Mechanism Explaining the Onset of Dendritic Li Electrodeposition Via Considerations of Li-Ion Transport within the SEI Layer

Abstract

High-energy density batteries are critically needed to power future electric vehicles. One major hurdle for rechargeable energy-dense Li-metal batteries is the dendritic electrodeposition of Li during the battery charging process. In this contribution, we report on our investigations of the onset time for dendritic growth during galvanostatic Li electrodeposition. Using electrochemical techniques combined with optical microscopy, we show that Li dendrites initiate at a time when the surface overpotential during galvanostatic electrodeposition reaches a maximum value. This observation is explained using an analytical transport model wherein the Li+ concentration within the solid electrolyte interphase (SEI) at the Li–SEI interface decreases gradually as the SEI thickens and its transport resistance increases with time. At the dendrite onset time (τonset), the Li+ concentration at the Li–SEI interface approaches zero – a condition under which surface roughness on the Li electrode amplifies, producing dendrites. Once dendrites form, they rupture the SEI, lowering the surface resistance for plating. Model predictions of how τonset varies with current density and soak time are shown to be in qualitative agreement with experimental observations. Pulsed currents are applied to mitigate Li+ concentration depletion within the SEI, thereby delaying or preventing altogether the formation of dendrites. The effect of temperature on τonset is also investigated and this temperature-dependence is discussed in the framework of the aforementioned transport model.