lithium surface, uniform SEI layer, and low SEI resistivity are important to reduce dendrite formation.